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W. N. Brandt

Publications and source records attributed to W. N. Brandt.

At least 145 records · Page 8Linked to original sources

Modeling Quasar UV/Optical Variability with the Corona-heated Accretion-disk Reprocessing (CHAR) Model

The rest-frame UV/optical variability of the quasars in the Sloan Digital Sky Survey (SDSS) Stripe 82 is used to test the Corona-Heated Accretion-disk Reprocessing (CHAR) model of Sun et al. 2020. We adopt our CHAR model and the observed black-hole masses ($M_{\mathrm{BH}}$) and luminosities ($L$) to generate mock light curves that share the same measurement noise and sampling as the real observations. Without any fine-tuning, our CHAR model can satisfactorily reproduce the observed ensemble structure functions for different $M_{\mathrm{BH}}$, $L$, and rest-frame wavelengths. Our analyses reveal that a luminosity-dependent bolometric correction is disfavored over the constant bolometric correction for UV/optical luminosities. Our work demonstrates the possibility of extracting quasar properties (e.g., the bolometric correction or the dimensionless viscosity parameter) by comparing the physical CHAR model with quasar light curves.

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On the Fraction of X-ray Weak Quasars from the Sloan Digital Sky Survey

We investigate systematically the X-ray emission from type 1 quasars using a sample of 1825 Sloan Digital Sky Survey (SDSS) non-broad absorption line (non-BAL) quasars with Chandra archival observations. A significant correlation is found between the X-ray-to-optical power-law slope parameter ($α_{\rm OX}$) and the 2500 $Å$ monochromatic luminosity ($L_{\rm 2500~Å}$), and the X-ray weakness of a quasar is assessed via the deviation of its $α_{\rm OX}$ value from that expected from this relation. We demonstrate the existence of a population of non-BAL X-ray weak quasars, and the fractions of quasars that are X-ray weak by factors of $\ge6$ and $\ge10$ are $5.8\pm0.7\%$ and $2.7\pm0.5\%$, respectively. We classify the X-ray weak quasars (X-ray weak by factors of $\ge6$) into three categories based on their optical spectral features: weak emission-line quasars (WLQs; CIV REW $<16~Å$), red quasars ($Δ(g-i)>0.2$), and unclassified X-ray weak quasars. The X-ray weak fraction of $35_{- 9}^{+12}\%$ within the WLQ population is significantly higher than that within non-WLQs, confirming previous findings that WLQs represent one population of X-ray weak quasars. The X-ray weak fraction of $13_{- 3}^{+ 5}\%$ within the red quasar population is also considerably higher than that within the normal quasar population. The unclassified X-ray weak quasars do not have unusual optical spectral features, and their X-ray weakness may be mainly related to quasar X-ray variability.

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Intensive disc-reverberation mapping of Fairall 9: 1st year of Swift & LCO monitoring

We present results of time-series analysis of the first year of the Fairall 9 intensive disc-reverberation campaign. We used Swift and the Las Cumbres Observatory global telescope network to continuously monitor Fairall 9 from X-rays to near-infrared at a daily to sub-daily cadence. The cross-correlation function between bands provides evidence for a lag spectrum consistent with the $τ\proptoλ^{4/3}$ scaling expected for an optically thick, geometrically thin blackbody accretion disc. Decomposing the flux into constant and variable components, the variable component's spectral energy distribution is slightly steeper than the standard accretion disc prediction. We find evidence at the Balmer edge in both the lag and flux spectra for an additional bound-free continuum contribution that may arise from reprocessing in the broad-line region. The inferred driving light curve suggests two distinct components, a rapidly variable ($<4$ days) component arising from X-ray reprocessing, and a more slowly varying ($>100$ days) component with an opposite lag to the reverberation signal.

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The Sloan Digital Sky Survey Reverberation Mapping Project: MgII Lag Results from Four Years of Monitoring

We present reverberation mapping results for the MgII 2800 A broad emission line in a sample of 193 quasars at 0.35<z<1.7 with photometric and spectroscopic monitoring observations from the Sloan Digital Sky Survey Reverberation Mapping project during 2014 - 2017. We find significant time lags between the MgII and continuum lightcurves for 57 quasars and define a "gold sample" of 24 quasars with the most reliable lag measurements. We estimate false-positive rates for each lag that range from 1-24%, with an average false-positive rate of 11% for the full sample and 8% for the gold sample. There are an additional ~40 quasars with marginal MgII lag detections which may yield reliable lags after additional years of monitoring. The MgII lags follow a radius -- luminosity relation with a best-fit slope that is consistent with alpha=0.5 but with an intrinsic scatter of 0.36dex that is significantly larger than found for the Hb radius -- luminosity relation. For targets with SDSS-RM lag measurements of other emission lines, we find that our MgII lags are similar to the Hb lags and ~2-3 times larger than the CIV lags. This work significantly increases the number of MgII broad-line lags and provides additional reverberation-mapped black hole masses, filling the redshift gap at the peak of supermassive black hole growth between the Hb and CIV emission lines in optical spectroscopy.

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The Sloan Digital Sky Survey Quasar Catalog: Sixteenth Data Release

We present the final Sloan Digital Sky Survey IV (SDSS-IV) quasar catalog from Data Release 16 of the extended Baryon Oscillation Spectroscopic Survey (eBOSS). This catalog comprises the largest selection of spectroscopically confirmed quasars to date. The full catalog includes two sub-catalogs: a "superset" of all SDSS-IV/eBOSS objects targeted as quasars containing 1,440,615 observations and a quasar-only catalog containing 750,414 quasars, including 225,082 new quasars appearing in an SDSS data release for the first time, as well as known quasars from SDSS-I/II/III. We present automated identification and redshift information for these quasars alongside data from visual inspections for 320,161 spectra. The quasar-only catalog is estimated to be 99.8% complete with 0.3% to 1.3% contamination. Automated and visual inspection redshifts are supplemented by redshifts derived via principal component analysis and emission lines. We include emission line redshifts for H$α$, H$β$, Mg II, C III], C IV, and Ly$α$. Identification and key characteristics generated by automated algorithms are presented for 99,856 Broad Absorption Line quasars and 35,686 Damped Lyman Alpha quasars. In addition to SDSS photometric data, we also present multi-wavelength data for quasars from GALEX, UKIDSS, WISE, FIRST, ROSAT/2RXS, XMM-Newton, and Gaia. Calibrated digital optical spectra for these quasars can be obtained from the SDSS Science Archive Server.

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The Sloan Digital Sky Survey Reverberation Mapping Project: Photometric g and i Light Curves

The Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) program monitors 849 active galactic nuclei (AGN) both spectroscopically and photometrically. The photometric observations used in this work span over four years and provide an excellent baseline for variability studies of these objects. We present the photometric light curves from 2014 to 2017 obtained by the Steward Observatory's Bok telescope and the CFHT telescope with MegaCam. We provide details on the data acquisition and processing of the data from each telescope, the difference imaging photometry used to produce the light curves, and the calculation of a variability index to quantify each AGN's variability. We find that the Welch-Stetson J-index provides a useful characterization of AGN variability and can be used to select AGNs for further study.

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$NuSTAR$ Observations of Four Nearby X-ray Faint AGN: Low Luminosity or Heavy Obscuration?

We present $NuSTAR$ observations of four active galactic nuclei (AGN) located within 15 Mpc. These AGN, namely ESO 121-G6, NGC 660, NGC 3486 and NGC 5195, have observed X-ray luminosities of $L_{\rm 2-10\ keV, obs} \lesssim$ 10$^{39}$ erg s$^{-1}$, classifying them as low luminosity AGN (LLAGN). We perform broadband X-ray spectral analysis for the AGN by combining our $NuSTAR$ data with $Chandra$ or $XMM-Newton$ observations to directly measure their column densities ($N_{\rm H}$) and infer their intrinsic power. We complement our X-ray data with archival and new high angular resolution mid-infrared (mid-IR) data for all objects, except NGC 5195. Based on our X-ray spectral analysis, we found that both ESO 121-G6 and NGC 660 are heavily obscured ($N_{\rm H}$ > 10$^{23}$ cm$^{-2}$; $L_{\rm 2-10\ keV,\ int} \sim$ 10$^{41}$ erg s$^{-1}$), and NGC 660 may be Compton-thick. We also note that the X-ray flux and spectral slope for ESO 121-G6 have significantly changed over the last decade, indicating significant changes in the obscuration and potentially accretion rate. On the other hand, NGC 3486 and NGC 5195 appear to be unobscured and just mildly obscured, respectively, with $L_{\rm 2-10\ keV,\ int} <$ 10$^{39}$ erg s$^{-1}$; i.e., genuine LLAGN. Both of the heavily obscured AGN have $L_{\rm bol} >$ 10$^{41}$ erg s$^{-1}$ and $λ_{\rm Edd} \gtrsim$ 10$^{-3}$, and are detected in high angular resolution mid-IR imaging, indicating the presence of obscuring dust on nuclear scale. NGC 3486 however, is undetected in high-resolution mid-IR imaging, and the current data do not provide stringent constraints on the presence or absence of obscuring nuclear dust in the AGN.

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The $L_\mathrm{x}$-$L_\mathrm{uv}$-$L_\mathrm{radio}$ relation and corona-disk-jet connection in optically selected radio-loud quasars

Radio-loud quasars (RLQs) are more X-ray luminous than predicted by the X-ray-optical/UV relation (i.e. $L_\mathrm{x}\propto L_\mathrm{uv}^γ$) for radio-quiet quasars (RQQs). The excess X-ray emission depends on the radio-loudness parameter ($R$) and radio spectral slope ($α_\mathrm{r}$). We construct a uniform sample of 729 optically selected RLQs with high fractions of X-ray detections and $α_\mathrm{r}$ measurements.We find that steep-spectrum radio quasars (SSRQs; $α_\mathrm{r}\le-0.5$) follow a quantitatively similar $L_\mathrm{x}\propto L_\mathrm{uv}^γ$ relation as that for RQQs, suggesting a common coronal origin for the X-ray emission of both SSRQs and RQQs. However, the corresponding intercept of SSRQs is larger than that for RQQs and increases with $R$, suggesting a connection between the radio jets and the configuration of the accretion flow. Flat-spectrum radio quasars (FSRQs; $α_\mathrm{r}>-0.5$) are generally more X-ray luminous than SSRQs at given $L_\mathrm{uv}$ and $R$, likely involving more physical processes. The emergent picture is different from that commonly assumed where the excess X-ray emission of RLQs is attributed to the jets. We thus perform model selection to comparecritically these different interpretations, which prefers the coronal scenario with a corona-jet connection. A distinct jet component is likely important for only a small portion of FSRQs.The corona-jet, disk-corona, and disk-jet connections of RLQs are likely driven by independent physical processes. Furthermore, the corona-jet connection implies that small-scale processesin the vicinity of SMBHs, probably associated with the magnetic flux/topology instead of black-hole spin, are controlling the radio-loudness of quasars.

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Space Telescope and Optical Reverberation Mapping Project. XI. Disk-wind characteristics and contributions to the very broad emission lines of NGC 5548

In 2014 the NGC 5548 Space Telescope and Optical Reverberation Mapping campaign discovered a two-month anomaly when variations in the absorption and emission lines decorrelated from continuum variations. During this time the soft X-ray part of the intrinsic spectrum had been strongly absorbed by a line-of-sight (LOS) obscurer, which was interpreted as the upper part of a disk wind. Our first paper showed that changes in the LOS obscurer produce the decorrelation between the absorption lines and the continuum. A second study showed that the base of the wind shields the BLR, leading to the emission-line decorrelation. In that study, we proposed the wind is normally transparent with no effect on the spectrum. Changes in the wind properties alter its shielding and affect the SED striking the BLR, producing the observed decorrelations. In this work, we investigate the impact of a translucent wind on the emission lines. We simulate the obscuration using XMM-Newton, NuSTAR, and HST observations to determine the physical characteristics of the wind. We find that a translucent wind can contribute a part of the He II and Fe K? emission. It has a modest optical depth to electron scattering, which explains the fainter far-side emission in the observed velocity delay maps. The wind produces the very broad base seen in the UV emission lines and may also be present in the Fe K? line. Our results highlight the importance of accounting for the effects of such winds in the analysis of the physics of the central engine.

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The Sloan Digital Sky Survey Reverberation Mapping Project: The M_BH-Host Relations at 0.2<z<0.6 from Reverberation Mapping and Hubble Space Telescope Imaging

We present the results of a pilot Hubble Space Telescope (HST) imaging study of the host galaxies of ten quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Probing more than an order of magnitude in BH and stellar masses, our sample is the first statistical sample to study the BH-host correlations beyond z>0.3 with reliable BH masses from reverberation mapping rather than from single-epoch spectroscopy. We perform image decomposition in two HST bands (UVIS-F606W and IR-F110W) to measure host colors and estimate stellar masses using empirical relations between broad-band colors and the mass-to-light ratio. The stellar masses of our targets are mostly dominated by a bulge component. The BH masses and stellar masses of our sample broadly follow the same correlations found for local RM AGN and quiescent bulge-dominant galaxies, with no strong evidence of evolution in the M_BH-M_*,bulge relation to z~0.6. We further compare the host light fraction from HST imaging decomposition to that estimated from spectral decomposition. We found a good correlation between the host fractions derived with both methods. However, the host fraction derived from spectral decomposition is systematically smaller than that from imaging decomposition by ~30%, indicating different systematics in both approaches. This study paves the way for upcoming more ambitious host galaxy studies of quasars with direct RM-based BH masses at high redshift.

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The Sixteenth Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra

This paper documents the sixteenth data release (DR16) from the Sloan Digital Sky Surveys; the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the southern hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey (TDSS) and new data from the SPectroscopic IDentification of ERosita Survey (SPIDERS) programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).

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The Sloan Digital Sky Survey Reverberation Mapping Project: The H$β$ Radius-Luminosity Relation

Results from a few decades of reverberation mapping (RM) studies have revealed a correlation between the radius of the broad-line emitting region (BLR) and the continuum luminosity of active galactic nuclei. This "radius-luminosity" relation enables survey-scale black-hole mass estimates across cosmic time, using relatively inexpensive single-epoch spectroscopy, rather than intensive RM time monitoring. However, recent results from newer reverberation mapping campaigns challenge this widely used paradigm, reporting quasar BLR sizes that differ significantly from the previously established radius-luminosity relation. Using simulations of the radius--luminosity relation with the observational parameters of SDSS-RM, we find that this difference is not likely due to observational biases. Instead, it appears that previous RM samples were biased to a subset of quasar properties, and the broader parameter space occupied by the SDSS-RM quasar sample has a genuinely wider range of BLR sizes. We examine the correlation between the deviations from the radius-luminosity relation and several quasar parameters; the most significant correlations indicate that the deviations depend on UV/optical SED and the relative amount of ionizing radiation. Our results indicate that single-epoch black-hole mass estimates that do not account for the diversity of quasars in the radius-luminosity relation could be overestimated by an average of ~0.3 dex.

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Corona-Heated Accretion-disk Reprocessing (CHAR): A Physical Model to Decipher the Melody of AGN UV/optical Twinkling

Active galactic nuclei (AGNs) have long been observed to "twinkle" (i.e., their brightness varies with time) on timescales from days to years in the UV/optical bands. Such AGN UV/optical variability is essential for probing the physics of supermassive black holes (SMBHs), the accretion disk, and the broad-line region. Here we show that the temperature fluctuations of an AGN accretion disk, which is magnetically coupled with the corona, can account for observed high-quality AGN optical light curves. We calculate the temperature fluctuations by considering the gas physics of the accreted matter near the SMBH. We find that the resulting simulated AGN UV/optical light curves share the same statistical properties as the observed ones as long as the dimensionless viscosity parameter $α$, which is widely believed to be controlled by magnetohydrodynamic (MHD) turbulence in the accretion disk, is about $0.01$---$0.2$. Moreover, our model can simultaneously explain the larger-than-expected accretion disk sizes and the dependence of UV/optical variability upon wavelength for NGC 5548. Our model also has the potential to explain some other observational facts of AGN UV/optical variability, including the timescale-dependent bluer-when-brighter color variability and the dependence of UV/optical variability on AGN luminosity and black hole mass. Our results also demonstrate a promising way to infer the black-hole mass, the accretion rate, and the radiative efficiency, thereby facilitating understanding of the gas physics and MHD turbulence near the SMBH and its cosmic mass growth history by fitting the AGN UV/optical light curves in the era of time-domain astronomy.

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Extended H$α$ over compact far-infrared continuum in dusty submillimeter galaxies -- Insights into dust distributions and star-formation rates at $z\sim2$

Using data from ALMA and near-infrared (NIR) integral field spectrographs including both SINFONI and KMOS on the VLT, we investigate the two-dimensional distributions of H$α$ and rest-frame far-infrared (FIR) continuum in six submillimeter galaxies at $z\sim2$. At a similar spatial resolution ($\sim$0.5" FWHM; $\sim$4.5 kpc at $z=2$), we find that the half-light radius of H$α$ is significantly larger than that of the FIR continuum in half of the sample, and on average H$α$ is a median factor of $2.0\pm0.4$ larger. Having explored various ways to correct for the attenuation, we find that the attenuation-corrected H$α$-based SFRs are systematically lower than the IR-based SFRs by at least a median factor of $3\pm1$, which cannot be explained by the difference in half-light radius alone. In addition, we find that in 40% of cases the total $V$-band attenuation ($A_V$) derived from energy balance modeling of the full ultraviolet(UV)-to-FIR spectral energy distributions (SEDs) is significantly higher than that derived from SED modeling using only the UV-to-NIR part of the SEDs, and the discrepancy appears to increase with increasing total infrared luminosity. Finally, considering all our findings along with the studies in the literature, we postulate that the dust distributions in SMGs, and possibly also in less IR luminous $z\sim2$ massive star-forming galaxies, can be decomposed into three main components; the diffuse dust heated by older stellar populations, the more obscured and extended young star-forming HII regions, and the heavily obscured central regions that have a low filling factor but dominate the infrared luminosity in which the majority of attenuation cannot be probed via UV-to-NIR emissions.

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X-CIGALE: fitting AGN/galaxy SEDs from X-ray to infrared

CIGALE is a powerful multiwavelength spectral energy distribution (SED) fitting code for extragalactic studies. However, the current version of CIGALE is not able to fit X-ray data, which often provide unique insights into AGN intrinsic power. We develop a new X-ray module for CIGALE, allowing it to fit SEDs from the X-ray to infrared (IR). We also improve the AGN fitting of CIGALE from UV-to-IR wavelengths. We implement a modern clumpy two-phase torus model, SKIRTOR. To account for moderately extincted type 1 AGNs, we implement polar-dust extinction. We publicly release the source code (named X-CIGALE). We test X-CIGALE with X-ray detected AGNs in SDSS, COSMOS, and AKARI-NEP. The fitting quality (as indicated by reduced $χ^2$) is good in general, indicating that X-CIGALE is capable of modelling the observed SED from X-ray to IR. We discuss constrainability and degeneracy of model parameters in the fitting of AKARI-NEP, for which excellent mid-IR photometric coverage is available. We also test fitting a sample of AKARI-NEP galaxies for which only X-ray upper limits are available from Chandra observations, and find that the upper limit can effectively constrain the AGN SED contribution for some systems. Finally, using X-CIGALE, we assess the ability of Athena to constrain the AGN activity in future extragalactic studies.

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An Extreme X-ray Variability Event of a Weak-Line Quasar

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.

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The Origins of X-ray Line Emissions in Circinus~X-1 at Very Low X-ray Flux

Accretion conditions and morphologies of X-ray transients containing neutron stars are still poorly understood. Circinus X-1 is an enigmatic case where we observe X-ray flux changes covering four orders of magnitude. We observed Circinus X-1 several times at its very lowest X-ray flux using the high energy transmission grating spectrometer on board the Chandra X-ray Observatory. At a flux of 1.8$\times10^{-11}$ \ergcm we observed a single 1.6 keV blackbody spectrum. The observed continuum luminosity of 10$^{35}$ \ergsec is about two orders of magnitude too low to explain the observed photoionized luminosity suggesting a much more complex structure of the X-ray source which is partially or entirely obscured as had been previously suggested. This affects most emissions from the accretion disk including previously observed accretion disk coronal line emissions. Instead, the strongest observed photoionized lines are blueshifted by about $\sim 400$ \kms\ and we suggest that they originate in the ionized wind of a B5Ia supergaint companion supporting a previous identification. The neutron star in Cir X-1 is very young and should have a high magnetic field. At the observed luminosity the emission radius of the blackbody is small enough to be associated with the accretion hot spot as the X-ray emitting region. The small emission radius then points to a field strength below $10^{12}$ G which would be consistent with the observation of occasional type I X-ray bursts at high magnetic fields. We discuss Cir X-1 in the context of being a high-mass X-ray binary with some emphasis on a possible Be-star X-ray binary nature.

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Probing the circumnuclear absorbing medium of the buried AGN in NGC 1068 through NuSTAR observations

We present the results of the latest NuSTAR monitoring campaign of the Compton-thick Seyfert 2 galaxy NGC 1068, composed of four $\sim$50 ks observations performed between July 2017 and February 2018 to search for flux and spectral variability on timescales from 1 to 6 months. We detect one unveiling and one eclipsing event with timescales less than 27 and 91 days, respectively, ascribed to Compton-thick material with $N_H=(1.8\pm0.8)\times10^{24}$ cm$^{-2}$ and $N_H\geq(2.4\pm0.5)\times10^{24}$ cm$^{-2}$ moving across our line of sight. This gas is likely located in the innermost part of the torus or even further inward, thus providing further evidence of the clumpy structure of the circumnuclear matter in this source. Taking advantage of simultaneous Swift-XRT observations, we also detected a new flaring ULX, at a distance $d\sim$30" (i.e. $\sim$2 kpc) from the nuclear region of NGC 1068, with a peak X-ray intrinsic luminosity of $(3.0\pm0.4)\times10^{40}$ erg s$^{-1}$ in the 2-10 keV band.

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