2011/06/17 by Dacheng Lin, Eleazar R. Carrasco, E. R. Carrasco +8 · 2 citations
Engineering · Physics and Astronomy · #Accretion (finance) #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Black hole (networking) #Black-body radiation #Flux (metallurgy) #Galaxy #Gamma-ray bursts and supernovae #Luminosity #Mechanics and Biomechanics Studies #Neutron star #Physics #Power law #RADIUS #ROSAT #Radiation #Redshift #X-ray transient #astro-ph.GA #astro-ph.HE
paper · pdf · doi:10.1088/0004-637x/738/1/52
12 pages, 6 figures. Accepted for publication in ApJ
arxiv created 2011/06/17 · openalex publication_date 2011/08/11 · arxiv updated 2015/05/28 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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 ≳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 ∼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 ∝ T 4 relation. This would indicate that the black hole mass is about 10 5 –10 6 M ☉ 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 ≳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 ≳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.