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Optical classification of 5 Swift hard X-ray sources

ATel #735; N. Masetti, L. Bassani (IASF/INAF, Bologna), A. J. Dean (Univ. Southampton), P. Ubertini (IASF/INAF, Rome) and R. Walter (ISDC, Versoix)
on 13 Feb 2006; 16:42 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Optical, X-ray, AGN, Binary

Referred to by ATel #: 757, 760, 763, 765

Between February 6 and 10, 2006, we used the instrument BFOSC mounted on the `G.D. Cassini' 1.5m telescope of the Astronomical Observatory of Bologna located in Loiano (Italy) to acquire optical spectra of the counterparts to 5 hard X-ray sources recently discovered by Swift (see ATels #669, #677 and #697) in order to assess their nature. Total exposure times ranged from 40 to 80 minutes, depending on the object brightness.

In the following table, we report the classification and redshift of these 5 sources:

Source                Class    z    ATel ref. 

SWIFT J0444.1+2813 Sy2 0.010 #669
SWIFT J0732.5-1331 LMXB 0 #697
SWIFT J0823.4-0457 Sy2 0.023 #697
SWIFT J0918.5+1618 Sy1 0.029 #697
SWIFT J1200.8+0650 Sy2 0.035 #677

Notes on individual sources.

SWIFT J0444.1+2813: narrow Halpha, Hbeta, [NII], [SII], [OI] and [OIII] emissions on a reddened continuum are detected in the spectrum of the optical counterpart. All features are at redshift z = 0.010+-0.001, consistently with the 21cm radio measurement of Pantoja et al. (1997, AJ, 113, 905). The line narrowness and their ratios indicate that this is a Seyfert 2 galaxy.

SWIFT J0732.5-1331: the Swift/XRT error box encompasses two objects. The brighter one (mag. R ~ 12.5) has the spectrum of a normal G/K type Galactic star. The fainter one (mag. R ~ 14), located ~10 arcsec SE, is the true optical counterpart: spectroscopy reveals Halpha, Hbeta, HeI 5875 and HeII 4686 in emission at redshift zero, superimposed on a very blue and otherwise featureless continuum. These signatures are typical of an accretion disk of a Galactic X-ray binary, most likely a Low Mass X-ray Binary.

SWIFT J0823.4-0457: the optical spectrum of the counterpart, galaxy LEDA 23515, shows narrow Halpha, Hbeta, [NII], [SII], [OI] and [OIII] emission lines over a flat continuum. These features are at redshift z = 0.023+-0.001, consistent with Fouque et al. (1992, Catalogue of optical radial velocities, Obs. Lyon et Paris-Meudon, Vol. 1, p. 1). Here also, the line shapes and ratios indicate that this is a Seyfert 2 galaxy.

SWIFT J0918.5+1618: broad Balmer emissions at least up to Hdelta, as well as narrow Halpha, Hbeta, [NII], [SII], [OI], [OIII], HeI and HeII emission lines are detected in the optical spectrum of the counterpart, which moreover shows a blue continuum. From the observed wavelengths of narrow features, we determine a redshift z = 0.029 +- 0.001. The overall spectral appearance allows us to classify this object as a Seyfert 1 galaxy.

SWIFT J1200.8+0650: here, Halpha, Hbeta, [NII], [SII], and [OIII] narrow emission lines are detected over the relatively flat spectral continuum of the optical counterpart, the galaxy LEDA 37894. The features are at redshift z = 0.035+-0.001, consistently with Grogin et al. (1998, ApJS, 119, 277). The line shape and ratios indicate that this also is a Seyfert 2 galaxy.

We thank R. Gualandi and S. Bernabei for the night assistance at the telescope.