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Swift observations of ASSASN-16oh

ATel #9866; Thomas J Maccarone (Texas Tech), Peter Brown (Texas A&M), Koji Mukai (NASA GSFC)
on 16 Dec 2016; 02:25 UT
Credential Certification: Tom Maccarone (thomas.maccarone@ttu.edu)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Cataclysmic Variable, Nova, Transient

Referred to by ATel #: 9867

We report on Swift observations of ASSASN-16oh. This is a transient near the Small Magellanic Cloud that was first reported in ATel #9859. Observations began 2016-12-15 10:25:45 UT. The following UVOT magnitudes were measured: v = 16.8 +/- 0.1 (79 s exposure time), b = 16.9 +/- 0.1 (78 s), u = 15.5 +/- 0.1 (79 s), uvw1 = 15.0 +/- 0.1 (157 s), uvm2 = 14.9 +/- 0.1 (247 s), and uvw2 = 14.9 +/- 0.1 (315 s). These magnitudes are reduced using the method outlined for the Swift Optical Ultraviolet Supernova Archive (SOUSA; Brown et al 2014, Ap&SS, 354, 89) on the Swift/UVOT Vega-based photometric system (Poole et al. 2008, Breeveld et al. 2011). The X-ray data were also analyzed. The data fit to a blackbody spectrum of kT = 0.08 keV with foreground N_H = 5.2e20. The absorbed flux from this model is 5.2e-12 cgs from 0.1 to 8 keV. The unabsorbed flux is 2e-11 cgs, with considerable uncertainty due to lack of tight constraints on N_H. The redshift of the object, large for the Galaxy, but consistent with being in the SMC, and the high ratio of X-rays to optical make this object unlikely to be a foreground CV. At SMC distances the intrinsic X-ray luminosity is about 10^37 erg/sec. The candidate precursor reported in Atel 9859 is then has properties consistent with those of an early A star,using the Mamajek tables at (http://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt). The X-ray properties are then very reasonable for a classical nova. On the other hand, the optical brightness of the transient event is extremely faint relative to most classical novae, and its blue color means it is unlikely to be extincted by many mags. Additionally, the reported upper limits on the optical line widths, in ATel 9859, being unresolved at R~1000, are narrowed than those of a large fraction of classical novae. Li et al. 2012, ApJ, 761, 99 discussed an object that showed some similarities to the object considered here. Li et al reported on a Be+WD binary, MAXI J0158-744. MAXI J0158-744 was about 1.5 magnitudes brighter at peak than this object, and much brighter in quiescence, due to the early B counterpart. If one wishes to apply the same scenario here, the question is why there is not a classical nova event seen as the system turns on as a fusion-powered source. A rather contrived, but not altogether unreasonable, possibility is that the object is undergoing steady nuclear fusion on the surface of a white dwarf, due to a rapid turn-on of mass transfer. If the mass donor is the A star, then it is more massive than the white dwarf, and mass transfer may be expected to be unstable, proceeding on a timescale of order the thermal timescale of the star's, which would yield a mass transfer rate of order a few * 10^-7 solar masses per year, which is in the regime in which steady nuclear fusion can occur on a white dwarf without driving strong outflows (Kato 2010, AN, 331, 140). The rather large questions in this scenario, which we leave for later work, are how likely it would be for one of the many present-day transient surveys to catch the turn-on of mass transfer in such a system at exactly the right time; and whether the turn-on really would be so abrupt as to transition the source from an unremarkable A star+WD binary into one with a hyperaccreting white dwarf, so that there was not enough accretion to cause a classical nova as the turn-on took place.