Classification of Chandra X-Ray Sources in Cygnus OB2

Abstract

We have devised a predominantly Naive Bayes−based method to classify X-ray sources detected by Chandra in the Cygnus OB2 association into members, foreground objects, and background objects. We employ a variety of X-ray, optical, and infrared characteristics to construct likelihoods using training sets defined by well-measured sources. Combinations of optical photometry from the Sloan Digital Sky Survey (riz) and Isaac Newton Telescope Photometric Hα Survey (r I i I Hα), infrared magnitudes from United Kingdom Infrared Telescope Deep Sky Survey and Two-Micron All Sky Survey (JHK), X-ray quantiles and hardness ratios, and estimates of extinction A v are used to compute the relative probabilities that a given source belongs to one of the classes. Principal component analysis is used to isolate the best axes for separating the classes for the photometric data, and Gaussian component separation is used for X-ray hardness and extinction. Errors in the measurements are accounted for by modeling as Gaussians and integrating over likelihoods approximated as quartic polynomials. We evaluate the accuracy of the classification by inspection and reclassify a number of sources based on infrared magnitudes, the presence of disks, and spectral hardness induced by flaring. We also consider systematic errors due to extinction. Of the 7924 X-ray detections, 5501 have a total of 5597 optical/infrared matches, including 78 with multiple counterparts. We find that ≈6100 objects are likely association members, ≈1400 are background objects, and ≈500 are foreground objects, with an accuracy of 96%, 93%, and 80%, respectively, with an overall classification accuracy of approximately 95%.

Optical and Infrared Counterparts of the X-Ray Sources Detected in the Chandra Cygnus OB2 Legacy Survey

“…The X-ray sources in our survey need to be classified also according to their OIR properties (Kashyap et al 2023). Erroneous matches between the OIR and X-ray catalogs will result in wrong classifications, affecting the scientific outcome of our survey.…”

“…The analysis of the X-ray properties of the X-ray sources with an OIR counterpart is beyond the scope of this paper, and it will be part of dedicated studies (Flaccomio et al 2023;Kashyap et al 2023). A brief comparison of the observed X-ray photons' energy between the X-ray sources with and without an OIR counterpart is shown in the next section.…”

“…Because of the multiple matches between X-ray and OIR counterparts, ID is the truly unique source identifier. A1) includes stellar positions, physical properties, and how the source is classified according to Kashyap et al (2023). Stellar parameters such as mass, age, and extinction are provided only for members.…”

The Chandra Cygnus OB2 Legacy Survey: Design and X-Ray Point-source Catalog

“…An analysis of the sensitivity of our survey to a number of observational and stellar parameters is presented in Wright et al (2023). Optical and infrared counterparts to these sources are presented in Guarcello et al (2023), and an analysis of the likely Cyg OB2 members and contaminants is presented in Kashyap et al (2023). X-ray spectroscopy of the low- (Flaccomio et al 2023) and high-mass (Rauw et al 2015) members of the association is presented elsewhere, along with analysis and discussion.…”

“…An analysis of the completeness limits of the survey as a function of various observational and stellar parameters is presented in Wright et al (2023). Other papers in this series present a catalog of optical and near-IR counterparts to the X-ray catalog (Guarcello et al 2023) and an analysis of Cyg OB2 members and contaminants in the X-ray catalog (Kashyap et al 2023).…”