Multiwavelength observations of MASTER OT 075353.88+174907.6: a likely superoutburst of a long period dwarf nova system

Abstract

MASTER OT 075353.88+174907.6 was a blue optical transient reported by the MASTER-Net project on 2017 Oct 31. This source was previously detected by {\it GALEX} in its NUV band but not by the Sloan Digital Sky Survey (in the optical). We carried out multiwavelength follow-up observations of this source during its 2017 outburst using {\it Swift} and RATIR. The source was found to be $\gtrsim$4.4 mag above its quiescent level during the peak of the outburst and the outburst lasted $\gtrsim$19 days. Our observations suggest that it was a superoutburst of a long orbital period U Geminorum type dwarf nova system. The spectral energy distribution during the initial slow decay phase of the outburst was consistent with a disk-dominated spectra (having spectral indices $\Gamma \! \sim$1.5--2.3). After this phase, the UV flux decreased slower than the optical and the spectral energy distribution was very steep with indices $\Gamma \! \sim$3.7$\pm$0.7. This slow decay in the UV may be the emission from a cooling white dwarf heated during the outburst. The spectral shape determined from the assumed pre-outburst quiescent level was also steep ($\Gamma \! \gtrsim$2.5) indicating that the white dwarf is still hot in quiescence (even after the cooling due to the potential accretion-induced heating has halted). No X-ray emission was detected from the source since it is likely located at a large distance $>$2.3 kpc.