Abstract:
We present the result of our extensive intranight optical monitoring of the well-known low-
energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of
R-band monitoring for a minimum duration of ∼4 hours, on 17 nights spanning 11 years
(1998–2008). Using the CCD as an N-star photometer, a detection limit of around 1 per cent
was attained for the intranight optical variability (INOV). Remarkably, an INOV amplitude
of ≥3 per cent on hour-like time-scale was not observed on any of the 17 nights, even
though the likelihood of a typical LBL showing such INOV levels in a single session of
4 hours duration is known to be high (∼50 per cent). Our observations have thus established
a peculiar long-term INOV quiescence of this radio-selected BL Lac object. Moreover, the
access to unpublished optical monitoring data of similarly high sensitivity, acquired in another
programme, has allowed us to confirm the same anomalous INOV quiescence of this LBL all
the way back to 1989, the epoch of its historically largest radio outburst. Here, we present
observational evidence revealing the very unusual INOV behaviour of this classical BL Lac object and discuss this briefly in the context of its other known exceptional properties.
