The centrifugal force reversal and X-ray bursts

¹ Department of Astronomy and Astrophysics, Chalmers University, 412-96 Göteborg, Sweden
² Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
³ Université Pierre et Marie Curie, Paris VI, France
⁴ Institute of Astronomy , University of Zielona Góra, Lubuska 2, 65-265 Zielona Góra, Poland

Corresponding author: J. P. Lasota, lasota@iap.fr

Received: 18 May 2001
Accepted: 13 June 2001

Abstract

Heyl (2000) made an interesting suggestion that the observed shifts in QPO frequency in type I X-ray bursts could be influenced by the same geometrical effect of strong gravity as the one that causes centrifugal force reversal discovered by Abramowicz & Lasota (1974). However, his main result contains a sign error. Here we derive the correct formula and conclude that constraints on the relation for neutron stars deduced from the rotational-modulation model of QPO frequency shifts are of no practical interest because the correct formula implies a weak condition , where R_S is the Schwarzschild radius. We also argue against the relevance of the rotational-modulation model to the observed frequency modulations.