Microquasars: hard X-ray/γ-ray emission

I review some of the basic observational details of jets from X-ray binaries, or ‘microquasars’. It is shown that in both (Z and Atoll) NS and BHC systems radio emission, and therefore jet formation, is correlated with the presence of hard (30–500 keV) X-ray emission. At γ-ray (> 500 keV) energies, the relation is not so clear. Possible physical connections between the presence of a jet, with a population of relativistic electrons, and the emission of hard X-rays, are briefly discussed. INTRODUCTION – JETS FROM X-RAY BINARIES The study of ‘microquasars’, or the phenomenon of relativistic jets from X-ray binary systems, is one of the most vigorously pursued fields in observational highenergy astrophysics in recent years. Once Mirabel & Rodriguez (1994) had shown that X-ray binary systems were capable of producing relativistic (bulk Lorentz factor Γ ≥ 2) outflows, whose primary signature was in the radio band, it was recognised that such ‘jets’ may constitute an important aspect of the ‘accretion’ process onto compact objects. In recent years observations have clearly demonstrated the presence of jets in most classes of X-ray binary, and furthermore have clearly indicated a strong coupling with the accretion ‘state’ of the source, whether neutron star (e.g. Penninx et al. 1988) or black hole accretor (e.g. Fender et al. 1999b). See e.g. Hjellming & Han (1995), Mirabel & Rodriguez (1999), and other papers by this author for reviews of radio emission and jets from X-ray binaries (although note that new observations are coming thick and fast all the time!). See e.g. Lewin, van Paradijs & van den Heuvel (1995) for an excellent volume containing reviews of the models for the different ‘states’ of accreting neutron stars and black holes (admittedly from the era before the ubiquity and significance of jets was widely recognised). In Fig 1 and 2 I summarise the current understanding of the empirical relation of radio emission to X-ray ‘state’ of the neutron star (NS) and black hole candidate (BHC) systems respectively. In addition to the relations indicated in these figures, it seems that discrete, radio-emitting ejection events (in which the radio spectrum NO radio emission disc truncated > 1000 km m ‘soft’ colour ‘ha rd’ co lou r Radio on ‘Atoll’ sources m ~ 0.01-0.1 Edd ‘soft’ colour ‘ha rd’ co lou r X-ray col-col diag. Radio