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Search Results: 1 - 10 of 195150 matches for " Roger D. Blandford "
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Relativistic Accretion
Roger. D. Blandford
Physics , 1999,
Abstract: A brief summary of the properties of astrophysical black holes is presented. Various modes of accretion are distinguished, corresponding to accretion at rates from well below to well above the Eddington rate. The importance of mass loss is emphasized when the accreting gas cannot radiate and it is asserted that a strong wind is likely to be necessary to carry off mass, angular momentum and energy from the accreting gas. The possible importance of the black hole spin in the formation of jets and in dictating the relative importance of non-thermal emission over thermal radiation is discussed.
Gravitational Lensing and the Extragalactic Distance Scale
Roger D. Blandford,Tomislav Kundic
Physics , 1996,
Abstract: The potential of gravitational lenses for providing direct, physical measurements of the Hubble constant, free from systematic errors associated with the traditional distance ladder, has long been recognized. However, it is only recently that there has been a convincing measurement of a time delay sufficiently accurate to carry out this program. By itself, an accurate time delay measurement does not produce an equivalently definite Hubble constant and the errors associated with models of the primary lens, propagation through the potential fluctuations produced by the large-scale structure and the global geometry of the universe must also be taken into account. The prospects for measuring several more time delays and the feasibility of making the corresponding estimates of the Hubble constant with total error smaller than ten percent are critically assessed.
On the Formation of Jets
Annalisa Celotti,Roger D. Blandford
Physics , 2000, DOI: 10.1007/10720995_43
Abstract: The phenomenology of jets associated with a variety of black hole systems is summarized, emphasizing the constraints imposed on their origin. Models of jet formation are reviewed, focusing in particular on recent ideas concerning MHD models. Finally, the potential for advancing our understanding of jets both through future observations - especially forthcoming X-ray missions - and for elucidating some crucial theoretical questions is highlighted.
Roche Accretion of stars close to massive black holes
Lixin Dai,Roger D. Blandford
Physics , 2011,
Abstract: In this paper we consider Roche accretion in an Extreme Mass-Ratio Inspiral (EMRI) binary system formed by a star orbiting a massive black hole. The ultimate goal is to detect the mass and spin of the black hole and provide a test of general relativity in the strong-field regime from the resultant quasi-periodic signals. Before accretion starts, the stellar orbit is presumed to be circular and equatorial, and shrinks due to gravitational radiation. New fitting formulae are presented for the inspiral time and the radiation-reaction torque in the relativistic regime. If the inspiralling star fills its Roche lobe outside the Innermost Stable Circular Orbit (ISCO) of the hole, gas will flow through the inner Lagrange point (L1) to the hole. We give new relativistic interpolation formulae for the volume enclosed by the Roche lobe. If this mass-transfer happens on a time scale faster than the thermal time scale but slower than the dynamical time scale, the star will evolve adiabatically, and, in most cases, will recede from the hole while filling its Roche lobe. We calculate how the stellar orbital period and mass-transfer rate will change through the "Roche evolution" for various types of stars in the relativistic regime. We envisage that the mass stream eventually hits the accretion disc, where it forms a hot spot orbiting the hole and may ultimately modulate the luminosity with the stellar orbital frequency. The observability of such a modulation is discussed along with a possible interpretation of an intermittent 1 hour period in the X-ray emission of RE J1034+396.
Understanding the Geometry of Astrophysical Magnetic Fields
Avery E. Broderick,Roger D. Blandford
Physics , 2009, DOI: 10.1088/0004-637X/718/2/1085
Abstract: Faraday rotation measurements have provided an invaluable technique with which to measure the properties of astrophysical magnetized plasmas. Unfortunately, typical observations provide information only about the density-weighted average of the magnetic field component parallel to the line of sight. As a result, the magnetic field geometry along the line of sight, and in many cases even the location of the rotating material, is poorly constrained. Frequently, interpretations of Faraday rotation observations are dependent upon underlying models of the magnetic field being probed (e.g., uniform, turbulent, equipartition). However, we show that at sufficiently low frequencies, specifically below roughly 13 (RM/rad m^-2)^(1/4) (B/G)^(1/2) MHz, the character of Faraday rotation changes, entering what we term the ``super-adiabatic regime'' in which the rotation measure is proportional to the integrated absolute value of the line-of-sight component of the field. As a consequence, comparing rotation measures at high frequencies with those in this new regime provides direct information about the geometry of the magnetic field along the line of sight. Furthermore, the frequency defining the transition to this new regime, nu_SA, depends directly upon the local electron density and magnetic field strength where the magnetic field is perpendicular to the line of sight, allowing the unambiguous distinction between Faraday rotation within and in front of the emission region. Typical values of nu_SA range from 10 kHz to 10 GHz, depending upon the details of the Faraday rotating environment. In particular, for resolved AGN, including the black holes at the center of the Milky Way (Sgr A*) and M81, nu_SA ranges from roughly 10 MHz to 10 GHz, and thus can be probed via existing and up-coming ground-based radio observatories.
On the Energy Spectra of GeV/TeV Cosmic Ray Leptons
Lukasz Stawarz,Vahe Petrosian,Roger D. Blandford
Physics , 2009, DOI: 10.1088/0004-637X/710/1/236
Abstract: Recent observations of cosmic ray electrons from several instruments have revealed various degrees of deviation in the measured electron energy distribution from a simple power-law, in a form of an excess around TeV energies. An even more prominent deviation has been observed in the fraction of cosmic ray positrons around 100 GeV energies. In this paper we show that the observed excesses in the electron spectrum may be easily re-produced without invoking any unusual sources other than the general diffuse Galactic components of cosmic rays. The primary physical effect involved is the Klein-Nishina suppression of the electron cooling rate around TeV energies. With a very reasonable choice of the model parameters characterizing the local interstellar medium, we can reproduce the most recent observations by Fermi and HESS experiments. We also find that high positron fraction increasing with energy, as claimed by the PAMELA experiment, cannot be explained in our model with the conservative set of the model parameters. We are able, however, to reproduce the PAMELA results assuming high values of the starlight and interstellar gas densities, which would be more appropriate for vicinities of supernova remnants. A possible solution to this problem may be that cosmic rays undergo most of their interactions near their sources due to the efficient trapping in the far upstream of supernova shocks by self-generated, cosmic ray-driven turbulence.
Will GRB 990123 Perform an Encore?
Roger. D. Blandford,David J. Helfand
Physics , 1999, DOI: 10.1046/j.1365-8711.1999.02530.x
Abstract: The recent gamma ray burst, GRB 990123, has an absorption redshift z_s=1.60, implying an apparent energy $E \ge 3 \times 10^{54} erg$, and a peak luminosity $L_{max} \ge 6 \times10^{53}erg/s$, assuming isotropic emission. This energy is ten times larger than hitherto measured and in excess of the rest mass of the sun. Optical observations have revealed an associated galaxy displaced from the line of sight by $\sim 0.6''$. This raises the possibility that the burst is enhanced by gravitational lensing. We argue that existing observations probably only allow magnifications $\mu>400$ if the galaxy is at z_d=1.60 and the burst originates at much higher redshift. It should be possible to exclude this possibility by examining the burst time structure. If, as we anticipate, multiple imaging can be excluded, GRB 990123 remains the most intrinsically luminous event yet observed in its entirety.
Black hole mass and spin coevolution by mergers
Scott A. Hughes,Roger D. Blandford
Physics , 2002, DOI: 10.1086/375495
Abstract: Massive black holes appear to be present in the nuclei of almost all galaxies, but their genesis and evolution are not well understood. As astrophysical black holes are completely characterized by their masses and spins, the observed joint distribution of these quantities contains important clues to their history. We examine the coevolution of mass and spin in binary merger growth scenarios. We find that holes are typically spun down by mergers. Rapid rotation results only if the binary's larger member already spins quickly and the merger with the smaller hole is consistently near prograde; or, if the binary's mass ratio approaches one. If, as some observations have suggested, observed black holes spin rapidly, then this limits the importance of merger scenarios for the growth of black holes.
Intrinsic and Extrinsic Galaxy Alignment
Paolo Catelan,Marc Kamionkowski,Roger D. Blandford
Physics , 2000, DOI: 10.1046/j.1365-8711.2001.04105.x
Abstract: We show with analytic models that the assumption of uncorrelated intrinsic ellipticities of target sources that is usually made in searches for weak gravitational lensing due to large-scale mass inhomogeneities (``field lensing'') is unwarranted. If the orientation of the galaxy image is determined either by the angular momentum or the shape of the halo in which it forms, then the image should be aligned preferentially with the component of the tidal gravitational field perpendicular to the line of sight. Long-range correlations in the tidal field will thus lead to long-range ellipticity-ellipticity correlations that mimic the shear correlations due to weak gravitational lensing. We calculate the ellipticity-ellipticity correlation expected if halo shapes determine the observed galaxy shape, and we discuss uncertainties (which are still considerable) in the predicted amplitude of this correlation. The ellipticity-ellipticity correlation induced by angular momenta should be smaller. We consider several methods for discriminating between the weak-lensing (extrinsic) and intrinsic correlations, including the use of redshift information. An ellipticity--tidal-field correlation also implies the existence of an alignment of images of galaxies near clusters. Although the intrinsic alignment may complicate the interpretation of field-lensing results, it is inherently interesting as it may shed light on galaxy formation as well as on structure formation.
On the Fate of Gas Accreting at a Low Rate onto a Black Hole
Roger D. Blandford,Mitchell C. Begelman
Physics , 1998, DOI: 10.1046/j.1365-8711.1999.02358.x
Abstract: Gas supplied conservatively to a black hole at rates well below the Eddington rate may not be able to radiate effectively and the net energy flux, including the energy transported by the viscous torque, is likely to be close to zero at all radii. This has the consequence that the gas accretes with positive energy so that it may escape. Accordingly, we propose that only a small fraction of the gas supplied actually falls onto the black hole and that the binding energy it releases is transported radially outward by the torque so as to drive away the remainder in the form of a wind. This is a generalization of and an alternative to an "ADAF" solution. Some observational implications and possible ways to distinguish these two types of flow are briefly discussed.
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