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Optical identification of IGR J13091+1137 as a heavily obscured AGN

ATel #719; N. Masetti, E. Palazzi, A. Malizia (INAF-IASF, Bologna), A. J. Bird (Univ. Southampton), L. Norci (Dunsink Obs.), I. Bruni (INAF-Oss. Astron. Bologna), A. Bazzano and A. De Rosa (INAF/IASF, Rome)
on 3 Feb 2006; 14:42 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Optical, X-ray, AGN

In order to identify the nature of the active nucleus of spiral galaxy NGC 4992, associated with the Chandra and INTEGRAL high-energy sources CXOU J130905.6+113803 and IGR J13091+1137 (see Halpern, Atel #572 and Sazonov et al., 2005, A&A, 444, L37), we acquired a 15-minute optical spectrum on February 1, 2006 with the instrument BFOSC mounted on the `G.D. Cassini' 1.5m telescope of the Astronomical Observatory of Bologna located in Loiano (Italy).

The spectral continuum in the range 4000-8000 Angstroms is typical of a normal spiral galaxy, with CaII H+K doublet, G-band at 4304A, Hbeta, Mg b band at 5175A, FeI at 5270A and NaI doublet at 5890A absorptions, and a weak [NII] 6583A emission. All of these features are at a redshift z = 0.025 (consistent with that reported on the Hyperleda catalogue).

The X-ray (0.5-8 keV) luminosity (1.2*10^42 erg cm-2 s-1; Sazonov et al. 2005) combined with the optical spectrum properties suggests that NGC 4992 belongs to the class of X-ray Bright, Optically Normal Galaxies (XBONGs; e.g., Comastri et al. 2002, astro-ph/0203019; see also Severgnini et al. 2003, A&A, 406, 483). The intrinsic absorption measured by Chandra and the INTEGRAL detection further indicate that in this case the best explanation for the nature of this source is a heavily absorbed AGN. To the best of our knowledge, this is the first XBONG detected at energies above 20 keV, and its closeness to Earth makes NGC 4992 an ideal laboratory for the study of this enigmatic class of galaxies.

We acknowledge the Astronomical Observatory of Bologna for the Service Programme in Loiano.