Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 11 June 2022 (MN L A T E X style file v2.2) Hints of Quasi-Periodic Oscillations from Unstable Accretion: 3D MHD Simulations A. K. Kulkarni ⋆ , M. M. Romanova† Dept. of Astronomy, Cornell University, Ithaca, NY 14853 11 June 2022 ABSTRACT We investigate the photometric variability of magnetized stars, particularly neu- tron stars, accreting through a magnetic Rayleigh-Taylor-type instability at the disk- magnetosphere interface, and compare it with the variability during stable accretion, with the goal of looking for possible quasi-periodic oscillations. The lightcurves dur- ing unstable accretion generally show periodicity at the star’s rotation frequency, and also signs of quasi-periodic variability, except in the most strongly unstable cases. The power spectra are noisier than during stable accretion, with the result that the frac- tional rms amplitudes of the Fourier peaks are smaller. We also study in detail the effect of the misalignment angle between the rotation and magnetic axes of the star on the variability, and find that at misalignment angles  25 ◦ , the star’s period always appears in the lightcurves. Key words: accretion, accretion discs; instabilities; MHD; stars:neutron; stars: oscil- lations; stars: magnetic fields 1 INTRODUCTION Accretion to magnetized stars occurs in different systems, in- cluding classical T Tauri stars (CTTSs), which are the progen- itors of solar-type stars (e.g., Bouvier et al. 2007), in magne- tized cataclysmic variables, which are accreting white dwarfs (e.g., Warner 1995), and in millisecond pulsars, which are weakly magnetized accreting neutron stars (e.g., van der Klis 2004). The lightcurves of these stars are expected to show definite periodicity corresponding to the star’s rotation pe- riod, but in addition to this they often show interesting fea- tures like quasi-periodic oscillations (QPOs), or lack any pe- riodicity altogether. The reasons for this behaviour are not yet perfectly clear. The lack of periodicity in many CTTSs was thought to be due to the lack of a dynamically important mag- netic field, but recent observations have shown that that is not always the case (e.g., Donati et al. 2007). QPO models in neutron star systems usually attempt to look towards charac- teristic frequencies in the disk, and their beats with the stellar rotation frequency, for explanations (van der Klis 2004). The variability characteristics of dwarf novae, which are a sub- class of cataclysmic variables, show close similarity to those of neutron stars (Warner et al. 2003). These observations hint at a common explanation for the variability features in these systems, perhaps based on the properties of the accretion flow ⋆ E-mail: akshay@astro.cornell.edu † E-mail: romanova@astro.cornell.edu itself. This leads us to investigate the effect of the accretion flow on the variability. Accretion to magnetized stars can be in one of two regimes: (i) the stable regime, in which the accretion disk is stopped by the star’s magnetic field, and the accreting matter flows around the magnetosphere, forming two funnels that deposit matter near the star’s magnetic poles (Ghosh & Lamb 1979; K¨ onigl 1991; Koldoba et al. 2002; Romanova et al. 2003, 2004), or (ii) the unstable regime, in which Rayleigh- Taylor-type instabilities occur at the disk-magnetosphere in- terface (Arons & Lea 1976; Elsner & Lamb 1977; Wang & Robertson 1985). The instabilities produce tongues of matter that penetrate the magnetosphere and deposit matter much closer to the star’s equator (Romanova et al. 2008; Kulkarni & Romanova 2008, hereafter KR08). The tongues rotate around the star with an angular velocity close to the inner-disk orbital frequency, and their number is of the order of a few. In KR08 it was noticed that the lightcurves during unstable accretion can lack the clear periodicity seen in the lightcurves during stable accretion (see, e.g., Poutanen & Gierli´ nski 2003; Kulka- rni & Romanova 2005, hereafter KR05). Here we investigate this behaviour in more detail in accreting magnetized stars, particularly millisecond pulsars. Our goal is to look for signs of QPOs in the unstable regime. KR08 also found that the instability is suppressed at high misalignment angles between the star’s rotation and mag- netic axes. Here we perform a detailed investigation of the boundary between stable and unstable regimes as a function c  0000 RAS arXiv:0812.0241v3 [astro-ph] 12 May 2009