Super-soft X-ray emission on day 6.4 from nova LMC N1968-12a strongly suggests a very high mass white dwarf
ATel #8615; K. L. Page, J. P. Osborne (U. Leicester), M. J. Darnley (LJMU), M. Henze (IEEC/CSIC), N. P.M. Kuin (MSSL/UCL), G. J. Schwarz (AAS), S. N. Shore (Universita di Pisa), S. Starrfield (ASU) & S. C. Williams (U. Lancaster)
on 30 Jan 2016; 21:22 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
Subjects: Ultra-Violet, X-ray, Nova
Referred to by ATel #: 8616
Swift observations of the 2016 outburst of LMCN 1968-12a (ATel #8578,
#8586, #8587) have been continuing daily. The early XRT spectrum, covering
2016 Jan 23 - 25 (days 2.07-4.26 after outburst), can be modelled by a
single temperature optically thin emission component of kT > 0.9 keV
and NH = (8.3 +21.7/-7.8)x1021 cm-2, with a mean
0.3-10 keV observed flux of (5.1 +8.2/-2.3)x10-13 erg
cm-2 s-1.
After this time, an increase in soft emission (counts below ~1 keV) was
seen, although at first this appeared to be related to a decrease in the
absorbing column; no corresponding increase in X-ray count rate was found,
remaining around 0.016 count s-1.
On 2016 Jan 27 (day 6.4), the X-ray count rate rose to 0.05 count
s-1 with the emission being dominated by counts below 1 keV. We
identify this as the start of the super-soft source (SSS) phase. On 2016
Jan 30 (day 9.3) the nova was at 0.14 ± 0.01 count s-1. This
spectrum can be fitted with a BB kT = 64 +14/-12 eV and NH = (2.8
+2.1/-1.5)x1021 cm-2, providing an observed flux
measurement of (3.3 +0.6/-0.5)x10-12 erg cm-2
s-1. There is no requirement for an additional optically thin
thermal model component at this time. By 2016 Jan 30 the uvw1 magnitude had faded to ~15.
This early discovery of soft emission on day 6.4 is comparable to the switch-on of the SSS in V745 Sco (at ~4 days after outburst; Page et al. 2015) and M31N 2008-12a (at ~6 days after outburst; Henze et al. 2015), and suggests a high white dwarf mass. However, this system may best be compared to U Sco, which, during the 2010 outburst, showed a SSS turn-on between days 7.6 and 12
(Pagnotta et al. 2015).
We encourage ground-based follow-up, including spectroscopic observations,
of this recurrent nova in outburst.
We thank the Swift PI, Neil Gehrels, for approving the observations, and
the Swift planning and operations teams for their ongoing support.
