DSpace Angular :: Browsing by Author "Luo, B."

Browsing by Author "Luo, B."

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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 Extreme X-Ray Variability Event of a Weak-line Quasar
(2020) Ni, Q.; Brandt, W. N.; Yi, W.; Luo, B.; Timlin, J. D., III; Hall, P. B.; Liu, Hezhen; Plotkin, R. M.; Shemmer, O.; Vito, F.; Wu, Jianfeng
We report the discovery of an extreme X-ray flux rise (by a factor of greater than or similar to 20) of the weak-line quasar Sloan Digital Sky Survey (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 active galactic nuclei (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 ultraviolet (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.
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.
Black Hole Growth Is Mainly Linked to Host-galaxy Stellar Mass Rather Than Star Formation Rate
(2017) Wang, J.; Yang, G.; Chen, C.; Vito, F.; Brandt, W.; Alexander, D.; Luo, B.; Sun, M.; Xue, Y.; Bauer, Franz Erik; Koekemoer, A.; Lehmer, B.; Liu, T.; Schneider, D.; Shemmer, O.; Trump J.; Vignali, C.
Deep-Survey Constraints on X-Ray Outbursts from Galactic Nuclei
(2008) Luo, B.; Bauer, Franz Erik
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.
Discovery of the Most-Distant Double-Peaked Emitter at z=1.369
(2009) Luo, B.; Bauer, Franz Erik
Does black hole growth depend fundamentally on host-galaxy compactness?
(2019) Ni, Q.; Yang, G.; Brandt, W. N.; Alexander, D. M.; Chen, C-T J.; Luo, B.; Vito, F.; Xue, Y. Q.
Possible connections between central black hole (BH) growth and host-galaxy compactness have been found observationally, which may provide insight into BH-galaxy coevolution: compact galaxies might have large amounts of gas in their centres due to their high mass-to-size ratios, and simulations predict that high central gas density can boost BH accretion. However, it is not yet clear if BH growth is fundamentally related to the compactness of the host galaxy, due to observational degeneracies between compactness, stellar mass (M-star) and star formation rate (SFR). To break these degeneracies, we carry out systematic partial-correlation studies to investigate the dependence of sample-averaged BH accretion rate ((BHAR) over bar) on the compactness of host galaxies, represented by the surface-mass density, Sigma(e), or the projected central surface-mass density within 1 kpc, Sigma(1). We utilize 8842 galaxies with H < 24.5 in the five CANDELS fields at z = 0.5-3. We find that <(BHAR)over bar> does not significantly depend on compactness when controlling for SFR or M-star among bulge-dominated galaxies and galaxies that are not dominated by bulges, respectively. However, when testing is confined to star-forming galaxies at z = 0.5-1.5, we find that the (BHAR) over bar-Sigma(1) relation is not simply a secondary manifestation of a primary (BHAR) over bar -M-star relation, which may indicate a link between BH growth and the gas density within the central 1 kpc of galaxies.
Extragalactic fast X-ray transient candidates discovered by Chandra (2000-2014)
(2022) Quirola-Vasquez, J.; Bauer, F. E.; Jonker, P. G.; Brandt, W. N.; Yang, G.; Levan, A. J.; Xue, Y. Q.; Eappachen, D.; Zheng, X. C.; Luo, B.
Context. Extragalactic fast X-ray transients (FXRTs) are short flashes of X-ray photons of unknown origin that last a few seconds to hours.
Extragalactic fast X-ray transient candidates discovered by Chandra (2014-2022)
(2023) Quirola-Vasquez, J.; Bauer, F. E.; Jonker, P. G.; Brandt, W. N.; Yang, G.; Levan, A. J.; Xue, Y. Q.; Eappachen, D.; Camacho, E.; Ravasio, M. E.; Zheng, X. C.; Luo, B.
Context. Extragalactic fast X-ray transients (FXTs) are short flashes of X-ray photons of unknown origin that last a few minutes to hours.
GOODS-Herschel: the far-infrared view of star formation in active galactic nucleus host galaxies since z ∼ 3
(2012) Mullaney, J. R.; Pannella, M.; Daddi, E.; Alexander, D. M.; Elbaz, D.; Hickox, R. C.; Bournaud, F.; Altieri, B.; Aussel, H.; Coia, D.; Dannerbauer, H.; Dasyra, K.; Dickinson, M.; Hwang, H. S.; Kartaltepe, J.; Leiton, R.; Magdis, G.; Magnelli, B.; Popesso, P.; Valtchanov, I.; Bauer, F. E.; Brandt, W. N.; Del Moro, A.; Hanish, D. J.; Ivison, R. J.; Juneau, S.; Luo, B.; Lutz, D.; Sargent, M. T.; Scott, D.; Xue, Y. Q.
We present a study of the infrared properties of X-ray selected, moderate-luminosity (i.e. L-X = 10(42)-10(44) erg s(-1)) active galactic nuclei (AGNs) up to z approximate to 3, in order to explore the links between star formation in galaxies and accretion on to their central black holes. We use 100 and 160 mu m fluxes from GOODS-Herschel - the deepest survey yet undertaken by the Herschel telescope - and show that in the vast majority of cases (i.e. > 94 per cent) these fluxes are dominated by emission from the host galaxy. As such, these far-infrared bands provide an uncontaminated view of star formation in the AGN host galaxies. We find no evidence of any correlation between the X-ray and infrared luminosities of moderate AGNs at any redshift, suggesting that global star formation is decoupled from nuclear (i.e. AGN) activity in these galaxies. On the other hand, we confirm that the star formation rates of AGN hosts increase strongly with redshift, by a factor of 43(-18)(+27) from z < 0.1 to z = 2-3 for AGNs with the same range of X-ray luminosities. This increase is entirely consistent with the factor of 25-50 increase in the specific star formation rates (SSFRs) of normal, star-forming (i.e. main-sequence) galaxies over the same redshift range. Indeed, the average SSFRs of AGN hosts are only marginally (i.e. approximate to 20 per cent) lower than those of main-sequence galaxies at all surveyed redshifts, with this small deficit being due to a fraction of AGNs residing in quiescent (i.e. low SSFR) galaxies. We estimate that 79 +/- 10 per cent of moderate-luminosity AGNs are hosted in main-sequence galaxies, 15 +/- 7 per cent in quiescent galaxies and < 10 per cent in strongly starbursting galaxies. We derive the fractions of all main-sequence galaxies at z < 2 that are experiencing a period of moderate nuclear activity, noting that it is strongly dependent on galaxy stellar mass (M-stars), rising from just a few per cent at M-stars similar to 10(10) M-circle dot to greater than or similar to 20 per cent at M-stars >= 10(11) M-circle dot. Our results indicate that it is galaxy stellar mass that is most important in dictating whether a galaxy hosts a moderate-luminosity AGN. We argue that the majority of moderate nuclear activity is fuelled by internal mechanisms rather than violent mergers, which suggests that high-redshift disc instabilities could be an important AGN feeding mechanism.
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
Hard X-Ray-selected AGNs in Low-mass Galaxies from the NuSTAR Serendipitous Survey
(2017) Chen, C. T. J.; Brandt, W. N.; Reines, A. E.; Lansbury, G.; Stern, D.; Alexander, D. M.; Bauer, Franz Erik; Ricci, Claudio; Del Moro, A.; Gandhi, P.; Harrison, F. A.; Hickox, R. C.; Koss, M. J.; Lanz, L.; Koss, M. J.; Lanz, L.; Luo, B.; 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.
IDENTIFICATIONS AND PHOTOMETRIC REDSHIFTS OF THE 2 Ms CHANDRA DEEP FIELD-SOUTH SOURCES
(2010) Luo, B.; Bauer, Franz Erik
Long-term x-ray variability of typical active galactic nuclei in the distant universe.
(2016) Yang, G.; Kim, Sam; Brandt, W N.; Bauer, Franz Erik; Schulze, Steve.; Luo, B.; Xue, Y. Q.; Sun, M. Y.; Zheng, X. C.; Paolillo, M.; Shemmer, O.
Mid-infrared luminous quasars in the GOODS–Herschel fields: a large population of heavily obscured, Compton-thick quasars at z ≈ 2
(2016) Del Moro, A.; Alexander, D. M.; Bauer, Franz Erik; Daddi, E.; Kocevski, D. D.; McIntosh, D. H.; Stanley, F.; Brandt, W. N.; Elbaz, D.; Harrison, C. M.; Luo, B.; Mullaney, J. R.; Xue, Y. Q.
NuSTAR Observations of Intrinsically X-Ray Weak Quasar Candidates: An Obscuration-only Scenario
(2022) Wang, Chaojun; Luo, B.; Brandt, W. N.; Alexander, D. M.; Bauer, F. E.; Gallagher, S. C.; Huang, Jian; Liu, Hezhen; Stern, D.
We utilize recent NuSTAR observations (co-added depth approximate to 55-120 ks) of PG 1001+054, PG 1254+047, and PHL 1811 to constrain their hard X-ray (greater than or similar to 5 keV) weakness and spectral shapes and thus to investigate the nature of their extreme X-ray weakness. These quasars showed very weak soft X-ray emission, and they were proposed to be intrinsically X-ray weak, with the X-ray coronae producing weak continuum emission relative to their optical/UV emission. However, the new observations suggest an alternative explanation. The NuSTAR 3-24 keV spectral shapes for PG 1001+054 and PHL 1811 are likely flat (effective power-law photon indices Gamma(eff )= 1.0(-0.6)(+0.5) and Gamma(eff )= 1.4(-0.7)(+0.8) respectively), while the shape is nominal for PG 1254+047 ( Gamma(eff) = 1.8 +/- 0.3). PG 1001+054 and PHL 1811 are significantly weak at hard X-ray energies (by factors of approximate to 26-74 at rest-frame 8 keV) compared to the expectations from their optical/UV emission, while PG 1254+047 is only hard X-ray weak by a factor of approximate to 3. We suggest that X-ray obscuration is present in all three quasars. We propose that, as an alternative to the intrinsic X-ray weakness + X-ray obscuration scenario, the soft and hard X-ray weakness of these quasars can be uniformly explained under an obscuration-only scenario. This model provides adequate descriptions of the multiepoch soft and hard X-ray data of these quasars, with variable column density and leaked fraction of the partial covering absorber. We suggest that the absorber is the clumpy dust-free wind launched from the accretion disk. These quasars probably have super-Eddington accretion rates that drive powerful and high-density winds.
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.
PHOTOMETRIC REDSHIFTS IN THE HAWAII-HUBBLE DEEP FIELD-NORTH (H-HDF-N)
(2014) Bauer, Franz Erik; Yang, G.; Xue, Y.; Luo, B.; Brandt, W.; Alexander, D.; Cui, W.; Kong, X.

Browsing by Author "Luo, B."

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