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MAXI J1932+091: Swift/XRT Observations of the MAXI error circle

ATel #6177; J. A. Kennea, D. N. Burrows, V. Mangano (PSU), H. A. Krimm (CRESST/GSFC/USRA), P. A. Evans (U Leicester), P. Romano, M. De Pasquale, A. Masselli (INAF-IASF PA), P. Curran (Curtin), K. Yamaoka (Nagoya U) and H. Negoro (Nihon U.)
on 28 May 2014; 01:06 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 6184, 6186

After the detection of the short lived transient MAXI J1932+091, tentatively suggested to be associated with the BL Lac object 2FGL J1931.1+0938 (Yamaoka et al., ATEL #6174), Swift performed two target of opportunity observations of the MAXI error circle. The first observation was a single 500s long observation in Photon Counting mode, taken starting 01:54UT on May 27, 2014. The second observation consisted of four ~500s exposures tiling the MAXI error circle that began at 11:05UT on May 27, 2014. In these observations a single uncatalogued X-ray source was detected inside the MAXI error circle. This source has RA/Dec (J2000) = 293.29267, 9.19492, equivalent to:

RA (J2000): 19h 33m 10.24s (293.29334)
Dec (J2000): +09d 11m 41.7s (9.19473)

with an error radius of 2.8 arc-seconds radius (90% confidence). We refer to this source from now on as Swift J193310.4+091141. The position is consistent with a V=13.9 magnitude star, categorized as such due to having a measured proper motion. This X-ray source does not appear in any catalogues, and we could find no archival observations of this field that would confirm whether or not this source is transient. Swift J193310.4+091141 is 7.6 arc-minutes from the center of the MAXI error ellipse, consistent with MAXI J1932+091.

Swift J193310.4+091141 was detected in 3 of the pointings: The initial first pointing, and in two of the later tiled pointings. In the earlier pointing, the count rate is ~0.1 XRT count s-1, and in the later pointings, it has brightened to ~0.35 XRT count s-1, with evidence of fading during this window, suggestive of a flare. The average spectrum of Swift J193310.4+091141, fitted to an absorbed power-law model, has high absorption, 2.3 (+4.1/-2.1) x 1022 cm-2 and is hard, with a fitted photon index of -0.5 (+0.5/-0.6). The average observed flux is ~ 6 x 10-11 erg s-1 cm-2 (0.5 - 10 keV).

Given the hardness of the spectrum, spatial coincidence and variability of Swift J193310.4+091141, we suggest that it may be the counterpart of MAXI J1932+091. We note that the position of 2FGL J1931.1+0938 is 40 arc-minutes from the position of this source, and is not covered by any of the Swift observations. Therefore we cannot comment on the state of 2FGL J1931.1+0938 in this ATEL, nor can we definitively rule out that it is MAXI J1932+091.

Further observations of Swift J193310.4+091141 are encouraged in order to determine the source type.