Discovery of absorption lines in X-ray burst spectra from X1636-536.

The detection of thirteen X-ray bursts from X1636 -536, observed with gas-scintillation proportional counters on board the X-ray astronomy satellite, Tenma, is reported. Significant dips in the X-ray spectra are detected in four different bursts and are described by absorption lines from X-ray bursts. The largest burst, both in the peak flux and in the intergrated flux, shows an absorption line near 5.7 keV for the first 8-s interval near the burst peak and an absorption line near 4.1 keV in the decay part, while three other bursts reveal absorption lines near 4 keV. The observed absorption-line features are atributed to the gravitationally redshifted absorption lines of some heavy elements. Although the identification of the element responsible for the absorption is uncertain, this result offers a new observational approach to the problem of the mass and radius of the neutron stars.