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Refined X-ray Position and Spectrum and Near-IR Identification of IGR J14091-6108

ATel #6793; John A. Tomsick (SSL/UCB), Farid Rahoui (ESO and Harvard), Roman Krivonos (SSL/UCB), Jerome Rodriguez (AIM and CEA Saclay), Arash Bodaghee (Georgia College), Sylvain Chaty (AIM and CEA Saclay)
on 8 Dec 2014; 21:42 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

Subjects: Infra-Red, X-ray, Binary, Cataclysmic Variable, Neutron Star

IGR J14091-6108 was discovered by INTEGRAL as a hard X-ray source in the Galactic plane (l,b = 312.16,+0.33; Krivonos et al., 2012, A&A, 545, 27). A Swift/XRT source with a hard X-ray spectrum was identified as the likely counterpart, but the 4 arcsecond error on the position did not allow for identification of a unique optical or near-IR counterpart (Landi et al., 2012, ATEL#4165).

On 2013 December 7-8, we obtained a 4.9 ks observation with Chandra/ACIS-I to refine the position measurement. We strongly detect IGR J14091-6108 (403 ACIS counts) at a position consistent with the Swift position. We also detect seven other sources with near-IR counterparts in the VISTA catalog, and we use these matches to register the ACIS image. For IGR J14091-6108, we find a position of R.A. (J2000) = 14h 08m 45.99s, Decl. (J2000) = -61d 07m 54.3s (the 2-sigma error radius is 0.4 arcseconds based on the residual differences between the Chandra and VISTA positions for the seven sources).

The Chandra position is not consistent with either of the USNO sources suggested to be the counterpart by Landi et al. (2012), but it is consistent (within 0.14 arcseconds) with a VISTA source (ID number = 515845105705) with Z = 16.190+/-0.006, Y = 15.797+/-0.007, J = 15.240+/-0.006, H = 14.809+/-0.007, and Ks = 14.376+/-0.011, and it is also within 0.34 arcseconds of the Spitzer/GLIMPSE source G312.1289+00.3516 (m3.6 = 13.76+/-0.09, m4.5 = 13.15+/-0.13). Thus, IGR J14091-6108 is a moderately bright near-IR source with modest extinction (J-Ks = 0.86+/-0.01).

The Chandra 0.3-10 keV energy spectrum is fitted with an absorbed power-law plus a Gaussian iron line. The column density is N_H = 6(+5)(-4)e21 cm^-2 (Wilms et al. 2000 abundances and 90% confidence errors), and the power-law photon index is Gamma = 0.3+/-0.3. The iron line energy is 6.51(+0.18)(-0.12) keV, the width is <0.38 keV, and the equivalent width is 900(+650)(-450) eV. The 0.3-10 keV unabsorbed flux is 3.3e-12 erg/cm2/s, which corresponds to a 4e32-4e34 erg/s luminosity range for distances of 1-10 kpc. The distance to the source is highly uncertain, but the Galactic column density (atomic and molecular Hydrogen) along the line of sight is 5.4e22 cm^-2, so the value we measure (an order of magnitude lower) likely indicates that the distance is closer to 1 kpc than 10 kpc.

The low column density and extinction, the apparently point-like near-IR counterpart in the VISTA images, and especially the zero-redshift iron line show that IGR J14091-6108 is a Galactic source. The very hard power-law index would be possible for a magnetic Cataclysmic Variable (CV) or a High-Mass X-ray Binary (HMXB). The likely low X-ray luminosity and strong iron line lead us to slightly favor the CV possibility, but the properties would not be unprecedented for an HMXB. Near-IR (or perhaps optical) spectroscopy would very likely answer this question, and more could be learned (e.g., periodicity search and broadband spectrum) about the source from further X-ray observations.