Astron. Astrophys. 349, L45-L48 (1999)

Astron. Astrophys. 349, L45-L48 (1999)

1. Introduction

Giant X-ray outbursts among galaxies, even the active ones, are rare. `Normal' galaxies, starbursts, and most LINERs show constant soft X-ray emission (e.g., Fabbiano 1989, Vogler & Pietsch 1999, Komossa et al. 1999) and X-ray luminosities of typically 10^38-40 erg s^-1. Some off-nuclear, X-ray-bright point sources, several of them variable, have been detected recently but they seldom exceed 10³⁹ erg s^-1 (e.g., Immler et al. 1998, Komossa & Schulz 1998, and references therein). In contrast, many active galactic nuclei (AGN; [FORMULA] erg s^-1) are known to be X-ray variable by typically a factor 2-3 (e.g., Mushotzky et al. 1993). Outbursts with amplitudes exceeding a factor 10-20 are very rare even among AGN, though. Therefore, it was quite surprising when giant X-ray outbursts (factors 100 in countrate) from the two optically `rather' inactive galaxies IC 3599 (Brandt et al. 1995, Grupe et al. 1995) and NGC 5905 (Bade et al. 1996, Komossa & Bade 1999) were discovered. Both outbursters were characterized by very soft X-ray spectra and reached high outburst luminosities.

The ROSAT all-sky survey (RASS; Voges et al. 1996) provides an excellent data base to search for further cases of giant X-ray variability. Such outbursts provide a powerful tool to probe for the existence of SMBHs in nearby galaxies and to study the physics of accretion events. In particular, Rees (1988,1989,1990) proposed to use the UV-X-ray flares expected from the tidal disruption events of stars swallowed by SMBHs to detect these SMBHs in nearby, non-active galaxies.

RXJ1242-11, serendipituously located in the field-of-view of a ROSAT PSPC pointing, was originally selected for optical follow-up observations due to its very soft X-ray spectrum in the course of the extension of the identification program of supersoft X-ray sources from the RASS (Greiner 1996) to pointed observations of nearby galaxies. We report here the optical and X-ray properties of this previously unknown source (Sect. 2) and discuss scenarios to account for its very peculiar variability behavior (Sect. 3). Luminosities given below are calculated assuming [FORMULA] km/s/Mpc.

Online publication: September 2, 1999
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