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Swift/XRT Follow-up of the Fermi-LAT Galactic Plane Transient J1057-6027

ATEL # 2083; C.C. Cheung (NASA GSFC), on behalf of the Fermi-LAT collaboration
on 15 Jun 2009; 2:18 UT
Password Certification: Teddy Cheung (ccheung@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Gamma Ray, >GeV, Transients

Following the Fermi-LAT detection of the Galactic Plane transient J1057-6027 (l, b = 289.301 deg, -0.645 deg) on 11 June 2009 (ATEL #2081), we obtained Swift observations beginning 1 day after the LAT detection. Combined with the 1.5 ksec XRT exposure obtained independently by Torres et al. (ATEL #2082), we report on analysis of the cumulative 9.4 ksec exposure obtained through 14 June.

Five low-significance (2.5-3.4 sigma) X-ray sources are detected in the ~24 arcmin XRT field. However, all of the X-ray sources are outside of the LAT 95% confidence error circle (r= 0.068 deg = 4.1 arcmin). Some sources are likely spurious. The X-ray source count rates, significance (snr), positions, and localization errors are tabulated below. Assuming a power-law with photon index = 2 and Galactic absorption = 1.45e22 cm-2, a 0.3-10 keV count rate of 1e-3 corresponds to an unabsorbed flux of 1.4e-13 erg/cm2/s.

One source of potential interest is Swift J105611.74-602714.1 (source #1 below) which is positionally coincident with the Wolf-Rayet star AG Car (WR 31b, HIP 53461) at RA(J2000)=10 56 11.58, Dec(J2000)=-60 27 12.8 (M.A.C. Perryman et al. 1997 A&A, 323, L49). AG Car is a luminous blue variable star (type B2 from SIMBAD) with a coincident radio source, MGPS2 J105611-602713 (218 mJy at 843 MHz; T. Murphy et al. 2007, MNRAS, 382, 382). AG Car is 7.7 arcmin from the LAT position centroid (1.9 * r95 of the LAT localization circle) and its potential association with the transient gamma-ray source should be further explored.

We thank the Swift team for their rapid scheduling of the observations.

source# count/s(err) snr RA(J2000) Dec(J2000) err_radius
1 1.18E-03(4.7E-04) 2.5 10:56:11.74 -60:27:14.1 5.6"
2 1.51E-03(5.2E-04) 2.9 10:57:36.92 -60:22:45.2 5.4"
3 1.54E-03(5.4E-04) 2.9 10:56:11.67 -60:33:03.0 6.0"
4 2.20E-03(6.4E-04) 3.4 10:57:20.88 -60:36:13.7 5.0"
5 1.30E-03(5.3E-04) 2.5 10:55:48.79 -60:32:31.2 5.8"


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