THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1995 AND 1996

2. Scientific Results during 1995 and 1996

2.2. Selected Scientific Results

2.2.4. CGS4 and X Ray Binaries

M.J. Coe (Univ. of Southampton)

A Be/X ray binary system consists of a neutron star in orbit around a massive young O or B type star, which itself has orbiting about it a disk of hot gas. The neutron star's orbit is often highly elongated. When its orbit brings the neutron star close to the massive star, material is pulled away from the disk of the O/B star and spirals down onto the neutron star, causing a massive X ray outburst. This ouburst lasts until the distance between the two objects increases and mass transfer ceases.

One Be/X ray binary system, with the distinctly unalluring name A0535+262, was discovered almost two decades ago and, due to its relative brightness and the well known orbital period of the neutron star companion, has been extensively studied at optical wavelengths. The system was not been observed spectroscopically at infrared wavelengths until 1992, when we began a long term monitoring campaign of the source using CGS4 on UKIRT. Results from the period 1992 1995 have just been submitted to MNRAS for publication (Clark et al. 1997).

Of particular interest in the spectra of Be/X ray binaries are the emission lines of hydrogen and helium, which arise mainly in the disk of the O/B star. The relative intensities of the lines provides information on the density in the disk and how it varies with radius. The shape of each line reveals the velocities and geometry of the gas that emits it. Since different lines are emitted at different distances from the O/B star, studies of an assortment of lines reveal the the way in which gas is distributed in the vicinity of that star. The infrared lines allow one to look deeper into the disk than do the optical lines, and these are needed to provide a complete kinematic map of the disk. In Figure 5 the clear difference between the H (optical) and Br (infrared) line shapes in A0535+262 is due to the Br emission arising further inside the disk where the gas has a different range of velocities.

Together the optical and infrared spectra can provide a concise picture of the density, velocity, and temperature at different positions in the disk of the O/B star. We are in the process of modelling our spectra of A0535+262 in order to determine the above properties. Furthermore, it can be seen from Figure 6 that spectra obtained at different times show changes in the intensity ratios of some of the lines in A0535+262. These variations reflect changes in the physical conditions of the disk at different times during the orbit. Accurate measurements of these changes, including those at the time when disk material is being pulled away by the neutron star, will allow us to understand more of the details of the process which ultimately results in the spectacular X ray outbursts we see from these objects.

  
Figure 5: H and Br profiles

  
Figure 6: Time resolved spectra

I gratefully acknowledge my collaborators in this work: J.S. Clark (Univ. of Southampton), I.A. Steele (Southampton and Liverpool John Moores University), and P. Roche (University of Sussex).