arXiv subjects

Martin J. Rees

Publications and source records attributed to Martin J. Rees.

61 records · Page 4Linked to original sources

arXiv1997

Astrophysical Evidence for Black Holes

The case for collapsed objects in some X-ray binary systems continues to strengthen. But there is now even firmer evidence for supermassive black holes in galactic centres. Gravitational collapse seems to have occurred in the centres of most newly-forming galaxies, manifesting itself in a phase of quasar-like activity (which may be reactivated later). These phenomena (especially the gas-dynamical aspects) are still a daunting challenge to theorists, but there is `cleaner' evidence, based on stellar dynamics, for collapsed objects in the centres of most nearby galaxies. The current evidence does not tell us the spin of the collapsed objects -- nor, indeed, whether they are described by Kerr geometry, as general relativity theory predicts. There are now, however, several hopeful prospects of discovering observational signatures that will indeed probe the strong-gravity domain.

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arXiv1997

Gamma-ray bursts: Challenges to relativistic astrophysics

Although they were discovered more than 25 years ago, gamma-ray bursts are still a mystery. Even their characteristic distance is highly uncertain. All that we can be confident about is that they involve compact objects and relativistic plasma. Current ideas and prospects are briefly reviewed. There are, fortunately, several feasible types of observation that could soon clarify the issues.

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arXiv1997

High-Redshift Supernovae and the Metal-Poor Halo Stars: Signatures of the First Generation of Galaxies

Recent evidence on the metal content of the high-redshift Lyman alpha forest seen in quasar spectra suggests that an early generation of galaxies enriched the intergalactic medium (IGM) at z ~ 5. We calculate the number of supernovae that need to have taken place to produce the observed metallicity. The progenitor stars of the supernovae should have emitted $\sim 20$ ionizing photons for each baryon in the universe, i.e., more than enough to ionize the IGM. We calculate that the rate of these supernovae is such that about one of them should be observable at any time per square arc minute. Their fluxes are, of course, extremely faint: at z=5, the peak magnitude should be K=27 with a duration of ~ 1 year. However, these supernovae should still be the brightest objects in the universe beyond some redshift, because the earliest galaxies should form before quasars and they should have very low mass, so their luminosities should be much lower than that of a supernova. We also show that, under the assumption of a standard initial mass function, a significant fraction of the stars in the Galactic halo should have formed in the early galaxies that reionized and enriched the IGM, and which later must have merged with our Galaxy. These stars should have a more extended radial distribution than the observed halo stars.

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arXiv1996

The Universe at $Z > 5$: when and how did the `Dark Age' End?

This paper considers how the first subgalactic structures produced the UV radiation that ionized the intergalactic medium before $z = 5$, and the `feedback' effects of the UV radiation on structure formation. The relevance of pregalactic activity to heavy element production and the origin of magnetic fields is briefly addressed.

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arXiv1996

21-cm Tomography of the Intergalactic Medium at High Redshift

We investigate the 21-cm signature that may arise from the intergalactic medium (IGM) prior to the epoch of full reionization (z>5). In scenarios in which the IGM is reionized by discrete sources of photoionizing radiation, the neutral gas which has not yet been engulfed by an H II region may easily be preheated to temperatures well above that of the cosmic background radiation (CBR), rendering the IGM invisible in absorption against the CBR. We identify three possible preheating mechanisms: (1) photoelectric heating by soft X-rays from QSOs, (2) photoelectric heating by soft X-rays from early galactic halos, and (3) resonant scattering of the continuum UV radiation from an early generation of stars. In the presence of a sufficiently strong ambient flux of Lyman-alpha photons, the hyperfine transition in the warmed H I will be excited. A beam differencing experiment would detect a patchwork of emission, both in frequency and in angle across the sky. This patchwork could serve as a valuable tool for understanding the epoch, nature, and sources of the reionization of the universe, and their implications for cosmology. We demonstrate that isolated QSOs will produce detectable signals at meter wavelengths within their ``spheres of influence'' over which they warm the IGM. As a result of the redshifted 21-cm radiation emitted by warm H I bubbles, the spectrum of the radio extragalactic background will display frequency structure with velocity widths up to 10,000 km/s. Broad beam observations would reveal corresponding angular fluctuations in the sky intensity with (delta T/T)=1E-3 on degree-scales. Radio measurements near 235 and 150 MHz, as will be possible in the near future using the Giant Metrewave Radio Telescope, may provide the first detection of a neutral IGM at 5<z<10.

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arXiv1995

Nucleosynthesis Constraints on Defect-Mediated Electroweak Baryogenesis

In the defect-mediated electroweak baryogenesis scenario, baryons are produced in well separated regions of space. It is shown that between the electroweak phase transition at a temperature of $T \sim 100 {\rm GeV}$ and the end of nucleosynthesis at $T \sim 1 {\rm KeV}$ the baryon inhomogeneities dissipate, and that no constraints on defect-mediated electroweak baryogenesis can be derived from considerations of inhomogeneous nucleosynthesis.

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arXiv1993

Gamma-Ray Bursts from Blast Waves Around Galactic Nuetron Stars

If gamma-ray bursts originate in our Galaxy, they probably involve violent disturbances in the magnetospheres of neutron stars. Any event of this kind is likely to trigger the sudden expulsion of magnetic flux and plasma at relativistic speed. We show that such ejecta would be braked by the interstellar medium (ISM), and that a gamma-ray flash with duration $\sim 0.1 - 100 $ s may result from this interaction. We suggest that the stronger,``smooth"class of bursts, recently identified from analyses of BATSE data, could result from the deceleration of such a blast wave by the ISM. The radiative efficiency,and hence the detectability, of such bursts would depend on the density of the circumstellar ISM. Therefore, even if the neutron star ``triggers" were uniformly distributed in space (at least within 1--2 kpc of the Sun), the bserved locations of bursts would correlate with regions of above-average ISM density.

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