Violent intranight optical variability of the blazar S4 0954+65 during its unprecedented 2015 February outburst

Abstract

In this letter we present results from intra-night monitoring in three colors (BRI) of the blazar S4 0954+65 during its recent (2015 February) unprecedented high state. We find violent variations on very short time scales, reaching magnitude change levels of 0.1-0.2 mag/h. On some occasions, changes of ∼0.1 mag are observed even within ∼10 min. During the night of 14.02.2015 an exponential drop of ∼0.7 magnitudes is detected for about 5 hours. Cross-correlation between the light curves does not reveal any detectable wavelength-dependent time delays, larger than ∼5 min. Color changes "bluer-when-brighter" are observed on longer time scales. Possible variability mechanisms to explain the observations are discussed and a preference to the geometrical one is given.

Optical Variability of TeV Blazars on long time-scales

“…The source was in a low optical state during the period MJD 6400-7000, with the mean magnitude of this source during this period ∼17. After this period, the source flux increased significantly and reached up to an R magnitude of ∼ 13 as was also reported by Bachev (2015). Until MJD=7600 this source displayed many flare-like events.…”

“…The flatter slope of the colour-magnitude diagram could be revealing the presence of multiple variable components, each evolving differently. Hence, superposition of different spectral slopes from multiple components leads to a general reduction of the colour-magnitude correlations (Bonning et al 2012, Bachev 2015. It has been found in previous studies that the slope of the optical spectrum is often only weakly sensitive to the long-term trends while strictly following the bluer when brighter trends on short timescales (Villata et al 2002;Larionov et al 2010;Gaur et al 2012Gaur et al , 2015a.…”

“…S5 0954+658 is a BL Lac object with a claimed redshift of z = 0.367 (Stickel et al 1993); however, Landoni et al (2015) found a featureless spectrum and a limit of z > 0.45, which is based on non-detection of the host galaxy. This source is optically active and its optical variability was first studied by Bachev (2015) presented the intra-night monitoring of the blazar during February 2015 and found violent variations on very short timescales of ∼ 0.1 mag within ∼ 10 min.…”

Multi-Wavelength Intra-Day Variability and Quasi-Periodic Oscillation in Blazars

“…Indian groups also performed extensive searches for optical/IR IDV in blazars with collaborators around the globe (e.g., [11,20,23,24,26,27,[29][30][31][32][38][39][40][89][90][91][92][93][94]; and the references therein). There are some other collaborations world wide that are performing searches for optical/NIR IDV of blazars (e.g., [19,[95][96][97][98][99][100][101][102][103][104][105][106][107][108][109]; and the references therein). The important results reported in these papers are: (i) LBLs and IBLs show large amplitude IDV with high duty cycles; (ii) HBLs either do not show IDV or if they do, the amplitude of variability and the duty cycle of IDV detection are less compared to LBLs and IBLs; (iii) cross-correlation analysis in different optical bands on IDV timescales in general shows strong correlation with zero lag, which implies that the emissions in different optical bands are co-spatial, but occasionally, a few minute time lags are also reported; (iv) optical color variation on IDV timescales shows a range of properties, e.g.…”

Radiografía X del cielo γ

“…En el rango óptico, los blazares de todas las clases muestran el mismo comportamiento: son variables en la escala de años (Gaur et al, 2019), semanas (Marchesini et al, 2016b), o incluso minutos (Cellone et al, 2007;Bachev, 2015). Una situación similar ocurre cuando se analiza su polarización (Andruchow et al, 2003(Andruchow et al, , 2005.…”