Authors:Yi-Chi (Judy) Chang (張怡琪), R. Soria, Albert K.H. Kong, A.Graham, Kiril Grishin, I.V. Chiligarian
Abstract: We report on the intriguing properties of a variable X-ray source projected at the outskirts of the elliptical galaxy NGC 6099 (d≈139 Mpc). If truly located near NGC 6099, this is a hyperluminous X-ray source that reached an X-ray luminosity LX ≈ a few times 1042 erg s−1 in 2012 February (XMM-Newton data), about 50 to 100 times brighter than in 2009 May (Chandra) and 2023 August (XMM-Newton). The X-ray spectrum was soft at all three epochs, with a thermal component at kT ≈ 0.2 keV and a power-law photon index > 3. Such properties make it a strong candidate for an intermediate mass black hole (IMBH). We also discovered a point-like, blue optical counterpart (mg,Vega≈24.7 mag, Mg,Vega≈−11.2 mag), from images taken by the Canada-France-Hawaii Telescope, and later confirmed with Hubble Space Telescope observations. The optical continuum can be modeled as stellar emission from a compact star cluster or an X-ray-irradiated accretion disk, consistent with the IMBH scenario. We discuss alternative explanations for the nature of this system. A possible scenario is tidal stripping of an orbiting star, with repeated X-ray outbursts every few years. An alternative possibility is that the thermal X-ray emission seen in 2009 was from shocked gas in the self-intersecting tidal stream during the rising phase of a tidal disruption event, while the 2012 and 2023 emissions were from the fully-formed accretion disk.