An Ultraluminous Supersoft Source in a Dwarf Galaxy of Abell 85: An Intermediate-Mass Black Hole Candidate

TL;DR

This paper reports the discovery of a candidate intermediate-mass black hole in a dwarf galaxy of Abell 85, identified through its unique X-ray supersoft emission, suggesting a new method for detecting such black holes in low-mass galaxies.

Contribution

The study presents the first detection of an ultraluminous supersoft X-ray source in a dwarf galaxy, indicating a potential intermediate-mass black hole and proposing a link between galaxy interactions and black hole activity.

Findings

X-ray source with luminosity ~10^41 erg/s detected in a dwarf galaxy.

The source exhibits supersoft X-ray emission under 1 keV.

Estimated black hole mass falls within the intermediate regime.

Abstract

We study a large sample of dwarf galaxies using archival Chandra X-ray observations, with the aim of detecting accreting intermediate-mass black holes (IMBHs). IMBHs are expected to inhabit dwarf galaxies and to produce specific signatures in terms of luminosity and X-ray spectra. We report the discovery of an X-ray source associated with an Abell 85 dwarf galaxy that fits the IMBH description. The stellar mass of the host galaxy is estimated to be 2 $\times$ 10$^8$ $M_\odot$, which makes it one of the least massive galaxies to potentially host an accreting black hole. The source is detected in the soft band, under 1 keV, while undetected at higher energies. The X-ray luminosity is $\approx$ 10$^{41}$ erg s$^{-1}$, making it almost three orders of magnitude more luminous than the most luminous stellar-mass supersoft emitters. From the galaxy stellar mass vs. black hole mass relation, we estimate the mass to be within the intermediate regime. Another method that resulted in an intermediate mass relies on the fact that supersoft emission is expected to be associated with high accretion rates, approaching the Eddington limit. We suggest that the observed offset of the X-ray source from the galactic center ($\approx$ 1.8 kpc) is due to galaxy interactions, and we present evidence from the literature that supports the relation between black hole activity and galaxy interactions.