MAXI J1930+093: Likely a flare of the BL Lac 2FGL J1931.1+0938

ATel #5946; J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), P. A. Evans (U Leicester), P. Romano (INAF-IASF PA), V. Mangano (PSU), P. Curran (Curtin), K. Yamaoka (Nagoya U) and H. Negoro (Nihon U.)
on 4 Mar 2014; 16:42 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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As part of the Swift GI program to localize MAXI Galactic Transients, starting at 19:21UT on March 3rd, 2014, we observed the error ellipse of MAXI J1930+093 (Negoro et al., ATEL #5943), with Swift's X-ray Telescope, utilizing a 4-point tiling observation in order to cover the error ellipse. Each tile was observed for 0.5 ks in Photon Counting (PC) mode. We find one bright point source in the four tiled pointings, at the following location, which is corrected for astrometric errors by UVOT using the methods of Goad et al (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177): RA/Dec(J2000) = 292.78859, 9.62051, which is equivalent to:

RA(J2000) = 19h 31m 09.26m,
Dec(J2000) = +09d 37m 13.8s,

with an estimated error of 2 arc-seconds radius (90% confidence). This position lies 2.6 arc-seconds from NED position of the BL Lac object BZB J1931+0937, a BL Lac object also known by the Fermi name 2FGL J1931.1+0938 (Ackermann et al., 2011), and the ROSAT point source catalogue name 1RXS J193109.5+093714 (Voges et al., 1999). The source lies inside the MAXI error ellipse, as noted by Negoro et al. (ATEL #5943).

Looking at the archival Swift XRT data on this source, the object was last observed on February 22nd, 2010, at which point the source was at a birghtness of ~0.35 XRT count s^-1. In the observation taken on March 3rd, the source was highly elevated at a brightness of 4.8 +/- 0.5 XRT count s^-1, showing clear flaring activity. We therefore suggest that MAXI J1930+093 is a detection of an X-ray flare from 2FGL J1931.1+0938.

The PC mode spectrum can be well fit by a simple absorbed power-law model, with N_H = 4 +/- 1 x 10²¹ cm^-2, consistent with the Galactic value of 3.13 x 10²¹ cm^-2 (Kalbera et al, 2005), and a photon index of 1.98 +/- 0.25. The X-ray flux is 2.5 +/- 0.3 x 10^-10 erg s^-1 cm^-2 (0.5-10 keV, uncorrected for absorption).

We examined archival daily images from the Swift/BAT hard X-ray transient monitor in the 15-50 keV band from February 1, 2014 to the present. We see no significant emission from the direction of 2FGL J1931.1+0938 to an average limiting count rate of 0.002 count s^-1 cm^-2.