Browsing by Author "Narayan, Ramesh"
Now showing items 21-40 of 99
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Electron Heating by the Ion Cyclotron Instability in Collisionless Accretion Flows. I. Compression-Driven Instabilities and the Electron Heating Mechanism
Sironi, Lorenzo; Narayan, Ramesh (IOP Publishing, 2015)In systems accreting well below the Eddington rate, such as the central black hole in the Milky Way (Sgr A∗), the plasma in the innermost regions of the disk is believed to be collisionless and have two temperatures, with ... -
Energy Extraction from Spinning Black Holes Via Relativistic Jets
Narayan, Ramesh; McClintock, Jeffrey E.; Tchekhovskoy, Alexander (2014)It has for long been an article of faith among astrophysicists that black hole spin energy is responsible for powering the relativistic jets seen in accreting black holes. Two recent advances have strengthened the case. ... -
Energy flows in thick accretion discs and their consequences for black hole feedback
Sadowski, Aleksander; Lasota, Jean-Pierre; Abramowicz, Marek A.; Narayan, Ramesh (Oxford University Press (OUP), 2016)We study energy flows in geometrically thick accretion discs, both optically thick and thin, using general relativistic, three-dimensional simulations of black hole accretion flows. We find that for non-rotating black holes ... -
Equilibrium configurations from gravitational collapse
Joshi, Pankaj S; Malafarina, Daniele; Narayan, Ramesh (IOP Publishing, 2011)We develop here a new procedure within Einstein's theory of gravity to generate equilibrium configurations that result as the final state of gravitational collapse from regular initial conditions. As a simplification, we ... -
Erratum: "Precise Measurement of the Spin Parameter of the Stellar-Mass Black hole M33 X-7” (2008, ApJ, 679, L37)
Liu, Jifeng; McClintock, Jeffrey E.; Narayan, Ramesh; Davis, Shane W.; Orosz, Jerome A. (IOP Publishing, 2010)In prior work, Chandra and Gemini-North observations of the eclipsing X-ray binary M33 X-7 have yielded measurements of the mass of its black hole primary and the system’s orbital inclination angle of unprecedented accuracy. ... -
Erratum: “The Power of Imaging: Constraining the Plasma Properties of GRMHD Simulations using EHT Observatioins of SGR A*” (2015, ApJ, 799, 1)
Chan, Chi-kwan; Psaltis, Dimitrios; Özel, Feryal; Narayan, Ramesh; Sadowski, Aleksander (IOP Publishing, 2015) -
Estimating the Spins of Stellar-Mass Black Holes by Fitting Their Continuum Spectra
Narayan, Ramesh; McClintock, Jeffrey E.; Shafee, Rebecca; Yuan, Ye-Fei; Li, Xiang-Dong; Lai, Dong (AIP Publishing, 2008)We have used the Novikov‐Thorne thin disk model to fit the continuum X‐ray spectra of three transient black hole X‐ray binaries in the thermal state. From the fits we estimate the dimensionless spin parameters of the black ... -
The Event Horizon of M87
Broderick, Avery E.; Narayan, Ramesh; Kormendy, John; Perlman, Eric S.; Rieke, Marcia J.; Doeleman, Sheperd S. (IOP Publishing, 2015)The 6 × 109M⊙ supermassive black hole at the center of the giant elliptical galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the ... -
The event horizon of Sagittarius A*
Broderick, Avery E.; Loeb, Abraham; Narayan, Ramesh (IOP Publishing, 2009)Black hole event horizons, causally separating the external universe from compact regions of spacetime, are one of the most exotic predictions of general relativity. Until recently, their compact size has prevented efforts ... -
Event Horizon Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way With the Inner Stellar Disk
Psaltis, Dimitrios; Narayan, Ramesh; Fish, Vincent L.; Broderick, Avery E.; Loeb, Abraham; Doeleman, Sheperd S. (IOP Publishing, 2014)Observations of the black hole in the center of the Milky Way with the Event Horizon Telescope at 1.3 mm have revealed a size of the emitting region that is smaller than the size of the black-hole shadow. This can be ... -
Extracting black-hole rotational energy: The generalized Penrose process
Lasota, J.-P.; Gourgoulhon, E.; Abramowicz, M.; Tchekhovskoy, A.; Narayan, Ramesh (American Physical Society (APS), 2014)In the case involving particles the necessary and sufficient condition for the Penrose process to extract energy from a rotating black hole is absorption of particles with negative energies and angular momenta. No torque ... -
Extreme Brightness Temperatures and Refractive Substructure in 3C 273 with RadioAstron
Johnson, Michael D.; Kovalev, Yuri Y.; Gwinn, Carl R.; Gurvits, Leonid I.; Narayan, Ramesh; Macquart, Jean-Pierre; Jauncey, David L.; Voitsik, Peter A.; Anderson, James M.; Sokolovsky, Kirill V.; Lisakov, Mikhail M. (American Astronomical Society, 2016)Earth–space interferometry with RadioAstron provides the highest direct angular resolution ever achieved in astronomy at any wavelength. RadioAstron detections of the classic quasar 3C 273 on interferometric baselines up ... -
The Extreme Spin of the Black Hole in Cygnus X-1
Gou, Lijun; McClintock, Jeffrey E.; Reid, Mark J.; Orosz, Jerome A.; Steiner, James F.; Narayan, Ramesh; Xiang, Jingen; Remillard, Ronald A.; Arnaud, Keith A.; Davis, Shane W. (IOP Publishing, 2011)The compact primary in the X-ray binary Cygnus X-1 was the first black hole to be established via dynamical observations. We have recently determined accurate values for its mass and distance, and for the orbital inclination ... -
The eye of the storm: light from the inner plunging region of black hole accretion discs
Zhu, Yucong; Davis, Shane W.; Narayan, Ramesh; Kulkarni, Akshay K.; Penna, Robert Francis; McClintock, Jeffrey E. (Oxford University Press (OUP), 2012)It is generally thought that the light coming from the inner plunging region of black hole accretion discs contributes negligibly to the disc's overall spectrum, i.e. the plunging fluid is swallowed by the black hole before ... -
Fast Variability and Millimeter/IR Flares in GRMHD Models of Sgr A* From Strong-Field Gravitational Lensing
Chan, Chi-kwan; Psaltis, Dimitrios; Özel, Feryal; Medeiros, Lia; Marrone, Daniel; Sadowski, Aleksander; Narayan, Ramesh (IOP Publishing, 2015)We explore the variability properties of long, high cadence GRMHD simulations across the electromagnetic spectrum using an efficient, GPU-based radiative transfer algorithm. We focus on both disk- and jet-dominated simulations ... -
General relativistic magnetohydrodynamic simulations of Blandford-Znajek jets and the membrane paradigm
Penna, R. F.; Narayan, Ramesh; Sadowski, A. (Oxford University Press (OUP), 2013)Recently it has been observed that the scaling of jet power with black hole spin in galactic X-ray binaries is consistent with the predictions of the Blandford-Znajek (BZ) jet model. These observations motivate us to revisit ... -
General Relativistic Modeling of Magnetized Jets from Accreting Black Holes
Tchekhovskoy, Alexander; McKinney, Jonathan C; Narayan, Ramesh (Institute of Physics, 2012)Recent advances in general relativistic magnetohydrodynamic modeling of jets offer unprecedented insights into the inner workings of accreting black holes that power the jets in active galactic nuclei (AGN) and other ... -
Global simulations of axisymmetric radiative black hole accretion discs in general relativity with a mean-field magnetic dynamo
Sadowski, Aleksander; Narayan, Ramesh; Tchekhovskoy, Alexander; Abarca, David; Zhu, Yucong; McKinney, Jonathan C. (Oxford University Press (OUP), 2014)We present a sub-grid model that emulates the magnetic dynamo operating in magnetized accretion disks. We have implemented this model in the general relativisic radiation magnetohydrodynamic (GRRMHD) code KORAL, using ... -
Global Structure of Optically Thin, Magnetically Supported, Two-Temperature, Black Hole Accretion Disks
Oda, Hiroshi; Machida, Mami; Nakamura, Kenji E.; Matsumoto, Ryoji; Narayan, Ramesh (Oxford University Press (OUP), 2012)We present global solutions of optically thin, two-temperature black hole accretion disks incorporating magnetic fields. We assume that the ϖφ-component of the Maxwell stress is proportional to the total pressure, and ... -
Globular Clusters and Dark Satellite Galaxies Through the Stream Velocity
Naoz, Smadar; Narayan, Ramesh (IOP Publishing, 2014)The formation of purely baryonic globular clusters with no gravitationally bound dark matter is still a theoretical challenge. We show that these objects might form naturally whenever there is a relative stream velocity ...