Browsing by Author "Becker, Peter A."

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Angular momentum transport in quasi-Keplerian accretion disks
(2011-07-06) Subramanian, Prasad; Pujari, B. S.; Becker, Peter A.
We reexamine arguments advanced by Hayashi &Matsuda (2001), who claim that several simple, physically motivated deriva- tions based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the na- ture of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expres- sion for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The deriva- tion is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.
Further constraints on electron acceleration in solar noise storms
(2006-02-28) Subramanian, Prasad; Becker, Peter A.
We reexamine the energetics of nonthermal electron acceleration in solar noise storms. A new result is obtained for the minimum nonthermal electron number density required to produce a Langmuir wave population of sufficient intensity to power the noise storm emission. We combine this constraint with the stochastic electron acceleration formalism developed by Subramanian & Becker (2005) to derive a rigorous estimate for the efficiency of the overall noise storm emission process, beginning with nonthermal electron acceleration and culminating in the observed radiation. We also calculate separate efficiencies for the electron acceleration - Lang-muir wave generation stage and the Langmuir wave - noise storm production stage. In addition, we obtain a new theoretical estimate for the energy density of the Langmuir waves in noise storm continuum sources.
Noise storm continua: power estimates for electron acceleration
(2011-07-06) Subramanian, Prasad; Becker, Peter A.
We use a generic stochastic acceleration formalism to examine the power Lin (erg s −1 ) input to nonthermal electrons that cause noise storm continuum emission. The analytical approach includes the derivation of the Green’s function for a general second-order Fermi process, and its application to obtain the particular solution for the nonthermal electron distribution resulting from the acceleration of a Maxwellian source in the corona. We compare Lin with the power Lout observed in noise storm radiation. Using typical values for the various parameters, we ﬁnd that Lin ∼ 1023−26 erg s −1 , yielding an eﬃciency estimate η ≡ Lout/Lin in the range 10−10 ∼ < η ∼ < 10−6 for this nonthermal acceleration/radiation process. These results reﬂect the eﬃciency of the overall process, starting from electron acceleration and culminating in the observed noise
Restrictions on the Physical Prescription for the Viscosity in Advection-Dominated Accretion Disks
(2011-07-06) Becker, Peter A.; Subramanian, Prasad
It has recently been demonstrated that the Shakura-Sunyaev prescription for the kinematic viscosity in an advection-dominated accretion disk yields physically reasonable solutions for the structure of the inﬂow close to the event horizon. In particular, no violations of relativistic causality occur at the horizon. This is somewhat surprising considering the diﬀusive nature of the angular momentum transport in the Shakura-Sunyaev scenario, and it is therefore natural to ask whether one can also obtain acceptable solutions for the disk structure based on the various alternative models for the viscosity that have been proposed, includ- ing the “deterministic” forms. In this paper we perform a rigorous asymptotic analysis of the structure of an advection-dominated accretion disk close to the event horizon of a nonrotating black hole based on three of the alternative pre- scriptions for the viscosity that have been suggested in the literature. We constrain the physical disk model by stipulating that the stress must van- ish at the horizon, which is the fundamental inner boundary condition imposed by general relativity. Surprisingly, we ﬁnd that none of the three alternative viscosity prescriptions yield physically acceptable disk structures close to the horizon when the zero-torque condition is applied, whether the ﬂow is in vertical hydrostatic equilibrium or free-fall. Hence we conclude that the original Shakura- Sunyaev prescription is the only one proposed so far that is physically consistent close to the event horizon. We argue that, somewhat ironically, it is in fact the diﬀusive nature of the Shakura-Sunyaev form that is the reason for its success in this application. Our focus here is on advection-dominated accretion disks, but we expect that our results will also apply to generalized disks provided that losses of matter and energy become negligible as the gas approaches the event horizon.