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

Abstract

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.

Classical nova V339 Del (Nova Del 2013) - a short review

“…Large amplitude variations can be explained either by clumps in the ejecta, or WD photosphere temperature variations. QPO with a periodicity of 54 s, later confirmed by Newton-XMM X-ray observations for day 97 AM [24], were also present in the data and interpreted by [25] as non radial g-mode pulsations of the WD. The high resolution X-ray spectrum (22-44 Å range) taken by the Chandra observatory on day 86 revealed a rich system of absorption lines superimposed on a supersoft source continuum without any emission lines.…”

The Supersoft X-Ray Transient ASASSN-16oh as a Thermonuclear Runaway without Mass Ejection

“…We examined the Chandra zero-order light curves measured with the HRC-S camera in the exposure described in Section 2, Maccarone et al (2019), because thermonuclear burning SSS seem to be associated either with phenomena of short pulsations lasting around a minute or less (see Ness et al 2015, and references therein), or with longer periodicities of the order of half an hour. For the latter, examples are V1494 Aql (Drake et al 2003), V4743 Sgr (Leibowitz et al 2006;Zemko et al 2016;Dobrotka & Ness 2017), V2491 Cyg (Ness et al 2011;Zemko et al 2015), V959 Mon (Peretz et al 2016).…”

Multiwavelength observations of nova SMCN 2016-10a – one of the brightest novae ever observed

“…A number of novae with bright X-ray emission have shown short period (< 100 s) oscillations (e.g. Osborne et al 2011;Beardmore et al 2010;Ness et al 2015). We have therefore searched for the presence of such oscillations in the soft X-ray light-curves of nova SMCN 2016-10a, following the procedure outlined in Beardmore et.…”