Chandra Position and Optical/IR Identification for IGR J11435-6109

ATel #1231; J. A. Tomsick (SSL/UC Berkeley), S. Chaty (AIM CEA Saclay/University Paris 7), J. Rodriguez (AIM CEA Saclay), R. Walter (ISDC), P. Kaaret (University of Iowa)
on 8 Oct 2007; 18:51 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

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IGR J11435-6109 is a hard X-ray source that is located within 0.7 degrees of the Galactic plane. X-ray pulsations with a period of 161.76 seconds have been detected from this source (ATELs #359 and #362) as well as a long-term X-ray modulation with a period of 52.36 days (ATELs #359 and #362 and Wen et al. 2006). These periodicities strongly suggest that the system is a High Mass X-ray Binary (HMXB) with a neutron star orbiting its companion with an orbital period of 52.36 days. Like many IGR sources, this source has a high column density of N_H = 9x10^22 cm^-2 (ATEL #362).

In order to localize the X-ray source, we obtained a 5 ks observation with the Chandra X-ray Observatory on 2007 September 23. The 0.3-10 keV Chandra/ACIS image shows only one bright source in the INTEGRAL error circle at a position of R.A. = 11h 44m 00s.31, Decl. = -61d 07' 36".5 (equinox 2000.0, 90% confidence uncertainty = 0".6). The energy spectrum of this 381 count source is consistent with an absorbed power-law with an photon index of 1.0 +/- 0.6, a column density of (10 +/- 3)x10^22 cm^-2, and a flux of 4.8x10^-12 ergs/cm^2/s (0.3-10 keV, absorbed). The coincidence with the INTEGRAL position (as well as the BeppoSAX and Einstein positions, ATEL #362) and the high column density make it nearly certain that this Chandra source is the correct counterpart for IGR J11435-6109.

The Chandra position for IGR J11435-6109 is consistent with that of USNO-B1.0 0288-0337502, which has magnitudes of B = 16.6 +/- 0.3, R = 15.7 +/- 0.3, and I = 14.8 +/- 0.3. This source also appears in the UCAC catalog (UCAC 04813819), the 2MASS catalog (2MASS J11440030-6107364 with J = 13.00 +/- 0.02, H = 12.34 +/- 0.02, and K = 11.852 +/- 0.02), and the DENIS catalog (DENIS J114400.2-610736 with I = 14.51 +/- 0.03, J = 13.02 +/- 0.08, and K = 11.81 +/- 0.10). These magnitudes are consistent with the source being an HMXB, but the relatively bright optical magnitudes suggest a much lower Galactic extinction than is implied by the fits to the X-ray spectrum. This probably indicates a significant level of absorption that is local to the source.

Finally, we note that a different USNO source (USNO-B1.0 0288-0337948) was previously suggested as a possible optical counterpart to IGR J11435-6109 (ATEL #370). However, this USNO source is 1.4 arcminutes away from the Chandra source, and there is no X-ray source detected at its position, indicating that this source is unlikely to be the correct counterpart.