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

Abstract

While there are a lot of investigations on the optical variability of flat-spectrum radio quasars (FSRQs), not much work has been done on the optical variability of steep-spectrum radio quasars (SSRQs). We investigate the optical variability of 18 SSRQs and 15 FSRQs in the SDSS Stripe 82 region using SDSS DR7 released multi-epoch data covering about nine years. For all SSRQs studied, we detect variations in r band flux of overall amplitude between 0.22 mag and 0.92 mag in different sources. Eight of 18 SSRQs display a bluer-when-brighter (BWB) trend. In contrast, the variability amplitude of 15 FSRQs in r band ranges from 0.18 to 0.97 mag, and 11 of 15 sources show a BWB trend. We found an anti-correlation between the Eddington ratio and the variability amplitude in r band for SSRQs, which is similar to that in radio quiet AGNs. This implies that the thermal emission from accretion disk may be responsible for the variability in SSRQs, although the jet nonthermal emission cannot be excluded. In contrast, there is no correlation in FSRQs, which implies that the mechanisms of variability in FSRQs may be different from that in SSRQs. The optical continuum variability of radio loud broad absorption line quasars (BALQs) are investigated for the first time here on two sources with steep radio spectrum. Both radio loud BALQs show variations with amplitude of about 0.3 mag at r band. Their spectral variability all show a BWB trend. In a combined sample (18 SSRQs, and 44 FSRQs) of our sample with the FSRQs in Gu & Ai (2011), we found a trend of a broader line width of broad Mg II emission line with steeper radio spectral index. It implies a disc-like broad line region (BLR) geometry may present in these quasars. In these 62 quasars, we found a ~ 57% source percentage showing BWB trend in FSRQs, whereas it is ~ 44% in SSRQs.