Population of X-ray Sources in the Intermediate-Age Cluster NGC 3532: a Test Bed for Machine-Learning Classification

Abstract

Open clusters are thought to be the birth place of most stars in the Galaxy. Thus, they are excellent laboratories for investigating stellar evolution, and X-ray properties of various types of stars (including binary stars, evolved stars, and compact objects). In this work, we investigate the population of X-ray sources in the nearby 300-Myr-old open cluster NGC 3532 using Chandra X-ray Observatory and multi-wavelength data from several surveys. We apply a random-forest machine-learning pipeline (MUWCLASS) to classify all confidently detected X-ray sources (S/N$>5$) in the field of NGC 3532. We also perform a more detailed investigation of brighter sources, including their X-ray spectra and lightcurves. Most X-ray sources are confirmed as coronally-active low-mass stars, many of which are confidently identified by MUWCLASS. Several late B or early A-type \textbf{stars} are relatively bright in X-rays, most of which are likely binaries. We do not find any compact objects among X-ray sources reliably associated with NGC 3532, down to the limiting X-ray flux of $\sim 2\times 10^{-15}$ erg s${}^{-1}$ cm${}^{-2}$, corresponding to $L_X\sim 6\times 10^{28}$ erg s${}^{-1}$ at the cluster's distance. We also identify several Galactic sources beyond NGC 3532 that differ from typical coronally active stars, and were classified by MUWCLASS as potential compact objects. Detailed investigation reveals that these sources may indeed belong to rarer classes, and deserve follow up observations.

Accepted for publication in Astrophysical Journal, 41 pages, 23 figures

References in corpus (20)

• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• astroquery: An Astronomical Web-Querying Package in Python
• The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging
• The CatWISE2020 Catalog
• X-Ray Spectroscopy of Stars
• The VST Photometric Halpha Survey of the Southern Galactic Plane and Bulge (VPHAS+)
• Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -
• Chandra Multiwavelength Project X-ray Point Source Catalog
• The Evolution of Flare Activity with Stellar Age
• The Dark Energy Camera Plane Survey 2 (DECaPS2): More Sky, Less Bias, and Better Uncertainties
• A new detailed examination of white dwarfs in NGC3532 and NGC2287
• Large-amplitude variables in Gaia Data Release 2. Multi-band variability characterization
• Combined analysis of neutron star natal kicks using proper motions and parallax measurements for radio pulsars and Be X-ray binaries
• Rotation periods for cool stars in the open cluster NGC 3532. The transition from fast to slow rotation
• The difficult early stages of embedded star clusters and the importance of the pre-gas expulsion virial ratio
• Classifying Unidentified X-ray Sources in the Chandra Source Catalog Using a Multiwavelength Machine-learning Approach
• Membership lists for 431 open clusters in Gaia DR2 using extreme deconvolution gaussian mixture models
• Spectroscopic membership for the populous 300 Myr-old open cluster NGC 3532
• Confirming NGC 6231 as the parent cluster of the runaway high-mass X-ray binary HD 153919/4U 1700-37 with Gaia DR2
• Wide binary pulsars from electron-capture supernovae

Cited by in corpus (2)

• Unsupervised Machine Learning for the Classification of Astrophysical X-ray Sources
• Chandra X-ray Observatory Observations of 13 Fermi LAT Sources