Quasi Periodic Oscillations due to Axisymmetric and Non-Axisymmetric Shock Oscillations in Black Hole Accretion

arXiv:0903.1482v1 [astro-ph.HE] 9 Mar 2009 October 26, 2018 6:6 WSPC - Proceedings Trim Size: 9.75in x 6.5in apo3review QUASI PERIODIC OSCILLATIONS DUE TO AXISYMMETRIC AND NON-AXISYMMETRIC SHOCK OSCILLATIONS IN BLACK HOLE ACCRETION SANDIP K. CHAKRABARTI1,2, D. DEBNATH2, P.S. PAL2, A. NANDI2, R. SARKAR2, M.M. SAMANTA2, P.J. WIITA3, H. GHOSH1 and D. SOM1 1S. N. Bose National Centre for Basic Sciences, JD-Block, Sector III, Salt Lake, Kolkata, 700 098, India. E-mail:chakraba@bose.res.in 2 Centre for Space Physics, Chalantika 43, Garia Station Rd., Kolkata 700084, India. 3 Department of Physics & Astronomy, Georgia State University, PO Box 4106, Atlanta, GA 30302-4106, USA Quasi-Periodic Oscillations (QPOs) are very puzzling since they remain totally un- explained by popular earlier models of accretion disks. The significant rms value in power density spectrum implies that the oscillation involves in the dynamical and non-linear variation of certain region of the accretion disk itself. The nature of the energy depen- dence implies that the region which produces Comptonized hard tail is also responsible for QPOs. Similarly, the occurrences of the QPOs are strongly related to the jet forma- tion and the spectral states. These features are the natural consequences of the advective disk paradigm that we are advocating. In the mid 90s, some of the present authors first pointed out that the QPOs in all possible types of black holes may be simply due to the oscillations of the CENBOL, the CENtrifugal pressure supported BOundary Layer which is formed in the sub-Keplerian flows around a black hole. This CENBOL could be axi-symmetric as well as non-axisymteric in nature since its boundary, namely, the centrifugally driven shocks could be axi-symmetric or non-axisymmetric. In addition, we pointed out that the transition radius where the flow becomes Keplerian to sub- Keplerian, as well as the location of the inner sonic point can also oscillate and produce the QPOs. Since the shock locations are functions of the specific angular momentum (λ) and specific energy (E) of the flow, our model naturally predicts that the QPO frequency should vary with mass, spin, λ and E. The QPO frequencies with specific ratios, such as, 2:3 must be due to non-axisymmetric effects when the shock switches between the two-armed and the three-armed spirals. We also discuss the possible effects that the disk inclination might have with the line of sight. Keywords: accretion, accretion disk – black hole physics– X-ray variabilities - shock waves. 1. Introduction Quasi-Periodic Oscillations (QPOs) of radiations emitted from the accretion disks of black hole candidates remain totally unexplained by the ‘standard’ model of accretion disk (Shakura & Sunyaev, 1973) or Advection Dominated Accretion Disk (Esin et al. 1998) or any of their variations. The oscillations manifest themselves so powerfully that they leave little doubt that they are not merely due to vibrations of the disks or jets. Rather, regions of the disks are dynamically oscillating and causing the modulation in radiation intensity. Moreover, while the thickness of the region determines the ‘Q’ value of the QPO, the size of that region determines the frequency itself. The movement of that region causes drifting of the frequency. Thus a gradual or rapid increase of frequency means that the region moves towards the black hole, while the decrease of frequency means that the region is drifting away. 1 October 26, 2018 6:6 WSPC - Proceedings Trim Size: 9.75in x 6.5in apo3review 2 Another interesting and quite general property of QPOs is that with the increase in luminosity, the frequency goes up, as if the region moves closer to the black hole. Also, the higher energy photons appear to have sharper QPOs. In very soft states QPOs disappear. What is this ‘region’ which causes QPOs in black hole and neutron star candi- dates? Guesses and counter-guesses are rampant ranging from blobby disks, grav- itational lensing, some instability at inner stable orbit to elliptical precessing tra- jectories in the disks. In reality, the QPOs are the most natural manifestation of a non-steady sub-Keplerian flow in which cooling through radiative effects or dynam- ical cooling through jets is important. We have argued for over a decade now that QPOs are due to oscillations of the CENBOL, the CENtrifugal pressure supported BOundary Layer which is bounded by a shock and the sonic point. With every new observation this assertion is vindicated even further. The oscillation frequency is roughly the infall time of matter in the CENBOL. With increase in luminosity,the post-shock region cools down and the shock moves closer and the frequency is also increased. This review is to show the present status of the QPOs in black hole candidates, especially emphasizing the occurrences of QPO frequencies in 2:3 ratio. Even though there are popular ‘models’ which can ‘explain’ the ‘frequencies’ of QPOs by vibrations of the disk, or Kepleria

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