Asymptotic giant branch stars in the eROSITA-DE eRASS1 catalog

• Published in: Astronomy and astrophysics (Berlin) Vol. 689; p. A62
• Main Authors: Guerrero, M. A., Montez, R., Ortiz, R., Toalá, J. A., Kastner, J. H.
• Format: Journal Article
• Language: English
• Published: Heidelberg EDP Sciences 01.09.2024
• Subjects: Accretion disks; Asymptotic giant branch stars; Boundary layers; Companion stars; Emission; Emitters; Luminosity; Near infrared radiation; Stellar coronas; Symbiotic stars; White dwarf stars; X ray sources; XMM (spacecraft)
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202450155

Context . Asymptotic giant branch (AGB) stars are not expected to be X-ray-emitters, yet a small fraction of them, the so-called X-AGBs, show X-ray emission that can be attributed to coronal activity of a companion or accretion onto one. Aims . By searching the recently released SRG eROSITA-DE eRASS1 source catalog, we aim to increase the sample of known X-AGBs and investigate their X-ray and far-UV properties. So far, 36 X-AGBs have been reported, which includes 21 previous detections from ROSAT RASS, Chandra, and XMM-Newton and 15 recent detections from eROSITA eRASS1. Methods . We cross-correlated the position of sources in the eROSITA-DE eRASS1 catalog with the largest available samples of AGB stars in order to find possible X-ray counterparts. We carefully checked the possible counterparts by comparing X-ray and near-IR K images, disregarding those affected by optical loading, those found to be diffuse sources, or those simply considered unreliable positional associations. Results . We have found seven high-confidence X-AGBs and another seven possible ones. Accounting for previous X-ray detections, the sample of X-AGBs is increased by 11 new sources, increasing the sample of X-AGBs from 36 up to 47. Adding these sources to previous eROSITA-DE eRASS1 X-AGB detections, eROSITA has so far discovered 26 new X-AGBs, more than doubling the number of known X-AGBs. This demonstrates eROSITA’s capability to detect X-AGBs despite the challenge posed by the optical loading caused by their near-IR brightness, which makes the X-ray detection untrustworthy in a number of cases. Conclusions . The eRASS1 X-AGBs tend to have a higher X-ray luminosity than that of previously detected X-AGBs, suggesting a bias toward brighter sources that is very likely due to the short exposure time of eRASS1 sources. A comparison of the X-ray and far-UV luminosity of X-AGBs with those of X-ray-emitter symbiotic stars (X-SySts) revealed an overlap in the X-ray luminosity range 10 29.5 < L X (erg s −1 ) <10 33.0 . The average higher X-ray luminosity of X-SySts AGBs (≈10 32 erg s −1 ) can be interpreted as X-ray emission arising from a boundary layer between an accretion disk and a white dwarf, whereas the average lower X-ray luminosity (≈5 × 10 30 erg s −1 ) of X-AGBs would arise from an accretion disk around main-sequence or subgiant F-K companion stars.