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Optical spectroscopic followup of XMMSL1 J164303.7+653253 in the error box of IGR J16426+6536

ATel #1800; P. Parisi, N. Masetti, A. Malizia (INAF/IASF, Bologna), L. Morelli (Univ. Padova), E. Mason (ESO, Chile), A. J. Dean (Univ. Southampton) and P. Ubertini (INAF/IASF, Rome)
on 22 Oct 2008; 15:38 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Optical, X-ray, AGN

We report on a spectroscopic observation of the optical object USNO-A2.0 1500-06133361 (with J2000 coordinates RA = 16 43 04.07, Dec = +65 32 50.9 and magnitude R ~ 18.9) inside the error circle of the XMM-Newton slew source XMMLS1 J164303.7+653253 (see Ibarra et al., ATel #1397), possibly associated with the unidentified INTEGRAL source IGR J16426+6536 (Bird et al. 2007, ApJS, 170, 175).

The observations were performed on 2008 February 04, starting at 06:10 UT, with DOLORES, a focal reducer instrument installed on the 3.58m Telescopio Nazionale Galileo (TNG) in the Astronomical Observatory of Roque de Los Muchachos (Santa Cruz de La Palma, Spain), for a total exposure time of 1800 s.
We found that the optical spectrum of the source shows a blue continuum with superimposed several redshifted emission features, among which Hbeta, Hgamma, Hdelta, and Hepsilon Balmer lines, as well as narrow [OII] and [OIII] lines. Using the narrow emission features, we determine a redshift z=0.323+-0.001 for the source.

From all of the above we identify the soft X-ray source XMMLS1 J164303.7+653253 as a Seyfert 1 Active Galactic Nucleus (AGN).

We remark that, as reported in Ibarra et al. (ATel #1397), in addition to the XMMLS1 source, there are two faint ROSAT sources within the error box of IGR J16426+6536. Therefore one cannot be completely sure that XMMLS1 J164303.7+653253 is the actual X-ray counterpart of this INTEGRAL source.
However, given that (1) our results demonstrate that XMMSL1 J164303.7+653253 is an AGN, and (2) according to Ibarra et al. (ATel #1397), the estimate of the false detection probability of a random XMM source within the IBIS error box of IGR J16426+6536 is about 0.5%, we can confidently state that these two X-ray sources are the same object, which we identified here as a Seyfert 1 AGN.


We thank F. Ghinassi for the observation performed in service mode at TNG.