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Investigating Transition States in Bright Chandra ACIS X-ray Binaries Exhibiting Intra-Observational Variation

Presentation #340.08 in the session “X-ray Pulsars and Black Holes”.

Published onJan 11, 2021
Investigating Transition States in Bright Chandra ACIS X-ray Binaries Exhibiting Intra-Observational Variation

As part of a broader survey into the population of X-ray binaries, we investigated X-ray spectra detected by Chandra X-ray Observatory that exhibited intra-observational spectral, hardness ratio, or flux rate changes. In accordance with the survey, our sources were chosen from a database of archival Chandra ACIS sources spatially coincident with resolvable galaxies presumed to be X-ray binaries. This database encompasses a sample of over 12,000 X-ray sources compiled from over 200 Chandra observations. Sources were chosen for the sample based on time and distance criteria. Only observations longer than 5 kiloseconds of galaxies closer than 15 megaparsecs were considered. These sources were most often not the primary target of the observation, and thus had to be corrected for off-axis biases. From this sample, we considered only sources between 1000 and 2000 total observational counts. 135 unique sources which met these criteria were identified. Light curves were made, and the sources were fit with spectral models in order to identify sources variable in flux rate or spectrum. Once identified, Bayesian estimation of hardness ratio (BEHR) methods were used to compute hardness ratios in bins defined by both time and number of source counts. BEHR is especially useful in low count regimes where model fits cannot be done with any statistical significance. Investigation of these variable sources can provide information regarding the spectral transition states of the X-ray binaries. These transitions tell us about the nature of the compact object and its accretion from the companion star. This information can give us insight into the larger population of X-ray binaries including those who are too faint to be fit with spectral models and those who are not observed during spectral transitions. We discuss the variation of five sources in detail and consider their implications in the survey as a whole.


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