Browsing by Author "Brandt, W. N."

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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.
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    A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger
    (2019) Xue, Yongquan; Zheng, X. C.; Li, Y.; Brandt, W. N.; Zhang, Bryna; Luo, Bruce; Zhang, Boce; Bauer, Franz Erik; Sun, H.; Lehmer, B. D.; Wu, Xuefeng; Yang, Guang; Kong, Xu; Li, J. Y.; Sun, M. Y.; Wang, Junxian; Vito, Fabio
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    A random forest-based selection of optically variable AGN in the VST-COSMOS field
    (2021) De Cicco, D.; Bauer, F. E.; Paolillo, M.; Cavuoti, S.; Sanchez-Saez, P.; Brandt, W. N.; Pignata, G.; Vaccari, M.; Radovich, M.
    Context. The survey of the COSMOS field by the VLT Survey Telescope is an appealing testing ground for variability studies of active galactic nuclei (AGN). With 54 r-band visits over 3.3 yr and a single-visit depth of 24.6 r-band mag, the dataset is also particularly interesting in the context of performance forecasting for the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST).Aims. This work is the fifth in a series dedicated to the development of an automated, robust, and efficient methodology to identify optically variable AGN, aimed at deploying it on future LSST data.Methods. We test the performance of a random forest (RF) algorithm in selecting optically variable AGN candidates, investigating how the use of different AGN labeled sets (LSs) and features sets affects this performance. We define a heterogeneous AGN LS and choose a set of variability features and optical and near-infrared colors based on what can be extracted from LSST data.Results. We find that an AGN LS that includes only Type I sources allows for the selection of a highly pure (91%) sample of AGN candidates, obtaining a completeness with respect to spectroscopically confirmed AGN of 69% (vs. 59% in our previous work). The addition of colors to variability features mildly improves the performance of the RF classifier, while colors alone prove less effective than variability in selecting AGN as they return contaminated samples of candidates and fail to identify most host-dominated AGN. We observe that a bright (r less than or similar to 21 mag) AGN LS is able to retrieve candidate samples not affected by the magnitude cut, which is of great importance as faint AGN LSs for LSST-related studies will be hard to find and likely imbalanced. We estimate a sky density of 6.2x10(6) AGN for the LSST main survey down to our current magnitude limit.
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    A structure function analysis of VST-COSMOS AGN
    (2022) De Cicco, D.; Bauer, F. E.; Paolillo, M.; Sanchez-Saez, P.; Brandt, W. N.; Vagnetti, F.; Pignata, G.; Radovich, M.; Vaccari, M.
    Context. We present our sixth work in a series dedicated to variability studies of active galactic nuclei (AGN), based on the survey of the COSMOS field by the VLT Survey Telescope (VST). Its 54 r-band visits over 3.3 yr and single-visit depth of 24.6 r-band mag make this dataset a valuable scaled-down version that can help forecast the performance of the Rubin Observatory Legacy Survey of Space and Time (LSST).
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    An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: radio properties and the far-infrared/radio correlation
    (2014) Thomson, A. P.; Ivison, R. J.; Simpson, J. M.; Swinbank, A. M.; Smail, Ian; Arumugam, V.; Alexander, D. M.; Beelen, A.; Brandt, W. N.; Chandra, I.; Dannerbauer, H.; Greve, T. R.; Hodge, J. A.; Ibar, E.; Karim, A.; Murphy, E. J.; Schinnerer, E.; Sirothia, S.; Walter, F.; Wardlow, J. L.; van der Werf, P.
    We present a study of the radio properties of 870 mu m-selected submillimetre galaxies (SMGs), observed at high resolution with Atacama Large Millimeter Array (ALMA) in the Extended Chandra Deep Field South. From our initial sample of 76 ALMA SMGs, we detect 52 SMGs at >3 sigma significance in Karl G. Jansky Very Large Array 1400 MHz imaging, of which 35 are also detected at >3 sigma in new 610 MHz Giant Metre-Wave Radio Telescope imaging. Within this sample of radio-detected SMGs, we measure a median radio spectral index a alpha(1400)(610) = -0.79 +/- 0.06, (with inter-quartile range alpha = [-1.16, -0.56]) and investigate the far-infrared/radio correlation via the parameter q(IR), the logarithmic ratio of the rest-frame 8-1000 mu m flux and monochromatic radio flux. Our median q(IR) = 2.56 +/- 0.05 (inter-quartile range q(IR) = [2.42, 2.78]) is higher than that typically seen in single-dish 870 mu m-selected sources (q(IR) similar to 2.4), which may reflect the fact that our ALMA-based study is not biased to radio-bright counterparts, as previous samples were. Finally, we search for evidence that q(IR) and alpha evolve with age in a codependent manner, as predicted by starburst models: the data populate the predicted region of parameter space, with the stellar mass tending to increase along tracks of q(IR) versus alpha in the direction expected, providing the first observational evidence in support of these models.
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    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.
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    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.
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    Broad Absorption Line Disappearance/Emergence in Multiple Ions in a Weak Emission-line Quasar
    (2019) Yi, W.; Vivek, M.; Brandt, W. N.; Wang, T.; Timlin, J.; Ak, N. Filiz; Schneider, D. P.; Fynbo, J. P. U.; Ni, Q.; Vito, F.; Indahl, B. L.; Sameer
    We report the discovery of the disappearance of Mg II, Al III, C IV, and Si IV broad absorption lines (BALs) at the same velocity (0.07c), accompanied by a new C IV BAL emerging at a higher velocity (up to 0.11c), 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 Sloan Digital Sky Survey and one follow-up spectrum from Hobby-Eberly Telescope/Low-Resolution Spectrograph-2. The simultaneous C IV BAL disappearance and emergence at different velocities, together with no variations in the Catalina Real-Time Transient Survey 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.
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    Compact white dwarf binaries in the combined SRG/eROSITA/SDSS eFEDS survey
    (2024) Schwope, A.; Kurpas, J.; Baecke, P.; Knauff, K.; Stuetz, L.; Tubin-Arenas, D.; Standke, A.; Anderson, S. F.; Bauer, F.; Brandt, W. N.; Covey, K.; Demasi, S.; Dwelly, T.; Freund, S.; Friedrich, S.; Gaensicke, B. T.; Maitra, C.; Merloni, A.; Munoz-Giraldo, D.; Rodriguez, A.; Salvato, M.; Stassun, K.; Stelzer, B.; Strong, A.; Morrison, S.
    Context. X-ray surveys combined with optical follow-up observations are used to generate complete flux-limited samples of the main X-ray emitting source classes. eROSITA on the Spectrum-Roentgen-Gamma mission provides sufficient sensitivity to build significantly enhanced samples of rare X-ray emitting sources. Aims. We strive to identify and classify compact white dwarf binaries, cataclysmic variables (CVs), and related objects, which were detected in the sky area of eFEDS, the eROSITA Final Equatorial Depths Survey, and they were observed in the plate program of SDSS-V. Methods. Compact white dwarf binaries were selected from spectra obtained in the early SDSS-V plate program. A dedicated set of SDSS plate observations were carried out in the eFEDS field, providing spectroscopic classifications for a significant fraction of the optically bright end (r < 22.5) of the X-ray sample. The identification and subclassification rests on visual inspections of the SDSS spectra, spectral variability, color-magnitude and color-color diagrams involving optical and X-ray fluxes, optical variability, and literature work. Results. Upon visual inspection of SDSS spectra and various auxiliary data products, we have identified 26 accreting compact white dwarf binaries (aCWDBs) in eFEDS, of which 24 are proven X-ray emitters. Among those 26 objects, there are 12 dwarf novae, three WZ Sge-like disk-accreting nonmagnetic CVs with low accretion rates, five likely nonmagnetic high accretion rate nova-like CVs, two magnetic CVs of the polar subcategory, and three double degenerates (AM CVn objects). Period bouncing candidates and magnetic systems are rarer than expected in this sample, but it is too small for a thorough statistical analysis. Fourteen of the systems are new discoveries, of which five are fainter than the Gaia magnitude limit. Thirteen aCWDBs have measured or estimated orbital periods, of which five were presented here. Through a Zeeman analysis, we revise the magnetic field estimate of the polar system J0926+0105, which is likely a low-field polar at B = 16 MG. We quantified the success of X-ray versus optical/UV selection of compact white dwarf binaries which will be relevant for the full SDSS-V survey. We also identified six white dwarf main sequence (WDMS) systems, among them there is one confirmed pre-CV at an orbital period of 17.6 h and another pre-CV candidate. Conclusions. This work presents successful initial work in building large samples of all kinds of accreting and X-ray emitting compact white dwarf binaries that will be continued over the full hemisphere in the years to come.
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    Constraining the shielded wind scenario in PG2112+059
    (2016) Sáez, C.; Brandt, W. N.; Bauer, Franz Erik
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    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.
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    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.
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    Enhanced star formation rates in AGN hosts with respect to inactive galaxies from PEP-Herschel observations
    (2012) Santini, P.; Rosario, D. J.; Shao, L.; Lutz, D.; Maiolino, R.; Alexander, D. M.; Altieri, B.; Andreani, P.; Aussel, H.; Bauer, F. E.; Berta, S.; Bongiovanni, A.; Brandt, W. N.; Brusa, M.; Cepa, J.; Cimatti, A.; Daddi, E.; Elbaz, D.; Fontana, A.; Schreiber, N. M. Foerster; Genzel, R.; Grazian, A.; Le Floc'h, E.; Magnelli, B.; Mainieri, V.; Nordon, R.; Garcia, A. M. Perez; Poglitsch, A.; Popesso, P.; Pozzi, F.; Riguccini, L.; Rodighiero, G.; Salvato, M.; Sanchez-Portal, M.; Sturm, E.; Tacconi, L. J.; Valtchanov, I.; Wuyts, S.
    We compare the average star formation (SF) activity in X-ray selected AGN hosts with a mass-matched control sample of inactive galaxies, including both star forming and quiescent sources, in the 0.5 < z < 2.5 redshift range. Recent observations carried out by PACS, the 60-210 mu m photometric camera on board the Herschel Space Observatory, in GOODS-S, GOODS-N and COSMOS allow us to obtain an unbiased estimate of the far-IR luminosity, and hence of the SF properties, of the two samples. Accurate AGN host stellar mass estimates are obtained by decomposing their total emission into the stellar and the nuclear components. We report evidence of a higher average SF activity in AGN hosts with respect to the control sample of inactive galaxies. The level of SF enhancement is modest (similar to 0.26 dex at similar to 3 sigma confidence level) at low X-ray luminosities (L-X less than or similar to 10(43.5) erg s(-1)) and more pronounced (0.56 dex at >10 sigma confidence level) in the hosts of luminous AGNs. However, when comparing to star forming galaxies only, AGN hosts are found broadly consistent with the locus of their "main sequence". We investigate the relative far-IR luminosity distributions of active and inactive galaxies, and find a higher fraction of PACS detected, hence normal and highly star forming systems among AGN hosts. Although different interpretations are possible, we explain our findings as a consequence of a twofold AGN growth path: faint AGNs evolve through secular processes, with instantaneous AGN accretion not tightly linked to the current total SF in the host galaxy, while the luminous AGNs co-evolve with their hosts through periods of enhanced AGN activity and star formation, possibly through major mergers. While an increased SF activity with respect to inactive galaxies of similar mass is expected in the latter, we interpret the modest SF offsets measured in low-L-X AGN hosts as either a) generated by non-synchronous accretion and SF histories in a merger scenario or b) due to possible connections between instantaneous SF and accretion that can be induced by smaller scale (non-major merger) mechanisms. Far-IR luminosity distributions favour the latter scenario.
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    Evident black hole-bulge coevolution in the distant universe
    (2019) Yang, G.; Brandt, W. N.; Alexander, D. M.; Chen, C. -T. J.; Ni, Q.; Vito, F.; Zhu, F. -F.
    Observations in the local universe show a tight correlation between the masses of supermassive black holes (SMBHs; M-BH) and host-galaxy bulges (M-bulge), suggesting a strong connection between SMBH and bulge growth. However, direct evidence for such a connection in the distant universe remains elusive. We have studied sample-averaged SMBH accretion rate ((BHAR) over bar) for bulge-dominated galaxies at z = 0.5-3. While previous observations found (BHAR) over bar is strongly related to host-galaxy stellar mass (M-star) for the overall galaxy population, our analyses show that, for the bulge-dominated population, (BHAR) over bar is mainly related to SFR rather than M-star. This (BHAR) over bar -SFR relation is highly significant, e.g. 9.0 sigma (Pearson statistic) at z = 0.5-1.5. Such a (BHAR) over bar -SFR connection does not exist among our comparison sample of galaxies that are not bulge dominated, for which M-star appears to be the main determinant of SMBH accretion. This difference between the bulge-dominated and comparison samples indicates that SMBHs only coevolve with bulges rather than the entire galaxies, explaining the tightness of the local M-BH - M-bulge correlation. Our best-fitting (BHAR) over bar -SFR relation for the bulge-dominated sample is log (BHAR) over bar = log SFR - (2.48 +/- 0.05) (solar units). The best-fitting (BHAR) over bar /SFR ratio (10(-2.48)) for bulge-dominated galaxies is similar to the observed M-BH/M-bulge values in the local universe. Our results reveal that SMBH and bulge growth are in lockstep, and thus non-causal scenarios of merger averaging are unlikely the origin of the M-BH-M-bulge correlation. This lockstep growth also predicts that the M-BH-M-bulge relation should not have strong redshift dependence.
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    Exploring Changing-look Active Galactic Nuclei with the Sloan Digital Sky Survey V: First Year Results
    (2024) Zeltyn, Grisha; Trakhtenbrot, Benny; Eracleous, Michael; Yang, Qian; Green, Paul; Anderson, Scott F.; LaMassa, Stephanie; Runnoe, Jessie; Assef, Roberto J.; Bauer, Franz E.; Brandt, W. N.; Davis, Megan C.; Frederick, Sara E.; Fries, Logan B.; Graham, Matthew J.; Grogin, Norman A.; Guolo, Muryel; Hernandez-Garcia, Lorena; Koekemoer, Anton M.; Krumpe, Mirko; Liu, Xin; Martinez-Aldama, Mary Loli; Ricci, Claudio; Schneider, Donald P.; Shen, Yue; Sniegowska, Marzena; Temple, Matthew J.; Trump, Jonathan R.; Xue, Yongquan; Brownstein, Joel R.; Dwelly, Tom; Morrison, Sean; Bizyaev, Dmitry; Pan, Kaike; Kollmeier, Juna A.
    "Changing-look" active galactic nuclei (CL-AGNs) challenge our basic ideas about the physics of accretion flows and circumnuclear gas around supermassive black holes. Using first-year Sloan Digital Sky Survey V (SDSS-V) repeated spectroscopy of nearly 29,000 previously known active galactic nuclei (AGNs), combined with dedicated follow-up spectroscopy, and publicly available optical light curves, we have identified 116 CL-AGNs where (at least) one broad emission line has essentially (dis-)appeared, as well as 88 other extremely variable systems. Our CL-AGN sample, with 107 newly identified cases, is the largest reported to date, and includes similar to 0.4% of the AGNs reobserved in first-year SDSS-V operations. Among our CL-AGNs, 67% exhibit dimming while 33% exhibit brightening. Our sample probes extreme AGN spectral variability on months to decades timescales, including some cases of recurring transitions on surprisingly short timescales (less than or similar to 2 months in the rest frame). We find that CL events are preferentially found in lower-Eddington-ratio (f Edd) systems: Our CL-AGNs have a f Edd distribution that significantly differs from that of a carefully constructed, redshift- and luminosity-matched control sample (Anderson-Darling test yielding p AD approximate to 6 x 10-5; median f Edd approximate to 0.025 versus 0.043). This preference for low f Edd strengthens previous findings of higher CL-AGN incidence at lower f Edd, found in smaller samples. Finally, we show that the broad Mg ii emission line in our CL-AGN sample tends to vary significantly less than the broad H beta emission line. Our large CL-AGN sample demonstrates the advantages and challenges in using multi-epoch spectroscopy from large surveys to study extreme AGN variability and physics.
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    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.
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    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.
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    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.
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    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
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    High-quality Extragalactic Legacy-field Monitoring (HELM) with DECam: Project Overview and First Data Release
    (2024) Zhuang, Ming-Yang; Yang, Qian; Shen, Yue; Adamow, Monika; Friedel, Douglas N.; Gruendl, R. A.; Stone, Zachary; Li, Junyao; Liu, Xin; Martini, Paul; Abbott, Timothy M. C.; Anderson, Scott F.; Assef, Roberto J.; Bauer, Franz E.; Bielby, Richard; Brandt, W. N.; Burke, Colin J.; Casares, Jorge; Chen, Yu-Ching; De Rosa, Gisella; Drlica-Wagner, Alex; Dwelly, Tom; Eltvedt, Alice; Alvarez, Gloria Fonseca; Fu, Jianyang; Fuentes, Cesar; Graham, Melissa L.; Grier, Catherine J.; Golovich, Nathan; Hall, Patrick B.; Hartigan, Patrick; Horne, Keith; Koekemoer, Anton M.; Krumpe, Mirko; Li, Jennifer I.; Lidman, Chris; Malik, Umang; Mangian, Amelia; Merloni, Andrea; Ricci, Claudio; Salvato, Mara; Sharp, Rob; Trilling, David E.; Tucker, Brad E.; Wen, Di; Wideman, Zachary; Xue, Yongquan; Yu, Zhefu; Zucker, Catherine
    High-quality Extragalactic Legacy-field Monitoring (HELM) is a long-term observing program that photometrically monitors several well-studied extragalactic legacy fields with the Dark Energy Camera (DECam) imager on the CTIO 4 m Blanco telescope. Since 2019 February, HELM has been monitoring regions within COSMOS, XMM-LSS, CDF-S, S-CVZ, ELAIS-S1, and SDSS Stripe 82 with few-day cadences in the (u)gri(z) bands, over a collective sky area of similar to 38 deg2. The main science goal of HELM is to provide high-quality optical light curves for a large sample of active galactic nuclei (AGNs), and to build decades-long time baselines when combining past and future optical light curves in these legacy fields. These optical images and light curves will facilitate the measurements of AGN reverberation mapping lags, as well as studies of AGN variability and its dependencies on accretion properties. In addition, the time-resolved and coadded DECam photometry will enable a broad range of science applications from galaxy evolution to time-domain science. We describe the design and implementation of the program and present the first data release that includes source catalogs and the first similar to 3.5 yr of light curves during 2019A-2022A.