Chandra Observation of NGC 1559: Eight Ultraluminous X-ray Sources Including a Compact Binary Candidate

TL;DR

This study used Chandra X-ray observations of NGC 1559 to identify eight ultraluminous X-ray sources, including a potential stellar-mass black hole binary candidate exhibiting periodicity, thus expanding the ULX sample and understanding their nature.

Contribution

First detailed analysis of ULX population in NGC 1559 with identification of a potential compact binary ULX candidate based on periodicity.

Findings

Eight ULXs identified with luminosity >10^39 erg/s

One ULX shows a periodicity of ~7500 seconds

Most sources have non-thermal, hard X-ray spectra

Abstract

Despite the 30-year history of ultra-luminous X-ray sources (ULXs) studies, issues like the majority of their physical natures (i.e., neutron stars, stellar-mass black holes, or intermediate black holes) as well as the accretion mechanisms are still under debate. Expanding the ULX sample size in the literature is clearly a way to help. To this end, we investigated the X-ray source population, ULXs in particular, in the barred spiral galaxy NGC 1559 using a Chandra observation made in 2016. In this 45-ks exposure, 33 X-ray point sources were detected within the 2.'7 isophotal radius of the galaxy. Among them, 8 ULXs were identified with the criterion of the X-ray luminosity $L_x>10^{39}$ erg s$^{-1}$ (0.3-7~keV). Both X-ray light curves and spectra of all the sources were examined. Except for some low-count spectra that only provide ambiguous spectral fitting results, all the X-ray sources were basically spectrally hard and therefore likely have non-thermal origins. While no strong X-ray variability was present in most of the sources owing to the relatively short exposure of the observation, we found an intriguing ULX, named X-24, exhibiting a periodicity of $\sim$7500s with a detection significance of 2.7$\sigma$. We speculate that it is the orbital period of the system. Roche-lobe over flow and Roche limit are consistent with the speculation. Thus, we suggest that X-24 may be the one of the rare compact binary ULXs, and hence, a good candidate as a stellar-mass black hole.