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Spectroscopy of six optical candidates to unidentified INTEGRAL sources

ATel #1033; N. Masetti (INAF/IASF, Bologna), L. Morelli (PUC, Santiago), S. A. Cellone (UNLP, La Plata), L. Bassani, A. Malizia (INAF/IASF, Bologna), A. Bazzano (INAF/IASF, Rome), A. J. Bird (Univ. Southampton), G. Galaz (PUC, Santiago), G. E. Romero (UNLP, La Plata), R. Walter (ISDC, Versoix)
on 27 Mar 2007; 16:01 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Optical, X-ray, AGN

We obtained optical spectra of the putative counterparts of six unidentified INTEGRAL X-ray sources (Bird et al., astro-ph/0611493; Krivonos et al., astro-ph/0701836) using the BFOSC instrument of the 1.5m 'Cassini' telescope of the Astronomical Observatory of Bologna in Loiano (Italy) on 13 Jan 2007, the R-C spectrograph of the CTIO 1.5m telescope in Cerro Tololo (Chile) on 18-19 Feb 2007, and the REOSC spectrograph of the 2.15m 'Jorge Sahade' telescope at CASLEO (Argentina) on 10-11 Mar 2007.

As usually performed, and as proven very effective in this investigation (see e.g. Masetti et al. 2006, A&A, 459, 21), in order to reduce the X-ray error box size to a few arcsec at most, we selected sources with a single ROSAT Bright Source (Voges et al. 1999, A&A, 349, 389) within the INTEGRAL error box.
In a few cases we observed peculiar objects falling within the INTEGRAL error circle of the X-ray source of interest. For these cases, we stress once more that pointed soft X-ray observations with satellites affording arcsecond localizations are essential to confirm the proposed association.
Exposure times ranged from 40 to 90 min, depending on the object brightness.

Notes on individual sources (coordinates refer to epoch J2000):

PKS 0611-663: the spectrum of USNO-A2.0 object U0225_02842370 (RA = 06 11 48.32, Dec = -66 24 34.0), the only optical source in the 6" radius X-ray error circle of ROSAT source 1RXS J061148.5-662430, shows broad Balmer and narrow [OIII] emission lines at redshift z = 0.230. The line shapes and ratios indicate that this object is a Seyfert 1.5 AGN. We remark that this source, which most likely is the optical counterpart of the INTEGRAL hard X-ray emission, is actually not the object PKS 0611-663, lying more than 20" away and thus positionally inconsistent with the ROSAT source. Thus, we suggest to change the name of this newly-identified INTEGRAL source to IGR J06117-6625, following the usual naming convention for INTEGRAL objects.

IGR J07437-5137: the optical spectrum of galaxy LEDA 21656, at z = 0.025 (at odds with the redshift z = 0.0086 from the Hyperleda catalogue, http://leda.univ-lyon1.fr/ ) shows narrow Halpha, [NII], and [OII] emission lines superimposed on the stellar continuum of the galaxy. The Halpha/[NII] line ratio is consistent with that of Seyfert 2 AGNs.

NGC 4180: from the nucleus of this radio-emitting galaxy (Condon et al. 1998, AJ, 115, 1693) we detect Halpha, [NII], [SII] and [OIII] lines in emission at redshift z = 0.006 (consistent with the Hyperleda catalogue value). Line ratios point to the fact that this most likely is a Seyfert 2 galaxy.

IGR J14561-3738: the spectrum of galaxy LEDA 53392, at redshift z = 0.025 (consistent with that in the Hyperleda catalogue), shows Halpha, [NII] and [SII] in emission superimposed on the galaxy stellar continuum. Line ratios are consistent with those of Seyfert 2 AGNs.

IC 4518a: we detect strong and narrow Balmer, [OI], [OIII], [NII] and [SII] emission lines in the optical spectrum of the nucleus of this radio-emitting galaxy (Mauch et al. 2003, MNRAS, 342, 1117), at redshift z = 0.016 (coincident with the result of West et al. 1981, A&AS, 46, 57). Line ratios confirm that this galaxy hosts a Seyfert 2 AGN.

IGR J16056-6110: spectroscopy of the galaxy LEDA 3079933 within the 8" radius error box of ROSAT source 1RXS J160551.9-611142 shows that this object has broad Balmer and narrow [OIII] emissions at redshift z = 0.025. Line width and ratios allow us to identify this source as a Seyfert 1.5 AGN.

We thank S. Galleti and S. Bernabei for the service mode observations made in Loiano, C. Aguilera and A. Gomez for assistance at CTIO, and A. De Franceschi for assistance at CASLEO.