Search

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

The large total infrared (TIR) luminosities ($L_{\rm TIR} \gtrsim 10^{12}~L_\odot$) observed in $z \sim 6$ quasars are generally converted into high star formation rates ($SFR \gtrsim 10^2~M_\odot$ yr$^{-1}$) of their host galaxies. However, these estimates rely on the assumption that dust heating is dominated by stellar radiation, neglecting the contribution from the central Active Galactic Nuclei (AGN). We test the validity of this assumption by combining cosmological hydrodynamic simulations with radiative transfer calculations. We find that, when AGN radiation is included in the simulations, the mass (luminosity)-weighted dust temperature in the host galaxies increases from $T\approx 50$ K ($T \approx 70$ K) to $T\approx 80$ K ($T\approx 200$ K), suggesting that AGN effectively heat the bulk of dust in the host galaxy. We compute the AGN-host galaxy $SFR$ from the synthetic spectral energy distribution by using standard $SFR - L_{\rm TIR}$ relations, and compare the results with the "true" values in the simulations. We find that the $SFR$ is overestimated by a factor of $\approx 3$ ($\gtrsim 10$) for AGN bolometric luminosities of $L_{\rm bol} \approx 10^{12}~L_\odot$ ($\gtrsim 10^{13}~ L_\odot$), implying that the star formation rates of $z\sim 6$ quasars can be overestimated by over an order of magnitude., Comment: 9 pages, 6 figures. Accepted for publication by MNRAS

Recent studies have revealed a strong relation between sample-averaged black-hole (BH) accretion rate (BHAR) and star formation rate (SFR) among bulge-dominated galaxies, i.e., "lockstep" BH-bulge growth, in the distant universe. This relation might be closely related to the BH-bulge mass correlation observed in the local universe. To understand further BH-bulge coevolution, we present ALMA CO(2-1) or CO(3-2) observations of 7 star-forming bulge-dominated galaxies at z=0.5-2.5. Using the ALMA data, we detect significant ($>3σ$) CO emission from 4 objects. For our sample of 7 galaxies, we measure (or constrain with upper limits) their CO line fluxes and estimate molecular gas masses ($M_{gas}$). We also estimate their stellar masses ($M_{star}$) and SFRs by modelling their spectral energy distributions (SEDs). Using these physical properties, we derive the gas-depletion timescales ($t_{dep} = M_{gas}/SFR$) and compare them with the bulge/BH growth timescales ($t_{grow} = M_{star}/SFR \sim M_{BH}/BHAR$). Our sample generally has $t_{dep}$ shorter than $t_{grow}$ by a median factor of $\gtrsim 4$, indicating that the cold gas will be depleted before significant bulge/BH growth takes place. This result suggests that the BH-bulge lockstep growth is mainly responsible for maintaining their mass relation, not creating it. We note that our sample is small and limited to $z, 15 pages, 7 figures, and 3 tables. Accepted by ApJ

W-CDF-S, ELAIS-S1, and XMM-LSS will be three Deep-Drilling Fields (DDFs) of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), but their extensive multi-wavelength data have not been fully utilized as done in the COSMOS field, another LSST DDF. To prepare for future science, we fit source spectral energy distributions (SEDs) from X-ray to far-infrared in these three fields mainly to derive galaxy stellar masses and star-formation rates. We use CIGALE v2022.0, a code that has been regularly developed and evaluated, for the SED fitting. Our catalog includes 0.8 million sources covering $4.9~\mathrm{deg^2}$ in W-CDF-S, 0.8 million sources covering $3.4~\mathrm{deg^2}$ in ELAIS-S1, and 1.2 million sources covering $4.9~\mathrm{deg^2}$ in XMM-LSS. Besides fitting normal galaxies, we also select candidates that may host active galactic nuclei (AGNs) or are experiencing recent star-formation variations and use models specifically designed for these sources to fit their SEDs; this increases the utility of our catalog for various projects in the future. We calibrate our measurements by comparison with those in well-studied smaller regions and briefly discuss the implications of our results. We also perform detailed tests of the completeness and purity of SED-selected AGNs. Our data can be retrieved from a public website., Comment: 49 pages, 39 figures, 9 tables, accepted for publication in ApJS

The Chandra Deep Field-South and North surveys (CDFs) provide unique windows into the cosmic history of X-ray emission from normal (nonactive) galaxies. Scaling relations of normal-galaxy X-ray luminosity (L X) with star formation rate (SFR) and stellar mass (M ⋆) have been used to show that the formation rates of low-mass and high-mass X-ray binaries (LMXBs and HMXBs, respectively) evolve with redshift across z ≈ 0–2 following L HMXB/SFR ∝ (1 + z) and L LMXB/M ⋆ ∝ (1 + z)2−3. However, these measurements alone do not directly reveal the physical mechanisms behind the redshift evolution of X-ray binaries (XRBs). We derive star formation histories for a sample of 344 normal galaxies in the CDFs, using spectral energy distribution (SED) fitting of FUV-to-FIR photometric data, and construct a self-consistent, age-dependent model of the X-ray emission from the galaxies. Our model quantifies how X-ray emission from hot gas and XRB populations vary as functions of host stellar-population age. We find that (1) the ratio L X/M ⋆ declines by a factor of ∼1000 from 0 to 10 Gyr and (2) the X-ray SED becomes harder with increasing age, consistent with a scenario in which the hot gas contribution to the X-ray SED declines quickly for ages above 10 Myr. When dividing our sample into subsets based on metallicity, we find some indication that L X/M ⋆ is elevated for low-metallicity galaxies, consistent with recent studies of X-ray scaling relations. However, additional statistical constraints are required to quantify both the age and metallicity dependence of X-ray emission from star-forming galaxies.

The cosmic black hole accretion density (BHAD) is critical for our understanding of the formation and evolution of supermassive black holes (BHs). However, at high redshifts ($z>3$), X-ray observations report BHADs significantly ($\sim 10$ times) lower than those predicted by cosmological simulations. It is therefore paramount to constrain the high-$z$ BHAD using independent methods other than direct X-ray detections. The recently established relation between star formation rate and BH accretion rate among bulge-dominated galaxies provides such a chance, as it enables an estimate of the BHAD from the star-formation histories (SFHs) of lower-redshift objects. Using the CANDELS Lyman-$\alpha$ Emission At Reionization (CLEAR) survey, we model the SFHs for a sample of 108 bulge-dominated galaxies at $z=$0.7-1.5, and further estimate the BHAD contributed by their high-$z$ progenitors. The predicted BHAD at $z\approx 4$-5 is consistent with the simulation-predicted values, but higher than the X-ray measurements (by $\approx$3-10 times at $z=$4-5). Our result suggests that the current X-ray surveys could be missing many heavily obscured Compton-thick active galactic nuclei (AGNs) at high redshifts. However, this BHAD estimation assumes that the high-$z$ progenitors of our $z=$0.7-1.5 sample remain bulge-dominated where star formation is correlated with BH cold-gas accretion. Alternatively, our prediction could signify a stark decline in the fraction of bulges in high-$z$ galaxies (with an associated drop in BH accretion). JWST and Origins will resolve the discrepancy between our predicted BHAD and the X-ray results by constraining Compton-thick AGN and bulge evolution at high redshifts., Comment: 10 pages and 5 figures; accepted for publication in ApJ

arXiv:2106.08345v1, SPICA Collaboration Team., In the black hole (BH)–galaxy co-evolution framework, most of the star formation (SF) and the BH accretion are expected to take place in highly obscured conditions. The large amount of gas and dust absorbs most of the UV-to-soft-X radiation and re-emits it at longer wavelengths, mostly in the IR. Thus, obscured active galactic nuclei (AGN) are very difficult to identify in optical or X-ray bands but shine bright in the IR. Moreover, X-ray background (XRB) synthesis models predict that a large fraction of the yet-unresolved XRB is due to the most obscured (Compton thick, CT: NH ≥ 1024 cm−2) of these AGN. In this work, we investigate the synergies between putative IR missions [using SPace Infrared telescope for Cosmology and Astrophysics (SPICA), proposed for European Space Agency (ESA)/M5 but withdrawn in 2020 October, and Origins Space Telescope, OST, as ‘templates’] and the X-ray mission Athena (Advanced Telescope for High ENergy Astrophysics), which should fly in early 2030s, in detecting and characterising AGN, with a particular focus on the most obscured ones. Using an XRB synthesis model, we estimated the number of AGN and the number of those which will be detected in the X-rays by Athena. For each AGN, we associated an optical-to-Far InfraRed (FIR) spectral energy distribution (SED) from observed AGN with both X-ray data and SED decomposition and used these SEDs to check if the AGN will be detected by SPICA-like or OST at IR wavelengths. We expect that, with the deepest Athena and SPICA-like (or OST) surveys, we will be able to photometrically detect in the IR more than 90% of all the AGN (down to L2−10keV ∼ 1042 erg s−1 and up to z ∼ 10) predicted by XRB synthesis modeling, and we will detect at least half of them in the X-rays. The spectroscopic capabilities of the OST can provide ≈51 000 and ≈3 400 AGN spectra with R = 300 at 25–588µm in the wide and deep surveys, respectively, the last one up to z ≈ 4. Athena will be extremely powerful in detecting and discerning moderate- and high-luminosity AGN, allowing us to properly select AGN even when the mid-IR torus emission is ‘hidden’ by the host galaxy contribution. We will constrain the intrinsic luminosity and the amount of obscuration for ∼20% of all the AGN (and ∼50% of those with L2−10keV > 3.2 × 1043 erg s−1) using the X-ray spectra provided by Athena WFI. We find that the most obscured and elusive CT-AGN will be exquisitely sampled by SPICA-like mission or OST and that Athena will allow a fine characterisation of the most luminous ones. This will provide a significant step forward in the process of placing stronger constraints on the yet-unresolved XRB and investigating the BH accretion rate evolution up to very high redshift (z ≥ 4)., FP, CV, CG, LB, and LS acknowledge financial support by the Agenzia Spaziale Italiana (ASI) under the research contract 2018-31-HH.0. FJC acknowledges financial support from the Spanish Ministry MCIU under project RTI2018-096686-B-C21 (MCIU/AEI/FEDER/UE), cofunded by FEDER funds and from the Agencia Estatal de Investigación, Unidad de Excelencia María de Maeztu, ref. MDM-2017-0765. MPS acknowledges support from the Comunidad de Madrid, Spain, through Atracción de Talento Investigador Grant 2018-T1/TIC-11035 and PID2019-105423GA-I00 (MCIU/AEI/FEDER,UE). AF acknowledges the support from grant PRIN MIUR2017-20173ML3WW_001.

The discovery of hundreds of QSOs in the first Gyr of the Universe powered by already grown SMBHs challenges our knowledge of SMBH formation. In particular, investigations of $z>6$ QSOs presenting notable properties can provide unique information on the physics of fast SMBH growth in the early universe. We present the results of follow-up observations of the $z=6.515$ radio-quiet QSO PSO167-13, which is interacting with a close companion galaxy. The PSO167-13 system has been recently proposed to host the first heavily obscured X-ray source at high redshift. We observed PSO167-13 with Chandra/ACIS-S (177 ks), and obtained new spectroscopic observations (7.2 h) with Magellan/FIRE. No significant X-ray emission is detected from the PSO167-13 system, suggesting that the obscured X-ray source previously tentatively detected was either due to a strong background fluctuation or is highly variable. The upper limit (90% confidence level) on the X-ray emission of PSO167-13 ($L_{2-10\,\mathrm{keV}}6$ QSO. The ratio between the X-ray and UV luminosity of $\alpha_{ox}6$ times weaker than the expectation based on its UV luminosity. The new Magellan/FIRE spectrum of PSO167-13 is strongly affected by the unfavorable sky conditions, but the tentatively detected C IV and Mg II emission lines appear strongly blueshifted. The most plausible explanations for the X-ray weakness of PSO167-13 are intrinsic weakness or small-scale absorption by Compton-thick material. The possible strong blueshift of its emission lines hints at the presence of nuclear winds, which could be related to its X-ray weakness., Comment: Accepted for publication on A&A

By using a large highly obscured ($N_{\rm H} > 10^{23}\ \rm cm^{-2}$) AGN sample (294 sources at $z \sim 0-5$) selected from detailed X-ray spectral analyses in the deepest Chandra surveys, we explore distributions of these X-ray sources in various optical/IR/X-ray color-color diagrams and their host-galaxy properties, aiming at characterizing the nuclear obscuration environment and the triggering mechanism of highly obscured AGNs. We find that the refined IRAC color-color diagram fails to identify the majority of X-ray selected highly obscured AGNs, even for the most luminous sources with ${\rm log}\,L_{\rm X}\, \rm (erg\ s^{-1}) > 44$. Over 80% of our sources will not be selected as heavily obscured candidates using the flux ratio of $f_{\rm 24 ��m}\, /\,f_R > 1000$ and $R - K > 4.5$ criteria, implying complex origins and conditions for the obscuring materials that are responsible for the heavy X-ray obscuration. The average star formation rate of highly obscured AGNs is similar to that of stellar mass- ($M_*$-) and $z$-controlled normal galaxies, while the lack of quiescent hosts is observed for the former. Partial correlation analyses imply that highly obscured AGN activity (traced by $L_{\rm X}$) appears to be more fundamentally related to $M_*$, and no dependence of $N_{\rm H}$ on either $M_*$ or SFR is detected. Morphology analyses reveal that 61% of our sources have a significant disk component, while only 27% of them exhibit irregular morphological signatures. These findings together point toward a scenario where secular processes (e.g., galactic-disk instabilities), instead of mergers, are most probable to be the leading mechanism that triggers accretion activities of X-ray-selected highly obscured AGNs., 23 pages, 12 figures, 3 tables, resubmitted to ApJ after addressing referee's comments

We present the X-ray source catalog for the ∼479 ks Chandra exposure of the SDSS J1030+0524 field, which is centered on a region that shows the best evidence to date of an overdensity around a z > 6 quasar, and also includes a galaxy overdensity around a Compton-thick Fanaroff-Riley type II (FRII) radio galaxy at z = 1.7. Using wavdetect for initial source detection and ACIS Extract for source photometry and significance assessment, we create preliminary catalogs of sources that are detected in the full (0.5-7.0 keV), soft (0.5-2.0 keV), and hard (2-7 keV) bands, respectively. We produce X-ray simulations that mirror our Chandra observation to filter our preliminary catalogs and achieve a completeness level of > 91% and a reliability level of ∼95% in each band. The catalogs in the three bands are then matched into a final main catalog of 256 unique sources. Among them, 244, 193, and 208 are detected in the full, soft, and hard bands, respectively. The Chandra observation covers a total area of 335 arcmin2 and reaches flux limits over the central few square arcmins of ∼3 × 10-16, 6 × 10-17, and 2 × 10-16 erg cm-2 s-1 in the full, soft, and hard bands, respectively This makes J1030 field the fifth deepest extragalactic X-ray survey to date. The field is part of the Multiwavelength Survey by Yale-Chile (MUSYC), and is also covered by optical imaging data from the Large Binocular Camera (LBC) at the Large Binocular Telescope (LBT), near-infrared imaging data from the Canada France Hawaii Telescope WIRCam (CFHT/WIRCam), and Spitzer IRAC. Thanks to its dense multi-wavelength coverage, J1030 represents a legacy field for the study of large-scale structures around distant accreting supermassive black holes. Using a likelihood ratio analysis, we associate multi-band (r, z, J, and 4.5 μm) counterparts for 252 (98.4%) of the 256 Chandra sources, with an estimated reliability of 95%. Finally, we compute the cumulative number of sources in each X-ray band, finding that they are in general agreement with the results from the Chandra Deep Fields., We acknowledge the referee for a prompt and constructive report. We acknowledge financial contribution from the agreement ASI-INAF n. 2017-14-H.O. We thank P. Broos for providing great support for the analysis of our simulations with AE, and H. M. Günther for the support provided for using MARX. We also thank B. Luo for providing us the log(N)–log(S) of the 7Ms CDF-S. FV acknowledges financial support from CONICYT and CASSACA through the Fourth call for tenders of the CAS-CONICYT Fund, and CONICYT grants Basal-CATA AFB-170002. DM and MA acknowledge support by grant number NNX16AN49G issued through the NASA Astrophysics Data Analysis Program (ADAP). Further support was provided by the Faculty Research Fund (FRF) of Tufts University.

We report the discovery of an extreme X-ray flux rise (by a factor of > 20) of the weak-line quasar SDSS J153913.47+395423.4 (hereafter SDSS J1539+3954) at z = 1.935. SDSS J1539+3954 is the most-luminous object among radio-quiet type 1 AGNs where such dramatic X-ray variability has been observed. Before the X-ray flux rise, SDSS J1539+3954 appeared X-ray weak compared with the expectation from its UV flux; after the rise, the ratio of its X-ray flux and UV flux is consistent with the majority of the AGN population. We also present a contemporaneous HET spectrum of SDSS J1539+3954, which demonstrates that its UV continuum level remains generally unchanged despite the dramatic increase in the X-ray flux, and its C iv emission line remains weak. The dramatic change only observed in the X-ray flux is consistent with a shielding model, where a thick inner accretion disk can block our line of sight to the central X-ray source. This thick inner accretion disk can also block the nuclear ionizing photons from reaching the high-ionization broad emission-line region, so that weak high-ionization emission lines are observed. Under this scenario, the extreme X-ray variability event may be caused by slight variations in the thickness of the disk. This event might also be explained by gravitational light-bending effects in a reflection model., 8 pages, 3 figures. Accepted for publication in ApJL

We present a series of new, publicly available mock catalogs of X-ray selected active galactic nuclei (AGNs), non-active galaxies, and clusters of galaxies. They are based on up-to-date observational results on the demographic of extragalactic X-ray sources and their extrapolations.These mocks reach fluxes below 1E-20 erg s-1 cm-2 in the 0.5-2 keV band, i.e., more than an order of magnitude below the predicted limits of future deep fields, and therefore represent an important tool for simulating extragalactic X-ray surveys with both current and future telescopes. We use our mocks to perform a set of end-to-end simulations of X-ray surveys with the forthcoming Athena mission and with the AXIS probe, a sub-arcsecond resolution X-ray mission concept proposed to the Astro 2020 Decadal Survey. We find that these proposed, next generation surveys may transform our knowledge of the deep X-ray Universe. As an example, in a total observing time of 15\,Ms, AXIS would detect ~225,000 AGNs and ~$50,000 non-active galaxies, reaching a flux limit f_0.5-2~5E-19 erg/s/cm2 in the 0.5-2 keV band, with an improvement of over an order of magnitude with respect to surveys with current X-ray facilities. Consequently, 90% of these sources would be detected for the first time in the X-rays. Furthermore, we show that deep and wide X-ray surveys with instruments like AXIS and Athena are expected to detect ~20,000 z>3 AGNs and ~250 sources at redshift z>6, thus opening a new window of knowledge on the evolution of AGNs over cosmic time and putting strong constraints on the predictions of theoretical models of black hole seed accretion in the early universe., 19 pages, 13 figures. Accepted for publication in A&A. Mock catalogs available online at http://cxb.oas.inaf.it/mock.html

We present a procedure to constrain the redshifts of obscured ($N_H > 10^{22}$ cm$^{-2}$) Active Galactic Nuclei (AGN) based on low-count statistics X-ray spectra, which can be adopted when photometric and/or spectroscopic redshifts are unavailable or difficult to obtain. We selected a sample of 54 obscured AGN candidates on the basis of their X-ray hardness ratio, $HR>-0.1$, in the Chandra deep field ($\sim$479 ks, 335 arcmin$^2$) around the $z=6.3$ QSO SDSS J1030+0524. The sample has a median value of $\approx80$ net counts in the 0.5-7 keV energy band. We estimate reliable X-ray redshift solutions taking advantage of the main features in obscured AGN spectra, like the Fe 6.4 keV K$\mathrm{\alpha}$ emission line, the 7.1 keV Fe absorption edge and the photoelectric absorption cut-off. The significance of such features is investigated through spectral simulations, and the derived X-ray redshift solutions are then compared with photometric redshifts. Both photometric and X-ray redshifts are derived for 33 sources. When multiple solutions are derived by any method, we find that combining the redshift solutions of the two techniques improves the rms by a factor of two. Using our redshift estimates ($0.1\lesssim z \lesssim 4$), we derived absorbing column densities in the range $\sim 10^{22}-10^{24}$ cm$^{-2}$ and absorption-corrected, 2-10 keV rest-frame luminosities between $\sim 10^{42}$ and $10^{45}$ erg s$^{-1}$, with median values of $N_H = 1.7 \times 10^{23}$ cm$^{-2}$ and $L_{\mathrm{2-10\, keV}} = 8.3\times10^{43}$ erg s$^{-1}$, respectively. Our results suggest that the adopted procedure can be applied to current and future X-ray surveys, for sources detected only in the X-rays or that have uncertain photometric or single-line spectroscopic redshifts., Comment: 22 pages, 18 figures

We report on the spectroscopic confirmation of a large-scale structure around the luminous z = 6.31 quasi-stellar object (QSO) SDSS J1030+0524, powered by a one billion solar mass black hole. The structure is populated by at least six members, namely, four Lyman-break galaxies (LBGs), and two Lyman alpha emitters (LAEs). The four LBGs were identified among a sample of 21 i-band dropouts with zAB < 25.5 selected up to projected separations of 5 physical Mpc (15 arcmin) from the QSO. Their redshifts were determined through multi-object spectroscopic observations at 8-10 m class telescopes lasting up to eight hours. The two LAEs were identified in a 6 h VLT/MUSE observation centered on the QSO. The redshifts of the six galaxies cover the range between 6.129-6.355. Assuming that the peculiar velocities are negligible, this range corresponds to radial separations of ±5 physical Mpc from the QSO, that is comparable to the projected scale of the observed LBG distribution on the sky. We conservatively estimate that this structure is significant at a level > 3.5σ and that the level of the galaxy overdensity is at least 1.5-2 within the large volume sampled (∼780 physical Mpc3). The spectral properties of the six member galaxies (Lyα strength and UV luminosity) are similar to those of field galaxies at similar redshifts. This is the first spectroscopic identification of a galaxy overdensity around a supermassive black hole in the first billion years of the Universe. Our finding lends support to the idea that the most distant and massive black holes form and grow within massive (>1012 M) dark matter halos in large-scale structures and that the absence of earlier detections of such systems is likely due to observational limitations., We acknowledge funding from the INAF main-stream (1.05.01.86.31) and from the agreement ASI-INAF n. 2017-14-H.O. This work is based on ESO program 0102.A-0263(A), on observations obtained at the W. M. Keck Observatory, and on data acquired using the Large Binocular Telescope (ID 2017/2018 #18).

For over 60 years, the scientific community has studied actively growing central super-massive black holes (active galactic nuclei -- AGN) but fundamental questions on their genesis remain unanswered. Numerical simulations and theoretical arguments show that black hole growth occurs during short-lived periods ($\sim$ 10$^{7}$ -10$^{8}$ yr) of powerful accretion. Major mergers are commonly invoked as the most likely dissipative process to trigger the rapid fueling of AGN. If the AGN-merger paradigm is true, we expect galaxy mergers to coincide with black hole accretion during a heavily obscured AGN phase (N$_H$ $ > 10^{23}$ cm$^{-2}$). Starting from one of the largest samples of obscured AGN at 0.5 $, Comment: 19 pages, 11 figures, accepted in ApJ

We investigate the population of high-redshift ($3\leq z < 6$) AGN selected in the two deepest X-ray surveys, the 7 Ms \textit{Chandra} Deep Field-South and 2 Ms \textit{Chandra} Deep Field-North. Their outstanding sensitivity and spectral characterization of faint sources allow us to focus on the sub-$L_*$ regime (log$L_{\mathrm{X}}\lesssim44$), poorly sampled by previous works using shallower data, and the obscured population. Taking fully into account the individual photometric-redshift probability distribution functions, the final sample consists of $\approx102$ X-ray selected AGN at $3\leq z < 6$. The fraction of AGN obscured by column densities log$N_{\mathrm{H}}>23$ is $\sim0.6-0.8$, once incompleteness effects are taken into account, with no strong dependence on redshift or luminosity. We derived the high-redshift AGN number counts down to $F_{\mathrm{0.5-2\,keV}}=7\times10^{-18}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$, extending previous results to fainter fluxes, especially at $z>4$. We put the tightest constraints to date on the low-luminosity end of AGN luminosity function at high redshift. The space-density, in particular, declines at $z>3$ at all luminosities, with only a marginally steeper slope for low-luminosity AGN. By comparing the evolution of the AGN and galaxy densities, we suggest that such a decline at high luminosities is mainly driven by the underlying galaxy population, while at low luminosities there are hints of an intrinsic evolution of the parameters driving nuclear activity. Also, the black-hole accretion rate density and star-formation rate density, which are usually found to evolve similarly at $z\lesssim3$, appear to diverge at higher redshifts., Accepted 2017 September 15 (MNRAS). 31 Pages, 25 figures

We report the discovery of disappearance of Mg ii, Al iii, C iv, and Si iv broad absorption lines (BALs) at the same velocity, accompanied by a new Civ BAL emerging at a higher velocity, in the quasar J0827+4252 at z = 2.038. This is the first report of BAL disappearance (i) over Mg ii, Al iii, C iv, and Si iv ions and (ii) in a weak emission-line quasar (WLQ). The discovery is based on four spectra from the SDSS and one follow-up spectrum from HET/LRS2. The simultaneous Civ BAL disappearance and emergence at different velocities, together with no variations in the CRTS light curve, indicate that ionization changes in the absorbing material are unlikely to cause the observed BAL variability. Our analyses reveal that transverse motion is the most likely dominant driver of the BAL disappearance/emergence. Given the presence of mildly relativistic BAL outflows and an apparently large C iv emission-line blueshift that is likely associated with strong bulk outflows in this WLQ, J0827+4252 provides a notable opportunity to study extreme quasar winds and their potential in expelling material from inner to large-scale regions., Comment: Published in 2019 ApJL, 870, 2

Population synthesis models of actively accreting super-massive black holes (or active galactic nuclei -- AGN) predict a large fraction that must grow behind dense, obscuring screens of gas and dust. Deep X-ray surveys are thought to have provided the most complete and unbiased samples of AGN, but there is strong observational evidence that a portion of the population of obscured AGN is being missed. In this paper we use a sample of AGN derived from the deepest X-ray survey to date, the Chandra 7Ms GOODS-South Survey, to investigate the nature of low flux X-ray sources. We make full use of the extensive multi-wavelength coverage of the GOODS-South field, and cross-match our objects with wavelengths from the Radio to the IR. We find the low X-ray flux AGN in our sample have X-ray luminosities that indicate low-luminosity AGN classification, while their radio, infrared and optical counterparts indicate moderate to powerful AGN classification. We find the predicted column densities is on average an order of magnitude higher than the calculated column densities via X-ray detections for X-ray faint sources. We interpret our results as evidence of obscured AGN disguising as low-luminosity AGN via their X-ray luminosities. When we compare the estimation of the obscured AGN space density with and without these objects, we find a difference of 40% in the lowest X-ray luminosity regime probed by our sample., Comment: 23 pages, 16 figures, Accepted to the Astrophysical Journal

We present an X-ray point-source catalog from the XMM-Large Scale Structure survey region (XMM-LSS), one of the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS) fields. We target the XMM-LSS region with $1.3$ Ms of new XMM-Newton AO-15 observations, transforming the archival X-ray coverage in this region into a 5.3 deg$^2$ contiguous field with uniform X-ray coverage totaling $2.7$ Ms of flare-filtered exposure, with a $46$ ks median PN exposure time. We provide an X-ray catalog of 5242 sources detected in the soft (0.5-2 keV), hard (2-10 keV), and/or full (0.5-10 keV) bands with a 1% expected spurious fraction determined from simulations. A total of 2381 new X-ray sources are detected compared to previous source catalogs in the same area. Our survey has flux limits of $1.7\times10^{-15}$, $1.3\times10^{-14}$, and $6.5\times10^{-15}$ erg cm$^{-2}$ s$^{-1}$ over 90% of its area in the soft, hard, and full bands, respectively, which is comparable to those of the XMM-COSMOS survey. We identify multiwavelength counterpart candidates for 99.9% of the X-ray sources, of which 93% are considered as reliable based on their matching likelihood ratios. The reliabilities of these high-likelihood-ratio counterparts are further confirmed to be $\approx 97$% reliable based on deep Chandra coverage over $\approx 5$% of the XMM-LSS region. Results of multiwavelength identifications are also included in the source catalog, along with basic optical-to-infrared photometry and spectroscopic redshifts from publicly available surveys. We compute photometric redshifts for X-ray sources in 4.5 deg$^2$ of our field where forced-aperture multi-band photometry is available; $>70$% of the X-ray sources in this subfield have either spectroscopic or high-quality photometric redshifts., MNRAS, accepted. 34 pages, 25 figures, and 8 tables. The data products are available at this http url: http://personal.psu.edu/wnb3/xmmservs/xmmservs.html

It is well known that environment affects galaxy evolution, which is broadly related to supermassive black hole (SMBH) growth. We investigate whether SMBH evolution also depends on host-galaxy local (sub-Mpc) and global ($\approx 1-10$ Mpc) environment. We construct the surface-density field (local environment) and cosmic web (global environment) in the COSMOS field at $z=0.3-3.0$. The environments in COSMOS range from the field to clusters ($M_\mathrm{halo} \lesssim 10^{14}\ M_\odot$), covering the environments where ${\approx 99\%}$ of galaxies in the Universe reside. We measure sample-averaged SMBH accretion rate ($\overline{\mathrm{BHAR}}$) from X-ray observations, and study its dependence on overdensity and cosmic-web environment at different redshifts while controlling for galaxy stellar mass ($M_\star$). Our results show that $\overline{\mathrm{BHAR}}$ does not significantly depend on overdensity or cosmic-web environment once $M_\star$ is controlled, indicating that environment-related physical mechanisms (e.g. tidal interaction and ram-pressure stripping) might not significantly affect SMBH growth. We find that $\overline{\mathrm{BHAR}}$ is strongly related to host-galaxy $M_\star$, regardless of environment., 24 pages, 17 figures, 7 tables. Accepted by MNRAS

We present the results from a $\sim500$ ks Chandra observation of the $z=6.31$ QSO SDSS J1030+0524. This is the deepest X-ray observation to date of a $z\sim6$ QSO. The QSO is detected with a total of 125 net counts in the full ($0.5-7$ keV) band and its spectrum can be modeled by a single power-law model with photon index of $\Gamma = 1.81 \pm 0.18$ and full band flux of $f=3.95\times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$. When compared with the data obtained by XMM-Newton in 2003, our Chandra observation in 2017 shows a harder ($\Delta \Gamma \approx -0.6$) spectrum and a 2.5 times fainter flux. Such a variation, in a timespan of $\sim2$ yrs rest-frame, is unexpected for such a luminous QSO powered by a $> 10^9 \: M_{\odot}$ black hole. The observed source hardening and weakening could be related to an intrinsic variation in the accretion rate. However, the limited photon statistics does not allow us to discriminate between an intrinsic luminosity and spectral change, and an absorption event produced by an intervening gas cloud along the line of sight. We also report the discovery of diffuse X-ray emission that extends for 30"x20" southward the QSO with a signal-to-noise ratio of $\sim$6, hardness ratio of $HR=0.03_{-0.25}^{+0.20}$, and soft band flux of $f_{0.5-2 \: keV}= 1.1_{-0.3}^{+0.3} \times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$, that is not associated to a group or cluster of galaxies. We discuss two possible explanations for the extended emission, which may be either associated with the radio lobe of a nearby, foreground radio galaxy (at $z \approx 1-2$), or ascribed to the feedback from the QSO itself acting on its surrounding environment, as proposed by simulations of early black hole formation., Comment: 13 pages, 9 figures, A&A accepted

Hot Dust-Obscured Galaxies (Hot DOGs) are hyperluminous ($L_{\mathrm{8-1000\,��m}}>10^{13}\,\mathrm{L_\odot}$) infrared galaxies with extremely high (up to hundreds of K) dust temperatures. The sources powering both their extremely high luminosities and dust temperatures are thought to be deeply buried and rapidly accreting supermassive black holes (SMBHs). Hot DOGs could therefore represent a key evolutionary phase in which the SMBH growth peaks. X-ray observations can be used to study their obscuration levels and luminosities. In this work, we present the X-ray properties of the 20 most-luminous ($L_{\mathrm{bol}}\gtrsim10^{14}\, L_\odot$) known Hot DOGs at $z=2-4.6$. Five of them are covered by long-exposure ($10-70$ ks) Chandra and XMM-Newton observations, with three being X-ray detected, and we study their individual properties. One of these sources (W0116$-$0505) is a Compton-thick candidate, with column density $N_H=(1.0-1.5)\times10^{24}\,\mathrm{cm^{-2}}$ derived from X-ray spectral fitting. The remaining 15 Hot DOGs have been targeted by a Chandra snapshot (3.1 ks) survey. None of these 15 is individually detected; therefore we applied a stacking analysis to investigate their average emission. From hardness-ratio analysis, we constrained the average obscuring column density and intrinsic luminosity to be log$N_H\,\mathrm{[cm^{-2}]}>23.5$ and $L_X\gtrsim10^{44}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$, which are consistent with results for individually detected sources. We also investigated the $L_X-L_{6��\mathrm{m}}$ and $L_X-L_{bol}$ relations, finding hints that Hot DOGs are typically X-ray weaker than expected, although larger samples of luminous obscured QSOs are needed to derive solid conclusions., MNRAS, accepted 2017 November 29 . Received 2017 November 29 ; in original form 2017 October 11. 15 pages, 6 figures

In deep X-ray surveys, active galactic nuclei (AGNs) with a broad range of luminosities have been identified. However, cosmologically distant low-luminosity AGN (LLAGN, $L_{\mathrm{X}} \lesssim 10^{42}$ erg s$^{-1}$) identification still poses a challenge due to significant contamination from host galaxies. Based on the 7 Ms Chandra Deep Field-South (CDF-S) survey, the longest timescale ($\sim 17$ years) deep X-ray survey to date, we utilize an X-ray variability selection technique to search for LLAGNs that remain unidentified among the CDF-S X-ray sources. We find 13 variable sources from 110 unclassified CDF-S X-ray sources. Except for one source which could be an ultraluminous X-ray source, the variability of the remaining 12 sources is most likely due to accreting supermassive black holes. These 12 AGN candidates have low intrinsic X-ray luminosities, with a median value of $7 \times10^{40}$ erg s$^{-1}$. They are generally not heavily obscured, with an average effective power-law photon index of 1.8. The fraction of variable AGNs in the CDF-S is independent of X-ray luminosity and is only restricted by the total number of observed net counts, confirming previous findings that X-ray variability is a near-ubiquitous property of AGNs over a wide range of luminosities. There is an anti-correlation between X-ray luminosity and variability amplitude for high-luminosity AGNs, but as the luminosity drops to $\lesssim 10^{42}$ erg s$^{-1}$, the variability amplitude no longer appears dependent on the luminosity. The entire observed luminosity-variability trend can be roughly reproduced by an empirical AGN variability model based on a broken power-law power spectral density function., Comment: 18 pages, 11 figures, accepted for publication in ApJ

We systematically analyze X-ray variability of active galactic nuclei (AGNs) in the 7~Ms \textit{Chandra} Deep Field-South survey. On the longest timescale ($\approx~17$ years), we find only weak (if any) dependence of X-ray variability amplitudes on energy bands or obscuration. We use four different power spectral density (PSD) models to fit the anti-correlation between normalized excess variance ($\sigma^2_{\rm nxv}$) and luminosity, and obtain a best-fit power law index $\beta=1.16^{+0.05}_{-0.05}$ for the low-frequency part of AGN PSD. We also divide the whole light curves into 4 epochs in order to inspect the dependence of $\sigma^2_{\rm nxv}$ on these timescales, finding an overall increasing trend. The analysis of these shorter light curves also infers a $\beta$ of $\sim 1.3$ that is consistent with the above-derived $\beta$, which is larger than the frequently-assumed value of $\beta=1$. We then investigate the evolution of $\sigma^2_{\rm nxv}$. No definitive conclusion is reached due to limited source statistics but, if present, the observed trend goes in the direction of decreasing AGN variability at fixed luminosity toward large redshifts. We also search for transient events and find 6 notable candidate events with our considered criteria. Two of them may be a new type of fast transient events, one of which is reported here for the first time. We therefore estimate a rate of fast outbursts $\langle\dot{N}\rangle = 1.0^{+1.1}_{-0.7}\times 10^{-3}~\rm galaxy^{-1}~yr^{-1}$ and a tidal disruption event~(TDE) rate $\langle\dot{N}_{\rm TDE}\rangle=8.6^{+8.5}_{-4.9}\times 10^{-5}~\rm galaxy^{-1}~yr^{-1}$ assuming the other four long outbursts to be TDEs., Comment: 20 pages, 16 figures. Accepted for publication in ApJ

Many cosmological studies predict that early supermassive black holes (SMBHs) can only form in the most massive dark matter halos embedded within large scale structures marked by galaxy over-densities that may extend up to 10 physical Mpc. This scenario, however, has not been confirmed observationally, as the search for galaxy over-densities around high-z quasars has returned conflicting results. The field around the z=6.28 quasar SDSSJ1030+0524 (J1030) is unique for multi-band coverage and represents an excellent data legacy for studying the environment around a primordial SMBH. In this paper we present wide-area (25x25 arcmin), Y- and J-band imaging of the J1030 field obtained with the near infrared camera WIRCam at the Canada-France-Hawaii Telescope (CFHT). We built source catalogues in the Y- and J-band, and matched those with our photometric catalogue in the r, z, i bands presented in Morselli et al. (2014). We used these new infrared data together with H and K and Spitzer/IRAC data to refine our selection of Lyman Break Galaxies (LBGs), extending our selection criteria to galaxies in the range 25.2=1.3. We found a significant asymmetry in the distribution of the high-z galaxies in J1030, supporting the existence of a coherent large-scale structure around the quasar. We compared our results with those of Bowler et al. (2015), who adopted similar LBGs selection criteria, and estimated an over-density of galaxies in the field of delta = 2.4, which is significant at >4 sigma. The over-density value and its significance are higher than those found in Morselli et al. (2014), and we interpret this as evidence of an improved LBG selection., Comment: 13 pages, 8 figures, accepted for publication in A&A

We investigate the dependence of black-hole accretion rate (BHAR) on host-galaxy star formation rate (SFR) and stellar mass ($M_*$) in the CANDELS/GOODS-South field in the redshift range of $0.5\leq z < 2.0$. Our sample consists of $\approx 18000$ galaxies, allowing us to probe galaxies with $0.1 \lesssim \mathrm{SFR} \lesssim 100\ M_\odot\ \mathrm{yr}^{-1}$ and/or $10^8 \lesssim M_* \lesssim 10^{11}\ M_{\odot}$. We use sample-mean BHAR to approximate long-term average BHAR. Our sample-mean BHARs are derived from the $Chandra$ Deep Field-South 7 Ms observations, while the SFRs and $M_*$ have been estimated by the CANDELS team through SED fitting. The average BHAR is correlated positively with both SFR and $M_*$, and the BHAR-SFR and BHAR-$M_*$ relations can both be described acceptably by linear models with a slope of unity. However, BHAR appears to be correlated more strongly with $M_*$ than SFR. This result indicates that $M_*$ is the primary host-galaxy property related to black-hole growth, and the apparent BHAR-SFR relation is largely a secondary effect due to the star-forming main sequence. Among our sources, massive galaxies ($M_* \gtrsim 10^{10} M_{\odot}$) have significantly higher BHAR/SFR ratios than less-massive galaxies, indicating the former have higher black-hole fueling efficiency and/or higher SMBH occupation fraction than the latter. Our results can naturally explain the observed proportionality between $M_{\rm BH}$ and $M_*$ for local giant ellipticals, and suggest their $M_{\rm BH}/M_*$ is higher than that of local star-forming galaxies. Among local star-forming galaxies, massive systems might have higher $M_{\rm BH}/M_*$ compared to dwarfs., 18 pages, 11 figures, 3 tables; accepted for publication in ApJ

We present X-ray source catalogs for the $\approx7$ Ms exposure of the Chandra Deep Field-South (CDF-S), which covers a total area of 484.2 arcmin$^2$. Utilizing WAVDETECT for initial source detection and ACIS Extract for photometric extraction and significance assessment, we create a main source catalog containing 1008 sources that are detected in up to three X-ray bands: 0.5-7.0 keV, 0.5-2.0 keV, and 2-7 keV. A supplementary source catalog is also provided including 47 lower-significance sources that have bright ($K_s\le23$) near-infrared counterparts. We identify multiwavelength counterparts for 992 (98.4%) of the main-catalog sources, and we collect redshifts for 986 of these sources, including 653 spectroscopic redshifts and 333 photometric redshifts. Based on the X-ray and multiwavelength properties, we identify 711 active galactic nuclei (AGNs) from the main-catalog sources. Compared to the previous $\approx4$ Ms CDF-S catalogs, 291 of the main-catalog sources are new detections. We have achieved unprecedented X-ray sensitivity with average flux limits over the central $\approx1$ arcmin$^2$ region of $\approx1.9\times10^{-17}$, $6.4\times10^{-18}$, and $2.7\times10^{-17}$ erg cm$^{-2}$ s$^{-1}$ in the three X-ray bands, respectively. We provide cumulative number-count measurements observing, for the first time, that normal galaxies start to dominate the X-ray source population at the faintest 0.5-2.0 keV flux levels. The highest X-ray source density reaches $\approx50\,500$ deg$^{-2}$, and $47\%\pm4\%$ of these sources are AGNs ($\approx23\,900$ deg$^{-2}$)., Comment: 31 pages, 31 figures, 8 tables, ApJS in press, minor text updates; full-resolution PDF version and data products available at http://www.astro.psu.edu/users/niel/cdfs/cdfs-chandra.html

We present results from the deep XMM-Newton observations of the two brightest X-ray sources in the Chandra Deep Field South (CDFS), PID 203 (z = 0.544) and PID 319 (z = 0.742). The long exposure of 2.5 Ms over a 10 year period (net 4 yr with a 6 yr gap) makes it possible to obtain high quality X-ray spectra of these two Type I AGN with X-ray luminosity of 1044 erg s-1, which is the typical luminosity for low-redshift PG quasars, and track their X-ray variability both in flux and spectral shape. Both sources showed X-ray flux variability of ~10−20% in rms, which is similar in the soft (0.5−2 keV) and hard (2−7 keV) bands. PID 203, which has evidence for optical extinction, shows modest amount of absorption (NH≤ 1 × 1021 cm-2) in the X-ray spectrum. Fe K emission is strongly detected in both objects with EW ~ 0.2 keV. The lines in both objects are moderately broad and exhibit marginal evidence for variability in shape and flux, indicating that the bulk of the line emission comes from their accretion disks rather than distant tori., K.I. acknowledges support by DGI of the Spanish Ministerio de Economía y Competitividad (MINECO) under grant AYA2013-47447-C3-2-P. W.N.B. thanks the NASA ADP grant NNX10AC99G. We acknowledge financial contribution from the agreement ASI-INAF I/009/10/0.

Gas outflows are believed to play a pivotal role in shaping galaxies, as they regulate both star formation and black hole growth. Despite their ubiquitous presence, the origin and the acceleration mechanism of such powerful and extended winds is not yet understood. Direct observations of the cold gas component in objects with detected outflows at other wavelengths are needed to assess the impact of the outflow on the host galaxy interstellar medium (ISM). We observed with the Plateau de Bure Interferometer an obscured quasar at z~1.5, XID2028, for which the presence of an ionised outflow has been unambiguously signalled by NIR spectroscopy. The detection of CO(3-2) emission in this source allows us to infer the molecular gas content and compare it to the ISM mass derived from the dust emission. We then analyze the results in the context of recent insights on scaling relations, which describe the gas content of the overall population of star-forming galaxies at a similar redshifts. The Star formation efficiency (~100) and gas mass (M_gas=2.1-9.5x10^{10} M_sun) inferred from the CO(3-2) line depend on the underlying assumptions on the excitation of the transition and the CO-to-H2 conversion factor. However, the combination of this information and the ISM mass estimated from the dust mass suggests that the ISM/gas content of XID2028 is significantly lower than expected for its observed M$_\star$, sSFR and redshift, based on the most up-to-date calibrations (with gas fraction, 8 pages, 4 figures. Astronomy and Astrophysics, in press

We report ALMA Cycle 0 observations at 1.3mm of LESS J033229.4-275619 (XID403), an Ultraluminous Infrared Galaxy at $z=4.75$ in the Chandra Deep Field South hosting a Compton-thick QSO. The source is not resolved in our data at a resolution of $\sim$0.75 arcsec, placing an upper-limit of 2.5 kpc to the half-light radius of the continuum emission from heated-dust. After deconvolving for the beam size, however, we found a $\sim3\sigma$ indication of an intrinsic source size of $0.27\pm0.08$ arcsec (Gaussian FWHM), which would correspond to $r_{half}\sim0.9\pm0.3$ kpc. We build the far-IR SED of XID403 by combining datapoints from both ALMA and Herschel and fit it with a modified blackbody spectrum. For the first time, we measure the dust temperature $T_d=58.5\pm5.3$ K in this system, which is comparable to what has been observed in other high-z submillimeter galaxies. The measured star formation rate is SFR=$1020\pm150$ $M_{\odot}$ yr$^{-1}$, in agreement with previous estimates at lower S/N. Based on the measured SFR and source size, we constrain the SFR surface density to be $\Sigma_{SFR}>26\;M_{\odot}$yr$^{-1}$kpc$^{-2}$ ($\sim200\;M_{\odot}$yr$^{-1}$kpc$^{-2}$ for $r_{half}\sim0.9$ kpc). The compactness of this starburst is comparable to what has been observed in other local and high-z starburst galaxies. If the gas mass measured from previous [CII] and CO(2-1) observations at low resolution is confined within the same dust region, assuming $r_{half}\sim0.9\pm0.3$ kpc, this would produce a column density of $N_H\sim0.3-1.1\times10^{24}$cm$^{-2}$ towards the central SMBH, similar to the column density of $\approx1.4\times10^{24}$cm$^{-2}$ measured from the X-rays. Then, in principle, if both gas and dust were confined on sub-kpc scales, this would be sufficient to produce the observed X-ray column density without any need of a pc-scale absorber [abridged]., Comment: 11 pages, 5 figures. Accepted for publication in A&A

We have investigated the gas content of a sample of several hundred AGN host galaxies at z$, Comment: Accepted for pubblication in MNRAS. 7 pages, 4 figures

Improving the capabilities of detecting faint X-ray sources is fundamental to increase the statistics on faint high-z AGN and star-forming galaxies. We performed a simultaneous Maximum Likelihood PSF fit in the [0.5-2] keV and [2-7] keV energy bands of the 4 Ms{\em Chandra} Deep Field South (CDFS) data at the position of the 34930 CANDELS H-band selected galaxies. For each detected source we provide X-ray photometry and optical counterpart validation. We validated this technique by means of a raytracing simulation. We detected a total of 698 X-ray point-sources with a likelihood $\mathcal{L}$$>$4.98 (i.e. $>$2.7$��$). We show that the prior knowledge of a deep sample of Optical-NIR galaxies leads to a significant increase of the detection of faint (i.e. $\sim$10$^{-17}$ cgs in the [0.5-2] keV band) sources with respect to "blind" X-ray detections. By including previous X-ray catalogs, this work increases the total number of X-ray sources detected in the 4 Ms CDFS, CANDELS area to 793, which represents the largest sample of extremely faint X-ray sources assembled to date. Our results suggest that a large fraction of the optical counterparts of our X-ray sources determined by likelihood ratio actually coincides with the priors used for the source detection. Most of the new detected sources are likely star-forming galaxies or faint absorbed AGN. We identified a few sources sources with putative photometric redshift z$>$4. Despite the low number statistics and the uncertainties on the photo-z, this sample significantly increases the number of X--ray selected candidate high-z AGN., 15 Pages, 10 Figures, Accepted by ApJ, version before editing, Catalog available at: http://www.astrodeep.eu/chandra-counterparts/ , CDS and www.astrodeep.edu

We perform long-term ($\approx 15$ yr, observed-frame) X-ray variability analyses of the 68 brightest radio-quiet active galactic nuclei (AGNs) in the 6 Ms $Chandra$ Deep Field-South (CDF-S) survey; the majority are in the redshift range of $0.6-3.1$, providing access to penetrating rest-frame X-rays up to $\approx 10-30$ keV. Twenty-four of the 68 sources are optical spectral type I AGNs, and the rest (44) are type II AGNs. The time scales probed in this work are among the longest for X-ray variability studies of distant AGNs. Photometric analyses reveal widespread photon-flux variability: $90\%$ of AGNs are variable above a 95% confidence level, including many X-ray obscured AGNs and several optically classified type II quasars. We characterize the intrinsic X-ray luminosity ($L_{\rm{X}}$) and absorption ($N_{\rm{H}}$) variability via spectral fitting. Most (74%) sources show $L_{\rm{X}}$ variability; the variability amplitudes are generally smaller for quasars. A Compton-thick candidate AGN shows variability of its high-energy X-ray flux, indicating the size of reflecting material to be $\lesssim 0.3$ pc. $L_{\rm{X}}$ variability is also detected in a broad absorption line (BAL) quasar. The $N_{\rm{H}}$ variability amplitude for our sample appears to rise as time separation increases. About 16% of sources show $N_{\rm{H}}$ variability. One source transitions from an X-ray unobscured to obscured state while its optical classification remains type I; this behavior indicates the X-ray eclipsing material is not large enough to obscure the whole broad-line region., 21 pages, 17 figures, accepted for publication in ApJ