SPECTRAL AND TEMPORAL FEATURES IN BURSTS FROM 2S 1636-536 OBSERVED WITH EXOSAT

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, comprising a slow rise (∼ 5 s) and a slow decay (e-fold time ∼ 20 s) with peak luminosity ∼ 2 × 1038 erg s−1 (2−20 keV). A step is seen on the leading edge of each burst profile. The temperature and the ‘blackbody’ radius appear to increase in two steps during the rise. These results suggest that each burst has two components with an initial helium thermonuclear flash triggering a stronger hydrogen flash which then determines the observed slow decay; the ratio of continuum to burst flux for these bursts (α ∼ 50) 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 over-laying 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 Mo star. © Royal Astronomical Society • Provided by the NASA Astrophysics Data System.