---
abstract: |
    CRTS J084133.15+200525.8 is an optically bright quasar at $z =
    2.345$ that has shown extreme spectral variability over the past decade. Photometrically, the source had a visual magnitude of $V
    \sim 17.3$ between 2002 and 2008. Then, over the following five years, the source slowly brightened by approximately one magnitude, to $V \sim 16.2$. Only $\sim 1$ in 10,000 quasars show such extreme variability, as quantified by the extreme parameters derived for this quasar assuming a damped random walk model. A combination of archival and newly acquired spectra reveal the source to be an iron low-ionization broad absorption line (FeLoBAL) quasar with extreme changes in its absorption spectrum. Some absorption features completely disappear over the 9 years of optical spectra, while other features remain essentially unchanged. We report the first definitive redshift for this source, based on the detection of broad H$\alpha$ in a Keck/MOSFIRE spectrum. Absorption systems separated by several 1000 km s$^{-1}$ in velocity show coordinated weakening in the depths of their troughs as the continuum flux increases. We interpret the broad absorption line variability to be due to changes in photoionization, rather than due to motion of material along our line of sight. This source highlights one sort of rare transition object that astronomy will now be finding through dedicated time-domain surveys.
author:
- 'Daniel Stern, Matthew J. Graham, Nahum Arav, S. G. Djorgovski, Carter Chamberlain, Aaron J. Barth, Ciro Donalek, Andrew J. Drake, Eilat Glikman, Hyunsung D. Jun, Ashish A. Mahabal, Charles. C. Steidel'
title: Extreme Variability in a Broad Absorption Line Quasar
---

Introduction
============

For galaxies hosting active galactic nuclei (AGNs), time-domain surveys have long proven to be fertile avenues of research. Indeed, optical continuum variability was recognized as a common feature of quasars shortly after their initial discovery [@Matthews:63], and has since been exploited for purposes ranging from identifying quasars [, @vandenBergh:73], to determining black hole masses through reverberation mapping [, @Blandford:82; @Bentz:09], to studying the inner circumnuclear environment [, @Risaliti:02]. Recent efforts using wide-area, time-domain surveys have vastly extended this avenue of research by exploring the optical variability of extremely large samples of quasars, numbering in the tens to hundreds of thousands [, @MacLeod:12; @Graham:14]. Besides determining the light curve properties of typical quasars, such work has identified interesting new phenomenology such as candidate periodic light curves suggestive of sub-parsec binary super-massive black hole systems [, @DOrazio:15a; @DOrazio:15b; @Graham:15; @Graham:15b; @Jun:15b; @Liu:15], AGN undergoing major flaring suggestive of microlensing or explosive activity in the accretion disk such as superluminous supernovae, mergers, or tidal disruption events [, @Drake:11; @Lawrence:16 Graham , submitted], and changing look AGN with the abrupt appearance or disappearance of broad emission lines [, @LaMassa:15; @Gezari:17].

One topic where quasar variability has received particular attention has been the temporal characteristics of broad absorption line (BAL) quasars. Specifically, over the past few years, several teams have reported on multi-epoch spectroscopic observations of BAL quasars [, @Barlow:92; @Lundgren:07; @Gibson:08; @Gibson:10; @Capellupo:11; @Capellupo:12; @Capellupo:13; @FilizAk:12; @FilizAk:13; @FilizAk:14; @Vivek:12; @He:14; @He:15; @Joshi:14; @Wildy:14; @Wildy:15; @Grier:15; @Zhang:15]. While variability in BAL trough strengths is relatively common, large ($> 50\%$) changes in the absorption equivalent width is quite rare [, @Hall:11]. A primary question in BAL variability studies has been whether observed changes in BAL trough strengths are primarily due to changes in the ionization state of the outflowing wind [, @Wang:15], or whether they are due to high column density BAL clouds moving through our line of sight [, @McGraw:15].

For example, @FilizAk:13 present a detailed analysis of $\approx 650$ BAL troughs identified in 291 quasars observed by the Sloan Digital Sky Survey (SDSS), sampling rest-frame timescales between 1 and 3.7 years. They estimate that the average lifetime of a BAL trough is a few thousand years, and that the emergence/disappearance of BAL features are extremes of general BAL variability. @FilizAk:13 also report coordinated BAL variability across multiple troughs at different velocities. They argue that changes in the opacity of the shielding gas producing changes in the ionizing radiation incident on the BAL material are the most probable cause for such coordinated variability.

@Grier:15 and @Wildy:15 reach similar conclusions based on the highly variable BAL lines seen in a spectroscopic monitoring campaigns. With variability seen on time-scales of just a few days, both authors conclude that the most likely cause of such rapid changes is the BAL gas responding to changes in the incident ionizing continuum.

Leading to an alternative explanation of BAL variability, @Capellupo:11 [@Capellupo:12] report on an ongoing monitoring campaign of a sample of 24 BAL quasars at $1.2 < z < 2.9$ on timescales ranging from $\sim 4$ months to $\sim 8$ years. Studying the BAL feature, @Capellupo:11 found variability in 40% of their sample on month-long timescales, and in 65% of their sample on year-long timescales. They find that higher-velocity BALs are more likely to vary than lower-velocity BALs, and that weaker BALs are more likely to vary than stronger BALs. They suggest that the observations are best understood as the movement of clouds within 6 pc of the central engine across the line of sight. In a detailed study of the first observation of $\lambda
\lambda 1118, 1128$ BAL variability in a quasar, @Capellupo:14 argue that the observations are best described by a BAL cloud at a distance of $\simlt 3.5$ pc moving across the line sight. The implied kinetic energy of the outflow would be $\sim 2\%$ of the quasar bolometric luminosity, which is sufficient to cause substantial feedback.

Also supporting this interpretation that BAL variability is not dominated by photoionization, @He:14 report on 18 epochs of SDSS/BOSS spectroscopy of a BAL quasar at $z = 2.72$. They find only a weak correlation between the BAL variability and the continuum luminosity, suggesting that continuum changes are not driving changes in the BAL trough amplitudes.

Here, we report on CRTS J084133.15+200525.8 (), an optically bright quasar that has shown extreme variability over the past decade (Figure \[fig:lc\]). The quasar transitioned from having a relatively stable visual magnitude of $V \sim 17.3$ between 2002 and 2008, to slowly brightening by a factor of $\sim 2.5$ over the course of 5 years and then plateauing at $V \sim 16.2$. As detailed below, a combination of archival and newly acquired spectroscopy reveal this source to be an iron low-ionization broad absorption line (FeLoBAL) quasar exhibiting extreme spectroscopic changes over the same time period, and the nature of these variations allow us to assess the likely cause of the BAL trough variability.

Independent of our own work on , @Rafiee:16 recently reported on this same source as part of a sample of three FeLoBAL quasars that have shown significant spectroscopic variability over the past decade. Interestingly, all three show decreasing strength of their low-ionization iron absorption. The current paper has several additions relative to that work. Specifically, we provide new data on , including a new epoch of optical spectroscopy which demonstrates continued spectral changes, and a near-infrared spectrum which provides the first precise redshift for the quasar as well as an estimate of its black hole mass. Finally, @Rafiee:16 remain agnostic as to whether absorber transverse motion or ionization variability is the more likely cause of the changes in the absorption troughs of this source. In contrast, the additional epoch of Palomar spectroscopy presented here allows us to argue that ionization variability is the more likely cause of the extreme absorption variability seen in .

Throughout this paper, we use Vega magnitudes unless otherwise indicated and we adopt the concordance cosmology, $\Omega_{\rm M}
= 0.3$, $\Omega_\Lambda = 0.7$ and $H_0 = 70\, \kmsMpc$.

Data and Results
================

Optical Light Curve
-------------------

The Catalina Real-time Transient Survey[^1] [CRTS; @Drake:09] leverages the Catalina Sky Survey, designed to search for near-Earth objects, as a probe of the time-variable universe. CRTS has used three telescopes for much of the past decade, two in the northern hemisphere and one in Australia, to cover up to $\sim 2500\, {\rm deg}^2$ per night. The filterless observations are broadly calibrated to Johnson $V$ [for details, see @Drake:13] with a nominal depth of $V
\sim 20$. The full CRTS data set contains time series for approximately 500 million sources.[^2]

CRTS represents the best data set currently available with which to systematically study quasar variability with large samples over a decade-length timescale. In an analysis of characteristic timescales of 240,000 known spectroscopically confirmed objects using Slepian wavelet variance, Graham et al. (submitted) originally identified  as an extreme outlier in the plane defined by a linear trend (the Thiel-Sen statistic) and deviation from the median Slepian wavelet variance fit. In that analysis,  has a characteristic timescale $\tau = 109.9$ days, which is significantly larger than expected for a quasar of its magnitude, $\tau = 48.0 \pm 5.9$ days.

If we instead characterize quasar light curves with a Gaussian process damped random walk model and only consider the subset of 79,749 quasars with at least 200 CRTS photometric measurements,  again stands out. The two parameters from this model are the amplitude, $\sigma$, and the characteristic timescale, $\tau$, of the damped random walk [, @Kelly:09]. We use a kernel density estimator to determine the distribution of sources in the $\sigma-\tau$ plane, and we find that  resides in an extreme location in this plane ($\log \Sigma = -7.8$, where $\Sigma$ is the density of sources in this plane). Only seven quasars stand out at this level or more from the population distribution, implying that only $\sim 1$ in 10,000 quasars show variability behavior as extreme as . Further inspection of the CRTS light curve of  (Figure \[fig:lc\]) also indicates that the variable behavior is different from the expected stochastic damped random walk model that describes most quasars, and instead appears more consistent with a state change. Further support for this interpretation comes from earlier photometry of  reported in @Rafiee:16 from the Palomar Sky Surveys (POSS-I, Palomar Quick V, and POSS-II), reaching back to the mid-1950s. @Rafiee:16 reports no evidence for a significant change in the optical brightness of  prior to 2000.

SDSS imaged  on UT 2004 December 12 (MJD = 53351), which is prior to the brightening episode. The source was unresolved, and based on its unusual and red colors, SDSS targeted  for spectroscopic observations as a high-redshift quasar candidate.

Optical Spectroscopy
--------------------

 was first observed spectroscopically by SDSS on UT 2005 December 1 [MJD = 53705; @Blanton:03] and was then re-observed by SDSS-III BOSS on UT 2011 January 4 [MJD = 55565; @Dawson:13]. The spectra, shown in Figure \[fig:optical\], show a source with many absorption features, making redshift identification challenging. Indeed, the SDSS data releases have reported a variety of redshifts for , always with warning flags, ranging from $z = 0.859$ (DR8; Warning = Many Outliers) to $z = 1.295$ (DR7; zStatus = Failed) to $z = 3.195$ (DR9; Warning = Negative Emission). Our visual inspection of the BOSS spectrum tentatively identified and blends in the region around 9400 Å, implying $z \sim 2.3$, consistent with both the visual inspection value of $z = 2.342$ in the SDSS DR12 quasar catalog [DR12Q; @Paris:14] and the results of our Keck infrared spectrum described in §2.4.

We obtained additional optical spectroscopy of  using the Double Spectrograph on the Hale 200” Telescope at Palomar Observatory on UT 2014 April 22 (MJD = 56769). We obtained two 600 s exposures using the 1.0 slit in cloudy conditions. The data were reduced using standard procedures and relative spectrophotometric calibration was achieved using observations of standard stars obtained on the same night. Figure \[fig:optical\] presents the Palomar data, where we have scaled the spectra so that the long wavelength ($\simgt 5500$ Å) part of the spectra is of comparable flux density to the BOSS spectrum at the same wavelengths.

The multi-epoch spectra show the extreme variability exhibited by , as well as multiple strong absorption features, characteristic of an FeLoBAL quasar. FeLoBALs are notoriously challenging targets for redshift identification [, @Becker:97; @Brunner:03]. We see dramatic changes across the full spectrum, particularly in the spectral region between redshifted Ly$\alpha$ and . Some features do not change across the near-decade timescale of the spectroscopy, such as the saturated absorption at 5150 Å. Other features completely disappear, such as absorption lines at $\approx$ 5450 and 8500 Å. There is an overall uncovering of blue continuum emission, with the flux around Ly$\alpha$ increasing by an order of magnitude over the $>8$ years spanned by the spectroscopy. In addition, while the continuum between \] and is extremely choppy in the 2005 spectrum, by 2014 it is smoother, which is more typical of normal quasar spectra.

Imaging at Other Wavelengths
----------------------------

 is a bright near- to mid-infrared source, well detected by both the Two Micron All Sky Survey [$K_s = 13.62 \pm 0.04$ — 2MASS; @Skrutskie:06] and the [*Wide-field Infrared Survey Explorer*]{} [$W3 = 9.58 \pm 0.06$ — ; @Wright:10]. With $W1 - W2 = 0.65$,  is slightly bluer than the mid-infrared AGN selection criteria of @Stern:12, which are $W1 - W2 \geq
0.8$ and $W2 \leq 15.05$. However, as shown in @Assef:13, the AGN selection color can be relaxed for brighter sources.

There is little variability detected at longer wavelengths in this source. In AB magnitudes, the $z$-band magnitude recorded by SDSS was $z = 16.37 \pm 0.01$ on MJD 53351, closely matching the $z$-band magnitude of $Z = 16.34 \pm 0.01$ recorded by UKIRT Infrared Deep Sky Survey [UKIDSS; @Lawrence:07] on MJD 55141. In the near-infrared (in Vega magnitudes), 2MASS recorded $H = 14.41 \pm
0.05$ and $K_s = 13.62 \pm 0.04$ on MJD 51105, closely matching the UKIDSS values of $H = 14.32 \pm 0.02$ on MJD 54061 and $K = 13.57
\pm 0.02$ on both MJD 54061 and MJD 55238, where we have assumed a 2% floor on the UKIDSS photometric calibration [, @Hodgkin:09]. Similarly, the mid-infrared flux measured by [*WISE*]{} and [*NEOWISE*]{} [@Mainzer:14] varies by only $\sim 0.04$ mag, comparable to the typical uncertainty.

 is not detected by [*ROSAT*]{}, nor was it (serendipitously) observed by either the [*Chandra X-Ray Observatory*]{} or [*XMM-Newton*]{}.  is also not detected by the Faint Images of the Radio Sky at Twenty cm survey [FIRST; @Becker:95], implying $S_{\rm 1.4~GHz} \simlt 1$ mJy ($5 \sigma$). Finally, as expected, observations by the [*Galaxy Evolution Explorer*]{} [[*GALEX*]{}; @Martin:05], which sample below the Lyman limit for $z = 2.35$, do not detect .

Near-infrared Spectroscopy
--------------------------

We obtained a $K$-band (1.95-2.39 $\mu$m) spectrum of  with the Multi-Object Spectrometer for InfraRed Exploration [MOSFIRE; @McLean:12; @Steidel:14] on UT 2014 May 5 (MJD=56782) in longslit mode. We obtained three dithered exposures of 180 s each through a 07 entrance slit under clear conditions with good seeing. The spectrum was reduced using a combination of the MOSFIRE data reduction pipeline (DRP) and custom routines [for details, see @Steidel:14]. Wavelength calibration was based on a combination of OH emission lines in the night sky and an internal Ne arc lamp. Flux calibration and telluric absorption removal was accomplished using spectra of an A0V star (Vega analog) observed at similar airmass. The final extracted spectrum (Figure \[fig:nearIR\]) shows strong continuum and a single broad emission line with a peak at 2.1956 $\mu$m, which we identify as H$\alpha$ at $z=2.3446$. The apparent asymmetry in the continuum straddling the line is well modeled by the @Boroson:92 template on the blue side of the line.

We use the broad H$\alpha$ emission line to estimate the mass of the black hole in . First, we apply a multiplicative correction to the $K$-band spectrum to match the $K$-band photometry from the UKIDSS observations. We then approximate the uncertainties in the spectrum by considering the standard deviation of the spectrum outside the strong emission line. We model the H$\alpha$ spectral region as the sum of two broad Gaussian lines, a single narrow Gaussian, an iron template (which elevates the continuum on the blue side of the emission line), and a power-law continuum. The full-width at half-maximum (FWHM) of the broad H$\alpha$ emission is $6086 \pm 42\, \kms$, and the combined luminosity of the broad H$\alpha$ components is $L_{\rm H\alpha} = (5.56 \pm 0.05) \times
10^{45} \ergs$. Modeling the broad-band (3000Å to 7$\mu$m) spectral energy distribution of the quasar as a sum of a power-law continuum, two blackbody thermal components (500 and 1250 K, to model the rest-frame IR emission), and line emission from H$\alpha$ and as determined from the Keck spectrum, we derive $L_{5100} = (1.24 \pm 0.02) \times 10^{47} \ergs$. Following @Jun:15a, we derive $\log(M_{\rm BH}/M_\odot) = 10.36 \pm
0.16$ using the $L_{5100}$ estimator and $\log(M_{\rm BH}/M_\odot)
= 10.29 \pm 0.17$ using the $L_{\rm H\alpha}$ estimator. We note that these statistical error bars underestimate the true uncertainty, both due to the non-simultaneity of the imaging and near-infrared spectroscopy and, more importantly, the systematic uncertainty in the virial scale factor, $f$, which is the typically the dominant source of uncertainty in black hole mass measurements; in this case, we adopt $f = 5.1 \pm 1.3$ from @Woo:13, as per @Jun:15a.

For comparison, without access to any well-detected emission features, @Rafiee:16 simply adopted a black hole mass of $M_{\rm BH}
= 6 \times 10^9\, M_\odot$ as a typical value. Adopting their value for the bolometric luminosity of , $L_{\rm bol} = (3.36 \pm
0.69) \times 10^{47}\, {\rm erg}\, {\rm s}^{-1}$ [based on the observed rest-frame 2900 Å flux density and a bolometric correction of ${\rm BC}_{2900} = 5 \pm 1$ from @Richards:06], we determine an Eddington ratio of $L_{\rm bol}/L_{\rm Edd} \sim 0.15$ (in comparison to their value of 0.45).

Discussion
==========

Figure \[fig:Fe\_Mg\] shows a comparison between the long wavelength portion of the three epochs of optical spectroscopy. In the spectral region beyond $\sim 7300$ Å there is much less blending of troughs from different ions, making the variability changes simpler to interpret. We identify two primary absorption systems. The first system, A, shows several troughs of UV absorption between 7500 and 8900 Å, as well as absorption at 9050 Å. The other system, B, shows absorption at 9300 Å. The two absorption systems are separated by 9000 , yet show coordinated reductions in the depth of their troughs as the quasar brightens. This is the expected behavior if the BAL spectral variability is driven by changes in the photoionization: as the ionizing continuum flux increases, the column densities of and decrease for all clouds along the line of sight. (Note that this expectation assumes that the ionizing continuum changes are correlated with the flux changes around 2500 Å). The scenario of clouds moving across our line of sight is hard pressed to explain both the coordinated changes of the trough depths as well as the observed trough weakening with increasing UV flux. A priori, there is no reason that troughs as widely separated in velocity as A and B would be correlated since they are different parcels of gas. Even more so, there is no reason in this scenario for changes in the trough depths to be correlated with flux changes. Therefore, we interpret the variability in the absorption troughs to be due to changes in photoionization, rather than motion of material into our line of sight. A follow-up paper will more carefully model the full multi-epoch spectroscopic data set, including additional spectroscopy from our continuing monitoring, with the goal of understanding the location and energetics of the outflow, and its impact on the host galaxy (Chamberlain , in preparation).

 appears to be an FeLoBAL quasar in the process of transitioning to a more common low-ionization BAL (LoBAL) quasar, similar to FBQS J1408+3054 reported by @Hall:11. We note, however, that @Hall:11 interpreted the variability in that source as being related to structure in the BAL outflow moving out of our line of sight rather than being related to photoionzation changes.

 highlights the sort of rare, extremely variable quasars that can be used to probe the physics of quasar outflows. We expect to find many more such examples with the new generation of wide-area, sensitive, high-cadence synoptic surveys. We were fortuitous in this case that multi-epoch archival spectroscopy was available for this source. In the future, it will be exciting to find similar major events in real time, allowing real-time multi-wavelength follow-up in order to more fully dissect the internal workings of AGN engines.

We thank the anonymous referee for a prompt and helpful referee report. CRTS was supported by the NSF grants AST-1313422, AST-1413600, and AST-1518308. The work of DS and HJ was carried out at Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. DS also acknowledges support from NASA through ADAP award 12-ADAP12-0109. NA and CC acknowledge support from NSF through grant AST 1413319, and from NASA through STScI grants GO 11686 and GO 12022. Research by AJB was supported by NSF grant AST-1412693. EG acknowledges the generous support of the Cottrell College Award through the Research Corporation for Science Advancement. HJ is supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Universities Space Research Association under contract with NASA. The authors a grateful to the staff at the Palomar and Keck observatories, where some of the data presented here were obtained. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

, , , , ,

©2017. All rights reserved.

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