DISCOVERY OF AN ULTRASOFT X-RAY TRANSIENT SOURCE IN THE 2XMM CATALOG: A TIDAL DISRUPTION EVENT CANDIDATE
- Universite de Toulouse, UPS-OMP, IRAP, Toulouse (France)
- Gemini Observatory/AURA, Southern Operations Center, Casilla 603, La Serena (Chile)
- Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802 (United States)
We have discovered an ultrasoft X-ray transient source, 2XMMi J184725.1-631724, which was detected serendipitously in two XMM-Newton observations in the direction of the center of the galaxy IC 4765-f01-1504 at a redshift of 0.0353. These two observations were separated by 211 days, with the 0.2-10 keV absorbed flux increasing by a factor of about nine. Their spectra are best described by a model dominated by a thermal disk or a single-temperature blackbody component (contributing {approx}>80% of the flux) plus a weak power-law component. The thermal emission has a temperature of a few tens of eV, and the weak power-law component has a photon index of {approx}3.5. Similar to the black hole X-ray binaries in the thermal state, our source exhibits an accretion disk whose luminosity appears to follow the L{proportional_to}T {sup 4} relation. This would indicate that the black hole mass is about 10{sup 5}-10{sup 6} M{sub sun} using the best-fitting inner disk radius. Both XMM-Newton observations show variability of about 21% on timescales of hours, which can be explained as due to fast variations in the mass accretion rate. The source was not detected by ROSAT in an observation in 1992, indicating a variability factor of {approx}>64 over longer timescales. The source was not detected again in X-rays in a Swift observation in 2011 February, implying a flux decrease by a factor of {approx}>12 since the last XMM-Newton observation. The transient nature, in addition to the extreme softness of the X-ray spectra and the inactivity of the galaxy implied by the lack of strong optical emission lines, makes it a candidate tidal disruption event. If this is the case, the first XMM-Newton observation would have been in the rising phase and the second one in the decay phase.
- OSTI ID:
- 21582936
- Journal Information:
- Astrophysical Journal, Vol. 738, Issue 1; Other Information: DOI: 10.1088/0004-637X/738/1/52; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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