Time lags of the flickering in cataclysmic variables as a function of wavelength

Abstract

Context. Flickering is a ubiquitous phenomenon in cataclysmic variables (CVs). Although the underlying light source is one of the main contributors to the optical radiation, the mechanism leading to flickering is not understood as yet. Aims. The present study aims to contribute to the set of boundary conditions, defined by observations, which must be met by physical models that describe the flickering. In particular, time lags in the occurrence of flickering events at different wavelengths over the optical range are examined. Methods. To this end, the cross-correlation functions (CCFs) of numerous light curves of a sample of CVs are analysed that were observed simultaneously or quasi-simultaneously in different bands of various photometric systems. Results. Deviations of the maxima of the CCFs from zero time-shift indicate a dependence of the flickering activity on the wavelength in the sense that flickering flares reach their maxima slightly earlier in the blue range than in the red. While the available observational material does not permit detecting this individually in all observed systems, the ensemble of all data clearly shows this effect. Particularly instructive are the cases of V603 Aql and TT Ari, where time lags of 15. s 1 and 4. s 3, respectively, are observed between the U and R bands. In principle this can be understood if during the development of a flickering flare the radiation characteristics of the light source responsible for flickering change such that in the early phases of a flare more short-wavelength radiation is emitted, and later on, the peak of the emission shifts to the red. Respective scenarios are discussed and shown to be in qualitative and quantitative agreement with observations.

XMM–Newton observation of MV Lyr and the sandwiched model confirmation

“…-100 to 100 14.6 ± 1.0 −0.3 ± 1.9 18.6 ± 1.3 -200 to 200 20.3 ± 1.2 5.0 ± 1.2 31.5 ± 1.7 -300 to 300 22.9 ± 1.2 3.7 ± 1.0 33.5 ± 0.9 -400 to 400 24.2 ± 1.2 1.3 ± 1.5 36.0 ± 1.2 UV and X-rays using a cross-correlation function (CCF) as described in Section 2 of Bruch (2015). Because of the noncontinuous nature of the OM light curve, we calculated a separate CCF for each of the 13 continuous OM light curves ( Fig.…”

“…Such penetration can also be understood as an expansion which approaches the eddy to the disc surface. The idea of an expanding and cooling sphere, the so-called fireball model, was proposed by Pearson et al (2005) and used by Bruch (2015) to explain a typical color delay in CVs flickering. This mechanism does not agree with the interpretation by Scaringi (2014) of L1 that the variability is generated by the corona, but it has also problems to explain the detected soft lag behind the hard X-ray.…”

“…Uttley et al 2002, Markowitz et al 2003, McHardy et al 2004). Flickering has three basic observational characteristics; 1) linear correlation between variability amplitude and lognormally distributed flux (so called rms-flux relation) observed in a variety of accreting systems such as Xray binaries or active galactic nuclei (Uttley et al 2005), CVs , Van de Sande et al 2015) and symbiotic systems (Zamanov et al 2015), 2) time lags where flares reach their maxima slightly earlier in the blue than in the red (Bruch 2015) and 3) red noise in power density spectra (PDS). The shape of such PDS can be a simple power law (see e.g.…”

Accretion disk coronae of intermediate polar cataclysmic variables

“…Observations suggest that in some systems flickering may arise in the turbulent inner region of the disk, or from the bright spots on the surface of the white dwarf (Hellier 2001). Actually, the analysis of eclipsing lightcurves (Horne & Stiening 1985) demonstrated that the flickering source must be concentrated in the inner portion of the accretion disk but it is not necessarily confined to the immediate vicinity of the white dwarf, possibly spreading out in the inner part of the accretion disk (Bruch 2015).…”

Using Gaussian processes to model light curves in the presence of flickering: the eclipsing cataclysmic variable ASASSN-14ag

“…This can be due to such systems containing a high amount of flickering, seen as random variability in CV light curves with amplitudes reaching the same order of magnitude as the bright spot eclipse features. Flickering in CVs is found to originate in both the bright spot and the inner accretion disc, and is due to the turbulent nature of the transferred material within the system (Bruch 2000(Bruch , 2015Baptista & Bortoletto 2004;Scaringi et al 2012;Scaringi 2014).…”