IGR J16358-4726 and IGR J16393-4643: two new symbiotic X-ray binaries

ATel #1450; Elisa Nespoli, Juan Fabregat (University of Valencia, Spain) and Ronald E. Mennickent (University of Concepcion, Chile)
on 31 Mar 2008; 11:30 UT
Credential Certification: Juan Fabregat (juan.fabregat@uv.es)

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We report results from Ks band spectroscopy of the INTEGRAL sources IGR J16358-4726 and IGR J16393-4643. The proposed infrared counterparts 2MASS J16355369-4725398 (Kouveliotou et al. 2003, IAUC 8109 ) and 2MASS J16390535-4642137 (Bodaghee et al. 2006, A&A 447, 1027) were observed in service mode on 2007 April 5th with the ISAAC spectrograph on UT1 at Paranal (ESO proposal 079.D-0668-B). The sky was clear during the observation, the seeing was ≤1.4'' and the targets were observed at airmass 1.08 and 1.09 respectively. Data were taken in the Short Wavelength - Low Resolution mode, with a pixel scale of 0.147'' and a resolution of ~ 500. The snr averaged 150 for both spectra.

The two spectra are very similar, being their most prominent features the series of strong CO absorption bands between 2.29 and 2.40 µm, which are characteristic of late type stars. Several metallic lines are clearly recognizable as well (see details below), although at our resolution they are generally blended. He I 20581 Å and Br gamma are also present in faint emission in both spectra; since they are not a proper spectral feature of late-type stars, these lines might be the signature of a circumstellar disk around a compact object. However, they are very weak, and they might be as well a residual artifact from the telluric line removal procedure.

The most remarkable lines in each spectrum are the following:

IGR J16358-4726: He I 20581 Å (EW = -0.03 Å), Mg I 21100 Å (EW = 5.62 Å), Br gamma 21661 Å (EW = -2.80 Å), Na I 22100 Å (EW = 2.58 Å), Ca I 22600 Å (EW = 1.35 Å), Mg I 22800 Å (EW = 2.75 Å), 12CO 22900 Å (EW = 9.91 Å), 12CO 23200 Å (EW = 13.04 Å), 12CO 23500 Å (EW = 7.84 Å), 13CO 23500 (2, 0) Å (EW = 2.53 Å).

IGR J16393-4643: He I 20581 Å (EW = -4.10 Å), Mg I 21100 Å (EW = 4.98 Å), Br gamma 21661 Å (EW = -0.94 Å), Na I 22100 Å (EW = 2.63 Å), Ca I 22600 Å (EW = 1.11 Å), Mg I 22800 Å (EW = 1.58 Å), 12CO 22900 Å (EW = 11.97 Å), 12CO 23200 Å (EW = 12.92 Å), 12CO 23500 Å (EW = 10.09 Å), 13CO (2, 0) 23500 Å (EW = 3.32 Å).

Our ISAAC spectra clearly point to a late type star nature of both objects, and exclude that the systems are HMXBs with OB companions. From the identified spectral lines, our conclusion is that both stars are of K or M spectral type. The general morphology of the spectra and the comparison between the relative strength of CO 22900 Å and both Na I and Na lines, which are luminosity indicators, reveal that both observed stars are giants. The forbidden [Fe II] transition at 22200 Å detected in IGR J16358-4726 by Chaty et al. (2008, arXiv:0802.1774) is not present in our spectrum.

We propose that both systems belong to the class of symbiotic X-ray binaries, in which a compact object accretes from the stellar wind of a red giant or supergiant companion.