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

Letter to the Editor

Discovery of a giant and luminous X-ray outburst from the optically inactive galaxy pair RX J1242.6-1119 ^*

Stefanie Komossa ¹ and Jochen Greiner <sup>2

1</sup> Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany

Received 30 July 1999 / Accepted 18 August 1999

Abstract

We report the discovery of large-amplitude X-ray variability from the direction of the previously unknown, optically inactive galaxy pair RX J1242.6-1119. The X-ray source shows variability by a factor 20 between the ROSAT all-sky survey and a later pointed observation separated by 1.5 yr. Its spectrum is extremely soft with photon index , among the steepest ever observed among galaxies. Based on the redshift derived from the optical spectra, z=0.05, the source's intrinsic luminosity is large, 9 10⁴³ erg s^-1. Surprisingly, the optical spectra of both galaxies are characterized by absorption lines and do not show signs of (Seyfert) activity. This makes RX J1242-11 the third candidate for giant-amplitude variability in an otherwise non-active galaxy, the first two being NGC 4552 (in the UV; Renzini et al. 1995) and NGC 5905 (in X-rays; Bade et al. 1996, Komossa & Bade 1999). Several mechanisms to explain this unexpected and peculiar behavior are investigated. The most likely one seems to be an accretion event onto an otherwise dormant supermassive black hole (SMBH), e.g., by a tidal disruption event.

Key words: accretion, accretion disks galaxies: active galaxies: individual: RX J1242.6 1119 galaxies: nuclei X-rays: galaxies

* Partly based on observations obtained at the European Southern Observatory, La Silla, Chile.

Send offprint requests to: St. Komossa (skomossa@mpe.mpg.de)

SIMBAD Objects

Contents

• 1. Introduction
• 2. Data reduction and analysis
  • 2.1. Optical observations
  • 2.2. X-ray observations
    • 2.2.1. Temporal analysis
    • 2.2.2. Spectral analysis
• 3. Discussion
  • 3.1. Previous observations of large-amplitude X-ray outbursts in optically non-active galaxies
  • 3.2. Variability mechanisms
• 4. Summarizing conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999
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