Statistics of optical intraday variability in a complete sample of radio-selected BL Lac objects

TL;DR

This study analyzes the intraday variability of radio-selected BL Lac objects, revealing high duty cycles and typical variability timescales of 0.5 to 3 days, with some observations challenging standard models.

Contribution

It provides the first comprehensive statistical analysis of intraday variability in a complete radio-selected BL Lac sample, highlighting high duty cycles and variability timescales.

Findings

82% of BL Lac objects showed intraday variability

Typical variability amplitudes reached 30%

Most variability timescales ranged from 0.5 to 3 days

Abstract

We present a study of the intraday variability behaviour of a complete sample of radio-selected BL Lac objects taken from the 1 Jy catalogue. In 28 out of 34 BL Lac objects (82\%) we detected intraday variability. 7 objects were observed during several campaigns. None of them changed its variability behaviour. The duty cycle in radio-loud BL Lac objects is very high, i.e. at least 0.8. The typical peak-to-peak amplitudes of the variability were 30\%. By means of structure function and autocorrelation analyses we investigated the typical time-scales of the variability and determined the activity of the BL Lac objects. In 21 of the variable BL Lac objects we were able to measure a typical time-scale, which lies in the range between 0.5 and 3 days. In general, the results can be explained by the standard model, where shocks are propagating down a relativistic jet. However, in the two most densely sampled observations we found variability characteristics, which cannot easily be explained by the standard model alone, demonstrating that at least in these objects alternative models should be taken into account. Nevertheless, all models, which are able to explain variability on time-scales of days must be able to consider the high duty cycle of these BL Lac objects.