White Dwarfs as X-ray Sources

ACCORDING to Uhuru data, the 1.24 s period in the X-ray emission of Hercules X-l decreased at a rate of 9 µs yr^-1 during the first half of 1972 (ref. 1). Further study has shown that the period variation is rather complex, with both increases and decreases occurring, although the net result over a full year remains a period decrease². Most authors have interpreted the period decrease in terms of models involving accretion of matter with angular momentum on to a rotating neutron star. We point out here that models involving pulsating white dwarfs should not be prematurely rejected and that they may account quantitatively for the overall period change.