Detection of X-ray elongated emission from a ultraluminous X-ray source in the interacting pair of galaxies NGC 5953/5954

TL;DR

This study investigates a bright ultraluminous X-ray source in NGC 5953/5954, revealing its elongated shape, optical counterpart, and complex emission features, suggesting a massive black hole binary system rather than stellar-mass black hole beaming.

Contribution

First detailed multiwavelength analysis of an elongated ULX in a starburst galaxy pair, proposing a massive black hole binary as the most plausible explanation.

Findings

ULX shows elongated X-ray morphology with a 53-degree inclination.

Optical counterpart is consistent with a young star cluster or O-type supergiant.

Mild beaming from a >50 solar mass black hole best explains the observed luminosity.

Abstract

We present radio through X-ray results of a bright (10^{40} erg/s in the 0.5 to 8.0 keV band) ultraluminous X-ray source (ULX), CXOU J153434.9+151149, in the starburst, interacting pair of galaxies NGC 5953/5954. Chandra image of this ULX shows that it is elongated. From HST/WFPC2/F606W data we have detected a counterpart of the ULX system with M_{F606W} ~-7.1 \pm 0.7 mag. This optical counterpart may be either an O-type supergiant star or a young star cluster. From our Fabry-Perot interferometric observations, we have detected Halpha and [NII](6584 A) diffuse emission, with velocity gradients up to 60 km/s at the astrometric corrected Chandra position of the ULX. Different scenarios have been invoked as to explain the possible nature of CXOU J153434.9+151149. Based on the observed X-ray morphology of the ULX, we determine that the inclination angle to the elongated emission will be ~53 deg. Beaming with this geometry from a stellar-mass black hole system will be inadequate to explain the observed X-ray luminosity of this ULX. Finally, we suggest that mild-beaming from a binary black hole with mass more than 50 solar masses, associated with a young star cluster, is the most favorable scenario that describes the multiwavelength properties of this ULX. Future observations are highly essential to determine the nature of this rare object.