Discovery of a likely counterpart to IGR J15391-5307 with Chandra: X-ray localization and near-IR identification

ATel #3293; John A. Tomsick (SSL/UC Berkeley), Sylvain Chaty (AIM - Univ. Paris 7 and CEA Saclay), Arash Bodaghee (SSL/UC Berkeley), Farid Rahoui (Harvard University, Department of Astronomy), Jerome Rodriguez (CEA-SAp/Lab. AIM Saclay), Roland Walter (ISDC/Univ. de Geneve)
on 18 Apr 2011; 22:16 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

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As part of our Chandra program to localize hard X-ray sources discovered by INTEGRAL in the Galactic Plane, we obtained a 5.0 ks ACIS exposure of the IGR J15391-5307 field on UT 2011 April 14, 21.2-23.1 h. We detect five 0.3-10 keV sources within the 4.5 arcminute INTEGRAL error circle (Bird et al. 2010, ApJS, 186, 1). The source that is brightest (47 ACIS counts compared to 3 to 6 counts for the other sources) and closest to the center of the INTEGRAL error circle is CXOU J153916.7-530815. This source also has a hard X-ray spectrum with all 47 counts above 2.9 keV. Based on these characteristics, we suggest that it is likely to be the IGR J15391-5307 counterpart.

The exact Chandra position of CXOU J153916.7-530815 is R.A. = 15h 39m 16s.77, Decl. = -53d 08' 15".9 (equinox 2000.0, 90% confidence uncertainty = 0".64). This is consistent with the position of 2MASS J15391681-5308158, which has near-IR magnitudes of H = 13.95 +/- 0.08 and Ks = 12.99 +/- 0.07, and the position of DENIS J153916.8-530815 with J = 15.41 +/- 0.16 and Ks = 13.06 +/- 0.15. This source was not present in the USNO optical catalogs. With only 47 counts, the Chandra energy spectrum is not well-constrained, but a fit with an absorbed power-law indicates a photon index of Gamma = 3.6(+3.1)(-2.4), a column density of 4(+3)(-2)x10^23 cm^-2, and a 0.3-10 keV absorbed flux of 8x10^-13 ergs cm^-2 s^-1.

The near-IR magnitudes give J-Ks = 2.4 +/- 0.2, which implies a high level of interstellar extinction. Given the location of the source at l,b = 326.6, +1.9 degrees, this could either indicate a relatively distant source within the Galaxy (e.g., a High-Mass X-ray Binary) or that the source is a background Active Galactic Nucleus. As the column density along this line of sight is near 1e22 cm^-2, the high X-ray column density indicates intrinsic absorption, but this is possible for either HMXBs or AGN. The HMXB interpretation may be slightly more probable given the location of the source close to the Galactic Plane and the indication in Bird et al. (2010) that the source is variable in the hard X-ray band. IR spectroscopy could lead to a definitive classification.