Letter to the Editor

Phase-dependent X-ray observations of the β Lyrae system

No eclipse in the soft band

¹ Department of Physics, Astronomy, & Geology, East Tennessee State University, Johnson City, TN 37614, USA e-mail: ignace@etsu.edu
² Lehrstuhl Astrophysik der Universität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany
³ Eureka Scientific, Inc., 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
⁴ Department of Astronomy, UC Berkeley, 601 Campbell Hall, Berkeley, CA 94530, USA

Received: 18 October 2007
Accepted: 26 November 2007

Abstract

Aims.We report on observations of the eclipsing and interacting binary β Lyrae from the Suzaku X-ray telescope. This system involves an early B star embedded in an optically and geometrically thick disk that is siphoning atmospheric gases from a less massive late B II companion.

Methods.Motivated by an unpublished X-ray spectrum from the Einstein X-ray telescope suggesting unusually hard emission, we obtained time with Suzaku for pointings at three different phases within a single orbit.

Results.From the XIS detectors, the softer X-ray emission appears typical of an early-type star. What is surprising is the remarkably unchanging character of this emission, both in luminosity and in spectral shape, despite the highly asymmetric geometry of the system. We see no eclipse effect below 10 keV. The constancy of the soft emission is plausibly related to the wind of the embedded B star and Thomson scattering of X-rays in the system, although it might be due to extended shock structures arising near the accretion disk as a result of the unusually high mass-transfer rate. There is some evidence from the PIN instrument for hard emission in the 10–60 keV range. Follow-up observations with the RXTE satellite will confirm this preliminary detection.