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Search Results: 1 - 10 of 2867 matches for " Ramesh Narayan "
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Accretion Flows Around Black Holes
Ramesh Narayan
Physics , 1995,
Abstract: Most accreting black holes emit a substantial fraction of their luminosity in hard X-rays/soft $\gamma$-rays of up to $\sim{\rm few}\times100$ keV. The observations suggest that the radiating gas is very hot, $T>10^9$ K, and optically thin. Theoretical attempts to explain the observations are reviewed and unresolved issues are highlighted.
Advection-Dominated Accretion Flows: Optically Thin Solutions
Ramesh Narayan
Physics , 1995,
Abstract: General properties of advection-dominated accretion flows are discussed. Special emphasis is given to the optically thin branch of solutions, which has very high ion and electron temperatures and is thermally stable. This solution branch has been applied to a number of low-luminosity accreting black holes. The models have resolved some puzzles and have provided a straightforward explanation of the observed spectra. The success of the models confirms that the central objects in these low-luminosity sources are black holes. There is some indication that advection-dominated models may be relevant also for higher luminosity systems. The properties of the Low state of accreting black holes, the transition from the Low state to the High state, and the similarity of hard X-ray/gamma-ray spectra of black hole X-ray binaries and active galactic nuclei, are explained.
Advective Disks
Ramesh Narayan
Physics , 1996,
Abstract: Recent work on advection-dominated accretion flows (ADAFs) is reviewed. The article concentrates on an optically thin branch of ADAFs which is present at mass accretion rates below a critical value $\sim(10^{-2}-10^{-1})$ the Eddington rate. Models based on this branch have been quite successful at explaining a number of low-luminosity X-ray binaries and galactic nuclei, and some brighter systems. Some progress has also been made toward understanding the various spectral states of accreting black holes. It is argued that ADAFs may provide one of the best techniques for demonstrating the reality of event horizons in black holes.
Advection-Dominated Models of Luminous Accreting Black Holes
Ramesh Narayan
Physics , 1995, DOI: 10.1086/177136
Abstract: It has been found that a class of optically-thin two-temperature advection-dominated accretion solutions explains many observations of low-luminosity accreting black holes. Here it is shown that these models give a satisfactory description also of higher luminosity systems, provided the viscosity parameter $\alpha$ is large. The models reproduce the spectra of black hole X-ray binaries in the Low State, and explain the transition from the Low State to the High State at a critical mass accretion rate. The models also show that the X-ray/$\gamma$-ray spectra of X-ray binaries and active galactic nuclei should be similar, as confirmed by observations.
Why Do AGN Lighthouses Switch Off?
Ramesh Narayan
Physics , 2002, DOI: 10.1007/10856495_60
Abstract: Nearby galactic nuclei are observed to be very much dimmer than active galactic nuclei in distant galaxies. The Chandra X-ray Observatory has provided a definitive explanation for why this is so. With its excellent angular resolution, Chandra has imaged hot X-ray-emitting gas close to the gravitational capture radius of a handful of supermassive black holes, including Sgr A* in the nucleus of our own Galaxy. These observations provide direct and reliable estimates of the Bondi mass accretion rate in these nuclei. It is found that the Bondi accretion rate is significantly below the Eddington mass accretion rate, but this alone does not explain the dimness of the accretion flows. In all the systems observed so far, the accretion luminosity is orders of magnitude less than Mdot_{Bondi} times c^2, which means that the accretion must occur via a radiatively inefficient mode. This conclusion, which was strongly suspected for many years, is now inescapable. Furthermore, if the accretion in these nuclei occurs via either a Bondi flow or an advection-dominated accretion flow, the accreting plasma must be two-temperature at small radii, and the central mass must have an event horizon. Convection, winds and jets may play a role, but observations do not yet permit definite conclusions.
Evidence for the Black Hole Event Horizon
Ramesh Narayan
Physics , 2003,
Abstract: Astronomers have discovered many candidate black holes in X-ray binaries and in the nuclei of galaxies. The candidate objects are too massive to be neutron stars, and for this reason they are considered to be black holes. While the evidence based on mass is certainly strong, there is no proof yet that any of the objects possesses the defining characteristic of a black hole, namely an event horizon. Type I X-ray bursts, which are the result of thermonuclear explosions when gas accretes onto the surface of a compact star, may provide important evidence in this regard. Type I bursts are commonly observed in accreting neutron stars, which have surfaces, but have never been seen in accreting black hole candidates. It is argued that the lack of bursts in black hole candidates is compelling evidence that these objects do not have surfaces. The objects must therefore possess event horizons.
Hydrodynamic Drag on a Compact Star Orbiting a Supermassive Black Hole
Ramesh Narayan
Physics , 1999, DOI: 10.1086/308956
Abstract: The proposed Laser Interferometer Space Antenna is expected to detect gravitational waves from neutron stars and stellar-mass black holes spiraling into supermassive black holes in distant galactic nuclei. Analysis of the inspiral events will require careful comparison of the observed signals with theoretical waveform templates. The comparison could be seriously compromised if non-gravitational torques modify the orbit of the star. This paper estimates the torque exerted on an orbiting star as a result of hydrodynamic interactions with an accretion flow around the supermassive black hole. It is argued that the majority of inspiral events will take place in low luminosity galactic nuclei in which the mass accretion rate is low and the accretion occurs via an advection-dominated flow. The hydrodynamic torque is negligibly small in such systems and will have no effect on gravitational wave experiments.
Black Holes in Astrophysics
Ramesh Narayan
Physics , 2005, DOI: 10.1088/1367-2630/7/1/199
Abstract: This article reviews the current status of black hole astrophysics, focusing on topics of interest to a physics audience. Astronomers have discovered dozens of compact objects with masses greater than 3 solar masses, the likely maximum mass of a neutron star. These objects are identified as black hole candidates. Some of the candidates have masses of 5 to 20 solar masses and are found in X-ray binaries, while the rest have masses from a million to a billion solar masses and are found in galactic nuclei. A variety of methods are being tried to estimate the spin parameters of the candidate black holes. There is strong circumstantial evidence that many of the objects have event horizons. Recent MHD simulations of magnetized plasma accreting on rotating black holes seem to hint that relativistic jets may be produced by a magnetic analog of the Penrose process.
Review of thermo-physical properties, wetting and heat transfer characteristics of nanofluids and their applicability in industrial quench heat treatment
Ramesh Gopalan,Prabhu Narayan
Nanoscale Research Letters , 2011,
Abstract: The success of quenching process during industrial heat treatment mainly depends on the heat transfer characteristics of the quenching medium. In the case of quenching, the scope for redesigning the system or operational parameters for enhancing the heat transfer is very much limited and the emphasis should be on designing quench media with enhanced heat transfer characteristics. Recent studies on nanofluids have shown that these fluids offer improved wetting and heat transfer characteristics. Further water-based nanofluids are environment friendly as compared to mineral oil quench media. These potential advantages have led to the development of nanofluid-based quench media for heat treatment practices. In this article, thermo-physical properties, wetting and boiling heat transfer characteristics of nanofluids are reviewed and discussed. The unique thermal and heat transfer characteristics of nanofluids would be extremely useful for exploiting them as quench media for industrial heat treatment.
Lectures on Gravitational Lensing
Ramesh Narayan,Matthias Bartelmann
Physics , 1996,
Abstract: These lectures give an introduction to Gravitational Lensing. We discuss lensing by point masses, lensing by galaxies, and lensing by clusters and larger-scale structures in the Universe. The relevant theory is developed and applications to astrophysical problems are discussed.
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