Person: Narayan, Ramesh
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Narayan
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Ramesh
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Narayan, Ramesh
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Publication General Relativistic Modeling of Magnetized Jets from Accreting Black Holes(Institute of Physics, 2012) Tchekhovskoy, Alexander; McKinney, Jonathan C; Narayan, RameshRecent 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 accretion systems. I will present the results of recent studies that determine spin-dependence of jet power and discuss the implications for the AGN radio loud/quiet dichotomy and recent observations of high jet power in a number of AGN.Publication Radio Synchrotron Emission from a Bow Shock Around the Gas Cloud G2 Heading Toward the Galactic Center(American Astronomical Society, 2012) Narayan, Ramesh; Özel, Feryal; Sironi, LorenzoA dense ionized cloud of gas has been recently discovered to be moving directly toward the supermassive black hole, Sgr A*, at the Galactic center. In 2013 June, at the pericenter of its highly eccentric orbit, the cloud will be approximately 3100 Schwarzschild radii from the black hole and will move supersonically through the ambient hot gas with a velocity of vp ≈ 5400 km s–1. A bow shock is likely to form in front of the cloud and could accelerate electrons to relativistic energies. We estimate via particle-in-cell simulations the energy distribution of the accelerated electrons and show that the non-thermal synchrotron emission from these electrons might exceed the quiescent radio emission from Sgr A* by a factor of several. The enhanced radio emission should be detectable at GHz and higher frequencies around the time of pericentric passage and in the following months. The bow shock emission is expected to be displaced from the quiescent radio emission of Sgr A* by ~33 mas. Interferometric observations could resolve potential changes in the radio image of Sgr A* at wavelengths lsim 6 cm.Publication The eye of the storm: light from the inner plunging region of black hole accretion discs(Oxford University Press (OUP), 2012) Zhu, Yucong; Davis, Shane W.; Narayan, Ramesh; Kulkarni, Akshay K.; Penna, Robert; McClintock, JeffreyIt 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 it has time to radiate. In the standard disc model used to fit X-ray observations of accretion discs, the plunging region is assumed to be perfectly dark. However, numerical simulations that include the full physics of the magnetized flow predict that a small fraction of the disc's total luminosity emanates from the plunging region. We investigate the observational consequences of this neglected inner light. We compute radiative transfer based disc spectra that correspond to 3D general relativistic magnetohydrodynamic simulated discs (which produce light inside their plunging regions). In the context of black hole spin estimation, we find that the neglected inner light only has a modest effect (this bias is less than typical observational systematic errors). For rapidly spinning black holes, we find that the combined emission from the plunging region produces a weak power-law tail at high energies. This indicates that infalling matter is the origin for some of the `coronal' emission observed in the thermal dominant and steep power-law states of X-ray binaries.Publication GRMHD simulations of magnetized advection-dominated accretion on a non-spinning black hole: role of outflows(Oxford University Press (OUP), 2012) Narayan, Ramesh; Sadowski, Aleksander; Penna, Robert; Kulkarni, Akshay K.We present results from two long-duration GRMHD simulations of advection-dominated accretion around a non-spinning black hole. The first simulation was designed to avoid significant accumulation of magnetic flux around the black hole. This simulation was run for a time of 200,000GM/c^3 and achieved inflow equilibrium out to a radius \sim90GM/c^2. Even at this relatively large radius, the mass outflow rate \dot{M}_{out} is found to be only 60% of the net mass inflow rate \dot{M}_{BH} into the black hole. The second simulation was designed to achieve substantial magnetic flux accumulation around the black hole in a magnetically arrested disc. This simulation was run for a shorter time of 100,000GM/c^3. Nevertheless, because the mean radial velocity was several times larger than in the first simulation, it reached inflow equilibrium out to a radius \sim170GM/c^2. Here, \dot{M}_{out} becomes equal to \dot{M}_{BH} at r\sim 160GM/c^2. Since the mass outflow rates in the two simulations do not show robust convergence with time, it is likely that the true outflow rates are lower than our estimates. The effect of black hole spin on mass outflow remains to be explored. Neither simulation shows strong evidence for convection, though a complete analysis including the effect of magnetic fields is left for the future.Publication On the Mass Distribution and Birth Masses of Neutron Stars(American Astronomical Society, 2012) Özel, Feryal; Psaltis, Dimitrios; Narayan, Ramesh; Santos Villarreal, AntonioWe investigate the distribution of neutron star masses in different populations of binaries, employing Bayesian statistical techniques. In particular, we explore the differences in neutron star masses between sources that have experienced distinct evolutionary paths and accretion episodes. We find that the distribution of neutron star masses in non-recycled eclipsing high-mass binaries as well as of slow pulsars, which are all believed to be near their birth masses, has a mean of 1.28 M ☉ and a dispersion of 0.24 M ☉. These values are consistent with expectations for neutron star formation in core-collapse supernovae. On the other hand, double neutron stars, which are also believed to be near their birth masses, have a much narrower mass distribution, peaking at 1.33 M ☉, but with a dispersion of only 0.05 M ☉. Such a small dispersion cannot easily be understood and perhaps points to a particular and rare formation channel. The mass distribution of neutron stars that have been recycled has a mean of 1.48 M ☉ and a dispersion of 0.2 M ☉, consistent with the expectation that they have experienced extended mass accretion episodes. The fact that only a very small fraction of recycled neutron stars in the inferred distribution have masses that exceed ~2 M ☉ suggests that only a few of these neutron stars cross the mass threshold to form low-mass black holes.Publication Global Structure of Optically Thin, Magnetically Supported, Two-Temperature, Black Hole Accretion Disks(Oxford University Press (OUP), 2012) Oda, Hiroshi; Machida, Mami; Nakamura, Kenji E.; Matsumoto, Ryoji; Narayan, RameshWe 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 prescribe the radial dependence of the magnetic flux advection rate in order to complete the set of basic equations. We obtained magnetically supported (low-β) disk solutions, whose luminosity exceeds the maximum luminosity for an advection-dominated accretion flow (ADAF), L ≳ 0.4α2 LEdd, where LEdd is the Eddington luminosity. The accretion flow is composed of the outer ADAF, a luminous hot accretion flow (LHAF) inside the transition layer from the outer ADAF to the low-β disk, the low-β disk, and the inner ADAF. The low-β disk region becomes wider as the mass-accretion rate increases further. In the low-β disk, the magnetic heating balances the radiative cooling, and the electron temperature decreases from ∼ 109.5 K to ∼ 108 K as the luminosity increases. These results are consistent with the anti-correlation between the energy cutoff in X-ray spectra (hence the electron temperature) and the luminosity when L ≳ 0.1LEdd, observed in the bright/hard state during the bright hard-to-soft transitions of transient outbursts in galactic black hole candidates.Publication Variability in Blazars: Clues from PKS 2155−304(Oxford University Press (OUP), 2012) Narayan, Ramesh; Piran, TsviRapid variability on a time scale much faster than the light-crossing time of the central supermassive black hole has been seen in TeV emission from the blazar PKS 2155-304. The most plausible explanation of this puzzling observation is that the radiating fluid in the relativistic jet is divided into a large number of sub-regions which move in random directions with relativistic speeds. We consider two versions of this "jets in a jet" model. In the first, the "subjets" model, stationary regions in the mean jet frame emit relativistic subjets that produce the observed radiation. The variability time scale is determined by the size of the sub-regions in the mean jet frame. This model, which is motivated by magnetic reconnection, has great difficulty explaining the observations in PKS 2155-304. In the alternate "turbulence" model, various sub-regions move relativistically in random directions and the variability time scale is determined by the size of these regions in their own comoving frames. This model fits the data much more comfortably. We consider collisions between TeV photons emitted from different sub-regions and find that, in both the subjets and turbulence models, the mean bulk Lorentz factor of the jet needs to be greater than 25 to avoid the pair catastrophe.Publication Observational evidence for a correlation between jet power and black hole spin(Oxford University Press (OUP), 2012) Narayan, Ramesh; McClintock, JeffreyWe show that the 5-GHz radio flux of transient ballistic jets in black hole binaries correlates with the dimensionless black hole spin parameter a* estimated via the continuum-fitting method. The data suggest that jet power scales either as the square of a* or the square of the angular velocity of the horizon. This is the first direct evidence that jets may be powered by black hole spin energy. The observed correlation validates the continuum-fitting method of measuring spin. In addition, for those black holes that have well-sampled radio observations of ballistic jets, the correlation may be used to obtain rough estimates of their spins.Publication Measuring the spins of accreting black holes(IOP Publishing, 2011) McClintock, Jeffrey; Narayan, Ramesh; Davis, Shane W.; Gou, Lijun; Kulkarni, Akshay; Orosz, Jerome A; Penna, Robert; Remillard, Ronald A; Steiner, JamesA typical galaxy is thought to contain tens of millions of stellar-mass black holes, the collapsed remnants of once massive stars, and a single nuclear supermassive black hole. Both classes of black holes accrete gas from their environments. The accreting gas forms a flattened orbiting structure known as an accretion disk. During the past several years, it has become possible to obtain measurements of the spins of the two classes of black holes by modeling the x-ray emission from their accretion disks. Two methods are employed, both of which depend upon identifying the inner radius of the accretion disk with the innermost stable circular orbit, whose radius depends only on the mass and spin of the black hole. In the Fe Kα method, which applies to both classes of black holes, one models the profile of the relativistically broadened iron line with a special focus on the gravitationally redshifted red wing of the line. In the continuum-fitting (CF) method, which has so far only been applied to stellar-mass black holes, one models the thermal x-ray continuum spectrum of the accretion disk. We discuss both methods, with a strong emphasis on the CF method and its application to stellar-mass black holes. Spin results for eight stellar-mass black holes are summarized. These data are used to argue that the high spins of at least some of these black holes are natal, and that the presence or absence of relativistic jets in accreting black holes is not entirely determined by the spin of the black hole.Publication Constraints on cold magnetized shocks in gamma-ray bursts(Oxford University Press (OUP), 2011) Narayan, Ramesh; Kumar, Pawan; Tchekhovskoy, AlexanderWe consider a model in which the ultra-relativistic jet in a gamma-ray burst (GRB) is cold and magnetically accelerated. We assume that the energy flux in the outflowing material is partially thermalized via internal shocks or a reverse shock, and we estimate the maximum amount of radiation that could be produced in such magnetized shocks. We compare this estimate with the available observational data on prompt γ-ray emission in GRBs. We find that, even with highly optimistic assumptions, the magnetized jet model is radiatively too inefficient to be consistent with observations. One way out is to assume that much of the magnetic energy in the post-shock, or even pre-shock, jet material is converted to particle thermal energy by some unspecified process, and then radiated. This can increase the radiative efficiency sufficiently to fit observations. Alternatively, jet acceleration may be driven by thermal pressure rather than magnetic fields. In this case, which corresponds to the traditional fireball model, sufficient prompt GRB emission could be produced either from shocks at a large radius or from the jet photosphere closer to the center.