Swift detection of the super-swift switch-on of the super-soft phase in nova V745 Sco (2014)

TL;DR

This paper reports the earliest detection and rapid evolution of the super-soft X-ray phase in nova V745 Sco (2014), revealing insights into the white dwarf's mass and outflow characteristics through Swift observations.

Contribution

It presents the fastest follow-up and earliest super-soft phase detection in a nova, with detailed spectral evolution analysis and comparison to other rapid novae.

Findings

Super-soft emission detected ~4 days after eruption

Super-soft phase lasted only ~2 days

Indicates a high-mass white dwarf (>1.3 M_sun)

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.