Multiwavelength Observations of MASTER OT 075353.88+174907.6: A Likely Superoutburst of a Long Period Dwarf Nova System

MASTER OT 075353.88+174907.6 was a blue optical transient reported by the MASTER-Net project on 2017 Oct 31. We carried out multiwavelength follow-up observations of this source during its 2017 outburst using Swift and RATIR. The source was found to be ≳4.4 mag above its quiescent level during the peak of the outburst and the outburst lasted ≳19 days. Our observations suggest that it was a superoutburst of a long 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 spectrum (with spectral indices Γ≈1.5-2.3). Afterwards, the UV flux decreased slowlier than the optical and the spectral energy distribution was very steep with indices Γ≈3.7±0.7. This slow UV decay may be the emission from a cooling white dwarf heated during the outburst. No X-ray emission was detected from the source since it is likely located at a large distance >2.3 kpc.