Ten-year optical monitoring of PKS 0735+178: historical comparison, multiband behavior, and variability timescales

Abstract

Aims. New data and results on the optical behaviour of the prominent blazar PKS 0735+178 (also know as OI 158, S3 0735+17, DA 237, 1ES 0735+178, 3EG J0737+1721) are presented, through the most continuous BV RI data available in the period 1994-2004 (about 500 nights of observations). In addition, the whole historical light curve, and a new photometric calibration of comparison stars in the field of this source are reported. Methods. Several methods for timeseries analysis of sparse data sets are developed, adapted and applied to the reconstructed historical light curve and to each observing season of our unpublished optical database on PKS 0735+178. Optical spectral indexes are calculated from the multi-band observations and studied on long-term (years) durations as well. For the first time in this source, variability modes, characteristic timescales and the signal power spectrum are explored and identified over 3 decades in time with a sufficient statistics. The novel investigation of mid-term optical scales (days, weeks), could be also applied and compared to blazar gamma-ray light curves that will be provided, on the same timescales, by the forthcoming GLAST observatory. Results. In the last 10 years the optical emission of PKS 0735+178 exhibited a rather achromatic behaviour and a variability mode resembling the shot-noise. The source was in a intermediate or low brightness level, showing a mild flaring activity and a superimposition/succession of rapid and slower flares, without extraordinary and isolated outbursts but, at any rate, characterized by one major active phase in 2001. Several mid-term scales of variability were found, the more common falling into duration intervals of about 27-28 days, 50-56 days and 76-79 days. Rapid variability in the historical light curve appears to be modulated by a general, slower and rather oscillating temporal trend, where typical amplitudes of about 4.5, 8.5 and 11-13 years can be identified. This spectral and temporal analysis, accompanying our data publication, suggests the occurrence of distinctive signatures at mid-term durations that can likely be of transitory nature. On the other hand the possible pseudo-cyclical or multi-component modulations at long times could be more stable, recurrent and correlated to the bimodal radio flux behaviour and the twisted radio structure observed by several years in this blazar.

Redshift constraints for RGB 0136+391 and PKS 0735+178 from deep optical imaging

“…In December 2008 the R-band brightness of RGB 0136+391 was observed to be close to the lowest level since the beginning of our monitoring (fall 2002) and in January 2011 the brightness of PKS 0735+178 reached R = 16.7, a very low level considering the long-term historical light curve (Ciprini et al 2007). Observations were promptly triggered and performed at the NOT and the results are reported in this paper.…”

“…Therefore, the background of PKS 0735+178 and its point spread function (PSF) stars was subtracted by fitting a two-dimensional linear slope to the sky pixels surrounding the targets, effectively removing any remaining tilt in the background. Calibration of the field was done through star 5, for which Ciprini et al (2007) give I = 15.12 ± 0.06. This star was unsaturated in several exposures, which were used to derive I-band magnitudes for stars 1−3, which were then used to determine the zero point of the coadded image.…”

Understanding BL Lacertae objects

“…7. The observed morphology of 0735+178 is indicated by filled squares and ellipses, corresponding to the VLBI observations at 8.4 GHz (1990-1998) and at 15 GHz (2000-2007. The evolution of the trajectory of hotspots (hollowed square) from 1990 to 2009 is reproduced in terms of the non-ballistic model.…”

“…The BL Lac 0735+178 is known for its pronounced flux variability in both optical and radio bands (Ciprini et al 2007;Goyal et al 2009). The origin of this variability, however, is still debated.…”

“…Periodic features of 13.8-14.2 years were found by applying the Jurkevich method to about a century long optical monitoring data (Fan et al 1997;Qian & Tao 2004). A&A 515, A105 (2010) Ciprini et al (2007) reported possible detection of several shorter time scales. In their search for intra-night optical variability on 17 nights between 1998 and 2008, Goyal et al (2009) did not detect on any occasion intra-night variability with an amplitude ≥3%, though such amplitudes are typical for radio selected BL Lacs (Gopal-Krishna et al 2003).…”