A new outburst from LMXB 1A 1744-361

ATel #5301; Arash Bahramian, Craig O. Heinke, Gregory R. Sivakoff (University of Alberta)
on 20 Aug 2013; 17:46 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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Starting Aug. 3rd, 2013, the MAXI All-Sky Monitor fluxes showed enhanced X-ray activity from their extraction of 4U 1746-37 in NGC 6441 which the MAXI team suggested to be due to a new outburst of 1A 1744-361 (http://maxi.riken.jp/top/index.php?cid=1&jname=J1750-370). Data from the Swift/BAT team centred on 1A 1744-361 indicate an increase in 15-50 keV X-ray flux, starting from Aug. 1st, 2013 (http://swift.gsfc.nasa.gov/results/transients/weak/1A1744-361/). We performed four tiled observations using Swift/XRT in PC mode on Aug. 18 to identify the source of the MAXI/Swift emission, detecting the new transient in Obs.ID 00032917001.

We find the position of the outbursting source to be RA = 17:48:13.4, Dec= -36:07:57 (J2000), with a radial uncertainty of 3 arcseconds. This is consistent with the radio position (RA=17:48:13.148 ± 0.014, Dec= -36 07 57.02 ± 0.30 (J2000, ATel #210) and optical counterpart (ATel #267) of a transient X-ray binary in Nov. 2003, named XTE J1748-361 (ATel #210), which is probably 1A 1744-361 (ATel #204). The Ariel V-reported position, converted to J2000, was RA=17:48:13.4, Dec=-36:07:53, with a radial uncertainty of 40" (Wilson et al. 1977, ApJ, 215, L111). This is the sixth reported outburst associated with 1A 1744-361 in the past ten years, including outbursts in Nov. 2003, Apr. 2004, July 2005, June 2008, and Nov. 2009 (ATels #204, #265, #567, #1587, #2305). We also note that there are indications of an outburst in mid-2010 from the Swift/BAT transient monitor lightcurves.

The source is heavily piled up in our Swift/XRT observation. We extracted a spectrum from a annulus of 18 to 120 arcseconds for the source, and background from a nearby region. Fitting an absorbed power-law (PHABS*PEGPWRLW) to the spectrum, we found N_H= (5.1±0.7) × 1e21 cm^-2, and a photon index of 1.5±0.1 with an unabsorbed (0.5-10 keV) flux of (2.9±0.1) × 1e-9 erg cm² s^-1. This fit yields a χ² of 34.3 for 41 degrees of freedom; addition of a thermal component did not significantly improve the fit. For an assumed distance of 8 kpc, this flux corresponds to L_X ~ 2e37 erg s^-1.

We thank the Swift team for rapidly scheduling our observations and both the MAXI and Swift/BAT teams for making their data publicly available.