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Swift observation of the LMC nova candidate MASTER OT J051032.58-692130.4

ATel #9048; Kirill Sokolovsky (IAASARS NOA/SAI MSU/ASC Lebedev)
on 13 May 2016; 16:47 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

Subjects: Optical, Ultra-Violet, X-ray, Nova, Star, Transient, Variables

The optical transient MASTER OT J051032.58-692130.4 was discovered by MASTER-SAAO on 2016-05-10.72797 (ATel #9039). Pre-discovery images by OGLE-IV indicate that the outburst occurred some time between 2016-04-21.00403 and 28.97626 (ATel #9040). The object was observed by Swift for 2ks starting on 2016-05-12.07560 UT.

The transient is clearly visible in Swift/UVOT images at the position 05:10:32.58 -69:21:30.5 +/-0.1" J2000 measured with respect to UCAC4 stars within the UVOT field of view. The object's observed Vega magnitudes in three ultraviolet bands are white
JD(TT)2457520.59876 W1 11.52 +/-0.02
JD(TT)2457520.59166 M2 11.68 +/-0.02
JD(TT)2457520.58271 W2 11.76 +/-0.02
corresponding to a T~10^4 K black body after correcting for the Galactic extinction.

No X-ray source is detected by Swift/XRT at the position of the optical/ultraviolet transient above the background level of 2.6e-03 cts/s. Assuming kT=5 keV thermal bremsstrahlung emission and absorption with HI column density of 2e21 cm^-2, this corresponds to a 0.3-10 keV flux limit of 1.2e-13 ergs/cm^2/s.

The intriguing feature of the transient is its suggested OGLE-IV progenitor reportedly showing a 2.65 d eclipse-like periodicity (ATel #9040). Such period is unusually long for a classical nova system and makes one suspect a nova in a symbiotic binary or even a V1309 Sco-like stellar merger event. An object LMC_SC10_20872 offset by 0.4" from the reported OGLE-IV position and 0.6" from the UVOT position is present in the public OGLE-II photometric database, but a period search reveals no obvious periodicity in its lightcurve. The relation between the OGLE-II and OGLE-IV objects is unclear as this is a crowded field. Apart from the suggested eclipsing binary progenitor, observed properties of the transient so far are consistent with a classical nova outburst.

We thank Neil Gehrels and the Swift team for rapid approval and scheduling of this ToO observation.

Finding chart and additional plots