Showing 1–41 of 41 results for author: Signorini, M

Precision near-IR spectroscopy for understanding AGN physics and shed light on the H0 tension -- SHARP Science Book

Authors: M. Signorini, V. N. Bennert, E. Dalla Bontà, F. Ricci, T. Treu, L. Villafaña

Abstract: The persistent tension between early- and late-Universe measurements of the Hubble constant (H0) remains on of the most significant challenges in modern cosmology. The Spectroastrometry and Reverberation Mapping (SARM) method offers a promising, calibration-independent approach to address this issue by combining time-delay measurements of the Broad-Line Region (BLR) with interferometric angular si… ▽ More The persistent tension between early- and late-Universe measurements of the Hubble constant (H0) remains on of the most significant challenges in modern cosmology. The Spectroastrometry and Reverberation Mapping (SARM) method offers a promising, calibration-independent approach to address this issue by combining time-delay measurements of the Broad-Line Region (BLR) with interferometric angular size determinations. Current implementations of SARM, however, are limited by the difficulty of performing near-infrared reverberation mapping (RM) on the same emission lines observed by GRAVITY, restricting applications to only a few bright AGN. We propose using the capabilities of SHARP, the next-generation near-infrared spectrograph for the Extremely Large Telescope (ELT), to overcome these limitations. SHARP's sensitivity and multi-object spectroscopy will enable (1) efficient long-term monitoring of existing GRAVITY targets with minimal time investment, and (2) systematic RM campaigns for the fainter AGN that will be observed by GRAVITY+. These advances will give us precise infrared lags for tens of AGN, enabling geometric distance measurements and a robust, calibration-free determination of H0. Beyond cosmology, SHARP will allow detailed studies of BLR structure and kinematics in the infrared, advancing our understanding of AGN physics and with repercussion on the measurements of Supermassive Black Holes (SMBH) masses. △ Less

Submitted 30 June, 2026; originally announced July 2026.

Comments: Accepted for publication in the New Astronomy Special issue SHARP Science Book

Journal ref: New Astronomy, 102592, V. 128, 2026

No evolution in the number density of little red dots from cosmic dawn to cosmic noon

Authors: Federica Loiacono, Roberto Gilli, Marco Mignoli, Marcella Brusa, Francesco Calura, Marco Chiaberge, Andrea Comastri, Quirino D'Amato, Roberto Decarli, Ivan Delvecchio, Kazushi Iwasawa, Ignas Juodžbalis, Giorgio Lanzuisi, Roberto Maiolino, Stefano Marchesi, Giovanni Mazzolari, Colin Norman, Alessandro Peca, Isabella Prandoni, Matteo Sapori, Matilde Signorini, Paolo Tozzi, Eros Vanzella, Cristian Vignali, Fabio Vito , et al. (2 additional authors not shown)

Abstract: We present our search for little red dots (LRDs) in the "J1030 field", a region of the sky around the $z\sim 6.3$ quasar SDSS J1030+0524, observed by the JWST EIGER program. Over 154 point-like sources selected in a JWST-based photometric catalog, we find five broad line emitters (with $FWHM \gtrsim 1000\ \rm km s^{-1}$) that are red ($F200W - F356W > 0$) and are undetected in the X-rays. We use t… ▽ More We present our search for little red dots (LRDs) in the "J1030 field", a region of the sky around the $z\sim 6.3$ quasar SDSS J1030+0524, observed by the JWST EIGER program. Over 154 point-like sources selected in a JWST-based photometric catalog, we find five broad line emitters (with $FWHM \gtrsim 1000\ \rm km s^{-1}$) that are red ($F200W - F356W > 0$) and are undetected in the X-rays. We use these sources to derive the bolometric luminosity function (LF) of LRDs at $z = 2.4$ and $z = 4.5$. At $z = 2.4$, the space density of LRDs is only a factor of $\sim 2$ lower than that of all pre-JWST active galactic nuclei (AGNs) with bolometric luminosity $L_{\rm bol} \gtrsim 3 \times 10^{44}\ \rm erg\ s^{-1}$. At $z = 4.5$, our estimate is consistent with those derived for LRDs based on larger areas of the sky. A similar behaviour is observed in the black hole mass function. More importantly, we study the number density of LRDs from cosmic dawn to cosmic noon. We find that there is no significant evolution in the abundance of LRDs with $L_{\rm bol} \gtrsim 3 \times 10^{44}\ \rm erg\ s^{-1}$ at $z > 2$. We speculate that the drop at $z < 4$ seen by other studies is due to their sampling of only the bright-end of the LRDs LF. At cosmic noon, the abundance of LRDs is $n = 3.4^{+5.6}_{-2.4} \times 10^{-5}\ \rm Mpc^{-3}$, which is a factor of $\sim 350$ larger than recent model predictions and is comparable with that of X-ray selected AGNs with similar bolometric luminosity. Our result may imply that, if LRDs are the early, rapid stages of supermassive black hole growth, as suggested by some models, then the formation of black hole seeds can be efficient down to epochs as recent as cosmic noon. Alternatively, LRDs may simply be a high-accretion phase in already mature black holes. △ Less

Submitted 29 June, 2026; originally announced June 2026.

Comments: 19 pages, 10 figures. Submitted to A&A

Observation of A Solar Like Magnetic Reconnection Event in an AGN Corona with XRISM

Authors: Gal Vardi, Ehud Behar, Liyi Gu, Jelle Kaastra, Matteo Guainazzi, Missagh Mehdipour, Keigo Fukumura, Jon Miller, Ari Laor, Erin Kara, Megan E. Eckart, Misaki Mizumoto, Christos Panagiotou, Chen Li, Ogawa Shoji, Matilde Signorini, Keqin Zhao

Abstract: The X-ray source in AGN is commonly referred to as the corona by analogy to stellar coronae. The similarities between the two suggest that the heating mechanism of AGN coronae is magnetic reconnection -- as in cool stars -- but this has not yet been directly observed. This work presents the first observational evidence for a magnetic reconnection flare in an AGN corona. We report on a flare in NGC… ▽ More The X-ray source in AGN is commonly referred to as the corona by analogy to stellar coronae. The similarities between the two suggest that the heating mechanism of AGN coronae is magnetic reconnection -- as in cool stars -- but this has not yet been directly observed. This work presents the first observational evidence for a magnetic reconnection flare in an AGN corona. We report on a flare in NGC 3783, which was observed with XRISM/Xtend and XMM-Newton/EPIC-PN exhibiting distinct temporal evolution in soft ($<2.0\,$keV) and hard ($>2.0\,$keV) X-rays. An Ultra-Fast Outflow (UFO) was detected during the event. The flare features the Neupert effect -- a temporal signature of the hard light curve correlating with the time derivative of the soft light curve, which shows that the flare is powered by magnetic reconnection, The Neupert effect is widely observed in the Sun, with Coronal Mass Ejections (CMEs) playing a role analogous to the UFO. We derive an upper limit of $30 \, R_g$ on the height of the magnetic loop from which the flare originates. Using the UFO's measured properties to characterize the magnetic field, we obtain $B > 1.3 \times 10^4\,$G for the field annihilated during the flare from total energy considerations, and $B \approx 500\,$G for the momentary magnetic field during reconnection from a dynamical consideration. △ Less

Submitted 16 July, 2026; v1 submitted 24 June, 2026; originally announced June 2026.

Comments: Accepted to ApjL. Added Figure 4

Journal ref: Astrophys. J. Lett. 1007 (2026) L23

The X-ray-to-UV relation does not evolve in homogeneous quasar samples

Authors: G. Risaliti, E. Lusso, E. Nardini, C. Niccolai, M. Ralowski, A. Sacchi, A. Shlentsova, M. Signorini, B. Trefoloni

Abstract: We present a new, highly homogeneous quasar sample with X-ray and UV observations optimized to reliably estimate distances via the non-linear X-ray-to-UV relation. Cross-matching the Sloan Digital Sky Survey DR16 quasar catalog with the XMM-Newton serendipitous catalogue (4XMM--DR14), we employ strict selection criteria to build a robust sample: (1) UV and (2) X-ray colour constraints to avoid, re… ▽ More We present a new, highly homogeneous quasar sample with X-ray and UV observations optimized to reliably estimate distances via the non-linear X-ray-to-UV relation. Cross-matching the Sloan Digital Sky Survey DR16 quasar catalog with the XMM-Newton serendipitous catalogue (4XMM--DR14), we employ strict selection criteria to build a robust sample: (1) UV and (2) X-ray colour constraints to avoid, respectively, extinction and absorption; (3) removal of broad absorption line and radio-bright quasars; (4) exclusion of sources at z<0.7 to prevent galactic UV contamination; and (5) rejection of sources with shallow X-ray observations. The latter step, closely related to the Eddington bias, is critical because SDSS data are generally deeper than X-ray data for typical quasar spectral energy distributions: ignoring such a discrepancy introduces a spurious redshift dependence in the X-ray-to-UV relation parameters. Our final sample contains about 2,000 quasars at z=0.7--5. We demonstrate that the X-ray-to-UV relation is constant across this redshift range, with a mean slope of 0.58\,$\pm$\,0.01 and a dispersion of 0.15 dex. Our findings confirm the intrinsic stability of this relation over cosmic time, emphasizing that both homogeneity and robust Eddington bias corrections are vital for flux-limited samples. In fact, the impact of the preferential detection of X-ray brighter-than-average sources near the effective sensitivity limits significantly grows with redshift. Any resulting evolutionary trend in the X-ray-to-UV relation, especially in the form a slope flattening, is therefore either a largely spurious effect, or the result of mixing populations of quasars with intrinsically different spectral properties. △ Less

Submitted 5 June, 2026; originally announced June 2026.

Comments: 10 Pages, 8 Figures. Submitted to A&A. Comments welcome

Delving into the depths of NGC 3783 with XRISM: V. Broad-band modeling of ionized outflows

Authors: Keqin Zhao, Jelle S. Kaastra, Liyi Gu, Missagh Mehdipour, Megan E. Eckart, Keigo Fukumura, Matteo Guainazzi, Chen Li, Christos Panagiotou, Matilde Signorini

Abstract: The Seyfert 1 galaxy NGC 3783 hosts a multiphase warm absorber (WA) that has been extensively studied in the X-ray band. High-resolution spectra from 2000-2001 revealed a complex outflow with multiple ionization and velocity components. Two decades later, new XMM-Newton and XRISM observations allow us to investigate the long-term evolution of these outflows. We perform joint spectral modeling of t… ▽ More The Seyfert 1 galaxy NGC 3783 hosts a multiphase warm absorber (WA) that has been extensively studied in the X-ray band. High-resolution spectra from 2000-2001 revealed a complex outflow with multiple ionization and velocity components. Two decades later, new XMM-Newton and XRISM observations allow us to investigate the long-term evolution of these outflows. We perform joint spectral modeling of the XMM-Newton/RGS and XRISM/Resolve time-averaged spectra using the pion photoionization code within SPEX. We derive the ionization parameter, column density, turbulent velocity, and outflow velocity for each absorption component, and investigate their thermal stability and Absorption Measure Distribution (AMD) to characterize the physical and dynamical properties of the WA in NGC 3783 in 2024. We compare these results with the 2000-2001 epoch to assess long-term variability, stability, and possible changes in the absorber population. We identify eight WA components spanning log $ξ=$ 1.08-3.38 and outflow velocities of 480-1230 km s$^{-1}$. The ranges of column densities and turbulent velocities remain broadly consistent with the WAs from 2000-2001, but the earlier data contained more low-ionization, high-velocity components. The total column density in 2024 is 1.5 times larger than in 2000-2001, requiring replenishment by fresh material. The dominant Unresolved Transition Array (UTA) absorber (Comp. B3) has increased its column density by a factor of three while maintaining a similar ionization parameter. The WAs in NGC 3783 have undergone significant structural and dynamical evolution over the past 24 years. △ Less

Submitted 23 April, 2026; originally announced April 2026.

Journal ref: A&A 710, A192 (2026)

Resolved UV-Optical HST Imaging and Spectral Energy Distribution Modeling of Nearby BAT Active Galactic Nuclei

Authors: Connor Auge, Michael Koss, Kriti K. Gupta, Claudio Ricci, Benny Trakhtenbrot, Franz E. Bauer, Ezequiel Treister, Alessandro Peca, Brad Cenko, Kohei Ichikawa, Arghajit Janna, Darshan Kakkad, Richard Mushotzky, Kyuseok Oh, Alejandra Rojas Lilayú, David Sanders, Roberto Serafinelli, Matilde Signorini, Alessia Tortosa, C. Megan Urry

Abstract: We use high-resolution UV-to-optical imaging from the Hubble Space Telescope (HST) to construct spatially resolved spectral energy distributions (SEDs) for seven nearby ($z<0.07$) hard (14--195$\,$keV) X-ray-selected broad-line active galactic nuclei (AGN) with $L_{\rm bol}=10^{43.26}-10^{45.34}\,\rm{erg\,s^{-1}}$. The high spatial resolution of HST, which physically resolves structures on the sca… ▽ More We use high-resolution UV-to-optical imaging from the Hubble Space Telescope (HST) to construct spatially resolved spectral energy distributions (SEDs) for seven nearby ($z<0.07$) hard (14--195$\,$keV) X-ray-selected broad-line active galactic nuclei (AGN) with $L_{\rm bol}=10^{43.26}-10^{45.34}\,\rm{erg\,s^{-1}}$. The high spatial resolution of HST, which physically resolves structures on the scale of $\sim$50$\,$pc at $z=0.05$, enables the separation of AGN and host-galaxy emission through morphological decomposition with GALFIT, yielding improved measurements of AGN properties compared to those obtained with lower-resolution Swift UV/Optical Telescope (UVOT) data. AGN UV magnitudes derived from HST imaging (e.g., F225W) can differ by more than a magnitude from those from Swift/UVOT UVM2 due to extended nuclear emission. Additionally, the inclusion of high-resolution data at longer wavelengths (e.g., F814W) can significantly affect the resulting SED fit. Comparing fits of accretion disk and extinction models using HST and Swift/UVOT data, we find significant differences in the resulting parameters, with average differences of 2.0$\,$eV in the maximum disk temperature and 2.2$\,$mag in the AGN host-galaxy extinction. These differences ultimately lead to significant changes in bolometric luminosities and X-ray bolometric corrections, with the HST-based fits yielding average increases of $\sim$0.57$\,$dex and $\sim$0.66$\,$dex respectively. This demonstrates host-galaxy contamination in unresolved UV--optical data can strongly bias SED-based estimates of disk temperatures, extinction, bolometric luminosities, and X-ray bolometric corrections in AGN. Large-area, high-resolution imaging surveys from Euclid and the Nancy Grace Roman Space Telescope will extend these techniques to much larger AGN samples, enabling uniform, high-precision SED measurements in the near-IR. △ Less

Submitted 21 April, 2026; originally announced April 2026.

Comments: 14 pages, 8 figures, accepted to ApJ

BASS. LI. Cool gas supply of HI-massive local Seyfert galaxies

Authors: Jeein Kim, Aeree Chung, O. Ivy Wong, Junhyun Baek, Chandrashekar Murugeshan, Michael J. Koss, Kyuseok Oh, Franz Erik Bauer, Chin-Shin Chang, Yaherlyn Diaz, Kohei Ichikawa, Darshan Kakkad, Minjin Kim, Macon Magno, Ignacio del Moral-Castro, Richard Mushotzky, Alessandro Peca, Claudio Ricci, Matilde Signorini, Miguel Parra Tello, Benny Trakhtenbrot, Jong-Hak Woo

Abstract: We present neutral atomic hydrogen (HI) imaging observations of 22 HI-rich ($M_{\rm HI} \gtrsim 10^{9.7} M_\odot$), hard X-ray-selected local Seyferts to explore how cool gas is supplied to active galactic nuclei (AGN) hosts. The sample predominantly resides in group-like, gas-rich environments. About 80% (18/22) of the galaxies have HI-detected neighbors, 61% (11/18) of which clearly exhibit stro… ▽ More We present neutral atomic hydrogen (HI) imaging observations of 22 HI-rich ($M_{\rm HI} \gtrsim 10^{9.7} M_\odot$), hard X-ray-selected local Seyferts to explore how cool gas is supplied to active galactic nuclei (AGN) hosts. The sample predominantly resides in group-like, gas-rich environments. About 80% (18/22) of the galaxies have HI-detected neighbors, 61% (11/18) of which clearly exhibit strong lopsidedness, one-sided gas tails, and/or gas structures connecting to nearby companion galaxies, suggesting gas exchange histories. We examine the HI size-mass relation and star formation properties of these HI-rich AGN hosts, finding no systematic deviations from known scaling relations. In most cases, our samples are the most massive systems within their respective groups, implying that our sample is more likely to acquire gas rather than lose it. Interestingly, galaxies with more extended HI disks show stronger AGN activity. Considering that extended HI is often associated with external processes, this finding suggests that environmentally accreted gas - through galaxy interactions and gas exchange with neighboring systems - may have played a role in supplying additional fuel to the AGNs in our sample. Notably, the HI extent-AGN activity correlation becomes even tighter for those AGN hosts whose neighboring galaxies are gas poor or lack HI, further supporting externally supplied gas as a fuel source. △ Less

Submitted 16 March, 2026; originally announced March 2026.

Comments: 19 pages, 3 tables, 6+7 figures, accepted for publication in A&A

Radio selection of heavily obscured AGN in the J1030 field: unraveling a missing Compton-thick population

Authors: Giovanni Mazzolari, Roberto Gilli, Marco Mignoli, Marcella Brusa, Isabella Prandoni, Fabio Vito, Ivan Delvecchio, Giorgio Lanzuisi, Alessandro Peca, Andrea Comastri, Stefano Marchesi, Marco Chiaberge, Marisa Brienza, Cristian Vignali, Matilde Signorini, Quirino D'Amato, Fabrizio Gentile, Kazushi Iwasawa, Colin Norman, Alberto Traina, Federica Loiacono, Pietro Baldini, Marianna Annunziatella, Roberto Decarli

Abstract: We tested the effectiveness of radio selection to discover heavily obscured AGNs, particularly at high-z, and we measured their abundance for the first time from a radio perspective. We consider the radio sources detected in the J1030 field, which is one of the fields with the deepest combination of 1.4 GHz radio and X-ray observations. We defined a radio excess parameter as the ratio between the… ▽ More We tested the effectiveness of radio selection to discover heavily obscured AGNs, particularly at high-z, and we measured their abundance for the first time from a radio perspective. We consider the radio sources detected in the J1030 field, which is one of the fields with the deepest combination of 1.4 GHz radio and X-ray observations. We defined a radio excess parameter as the ratio between the star formation rate (SFR) that would correspond to the observed radio luminosity and the one directly derived from the spectral energy distribution (SED) fitting, $\rm REX=SFR_{1.4GHz}/SFR^{corr}_{SED}$. We then select as radio excess AGN those sources with $\rm REX>8.5$, corresponding to a $3σ$ excess above the median value. In this way, we find 145 radio-excess sources falling into the \textit{Chandra} X-ray image footprint but without X-ray detection. From the deep X-ray upper limits, we estimated a lower limit to the obscuration of each radio-excess AGN, finding on average $\log (N_H/\rm{cm^{-2}})>23.7$. A CTK AGN scenario is also supported by the results of the X-ray stacking analysis performed on sources at $z>1.5$, which revealed X-ray luminosities and hardness ratios compatible with very highly obscured AGN. Finally, we computed the number density of these radio-selected CTK AGN. While at $z\sim 2$ the radio number density agrees well with the CTK AGN predictions of different population synthesis models, at $z\sim3$ the radio selection returns a CTK AGN number density $\sim 2-3$ times larger than what is predicted by the X-ray models and observations. This result supports the effectiveness of radio emission in selecting the most obscured sources, unraveling a population of AGN potentially missed by X-rays surveys at $z>3$, paving the way to a synergistic use of the future radio and X-ray facilities such as the \textit{SKAO} and \textit{NewAthena}. △ Less

Submitted 10 March, 2026; originally announced March 2026.

Comments: Accepted for publication on A&A. 15 pages + appendix. Main figures: Fig. 3 (radio-excess selection), Fig. 5 (column density distribution of radio-excess sources non detected in the X-rays), Fig. 8 (radio-selected Compton-thick AGN number density)

BASS LV. Connecting X-ray variability with AGN physical properties and a new path to Cosmological distances

Authors: Matilde Signorini, Federica Ricci, Alessia Tortosa, Stefano Bianchi, Fabio La Franca, Franz E. Bauer, Fiona A. Harrison, Kohei Ichikawa, Arghajit Jana, Michael J. Koss, Tingting Liu, Kyuseok Oh, Alessandro Peca, Meredith Powell, Claudio Ricci, David B. Sanders, Roberto Serafinelli, Daniel Stern, Benny Trakhtenbrot, Ezequiel Treister, Megan Urry

Abstract: X-ray variability is a well-established characteristic of active galactic nuclei (AGN), known to correlate inversely with both the supermassive black hole mass and luminosity, although the degree of each remains a topic of debate. The potential of X-ray variability as a proxy for MBH or for intrinsic LX has led to proposals to use AGN as standard candles to test cosmological models. However, the l… ▽ More X-ray variability is a well-established characteristic of active galactic nuclei (AGN), known to correlate inversely with both the supermassive black hole mass and luminosity, although the degree of each remains a topic of debate. The potential of X-ray variability as a proxy for MBH or for intrinsic LX has led to proposals to use AGN as standard candles to test cosmological models. However, the large intrinsic dispersion in these relations has limited their practical applications. In this work, we investigate the dependence of X-ray variability on AGN physical properties using a sample of 134 Seyfert 1 galaxies from the BAT AGN Spectroscopic Survey (BASS), which is the largest sample to date, more than three times larger than those used in previous studies. Contrary to earlier findings, we observe that X-ray variability correlates with luminosity just as strongly as with MBH. Furthermore, we still do not find evidence for the expected anti-correlation between variability and Eddington ratio, even when using refined bolometric luminosities from SED fitting to compute the Eddington ratio. From a cosmological perspective, the increased sample size reduces the scatter in the log(L)-log(exvar) relation to ~0.63 dex - a significant improvement over previous results, but still too large to serve as competitive standard candles, when compared to SNIa (uncertainties on distances of ~5-10%) or the L(X)-L(UV) relation in quasars (uncertainties of 10-12%). We tested including the width of broad emission lines as additional parameters, but found that this does not significantly lower the observed dispersion, contrary to previous studies on smaller samples. Finally, we discuss how future X-ray missions such as AXIS and NewAthena will improve this scenario by enabling precise variability measurements for thousands of AGN up to redshift z~3, thereby enabling it as a new cosmological probe. △ Less

Submitted 25 May, 2026; v1 submitted 23 February, 2026; originally announced February 2026.

Comments: accepted for publication in A&A - final version

Journal ref: A&A 710, A9 (2026)

The X-ray properties of the most luminous quasars with strong emission-line outflows

Authors: Anastasia Shlentsova, Bartolomeo Trefoloni, Matilde Signorini, Guido Risaliti, Elisabeta Lusso, Emanuele Nardini, Franz E. Bauer, Matthew J. Temple, Amy L. Rankine, Gordon T. Richards

Abstract: Strong outflows from active galactic nuclei are frequently observed in objects with lower coronal X-ray luminosity. This intrinsic X-ray weakness is considered a requirement for the formation of radiatively driven winds. To obtain an unbiased view on the connection between X-ray emission and the presence of powerful winds in the most luminous quasar phase, we present an X-ray analysis of a sample… ▽ More Strong outflows from active galactic nuclei are frequently observed in objects with lower coronal X-ray luminosity. This intrinsic X-ray weakness is considered a requirement for the formation of radiatively driven winds. To obtain an unbiased view on the connection between X-ray emission and the presence of powerful winds in the most luminous quasar phase, we present an X-ray analysis of a sample of extremely luminous, radio-quiet quasars with signatures of strong outflows in their rest-frame ultraviolet (UV) emission spectra. We study the $Chandra$ X-ray spectral properties of 10 objects, selected from the Sloan Digital Sky Survey Data Release 16 quasar catalogue based on their UV luminosities and ${\rm C}_{\rm IV}$ emission line blueshifts, comparing them to typical optically blue quasars. Our analysis reveals that seven out of 10 quasars in our sample have photon indices $Γ>1.7$. Only two out of 10 objects exhibiting outflows with velocities exceeding 1400 km/s are X-ray 'weak', consistent with the fraction of X-ray 'weak' objects generally observed in quasar populations. Notably, one of the objects identified as X-ray 'weak' is likely an intrinsically X-ray 'normal' quasar that is heavily obscured. We observe a tentative indication at a $\sim$2$σ$ confidence level that the correlation between the excessively low X-ray flux level and the presence of ${\rm C}_{\rm IV}$ emission-line outflows might emerge at wind velocities greater than 3000 km/s. Our study provides additional evidence that the relationship between X-ray emission and the presence of winds is intricate. Our findings emphasise the need for X-ray observations of a larger sample of UV-selected quasars with confirmed strong emission-line outflows to unravel the nuanced interplay between winds and X-ray emission. △ Less

Submitted 6 February, 2026; originally announced February 2026.

Comments: 12 pages, 8 figures, accepted for publication in A&A

Journal ref: A&A 707, A313 (2026)

The accretion of quasars at the epoch of reionisation: $JWST$ catches the primeval monsters slowly feasting

Authors: B. Trefoloni, E. Nardini, S. Carniani, E. Lusso, A. Marconi, E. Parlanti, A. Sacchi, A. Shlentsova, M. Signorini, G. Risaliti, S. Zamora

Abstract: Quasars (QSOs) emit an enormous amount of light as a result of the accretion of gas onto supermassive black holes (SMBHs). Thanks to their luminosity, the most distant known QSOs allow us to trace the growth of SMBHs deep into the epoch of reionisation. In this work, we employed $JWST$/NIRSpec observations of eight luminous (log$(L_{3000\,A^{\circ}}/(erg \, s^{-1}))>$45.7) QSOs at $z\geq$5.9 to co… ▽ More Quasars (QSOs) emit an enormous amount of light as a result of the accretion of gas onto supermassive black holes (SMBHs). Thanks to their luminosity, the most distant known QSOs allow us to trace the growth of SMBHs deep into the epoch of reionisation. In this work, we employed $JWST$/NIRSpec observations of eight luminous (log$(L_{3000\,A^{\circ}}/(erg \, s^{-1}))>$45.7) QSOs at $z\geq$5.9 to constrain their accretion properties, namely black hole mass, accretion disc (AD) luminosity, and Eddington ratio ($M_{BH}$, $L_{AD}$, $λ_{Edd}$), by fitting the rest-frame UV and optical emission with different AD models. This method provided self-consistent measurements of both $M_{BH}$ and $L_{AD}$. The uncertainties on $M_{BH}$ and $L_{AD}$, obtained within the AD-modelling framework ($σ^{AD}_{M_{BH}}\sim$0.2 dex; $σ^{AD}_{L_{AD}}\sim$0.1 dex), are significantly smaller than the systematic uncertainties associated with single-epoch $M_{BH}$ ($\sim$0.4 dex) and $L_{AD}$ derived via bolometric corrections ($\sim$0.2 dex). Based on these results, in our sample we found an average Eddington ratio of $\langle \log(λ_{Edd}) \rangle=-0.9$, with a dispersion of $\sim$0.2 dex. Assuming that our high-z QSOs are representative of optically-selected bright blue QSOs, we derive a fraction of systems accreting above the Eddington limit of $\sim$0.2%. In conclusion, this work i) demonstrates the suitability of $JWST$ to test AD models on high-redshift ($z\gtrsim$4) QSOs, thanks to the large NIRSpec spectral coverage; ii) shows that AD modelling can yield robust $M_{\rm BH}$ and $L_{\rm AD}$ measurements, with smaller uncertainties than the typical calibrations; and iii) provides compelling evidence for sub-Eddington accretion in bright high-$z$ QSOs, challenging the widespread paradigm of near- or super-Eddington accretion occurring in these sources. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 19 pages, 8 figures. Submitted to A&A

Delving into the depths of NGC 3783 with XRISM IV. Mapping of the accretion flow with Fe K$α$ emission lines

Authors: Chen Li, Jelle S. Kaastra, Liyi Gu, Missagh Mehdipour, Megan E. Eckart, Matteo Guainazzi, Erin Kara, Laura W. Brenneman, Misaki Mizumoto, Jon Miller, Keigo Fukumura, Ehud Behar, Christos Panagiotou, Matilde Signorini, Keqin Zhao, Ralf Ballhausen, Camille M. Diez, Timothy R. Kallman, Shoji Ogawa, Atsushi Tanimoto, Yoshihiro Ueda

Abstract: Using XRISM/Resolve $439 \, \rm ks$ time-averaged spectra of the well-known Seyfert-1.5 active galactic nucleus (AGN) in NGC 3783, we investigate the nature of the Fe K$α$ emission line at 6.4 keV, the strongest and most common X-ray line observed in AGN. Even the narrow component of the line is resolved with evident Fe K$α_{1}$ (6.404 keV) and K$α_{2}$ (6.391 keV) contributions in a 2:1 flux rati… ▽ More Using XRISM/Resolve $439 \, \rm ks$ time-averaged spectra of the well-known Seyfert-1.5 active galactic nucleus (AGN) in NGC 3783, we investigate the nature of the Fe K$α$ emission line at 6.4 keV, the strongest and most common X-ray line observed in AGN. Even the narrow component of the line is resolved with evident Fe K$α_{1}$ (6.404 keV) and K$α_{2}$ (6.391 keV) contributions in a 2:1 flux ratio, fully consistent with a neutral gas with negligible bulk velocity. The narrow and intermediate-width components have a full-width at half maximum (FWHM) of 350 $\pm$ 50 km/s and $3510 \pm 470 \, \rm km/s$, respectively, suggesting that they arise in the outer disk/torus and/or BLR. We detect a $10\%$ excess flux around 4 $-$ 7 keV that is not well described by a symmetric Gaussian line, but is consistent with a relativistically broadened emission line. In this paper, we take the simplest approach to model the asymmetric line as a single emission line (assuming either neutral, He-like or H-like iron) convolved with a relativistic disk line model. As expected, the inferred inclination angle is highly sensitive to the assumed ionization state, and ranges between $i=17-44^{ \circ}$. This model also constrains the black hole spin via the extent of the red wing: the required gravitational redshift in the fitted disk-line profile disfavors a non-spinning (Schwarzschild) black hole. The derived inner radius is close to the radius of the innermost stable circular orbit $r_{\rm ISCO}$ and strongly correlated with the black hole spin. To better constrain the spin, we fix the inner radius at $r_{\rm ISCO}$ and derive a lower limit on the spin of $a \ge 0.29$ at the 3 $σ$ confidence level. A Compton shoulder is detected in our data as well as a $2-3 \, σ$ detection of the Cr K$α$ and Ni K$α$ lines. △ Less

Submitted 9 December, 2025; originally announced December 2025.

Comments: accepted by A\&A, 12 pages, 9 figures

Journal ref: A&A 706, A255 (2026)

Delving into the depths of NGC 3783 with XRISM III. Birth of an ultrafast outflow during a soft flare

Authors: Liyi Gu, Keigo Fukumura, Jelle Kaastra, Megan Eckart, Ralf Ballhausen, Ehud Behar, Camille Diez, Matteo Guainazzi, Timothy Kallman, Erin Kara, Chen Li, Missagh Mehdipour, Misaki Mizumoto, Shoji Ogawa, Christos Panagiotou, Matilde Signorini, Atsushi Tanimoto, Keqin Zhao, Hirofumi Noda, Jon Miller, Satoshi Yamada

Abstract: The 2024 X-ray/UV observation campaign of NGC 3783, led by XRISM, revealed the launch of an ultrafast outflow (UFO) with a radial velocity of 0.19c (57000 km/s). This event is synchronized with the sharp decay, within less than half a day, of a prominent soft X-ray/UV flare. Accounting for the look-elsewhere effect, the XRISM Resolve data alone indicate a low probability of 2e-5 that this UFO dete… ▽ More The 2024 X-ray/UV observation campaign of NGC 3783, led by XRISM, revealed the launch of an ultrafast outflow (UFO) with a radial velocity of 0.19c (57000 km/s). This event is synchronized with the sharp decay, within less than half a day, of a prominent soft X-ray/UV flare. Accounting for the look-elsewhere effect, the XRISM Resolve data alone indicate a low probability of 2e-5 that this UFO detection is due to random chance. The UFO features narrow H-like and He-like Fe lines with a velocity dispersion of 1000 km/s, suggesting that it originates from a dense clump. Beyond this primary detection, there are hints of weaker outflow signatures throughout the rise and fall phases of the soft flare. Their velocities increase from 0.05c to 0.3c over approximately three days, and they may be associated with a larger stream in which the clump is embedded. The radiation pressure is insuffcient to drive the acceleration of this rapidly evolving outflow. The observed evolution of the outflow kinematics instead closely resembles that of solar coronal mass ejections, implying magnetic driving and, conceivably, reconnection near the accretion disk as the likely mechanisms behind both the UFO launch and the associated soft flare. △ Less

Submitted 9 December, 2025; originally announced December 2025.

Comments: 13 pages, 12 figures, published on A&A

Journal ref: A&A, 704, A146 (2025)

The Advanced X-ray Imaging Satellite (AXIS) Community Science Book

Authors: Michael Koss, Nafisa Aftab, Steven W. Allen, Roberta Amato, Hongjun An, Igor Andreoni, Timo Anguita, Riccardo Arcodia, Thomas Ayres, Matteo Bachetti, Maria Cristina Baglio, Arash Bahramian, Marco Balboni, Ranieri D. Baldi, Solen Balman, Aya Bamba, Eduardo Banados, Tong Bao, Iacopo Bartalucci, Antara Basu-Zych, Rebeca Batalha, Lorenzo Battistini, Franz Erik Bauer, Andy Beardmore, Werner Becker , et al. (373 additional authors not shown)

Abstract: The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with ~1.5'' imaging over a wide 24' field of… ▽ More The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with ~1.5'' imaging over a wide 24' field of view and an order of magnitude greater collecting area than Chandra in the 0.3-12 keV band. Combining sharp imaging, high throughput, and rapid response capabilities, AXIS will open new windows on virtually every aspect of modern astrophysics, exploring the birth and growth of supermassive black holes, the feedback processes that shape galaxies, the life cycles of stars and exoplanet environments, and the nature of compact stellar remnants, supernova remnants, and explosive transients. This book compiles 138 community-contributed science cases developed by five Science Working Groups focused on AGN and supermassive black holes, galaxy evolution and feedback, compact objects and supernova remnants, stellar physics and exoplanets, and time-domain and multi-messenger astrophysics. Together, these studies establish the scientific foundation for next-generation X-ray exploration in the 2030s and highlight strong synergies with facilities of the 2030s, such as JWST, Roman, Rubin/LSST, SKA, ALMA, ngVLA, and next-generation gravitational-wave and neutrino networks. △ Less

Submitted 6 January, 2026; v1 submitted 31 October, 2025; originally announced November 2025.

Comments: 592 pages, 226 figures, version 2

BASS LIV. Physical Properties of AGN-Hosting Galaxy Mergers from Multiwavelength SED Fitting

Authors: Marco Troncoso, Ezequiel Treister, Alejandra Rojas, Médéric Boquien, Franz Bauer, Michael J. Koss, Roberto J. Assef, Miguel Parra Tello, Ignacio del Moral-Castro, Claudio Ricci, Sophia Dai, Kyuseok Oh, Frederica Ricci, Alessandro Peca, C. Megan Urry, Kriti Kamal Gupta, Giacomo Venturi, Matilde Signorini, Richard Mushotzky, David Sanders

Abstract: Galaxy mergers are believed to play an important role in triggering rapid supermassive black hole (SMBH) growth. As merging nuclei approach each other, the physical properties of the participating galaxies and the associated SMBH growth are expected to evolve significantly. This study measures and characterizes these physical properties throughout the merger sequence. We constructed multiwavelengt… ▽ More Galaxy mergers are believed to play an important role in triggering rapid supermassive black hole (SMBH) growth. As merging nuclei approach each other, the physical properties of the participating galaxies and the associated SMBH growth are expected to evolve significantly. This study measures and characterizes these physical properties throughout the merger sequence. We constructed multiwavelength Spectral Energy Distributions (SEDs) from hard X-rays to the far-infrared (FIR) for a sample of 72 nearby Active Galactic Nuclei (AGN) host galaxies. The sample comprises 64 interacting systems, including single AGNs in mergers and dual AGNs, with nuclear separations $\leq$30 kpc, as well as eight isolated active galaxies with merging features. We carefully adapted available photometric measurements at each wavelength to account for their complex morphologies and varying spatial resolutions, to perform SED fitting using CIGALE, aimed to derive critical physical properties. Our results reveal that merging galaxies hosting AGN(s) show deviations from the star-forming main sequence, and a wide range of star formation rates (SFRs). Both AGN activity and star formation are significantly influenced by the merger process, but these effects are more prominent in major, mass ratios $<$4:1, interactions. We find that the projected nuclear separation is not a good tracer of the merger stage. Instead, morphological classification accurately assesses the merger progression. Based on this morphological analysis, late-stage mergers exhibit elevated SFRs (5.1$\times$), AGN luminosities (2.4$\times$), and nuclear obscuration (2.8$\times$) compared to earlier stages, supporting previous findings and reinforcing the link between merger-driven galaxy evolution and SMBH growth. △ Less

Submitted 22 September, 2025; originally announced September 2025.

Comments: ApJ accepted, figuresets 2, 6 available on the online version of the Journal only

The X-ray$-$UV Luminosity Relation of eROSITA Quasars

Authors: Andrea Sacchi, Guido Risaliti, Matilde Signorini, Emanuele Nardini, Elisabeta Lusso, Bartolomeo Trefoloni

Abstract: The non-linear relation between the UV and X-ray luminosity in quasars has been studied for decades. However, as we lack a comprehensive model able to explain it, its investigation still relies on observational efforts. This work focuses on optically selected quasars detected by eROSITA. We present the properties of the sources collected in the eROSITA early data release (eFEDS) and those resultin… ▽ More The non-linear relation between the UV and X-ray luminosity in quasars has been studied for decades. However, as we lack a comprehensive model able to explain it, its investigation still relies on observational efforts. This work focuses on optically selected quasars detected by eROSITA. We present the properties of the sources collected in the eROSITA early data release (eFEDS) and those resulting from the first six months of eROSITA all-sky survey (eRASS1). We focus on the subset of quasars bright enough in the optical/UV band to avoid an ''Eddington bias'' towards X-ray brighter-than-average spectral energy distributions. The final samples include 1,248 and 519 sources for eFEDS and eRASS1, up to redshift $z\approx3$ and $z\approx1.5$, respectively. We found that the X-ray$-$UV luminosity relation shows no significant evolution with redshift, and its slope is in perfect agreement with previous compilations of quasar samples. The intrinsic dispersion of the relation is about 0.2 dex, which is small enough for possible cosmological applications. However, the limited redshift range and statistics of the current samples do not allow us to obtain significant cosmological constraints yet. We show how this is going to change with the future releases of the eROSITA data. △ Less

Submitted 16 October, 2025; v1 submitted 1 August, 2025; originally announced August 2025.

Comments: 8 pages, 8 figures, 1 table, Accepted for publication on A&A

Journal ref: A&A 703, A273 (2025)

BASS. XLIX. Characterization of highly luminous and obscured AGNs: local X-ray and [NeV]$λ$3426 emission in comparison with the high-redshift Universe

Authors: Alessandro Peca, Michael J. Koss, Kyuseok Oh, Claudio Ricci, Benny Trakhtenbrot, Richard Mushotzky, Ezequiel Treister, C. Megan Urry, Andrealuna Pizzetti, Kohei Ichikawa, Alessia Tortosa, Federica Ricci, Matilde Signorini, Darshan Kakkad, Chin-Shin Chang, Giovanni Mazzolari, Turgay Caglar, Macon Magno, Ignacio del Moral-Castro, Peter G. Boorman, Tonima T. Ananna, Fiona Harrison, Daniel Stern, David Sanders

Abstract: We present a detailed analysis of the most luminous and obscured Active Galactic Nuclei (AGNs) detected in the ultra-hard X-ray band (14-195 keV) by Swift/BAT. Our sample comprises 21 X-ray luminous (log $L_X/{\rm erg\,s^{-1}}>44.6$, 2-10 keV) AGNs at $z<0.6$, optically classified as Seyfert 1.9-2. Using NuSTAR, XMM-Newton, Suzaku, and Chandra, we constrain AGN properties such as absorption column… ▽ More We present a detailed analysis of the most luminous and obscured Active Galactic Nuclei (AGNs) detected in the ultra-hard X-ray band (14-195 keV) by Swift/BAT. Our sample comprises 21 X-ray luminous (log $L_X/{\rm erg\,s^{-1}}>44.6$, 2-10 keV) AGNs at $z<0.6$, optically classified as Seyfert 1.9-2. Using NuSTAR, XMM-Newton, Suzaku, and Chandra, we constrain AGN properties such as absorption column density $N_H$, photon index $Γ$, intrinsic $L_X$, covering factor, and iron K$α$ equivalent width. For sources with black hole mass estimates (12/20), we find a weak correlation between $Γ$ and Eddington ratio ($λ_{Edd}$). Of these, six ($50\pm13\%$) lie in the $N_H$-$λ_{Edd}$ "forbidden region'' and exhibit a combined higher prevalence of $N_H$ variability and outflow signatures, suggesting a transitional phase where AGN feedback may be clearing the obscuring material. For the 13/21 sources with multi-epoch X-ray spectra, $82^{+6}_{-16}\%$ exhibit variability in either 2-10 keV flux ($73^{+9}_{-16}\%$) or line-of-sight $N_H$ ($33^{+15}_{-10}\%$). For the 20/21 sources with available near-UV/optical spectroscopy, we detect [NeV]$λ$3426 in 17 ($85^{+5}_{-11}\%$), confirming its reliability to probe AGN emission even in heavily obscured systems. When normalized to the same [OIII]$λ$5007 peak flux as $z = 2$-$9$ narrow-line AGNs identified with JWST, our sample exhibits significantly stronger [NeV]$λ$3426 emission, suggesting that high-redshift obscured AGNs may be intrinsically weaker in [NeV]$λ$3426 or that [NeV]$λ$3426 is more challenging to detect in those environments. The sources presented here serve as a benchmark for high-redshift analogs, showing the potential of [NeV]$λ$3426 to reveal obscured AGNs and the need for future missions to expand X-ray studies into the high-redshift Universe. △ Less

Submitted 14 July, 2025; originally announced July 2025.

Comments: Accepted for publication in ApJ

AGN with massive black holes have closer galactic neighbors: k-Nearest-Neighbor statistics of an unbiased sample of AGN at z~0.03

Authors: A. Mhatre, M. C. Powell, S. Yuan, S. W. Allen, T. Caglar, M. Koss, I. del Moral-Castro, K. Oh, A. Peca, C. Ricci, F. Ricci, A. Rojas, M. Signorini

Abstract: The large-scale environments of active galactic nuclei (AGN) reveal important information on the growth and evolution of supermassive black holes (SMBHs). Previous AGN clustering measurements using 2-point correlation functions have hinted that AGN with massive black holes preferentially reside in denser cosmic regions than AGN with less-massive SMBHs. At the same time, little to no dependence on… ▽ More The large-scale environments of active galactic nuclei (AGN) reveal important information on the growth and evolution of supermassive black holes (SMBHs). Previous AGN clustering measurements using 2-point correlation functions have hinted that AGN with massive black holes preferentially reside in denser cosmic regions than AGN with less-massive SMBHs. At the same time, little to no dependence on the accretion rate is found. However, the significance of such trends have been limited. Here we present kth-nearest-neighbor (kNN) statistics of 2MASS galaxies around AGN from the Swift/BAT AGN Spectroscopic survey. These statistics have been shown to contribute additional higher-order clustering information on the cosmic density field. By calculating the distances to the nearest 7 galaxy neighbors in angular separation to each AGN within two redshift ranges(0.01 < z < 0.03 and 0.03 < z < 0.06), we compare their cumulative distribution functions to that of a randomly distributed sample to show the sensitivity of this method to the clustering of AGN. We also split the AGN into bins of bolometric luminosity, black hole mass, and Eddington ratio (while controlling for redshift) to search for trends between kNN statistics and fundamental AGN properties. We find that AGN with massive SMBHs have significantly closer neighbors than AGN with less-massive SMBHs (at the 99.98% confidence level), especially in our lower redshift range. We find less significant trends with luminosity or Eddington ratio. By comparing our results to empirical SMBH-galaxy-halo models implemented in N-body simulations, we show that small-scale kNN trends with black hole mass may go beyond stellar mass dependencies. This suggests that massive SMBHs in the local universe reside in more massive dark matter halos and denser regions of the cosmic web, which may indicate that environment is important for the growth of SMBHs. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: Accepted to A&A

Journal ref: A&A 701, A45 (2025)

Quasar cosmology II: joint analyses with Cosmic Microwave Background

Authors: M. Benetti, G. Bargiacchi, G. Risaliti, S. Capozziello, E. Lusso, M. Signorini

Abstract: Currently, the increasing availability of accurate cosmological probes leads to the emergence of tensions between data on the one hand and between theoretical predictions and direct observations on the other. Moreover, after 25 years since the discovery of the accelerated expansion of the Universe has elected the $Λ$CDM model as the reference model, resolving shortcomings of the standard cosmologi… ▽ More Currently, the increasing availability of accurate cosmological probes leads to the emergence of tensions between data on the one hand and between theoretical predictions and direct observations on the other. Moreover, after 25 years since the discovery of the accelerated expansion of the Universe has elected the $Λ$CDM model as the reference model, resolving shortcomings of the standard cosmological model seems to be an unpostponed priority. Hence, it is key to test alternative models and investigate new cosmological probes at distances that range from the late to the early Universe, namely between the cosmic microwave background (CMB) and type Ia supernovae and baryonic acoustic oscillations (BAO) data. Bargiacchi et al. (2022) for the first time analysed dark energy (DE) models using quasars (QSOs) while also testing their consistency with BAO. Here, we carry on by exploring the compatibility of QSOs with both CMB data and dark energy survey measurements against the standard cosmological model and some DE extensions, such as the $w$CDM and Chevallier-Polarski-Linder parameterisations. We also consider an interacting dark matter and vacuum energy scenario, where vacuum energy perturbations affect the evolution of the matter growth rate in a decomposed Chaplygin gas model. We implement the QSO probe in Cobaya Markov chain Monte Carlo algorithm, using Botzmann solver codes as Cosmic Linear Anisotropy Solving System (CLASS) for the theory predictions. Our work shows that simple DE deviations from $Λ$CDM model do not reconcile the data and that only more complex models of interaction in the dark sector can succeed in solving the discrepancies of probes at all scales. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 23 pages, 3 figures, accepted in Physics of Dark Universe

A big red dot at cosmic noon

Authors: Federica Loiacono, Roberto Gilli, Marco Mignoli, Giovanni Mazzolari, Roberto Decarli, Marcella Brusa, Francesco Calura, Marco Chiaberge, Andrea Comastri, Quirino D'Amato, Kazushi Iwasawa, Ignas Juodžbalis, Giorgio Lanzuisi, Roberto Maiolino, Stefano Marchesi, Colin Norman, Alessandro Peca, Isabella Prandoni, Matteo Sapori, Matilde Signorini, Paolo Tozzi, Eros Vanzella, Cristian Vignali, Fabio Vito, Gianni Zamorani

Abstract: We report the discovery of a little red dot (LRD), dubbed BiRD ('big red dot'), at $z=2.33$ in the field around the $z=6.3$ quasar SDSSJ1030+0524. Using NIRCam images, we identified it as a bright outlier in the $F200W-F356W$ color vs $F356W$ magnitude diagram of point sources in the field. The NIRCam/WFSS spectrum reveals the emission from HeI$λ10830$ and PaG line, both showing a narrow and a bro… ▽ More We report the discovery of a little red dot (LRD), dubbed BiRD ('big red dot'), at $z=2.33$ in the field around the $z=6.3$ quasar SDSSJ1030+0524. Using NIRCam images, we identified it as a bright outlier in the $F200W-F356W$ color vs $F356W$ magnitude diagram of point sources in the field. The NIRCam/WFSS spectrum reveals the emission from HeI$λ10830$ and PaG line, both showing a narrow and a broad ($FWHM\gtrsim 2000\ \rm kms^{-1}$) component. The HeI line is affected by an absorption feature, tracing dense gas with HeI column density in the $2^3S$ level $N\sim 0.5-1.2\times 10^{14}\rm cm^{-2}$, depending on the location of the absorber, which is outflowing at the speed of $Δv \sim -830\ \rm kms^{-1}$. As observed in the majority of LRDs, BiRD does not show X-ray or radio emission. The BH mass and the bolometric luminosity, both inferred from the PaG broad component, amount to $M_{\rm BH}\sim 10^8\rm M_{\odot}$ and $L_{\rm bol}\sim 2.9\times 10^{45}\rm ergs^{-1}$, respectively. Intriguingly, BiRD presents strict analogies with other two LRDs spectroscopically confirmed at cosmic noon, GN-28074 ("Rosetta Stone") at $z=2.26$ and RUBIES-BLAGN-1 at $z=3.1$. The blueshifted HeI absorption detected in all three sources suggests that gas outflows may be common in LRDs. We derive a first estimate of the space density of LRDs at $z<3$ based on JWST data, as a function of $L_{\rm bol}$ and BH mass. The space density is only a factor of $\sim 2-3$ lower than that of UV-selected quasars with comparable $L_{\rm bol}$ and $z$, meaning that the contribution of LRDs to the broader AGN population is also relevant at cosmic noon. A similar trend is also observed in terms of BH masses. If, as suggested by recent theories, LRDs probe the very first and rapid growth of black hole seeds, our finding may suggest that the formation of black hole seeds remains efficient at least up to cosmic noon. △ Less

Submitted 9 September, 2025; v1 submitted 13 June, 2025; originally announced June 2025.

Comments: 14 pages, 7 figures, accepted for publication in A&A

Journal ref: A&A 703, A36 (2025)

The radio properties of the JWST-discovered AGN

Authors: G. Mazzolari, R. Gilli, R. Maiolino, I. Prandoni, I. Delvecchio, C. Norman, E. F. Jimenez-Andrade, S. Belladitta, F. Vito, E. Momjian, M. Chiaberge, B. Trefoloni, M. Signorini, X. Ji, Q. D'Amato, G. Risaliti, R. D. Baldi, A. Fabian, H. Übler, F. D'Eugenio, J. Scholtz, I. Juodžbalis, M. Mignoli, M. Brusa, E. Murphy , et al. (1 additional authors not shown)

Abstract: We explore the radio emission of JWST-selected Broad Line AGN (BLAGN, or type 1) in the GOODS-N field. We use deep radio data at different frequencies (144\,MHz, 1.5\,GHz, 3\,GHz, 5.5\,GHz, 10\,GHz), and we find that none of the {37} sources investigated is detected at any of the aforementioned frequencies. Similarly, the radio stacking analysis does not reveal any detection down to an rms of… ▽ More We explore the radio emission of JWST-selected Broad Line AGN (BLAGN, or type 1) in the GOODS-N field. We use deep radio data at different frequencies (144\,MHz, 1.5\,GHz, 3\,GHz, 5.5\,GHz, 10\,GHz), and we find that none of the {37} sources investigated is detected at any of the aforementioned frequencies. Similarly, the radio stacking analysis does not reveal any detection down to an rms of ${\sim 0.15}μ$Jy beam$^{-1}$, corresponding to a $3σ$ upper limit at rest frame 5 GHz of $L_{5GHz}=2\times10^{39}$ erg s$^{-1}$ at the mean redshift of the sample $z\sim 5.1$. We compared this and individual sources upper limits with expected radio luminosities estimated assuming different AGN scaling relations, {to check whether these are consistent with the standard BLAGN spectral energy distribution}. For most of the sources the radio luminosity upper limits are still compatible with expectations for radio-quiet (RQ) AGN; nevertheless, the more stringent stacking upper limits and the fact that no detection is found {might suggest} that JWST-selected BLAGN are weaker than standard AGN even at radio frequencies. Indeed, the probability of having none of the BLAGN detected in none of the investigated radio images is expected to be on average very low ($P<10^{-4}$). We discuss some scenarios that could explain the possible radio weakness, such as free-free absorption from a dense medium, or the lack of either magnetic field or a corona, possibly as a consequence of super-Eddington accretion. These scenarios would also explain the observed X-ray weakness. We also conclude that $\sim$1 dex more sensitive radio observations are needed to better constrain the level of radio emission (or lack thereof) for the bulk of these sources. The Square Kilometer Array Observatory (SKAO) will likely play a crucial role in assessing the properties of this AGN population. △ Less

Submitted 7 November, 2025; v1 submitted 5 December, 2024; originally announced December 2024.

Comments: Accepted for publication on A&A

Journal ref: A&A 706, A372 (2026)

The near-infrared SED of blue quasars: what drives the evolution of the dusty torus?

Authors: Bartolomeo Trefoloni, Roberto Gilli, Elisabeta Lusso, Alessandro Marconi, Giovanni Mazzolari, Emanuele Nardini, Guido Risaliti, Matilde Signorini

Abstract: A fundamental ingredient in the unified model of active galactic nuclei (AGN) is the obscuring torus, whose innermost, hottest region dominates the near infrared (NIR) emission. Characterising the change in the torus properties and its interplay with the main AGN emission is key for our understanding of AGN physics, evolution and classification. Its covering factor ($CF$) is largely responsible fo… ▽ More A fundamental ingredient in the unified model of active galactic nuclei (AGN) is the obscuring torus, whose innermost, hottest region dominates the near infrared (NIR) emission. Characterising the change in the torus properties and its interplay with the main AGN emission is key for our understanding of AGN physics, evolution and classification. Its covering factor ($CF$) is largely responsible for the classification of AGN on the basis of the detection of broad emission lines. It is still not clear whether the torus properties evolve over time and how they relate with the accretion parameters of the nucleus. In this work, we aim at investigating the evolution of the NIR properties with the redshift ($z$) and the bolometric luminosity ($L_{\rm bol}$) of the AGN. To this end, we assembled a large dataset of $\sim$36,000 Type 1 AGN between $0.5<z<2.9$ and $45.0<\log(L_{\rm bol} / (\rm erg / s))<48.0$ with UV, optical and near-infrared photometry. We produced average spectral energy distributions (SED) in different bins of the $z-L_{\rm bol}$ parameter space to estimate how the NIR SED evolves according to these parameters. We find that the NIR luminosity decreases for increasing $L_{\rm bol}$ at any redshift. At the same, time the shape of the NIR SED in our sample is consistent with a non-evolution with $z$. As a consequence, all the explored proxies for the $CF$ exhibit significant anti-correlations with $L_{\rm bol}$, but not with $z$. Additionally, the $CF$ also shows a shallower anti-correlation with the Eddington ratio ($λ_{\rm Edd}$), yet current systematic uncertainties, as well as the limited dynamical range, do not allow us to precisely constrain the role of the Eddington ratio. Lastly, we derived the covering factor from the ratio between the NIR and optical luminosity and we employed it to set a lower limit for the X-ray obscuration at different redshifts. △ Less

Submitted 14 October, 2024; originally announced October 2024.

Comments: 22 pages, 15 figures

Journal ref: A&A 697, A223 (2025)

AGN STORM 2: IX. Studying the Dynamics of the Ionized Obscurer in Mrk 817 with High-resolution X-ray Spectroscopy

Authors: Fatima Zaidouni, Erin Kara, Peter Kosec, Missagh Mehdipour, Daniele Rogantini, Gerard A. Kriss, Ehud Behar, Jelle Kaastra, Aaron J. Barth, Edward M. Cackett, Gisella De Rosa, Yasaman Homayouni, Keith Horne, Hermine Landt, Nahum Arav, Misty C. Bentz, Michael S. Brotherton, Elena Dalla Bontà, Maryam Dehghanian, Gary J. Ferland, Carina Fian, Jonathan Gelbord, Michael R. Goad, Diego H. González Buitrago, Catherine J. Grier , et al. (23 additional authors not shown)

Abstract: We present the results of the XMM-Newton and NuSTAR observations taken as part of the ongoing, intensive multi-wavelength monitoring program of the Seyfert 1 galaxy Mrk 817 by the AGN Space Telescope and Optical Reverberation Mapping 2 (AGN STORM 2) Project. The campaign revealed an unexpected and transient obscuring outflow, never before seen in this source. Of our four XMM-Newton/NuSTAR epochs,… ▽ More We present the results of the XMM-Newton and NuSTAR observations taken as part of the ongoing, intensive multi-wavelength monitoring program of the Seyfert 1 galaxy Mrk 817 by the AGN Space Telescope and Optical Reverberation Mapping 2 (AGN STORM 2) Project. The campaign revealed an unexpected and transient obscuring outflow, never before seen in this source. Of our four XMM-Newton/NuSTAR epochs, one fortuitously taken during a bright X-ray state has strong narrow absorption lines in the high-resolution grating spectra. From these absorption features, we determine that the obscurer is in fact a multi-phase ionized wind with an outflow velocity of $\sim$5200 km s$^{-1}$, and for the first time find evidence for a lower ionization component with the same velocity observed in absorption features in the contemporaneous HST spectra. This indicates that the UV absorption troughs may be due to dense clumps embedded in diffuse, higher ionization gas responsible for the X-ray absorption lines of the same velocity. We observe variability in the shape of the absorption lines on timescales of hours, placing the variable component at roughly 1000 $R_g$ if attributed to transverse motion along the line of sight. This estimate aligns with independent UV measurements of the distance to the obscurer suggesting an accretion disk wind at the inner broad line region. We estimate that it takes roughly 200 days for the outflow to travel from the disk to our line of sight, consistent with the timescale of the outflow's column density variations throughout the campaign. △ Less

Submitted 24 June, 2024; originally announced June 2024.

JWST meets Chandra: a large population of Compton thick, feedback-free, and intrinsically X-ray weak AGN, with a sprinkle of SNe

Authors: Roberto Maiolino, Guido Risaliti, Matilde Signorini, Bartolomeo Trefoloni, Ignas Juodzbalis, Jan Scholtz, Hannah Uebler, Francesco D'Eugenio, Stefano Carniani, Andy Fabian, Xihan Ji, Giovanni Mazzolari, Elena Bertola, Marcella Brusa, Andrew J. Bunker, Stephane Charlot, Andrea Comastri, Giovanni Cresci, Christa Noel DeCoursey, Eiichi Egami, Fabrizio Fiore, Roberto Gilli, Michele Perna, Sandro Tacchella, Giacomo Venturi

Abstract: We investigate the X-ray properties of a sample of 71 broad line and narrow line AGN at 2$<$z$<$11 discovered by JWST in the GOODS fields, which have the deepest Chandra observations ever obtained. Despite the widespread presence of AGN signatures in their rest-optical and -UV spectra, the vast majority of them is X-ray undetected. The stacked X-ray data of the non-detected sources also results in… ▽ More We investigate the X-ray properties of a sample of 71 broad line and narrow line AGN at 2$<$z$<$11 discovered by JWST in the GOODS fields, which have the deepest Chandra observations ever obtained. Despite the widespread presence of AGN signatures in their rest-optical and -UV spectra, the vast majority of them is X-ray undetected. The stacked X-ray data of the non-detected sources also results in a non-detection. The upper limit on the X-ray emission for many of these AGN is one or even two orders of magnitude lower than expected from a standard AGN SED. X-ray absorption by clouds with large (Compton-thick) column density and low dust content, such as the Broad Line Region (BLR) clouds, can explain the X-ray weakness. In this scenario the BLR covering factor should be much larger than in low-z AGN or luminous quasars; this is supported by the larger equivalent width of the broad component of H$α$ in JWST-selected AGN. We also find that the JWST-discovered AGN lack prominent, fast outflows, suggesting that, in JWST-selected AGN, dense gas lingers in the nuclear region, resulting in large covering factors. We also note that a large fraction of JWST-selected AGN matches the definition of NLSy1, typically accreting at high rates and characterized by a steep X-ray spectrum -- this can further contribute to their observed weakness at high-z. Finally, we discuss that the broad Balmer lines used to identify type 1 AGN cannot be ascribed to Very Massive Stars or Supernovae, although we show that some of the faintest broad lines could potentially be associated with superluminous SNe. △ Less

Submitted 12 December, 2024; v1 submitted 1 May, 2024; originally announced May 2024.

Comments: 21 pages, 15 figures, 2 tables, resubmitted to MNRAS after responding to the referee's comments

Quasars as standard candles VI: spectroscopic validation of the cosmological sample

Authors: Bartolomeo Trefoloni, Elisabeta Lusso, Emanuele Nardini, Guido Risaliti, Alessandro Marconi, Giada Bargiacchi, Andrea Sacchi, Matilde Signorini

Abstract: A sample of quasars has been recently assembled to investigate the non-linear relation between their monochromatic luminosities at 2500Å, and 2 keV and to exploit quasars as a new class of standardized candles. The use of this technique for cosmological purposes relies on the non-evolution with redshift of the UV-optical spectral properties of quasars, as well as on the absence of possible contami… ▽ More A sample of quasars has been recently assembled to investigate the non-linear relation between their monochromatic luminosities at 2500Å, and 2 keV and to exploit quasars as a new class of standardized candles. The use of this technique for cosmological purposes relies on the non-evolution with redshift of the UV-optical spectral properties of quasars, as well as on the absence of possible contaminants such as dust extinction and host-galaxy contribution. We address these possible issues by analysing the spectral properties of our cosmological quasar sample. We produced composite spectra in different bins of redshift and accretion parameters (black hole mass, bolometric luminosity), to investigate any possible evolution of the spectral properties of the continuum of the composites with these parameters. We found a remarkable similarity amongst the various stacked spectra. The overall shape of the continuum does not show any statistically significant trend with the accretion parameters nor with the redshift. The composite spectrum of our quasar sample is consistent with negligible levels of both intrinsic reddening (with a colour excess E(B-V)< 0.01) and host-galaxy emission (less than 10%) in the optical. We tested whether unaccounted dust extinction could explain the discrepancy between our cosmographic fit of the Hubble-Lemaitre diagram and the concordance ΛCDM model. The average colour excess required to solve the tension should increase with redshift up to unphysically high values (E(B-V)=0.1 at z>3) that would imply that the intrinsic emission of quasars is much bluer and more luminous than ever reported in observed spectra. The similarity of quasar spectra across the parameter space excludes a significant evolution of the average continuum properties with any of the explored parameters, confirming the reliability of our sample for cosmological applications. △ Less

Submitted 10 April, 2024; originally announced April 2024.

Comments: 24 pages, 18 figures, accepted for publication on A&A

Journal ref: A&A 689, A109 (2024)

Building the high-redshift Hubble Diagram with quasars

Authors: Matilde Signorini, Guido Risaliti, Andrea Sacchi, Elisabeta Lusso, Emanuele Nardini

Abstract: Proceeding of the "HACK100" Conference, 6-10 June 2022, Trieste, Italy - In recent years, quasars have been shown to be reliable standardizable candles, thanks to the non-linear relation between their X-rays and ultraviolet luminosity. Quasars are also very numerous and they are found at all the observed redshifts: this allows us to investigate the expansion rate of the Universe and the cosmologic… ▽ More Proceeding of the "HACK100" Conference, 6-10 June 2022, Trieste, Italy - In recent years, quasars have been shown to be reliable standardizable candles, thanks to the non-linear relation between their X-rays and ultraviolet luminosity. Quasars are also very numerous and they are found at all the observed redshifts: this allows us to investigate the expansion rate of the Universe and the cosmological parameters in a previously almost untested redshift range ($z\sim2-7$). At redshift higher than 1.5, the Hubble Diagram of quasars shows a significant tension with the predictions of the $Λ$CDM model. I will show how detailed optical/UV and X-rays spectroscopic analysis can be used (i) to obtain more precise distance estimates, and (ii) to derive information about the physical process behind the luminosities relation, and discuss the cosmological implementations. △ Less

Submitted 15 January, 2024; originally announced January 2024.

Comments: Proceeding of the "HACK100" Conference, 6-10 June 2022, Trieste, Italy

Journal ref: Memorie della Società Astronomica Italiana, Vol. 94, n.2, 2023

Quasars as Standard Candles V. Evaluation of a $\leq$ 0.06 dex intrinsic dispersion in the LX-LUV relation

Authors: Matilde Signorini, Guido Risaliti, Elisabeta Lusso, Emanuele Nardini, Giada Bargiacchi, Andrea Sacchi, Bartolomeo Trefoloni

Abstract: A characteristic feature of quasars is the observed non-linear relationship between their monochromatic luminosities at rest-frame 2500 Å and 2 keV. This relationship is evident across all redshifts and luminosities and, due to its non-linearity, can be implemented to estimate quasar distances and construct a Hubble Diagram for quasars. Historically, a significant challenge in the cosmological app… ▽ More A characteristic feature of quasars is the observed non-linear relationship between their monochromatic luminosities at rest-frame 2500 Å and 2 keV. This relationship is evident across all redshifts and luminosities and, due to its non-linearity, can be implemented to estimate quasar distances and construct a Hubble Diagram for quasars. Historically, a significant challenge in the cosmological application of this relation has been its high observed dispersion. Recent studies have demonstrated that this dispersion can be reduced by excluding biased objects from the sample. Nevertheless, the dispersion remains considerable ($δ\sim 0.20$ dex), especially when compared to the Phillips relation for supernovae Ia. Given the absence of a comprehensive physical model for the relation, it remains unclear how much of the remaining dispersion is intrinsically tied to the relation itself and how much can be attributed to observational factors not addressed by the sample selection and by the choice of X-ray and UV indicators. Potential contributing factors include (i) the scatter produced by using X-ray photometric results instead of spectroscopic ones, (ii) the intrinsic variability of quasars, and (iii) the inclination of the accretion disc relative to our line of sight. In this study, we thoroughly examine these three factors and quantify their individual contributions to the observed dispersion. Based on our findings, we argue that the intrinsic dispersion of the X-ray/UV luminosity relation is likely below 0.06 dex. We also discuss why high-redshift subsamples can show a significantly lower dispersion than the average one. △ Less

Submitted 13 December, 2023; originally announced December 2023.

Comments: submitted

Journal ref: A&A 687, A32 (2024)

AGN STORM 2. VI. Mapping Temperature Fluctuations in the Accretion Disk of Mrk 817

Authors: Jack M. M. Neustadt, Christopher S. Kochanek, John Montano, Jonathan Gelbord, Aaron J. Barth, Gisella De Rosa, Gerard A. Kriss, Edward M. Cackett, Keith Horne, Erin A. Kara, Hermine Landt, Hagai Netzer, Nahum Arav, Misty C. Bentz, Elena Dalla Bonta, Maryam Dehghanian, Pu Du, Rick Edelson, Gary J. Ferland, Carina Fian, Travis Fischer, Michael R. Goad, Diego H. Gonzalez Buitrago, Varoujan Gorjian, Catherine J. Grier , et al. (27 additional authors not shown)

Abstract: We fit the UV/optical lightcurves of the Seyfert 1 galaxy Mrk 817 to produce maps of the accretion disk temperature fluctuations $δT$ resolved in time and radius. The $δT$ maps are dominated by coherent radial structures that move slowly ($v \ll c$) inwards and outwards, which conflicts with the idea that disk variability is driven only by reverberation. Instead, these slow-moving temperature fluc… ▽ More We fit the UV/optical lightcurves of the Seyfert 1 galaxy Mrk 817 to produce maps of the accretion disk temperature fluctuations $δT$ resolved in time and radius. The $δT$ maps are dominated by coherent radial structures that move slowly ($v \ll c$) inwards and outwards, which conflicts with the idea that disk variability is driven only by reverberation. Instead, these slow-moving temperature fluctuations are likely due to variability intrinsic to the disk. We test how modifying the input lightcurves by smoothing and subtracting them changes the resulting $δT$ maps and find that most of the temperature fluctuations exist over relatively long timescales ($\sim$100s of days). We show how detrending AGN lightcurves can be used to separate the flux variations driven by the slow-moving temperature fluctuations from those driven by reverberation. We also simulate contamination of the continuum emission from the disk by continuum emission from the broad line region (BLR), which is expected to have spectral features localized in wavelength, such as the Balmer break contaminating the $U$ band. We find that a disk with a smooth temperature profile cannot produce a signal localized in wavelength and that any BLR contamination should appear as residuals in our model lightcurves. Given the observed residuals, we estimate that only $\sim$20% of the variable flux in the $U$ and $u$ lightcurves can be due to BLR contamination. Finally, we discus how these maps not only describe the data, but can make predictions about other aspects of AGN variability. △ Less

Submitted 2 October, 2023; originally announced October 2023.

Comments: 23 pages, 18 figures, submitting to ApJ, comments welcome

Quasars as Standard Candles IV. Analysis of the X-ray and UV indicators of the disc-corona relation

Authors: Matilde Signorini, Guido Risaliti, Elisabeta Lusso, Emanuele Nardini, Giada Bargiacchi, Andrea Sacchi, Bartolomeo Trefoloni

Abstract: Context: A non-linear relation between quasar monochromatic luminosities at 2500A and 2 keV holds at all observed redshifts and luminosities, and it has been used to derive quasar distances and to build a Hubble Diagram of quasars. The choice of the X-ray and UV indicators has so far been somewhat arbitrary, and has typically relied on photometric data. Aims: We want to determine the X-ray and UV… ▽ More Context: A non-linear relation between quasar monochromatic luminosities at 2500A and 2 keV holds at all observed redshifts and luminosities, and it has been used to derive quasar distances and to build a Hubble Diagram of quasars. The choice of the X-ray and UV indicators has so far been somewhat arbitrary, and has typically relied on photometric data. Aims: We want to determine the X-ray and UV proxies that provide the smallest dispersion of the relation, in order to obtain more precise distance estimates, and to confirm the reliability of the X-ray to UV relation as a distance indicator. Methods: We performed a complete UV spectroscopic analysis of a sample of $\sim$1800 quasars with SDSS optical spectra and XMM- Newton X-ray serendipitous observations. In the X-rays, we analysed the spectra of all the sample objects at redshift z $>$1.9, while we relied on photometric measurements at lower redshifts. As done in previous studies, we analysed the relation in small redshift bins, using fluxes instead of luminosities. Results: We show that the monochromatic fluxes at 1 keV and 2500A are, respectively, the best X-ray and UV continuum indicators among those that are typically available. We also find a tight relation between soft X-ray and Mg ii2800A line fluxes, and a marginal dependence of the X-ray to UV relation on the width of the Mg ii line. Conclusions: Our analysis suggests that the physical quantities that are more tightly linked to one another are the soft X-ray flux at $\sim$1 keV and the ionizing UV flux blueward of the Lyman limit. However, the "usual" monochromatic fluxes at 2 keV and 2500A estimated from photometric data provide an almost as-tight X-ray to UV relation, and can be used to derive quasar distances. The Hubble diagram obtained using spectroscopic indicators is fully consistent with the one presented in previous papers, based on photometric data. △ Less

Submitted 28 June, 2023; originally announced June 2023.

Comments: accepted for publication in A&A

Journal ref: A&A 676, A143 (2023)

X-ray properties and obscured fraction of AGN in the J1030 Chandra field

Authors: Matilde Signorini, Stefano Marchesi, Roberto Gilli, Marcella Brusa, Andrea Comastri, Quirino D'Amato, Kazushi Iwasawa, Giorgio Lanzuisi, Giovanni Mazzolari, Marco Mignoli, Alessandro Peca, Isabella Prandoni, Paolo Tozzi, Cristian Vignali, Fabio Vito, Colin Norman

Abstract: The 500ks Chandra ACIS-I observation of the field around the $z=6.31$ quasar SDSS J1030+0524 is currently the 5th deepest extragalactic X-ray survey. The rich multi-band coverage of the field allowed for an effective identification and redshift determination of the X-ray source counterparts: to date a catalog of 243 extragalactic X-ray sources with either a spectroscopic or photometric redshift es… ▽ More The 500ks Chandra ACIS-I observation of the field around the $z=6.31$ quasar SDSS J1030+0524 is currently the 5th deepest extragalactic X-ray survey. The rich multi-band coverage of the field allowed for an effective identification and redshift determination of the X-ray source counterparts: to date a catalog of 243 extragalactic X-ray sources with either a spectroscopic or photometric redshift estimate in the range $z\approx0-6$ is available over a 355 arcmin$^2$ area. Given its depth and the multi-band information, this catalog is an excellent resource to investigate X-ray spectral properties of distant Active Galactic Nuclei (AGN) and derive the redshift evolution of their obscuration. We performed a thorough X-ray spectral analysis for each object in the sample, measuring its nuclear column density $N_{\rm H}$ and intrinsic (de-absorbed) 2-10 keV rest-frame luminosity, $L_{2-10}$. Whenever possible, we also used the presence of the Fe K$_α$ emission line to improve the photometric redshift estimates. We measured the fractions of AGN hidden by column densities in excess of $10^{22}$ and $10^{23}$cm$^{-2}$ ($f_{22}$ and $f_{23}$, respectively) as a function of $L_{2-10}$ and redshift, and corrected for selection effects to recover the intrinsic obscured fractions. At $z\sim 1.2$, we found $f_{22}\sim0.7-0.8$ and $f_{23}\sim0.5-0.6$, respectively, in broad agreement with the results from other X-ray surveys. No significant variations with X-ray luminosity were found within the limited luminosity range probed by our sample (log$L_{2-10}\sim 42.8-44.3$). When focusing on luminous AGN with log$L_{2-10}\sim44$ to maximize the sample completeness up to large cosmological distances, we did not observe any significant change in $f_{22}$ or $f_{23}$ over the redshift range $z\sim0.8-3$. Nonetheless, the obscured fractions we measure are significantly higher than ... △ Less

Submitted 22 May, 2023; originally announced May 2023.

Comments: A&A, in press

Journal ref: A&A 676, A49 (2023)

The most luminous blue quasars at 3.0<z<3.3 -- III. LBT spectra and accretion parameters

Authors: Bartolomeo Trefoloni, Elisabeta Lusso, Emanuele Nardini, Guido Risaliti, Giada Bargiacchi, Susanna Bisogni, Francesca M. Civano, Martin Elvis, Giuseppina Fabbiano, Roberto Gilli, Alessandro Marconi, Gordon T. Richards, Andrea Sacchi, Francesco Salvestrini, Matilde Signorini, Cristian Vignali

Abstract: We present the analysis of the rest frame ultraviolet and optical spectra of 30 bright blue quasars at $z\sim3$, selected to examine the suitability of AGN as cosmological probes. In our previous works, we found an unexpectedly high fraction ($\approx 25 \%$) of X-ray weak quasars in the sample. The latter sources also display a flatter UV continuum and a broader and fainter CIV profile in the arc… ▽ More We present the analysis of the rest frame ultraviolet and optical spectra of 30 bright blue quasars at $z\sim3$, selected to examine the suitability of AGN as cosmological probes. In our previous works, we found an unexpectedly high fraction ($\approx 25 \%$) of X-ray weak quasars in the sample. The latter sources also display a flatter UV continuum and a broader and fainter CIV profile in the archival UV data with respect to their X-ray normal counterparts. Here we present new observations with the LBT in both the $zJ$ (rest-frame $\simeq$2300-3100 $\rm \mathring{A}$) and the $K_S$ ($\simeq$4750-5350 $\rm \mathring{A}$) bands. We estimated black hole masses ($M_{\rm BH}$) and Eddington ratios ($λ_{\rm Edd}$) from the from the H$β$ and MgII emission lines, finding that our $z\sim3$ quasars are on average highly accreting ($\langle λ_{\rm Edd} \rangle\simeq 1.2$ and $\langle M_{\rm BH} \rangle\simeq 10^{9.7}M_\odot$), with no difference in $λ_{\rm Edd}$ or $M_{\rm BH}$ between X-ray weak and X-ray normal quasars. From the $zJ$ spectra, we derive flux and equivalent width of MgII and FeII, finding that X-ray weak quasars display higher FeII/MgII ratios with respect to typical quasars. FeII/MgII ratios of X-ray normal quasars are instead consistent with other estimates up to $z\simeq6.5$, corroborating the idea of already chemically mature BLRs at early cosmic time. From the $K_S$ spectra, we find that all the X-ray weak quasars present generally weaker [OIII] emission (EW<10 $\rm \mathring{A}$) than the normal ones. The sample as a whole, however, abides by the known X-ray/[OIII] luminosity correlation, hence the different [OIII] properties are likely due to an intrinsically weaker [OIII] emission in X-ray weak objects, associated to the shape of the spectral energy distribution. We interpret these results in the framework of accretion-disc winds. △ Less

Submitted 12 May, 2023; originally announced May 2023.

Comments: 25 pages, 17 figures, Accepted for publication on Astronomy & Astrophysics

Quasars as high-redshift standard candles

Authors: Guido Risaliti, Elisabeta Lusso, Emanuele Nardini, Giada Bargiacchi, Susanna Bisogni, Andrea Sacchi, Matilde Signorini, Bartolomeo Trefoloni

Abstract: In the past few years, we built a Hubble diagram of quasars up to redshift z$\sim$7, based on the nonlinear relation between quasars' x-ray and UV luminosities. Such a Hubble diagram shows a >4$σ$ deviation from the standard flat $Λ$CDM model at z>1.5. Given the important consequences of this result, it is fundamental to rule out any systematic effect in the selection of the sample and/or in the f… ▽ More In the past few years, we built a Hubble diagram of quasars up to redshift z$\sim$7, based on the nonlinear relation between quasars' x-ray and UV luminosities. Such a Hubble diagram shows a >4$σ$ deviation from the standard flat $Λ$CDM model at z>1.5. Given the important consequences of this result, it is fundamental to rule out any systematic effect in the selection of the sample and/or in the flux measurements, and to investigate possible redshift dependences of the relation, that would invalidate the use of quasars as standard candles. Here we review all the observational results supporting our method: the match of the Hubble diagram of quasars with that of supernovae in the common redshift range, the constant slope of the relation at all redshifts, the redshift non-evolution of the spectral properties of our sources both in the x-rays and in the UV. An independent test of our results requires the observation of other standard candles at high redshift. In particular, we expect that future observations of supernovas at z>2 will confirm the deviation from the concordance model found with the Hubble diagram of quasars. △ Less

Submitted 26 April, 2023; originally announced April 2023.

Journal ref: published in Astronomische Nachrichten, 2023

LBT-MODS spectroscopy of high-redshift candidates in the Chandra J1030 field. A newly discovered z$\sim$2.8 large scale structure

Authors: Stefano Marchesi, Marco Mignoli, Roberto Gilli, Giovanni Mazzolari, Matilde Signorini, Marisa Brienza, Susanna Bisogni, Micol Bolzonella, Olga Cucciati, Quirino D'Amato, Alessandro Peca, Isabella Prandoni, Paolo Tozzi, Cristian Vignali, Fabio Vito, Andrea Comastri

Abstract: We present the results of a spectroscopic campaign with the Multi-Object Double Spectrograph (MODS) instrument mounted on the Large Binocular Telescope (LBT), aimed at obtaining a spectroscopic redshift for seven Chandra J1030 sources with a photometric redshift >=2.7 and optical magnitude r_AB=[24.5-26.5]. We obtained a spectroscopic redshift for five out of seven targets: all of them have z_spec… ▽ More We present the results of a spectroscopic campaign with the Multi-Object Double Spectrograph (MODS) instrument mounted on the Large Binocular Telescope (LBT), aimed at obtaining a spectroscopic redshift for seven Chandra J1030 sources with a photometric redshift >=2.7 and optical magnitude r_AB=[24.5-26.5]. We obtained a spectroscopic redshift for five out of seven targets: all of them have z_spec>=2.5, thus probing the reliability of the Chandra J1030 photometric redshifts. The spectroscopic campaign led to the serendipitous discovery of a z~2.78 large scale structure (LSS) in the J1030 field: the structure contains four X-ray sources (three of which were targeted in the LBT-MODS campaign) and two non-X-ray detected galaxies for which a VLT-MUSE spectrum was already available. The X-ray members of the LSS are hosted in galaxies that are significantly more massive (log(M_*/M_sun)=[10.0-11.1]) than those hosting the two MUSE-detected sources (log(M_*/M_sun)<10). Both observations and simulations show that massive galaxies, and particularly objects having log(M_*/M_sun)>10, are among the best tracers of large scale structures and filaments in the cosmic web. Consequently, our result can explain why X-ray-detected AGN have also been shown to be efficient tracers of large scale structures. △ Less

Submitted 23 March, 2023; originally announced March 2023.

Comments: 16 pages, 9 Figures. Accepted for publication in Astronomy and Astrophysics

Journal ref: A&A 673, A97 (2023)

Non-parametric analysis of the Hubble Diagram with Neural Networks

Authors: Lorenzo Giambagli, Duccio Fanelli, Guido Risaliti, Matilde Signorini

Abstract: The recent extension of the Hubble diagram of Supernovae and quasars to redshifts much higher than 1 prompted a revived interest in non-parametric approaches to test cosmological models and to measure the expansion rate of the Universe. In particular, it is of great interest to infer model-independent constraints on the possible evolution of the dark energy component. Here we present a new method,… ▽ More The recent extension of the Hubble diagram of Supernovae and quasars to redshifts much higher than 1 prompted a revived interest in non-parametric approaches to test cosmological models and to measure the expansion rate of the Universe. In particular, it is of great interest to infer model-independent constraints on the possible evolution of the dark energy component. Here we present a new method, based on a Neural Network Regression, to analyze the Hubble Diagram in a completely non-parametric, model-independent fashion. We first validate the method through simulated samples with the same redshift distribution as the real ones, and discuss the limitations related to the "inversion problem" for the distance-redshift relation. We then apply this new technique to the analysis of the Hubble diagram of Supernovae and quasars. We confirm that the data up to $z \sim 1-1.5$ are in agreement with a flat $ΛCDM$ model with $Ω_M \sim 0.3$, while $\sim 5$-sigma deviations emerge at higher redshifts. A flat $ΛCDM$ model would still be compatible with the data with $Ω_M > 0.4$. Allowing for a generic evolution of the dark energy component, we find solutions suggesting an increasing value of $Ω_M$ with the redshift, as predicted by interacting dark sector models. △ Less

Submitted 24 February, 2023; originally announced February 2023.

Quasars as high-redshift standard candles

Authors: A. Sacchi, G. Risaliti, M. Signorini, E. Lusso, E. Nardini, G. Bargiacchi, S. Bisogni, F. Civano, M. Elvis, G. Fabbiano, R. Gilli, B. Trefoloni, C. Vignali

Abstract: The non-linear relation between the X-ray and ultraviolet (UV) luminosity in quasars has been used to derive quasar distances and to build a Hubble diagram at redshifts up to $z\sim$ 7. This cosmological application is based on the assumption of independence of the relation on redshift and luminosity. We want to test the reliability of this hypothesis by studying the spectroscopic properties of hi… ▽ More The non-linear relation between the X-ray and ultraviolet (UV) luminosity in quasars has been used to derive quasar distances and to build a Hubble diagram at redshifts up to $z\sim$ 7. This cosmological application is based on the assumption of independence of the relation on redshift and luminosity. We want to test the reliability of this hypothesis by studying the spectroscopic properties of high-redshift quasars in the X-ray and UV bands. We performed a one-by-one analysis of a sample of 130 quasars at $z>$ 2.5 with high-quality X-ray and UV spectroscopic observations. We found that not only the X-ray to UV correlation still holds at these redshifts, but its intrinsic dispersion is as low as 0.12 dex (previous works reached 0.20$-$0.22 dex). For a sample of quasars at $z\sim$ 3 with particularly high-quality observations the dispersion further drops to 0.09 dex, a value entirely accountable for by intrinsic variability and source geometry effects. The composite spectra of these quasars, in both the X-rays and the UV, do not show any difference with respect to the average spectra of quasars at lower redshifts. The absence of any spectral difference between high- and low-$z$ quasars and the tightness of the X-ray to UV relation suggests that no evolutionary effects are present in the relation. Therefore, it can be safely employed to derive quasar distances. Under this assumption, we obtain a measurement of the luminosity distance at $z\sim$ 3 with 15 % uncertainty, and in a 4$σ$ tension with the concordance model. △ Less

Submitted 27 June, 2022; originally announced June 2022.

Comments: Accepted for publication in Astronomy & Astrophysics Letters, 8 pages, 6 figures

Journal ref: A&A 663, L7 (2022)

Quasar cosmology: dark energy evolution and spatial curvature

Authors: G. Bargiacchi, M. Benetti, S. Capozziello, E. Lusso, G. Risaliti, M. Signorini

Abstract: We analyse some open debates in cosmology in light of the most updated quasar (QSO) sample, covering a wide redshift range up to $z\sim7.5$, combined with type Ia supernovae (SNe) and baryon acoustic oscillations (BAO). Indeed, extending the cosmological analyses with high-redshift data is key to distinguishing between different cosmological models that are degenerate at low redshifts, and allowin… ▽ More We analyse some open debates in cosmology in light of the most updated quasar (QSO) sample, covering a wide redshift range up to $z\sim7.5$, combined with type Ia supernovae (SNe) and baryon acoustic oscillations (BAO). Indeed, extending the cosmological analyses with high-redshift data is key to distinguishing between different cosmological models that are degenerate at low redshifts, and allowing better constraints on a possible dark energy (DE) evolution. Also, we discuss combinations of BAO, SNe, and QSO data to understand their compatibility and implications for extensions of the standard cosmological model. Specifically, we consider a flat and non-flat $Λ\mathrm{CDM}$ cosmology, a flat and non-flat DE model with a constant DE equation of state parameter ($w$), and four flat DE models with variable $w$, namely the Chevallier-Polarski-Linder and Jassal-Bagla-Padmanabhan models, and an "exponential" and Barboza-Alcaniz parameterisations. We find that a joint analysis of QSO+SNe with BAO is only possible in the context of a flat Universe. Indeed BAO confirms the flatness condition assuming a curved geometry, whilst SNe+QSO show evidence of a closed space. We also find $Ω_{M,0}=0.3$ in all data sets assuming a flat $Λ\mathrm{CDM}$ model. Yet, all the other models show a statistically significant deviation at 2-3$σ$ with the combined SNe+QSO+BAO data set. In the models where DE density evolves with time, SNe+QSO+BAO data always prefer $Ω_{M,0}>0.3$, $w_{0}<-1$ and $w_{a}>0$. This DE phantom behaviour is mainly driven by SNe+QSO, while BAO are closer to the flat $Λ\mathrm{CDM}$ model. △ Less

Submitted 3 August, 2022; v1 submitted 3 November, 2021; originally announced November 2021.

Comments: 12 pages, 18 figures, 2 tables, published in MNRAS, comments are welcome

The most luminous blue quasars at 3.0<z<3.3. II. CIV/X-ray emission and accretion disc physics

Authors: Elisabeta Lusso, Emanuele Nardini, Susanna Bisogni, Guido Risaliti, Roberto Gilli, Gordon T. Richards, Francesco Salvestrini, Cristian Vignali, Giada Bargiacchi, Francesca Civano, Martin Elvis, Giuseppina Fabbiano, Alessandro Marconi, Andrea Sacchi, Matilde Signorini

Abstract: We analyse the properties of the CIV broad emission line in connection with the X-ray emission of 30 bright SDSS quasars at z~3.0-3.3 with pointed XMM-Newton observations, which were selected to test the suitability of AGN as cosmological tools. In our previous work, we found that a large fraction (~25%) of the quasars in this sample are X-ray underluminous by factors of >3-10. As absorbing column… ▽ More We analyse the properties of the CIV broad emission line in connection with the X-ray emission of 30 bright SDSS quasars at z~3.0-3.3 with pointed XMM-Newton observations, which were selected to test the suitability of AGN as cosmological tools. In our previous work, we found that a large fraction (~25%) of the quasars in this sample are X-ray underluminous by factors of >3-10. As absorbing columns of >10$^{23}$ cm$^{-2}$ can be safely ruled out, their weakness is most likely intrinsic. Here we explore possible correlations between the UV and X-ray features of these sources to investigate the origin of X-ray weakness. We fit their UV SDSS spectra and analyse their CIV properties (e.g., equivalent width, EW; line peak velocity, $\upsilon_{\rm peak}$) as a function of the X-ray photon index and 2-10 keV flux. We confirm the trends of CIV $\upsilon_{\rm peak}$ and EW with UV luminosity at 2500 angstrom for both X-ray weak and X-ray normal quasars, as well as the correlation between X-ray weakness and CIV EW. In contrast to some recent work, we do not observe any clear relation between the 2-10 keV luminosity and $\upsilon_{\rm peak}$. We find a correlation between the hard X-ray flux and the integrated CIV flux for X-ray normal quasars, whilst X-ray weak quasars deviate from the main trend by more than 0.5 dex. We argue that X-ray weakness might be interpreted in a starved X-ray corona picture associated with an ongoing disc-wind phase. If the wind is ejected in the vicinity of the black hole, the extreme-UV radiation that reaches the corona will be depleted, depriving the corona of seeds photons and generating an X-ray weak quasar. Yet, at the largest UV luminosities (>10$^{47}$ erg s$^{-1}$), there will still be an ample reservoir of ionising photons that can explain the excess CIV emission observed in the X-ray weak quasars with respect to normal sources of similar X-ray luminosities. △ Less

Submitted 6 July, 2021; originally announced July 2021.

Comments: 22 pages, 15 figures (with 3 more figures in the Appendix), abstract abridged. Accepted for publication in A&A

A Novel Framework for the Analysis of Unknown Transactions in Bitcoin: Theory, Model, and Experimental Results

Authors: Maurantonio Caprolu, Matteo Pontecorvi, Matteo Signorini, Carlos Segarra, Roberto Di Pietro

Abstract: Bitcoin (BTC) is probably the most transparent payment network in the world, thanks to the full history of transactions available to the public. Though, Bitcoin is not a fully anonymous environment, rather a pseudonymous one, accounting for a number of attempts to beat its pseudonimity using clustering techniques. There is, however, a recurring assumption in all the cited deanonymization technique… ▽ More Bitcoin (BTC) is probably the most transparent payment network in the world, thanks to the full history of transactions available to the public. Though, Bitcoin is not a fully anonymous environment, rather a pseudonymous one, accounting for a number of attempts to beat its pseudonimity using clustering techniques. There is, however, a recurring assumption in all the cited deanonymization techniques: that each transaction output has an address attached to it. That assumption is false. An evidence is that, as of block height 591,872, there are several millions transactions with at least one output for which the Bitcoin Core client cannot infer an address. In this paper, we present a novel approach based on sound graph theory for identifying transaction inputs and outputs. Our solution implements two simple yet innovative features: it does not rely on BTC addresses and explores all the transactions stored in the blockchain. All the other existing solutions fail with respect to one or both of the cited features. In detail, we first introduce the concept of Unknown Transaction and provide a new framework to parse the Bitcoin blockchain by taking them into account. Then, we introduce a theoretical model to detect, study, and classify -- for the first time in the literature -- unknown transaction patterns in the user network. Further, in an extensive experimental campaign, we apply our model to the Bitcoin network to uncover hidden transaction patterns within the Bitcoin user network. Results are striking: we discovered more than 30,000 unknown transaction DAGs, with a few of them exhibiting a complex yet ordered topology and potentially connected to automated payment services. To the best of our knowledge, the proposed framework is the only one that enables a complete study of the unknown transaction patterns, hence enabling further research in the fields -- for which we provide some directions. △ Less

Submitted 17 March, 2021; originally announced March 2021.

Cosmography by orthogonalized logarithmic polynomials

Authors: Giada Bargiacchi, G. Risaliti, M. Benetti, S. Capozziello, E. Lusso, A. Saccardi, M. Signorini

Abstract: Cosmography is a powerful tool to investigate the Universe kinematic and then to reconstruct dynamics in a model-independent way. However, recent new measurements of supernovae Ia and quasars have populated the Hubble diagram up to high redshifts ($z \sim 7.5$) and the application of the traditional cosmographic approach has become less straightforward due to the large redshifts implied. Here we i… ▽ More Cosmography is a powerful tool to investigate the Universe kinematic and then to reconstruct dynamics in a model-independent way. However, recent new measurements of supernovae Ia and quasars have populated the Hubble diagram up to high redshifts ($z \sim 7.5$) and the application of the traditional cosmographic approach has become less straightforward due to the large redshifts implied. Here we investigate this issue through an expansion of the luminosity distance-redshift relation in terms of "orthogonal" logarithmic polynomials. In particular we point out the advantages of a new procedure of "orthogonalization" and we show that such an expansion provides a very good fit in the whole $z=0÷7.5$ range to both real and mock data obtained assuming various cosmological models. Moreover, despite of the fact that the cosmographic series is tested well beyond its convergence radius, the parameters obtained expanding the luminosity distance - redshift relation for the $Λ$CDM model are broadly consistent with the results from a fit of mock data obtained with the same cosmological model. This provides a method to test the reliability of a cosmographic function to study cosmological models at high redshifts and it demonstrates that the logarithmic polynomial series can be used to test the consistency of the $Λ$CDM model with the current Hubble diagram of quasars and supernovae Ia. We confirm a strong tension (at $>4σ$) between the concordance cosmological model and the Hubble diagram at $z>1.5$. Such a tension is dominated by the contribution of quasars at $z>2$ and starts to be present also in the few supernovae Ia observed at $z>1$. △ Less

Submitted 16 February, 2021; v1 submitted 20 January, 2021; originally announced January 2021.

Comments: 11 pages, 7 figures, accepted for publication in A&A, comments are welcome

Journal ref: A&A 649, A65 (2021)

Quasars as standard candles III. Validation of a new sample for cosmological studies

Authors: E. Lusso, G. Risaliti, E. Nardini, G. Bargiacchi, M. Benetti, S. Bisogni, S. Capozziello, F. Civano, L. Eggleston, M. Elvis, G. Fabbiano, R. Gilli, A. Marconi, M. Paolillo, E. Piedipalumbo, F. Salvestrini, M. Signorini, C. Vignali

Abstract: We present a new catalogue of ~2,400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM-Newton. The sample can be used to investigate the non-linear relation between the UV and X-ray luminosity of quasars, and to build a Hubble diagram up to redshift z~7.5. We selected sources that are neither reddened by dust in the optical/UV nor obscured by… ▽ More We present a new catalogue of ~2,400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM-Newton. The sample can be used to investigate the non-linear relation between the UV and X-ray luminosity of quasars, and to build a Hubble diagram up to redshift z~7.5. We selected sources that are neither reddened by dust in the optical/UV nor obscured by gas in the X-rays, and whose X-ray fluxes are free from flux-limit related biases. After checking for any possible systematics, we confirm, in agreement with our previous works, that (i) the X-ray to UV relation provides distance estimates matching those from supernovae up to z~1.5, and (ii) its slope shows no redshift evolution up to z~5. We provide a full description of the methodology for testing cosmological models, further supporting a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat $Λ$CDM model up to z~1.5-2, but strong deviations emerge at higher redshifts. Since we have minimized all non-negligible systematic effects, and proven the stability of the $L_{\rm X}-L_{\rm UV}$ relation at high redshifts, we conclude that an evolution of the expansion rate of the Universe should be considered as a possible explanation for the observed deviation, rather than some systematic (redshift-dependent) effect associated with high-redshift quasars. △ Less

Submitted 19 August, 2020; originally announced August 2020.

Comments: 24 pages, 17 figures, accepted for publication in A&A

Journal ref: A&A 642, A150 (2020)

BAD: Blockchain Anomaly Detection

Authors: Matteo Signorini, Matteo Pontecorvi, Wael Kanoun, Roberto Di Pietro

Abstract: Anomaly detection tools play a role of paramount importance in protecting networks and systems from unforeseen attacks, usually by automatically recognizing and filtering out anomalous activities. Over the years, different approaches have been designed, all focused on lowering the false positive rate. However, no proposal has addressed attacks targeting blockchain-based systems. In this paper we p… ▽ More Anomaly detection tools play a role of paramount importance in protecting networks and systems from unforeseen attacks, usually by automatically recognizing and filtering out anomalous activities. Over the years, different approaches have been designed, all focused on lowering the false positive rate. However, no proposal has addressed attacks targeting blockchain-based systems. In this paper we present BAD: the first Blockchain Anomaly Detection solution. BAD leverages blockchain meta-data, named forks, in order to collect potentially malicious activities in the network/system. BAD enjoys the following features: (i) it is distributed (thus avoiding any central point of failure), (ii) it is tamper-proof (making not possible for a malicious software to remove or to alter its own traces), (iii) it is trusted (any behavioral data is collected and verified by the majority of the network) and (iv) it is private (avoiding any third party to collect/analyze/store sensitive information). Our proposal is validated via both experimental results and theoretical complexity analysis, that highlight the quality and viability of our Blockchain Anomaly Detection solution. △ Less

Submitted 3 July, 2020; v1 submitted 10 July, 2018; originally announced July 2018.