Long-Term Optical Variability of Radio-selected Quasars from the FIRST Survey

Helfand, D. J.; Stone, R. P. S.; Willman, Beth; White, R. L.; Becker, R. H.; Price, Trevor; Gregg, Michael D.; McMahon, R. G.

doi:10.1086/319971

Cited by 42 publications

(41 citation statements)

References 29 publications

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“…Recent work by Morganson et al (2014) confirm the results of Helfand et al (2000) that quasars are more variable in bluer bands -also consistent to what is found here.…”

Section: Flux Calibrationsupporting

confidence: 90%

An X-Shooter composite of bright 1 <z< 2 quasars from UV to infrared

Selsing

Fynbo

Christensen

et al. 2015

A&A

165

180

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Quasi-stellar object (QSO) spectral templates are important both to QSO physics and for investigations that use QSOs as probes of intervening gas and dust. However, combinations of various QSO samples obtained at different times and with different instruments so as to expand a composite and to cover a wider rest frame wavelength region may create systematic effects, and the contribution from QSO hosts may contaminate the composite. We have constructed a composite spectrum from luminous blue QSOs at 1 < z < 2.1 selected from the Sloan Digital Sky Survey (SDSS). The observations with X-Shooter simultaneously cover ultraviolet (UV) to nearinfrared (NIR) light, which ensures that the composite spectrum covers the full rest-frame range from Lyβ to 11 350 Å without any significant host contamination. Assuming a power-law continuum for the composite we find a spectral slope of α λ = 1.70 ± 0.01, which is steeper than previously found in the literature. We attribute the differences to our broader spectral wavelength coverage, which allows us to effectively avoid fitting any regions that are affected either by strong QSO emissions lines (e.g., Balmer lines and complex [Fe II] blends) or by intrinsic host galaxy emission. Finally, we demonstrate the application of the QSO composite spectrum for evaluating the reddening in other QSOs.

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“…Recent work by Morganson et al (2014) confirm the results of Helfand et al (2000) that quasars are more variable in bluer bands -also consistent to what is found here.…”

Section: Flux Calibrationsupporting

confidence: 90%

An X-Shooter composite of bright 1 <z< 2 quasars from UV to infrared

Selsing

Fynbo

Christensen

et al. 2015

A&A

165

180

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“…These results, and the SF method used to obtain them, have recently been verified for a sample of quasars observed over a 10 year period by MacLeod et al (2010). Furthermore, radio-loud quasars appear more variable than their radio-quiet counterparts (Helfand et al 2001), and the apparent increase in variability for higher redshift quasars is mainly due to the increase of variability with rest-frame frequency and to the increase of rest-frame frequency scanned at high redshift for a given frequency (di Clemente et al 1996).…”

Section: Variability Correlation Plotsmentioning

confidence: 56%

“…In addition, quasar variability is most probably a complex function of several (possibly interdependent) physical parameters such as luminosity, rest-frame wavelength, redshift, black hole mass and time-lag. Helfand et al (2001) have pointed out that "the complex interdependence of the observed variability (of quasars) on wavelength, redshift, bolometric luminosity, radio loudness and possibly other parameters makes it difficult to draw definitive conclusions regarding the physics of the central engine or quasar evolution". Thus, "caveat emptor" should be the watchwords for any discussion of quasar variability dependencies, and thus we employ a method by which these dependencies of variability can be plausibly disentangled.…”

Section: The Structure Functionmentioning

confidence: 99%

“…the time period between two sets of observations) and radio loudness (Helfand et al 2001;Vanden Berk et al 2004;de Vries et al 2005;Ramirez et al 2009). The establishment of such correlations is important since there is increasing evidence that the source of quasar variability is linked to the accretion disk surrounding the central supermassive black hole engine (Peterson et al 2004;Pereyra et al 2006;Wold et al 2007).…”

Section: Introductionmentioning

confidence: 99%

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GALEX observations of quasar variability in the ultraviolet

Welsh

Wheatley

Neil

2011

A&A

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Aims. Using archival observations recorded over a 5+ year timeframe with the NASA GALaxy Evolution eXplorer (GALEX) satellite, we present a study of the ultraviolet (UV) variability of 4360 quasars of redshifts up to z = 2.5 that have optical counterparts in the Sloan Digital Sky Survey DR5 spectroscopic catalog of Schneider et al. (2007, AJ, 134, 102). The observed changes in both the far UV (FUV: 1350−1785 Å) and near UV (NUV: 1770−2830 Å) AB magnitudes as a function of time may help differentiate between models of the emission mechanisms thought to operate in these active galaxies. Methods. A list of NUV and FUV variable quasars was derived from the UV light-curves of sources with 5 or more observational visits by GALEX that spanned a time-frame >3 months. By measuring the error in the derived mean UV magnitude from the series of GALEX observations for each source, quasars whose UV variability was greater than the 3-σ variance from the mean observed value were deemed to be (intrinsically) UV variable. This conservative selection criterion (which was applied to both FUV and NUV observations) resulted in identifying 550 NUV and 371 FUV quasars as being statistically significant UV variable objects. Results. Following the work of Vanden Berk et al. (2004, ApJ, 601, 692), we have performed a structure function (SF) analysis of these data to search for possible correlations between UV variability and parameters such as rest frame time-lag, redshift, luminosity and radio loudness. Firstly, we observe that the amplitudes of variability as a function of time-lag for both the NUV and FUV data are far larger than those observed at visible wavelengths. Secondly, the levels of FUV variability are greater than those observed in the NUV for a given value of time-lag. Also, the amplitudes of both the NUV and FUV variability of quasars increase as a function of rest frame time-lag, irrespective of the value of quasar redshift, for time-lags <200 days. For time-lags >300 days there is a pronounced rollover in the NUV SF for all redshift values, which is also observed (with a lower signiÞcance) in the FUV variability data. Although we find no strong relationship between UV variability and redshift, our data do show that higher redshift quasars appear to be more variable than their low redshift counterparts. Our data also show that, for all values of time-lag, the more luminous quasars tend to be slightly less UV variable, with perhaps the exception of FUV variable quasars for short time-lags. Although our data sample is small, we find that radio-loud quasars are marginally more variable than radio quiet ones by a factor ∼2 in the NUV and by a factor 1−3 in the FUV. Therefore, our present observations support the notion in which the radio properties of quasars have a limited influence on the observed UV variability of these objects. In summation, our present analysis favors a quasar model in which UV variability is mainly due to stochastic changes in the underlying continuum level, rather than models that favor gravitat...

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“…These relationships are summarized in Helfand et al (2001) and Giveon et al (1999). Numerous studies (e.g., Hawkins 2002;de Vries et al 2003) have shown variability to correlate with time lag.…”

Section: Introductionmentioning

confidence: 99%

Spectral Variability of Quasars in the Sloan Digital Sky Survey. I. Wavelength Dependence

Wilhite

Berk

Krön

et al. 2005

ApJ

137

219

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SDSS repeat spectroscopic observations have resulted in multiepoch spectroscopy for $2500 quasars observed more than 50 days apart. From this sample, calibrating against stars observed simultaneously, we identify 315 quasars that have varied significantly between observations (with respect to assumed nonvariable stars observed concurrently). These variable quasars range in redshift from 0.5 to 4.72. This is the first large quasar sample studied spectroscopically for variability and represents a potentially useful sample for future high-redshift reverberation mapping studies. This also marks the first time the precise wavelength dependence of quasar variability has been determined, allowing both the continuum and emission-line variability to be studied. We create an ensemble difference spectrum (bright phase minus faint phase) covering rest-frame wavelengths from 1000 to 6000 8. This average difference spectrum is bluer than the average single-epoch quasar spectrum; a power-law fit to the difference spectrum yields a spectral index k ¼ À2:00, compared to an index of k ¼ À1:35 for the single-epoch spectrum. This confirms that quasar continua are bluer when brighter. The difference spectrum also exhibits very weak or absent emission-line features; the strongest emission lines vary only 30% as much as the continuum. This small emission-line variability with respect to the continuum is consistent with the intrinsic Baldwin effect. Due to the lack of variability of the lines, measured photometric color is not always bluer in brighter phases but depends on redshift and the filters used. Lastly, the difference spectrum is bluer than the ensemble quasar spectrum only for k rest < 2500 8, indicating that the variability cannot result from a simple scaling of the average quasar spectrum.

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Long-Term Optical Variability of Radio-selected Quasars from the FIRST Survey

Cited by 42 publications

References 29 publications

An X-Shooter composite of bright 1 <z< 2 quasars from UV to infrared

An X-Shooter composite of bright 1 <z< 2 quasars from UV to infrared

GALEX observations of quasar variability in the ultraviolet

Spectral Variability of Quasars in the Sloan Digital Sky Survey. I. Wavelength Dependence

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