Rapid Flaring in the Galactic-plane Gamma-ray Transient Fermi J0035+6131.

We investigate the gamma-ray and X-ray emission from the transient gamma-ray source Fermi J0035+6131, which was discovered with the Fermi Large Area Telescope (LAT) near the Galactic plane at $b=1.3^\circ$, and we discuss potential multi-wavelength counterparts of the gamma-ray source. Our analysis of over 9 years of Fermi LAT data revealed two flaring events lasting 10-30 hr during which the gamma-ray flux increased by a factor of >300 compared to the long-term average. We also analyzed X-ray data obtained with XMM-Newton and Swift and identified several sources with a hard X-ray spectrum inside the Fermi LAT confidence region. The two brightest X-ray sources have known counterparts at other wavelengths and are associated with the compact radio source VCS4 J0035+6130 and the B1 IV:nn star HD 3191, respectively. VCS4 J0035+6130, which is also detected in the near infrared, is likely an active galaxy serendipitously located behind the Galactic disk and is the most compelling candidate for the counterpart of the gamma-ray source. HD 3191 appears to be part of an X-ray binary with a compact companion and is unlikely to be associated with Fermi J0035+6131.