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The symbiotic recurrent nova V745 Sco is confirmed as a bright super-soft source.

ATel #5877; K. L. Page, J. P. Osborne, A. P. Beardmore (U. Leicester), K. Mukai (NASA & UMBC) and S. Starrfield (ASU)
on 11 Feb 2014; 20:57 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

Subjects: X-ray, Nova

Referred to by ATel #: 5897, 5920

On 2014-02-10, Swift-XRT data collected on the recurrent nova V745 Sco showed an increase in soft counts, suggesting the onset of the super-soft source phase (Page et al., ATel #5870). Further data collected approximately 24 hours later showed a dramatic increase in the source count rate, almost entirely due to counts below 1 keV. We therefore confirm that V745 Sco is now in the supersoft emission phase.

The 0.3-1 and 1-10 keV keV count rates for the past couple of days are tabulated below. T0 is taken as 2014 Feb 6.694 UT (AAVSO Special Notice 380).

UT Date Day since detection 0.3-1 keV XRT count s-1 1-10 keV XRT count s-1
2014-02-10T03:07 3.44 0.095 +/- 0.007 1.17 +/- 0.02
2014-02-10T12:27 3.83 0.48 +/- 0.02 1.36 +/- 0.03
2014-02-10T17:16 4.03 4.28 +/- 0.16 1.64 +/- 0.10
2014-02-10T22:03 4.23 9.54 +/- 0.09 1.52 +/- 0.04
2014-02-10T03:08 4.44 13.4 +/- 0.1 1.54 +/- 0.04
2014-02-10T08:02 4.64 25.7 +/- 0.2 1.73 +/- 0.04

Duerbeck (1989, ESO Messenger 58, 34) notes that in the 1989 outburst V745 Sco appears similar to RS Oph, but the optical spectrum evolves about three times faster. The super-soft phase in RS Oph was first detected 26 days after the optical eruption (Osborne et al., 2011, ApJ, 727, 124); the much earlier detection of the super-soft phase in V745 Sco at ~3 days suggests an even faster evolution.

The supersoft component of V745 Sco is detected up to E ~ 1 keV, suggestive of a high temperature, and, consistent with the rapid turn-on, that the white dwarf is very massive (see e.g. Wolf et al., 2013 ApJ 777, 136).

We thank the Swift PI and operations team for their support of these observations.