Long-term variability of a complete sample of quasars

Cristiani, S.; Vio, R.; Andreani, P.

doi:10.1086/115489

Cited by 136 publications

(180 citation statements)

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“…We find no relation between the variability amplitude and redshift for radio-quiet quasars, which seems contrary to the negative or positive correlations reported in previous work (Cristiani et al 1990;Hook et al 1994;Trevese et al 1994). The anti-correlation found in Cristiani et al (1990) may reflect the anti-correlation between V and L bol , as their sample is too small to disentangle the relation.…”

Section: Comparison With Previous Workcontrasting

confidence: 99%

“…The anti-correlation found in Cristiani et al (1990) may reflect the anti-correlation between V and L bol , as their sample is too small to disentangle the relation. Only Trevese et al (1994) binned the magnitude limited sample within small luminosity and redshift ranges and found an anti-correlation, which is ascribed to the dependence of variability on wavelength.…”

Section: Comparison With Previous Workmentioning

confidence: 91%

“…This ignores the diversity of an individual quasar light curve, which may eventually fade out the actual relations. Due to the statistical incompleteness of small samples, previous studies found controversial evidences for the dependencies of variability on redshift and luminosity (Bonoli et al 1979;Netzer & Sheffer 1983;Cristiani et al 1990;Giallongo et al 1991;Hook et al 1994). Thus, the results may not be statistically reliable due to possible dependencies of variability on other parameters.…”

Section: Introductionmentioning

confidence: 95%

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The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82

Zuo¹,

Wu²,

Liu³

et al. 2012

Proc. IAU

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We investigate the optical variability of 7658 quasars from SDSS Stripe 82. Taking advantage of a larger sample and relatively more data points for each quasar, we estimate variability amplitudes and divide the sample into small bins of redshift, rest-frame wavelength, black hole mass, Eddington ratio and bolometric luminosity respectively, to investigate the relationships between variability and these parameters. An anti-correlation between variability and rest-frame wavelength is found. The variability amplitude of radio-quiet quasars shows almost no cosmological evolution, but that of radio-loud ones may weakly anticorrelate with redshift. In addition, variability increases as either luminosity or Eddington ratio decreases. However, the relationship between variability and black hole mass is uncertain; it is negative when the influence of Eddington ratio is excluded, but positive when the influence of luminosity is excluded. The intrinsic distribution of variability amplitudes for radio-loud and radio-quiet quasars are different. Both radio-loud and radio-quiet quasars exhibit a bluer-whenbrighter chromatism. Assuming that quasar variability is caused by variations of accretion rate, the Shakura-Sunyaev disk model can reproduce the tendencies of observed correlations between variability and rest-frame wavelength, luminosity as well as Eddington ratio, supporting that changes of accretion rate plays an important role in producing the observed optical variability. However, the predicted positive correlation between variability and black hole mass seems to be inconsistent with the observed negative correlation between them in small bins of Eddington ratio, which suggests that other physical mechanisms may still need to be considered in modifying the simple accretion disk model.

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Section: Comparison With Previous Workcontrasting

confidence: 99%

Section: Comparison With Previous Workmentioning

confidence: 91%

Section: Introductionmentioning

confidence: 95%

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The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82

Zuo¹,

Wu²,

Liu³

et al. 2012

Proc. IAU

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“…Examples of such composite spectra include template spectra of various classes of galaxies (Mannucci et al 2001;Shapley et al 2003;Dobos et al 2012), quasi-stellar object (QSO; Cristiani & Vio 1990;Boyle 1990;Francis et al 1991;Zheng et al 1997;Brotherton et al 2001;Vanden Berk et al 2001;Telfer et al 2002;Richards et al 2006a;Glikman et al 2006;Lusso et al 2015) and gamma-ray burst (GRB) afterglows (Christensen et al 2011). When investigating dust in QSOs or in galaxies along QSO sightlines, template spectra can be used to determine the amount of extinction by artificially reddening the template to match an observed spectrum (e.g., Glikman et al 2007;Urrutia et al 2009;Wang et al 2012;Fynbo et al 2013;Krogager et al 2015).…”

Section: Introductionmentioning

confidence: 99%

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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“…Cristiani & Vio (1990). ( The complete extragalactic (galaxy + AGN) template database used in this work.…”

Section: Template Selectionmentioning

confidence: 99%

Quasi-stellar objects in the ALHAMBRA survey

Matute

Márquez

Masegosa

et al. 2012

A&A

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Context. Even the spectroscopic capabilities of today's ground and space-based observatories can not keep up with the enormous flow of detections (>10 5 deg −2 ) unveiled in modern cosmological surveys as: i) would be required enormous telescope time to perform the spectroscopic follow-ups and ii) spectra remain unattainable for the fainter detected population. In the past decade, the typical accuracy of photometric redshift (photo-z) determination has drastically improved. Nowdays, it has become a perfect complement to spectroscopy, closing the gap between photometric surveys and their spectroscopic follow-ups. The photo-z precision for active galactic nuclei (AGN) has always lagged behind that for the galaxy population owing to the lack of proper templates and their intrinsic variability. Aims. Our goal is to characterize the ability of the Advanced Large, Homogeneous Area Medium-Band Redshift Astronomical (ALHAMBRA) survey in assigning accurate photo-z's to broad-line AGN (BLAGN) and quasi-stellar objects (QSOs) based on their ALHAMBRA very-low-resolution optical-near-infrared (NIR) spectroscopy. This will serve as a benchmark for any future compilation of ALHAMBRA selected QSOs and the basis for the statistical analysis required to derive luminosity functions up to z ∼ 5. Methods. We selected a sample of spectroscopically identified BLAGN and QSOs and used a library of templates (including the SEDs of AGN and both normal and starburst galaxies, as well as stars) to fit the 23 photometric data points provided by ALHAMBRA in the optical and NIR (20 medium-band optical filters plus the standard JHKs). Results. We find that the ALHAMBRA photometry is able to provide an accurate photo-z and spectral classification for ∼88% of the 170 spectroscopically identified BLAGN/QSOs over 2.5 deg 2 in different areas of the survey and brighter than m 678 = 23.5 (equivalent to r SLOAN ∼ 24.0). The derived photo-z accuracy is below 1% and is comparable to the most recent results in other cosmological fields that use photometric information over a wider wavelength range. The fraction of outliers (∼12%) is mainly caused by the larger photometric errors for the faintest sources and the intrinsic variability of the BLAGN/QSO population. A small fraction of outliers may have an incorrectly assigned spectroscopic redshift. Conclusions. The definition of the ALHAMBRA survey in terms of the number of filters, filter properties, areal coverage, and depth is able to provide photometric redshifts for BLAGN/QSOs with a precision similar to any previous survey that makes use of mediumband optical photometry. In agreement with previous literature results, our analysis also reveals that, in the 0 < z < 4 redshift interval, very accurate photo-z can be obtained without the use of NIR broadband photometry at the expense of a slight increase in the outliers. The importance of NIR data is expected to increase at higher z (z > 4). These results are relevant for the design of future optical follow-ups of surveys containing a large fractio...

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Long-term variability of a complete sample of quasars

Cited by 136 publications

References 0 publications

The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82

The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82

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

Quasi-stellar objects in the ALHAMBRA survey

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