Rapid optical variability of TeV blazars

Abstract

In this first systematic attempt to characterise the intranight optical\nvariability (INOV) of TeV detected blazars, we have monitored a well defined\nset of 9 TeV blazars on total 26 nights during 2004-2010. In this R (or V)-band\nmonitoring programme only one blazar was monitored per night for a minimum\nduration of 4 hours. Using the CCD, an INOV detection threshold of ~ 1-2 % was\nachieved in the densely sampled DLCs. We have further expanded the sample by\nincluding another 13 TeV blazars from literature. This enlarged sample of 22\nTeV blazars, monitored on a total of 116 nights (including 55 nights newly\nreported here), has enabled us to arrive at the first estimate of the INOV duty\ncycle of TeV detected blazars. Applying the C-test, the INOV DC is found to be\n59 %, which decreases to 47 % if only INOV fractional amplitudes above 3 % are\nconsidered. These observations also permit, for the first time, a comparison of\nthe INOV characteristics of the two major subclasses of TeV detected BL Lacs,\nnamely LBLs and HBLs, for which we find the INOV DCs to be ~ 63 % and ~ 38 %,\nrespectively. This demonstrates that the INOV differential between LBLs and\nHBLs persists even when only their TeV detected subsets are considered. Despite\ndense sampling, the intranight light curves of the 22 TeV blazars have not\nrevealed even a single feature on time scale substantially shorter than 1 hour,\neven though the inner jets of TeV blazars are believed to have exceptionally\nlarge bulk Lorentz factors (and correspondingly stronger time compression). An\nintriguing feature, clearly detected in the light curve of the HBL J1555+1111,\nis a 4 per cent `dip' on a 1 hour timescale. This unique feature could have\narisen from absorption in a dusty gas cloud, occulting a superluminally moving\noptical knot in the parsec scale jet of this relatively luminous BL Lacs\nobject.\n