Unusual optical quiescence of the classical BL Lac object PKS 0735+178 on intranight time scale

TL;DR

This study presents extensive intranight optical monitoring of BL Lac object PKS 0735+178 over 11 years, revealing an unusual long-term quiescence in optical variability despite expectations of frequent variability in such objects.

Contribution

It provides the first long-term evidence of persistent optical quiescence in a classical BL Lac object, challenging existing models of BL Lac variability behavior.

Findings

No intra-night optical variability ≥3% observed in 17 nights over 11 years.

Confirmed long-term optical quiescence back to 1989, even during radio outbursts.

Demonstrates unusual stability in optical emission of PKS 0735+178.

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 $\sim 4$ hours, on 17 nights spanning 11 years (1998-2008). Using the CCD as an N-star photometer, a detection limit of around 1% was attained for the intra-night optical variability (INOV). Remarkably, an INOV amplitude of $\geq 3%$ 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 $\ga 4$ hours duration is known to be high ($\sim50%$). 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 Moreover, the access to unpublished optical monitoring data of similarly high sensitivity, acquired in another programme, has allowed us to confirm the same anomalous INOV quiescence of this LBL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.