Three new X-ray emitting O-type subdwarf stars discovered with Chandra

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica - Milano, via Bassini 15, 20133 Milano, Italy
e-mail: nicola@iasf-milano.inaf.it
² IUSS-Istituto Universitario di Studi Superiori, piazza della Vittoria 15, 27100 Pavia, Italy
³ Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, via A. Bassi 6, 27100 Pavia, Italy

Received: 4 February 2014
Accepted: 19 April 2014

Abstract

Context. X-ray observations of sdO stars are a useful tool to investigate their properties, but so far only two sdO stars were detected at X-rays.

Aims. We aimed to perform the first systematic search for X-ray emission from sdO stars to characterize the X-ray emission from single and binary sdO stars.

Methods. We observed a complete flux-limited sample of 19 sdO stars with the Chandra HRC-I camera to measure the count rate of the detected sources or to set a tight upper limit on it for the undetected sources.

Results. We obtained a robust detection of BD+37° 1977 and Feige 34 and a marginal detection of BD+28° 4211. The estimated luminosity of BD+37° 1977 is above 10³¹ erg s^-1, which is high enough to suggest the possible presence of an accreting compact companion. This possibility is unlikely for all the other targets (both detected and undetected), since in their case L_X ≲ 10³⁰ erg s^-1. On the other hand, for all 19 targets the estimated value of L_X (or its upper limit) implies an X-ray/bolometric flux ratio that agrees with log(L_X/L_bol) = −6.7 ± 0.5, which is the range of values typical of main-sequence and giant O stars.

Conclusions. The observing campaign performed by Chandra has discovered three new X-ray emitting sdO stars; for one of them the observed X-ray flux might be emitted by an accreting compact companion, while for the other two stars it is most probably due to intrinsic emission. The same is possibile for the 16 undetected stars.