On the decay of outbursts in dwarf novae and X-ray novae

Abstract

view Abstract Citations (57) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Decay of Outbursts in Dwarf Novae and X-Ray Novae Cannizzo, John K. Abstract We perform computations using a time-dependent model for the accretion disk limit-cycle mechanism to examine the decay of the optical light following the peak of a dwarf nova outburst. We present the results of a parameter study of the physical input variables which affect the decay rate. In the model, the decay is brought about by a cooling transition front which begins at large radii in the disk and moves inward. The nature of the decay is strongly influenced by the radial dependence of the accretion disk viscosity parameter alpha. To obtain exponential decays for typical dwarf nova parameters, we require alpha proportional to repsilon0, where epsilon0 approximately = 0.3-0.4. The exact value of epsilon0 which produces exponential decays depends on factors such as the mass of the accreting star and the inner radius of the accretion disk. Therefore, the observed ubiquity of exponential decays in two different types of systems (dwarf novae and X-ray novae) leads us to believe that alpha is an unnatural scaling for the viscosity. The physics of the cooling transition front must be self-regulating in that the timescale (-parital derivative of lnSigma(r)/partial derivative +)-1 (where Sigma is the surface density) for mass extraction across the front remains constant. This may be consistent with a scaling alpha proportional to (h/r)n, where h is the local disk semi-thickness and n approximately 1-2. As regards the speed of the cooling front, we find vF(r) proportional to rp, where p approximately 3 at large radii, with an abrupt transition to p approximately 0 at some smaller radius. The r3 dependence is much steeper than has been found by previous workers and appears to result from the strong variation of specific heat within the cooling front when the front resides at a large radius in the disk. The outflow of disk material across the cooling front causes a significant departure of dln Tdff0/dln r from the standard value of -0.75 (expected from steady state accretion within about 0.2 dex in radius of the break associated with the cooling front -- Teff aproximately 103.9 K (r/1010 cm) -0.1. These effects should be observable with eclipse mapping. Finally, it appears that the relatively slow decay rate for the optical flux in the 1975 outburst of A0620-00 can be accounted for if the primary is a approximately 10 Solar mass black hole. Publication: The Astrophysical Journal Pub Date: November 1994 DOI: 10.1086/174821 Bibcode: 1994ApJ...435..389C Keywords: Accretion Disks; Cataclysmic Variables; Dwarf Novae; Stellar Mass Accretion; Stellar Mass Ejection; Stellar Models; Mathematical Models; Time Dependence; Astrophysics; ACCRETION; ACCRETION DISKS; STARS: NOVAE; CATACLYSMIC VARIABLES full text sources ADS | data products SIMBAD (6)