Probing disk truncation in Dwarf Novae and CVs with X-ray variations and comparisons with work on other wavelengths

Abstract

Flicker noise and its variations in accreting systems have been a diagnostic tool in understanding the structure in accretion disks. I study the nature of time variability of brightness of non-magnetic cataclysmic variables. DN systems demonstrate band limited noise in the UV and X-ray energy bands, which can be adequately explained in the framework of the model of propagating fluctuations. The detected frequency breaks in the range (1-6) mHz indicates an optically thick disk truncation in the inner disk of some dwarf novae systems. Analysis of other available data (SS Cyg, SU UMa and WZ Sge) indicate that during the outburst the inner disk radius moves towards the white dwarf and receeds as the outburst declines to quiescence. Cross-correlations between the simultaneous UV and X-ray light curves show time lags in the X-rays of 96-181 sec consistent with the travel time of matter from a truncated inner optically thick disk to the white dwarf surface. All this suggests that DN and other plausible nonmagnetic systems may have optically thick disk truncation where accretion may occur through coronal/hot flows in the inner disk region. I compare magnetic and nonmagnetic systems in terms of their broadband noise characteristics and summarize findings in other types of nonmagnetic CVs which in general show compliance with the model of propagating fluctuations. Finally, I discuss comparisons with X-ray binaries.

Accretion flows in nonmagnetic white dwarf binaries as observed in X-rays

“…Nonmagnetic CV PDS and broadband noise resembles PDS of high accretion rate states of BH/NS systems.The main PDS characteristics of nonmagnetic CVs indicate that regardless of the type of DN (and perhaps high state CVs), X-ray broad-band noise in quiescence and its evolution in outburst are similar, which is a result of the properties of the inner advective hot flow region. Narayan and Popham (1993) show that the optically thin BLs of accreting WDs in CVs can be radially extended and that they advect part of the viscously dissipated energy as a result of their inability to cool, therefore point out that optically thin BLs can act as ADAF-like hot accretion flows (see Balman, 2014b, for a more detailed discussion). However, no theoretical model has been attempted to derive characteristics of BL regions that are merged with advective hot flows for WDs.…”

“…The truncation radii for DN are calculated in a range ∼(3-10)×10 9 cm including errors (see Table 2 in Balman and Revnivtsev, 2012). Balman (2014bBalman ( , 2015Balman ( , 2019 presents PDS analysis of four more DNe with relevant break frequencies (see Figure 4 and Table 1).…”

THESEUS Science on Time-domain Studies of Compact Object Binaries

“…Compact Object Sciences with THESEUS Şölen Balman controversies and complexities can be explained in the context of advective hot flows and dominating hard X-ray emission in quiescence, outbursts and high states [8,22,23, and references therein].…”