Showing papers by "Roger Blandford published in 1992"

Cosmological Applications of Gravitational Lensing

Abstract: The discovery by Walsh et al (1979) of the first bona fide gravitational lens, the doubly-imaged quasar, Q0957 + 561, happened at an opportune time, following several prescient theoretical papers, and just preceding the completion of radio and optical quasar surveys that have since yielded over a dozen examples of this phenomenon. Interest in gravitational lenses stretches back over more than seventy years (Eddington 1919, Lodge 1919). Zwicky (1937a,b) appears to have been the first to realize that gravitational lensing ought to have a major impact on cosmology, specifically by "weighing" nebulae and providing crude telescopes to magnify lensed sources. The discovery of quasi-stellar "point" sources added two more possible uses of lenses, for distance measurement (Klimov 1963, Liebes 1964, Refsdal 1964b) and as probes of the stellar composition of lenses (Chang & Refsdal 1979), both of which may be just coming to fruition. These four topics constitute the primary theme of this review.

Magnetic acceleration of broad emission-line clouds in active galactic nuclei

Abstract: It is proposed that broad emission lines in active galactic nuclei are formed by dense clouds in a molecular, hydromagnetic wind accelerated radiatively and centrifugally away from an accretion disk orbiting a massive black hole. These clouds are supposed to be photoionized by the UV continuum produced in the innermost radii of the accretion disk where the radiation flux is sufficient to evaporate the dust grains, that is, at a radius ∼0.2L 1/2 ion,46 pc

Quasar evolution and the growth of black holes

Abstract: A 'minimalist' model of AGN evolution is analyzed that links the measured luminosity function to an elementary description of black hole accretion. The observed luminosity function of bright AGN is extrapolated and simple prescriptions for the growth and luminosity of black holes are introduced to infer quasar birth rates, mean fueling rates, and relict black hole distribution functions. It is deduced that the mean accretion rate scales as (M exp -1./5)(t exp -6.7) and that, for the most conservative model used, the number of relict black holes per decade declines only as M exp -0.4 for black hole masses between 3 x 10 exp 7 and 3 x 10 exp 9 solar masses. If all sufficiently massive galaxies pass through a quasar phase with asymptotic black hole mass a monotonic function of the galaxy mass, then it is possible to compare the space density of galaxies with estimated central masses to that of distant quasars.

On the Straight Arc in Abell 2390

Abstract: We investigate the long «straight arc» observed in the cluster of galaxies Abell 2390. The arc consists of three segments of comparable brightness, and its optical spectrum shows a nonmonotonic variation of velocity along its length. In addition to the optical data, recent infrared observations indicate that the longest segment of the arc is much brighter in infrared than the other two. We consider the formation of straight arcs analytically in terms of fold and cusp catastrophes and two singular sections of a cusp, the «beak-to-beak» and «lips» «calamities»

On the evolution of slowly accreting neutron stars

Abstract: In the light of recent calculations of pycnonuclear reaction rates for light elements, we consider the problem of slow interstellar accretion onto old, possibly magnetized neutron stars. We argue that accretion will occur at the Hoyle-Lyttleton rate after the star has spun down in less than about 10 exp 9 yr. A deep ocean of liquid hydrogen and helium, extending down to depths about 100 m, will cover the surface of the star once it has accreted about 10 exp 25 g of gas. Beneath the ocean will be a layer of almost pure solid O-16 which undergoes two-stage electron capture to C-16 above a pressure 2.7 x 10 exp 28 dyne/sq cm, corresponding to an accreted mass of about 10 exp 27 g. Taking into account the presence of multiple layers of distinct chemical composition, we conclude that the crust will be stable to small perturbations under the conditions envisaged for instellar accretion. A thick layer of up to 10 exp 27 g of metastable C-16 will then accumulate. We discuss the implications of these results to old Galactic neutron stars as sources of gamma-ray bursts.

Pulsars and Physics

Abstract: Twenty-five years of active pulsar research have yielded a quite unexpected harvest of physical effects. These range from pleasing corroborations of well-established physical principles in unfamiliar environments to direct verification of the general theory of relativity. Pulsars have the potential for advancing our understanding of high energy nuclear physics and relativistic plasma physics. They have also enlarged our horizons in atomic physics, quantum electrodynamics and solid state physics. As extremely compact sources of regularly pulsed, high brightness radio waves, they provide excellent probes of the interstellar medium and globular clusters. The large variety of connections between pulsars and physics is illustrated with examples taken from each of these areas.

A brief history of pulsar time

Abstract: Radio pulsars were discovered because their sources, spinning neutron stars, keep time with extraordinary precision. Indeed, some rival the very best atomic clocks. But they are not perfect clocks. Allowance must be made for their steady deceleration due to the action of an electromagnetic torque; and some undergo sudden, unpredictable increases in their frequencies, known as glitches. One of the best studied pulsars is PSR0531+21, more famous as the central pulsar of the Crab Nebula - the relic of a supernova explosion observed by Chinese astronomers in AD 1054. A large glitch on 29 August 1989 shortened this pulsar's 33-millisecond period by about 3 nanoseconds, a much bigger jump than in previous glitches. On page 706 of this issue, Lyne, Graham Smith and Pritchard present an analysis of the times of arrival of pulses from this pulsar which suggests a physical description of the interior of the neutron star that is both more complicated and more intriguing than any that has been seen so far.

Pulsars and physics.

Abstract: Twenty-five years of active pulsar research have yielded a quite unexpected harvest of physical effects. These range from pleasing corroborations of well-established physical principles in unfamiliar environments to direct verification of the general theory of relativity. Pulsars have the potential for advancing our understanding of high energy nuclear physics and relativistic plasma physics. They have also enlarged our horizons in atomic physics, quantum electrodynamics and solid state physics. As extremely compact sources of regularly pulsed, high brightness radio waves, they provide excellent probes of the interstellar medium and globular clusters. The large variety of connections between pulsars and physics is illustrated with examples taken from each of these areas.

On the straight arc in A2390

Abstract: We investigate the long “straight arc” observed in the cluster of galaxies Abell 2390. The arc consists of three segments of comparable optical brightness, and its optical spectrum shows a non-monotonic variation of velocity along its length. In addition to the optical data, recent infrared observations indicate that the longest segment of the arc is much brighter in infrared than the other two. Analytical investigation of the formation of straight arcs shows that single cusps always produce curved images, while fold catastrophes and “beak-to-beak” and “lips” “calamities” can form straight images. On the basis of these results we derive detailed models for the observed features. We argue that it is not possible to reproduce the straight arc without postulating some unseen matter not traced by the cluster. A fold model is consistent with the optical data, but it requires a large and very elongated unseen component, and it is not consistent with the infrared observations. A beak-to-beak model agrees with both infrared and optical data and requires only a small unseen galaxy. Future observations may distinguish between these possibilities.