Introduction

The microquasar, GRO J1655-40, was first discovered by the Compton Gamma Ray Observatory in 1994 (Zhang et al. 1994). It is one of only six Galactic superluminal sources observed so far, the others being 1E1740.7-2942 (Mirabel et al. 1992), GRS 1758-258 (Rodriguez Mirabel & Marti 1992), GRS 1915+105 (Mirabel & Rodriguez 1994), XTE J1819-284 (Hjellming et al. 1999) and XTE J1748-288 (Hjellming et al. 1998). This Low-Mass X-ray Binary (LMXB) has been extensively studied in most wavelengths (Harmon et al. 1995, Kroeger et al. 1996, Tavani et al. 1996, Hynes et al. 1998a and Hynes et al. 1998b). With a distance of only 3.2 0.2 kpc (Hjellming & Rupen 1995) and relatively large quiescent brightness compared to other LMXBs (V = 17.3 mag, Bailyn et al. 1995a), we have been able to study the companion star in this system with unprecedented coverage and accuracy. Therefore, we are able to derive the binary system parameters with great precision. Most other LMXBs reside within the plane of the Galaxy and are obscured by gas and dust, thereby blocking our views of their companion stars.

We have measured the rotational velocity of the companion star in GRO J1655-40 by fitting Kurucz model spectra to our spectrum of GRO J1655-40. We will discuss how the rotational velocity of the companion star can be used to calculate the mass of the compact object in Section 2 and what has been observed previously with respect to the companion star in Section 3. Section 4 will describe our observations and data reduction. We present radial velocities in Section 5 and our fitting code is described in Section 6. Section 7 shows our results and in Section 8 we discuss our results.

© Copyright Astronomical Society of Australia 1997