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

• Published in: The Astrophysical journal Vol. 956; no. 1; pp. 41 - 54
• Main Authors: Ma, Chen-Hsun, Li, Kwan-Lok, Chu, You-Hua, Kong, Albert K. H.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-10-2023; IOP Publishing
• Subjects: Astrophysics; Binary stars; Black holes; Deposition; Galaxies; Light curve; Luminosity; Neutron stars; Orbits; Overflow; Roche limit; Roche lobe overflow; Spectra; Spiral galaxies; Stellar mass black holes; Ultraluminous x-ray sources; X ray sources; X-ray astronomy; X-rays
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aced04

Despite the 30 yr history of ultraluminous X-ray sources (ULXs) studies, issues such as 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, eight 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 nonthermal 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 ∼7500 s with a detection significance of 2.7
σ
. We speculate that it is the orbital period of the system. Roche-lobe overflow and Roche limit are consistent with the speculation. Thus, we suggest that X-24 may be one of the rare compact binary ULXs, and hence, a good candidate as a stellar-mass black hole.