LSI+61 as an X-ray Binary

Our observations show that LSI+61 is a persistent yet highly variable source of x-rays, with amplitude and time scale of variations similar to the periodic radio outbursts. It is interesting to examine the properties of this system in relation to known classes of high-mass, x-ray emitting stars.

Meurs et al. (1992) analysed the ROSAT all-sky survey data for the majority of OB stars in the Bright Star Catalogue. Roughly 10% of all OB or OBe stars are detected with ROSAT, at luminosity ranging from erg . The detection rate increases toward earlier spectral types, reaching close to 50% at B0, the spectral type of LSI+61 . Meurs et al. attribute these x-ray luminosities to weak shocks in the stellar winds of OB stars. The x-ray luminosity of LSI+61 , at erg s, is at the high end of this range. However, the binary nature of LSI+61 and the high degree of modulation on the times scale of the orbital period, argues against this interpretation for the x-ray emission.

LSI+61 is one of a group of about 20 Be x-ray binary systems (van den Heuvel & Rappaport 1987). These systems have orbital periods of tens of days or longer, and the x-ray emission is considered to arise from wind accretion onto a neutron star companion. Approximately half of these objects show direct evidence for neutron star companions in the form of x-ray pulsations. The non-pulsed emission is generally transient or highly variable with outburst durations similar to the orbital periods, luminosities in the keV range of erg s and hard spectra (kT keV). These outbursts are thought to occur as a result of increased wind accretion rates on the neutron star companion due to a combination of orbital eccentricity and irregular episodes of enhanced equatorial mass loss from the Be stars (see van den Heuvel 1994). The x-ray properties of LSI+61 contrast markedly with the other members of this group. The x-ray outbursts have much lower luminosity and a much softer spectrum (kT keV). The single other exception is the Be star X Per, which has a similar keV, x-ray luminosity. However LSI+61 further distinguishes itself by its strong, outbursting radio emission.

The radio properties of LSI+61 are similar to those of the ``standard'' high-mass x-ray binaries such as SS 433, Cyg X-3 and Cir X-1. In these shorter period systems, the primary fills, or very nearly fills, its Roche Lobe and sustained, high mass accretion rates onto the compact companion can occur. However, the x-ray outbursts from this class in the keV range are similar to those of the Be x-ray binaries; much harder and more luminous by orders of magnitude than LSI+61 . If the COS-B and GRO -ray emission is associated with LSI+61 , then luminosities of in the MeV, and higher, energy range are inferred. While these luminosities are similar to the accretion generated keV luminosities of other high-mass x-ray binaries, the shift to higher photon energies would point to a fundamental difference in the accretion process.

LSI+61 does not fit easily into either of the two categories of high-mass x-ray binaries. If the x-ray outburst is accretion generated, then the data implies either a very low accretion rate compared to similar systems or a spectrum of accretion generated photons that emerges primarily at -rays. The peculiarity of LSI+61 relative to other x-ray binary systems, leads us to examine other possibilities for the source of x-ray emission.