The spectral and temporal properties of an Ultra-Luminous X-ray source in NGC 6946

TL;DR

This study analyzes the spectral and temporal variability of the ULX X-7 in NGC 6946, providing evidence for a black hole mass exceeding 100 solar masses based on X-ray spectral fitting and luminosity changes.

Contribution

The paper presents detailed spectral analysis of an ULX, estimating the black hole mass to be around 400 solar masses, which is higher than typical stellar-mass black holes.

Findings

Inner disk temperature decreased from ~0.29 to ~0.26 keV during variability.

Luminosity varied from 3.8 to 2.8 x 10^{39} erg/sec.

Black hole mass estimated at ~400 solar masses.

Abstract

We report variability of the X-ray source, X-7, in NGC 6946, during a 60 ksec Chandra observation when the count rate decreased by a factor of ~1.5 in ~5000 secs. Spectral fitting of the high and low count rate segments of the light curve reveal that the simplest and most probable interpretation is that the X-ray spectra are due to disk black body emission with an absorbing hydrogen column density equal to the Galactic value of 2.1 X 10^{21} cm^{-2}. During the variation, the inner disk temperature decreased from ~0.29 to ~0.26 keV while the inner disk radius remained constant at ~6 X 10^8 cm. This translates into a luminosity variation from 3.8 to 2.8 X 10^{39} ergs cm^{-2} sec^{-1} and a black hole mass of ~400 solar masses. More complicated models like assuming intrinsic absorption and/or the addition of a power-law component imply a higher luminosity and a larger black hole mass. Even if the emission is beamed by a factor of ~5, the size of the emitting region would be > 2.7 X 10^8 cm implying a black hole mass > 180 solar masses. Thus, these spectral results provide strong evidence that the mass of the black hole in this source is definitely > 100 solar masses and more probably ~400 solar masses.