Understanding the LMXB X2127+119 in M 15

I. X-ray eclipses and dips

¹ Department of Physics, Keele University, Keele, Staffordshire ST5 5BG, UK
² School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
³ Goddard Space Flight Centre, NASA, Greenbelt, MD 20771, USA
⁴ Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

Corresponding author: Z. Ioannou, zac@astro.as.utexas.edu

Received: 8 June 2001
Accepted: 7 November 2001

Abstract

We present X-ray observations of the high-inclination low-mass X-ray binary system X2127+119 (AC 211) in the globular cluster M 15 (NGC 7078). The observations consist of data acquired in 1996 with the RXTE satellite and in 1995 with the ASCA satellite. Also, the MPC1 data from the 1988 GINGA observations were de-archived and re-analysed. The phase-folded 2–10 keV hardness ratios from all three missions differ significantly indicating that the system can exhibit different spectral behaviours. We find that the X-ray eclipse profiles can be described relatively well using a simple model where the secondary star passes in front of a large X-ray emitting region. For this we require a mass ratio of about one. The radius of this X-ray emitting region is and its vertical extent above the orbital plane. We suggest that if this X-ray emitting region were an optically thick corona, it would explain various puzzling aspects of this system. We also show that the X-ray dip observed at phases around 0.65 does not conform with the idea that the dip is caused by vertically extended material associated with the stream/disc impact region, but that it could be due to structure in the inner parts of the disc.