Feeding The Little Monster: An Accreting Intermediate-Mass Black Hole In A Minor Merger Secondary Galaxy

We report the discovery of a luminous supersoft X-ray source associated with the secondary galaxy in a 10:1 minor merger at z =0.216. We identified the system in the 3D-HST survey and observed it serendipitously in more than 100 Chandra observations, providing a combined exposure of 6.6 Msec. The secondary galaxy ((M* /M)=8.24) exhibits X-ray emission detected exclusively below 2.0 keV, with no significant hard-band photons. Because the source is low-count, we characterize its spectrum using Bayesian hardness-ratio analysis combined with simulations of physically motivated spectral models folded through the Chandra response files. The observed count rates and hardness ratios are reproduced by a thermal accretion-disk spectrum with rest-frame inner-disk temperatures kT in =0.18-0.30 keV and luminosities log(L X /erg s-1)=40.5-41.2. Interpreted using a standard thin-disk framework, these properties imply a black hole mass of M about 8 times10 squared -- 5 times10 cubed,M accreting at about 0.2-0.5 of the Eddington rate. Alternative interpretations involving ultraluminous X-ray supersoft sources are disfavored by the source's luminosity, temperature, lack of variability, and the host galaxy's low star-formation rate. Numerical simulations predict that black holes in smaller, secondary galaxies could experience episodic rapid growth during minor mergers. This discovery provides observational support for this scenario and suggests that minor interactions may be an important channel for assembling supermassive black holes in dwarf galaxies.