| Full text | |
| Author(s): |
Guerrero, M. A.
;
Montez Jr, R.
;
Ortiz, R.
;
Toala, J. A.
;
Kastner, J. H.
Total Authors: 5
|
| Document type: | Journal article |
| Source: | Astronomy & Astrophysics; v. 689, p. 14-pg., 2024-09-04. |
| Abstract | |
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 (approximate to 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 (approximate to 5 x 10(30) erg s(-1)) of X-AGBs would arise from an accretion disk around main-sequence or subgiant F-K companion stars. (AU) | |
| FAPESP's process: | 23/05298-0 - Circumstellar shells around AGB stars |
| Grantee: | Roberto Pereira Ortiz |
| Support Opportunities: | Regular Research Grants |