Asymptotic giant branch stars in the eROSITA-DE eRASS1 catalog

¹ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía S/N, Granada 18008, Spain
e-mail: mar@iaa.es
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 01238, USA
³ Escola de Artes, Ciências e Humanidades, USP, Av. Arlindo Bettio 1000, 03828-000 São Paulo, Brazil
⁴ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, 58089 Morelia, Michoacán, Mexico
⁵ School of Physics and Astronomy and Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, Rochester, NY, USA
⁶ Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, USA

Received: 27 March 2024
Accepted: 28 June 2024

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 (≈10³² 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 × 10³⁰ erg s⁻¹) of X-AGBs would arise from an accretion disk around main-sequence or subgiant F-K companion stars.