MAXI J1807+132: UV/Optical counterpart, further XRT observations

ATel #10216; J. A. Kennea, M. H. Siegel (PSU), P. A. Evans, A. P. Beardmore (U Leicester), H. A. Krimm (NSF/USRA), P. Romano (INAF-IASFPA), K. Yamaoka (Nagoya U.), M. Serino (RIKEN) and H. Negoro (Nihon U.)
on 27 Mar 2017; 18:22 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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Following the detection and localization of the MAXI J1807+132 (Negoro et al., ATEL #10208) by Swift (Kennea et al., ATEL #10215), we requested follow-up observations with Swift with XRT in Windowed Timing (WT) mode (to avoid pile-up), and UVOT observing in 6-filter imaging mode.

Starting at 07:00UT on March 27th, 2017, Swift took a 2ks exposure of the MAXI J1807+132. In X-rays the source continues to be strongly detected. The WT spectrum is well described by a power-law model, with a photon index of 2.37 +/- 0.13, with an absorption N_H = 2.3 +/- 0.5 x 10²¹ cm^-2. The fitted flux is 3.4 +/- 0.2 x 10^-11 erg s^-1 cm^-2 (0.3 - 10 keV, uncorrected for absorption). We note that these values are consistent with fit to the the Photon Counting mode data taken on March 26, suggesting that the source is not evolving rapidly either in flux or spectrum.

Analyzing the UVOT data, we detect a new point source at a position consistent with the XRT position reported in ATEL #10215. The position of this source is RA/Dec(J2000) = 272.03142, 13.25150, which is equivalent to:

RA(J2000) = 18h 08m 07.549s,
Dec(J2000) = +13d 15m 05.40s,

with an estimated error of 0.16 arc-seconds (90% confidence). This position lies 0.9 arc-seconds from the center of the XRT error circle, reported in ATEL #10215, consistent with the XRT position. This source is UV bright, but not detected in the uvot v filter, the magnitudes (quoted in the Vega system) are:

v > 17.63
b = 18.4 +/- 0.2
u = 17.4 +/- 0.1
uvw1 = 17.2 +/- 0.1
uvw2 = 17.2 +/- 0.1
uvm2 = 17.3 +/- 0.1

This source does not match any catalogued optical object. Given the fact that the source is not on the Galactic Plane, has a soft X-ray spectrum and is UV bright, as suggested by Negoro et al. (ATEL #10208), MAXI J1807+132 could be an X-ray bright tidal disruption event.