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A Chandra X-ray localization, spectrum, and IR identification for IGR J17354-3255

ATel #2022; John A. Tomsick (SSL/UC Berkeley)
on 18 Apr 2009; 00:56 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

Subjects: Infra-Red, X-ray, Gamma Ray, Black Hole, Neutron Star, Transient

Referred to by ATel #: 2596

A recent report (ATEL#2017) of the candidate gamma-ray transient AGL J1734-3310, which may possibly be associated with the INTEGRAL source IGR J17354-3255, prompted a Swift follow-up observation of the field. Swift detected two sources in the 0.3-10 keV band in the INTEGRAL error circle: Source #1 is bright but transient (or at least highly variable), and Source #2 is fainter but persistent (ATEL#2019).

We further add to the follow-up of AGL J1734-3310 and IGR J17354-3255 that we observed the field with the Chandra ACIS-I instrument on 2009 February 6 for 4,700 seconds (ObsID 9050). We also detect two sources in the INTEGRAL error circle: CXOU J173527.5-325554 is 7.7 arcseconds Southwest of Swift Source #1 at R.A. = 17h 35m 27s.59, Decl. = -32d 55' 54".4 (equinox 2000.0, 90% confidence uncertainty = 0".64), and CXOU J173518.7-325428 is 7.8 arcseconds Southwest of Swift Source #2 at R.A. = 17h 35m 18s.73, Decl. = -32d 54' 28".7 (equinox 2000.0, 90% confidence uncertainty = 0".64). Although these are both somewhat outside the 90% confidence Swift error circles, we suggest that they are the same two sources.

The position of CXOU J173527.5-325554 is within 0.2 arcseconds of the IR source 2MASS J17352760-3255544, which has IR magnitudes of J=12.51+/-0.05, H=10.99+/-0.04, and Ks=10.27+/-0.03. This source does not appear in the USNO optical catalogs, suggesting a relatively high level of extinction. The 0.3-10 keV Chandra spectrum of CXOU J173527.5-325554 is well-described by an absorbed power-law with a column density of 7.5(+3.0)(-2.5)e22 cm^-2 (90% confidence errors) and a power-law photon index of 0.54(+0.60)(-0.55). The flux of this source is 1.3e-11 ergs/cm^2/s (0.3-10 keV, unabsorbed). This flux level is similar to that seen by Swift, but our results show that the emission is not necessarily thermal as suggested by the spectral analysis in ATEL#2019. The high optical/IR extinction and the position within 5 degrees of the Galactic Center may indicate a distance near 8.5 kpc, which would imply an X-ray luminosity of 1.1e35 ergs/s. The luminosity, the hard spectrum, and the variability reported in ATEL#2019 suggest the possibility that the source is a High Mass X-ray Binary.

Source #2, CXOU J173518.7-325428, does not appear in the 2MASS, DENIS, or USNO catalogs. The Chandra/ACIS-I count rate is about a factor of 3.4 lower for this source than for Source #1, and a fit with an absorbed power-law model indicates a flux of 1.4e-12 ergs/cm^2/s. Even though this source also has a relatively hard X-ray spectrum with a column density of 2.6(+1.4)(-1.2)e22 cm^-2 and a photon index of 0.79(+0.56)(-0.53), we agree with Vercellone et al. (ATEL#2019) that Source #1 is more likely to be the counterpart to IGR J17354-3255.