Identification of 2 INTEGRAL sources via Swift/XRT plus CASLEO followup

ATel #1034; N. Masetti (INAF/IASF, Bologna), S. A. Cellone (UNLP, La Plata), R. Landi, E. Palazzi, J. B. Stephen (INAF/IASF, Bologna), A. J. Dean (Univ. Southampton), D. Minniti (PUC, Santiago), G. E. Romero (UNLP, La Plata), P. Ubertini (INAF/IASF, Rome), N. Gehrels (NASA/GSFC), D. Burrows (PSU)
on 27 Mar 2007; 16:18 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

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For the purpose of the identification of their nature, we selected 2 unidentified INTEGRAL sources from the 3rd IBIS catalogue (Bird et al., APJS in press, astro-ph/0611493; see also Keek et al., ATel #810). We selected those which had also been observed with the XRT instrument onboard Swift, thereby giving an arcsec-sized X-ray error box (see http://www.asdc.asi.it ) so to ease the localization of the optical counterpart (see Masetti et al. 2006, A&A, 459, 21 and references therein for details on this procedure). We then obtained optical spectroscopy on 2007 March 10-11 with the REOSC spectrograph of the 2.15m 'Jorge Sahade' telescope at CASLEO (Argentina).
The total exposure time was 60 minutes for each object.

Details on the observed sources are given below (coordinates refer to epoch J2000).

IGR J15539-6142: Masetti et al. (ATel #815) proposed 2 possible candidates for this hard X-ray source; however, the XRT observation shows that, within the IBIS error circle, only the nucleus of galaxy ESO 136-6 emits X-rays, thus making it the most viable optical candidate for this INTEGRAL source while at the same time excluding the emission line star HD 141689. Optical spectra of the nucleus of ESO 136-6 show the presence of narrow Halpha, [NII] and [SII] emissions at redshift z = 0.015, fully consistent with the Hyperleda catalogue value (see http://leda.univ-lyon1.fr/ ). Line ratios are consistent with those of a Seyfert 2 AGN.

IGR J16500-3307: this INTEGRAL source is coincident with the ROSAT bright object 1RXS J164955.1-330713 (Voges et al. 1999, A&A, 349, 389), which is the only soft X-ray source within the IBIS error circle. There are several optical objects within its 14"-radius error box. The Swift/XRT pointing allowed us to reduce this uncertainty to 3.4 arcsec. The only optical object within the XRT error box is the USNO-A2.0 source U0525_24170526 (RA = 16 49 55.64; Dec = -33 07 01.8); its optical spectrum shows Halpha, Hbeta, HeI and HeII emissions at z = 0, thus indicating that this is a Galactic object. The spectral features above resemble those of a Cataclysmic Variable or of a Low-Mass X-ray Binary.

We thank A. De Franceschi for assistance at the telescope.