MAXI J1535-571: Swift detection and localization

ATel #10700; 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 3 Sep 2017; 04:10 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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As reported in GCN #21788, at 20:00:36UT Swift/BAT triggered on an outburst from a possible GRB (GRB 170902A) or, based upon its localization near the plane and the nature of the BAT trigger (long image trigger), a possible Galactic Transient. Negoro et al (ATEL #10699) reported a new transient, MAXI J1535-571 was detected by MAXI at 23:55UT on September 2nd, 2017, at a position coincident with the localization of the BAT detected transient (BAT and MAXI localizations lie 3.9 arc-minutes apart). We therefore conclude that the BAT detected transient is not a GRB, but a new Galactic Transient source, co-detected by Swift and MAXI: MAXI J1535-571.

Swift observed this new transient using both its normal automated burst response in Windowed Timing (WT) mode between 20:00UT and 21:51UT on September 2nd, 2017, and as a Target of Opportunity upload in Photon Counting (PC) mode between 22:13UT on September 2nd, 2017 and 01:15UT on September 3rd, 2017.

Using 3279 s of PC mode data and 5 UVOT images for MAXI J1535-571, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 233.83222, -57.23004 which is equivalent to:

RA (J2000): 15h 35m 19.73s
Dec (J2000): -57d 13' 48.1"

with an uncertainty of 1.5 arcsec (radius, 90% confidence).

Based on 1.6ks of WT mode data, we find the spectrum is well fit by an absorbed power-law model, with N_H = 3.6 +/- 0.2 x 10²² cm^-2 and a photon index of 1.53 +/- 0.07. The average flux, corrected for absorption, is 1.25 +/- 0.05 x 10^-9 erg/s/cm². In data taken from 20:07UT to 01:15UT there was no strong evidence of brightening or fading in the X-ray light-curve.

Examination of the Swift/UVOT data reveals no detection of any new optical transient at the above localization. However, given the location on the Galactic Plane and the high measured absorption, this is not surprising.

Further observations are encouraged in order to determine the nature of this transient.