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Astron. Astrophys. 317, 25-35 (1997)

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1. Introduction

In the First Extreme Ultraviolet Explorer Source Catalogue (Bowyer et al., 1994) the bright EUV source EUVE J0057-223 was identified with the UV-excess object KUV 00549-2239 of the Second Kiso Catalogue (Kondo, Noguchi, and Maehara, 1984). Bowyer et al.(1994) classified this object as a white dwarf. A close inspection of the sky field around KUV 00549-2239 (Hamuy, Maza, 1989; Bowen et al., 1994) revealed that the UV object is actually identical with the bright emission line object Ton S180 (Wisotzki et al., 1995).

Ton S180 is an optically bright galaxy with a Seyfert 1 spectrum (V=14.4) (Veròn-Cetty, 1984; Chavira, 1958) showing quite narrow permitted lines (FWHM = 900 km/s) and strong FeII emission. These features classify Ton S180 as a narrow line Seyfert 1 object (Boller, Brandt, Fink, 1995). With a redshift of [FORMULA] (Wisotzki et al., 1995) its absolute magnitude is [FORMULA] ( [FORMULA] ) qualifying the object as a QSO (Schmidt, Green, 1986). UBVRI photometry of Ton S180 revealed the quasar to be slightly variable with amplitudes of the order of 0.1 mag (Veròn-Cetty, 1984; Hamuy, Maza, 1987; Winkler, 1992). EUV flux measurements of EUVE J0057-223 reported by Vennes et al. (1995) yielded a flux density of [FORMULA] at [FORMULA] (0.14 keV) assuming a power law continuum with a photon index of [FORMULA] and Galactic absorption [FORMULA]. In addition, there are indications of large amplitude EUV variability. The ROSAT observations with their superb photon statistics are the first X-ray measurements of the narrow line QSO Ton S180.


Table 1. Observation log

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