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Optical spectroscopic identification of candidate counterparts to four INTEGRAL sources

ATel #957; N. Masetti, A. Malizia (INAF-IASF, Bologna), A. J. Dean (Univ. Southampton), A. Bazzano (INAF-IASF, Rome) and R. Walter (ISDC, Versoix)
on 4 Dec 2006; 16:42 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Optical, X-ray, AGN

For the purpose of the identification of their nature, we selected 4 unidentified INTEGRAL sources from the 3rd IBIS catalogue (Bird et al., astro-ph/0611493) for which a single NVSS radio source within the IBIS/INTEGRAL error box is found.

Observations on northern hemisphere sources were performed on 2006 October 2 with the `G.D. Cassini' 1.5m telescope of the Astronomical Observatory of Bologna located in Loiano (Italy), and on 2006 November 14 and 27 with the 1.8m `Copernicus' telescope of the Astronomical Observatory of Asiago (Italy).
We also retrieved spectra of southern hemisphere objects from the 6dF Galaxy Survey archive (http://www.aao.gov.au/local/www/6df/).

Details on each observed source are given below.

IGR J00040+7020: the error box of this X-ray source includes the radio object NVSS J000402+701916, associated with the 2MASS extended source 2MASX J00040192+7019185.
Spectroscopy of the optical object at the radio position, obtained with the Copernicus telescope, reveals Halpha, Hbeta, [OIII], [OI], [NII] and [SII] narrow emissions at redshift z = 0.096. Line ratio diagnostics, as per Ho et al. (1993, ApJ, 417, 63), indicate that this is a Seyfert 2 AGN.

IGR J02504+5443: the error box of this source (labeled as IGR J02501+5440 by Kuiper et al. 2006, ATel #939), as reported in the 3rd IBIS Survey, encompasses the radio-emitting galaxy LEDA 166445, which is also an IRAS mid-infrared source.
Optical spectra acquired with the Cassini and the Copernicus telescopes show a reddened continuum with Halpha, [OIII], [NII] and [SII] narrow emission lines at redshift z = 0.015, consistent with the value found by Nakanishi et al. (1997, ApJS, 112, 245). Line ratio diagnostics (Ho et al. 1993, ibid.) suggest that this object is consistent with being a Seyfert 2 AGN.

IGR J15161-3827: within the INTEGRAL error box of this object, the galaxy LEDA 2816946, associated with a NVSS and an IRAS source, is found.
A 6dF spectrum of this galaxy acquired on 2003 March 11 shows that the optical source a displays Halpha, Hbeta, Hgamma, [OIII], [NII] and [SII] narrow emission lines at redshift z = 0.036500. The line ratio diagnostics of Ho et al. (1993, ibid.) indicate that this a borderline source between a LINER and a Seyfert 2 AGN.

IGR J16385-2057: the galaxy Oph J163830-2055, positionally consistent with a NVSS and an IRAS object and with the ROSAT bright X-ray source 1RXS J163830.9-205520, is present in the INTEGRAL error circle.
6dF optical spectroscopy was acquired on 2003 June 8: the spectrum shows broad Halpha, Hbeta, Hgamma and Hdelta plus narrow [OIII] lines, all in emission; the redshift of the source is z = 0.026879. The relative narrowness of Hbeta and its flux ratio with respect to [OIII] allow one, according to the criteria of Osterbrock & Pogge (1985, ApJ, 297, 166), to classify this object as a Narrow-Line Seyfert 1 AGN.

As usual, we stress that pointed soft X-ray observations with satellites affording arcsecond localizations (such as Chandra, XMM-Newton or Swift) are essential to confirm the above associations concerning IGR J00040+7020, IGR J02504+5443 and IGR J16385-2057. Instead, the association of IGR J16385-2057 with a bright ROSAT source makes our identification secure.

We thank S. Galleti for the service observations performed in Loiano and H. Navasardyan for the service observations acquired in Asiago. We also acknowledge the use of the NVSS and 6dFGS archives. This research made use of the SIMBAD and NED online catalogues.