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

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Soft X-ray properties of the narrow line QSO Ton S180 k
(RX J0057.3-2222)

H.H. Fink 1, R. Walter 2, 3, N. Schartel 4 and D. Engels 5

1 Max- Planck-Institut für extraterrestrische Physik, D-85748 Garching bei München, Germany
2 INTEGRAL Science Data Centre, Chemin d'Écogia 16, CH-1290 Versoix, Switzerland
3 Observatoire de Geneve, Chemin des Maillettes 51, CH-1290 Sauverny, Switzerland
4 ESA, IUE Observatory, Vilspa, P.O.Box 50727, E-28080 Madrid, Spain
5 Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany

Received 8 March 1996 / Accepted 14 May 1996


Close to the celestial position of the QSO Ton S180 a bright X-ray source was detected by ROSAT during the all-sky survey. Pointed follow-up observations confirmed the positional coincidence of RX J0057.3-2222 and Ton S180. The X-ray radiation of the quasar is intensity variable on time scales of hours to days and years with amplitudes up to a factor of about two. During the observation in June 1992 an outburst of the X-ray emission occurred with a rise time of less than one day and a decay extending possibly over weeks. These intensity variations are not accompanied by corresponding spectral changes. On the other hand, a clear change of the quasar's spectrum was observed for the time interval between two observations separated by 23 days without noticeable intensity variations. For most observations the measured count rate spectrum of the QSO in the soft (0.1 - 2.4)keV energy band is best represented by a two- component spectrum consisting of a power law with a photon index [FORMULA] and a steep low energy component for which we choose a black body spectrum. Its temperature is unusually low, [FORMULA], causing this component to be dominant below 0.3 keV. During the observed spectral change the power law component flattens ( [FORMULA] ) and the low energy component vanishes. The low energy absorption obtained from the fit is inconsistently larger than the absorption due to cold matter in our Galaxy. Although intrinsic absorption by cold matter is therefore probable, there are no signs in the residuals of the spectral fits indicating the existence of ionized matter on the line of sight. With its variable X-ray emission and its steep soft spectrum the narrow line QSO Ton S180 shares the typical X-ray properties of the class of narrow line Seyfert 1 galaxies.

Key words: galaxies: active — galaxies: Seyfert — galaxies: quasars individual Ton S180 - X-rays: general

Send offprint requests to: H.H. Fink, MPE address

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