<i>XMM-Newton</i>and<i>Swift</i>observations of WZ Sagittae: spectral and timing analysis

Nucita, A. A.; Kuulkers, E.; Paolis, F. De; Mukai, K.; Ingrosso, G.; Maiolo, B.

doi:10.1051/0004-6361/201322875

Cited by 15 publications

(9 citation statements)

References 45 publications

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“…comm., publication in prep.). Similar X-ray periods were found in probably non-burning white dwarfs, for instance, a 33 s spin of AE Aqr (Patterson et al 1980), ∼29 s in WZ Sge (Nucita et al 2014, and references therein), or few-second quasi-periodic oscillations (QPOs) in a number of dwarf novae, e.g., SS Cyg, U Gem, (Cordova et al 1984); referred to as dwarf nova oscillations (DNOs), see for example, Mauche (2002), Warner et al (2003a) and references therein. Since the X-ray spectra of these systems reflect distinctly different production mechanisms from SSS spectra (e.g., accretion disc or the boundary layer), we do not include these CV systems in our study.…”

Section: Introductionsupporting

confidence: 77%

Short-period X-ray oscillations in super-soft novae and persistent super-soft sources

Ness

Beardmore

Osborne

et al. 2015

A&A

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Context. Transient short-period (<100 s) oscillations have been found in the X-ray light curves of three novae during their super-soft source (SSS) phase and in one persistent SSS. Aims. We pursue an observational approach to determine possible driving mechanisms and relations to fundamental system parameters such as the white dwarf mass. Methods. We performed a systematic search for short-period oscillations in all available XMM-Newton and Chandra X-ray light curves of persistent SSS and novae during their SSS phase. To study time evolution, we divided each light curve into short timesegments and computed power spectra. We then constructed a dynamic power spectrum from which we identified transient periodic signals even when only present for a short time. We base our confidence levels on simulations of false-alarm probability for the chosen oversampling rate of 16, corrected for multiple testing based on the number of time segments. From all time segments of each system, we computed fractions of time when periodic signals were detected. Results. In addition to the previously known systems with short-period oscillations, RS Oph (35 s), KT Eri (35 s), V339 Del (54 s), and Cal 83 (67 s), we found one additional system, LMC 2009a (33 s), and also confirm the 35 s period from Chandra data of KT Eri. The oscillation amplitudes are of about <15% of the respective count rates and vary without any clear dependence on the X-ray count rate. The fractions of the time when the respective periods were detected at 2σ significance (duty cycle) are 11.3%, 38.8%, 16.9%, 49.2%, and 18.7% for LMC 2009a, RS Oph, KT Eri, V339 Del, and Cal 83, respectively. The respective highest duty cycles found in a single observation are 38.1%, 74.5%, 61.4%, 67.8%, and 61.8%. Conclusions. Since fast rotation periods of the white dwarfs as origin of these transient oscillations are speculative, we concentrate on pulsation mechanisms. We present initial considerations predicting the oscillation period to scale linearly with the white dwarf radius (and thus mass), weakly with the pressure at the base, and luminosity. Estimates of the size of the white dwarf could be useful for determining whether these systems are more massive than typical white dwarfs, and thus whether they are growing from accretion over time. Signs of such mass growth may have implications for whether some of these systems are attractive as Type Ia supernova progenitors.

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Section: Introductionsupporting

confidence: 77%

Short-period X-ray oscillations in super-soft novae and persistent super-soft sources

Ness

Beardmore

Osborne

et al. 2015

A&A

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“…The primary mass required is twice the value determined by Smak (1993) and subsequently used by Hameury et al (1997), but the same value of the quiescent mass-transfer rate was deduced from optical observations by Smak (1993). There is little doubt that WZ Sge accretes in quiescence since it emits X-ray with a luminosity (1−2) × 10 30 erg s −1 (Nucita et al 2014).…”

Section: Wz Sgesupporting

confidence: 52%

Modelling rebrightenings, reflares, and echoes in dwarf nova outbursts

Hameury

Lasota

2021

A&A

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Context. The disc instability model (DIM) accounts well for most of the observed properties of dwarf novae and soft X-ray transients, but the rebrightenings, reflares, and echoes occurring at the end of outbursts or shortly after in WZ Sge stars or soft X-ray transients have not yet been convincingly explained by any model. Aims. We determine the additional ingredients that must be added to the DIM to account for the observed rebrightenings. Methods. We analyse in detail a recently discovered system, TCP J21040470+4631129, which has shown very peculiar rebrightenings. We also model the light curve of this system using our numerical code, including mass transfer variations from the secondary, inner–disc truncation, disc irradiation by a hot white dwarf and, in some cases, the mass-transfer stream over(under) flow. Results. We show that the luminosity in quiescence is dominated by a hot white dwarf that cools down on timescales of months. For a reason that remains elusive, the mass transfer rate from the secondary has to increase by several orders of magnitudes during the initial superoutburst. The mass transfer rate slowly returns to its secular average and causes the observed succession of outbursts with increasing quiescence durations until the disc can be steady, cold, and neutral; its inner parts are truncated either by the white dwarf magnetic field or by evaporation. The very short, quiescence phases between reflares are reproduced when the mass-transfer stream overflows the disc. Using similar additions to the DIM, we also produced light curves close to those observed in two WZ Sge stars, the prototype and EG Cnc. Conclusions. Our model successfully explains the reflares observed in WZ Sge systems. It requires, however, the inner disc truncation in dwarf novae to be due not only to the white dwarf magnetic field but, as in X-ray binaries, rather to evaporation of the inner disc. A similar model could also explain reflares observed in soft X-ray transients.

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“…Thus, 10 years after the superoutburst, the object has not yet returned to its pre-outburst level. In contrast to GW Lib, the observed quiescent X-ray flux of the prototype system WZ Sge showed similar values before and 10 years after its most recent 2001 superoutburst (Nucita et al 2014).…”

Section: Comparison With Gw Libmentioning

confidence: 74%

Superhumps linked to X-ray emission

Neustroev

Page

Kuulkers

et al. 2018

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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.

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XMM-NewtonandSwiftobservations of WZ Sagittae: spectral and timing analysis

Cited by 15 publications

References 45 publications

Short-period X-ray oscillations in super-soft novae and persistent super-soft sources

Short-period X-ray oscillations in super-soft novae and persistent super-soft sources

Modelling rebrightenings, reflares, and echoes in dwarf nova outbursts

Superhumps linked to X-ray emission

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