Study explores X-ray sources in globular cluster NGC 6528
Tomasz Nowakowski
astronomy writer
Stephanie Baum
scientific editor
Robert Egan
associate editor
By analyzing the data from NASA's Chandra spacecraft, astronomers have conducted the first X-ray study of a globular cluster known as NGC 6528. Results of the study, August 13 on the arXivpreprint server, yield important insights regarding the population of X-ray sources in this cluster.
Globular clusters (GCs) are spherical collections of tightly gravitationally bound stars. They contain from tens of thousands to millions of stars and investigating them could help us better understand the formation, history and evolution of early-type galaxies, as the origin of GCs seems to be closely linked to periods of intense star formation.
Located some 25,800 light years away, NGC 6528 is a galactic globular cluster with an estimated age of about 11 billion years. With a metallicity at a level of -0.1, NGC 6528 is one of the most metal-rich GCs in our galaxy.
In order to shed more light on the properties of NGC 6528, a team of astronomers, led by Bernard Leal of the Michigan State University, decided to explore this cluster in X-rays using the data from Chandra's Advanced CCD Imaging Spectrometer (ACIS). Their study was complemented by data from the Hubble Space Telescope (HST) and Gaia satellite.
"This paper has presented the first X-ray study of NGC 6528, one of the most metal-rich GCs in the galaxy," the researchers write.
The relatively deep archival imaging from Chandra allowed Leal's team to identify 18 X-ray sources (designated s01-s018) within the half-light radius of NGC 6528. Two of these sources, designated s01 and s08, appear to be located within the core radius of the cluster.
The brightest X-ray source from this sample is s01, with an unabsorbed X-luminosity at a level of 100 nonillion erg/s. The source exhibits a long-term variability and the astronomers assume that it is most likely a mass-segregated compact binary system.
The second brightest X-ray source reported in the paper is s14鈥攈aving an unabsorbed luminosity of approximately 46 nonillion erg/s. The X-ray emission of this source was found to be relatively soft, and given that it is located near the half-light radius of NGC 6528, it may be a field star.
In general, the authors of the paper emphasized that the identified X-ray sources are primarily cataclysmic variables (CVs) and active binaries (ABs). However, they noted that one or more of the brighter sources could be quiescent low-mass X-ray binaries (LMXBs).
Further observations, focused mainly on time-series optical or near-infrared photometry, could improve the classification of the detected X-ray sources.
Summing up the results, the astronomers emphasized what their findings can tell us regarding the general properties of NGC 6528.
"For this cluster, it appears that the X-ray binary-enhancing effects of high metallicity are outweighed by the cluster's advanced dynamical evolution, leading to a relatively modest X-ray source population," the scientists conclude.
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More information: Bernard Leal et al, The X-ray Source Population of the Metal-Rich Globular Cluster NGC 6528, arXiv (2025).
Journal information: arXiv
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