Research Note

Searching for X-ray emission from AGB stars^⋆

¹ Argelander Institute for Astronomy, University of Bonn, 53121 Bonn, Germany
e-mail: sofia@astro.uni-bonn.de
² Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA
³ Onsala Space Observatory, 43992 Onsala, Sweden

Received: 24 November 2011
Accepted: 18 June 2012

Abstract

Context. Magnetic fields have been measured around asymptotic giant branch (AGB) stars of all chemical types using maser polarization observations. If present, a large-scale magnetic field would lead to X-ray emission, which should be observable using current X-ray observatories.

Aims. The aim is to search the archival data for AGB stars that are intrinsic X-ray emitters.

Methods. We have searched the ROSAT, CXO, and XMM-Newton archives for serendipitous X-ray observations of a sample of ~500 AGB stars. We specifically searched for the AGB stars detected with GALEX. The data is calibrated, analyzed and the X-ray luminosities and temperatures are estimated as functions of the circumstellar absorption.

Results. We identify 13 AGB stars as having either serendipitous or targeted observations in the X-ray data archives, however for a majority of the sources the detailed analysis show that the detections are questionable. Two new sources are detected by ROSAT: T Dra and R UMa. The spectral analysis suggests that the emission associated with these sources could be due to coronal activity or interaction across a binary system.

Conclusions. Further observations of the detected sources are necessary to clearly determine the origin of the X-ray emission. Moreover, additional objects should be subject to targeted X-ray observations in order to achieve better constraints for the magnetic fields around AGB stars.