arXiv subjects

M. Signorini

Publications and source records attributed to M. Signorini.

10 recordsLinked to original sources

arXiv2026

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

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.

astro-ph.GA↗

arXiv2026

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

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.

astro-ph.HE↗

arXiv2025

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

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}$, $L_{AD}$, $\lambda_{Edd}$), 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 ($\sigma^{AD}_{M_{BH}}\sim$0.2 dex; $\sigma^{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 $\langle \log(\lambda_{Edd}) \rangle=-0.9$, 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.

astro-ph.GA↗

arXiv2025

Quasar cosmology II: joint analyses with Cosmic Microwave Background

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 $\Lambda$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 $\Lambda$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.

astro-ph.CO↗

arXiv2025

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

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.

astro-ph.GA↗

arXiv2024

The radio properties of the JWST-discovered AGN

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 ${\sim 0.15}\mu$Jy beam$^{-1}$, corresponding to a $3\sigma$ 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.

astro-ph.GA↗

arXiv2022

Quasars as high-redshift standard candles

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$\sigma$ tension with the concordance model.

astro-ph.CO↗

arXiv2021

Quasar cosmology: dark energy evolution and spatial curvature

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 $\Lambda \mathrm{CDM}$ 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 $\Omega_{M,0}=0.3$ in all data sets assuming a flat $\Lambda \mathrm{CDM}$ model. Yet, all the other models show a statistically significant deviation at 2-3$\sigma$ with the combined SNe+QSO+BAO data set. In the models where DE density evolves with time, SNe+QSO+BAO data always prefer $\Omega_{M,0}>0.3$, $w_{0}<-1$ and $w_{a}>0$. This DE phantom behaviour is mainly driven by SNe+QSO, while BAO are closer to the flat $\Lambda \mathrm{CDM}$ model.

astro-ph.CO↗

arXiv2021

Cosmography by orthogonalized logarithmic polynomials

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 $z=0\div 7.5$ 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 $\Lambda$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 $\Lambda$CDM model with the current Hubble diagram of quasars and supernovae Ia. We confirm a strong tension (at $>4\sigma$) 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 $z>1$.

astro-ph.CO↗

arXiv2020

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

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 $\Lambda$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.

astro-ph.GA↗