MAXI J1807+132: Swift/XRT localization

ATel #10215; J. A. Kennea (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 26 Mar 2017; 23:07 UT
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Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 08:42UT on March 26th, 2017, Swift began a target of opportunity observation of the error region of MAXI J1807+132 (Negoro et al., ATEL #10208). In order to cover the error region, Swift performed a 7-tile pattern, with approximately 200s per tile exposure. We detected one X-ray source inside the MAXI error region, at the following location: RA/Dec(J2000) = 272.03153, +13.25127, which is equivalent to:

RA(J2000) = 18h 08m 07.57s
Dec(J2000) = +13d 15m 04.6s

with an estimated uncertainty radius of 2.1 arc-seconds (90% confidence). This position lies 10.5 arc-minute from the center of the MAXI error ellipse, and so is consistent with being from that source. A search of the Simbad database reveals no known X-ray source at this location, and a search of the Vizier database finds no other catalogued object consistent with this object. Unfortunately at the source is at the edge of the XRT field of view for the 2 Swift pointings in which the object is detected, it lies outside of the UVOT FOV, so we cannot asses if an optical counterpart exists as yet.

The spectrum of this source can be well described by an absorbed power-law model, with a photon index of 2.4 +/- 0.5, and absorption 2.4 +/- 1.5 x 10²¹ cm^-2, consistent with the expected Galactic value (Willingale et al., 2013). The model fitted flux is 4.4 +/- 1.2 x 10^-11 erg s^-1 cm^-2 (0.3 - 10 keV), uncorrected for absorption.

Observations are encouraged in order to determine the nature of this event.