Abstract
Context. The rather elusive high-frequency quasi-periodic oscillations (HFQPOs) observed in the X-ray light curve of black holes have been seen in a wide range of frequencies, even within one source. Also notable is the detection of “pairs” of HFQPOs with a close-to-integer ratio between the frequencies. Aims. The aim of this paper is to investigate some of the possible observables that we could obtain from the Rossby wave instability (RWI) active in the accretion disc surrounding the compact object. Methods. Using the newly developed GR-AMRVAC code able to follow the evolution of the RWI in a full general relativistic framework, we explore how RWI can reproduce observed HFQPO frequency ratios and whether or not it is compatible with observations. In order to model the emission coming from the disc we have linked our general relativistic simulations to the general relativistic ray-tracing GYOTO code and delivered synthetic observables that can be confronted with actual data from binary systems hosting HFQPOs. Results. We demonstrate that some changes in the physical conditions prevailing in the part of the disc where RWI can be triggered lead to various dominant RWI modes whose ratio recovers frequency ratios observed in various X-ray binary systems. In addition we also show that when RWI is triggered near to the last stable orbit of a spinning black hole, the amplitude of the X-ray modulation increases with the spin of the black hole. Revisiting published data on X-ray binary systems, we show that this type of relationship actually exists in five systems where an indirect measurement of the spin of the black hole is available.
Highlights
While fast variability is often detected in black hole binaries, the most sought after observations are those of high-frequency quasi-periodic oscillations (HFQPOs)
These HFQPOs appear as narrow peak(s) in the X-ray powerdensity spectra (PDSs) of black hole binaries and have been detected in eight different black hole sources (GRO J1655−40, GRS 1915+105, XTE J1550−564, H1743−322, 4U 1630−47, XTE J1650−500, XTE J1859+226 IGR J17091−3624; e.g. Remillard & McClintock 2006; Altamirano & Belloni 2012; Belloni et al 2012, and references therein) ranging from as low as 27 Hz in GRS 1915+105 (Belloni et al 2001) up to a few hundred Hertz
Schwarzschild (Casse et al 2017) and Kerr (Casse & Varniere 2018) black hole. We take this a step further by adding general relativistic ray-tracing to our simulation in order to create numerical observations that can be translated into observables such as the root mean square amplitude of the modulation created by the instability, its quality factor, and the presence of integer ratio peaks in the PDS
Summary
While fast variability is often detected in black hole binaries, the most sought after observations are those of high-frequency quasi-periodic oscillations (HFQPOs). Much weaker and rarer, have stimulated much more interest than their low-frequency counterparts (LFQPO) Another reason behind the interest in HFQPOs is that they sometimes exhibit an integer ratio between detected peaks, such as the classical 1:2 in GRS 1915+105 Belloni & Altamirano (2013), and a 2:3 ratio in, for example, GRO J1655−40 (Remillard et al 2002) and both the 2:3 and potentially 3:4 ratios in XTE J1550−564 (Miller et al 2001; Varnière & Rodriguez 2018). We take this a step further by adding general relativistic ray-tracing to our simulation in order to create numerical observations that can be translated into observables such as the root mean square (rms) amplitude of the modulation created by the instability, its quality factor, and the presence of integer ratio peaks in the PDS
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