Journal Article

QPO emission from moving hot spots on the surface of neutron stars: a model

Matteo Bachetti, Marina M. Romanova, Akshay Kulkarni, Luciano Burderi and Tiziana di Salvo

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 403, issue 3, pages 1193-1205
Published in print April 2010 | ISSN: 0035-8711
Published online April 2010 | e-ISSN: 1365-2966 | DOI:
QPO emission from moving hot spots on the surface of neutron stars: a model

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We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misalignment angles, as regards the features in light curves produced by regular movements of the hot spots during accretion on to the star. In particular, we show that the variation of position of the hot spot created by the infalling matter, as observed in 3D simulations, can produce high-frequency quasi-periodic oscillations (QPOs) with frequencies associated with the inner zone of the disc. Previously reported simulations show that the usual assumption of a fixed hot spot near the polar region is valid only for misalignment angles Θ relatively large. Otherwise, two phenomena challenge the assumption: one is the presence of Rayleigh–Taylor instabilities at the disc-magnetospheric boundary, which produce tongues of accreting matter that can reach the star almost anywhere between the equator and the polar region; the other one is the motion of the hot spot around the magnetic pole during stable accretion. In this paper, we start by showing that both phenomena are capable of producing short-term oscillations in the light curves. We then use Monte Carlo techniques to produce model light curves based on the features of the movements observed, and we show that the main features of kHz QPOs can be reproduced. Finally, we show the behaviour of the frequencies of the moving spots as the mass accretion rate changes, and propose a mechanism for the production of double QPO peaks.

Keywords: accretion, accretion discs; instabilities; MHD; stars: magnetic fields; stars: neutron; stars: oscillations

Journal Article.  8519 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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