Ultraluminous X-ray sources (ULXs) in globular clusters (GCs) are low-mass X-ray binaries that achieve high X-ray luminosities through a currently uncertain accretion mechanism. Using archival Chandra and Hubble Space Telescope observations, we perform a volume-limited search (≲70 Mpc) of 21 of the most massive ($\gt 10^{11.5} \, \mathrm{M}_\odot$) early-type galaxies to identify ULXs hosted by GC candidates. We find a total of 34 ULX candidates above the expected background within five times the effective radius of each galaxy, with 10 of these ($\sim 29.4{{\ \rm per\ cent}}$) potentially hosted by a GC. A comparison of the spatial and luminosity distributions of these new candidate GC ULXs with previously identified GC ULXs shows that they are similar: both samples peak at LX ∼ a few × 1039 erg s−1 and are typically located within a few effective radii of their host galaxies.
A prime motivation for compiling catalogues of any celestial X-ray source is to increase our numbers of rare subclasses. In this work, we take a recent multimission catalogue of ultraluminous X-ray sources (ULXs) and look for hitherto poorly-studied ULX candidates that are luminous ($L_{\rm X} \ge 10^{40}\,{\rm erg\,s}^{-1}$), bright ($f_{\rm X} \ge 5 \times 10^{-13}\,{\rm erg\,cm}^{-2}\,{\rm s}^{-1}$), and have archival XMM–Newton data. We speculate that this luminosity regime may be ideal for identifying new pulsating ULXs (PULXs), given that the majority of known PULXs reach similar high luminosities. We find three sources that match our criteria and study them using archival data. We find 4XMM J165251.5−591503 to possess a bright and variable Galactic optical/IR counterpart, and so conclude it is very likely to be a foreground interloper. 4XMM J091948.8−121429 does appear to be an excellent ULX candidate associated with the dwarf irregular galaxy PGC 26378, but has only one detection to date with low data quality. The best data set belongs to 4XMM J112054.3+531040 which we find to be a moderately variable, spectrally hard (Γ ≈ 1.4) X-ray source located in a spiral arm of NGC 3631. Its spectral hardness is similar to known PULXs, but no pulsations are detected by accelerated pulsation searches in the available data. We discuss whether other missions provide objects for similar studies and compare this method to others suggested for identifying good PULX candidates.
more » « less- NSF-PAR ID:
- 10455165
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 525
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 3330-3343
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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