Spectral and temporal features in bursts from 2S 1636-536 observed with EXOSAT.

Abstract

The well-known X-ray burster 2S 1636-536 (Nor X-1) was observed for 7 hr by the ME instrument on EXOSAT. During this time 7 bursts of unusual shape were detected. The bursts have similar profiles. The results suggest that each burst has two components with an initial helium thermonuclear flash triggering a stronger hydrogen flash; the ratio of continuum to burst flux for these bursts supports the influence of hydrogen in the bursts. A dip in the spectrum at about 4.5 keV is seen during the rise of several bursts but not at the peak. This might be due to Fe absorption in the overlaying neutron star atmosphere redshifted by the gravitational field. If this is so, the redshift enables the ratio of mass to radius near the site of burst origin to be determined. The apparent redshift of this feature for 2S 1636-536 gives a radius of 5 - 6 km for a 1.4 M_sun; star.