Chandra Localizations and X-ray Spectra of Two INTEGRAL Sources in the Galactic Plane: IGR J16287-5021 and IGR J19267+1325
ATel #1649; J. A. Tomsick (SSL/UC Berkeley), J. Rodriguez (AIM CEA Saclay/Univ. Paris 7), S. Chaty (AIM CEA Saclay/Univ. Paris 7), R. Walter (ISDC/Univ. de Geneve), and P. Kaaret (Univ. of Iowa)
on 7 Aug 2008; 23:29 UT
Credential Certification: John A. Tomsick (firstname.lastname@example.org)
Subjects: Infra-Red, Optical, X-ray, Black Hole, Cataclysmic Variable, Neutron Star
As part of an on-going program to learn about the nature of the hard X-ray INTEGRAL sources in the Galactic Plane, we obtained, on 2008 Jan. 28, a 4935 s Chandra/ACIS exposure of the field of IGR J16287-5021, which is in the Norma region of the Galaxy at coordinates l = 334.1 deg and b = -1.1 deg and a 4698 s Chandra/ACIS exposure of the field of IGR J19267+1325 (l = 48.9 deg, b = -1.5 deg) on 2008 Feb. 27.
For IGR J16287-5021, the Chandra image revealed a single bright X-ray source in the 4'.4 radius INTEGRAL error circle. The position of this 490 count source is R.A. = 16h 28m 26s.85, Decl. = -50d 22' 39".6 (equinox 2000.0, 90% confidence uncertainty = 0".64). The 0.3-10 keV energy spectrum is reasonably well-described by a power-law with little to moderate interstellar absorption and a very hard photon index of Gamma = -0.9 +/- 0.4 (90% confidence errors). Absorption is not formally required, and we derive a 90% confidence upper limit of N_H < 1.5x10^22 cm^-2. The 0.3-10 keV flux is 7x10^-12 ergs/cm^2/s, and the hardness of the spectrum makes the identification of this Chandra source with the INTEGRAL source very likely. We note that the negative value for Gamma is surprising, but we do not find evidence that the spectrum is strongly affected by photon pile-up. Further investigation of the X-ray spectrum as seen by the Swift satellite will be presented in Rodriguez, Tomsick, & Chaty (2008, submitted to A&A). We do not find any counterparts to the Chandra source in the 2MASS, USNO, or DENIS optical and IR source catalogs, and further observations or analysis are required to determine the nature of this source.
For IGR J19267+1325, we did not find any bright sources in the 3'.7 INTEGRAL error circle, but we did find a Chandra source with 1435 counts nearby (4'.8 away from the best estimate for the INTEGRAL position). The position of this Chandra source is consistent with that of the Swift source reported by Landi et al. (2007, ATEL#1323). The refined (Chandra) position is R.A. = 19h 26m 26s.98, Decl. = +13d 22' 05".1 (equinox 2000.0, 90% confidence uncertainty = 0'.64). The 0.3-10 keV energy spectrum is well-described by a power-law with interstellar absorption. The spectral parameters are N_H = (2.1 +/- 0.9)x10^21 cm^-2 and Gamma = 0.68 +/- 0.13, and the unabsorbed 0.3-10 keV flux is 9x10^-12 ergs/cm^2/s. Although not as hard a spectrum as the previous source, the hardness of this spectrum still suggests that this Chandra source is the counterpart to the INTEGRAL source. The Chandra source has optical (USNO-B1.0) and IR (2MASS) counterparts with B = 17.7 +/- 0.3, R = 16.0 +/- 0.3, I = 15.1 +/- 0.3, J = 14.94 +/- 0.07, H = 14.46 +/- 0.08, and Ks = 14.10 +/- 0.09. This low level of extinction for a source in the Galactic Plane likely indicates that IGR J19267+1325 is a relatively nearby Galactic source.