X-Ray AGB Stars in the 4XMM-DR9 Catalog: Further Evidence for Companions

DOI: 
10.3847/1538-4357/abefd7
Publication date: 
24/05/2021
Main author: 
Ortiz, R.
IAA authors: 
Guerrero, M. A.
Authors: 
Ortiz, R.;Guerrero, M. A.
Journal: 
The Astrophysical Journal
Publication type: 
Article
Volume: 
912
Pages: 
93
Abstract: 
Single AGB stars are not normally expected to be X-ray emitters due to the lack of a corona capable of powering a hot plasma. Therefore, the detection of X-ray emission in AGB stars by the ROSAT, Chandra, and XMM-Newton observatories has been interpreted as evidence for binarity. The number of X-ray-emitting AGB stars is, however, very small, and statistically sound conclusions shall be considered tentative. In this paper we aim at increasing the number of X-ray-emitting AGB stars and at providing a consistent analysis of their X-ray emission to be compared to their UV and optical properties. The XMM-Newton 4XMM-DR9 catalog has been searched for X-ray counterparts of various types of AGB stars: nearby (i.e., listed in Hipparcos), mass-losing, and S- and C-types. Seventeen X-ray counterparts of AGB stars have been found in the 4XMM-DR9. Nine of them have pointed XMM-Newton observations, whereas eight are genuine serendipitous discoveries. Together with the AGB stars detected by ROSAT, this increases the number of X-ray AGB stars to 26. Most of their X-ray spectra can be fit by the emission from an optically thin single-temperature thermal plasma with temperatures typically larger than 10<SUP>7</SUP> K. There is no obvious correlation between the X-ray and bolometric luminosity of these stars, but the X-ray luminosity generally increases with the amount of far-UV excess. The high temperature of some X-ray-emitting plasma in AGB stars and the correlation of their X-ray luminosity with the far-UV emission supports the origin of this X-ray emission from accretion disks around unseen companions.
Database: 
ADS
URL: 
https://ui.adsabs.harvard.edu/#abs/2021ApJ...912...93O/abstract
ADS Bibcode: 
2021ApJ...912...93O
Keywords: 
Close binary stars;Asymptotic giant branch stars;Late-type giant stars;Single x-ray stars;Symbiotic binary stars;X-ray binary stars;254;2100;908;1461;1674;1811