The effect of cooling on time dependent behaviour of accretion flows around black holes

¹ S. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata 700098, India
² Centre for Space Physics, Chalantika 43, Garia Station Rd., Kolkata, 700084, India e-mail: space_phys@vsnl.com
³ Dipartimento di Fisica e Tecnologie Relative, Viale delle Scienze, 90128 Palermo, Italy e-mail: molteni@unipa.it

Corresponding author: S. K. Chakrabarti, chakraba@boson.bose.res.in

Received: 16 October 2003
Accepted: 16 January 2004

Abstract

We present the results of several numerical simulations of two dimensional axi-symmetric accretion flows around black holes using Smoothed Particle Hydrodynamics (SPH) in the presence of cooling effects. We consider both stellar black holes and super-massive black holes. We observe that due to both radial and vertical oscillation of shock waves in the accretion flow, the luminosity and average thermal energy content of the inner disk exhibit very interesting behaviour. When power density spectra are taken, quasi-periodic variabilities are seen at a few Hz and also occasionally at hundreds of Hz for stellar black holes. For super-massive black holes, the time scale of the oscillations ranges from hours to weeks. The power density spectra have a flat top behavior with average rms amplitude of a few percent and a broken power-law behavior. The break frequency is generally found to be close to the frequency where the shock oscillates.