Microvariability in BL Lacertae: “Zooming” into the Innermost Blazar Regions

Abstract

<jats:p>In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for four nights in 2016. We studied the intranight flux and spectral variability of the source in detail with an objective to characterize microvariability in the blazars, a frequently observed phenomenon in blazars. The results show that the source often displays a fast flux variability with an amplitude as large as ~0.2 magnitude within a few hours, and that the color variability in the similar time scales can be characterized as “bluer-when-brighter” trend. We also observed markedly curved optical spectrum during one of the nights. Furthermore, the correlation between multi-band emission shows that in general the emission in all the bands are highly correlated; and in one of the nights V band emission was found to lead the I band emission by ~13 min. The search for characteristic timescale using z-transformed auto-correlation function and the structure function analyses reveals characteristic timescale of ~50 min in one of the R band observations. We try to explain the observed results in the context of the passage of shock waves through the relativistic outflows in blazars.</jats:p>

Detection of Periodic Radio Signal from the Blazar J1043+2408

“…In the figure, it can be clearly seen that, not only does the source display long-term variability, but it also shows periodic flux modulation, such that, in each cycle, the flux nearly doubles between minimum and maximum. To quantify the observed long-term variability, we estimated a fractional variability [30,31] Figure 1. ∼10.5-year-long 15 GHz observations of blazar J1043+2408 from OVRO.…”

“…To further confirm the presence of the above periodic timescale using a different method, we performed discrete correlation function analysis as described in Reference [40]. The method has been mostly applied to investigate cross-correlation between two time series with uneven spacing (see References [10][11][12]31]). With only one light curve, the method becomes a discrete autocorrelation function (ACF), which can be exploited to reveal the periodic signals in a light curve.…”

Signatures of the Disk–Jet Coupling in the Broad-line Radio Quasar 4C+74.26

“…The discrete cross-correlation function (DCF) is one of the widely used methods to estimate cross correlations between unevenly sampled light curves of astrophysical sources (Edelson & Krolik 1988; see also Bhatta & Webb 2018). In the case of 4C +74.26, we evaluated the DCFs between the optical and the radio, the X-ray and the optical, and the X-ray and the radio flux changes.…”

The Microvariable Activity of BL Lacertae

“…Indeed, this object has become the prototype for an entire class of AGN, so understanding its behavior is very important. Multiband microvariability observations have been made by others [8,13]. Indeed, it has been observed in particular by Bhatta and Webb [13] that when brighter, BL Lac becomes bluer, and furthermore that emission is highly correlated across the various optical bands.…”

“…Multiband microvariability observations have been made by others [8,13]. Indeed, it has been observed in particular by Bhatta and Webb [13] that when brighter, BL Lac becomes bluer, and furthermore that emission is highly correlated across the various optical bands. Our observations were made in only one filter, R, but from previous work [13], we may assume that our documented outbursts were also well represented at other wavelengths.…”