arXiv subjects

M. J. Rees

Publications and source records attributed to M. J. Rees.

45 records · Page 3Linked to original sources

arXiv1996

Physical constraints on the sizes of dense clouds in the central magnetospheres of Active Galactic Nuclei

The range of microphysical and global dynamical timescales in the central regions of Active Galactic Nuclei (AGN) is sufficiently wide to permit the existence of multiphase structure. In particular, very dense, cool clouds can coexist with a hot, magnetically-dominated medium and can thereby efficiently reprocess the continuum radiation generated in this primary source region. The strong dynamical forces in this central magnetosphere can give rise to extremely small clouds. Microphysical processes then determine whether such clouds can indeed survive, in spite of their extremely contrasting properties relative to the surrounding environment, for long enough to produce potentially observable thermal reprocessing signatures. We examine specific physical constraints on the thicknesses of such reprocessing clouds. Our results are plotted to show the range of conditions that is representative of the central regions of AGN. We find a parameter subspace in the extreme high density regime for which the effects of microphysical diffusion processes can be overcome and for which cool gas can maintain pressure equilibrium with the ambient magnetosphere.

astro-ph↗

arXiv1996

Capture of stellar--mass compact objects by massive black holes in galactic cusps

A significant fraction of the stellar population in the cusp around central black holes of galaxies consists of compact remnants of evolved stars, such as white dwarfs, neutron stars and stellar mass black holes. We estimate the rate of capture of compact objects by massive central black holes, assuming most spiral galaxies have a central black hole of modest mass ($\sim 10^6 \msun$), and a cuspy spheroid. It is likely that the total capture rate is dominated by nucleated spirals. We estimate the flux of gravitational wave radiation from such coalescences, and the estimated detectable source count for proposed space--based gravitational wave observatories such as LISA. About one event per year should be detectable within $1 \, {\rm Gpc}$, given very conservative estimates of the black hole masses and central galactic densities. We expect $10^2$--$10^3$ detectable sources at lower frequencies ($10^{-4}$ Hz) ``en route'' to capture. If stellar mass black holes are ubiquitous, the signal may be dominated by stellar mass black holes coalescing with massive black holes. The rate of white dwarf--white dwarf mergers in the cores of nucleated spirals is estimated at $\sim10^{-6}$ per year per galaxy.

astro-ph↗

arXiv1996

Optical and Long Wavelength Afterglow from Gamma-Ray Bursts

We discuss the evolution of cosmological gamma-ray burst remnants, consisting of the cooling and expanding fireball ejecta together with any swept-up external matter, after the gamma-ray event. We show that significant optical emission is predicted which should be measurable for timescales of hours after the event, and in some cases radio emission may be expected days to weeks after the event. The flux at optical, X-ray and other long wavelengths decays as a power of time, and the initial value of the flux or magnitude, as well as the value of the time-decay exponent, should help to distinguish between possible types of dissipative fireball models.

astro-ph↗

arXiv1995

The accretion luminsoity of a massive black hole in an elliptical galaxy

Most large elliptical galaxies should now host a massive black hole leftover from an earlier quasar phase. Such galaxies also have an extensive hot gaseous halo. Here we consider why the nuclei of elliptical galaxies are not luminous sources due to the accretion of the hot gas by the central black hole, yet have weak radio sources. In particular we suggest that accretion from the hot medium has low radiative efficiency and forms a hot ion torus surrounding the black hole. Synchrotron emission from the torus can account for at least part of the weak radio sources commonly seen in otherwise normal elliptical galaxies. The inner funnel of the ion torus can also begin the collimation of a narrow jet. We speculate on the difference between jets from the nuclei of spiral and elliptical galaxies and suggest that the extensive hot atmosphere seen in ellipticals is essential for maintaining the collimation and provoking the observed radio structures of radio galaxies. We also note that low radiative efficiency, and not necessarity a large change in accretion rate, may be responsible for the demise of quasars and may contribute to the hard X-ray Background. Hot ion tori may be common in galactic nuclei.

astro-ph↗

arXiv1994

Delayed Gev Emission from Cosmological Gamma-Ray Bursts : Impact of a Relativistic Wind on External Matter

Sudden collapse of a compact object, or coalescence of a compact binary, can generate an unsteady relativistic wind that lasts for a few seconds. The wind is likely to carry a high magnetic field; and its Lorentz factor depends on the extent to which it is 'loaded' with baryons. If the Lorentz factor is $\sim 100$, internal dissipation and shocks in this wind produce a non-thermal gamma-ray burst, detectable in the range $0.1\MeV \siml E_γ\siml 0.1-1\GeV$ out to cosmological distances. The cooled wind ejecta would subsequently be decelerated by the external medium. The resultant blast wave and reverse shock can then give rise to a second burst component, mainly detectable in the GeV range, with a time delay relative to the MeV burst ranging from minutes to hours.

astro-ph↗

arXiv1994

Unsteady Outflow Models for Cosmological Gamma-Ray Bursts

The 'event' that triggers a gamma ray burst cannot last for more than a few seconds. This is, however, long compared with the dynamical timescale of a compact stellar-mass object ($\sim 10^{-3}$ seconds). Energy is assumed to be released as an outflow with high mean lorentz factor $Γ$. But a compact stellar-mass collapse or merger is, realistically, likely to generate a mass (or energy) flux that is unsteady on some timescales in the range $10^{-3}$ - 10 seconds. If $Γ$ fluctuates by a factor of $\sim 2$ around its mean value, relative motions within the outflowing material will themselves (in the comoving frame) be relativistic, and can give rise to internal shocks. For $Γ\sim 10^2$, the resultant dissipation occurs outside the 'photosphere' and can convert a substantial fraction of the overall outflow energy into non-thermal radiation. This suggests a mechanism for cosmological bursts that demands less extreme assumptions (in respect of $Γ$-values, freedom from baryonic contamination, etc) than earlier proposals.

astro-ph↗

arXiv1993

Spectral Properties of Blast Wave Models of Gamma-Ray Burst Sources

We calculate the spectrum of blast wave models of gamma-ray burst sources, for various assumptions about the magnetic field density and the relativistic particle acceleration efficiency. For a range of physically plausible models we find that the radiation efficiency is high, and leads to nonthermal spectra with breaks at various energies comparable to those observed in the gamma-ray range. Radiation is also predicted at other wavebands, in particular at X-ray, optical/UV and GeV/TeV energies. We discuss the spectra as a function of duration for three basic types of models, and for cosmological, halo and galactic disk distances. We also evaluate the gamma-ray fluences and the spectral characteristics for a range of external densities. Impulsive burst models at cosmological distances can satisfy the conventional X-ray paucity constraint $S_x/S_γ\siml$ few percent over a wide range of durations, but galactic models can do so only for bursts shorter than a few seconds, unless additional assumptions are made. The emissivity is generally larger for bursts in a denser external environment, with the efficiency increasing up to the point where all the energy input is radiated away.

astro-ph↗

arXiv1993

Gamma-Ray Bursts: Multiwaveband Spectral Predictions for Blast Wave Models

In almost any scenario for 'cosmological' gamma-ray bursts (and in many models where they originate in our own Galaxy), the initial energy density is so large that the resulting relativistic plasma expands with $v\sim c$ producing a blast wave ahead of it and a reverse shock moving into the ejecta, as it ploughs into the external medium. We evaluate the radiation expected from these shocks,for both cosmological and galactic bursts, for various assumptions about the strength of the magnetic field and the particle acceleration mechanisms in the shocks. The spectra are evaluated over the whole range from the IR to $>$ GeV, and are compared with the variety of spectral behavior reported by BATSE, and with the X-ray and optical constraints. For bursts of duration $\simg 1\s$ acceptable $γ$-ray spectra and $L_x/L_γ$ ratios are readily obtained for 'cosmological' models. Blast waves in galactic models can produce bursts of similar gamma-ray fluence and duration, but they violate the X-ray paucity constraint, except for the shorter bursts ($\siml 1\s$). We discuss the prospects for using O/UV and X-ray observations to discriminate among alternative models.

astro-ph↗

arXiv1993

Gasdynamics of Relativistically Expanding Gamma-Ray Burst Sources: Kinematics, Energetics, Magnetic Fields and Efficiency

We calculate both analytically and numerically the evolution of a highly relativistic fireball through the stages of free expansion and coasting, and determine the dependence of the thermodynamic and radiation variables in the comoving and laboratory frames. Magnetic fields may have been important in the original impulsive event. We discuss their effect on the fireball dynamics, and consider also their effects on the radiation emitted when the fireball runs into an external medium and is decelerated. The inverse synchro-Compton mechanism can then yield high radiative efficiency in the reverse shock, producing a burst of non-thermal radiation mainly in the MeV to GeV range whose total energy and duration agree with those of typical cosmic gamma ray bursts.

astro-ph↗