Quasar Spectral Slope Variability in the Optical Band

Trèvese, D.; Vagnetti, F.

doi:10.1086/324541

Cited by 73 publications

(104 citation statements)

References 29 publications

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“…On average, quasar continuum spectral slopes are steeper (bluer) in bright phases than faint phases. This is not surprising; previously detected color changes like those seen in VB04 and Trèvese & Vagnetti (2002) hinted at a change in power-law slope. The change in spectral shape appears to become stronger at wavelengths less than about 2500 8.…”

Section: Continuum Variabilitysupporting

confidence: 57%

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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Section: Continuum Variabilitysupporting

confidence: 57%

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

Wilhite

Berk

Krön

et al. 2005

ApJ

137

219

View full text Add to dashboard Cite

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“…10 indicate that there is a general decrease in both NUV and FUV variability as a function of increasing absolute magnitude for time-lags >100 d. The opposite effect is observed for time-lags <100d (apart from the weak exception in the NUV for time-lags <10 d). Figure 11 shows that this general decrease in UV variability as a function of luminosity is not strongly coupled to the UVQ redshift, in conflict with other (visible) studies that have claimed a redshiftvariability relationship (Trevese & Vagnetti 2002). Both of the NUV and FUV SFs shown in Fig.…”

Section: Luminosity Dependencementioning

confidence: 57%

“…It is mostly agreed that the level of quasar variability increases with decreasing rest wavelength, which is related to the observation that their spectra become bluer in their bright phases. Several authors have claimed a variability-redshift correlation in the visible, although the actual sign of the correlation in the studies is debated (Giveon et al 1999;Trevese & Vagnetti 2002). For example, Vanden Berk et al (2004) have claimed evidence for redshift evolution in quasar variability, with greater variability occurring at higher redshifts.…”

Section: Introductionmentioning

confidence: 99%

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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“…This is instead described by the spectral variability parameter β = ∂α/∂ log F (Trevese et al 2001;Trevese & Vagnetti 2002), which can be adapted to the optical-X-ray case to become…”

Section: Discussionmentioning

confidence: 99%

Variability and the X-ray/UV ratio of active galactic nuclei

Vagnetti¹,

Turriziani²,

Trèvese

et al. 2010

A&A

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Context. The observed relation between the X-ray radiation from active galactic nuclei, originating in the corona, and the optical/UV radiation from the disk is usually described by the anticorrelation between the UV to X-ray slope α ox and the UV luminosity. Many factors can affect this relation, including: i) enhanced X-ray emission associated with the jets of radio-loud AGNs, ii) X-ray absorption associated with the UV broad absorption line (BAL) outflows, iii) other X-ray absorption not associated with BALs, iv) intrinsic X-ray weakness, v) UV and X-ray variability, and non-simultaneity of UV and X-ray observations. The separation of these effects provides information about the intrinsic α ox − L UV relation and its dispersion, constraining models of disk-corona coupling. Aims. We use simultaneous UV/X-ray observations to remove the influence of non-simultaneous measurements from the α ox − L UV relation. Methods. We extract simultaneous data from the second XMM-Newton serendipitous source catalogue (XMMSSC) and the XMM-Newton Optical Monitor Serendipitous UV Source Survey catalogue (XMMOMSUSS), and derive the single-epoch α ox indices. We use ensemble structure functions to analyse multi-epoch data.Results. We confirm the anticorrelation of α ox with L UV , and do not find any evidence of a dependence of α ox on z. The dispersion in our simultaneous data (σ ∼ 0.12) is not significantly smaller than in previous non-simultaneous studies, suggesting that "artificial α ox variability" introduced by non-simultaneity is not the main cause of dispersion. "Intrinsic α ox variability", i.e., the true variability of the X-ray to optical ratio, is instead important, and accounts for ∼30% of the total variance, or more. "Inter-source dispersion", due to intrinsic differences in the average α ox values from source to source, is also important. The dispersion introduced by variability is mostly caused by the long timescale variations, which are expected to be driven by the optical variations.

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Quasar Spectral Slope Variability in the Optical Band

Cited by 73 publications

References 29 publications

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

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

GALEX observations of quasar variability in the ultraviolet

Variability and the X-ray/UV ratio of active galactic nuclei

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