Superhumps linked to X-ray emission

Abstract

The superoutbursts of SSS J122221.7−311525 and GW Lib. ABSTRACTContext. We present more than 4 years of Swift X-ray observations of the 2013 superoutburst, subsequent decline and quiescence of the WZ Sge-type dwarf nova SSS J122221.7−311525 (SSS J122222) from 6 days after discovery. Aims. Only a handful of WZ Sge-type dwarf novae have been observed in X-rays, and until recently GW Lib was the only binary of this type with complete coverage of an X-ray light curve throughout a superoutburst. We collected extensive X-ray data of a second such system to understand the extent to which the unexpected properties of GW Lib are common to the WZ Sge class. Methods. We collected 60 Swift-XRT observations of SSS J122222 between 2013 January 6 and 2013 July 1. Four follow-up observations were performed in 2014, 2015, 2016 and 2017. The total exposure time of our observations is 86.6 ks. We analysed the X-ray light curve and compared it with the behaviour of superhumps which were detected in the optical light curve. We also performed spectral analysis of the data. The results were compared with the properties of GW Lib, for which new X-ray observations were also obtained. Results. SSS J122222 was variable and around five times brighter in 0.3-10 keV X-rays during the superoutburst than in quiescence, mainly because of a significant strengthening of a high-energy component of the X-ray spectrum. The post-outburst decline of the X-ray flux lasted at least 500 d. The data show no evidence of the expected optically thick boundary layer in the system during the outburst. SSS J122222 also exhibited a sudden X-ray flux change in the middle of the superoutburst, which occurred exactly at the time of the superhump stage transition. A similar X-ray behaviour was also detected in GW Lib. Conclusions. We show that the X-ray flux exhibits changes at the times of changes in the superhump behaviour of both SSS J122222 and GW Lib. This result demonstrates a relationship between the outer disc and the white dwarf boundary layer for the first time, and suggests that models for accretion discs in high mass ratio accreting binaries are currently incomplete. The very long decline to X-ray quiescence is also in strong contrast to the expectation of low viscosity in the disc after outburst.

From outburst to quiescence: spectroscopic evolution of V1838 Aql imbedded in a bow-shock nebula

“…1). This pattern -the outburst flux is higher than in quiescence with a deep dip during the outburst rapid fadingis in contrast to ordinary dwarf novae which usually show a depression of the X-ray flux during outbursts, but is in agreement with X-ray observations of WZ Sge-type stars (see Neustroev et al 2018, and references therein).…”

“…The plateau-, decline-stage, and quiescent spectra of V1838 Aql consist of only 58, 6, and 12 counts, respectively, therefore no meaningful spectral analysis is possible. Nevertheless, assuming that the spectrum of V1838 Aql is similar to other WZ Sge-type stars such as SSS J122221.7−311525 and GW Lib (Neustroev et al 2018), and using the countrate as the scale-factor, one can estimate the X-ray flux of V1838 Aql in the 0.3-10 keV energy range during the superoutburst plateau and in quiescence to be ∼4.5×10 −13 and ∼2.3×10 −13 erg s −1 cm −2 , respectively, with the correspond- ing luminosity of ∼2.3×10 30 and ∼1.2×10 30 erg s −1 (adopting the distance of 202 pc, see Section 3.1). These luminosities are in agreement with those found for other WZ Sge-type stars and accreting white dwarfs (Reis et al 2013;Neustroev et al 2018).…”

Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. X: The Tenth Year (2017)

“…The refined relation was shown in Kato et al (2016b) and Kato et al (2017a) [we consider that Kato et al (2017a) to be the final regular summary of the statistics]. The stage A, B and C are now well-established and used in many publications by various authors: Katysheva et al (2014Katysheva et al ( ), B ąkowska et al (2014, Sklyanov et al (2016), Neustroev et al (2017), Ba ¸kowska et al (2017), Neustroev et al (2018), Sklyanov et al (2018), Littlefield et al (2018), Pala et al (2018), Pala et al (2019), Pavlenko et al (2019), McAllister et al (2019), Court et al (2019) (this work also illustrates the difficulty in determining superhump times in a deeply eclipsing system). The rapid growth of papers referring to our superhump stages indicates that this concept and application are now widely accepted in this field.…”

Spectroscopic Observations of V455 Andromedae Superoutburst in 2007: the Most Exotic Spectral Features in Dwarf Nova Outbursts

“…The evolution of early and ordinary superhumps are well established in time-resolved photometric studies (e.g., Kato et al 2009 and following series). Moreover, Neustroev et al (2018) found that the X-ray flux in DN outbursts is correlated with the superhump stages, and Niijima et al (2021) showed that the X-ray hard-soft transition in low-mass black hole X-ray binary ASASSN-18ey (=MAXI J1820+070) occurred simultaneously with the transition of optical superhump stages. Although the spectra during the outbursts of WZ Sgetype DNe dramatically changes as well (e.g., WZ Sge: Iijima 2004, GW Lib: Hiroi et al 2009), an association between the evolution of superhumps and the spectra has been less discussed.…”