The moving hotspot model for kHz QPOs in accreting neutron stars

Abstract

Abstract. 3D MHD simulation of accretion onto neutron stars have shown in the last few years that the footprint (hotspot) of the accretion flow changes with time. Two different kinds of accretion, namely the funnel flow and the equatorial accretion produced by instabilities at the inner disk, produce different kinds of motion of the hotspot. The funnel flow produces hotspots that move around the magnetic pole, while instabilities produce other hotspots that appear randomly and move along the equator or slightly above. The angular velocities of the two hotspots are different, the equatorial one being higher and both close to the Keplerian velocity in the inner region. Modeling of the lightcurves of these hotspots with Monte Carlo simulations show that the signatures produced in power specra by them, if observed, are QPOs plus low frequency components. Their frequencies, general behavior and features describe correctly most of the properties of kHz QPOs, if we assume the funnel flow hotspots as the origin of the lower kHz QPO and instabilities as the origin of the upper kHz QPO. THE KHZ QPO PHENOMENONThe lightcurves of X-ray binaries often show a number of variability phenomena, that populate the power spectra of their lightcurves with different kinds of noise and oscillatory components. They are normally fitted with Lorentzian curves [1] and classified in terms of their characteristic frequency ν and their width ∆ν. Depending on the quality factor Q = ν/∆ν, features are classified as broadband noise (Q 2), quasi-periodic oscillations (QPO, Q 2), coherent pulsations (∆ν smaller than the frequency bin). Many weakly-magnetized accreting neutron stars show a particular kind of QPOs, whose frequencies range from 300 to 1300 Hz, called for this reason kHz QPOs [2]. Very often kHz QPOs appear in pairs, with the two peaks, labelled "upper" and "lower", at frequencies differing by about 300 Hz. Lower and upper peak generally differ for their properties, the lower peak being normally more coherent and appearing in a shorter frequency range [3,4]. Their frequencies are correlated with the ones of lower-frequency phenomena. KHz QPOs tend to have higher frequencies at higher countrates, but the correlation is not straightforward. On small timescales, in fact, the variation of the frequency with the countrate is usually very rapid, with the frequency going up of hundreds of Hz with slight increases of the countrate. If one plots the frequency of the oscillations versus the countrate, it is evident that the variation happens on different tracks, almost parallel to each other but distinct. Instead, there is a good correlation between the frequency and the spectral state [5], and at the lower and upper limits of their range the frequency difarXiv:1011.0142v1 [astro-ph.HE]