Acoustic waves in the extended atmospheres of X-ray bursters

A possible sound origin for the second range photospheric oscillations detected in the X-ray bursting sources 1608-522 and 2127+119 is suggested. Basing on the recent models of radiatively driven winds from neutrons stars during powerful, super-Eddington thermonuclear bursts, it is shown that standing sound waves propagating in expanding spherical envelopes can have periods very close to the observed ones. Both the photosphere and the sonic surface may effectively reflect diverging spherical waves; the quite large ratio, approximately 10, of the periods in the two sources is explained in terms of different reflecting boundaries in either cases. It is proposed that the Quasi-Periodic Oscillation (QPO) observed in strongest type 2 bursts of the Rapid Burster are also related to the acoustic oscillations.