103 research outputs found
Exploratory X-Ray Monitoring of Luminous Radio-quiet Quasars at High Redshift: Extended Time-series Analyses and Stacked Imaging Spectroscopy
No full textWe present three new Chandra X-ray epochs along with new ground-based optical-UV observations as the third installment in a time-series analysis of four high-redshift (z ≈ 4.1-4.4) radio-quiet quasars. In total, we present nine epochs for these sources with rest-frame temporal baselines of ∼1300-2000 days. We utilize the X-ray data to determine basic variability properties, as well as produce mean spectra and stacked images based on effective exposure times of ∼40-70 ks per source. We perform time-series analyses in the soft and hard bands, separately, and compare variability properties to those of sources at lower redshifts and luminosities. The magnitude of X-ray variability of our sources remains consistent with or lower than that of similar sources at lower redshifts, in agreement with the variability-luminosity anticorrelation. The mean power-law photon indices in the stacked Chandra spectra of our sources are consistent with the values measured from their archival XMM-Newton spectra separated by about 3 yr in the rest frame. Along with the X-ray observations, we provide near-simultaneous optical monitoring of the sources in the optical-UV regime. The overall variability in the optical-to-X-ray spectral slope is consistent with sources at lower redshifts, and the optical-UV observations display mild variability on monthly timescales
No evidence for an Eddington-ratio dependence of X-ray weakness in BALQSOs
No full textSeveralworks have studied the relation between X-ray, ultraviolet (UV), and wind properties in broad absorption line quasars (BALQSOs), generally concluding that the formation of strong winds is tightly connected with the suppression of the ionizing extreme ultraviolet (EUV)/Xray emission. The Eddington ratio (λEdd), whichmeasures the accretion rate, is also known to be related with outflow and emission-line properties in the general quasar population. Moreover, models describing quasar accretion depend on λEdd, which can thus possibly affect the relative production of accelerating UV and ionizing EUV/X-ray radiation. In this work, for the first time, we investigated whether BALQSO X-ray properties are related with the Eddington ratio. We selected a sample of 30 BALQSOs with accurate measurements of black-hole mass and BAL properties from the literature, and we complemented it with four additional BALQSOs we observed with XMM-Newton to populate the low and high Eddington-ratio regimes. We did not find evidence for a strong relation between λEdd and X-ray suppression, which however shows a significant correlation with the strength of the UV absorption features. These findings are confirmed also by considering a sample of mini-BALQSOs collected from the literature
XMM-Newton spectroscopy of highly polarized and luminous broad absorption line quasar CSO 755
No full textThe Realm of the First Quasars in the Universe: the X-ray View
No full textWe review the X-ray studies of the highest redshift quasars, focusing on
the results obtained with Chandra and XMM-Newton.
Overall, the X-ray and broad-band properties of z>4 quasars and local
quasars are similar, suggesting that the small-scale X-ray emission regions
of Active Galactic Nuclei (AGN) are insensitive to the significant cosmic
changes occurring from z=0-6
The X-ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei
No full textThis article contains a detailed investigation of moderate- to high-quality X-ray spectra of 10 of the most luminous active galactic nuclei known at z > 4
Exploratory X-ray Monitoring of Luminous Radio-Quiet Quasars at High Redshift: Initial Results
No full textWe present initial results from an exploratory X-ray monitoring project of two groups of comparably luminous radio-quiet quasars (RQQs). The first consists of four sources at 4.10 <= z <= 4.35, monitored by Chandra, and the second is a comparison sample of three sources at 1.33 <= z <= 2.74, monitored by Swift. Together with archival X-ray data, the total rest-frame temporal baseline spans similar to 2-4 yr and similar to 5-13 yr for the first and second group, respectively. Six of these sources show significant X-ray variability over rest-frame timescales of similar to 10(2)-10(3) days; three of these also show significant X-ray variability on rest-frame timescales of similar to 1-10 days. The X-ray variability properties of our variable sources are similar to those exhibited by nearby and far less luminous active galactic nuclei (AGNs). While we do not directly detect a trend of increasing X-ray variability with redshift, we do confirm previous reports of luminous AGNs exhibiting X-ray variability above that expected from their luminosities, based on simplistic extrapolation from lower luminosity sources. This result may be attributed to luminous sources at the highest redshifts having relatively high accretion rates. Complementary UV-optical monitoring of our sources shows that variations in their optical-X-ray spectral energy distribution are dominated by the X-ray variations. We confirm previous reports of X-ray spectral variations in one of our sources, HS 1700+6416, but do not detect such variations in any of our other sources in spite of X-ray flux variations of up to a factor of similar to 4. This project is designed to provide a basic assessment of the X-ray variability properties of RQQs at the highest accessible redshifts that will serve as a benchmark for more systematic monitoring of such sources with future X-ray missions
Variability-selected low-luminosity Active Galactic Nuclei in the 4 Ms Chandra Deep Field-South
No full textThe 4 Ms Chandra Deep Field-South (CDF-S) and other deep X-ray surveys have been highly effective at selecting active galactic nuclei (AGNs). However, cosmologically distant low-luminosity AGNs (LLAGNs) have remained a challenge to identify due to significant contribution from the host galaxy. We identify long-term X-ray variability (~month-years, observed frame) in 20 of 92 CDF-S galaxies spanning redshifts z ≈ 0.08-1.02 that do not meet other AGN selection criteria. We show that the observed variability cannot be explained by X-ray binary populations or ultraluminous X-ray sources, so the variability is most likely caused by accretion onto a supermassive black hole (SMBH). The variable galaxies are not heavily obscured in general, with a stacked effective power-law photon index of Γstack ≈ 1.93 ± 0.13, and are therefore likely LLAGNs. The LLAGNs tend to lie a factor of ≈6-80 below the extrapolated linear variability-luminosity relation measured for luminous AGNs. This may be explained by their lower accretion rates. Variability-independent black hole mass and accretion-rate estimates for variable galaxies show that they sample a significantly different black hole mass-accretion-rate space, with masses a factor of 2.4 lower and accretion rates a factor of 22.5 lower than variable luminous AGNs at the same redshift. We find that an empirical model based on a universal broken power-law power spectral density function, where the break frequency depends on SMBH mass and accretion rate, roughly reproduces the shape, but not the normalization, of the variability-luminosity trends measured for variable galaxies and more luminous AGNs
The X-ray properties of z > 6 quasars: no evident evolution of accretion physics in the first Gyr of the Universe
No full textContext. X-ray emission from quasars (QSOs) has been used to assess supermassive black hole accretion properties up to z ≈ 6. However, at z > 6 only ≈15 QSOs are covered by sensitive X-ray observations, preventing a statistically significant investigation of the X-ray properties of the QSO population in the first Gyr of the Universe.
Aims: We present new Chandra observations of a sample of 10 z > 6 QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy (log {MBH}/{M_☉}=8.5-9.6) . Adding archival X-ray data for an additional 15 z > 6 QSOs, we investigate the X-ray properties of the QSO population in the first Gyr of the Universe. In particular, we focus on the LUV - LX relation, which is traced by the αox parameter, and the shape of their X-ray spectra.
Methods: We performed photometric analyses to derive estimates of the X-ray luminosities of our z > 6 QSOs, and thus their αox values and bolometric corrections (Kbol = Lbol/LX). We compared the resulting αox and Kbol distributions with the results found for QSO samples at lower redshift, and ran several statistical tests to check for a possible evolution of the LUV - LX relation. Finally, we performed a basic X-ray spectral analysis of the brightest z > 6 QSOs to derive their individual photon indices, and joint spectral analysis of the whole sample to estimate the average photon index.
Results: We detect seven of the new Chandra targets in at least one standard energy band, while two more are detected discarding energies E > 5 keV, where background dominates. We confirm a lack of significant evolution of αox with redshift, which extends the results from previous works up to z > 6 with a statistically significant QSO sample. Furthermore, we confirm the trend of an increasing bolometric correction with increasing luminosity found for QSOs at lower redshifts. The average power-law photon index of our sample (⟨Γ⟩ = 2.20-0.34+0.39 and ⟨Γ⟩ = 2.13-0.13+0.13 for sources with < 30 and > 30 net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs at z = 1 - 6.
Conclusions: All of these results indicate a lack of substantial evolution of the inner accretion-disk and hot-corona structure in QSOs from low redshift to z > 6. Our data hint at generally high Eddington ratios at z > 6
The X-Ray Properties of Typical High-Redshift Radio-Loud Quasars
No full textThis article reports spectral, imaging, and variability results from four XMM-Newton observations and two Chandra observations of high-redshift radio-loud quasars
Variability Selected Low-Luminosity Active Galactic Nuclei in the 4 Ms Chandra Deep Field-South
No full textThe 4 Ms Chandra Deep Field-South (CDF-S) and other deep X-ray surveys have been highly effective at selecting active galactic nuclei (AGN). However, cosmologically distant low-luminosity AGN (LLAGN) have remained a challenge to identify due to significant contribution from the host galaxy. We identify long-term X ray variability (approx. month years, observed frame) in 20 of 92 CDF-S galaxies spanning redshifts approx equals 00.8 - 1.02 that do not meet other AGN selection criteria. We show that the observed variability cannot be explained by X-ray binary populations or ultraluminous X-ray sources, so the variability is most likely caused by accretion onto a supermassive black hole. The variable galaxies are not heavily obscured in general, with a stacked effective power-law photon index of Gamma(sub Stack) approx equals 1.93 +/- 0.13, and arc therefore likely LLAGN. The LLAGN tend to lie it factor of approx equal 6-89 below the extrapolated linear variability-luminosity relation measured for luminous AGN. This may he explained by their lower accretion rates. Variability-independent black-hole mass and accretion-rate estimates for variable galaxies show that they sample a significantly different black hole mass-accretion-rate space, with masses a factor of 2.4 lower and accretion rates a factor of 22.5 lower than variable luminous AGNs at the same redshift. We find that an empirical model based on a universal broken power-law power spectral density function, where the break frequency depends on SMBH mass and accretion rate, roughly reproduces the shape, but not the normalization, of the variability-luminosity trends measured for variable galaxies and more luminous AGNs
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