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MAXI J1813-095: Swift detection and localization

ATel #11326; J. A. Kennea (PSU), D. M. Palmer (LANL), A. Y. Lien (GSFC/UMBC), P. A. Evans, A. P. Beardmore (U Leicester), H. A. Krimm (NSF/USRA), P. Romano (INAF-OAB), K. Yamaoka (Nagoya U.), M. Serino (RIKEN) and H. Negoro (Nihon U.)
on 21 Feb 2018; 00:38 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 11332, 11356, 11357

At 16:40UT on February 20th, 2018, Swift began a target of opportunity observation of the position of the new MAXI detected transient MAXI J1813-095 (Kawase et al. 2018; ATel #11323). We performed a 7-point tiling observation in order to cover the MAXI error region, with an exposure of 500s per tile. In the central tile we find a bright uncatalogued X-ray source at the following coordinates: RA/Dec (J2000) = 273.39169, -9.53307, which is equivalent to:

RA(J2000) = 18h 13m 34.0s,
Dec(J2000) = -09h 31m 59.0

with an estimated uncertainty of 2.2 arc-seconds radius (90% containment). This position is not consistent with the location of any previously know X-ray transient, and therefore we conclude that this is a new transient X-ray source, MAXI J1813-095.

The XRT Photon Counting mode spectrum is well fit by an absorbed power-law model, with a photon index of 1.45 +/- 0.23 and absorption column NH = 1.3 +/- 0.3 x 1022 cm-2. The average flux, corrected for absorption is 1.2 x 10-9 erg/s/cm2 (0.5 - 10 keV).

BAT has also detected the new X-ray transient MAXI J1813-095 as a sub-threshold source in its on-board images, with the earliest detection in a 17 minute image starting at 03:58 UT on 2018 Feb 19. The average location of these on-board detections is RA, Dec = 273.355, -9.478 which is

RA(J2000) = 18h 13m 25s,
Dec(J2000) = -9d 28' 42”,

with an uncertainty of 3 arc-minutes (radius, 90% containment, including systematic uncertainty). We note that this position is 4 arc-minutes away from the XRT position, however no other transient is present in the XRT data inside the BAT error circle, therefore we believe that the BAT and XRT source are consistent with both being MAXI J1813-095.

Continuing BAT ground analysis of the source will be available at https://swift.gsfc.nasa.gov/results/transients/weak/MAXIJ1813-095/

We encourage follow-up of this new transient in order to identify the source type.

This work was supported in part by the Swift GI program, NASA grant 80NSSC17K0313.

MAXI J1813-095 BAT Transient Monitor