The optical variability of flat-spectrum radio quasars in the SDSS stripe 82 region

¹ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, 200030 Shanghai PR China
e-mail: gumf@shao.ac.cn
² Department of Physics, University of California, Santa Barbara, CA 93106, USA
³ National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, PO Box 110, 650011 Kunming, Yunnan, PR China
⁴ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, PO Box 110, 650011 Kunming, Yunnan, PR China

Received: 7 December 2010
Accepted: 10 January 2011

Abstract

Context. Although a bluer-when-brighter trend is commonly observed in blazars, the opposite trend of redder-when-brighter has also been found in some blazars.

Aims. We investigate the frequency of the redder-when-brighter trend in flat-spectrum radio quasars (FSRQs).

Methods. We investigate the optical variability of 29 FSRQs in the SDSS Stripe 82 region using SDSS DR7 released multi-epoch data covering about nine years. We determined the spectral index by fitting a powerlaw to SDSS ugriz photometric data, and explored the relationship between the spectral index and source brightness.

Results. For all FSRQs studied, we detect variations in r band flux of overall amplitude between 0.24 mag and 3.46 mag in different sources. Fourteen of 29 FSRQs display a bluer-when-brighter trend. However, only one source exhibits a redder-when-brighter trend, which implies that this behavior is rare in our FSRQ sample. In this source, the thermal emission from the accretion disk may be responsible for the redder-when-brighter trend.