Abstract
V745 Sco is a recurrent nova, with the most recent eruption occurring in February 2014. V745 Sco was first observed by Swift a mere 3.7 hr after the announcement of the optical discovery, with the super-soft X-ray emission being detected around four days later and lasting for only ~two days, making it both the fastest follow-up of a nova by Swift and the earliest switch-on of super-soft emission yet detected. Such an early switch-on time suggests a combination of a very high velocity outflow and low ejected mass and, together with the high effective temperature reached by the super-soft emission, a high mass white dwarf (>1.3 M_sun). The X-ray spectral evolution was followed from an early epoch where shocked emission was evident, through the entirety of the super-soft phase, showing evolving column density, emission lines, absorption edges and thermal continuum temperature. UV grism data were also obtained throughout the super-soft interval, with the spectra showing mainly emission lines from lower ionization transitions and the Balmer continuum in emission. V745 Sco is compared with both V2491 Cyg (another nova with a very short super-soft phase) and M31N 2008-12a (the most rapidly recurring nova yet discovered). The longer recurrence time compared to M31N 2008-12a could be due to a lower mass accretion rate, although inclination of the system may also play a part. Nova V745 Sco (2014) revealed the fastest evolving super-soft source phase yet discovered, providing a detailed and informative dataset for study.
Highlights
Novae are thermonuclear explosions arising in interacting binary systems
It is noticeable that the rise in X-ray emission as the supersoft source began was monotonic, in contrast to the high-amplitude flux variability shown during this phase for some other novae well monitored by Swift (e.g. V458 Vul – Drake et al 2008, Ness et al 2009; KT Eri – Bode et al 2010, Beardmore et al 2010; RS Oph – Osborne et al 2011; LMC 2009a – Bode et al 2015)
Due to the speed of evolution of V745 Sco, no X-ray grating spectra were obtained during the Super-Soft Source (SSS) phase, meaning we cannot place firmer constraints on the ionization levels present at this time. The existence of these lines suggests that the white dwarf (WD) in the V745 Sco system may be of the ONeMg type, and unlikely to be a possible progenitor for a Type Ia supernova explosion
Summary
Novae are thermonuclear explosions arising in interacting binary systems. Material is transferred from the secondary star on to the white dwarf (WD) primary until the pressure and temperature at the. V745 Sco had previously been detected in outburst in 1937 (Plaut 1958) and 1989 (Schaefer 2010), with the International Ultraviolet Explorer (IUE) observing the 1989 eruption.. V745 Sco had previously been detected in outburst in 1937 (Plaut 1958) and 1989 (Schaefer 2010), with the International Ultraviolet Explorer (IUE) observing the 1989 eruption.2 This latest outburst in 2014 adds weight to the suggestion by Schaefer (2010) that there may have been an additional unobserved nova explosion around 1963, giving a recurrence duty cycle of ∼25 yr. Jean & Shore (2014b) reported a Fermi-LAT detection of V745 Sco, with high energy emission being found (at 2–3σ ) on 2014 February 6 and 7 (the day of outburst and the following day). The abundances from Wilms, Allen & McCray (2000) and photoelectric absorption cross-sections from Verner et al (1996) have been assumed for the X-ray spectral modelling
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