DSpace Angular :: Browsing by Author "Comastri, A."

Browsing by Author "Comastri, A."

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A GROWTH-RATE INDICATOR FOR COMPTON-THICK ACTIVE GALACTIC NUCLEI
(2016) Brightman, M.; Masini, A.; Ballantyne, D. R.; Balokovic, M.; Brandt, W. N.; Chen, C. -T.; Comastri, A.; Farrah, D.; Gandhi, P.; Harrison, F. A.; Ricci, C.; Stern, D.; Walton, D. J.
Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGNs) is difficult to measure. A statistically significant correlation between the Eddington ratio, lambda(Edd), and the X-ray power-law index, Gamma, observed in unobscured AGNs offers an estimate of their growth rate from X-ray spectroscopy (albeit with large scatter). However, since X-rays undergo reprocessing by Compton scattering and photoelectric absorption when the line of sight to the central engine is heavily obscured, the recovery of the intrinsic Gamma is challenging. Here we study a sample of local, predominantly CT megamaser AGNs, where the black hole mass, and thus Eddington luminosity, are well known. We compile results of the X-ray spectral fitting of these sources with sensitive high-energy (E > 10 keV) NuSTAR data, where X-ray torus models, which take into account the reprocessing effects have been used to recover the intrinsic Gamma values and X-ray luminosities, L-X. With a simple bolometric correction to L-X to calculate lambda(Edd), we find a statistically significant correlation between Gamma and lambda(Edd) (p = 0.007). A linear fit to the data yields Gamma = (0.41 +/- 0.18)log(10)lambda(Edd) + (2.38 +/- 0.20), which is statistically consistent with results for unobscured AGNs. This result implies that torus modeling successfully recovers the intrinsic AGN parameters. Since the megamasers have low-mass black holes (M-BH approximate to 10(6)-10(7) M-circle dot) and are highly inclined, our results extend the Gamma-lambda(Edd) relationship to lower masses and argue against strong orientation effects in the corona, in support of AGN unification. Finally this result supports the use of Gamma as a growth-rate indicator for accreting black holes, even for CT AGNs.
A new, faint population of X-ray transients.
(2017) Bauer, Franz Erik; Treister, Ezequiel; Schulze, Steve.; Schawinski, Kevin; Luo, B.; Alexander, D. M.; Brandt, W N.; Comastri, A.; Forster, Francisco; Gilli, Roberto
An X-ray fading, UV brightening QSO at z ≈ 6
(2022) Vito, F.; Mignoli, M.; Gilli, R.; Brandt, W. N.; Shemmer, O.; Bauer, F. E.; Bisogni, S.; Luo, B.; Marchesi, S.; Nanni, R.; Zamorani, G.; Comastri, A.; Cusano, F.; Gallerani, S.; Vignali, C.; Lanzuisi, G.
Explaining the existence of super massive black holes (SMBHs) with M-BH greater than or similar to 10(8) M-circle dot at z greater than or similar to 6 is a persistent challenge to modern astrophysics. Multiwavelength observations of z greater than or similar to 6 quasi-stellar objects (QSOs) reveal that, on average, their accretion physics is similar to that of their counterparts at lower redshift. However, QSOs showing properties that deviate from the general behavior can provide useful insights into the physical processes responsible for the rapid growth of SMBHs in the early universe. We present X-ray (XMM-Newton, 100 ks) follow-up observations of a z approximate to 6 QSO, J1641+3755, which was found to be remarkably X-ray bright in a 2018 Chandra dataset. J1641+3755 is not detected in the 2021 XMM-Newton observation, implying that its X-ray flux decreased by a factor greater than or similar to 7 on a notably short timescale (i.e., approximate to 115 rest-frame days), making it the z > 4 QSO with the largest variability amplitude. We also obtained rest-frame ultraviolet (UV) spectroscopic and photometric data with the Large Binocular Telescope (LBT). Surprisingly, comparing our LBT photometry with archival data, we found that J1641+3755 became consistently brighter in the rest-frame UV band from 2003 to 2016, while no strong variation occurred from 2016 to 2021. Its rest-frame UV spectrum is consistent with the average spectrum of high-redshift QSOs. Multiple narrow absorption features are present, and several of them can be associated with an intervening system at z = 5 :67. Several physical causes can explain the variability properties of J1641+3755, including intrinsic variations of the accretion rate, a small-scale obscuration event, gravitational lensing due to an intervening object, and an unrelated X-ray transient in a foreground galaxy in 2018. Accounting for all of the z > 6 QSOs with multiple X-ray observations separated by more that ten rest-frame days, we found an enhancement of strongly (i.e., by a factor >3) X-ray variable objects compared to QSOs at later cosmic times. This finding may be related to the physics of fast accretion in high-redshift QSOs.
Connecting X-ray nuclear winds with galaxy-scale ionised outflows in two z ∼ 1.5 lensed quasars
(2021) Tozzi, G.; Cresci, G.; Marasco, A.; Nardini, E.; Marconi, A.; Mannucci, F.; Chartas, G.; Rizzo, F.; Amiri, A.; Brusa, M.; Comastri, A.; Dadina, M.; Lanzuisi, G.; Mainieri, V.; Mingozzi, M.; Perna, M.; Venturi, G.; Vignali, C.
Aims. Outflows driven by active galactic nuclei (AGN) are expected to have a significant impact on host galaxy evolution, but the matter of how they are accelerated and propagated on galaxy-wide scales is still under debate. This work addresses these questions by studying the link between X-ray, nuclear ultra-fast outflows (UFOs), and extended ionised outflows, for the first time, in two quasars close to the peak of AGN activity (z similar to 2), where AGN feedback is expected to be more effective.Methods. Our selected targets, HS 0810+2554 and SDSS J1353+1138, are two multiple-lensed quasars at z similar to 1.5 with UFO detection that have been observed with the near-IR integral field spectrometer SINFONI at the VLT. We performed a kinematical analysis of the [O III]lambda 5007 optical emission line to trace the presence of ionised outflows.Results. We detected spatially resolved ionised outflows in both galaxies, extended more than 8 kpc and moving up to v>2000 km s(-1). We derived mass outflow rates of similar to 12 M-circle dot yr(-1) and similar to 2 M-circle dot yr(-1) for HS 0810+2554 and SDSS J1353+1138.Conclusions. Compared with the co-hosted UFO energetics, the ionised outflow energetics in HS 0810+2554 is broadly consistent with a momentum-driven regime of wind propagation, whereas in SDSS J1353+1138, it differs by about two orders of magnitude from theoretical predictions, requiring either a massive molecular outflow or a high variability of the AGN activity to account for such a discrepancy. By additionally considering our results together with those from the small sample of well-studied objects (all local but one) having both UFO and extended (ionised, atomic, or molecular) outflow detections, we found that in 10 out of 12 galaxies, the large-scale outflow energetics is consistent with the theoretical predictions of either a momentum- or an energy-driven scenario of wind propagation. This suggests that such models explain the acceleration mechanism of AGN-driven winds on large scales relatively well.
Determining the covering factor of compton-thick active galactic nuclei with NuSTAR
(2015) Brightman, M.; Balokovic, M.; Stern, D.; Arevalo, P.; Ballantyne, D. R.; Bauer, Franz Erik; Bogg, S. E.; Craig, W. W.; Christensen, F. E.; Comastri, A.; Gandhi, P.; Hailey, C. J.; Harrison, F. A.; Hickox, R. C.; Koss, M.; Lamassa, S.; Fuerst, F.; Puccetti, S.; Rivers, E.; Vasudevan, R.; Walton, D. J.; Zhang, W. W.
Discovery of a galaxy overdensity around a powerful, heavily obscured FRII radio galaxy at z=1.7: star formation promoted by large-scale AGN feedback?
(2019) Gilli, R.; Mignoli, M.; Peca, A.; Nanni, R.; Prandoni, I.; Liuzzo, E.; D'Amato, Q.; Brusa, M.; Calura, F.; Caminha, G. B.; Chiaberge, M.; Comastri, A.; Cucciati, O.; Cusano, F.; Grandi, P.; Decarli, R.; Lanzuisi, G.; Mannucci, F.; Pinna, E.; Tozzi, P.; Vanzella, E.; Vignali, C.; Vito, F.; Balmaverde, B.; Citro, A.; Cappelluti, N.; Zamorani, G.; Norman, C.
We report the discovery of a galaxy overdensity around a Compton-thick Fanaroff-Riley type II (FRII) radio galaxy at z = 1:7 in the deep multiband survey around the z = 6.3 quasi-stellar object (QSO) SDSS J1030 +0524. Based on a 6 h VLT/MUSE and on a 4 h LBT/LUCI observation, we identify at least eight galaxy members in this structure with spectroscopic redshift z = 1 .687 1 .699, including the FRII galaxy at z = 1.699. Most members are distributed within 400 kpc from the FRII core. Nonetheless, the whole structure is likely much more extended, as one of the members was serendipitously found at similar to 800 kpc projected separation. The classic radio structure of the FRII itself extends for similar to 600 kpc across the sky. Most of the identified overdensity members are blue, compact galaxies that are actively forming stars at rates of similar to 8-60 M-circle dot yr(-1). For the brightest of them, a half-light radius of 2 .2 similar to 0 .8 kpc at 8000A rest-frame was determined based on adaptive optics-assisted observations with LBT/SOUL in the Ks band. We do not observe any strong galaxy morphological segregation or concentration around the FRII core. This suggests that the structure is far from being virialized and likely constitutes the progenitor of a local massive galaxy group or cluster caught in its main assembly phase. Based on a 500 ks Chandra ACIS-I observation, we found that the FRII nucleus hosts a luminous QSO (L2-10 keV = 1 .3 similar to 10(44) erg s(-1), intrinsic and rest-frame) that is obscured by Compton-thick absorption (N-H = 1.5 +/- 0 .6 x 10(24) cm(-2)). Under standard bolometric corrections, the total measured radiative power (L-rad similar to 4 x 10(45) erg s(-1)) is similar to the jet kinetic power that we estimated from radio observations at 150MHz (P-kin = 6.3 x 10(45) erg s(-1)), in agreement with what is observed in powerful jetted AGN. Our Chandra observation is the deepest so far for a distant FRII within a galaxy overdensity. It revealed significant di ffuse X-ray emission within the region that is covered by the overdensity. In particular, X-ray emission extending for similar to 240 kpc is found around the eastern lobe of the FRII. Four out of the six MUSE star-forming galaxies in the overdensity are distributed in an arc-like shape at the edge of this di ffuse X-ray emission. These objects are concentrated within 200 kpc in the plane of the sky and within 450 kpc in radial separation. Three of them are even more concentrated and fall within 60 kpc in both transverse and radial distance. The probability of observing four out of the six z = 1.7 sources by chance at the edge of the di ffuse emission is negligible. In addition, these four galaxies have the highest specific star formation rates of the MUSE galaxies in the overdensity and lie above the main sequence of field galaxies of equal stellar mass at z = 1.7. We propose that the di ffuse X-rays originate from an expanding bubble of gas that is shock heated by the FRII jet, and that star formation is promoted by the compression of the cold interstellar medium of the galaxies around the bubble, which may be remarkable evidence of positive AGN feedback on cosmological scales.
Discovery of the first heavily obscured QSO candidate at z > 6 in a close galaxy pair
(2019) Vito, F.; Brandt, W. N.; Bauer, F. E.; Gilli, R.; Luo, B.; Zamorani, G.; Calura, F.; Comastri, A.; Mazzucchelli, C.; Mignoli, M.; Nanni, R.; Shemmer, O.; Vignali, C.; Brusa, M.; Cappelluti, N.; Civano, F.; Volonteri, M.
While theoretical arguments predict that most of the early growth of supermassive black holes (SMBHs) happened during heavily obscured phases of accretion, current methods used for selecting z > 6 quasars (QSOs) are strongly biased against obscured QSOs, thus considerably limiting our understanding of accreting SMBHs during the first gigayear of the Universe from an observational point of view. We report the Chandra discovery of the first heavily obscured QSO candidate in the early universe, hosted by a close (approximate to 5 kpc) galaxy pair at z = 6.515. One of the members is an optically classified type-1 QSO, PSO167-13. The companion galaxy was first detected as a [C II] emitter by Atacama large millimeter array (ALMA). An X-ray source is significantly (P = 0.9996) detected by Chandra in the 2-5 keV band, with < 1.14 net counts in the 0.5-2 keV band, although the current positional uncertainty does not allow a conclusive association with either PSO167-13 or its companion galaxy. From X-ray photometry and hardness-ratio arguments, we estimated an obscuring column density of N-H > 2 x 10(24) cm(-2) and N-H > 6 x 10(23) cm(-2) at 68% and 90% confidence levels, respectively. Thus, regardless of which of the two galaxies is associated with the X-ray emission, this source is the first heavily obscured QSO candidate at z > 6.
Finding rare AGN: X-ray number counts of Chandra sources in Stripe 82.
(2013) LaMassa, Stephanie M.; Treister, Ezequiel; Urry, C. Meg; Glikman, Eilat; Cappelluti, N.; Civano, Francesca; Comastri, A.; Böhringer, Hans; Cardamone, Carie; Chon, Gayoung; Kephart, Miranda
Finding rare AGN: XMM-Newton and Chandra observations of SDSS Stripe 82.
(2013) LaMassa, Stephanie M.; Treister, Ezequiel; Urry, C. Meg; Cappelluti, N.; Civano, Francesca; Ranalli, Piero; Glikman, Eilat; Richards, Gordon; Ballantyne, David; Stern, Daniel; Comastri, A.
Hard X-ray emission of the luminous infrared galaxy NGC 6240 as observed by NuSTAR
(2016) Puccetti, S.; Comastri, A.; Bauer, Franz Erik; Brandt, W.; Fiore, E.; Harrison, F.; Luo, B.; Stern, D.; Urry, C.; Alexander, D.; Annuar, A.; Arevalo, P.; Balokovic, M.; Boggs, S.; Brightman, M.; Craig, W.; Ricci, Claudio
Chandra and Magellan/FIRE follow-up observations of PSO167-13: An X-ray weak QSO at z=6.515
(2021) Vito, F.; Brandt, W. N.; Ricci, F.; Congiu, E.; Connor, T.; Banados, E.; Bauer, F. E.; Gilli, R.; Luo, B.; Mazzucchelli, C.; Mignoli, M.; Shemmer, O.; Vignali, C.; Calura, F.; Comastri, A.; Decarli, R.; Gallerani, S.; Nanni, R.; Brusa, M.; Cappelluti, N.; Civano, F.; Zamorani, G.
Context. The discovery of hundreds of quasi-stellar objects (QSOs) in the first gigayear of the Universe powered by already grown supermassive black holes (SMBHs) challenges our knowledge of SMBH formation. In particular, investigations of z>6 QSOs that present notable properties can provide unique information on the physics of fast SMBH growth in the early Universe.Aims. 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 recently been proposed to host the first heavily obscured X-ray source at high redshift. The goals of these new observations are to confirm the existence of the X-ray source and to investigate the rest-frame UV properties of the QSO.Methods. We observed the PSO167-13 system with Chandra/ACIS-S (177 ks) and obtained new spectroscopic observations (7.2 h) with Magellan/FIRE.Results. 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 (L2-10 keV<8.3x10(43) erg s(-1)) is the lowest available for a z>6 QSO. The ratio between the X-ray and UV luminosity of alpha (ox)<-1.95 makes PSO167-13 a strong outlier from the (ox)-L-UV and L-X-L-bol relations. In particular, its X-ray emission is more than six times weaker than the expectation based on its UV luminosity. The new Magellan/FIRE spectrum of PSO167-13 is strongly affected by unfavorable sky conditions, but the tentatively detected C IV and Mg II emission lines appear strongly blueshifted.Conclusions. 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.
Inferring Compton-thick AGN candidates at z > 2 with Chandra using the > 8 keV rest-frame spectral curvature.
(2017) Baronchelli, Linda; Ricci, Claudio; Treister, Ezequiel; Koss, M.; Schawinski, K.; Cardamone, C.; Civano, F.; Comastri, A.; Elvis, M.; Lanzuisi, G.; Marchesi, S.
New spectral model for constraining torus covering factors from broadband X-ray spectra of active galactic nuclei
(2018) Baloković, M.; Brightman, M.; Harrison, F.A.; Comastri, A.; Ricci, Claudio; Buchner, J.; Gandhi, P.; Farrah, D.; Stern, D.
NuSTAR J033202-2746.8: Direct Constraints on the Compton Reflection in a Heavily Obscured Quasar at z ≈ 2.
(2014) Del Moro, A.; Bauer, Franz Erik; Treister, Ezequiel; Mullaney, J. R.; Alexander, D. M.; Comastri, A.; Stern, D.; Civano, F.; Ranalli, P.; Vignali, C.; Aird, J. A.
NuSTAR observations of water megamaser AGN
(2016) Masini, A.; Comastri, A.; Baloković, M.; Zaw, I.; Puccetti, S.; Ballantyne, D. R.; Bauer, Franz Erik; Boggs, S. E.; Brandt, W. N.; Brightman, M.; Christensen, F. E.; Craig, W. W.; Gandhi, P.; Hailey, C. J.; Harrison, F. A.; Koss, M. J.; Madejski, G.; Ricci, Claudio; Rivers, E.; Stern, D.
NuSTAR reveals an intrinsically X-ray weak broad absorption line quasar in the ultraluminous infrared galaxy Markarian 231
(2014) Teng, Stacy H.; Brandt, W.N.; Harrison, F.A.; Luo, B.; Alexander, D.M.; Bauer, Franz Erik; Boggs, S.E.; Christensen, F.E.; Comastri, A.; Craig, W.W.
NuStar reveals extreme absorption in z < 0.5 type 2 quasars
(2015) Lansbury, G.; Gandhi, P.; Alexander, D.; Assef, R.; Aird, J.; Annuar, A.; Ballantyne, D.; Balokovic, M.; Bauer, Franz Erik; Boggs, S.; Brandt, W.; Brightman, M.; Christensen, F.; Civano, F.; Comastri, A.; Craig, W.
NuStar spectroscopy of multiI-component X-Ray reflection from NGC 1068
(2015) Bauer, Franz Erik; Arévalo, P.; Walton, D.; Koss, M.; Puccetti, S.; Gandhi, P.; Stern, D.; Alexander, D.; Balokovic, M.; Boggs, S.; Brandt, W.; Brightman, M.; Christensen, F.; Comastri, A.; Craig, W.; Del Moro, A.
NuSTAR. reveals the extreme properties of the super-Eddington accreting supermassive black hole in PG. 1247+267
(2016) Lanzuisi, G.; Perna, M.; Comastri, A.; Cappi, M.; Dadina, M.; Marinucci, A.; Masini, A.; Matt, G.; Vagnetti, F.; Bauer, Franz Erik; Vignali, C.; Ballantyne, D.; Boggs, S.; Brandt, W.; Brusa, M.; Craig, W.; Ricci, Claudio
REVEALING A POPULATION OF HEAVILY OBSCURED ACTIVE GALACTIC NUCLEI AT z ≈ 0.5-1 IN THE CHANDRA DEEP FIELD-SOUTH
(2011) Luo, B.; Brandt, W. N.; Xue, Y. Q.; Alexander, D. M.; Brusa, M.; Bauer, F. E.; Comastri, A.; Fabian, A. C.; Gilli, R.; Lehmer, B. D.; Rafferty, D. A.; Schneider, D. P.; Vignali, C.
Heavily obscured (NH greater than or similar to 3 x 10(23) cm(-2)) active galactic nuclei (AGNs) not detected even in the deepest X-ray surveys are often considered to be comparably numerous to the unobscured and moderately obscured AGNs. Such sources are required to fit the cosmic X-ray background (XRB) emission in the 10-30 keV band. We identify a numerically significant population of heavily obscured AGNs at z approximate to 0.5-1 in the Chandra Deep Field-South (CDF-S) and Extended Chandra Deep Field-South by selecting 242 X-ray undetected objects with infrared-based star-formation rates (SFRs) substantially higher (a factor of 3.2 or more) than their SFRs determined from the UV after correcting for dust extinction. An X-ray stacking analysis of 23 candidates in the central CDF-S region using the 4 Ms Chandra data reveals a hard X-ray signal with an effective power-law photon index of Gamma = 0.6(-0.4)(+ 0.3), indicating a significant contribution from obscured AGNs. Based on Monte Carlo simulations, we conclude that 74% +/- 25% of the selected galaxies host obscured AGNs, within which approximate to 95% are heavily obscured and approximate to 80% are Compton-thick (CT; N-H > 1.5 x 10(24) cm(-2)). The heavily obscured objects in our sample are of moderate intrinsic X-ray luminosity (approximate to(0.9-4) x 10(42) erg s(-1) in the 2-10 keV band). The space density of the CT AGNs is (1.6 +/- 0.5) x 10(-4) Mpc(-3). The z approximate to 0.5-1 CT objects studied here are expected to contribute approximate to 1% of the total XRB flux in the 10-30 keV band, and they account for approximate to 5%-15% of the emission in this energy band expected from all CT AGNs according to population-synthesis models. In the 6-8 keV band, the stacked signal of the 23 heavily obscured candidates accounts for <5% of the unresolved XRB flux, while the unresolved approximate to 25% of the XRB in this band can probably be explained by a stacking analysis of the X-ray undetected optical galaxies in the CDF-S (a 2.5 sigma stacked signal). We discuss prospects to identify such heavily obscured objects using future hard X-ray observatories.

Browsing by Author "Comastri, A."

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