HARD X-RAY-EMISSION FROM WEAKLY MAGNETIZED NEUTRON-STARS

In the light of recent results obtained with the hard X-ray/softy-ray telescope SIGMA, we review the present status of high-energy observations (E greater-than-or-equal-to 30 keV) of low-mass X-ray binaries containing weakly magnetized neutron stars, i.e., the high-luminosity X-ray sources, and low-luminosity X-ray bursters. By combining soft (1 less-than-or-equal-to E less-than-or-equal-to 20 keV) and hard X-ray observations, we found that for the brightest soft X-ray sources, there is no indication that they are capable of emitting substantial hard X-ray fluxes. On the other hand, hard power-law tails extending up to almost-equal-to 100-200 keV (or more), similar to those observed by SIGMA, are likely to arise in X-ray bursters, if they reach sufficiently low intensity states. This implies that the use of a high energy emission criterion alone, as a possible means of discriminating between black hole and neutron star systems, is not valid. However, it seems that unlike black hole candidates, neutron star systems are unable to emit hard X-rays when the overall X-rav luminosity (i.e., the accretion rate) is high.