Swift/XRT Follow-up of the Fermi-LAT Galactic Plane Transient J0902-4624

ATel #3973; C. C. Cheung (NRC/NRL), E. Hays, R. Ojha, D. Donato (NASA/GSFC), on behalf of the Fermi-LAT collaboration, J. A. Kennea (Penn State Univ.)
on 17 Mar 2012; 17:18 UT
Credential Certification: Teddy Cheung (ccheung@milkyway.gsfc.nasa.gov)

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Following the Fermi-LAT detection of the Galactic plane gamma-ray transient J0902-4624 (l, b = 267.47 deg, 0.07 deg) on 15 March 2012 (ATEL #3972), we obtained Swift observations on 17 March (04:31 - 09:59 UT) with 9.9 ks exposure.

Four X-ray sources are detected at 2.7-5.8 sigma significances in the r~12 arcmin XRT field covering most of the LAT 68% confidence error circle (r=13.8 arcmin). The brightest X-ray source (XRT1) is closest to the LAT centroid, but at this early stage, all four XRT sources can be considered potential counterparts to the gamma-ray transient source. The 0.3-10 keV count rates, significances (snr), positions, and localization errors (90% confidence) for the XRT sources are tabulated below. Assuming a power-law with photon index = 2 and Galactic hydrogen column density = 7.53e21 cm-2, a 0.3-10 keV count rate of 1e-3 cts s-1 corresponds to an observed (unabsorbed) flux of 5.3e-14 (1.0e-13) erg cm-2 s-1.

Within the XRT error circles, all four X-ray sources had optical and near-infrared counterparts in the USNO-B1.0 and 2MASS catalogs, respectively, and their source names along with their optical/near-infrared magnitudes are tabulated below.

Additional Swift observations are planned in the coming days. We thank the Swift team for their rapid scheduling of the observations.

source# cts/s(err) snr RA(J2000) Dec(J2000) err_radius
XRT1 4.89(0.85)e-3 5.8 09:02:05.90 -46:22:48.49 4.6"
XRT2 2.18(0.59)e-3 3.7 09:02:13.37 -46:30:36.10 5.3"
XRT3 2.11(0.62)e-3 3.4 09:02:45.99 -46:17:53.36 5.1"
XRT4 1.65(0.60)e-3 2.7 09:02:34.81 -46:14:00.05 5.6"

USNOB1.0name B2mag R2mag Imag 2MASSname Jmag Hmag Kmag
0436-0153682 14.73 13.58 13.37 09020589-4622452 11.709 11.209 11.035
0434-0160763 14.03 13.05 12.62 09021358-4630354 11.938 11.542 11.424
0437-0155984 13.41 12.39 12.79 09024562-4617509 11.628 11.321 11.247
0437-0155905 10.86 10.54 10.41 09023440-4613575 10.175 10.024 10.012