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Nova V745 Sco shows fastest rise of super-soft X-ray emission: 3-4 days after eruption

ATel #5870; K. L. Page, J. P. Osborne, A. P. Beardmore (U. Leicester) and K. Mukai (NASA & UMBC)
on 10 Feb 2014; 17:27 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

Subjects: X-ray, Nova

Referred to by ATel #: 5874, 5877, 6565

Swift started monitoring the recurrent nova V745 Sco on 2014-02-06, 0.16 days after the discovery of the most recent eruption (R. Stubbings, AAVSO Special Notice 380, CBET 3803). Mukai et al. reported the initial Swift observations in ATel #5862, finding that the X-ray emission was hard and highly absorbed, with the absorbing column decreasing with time, resulting in a corresponding increase in count rate.

Swift has continued following the nova, performing observations approximately every 5-6 hours. Data obtained on 2014-02-10 between about 03:00 and 03:30 UT show a highly significant excess of soft counts below ~1 keV, amounting to 0.03 count s-1, quite distinct from the harder emission. This suggests the onset of the super-soft X-ray phase. The previous observation (2014-02-09 at 22:18 UT) may also have shown a very slight excess in emission at these low energies. Earlier observations (up to and including 2014-02-09 at 15:55 UT) showed no evidence for this soft excess; the hard component has remained largely unchanged, except for the effect of decreasing NH.

If these observations signal the start of the super-soft X-ray phase, then such an early turn-on time (~3 days after discovery; the outburst could have begun up to 1 day earlier) suggests very high velocity ejecta (Schwarz et al., 2011, ApJS, 197, 31; see also ATel #5865), and thus a very high mass white dwarf. We believe this is the earliest nova super-soft turn-on yet observed, either within our own Galaxy or in M31 (e.g., Schwarz et al., 2011; Henze et al., 2011, A&A, 533, 52; Henze et al., 2014, arXiv:1401.2904).

We are continuing to monitor V745 Sco in both X-rays and UV, and thank the Swift PI and mission operations team for their ongoing support.