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Optical Spectroscopy of the IGR J16195-4945 Candidate Counterpart

ATel #804; Gagik Tovmassian (OAN, IA, UNAM, Ensanada, BC, Mexico), John A. Tomsick (CASS/UCSD), and Ronald Mennickent (Univ. de Concepcion, Chile)
on 5 May 2006; 23:00 UT
Credential Certification: John A. Tomsick (jtomsick@ucsd.edu)

Subjects: Optical, Binary, Neutron Star

On 2005 October 5, we observed the candidate optical counterpart of the INTEGRAL source IGR J16195-4945 with the EMMI spectrograph attached to the ESO 3.5m NTT in La Silla, Chile (see Tomsick et al. 2006; astro-ph/0603810 v2 and references therein for the details about this likely High-Mass X-ray Binary). The Chandra position of IGR J16195-4945 coincides with the infrared source 2MASS J16193220-4944305 (Ks = 11.00). Meanwhile, there is also an optical object (USNO-B1.0 0402-0529810, R ~ 16) that lies just outside the Chandra error circle. Being 0.9 +/- 0.3 arcseconds apart, the 2MASS and USNO sources are probably not the same object, but we have not yet been able to obtain an optical or IR image that resolves the two sources. The Spectral Energy Distribution for the IGR J16195-4945 counterpart reported in Tomsick et al. (2006) provides further evidence that there are two blended sources.

We obtained a series of 3 exposures of the optical source using Grism#5 in RILD mode that provides 0.83 A/pix spectral resolution and covers the 3800-7000 Angstrom range. Three 600 sec integrations were collected along with all necessary calibration images for bias subtraction, flat fielding, wavelength calibration, and flux calibration using the standard NTT procedures.

The spectrum of the optical source shows absorption lines but no emission lines and is consistent with that of a late F-type star. The S/N of the spectrum is not high (S/N ~ 4 around 4000A, increasing to ~40 near Halpha). Our best estimate is that its spectral type is between F6 and F8, but the important lines that could be used for better spectral classification and luminosity class determination tend to be at the blue side of the spectrum where the S/N is the lowest. Despite difficulties in unambiguously determining the spectral type, the spectrum appears to confirm that the optical source is not the INTEGRAL counterpart for several reasons. First, the source reddening is too low to be compatible with the very high extinction associated with IGR J16195-4945. Although the X-ray source could be intrinsically absorbed, the E(B-V) correction for the optical spectrum required to bring the continuum shape into the accordance with the spectral classification done by lines is at most 0.75, resulting in Av=2.3. Meanwhile, Tomsick et al. (2006) report Av=17.5 for the IR source. Although we cannot properly determine the luminosity class of the star in the line of sight, we can estimate that a B = 17.4 (USNO) F8V star will be at a distance 1.3 kpc, while F8I will be as far as 20 kpc. It is safe to assume that we observed a dwarf star in a relative vicinity rather than a remote giant judging from the mild reddening.

Finally, as IGR J16195-4945 is almost certainly an X-ray binary one would expect its counterpart to have emission lines from an X-ray-irradiated accretion disk or companion or from a stellar wind. Thus, the lack of optical emission lines confirms that the optical source observed is not the INTEGRAL source but a foreground object. IR spectroscopy and optical or IR imaging with higher angular resolution would be useful in further studies of this source.