Martin J. Rees

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.

Comptonization of diffuse ambient radiation by a relativistic jet: The source of gamma rays from blazars?

Recent Energy Gamma Ray Experiment Telescope (EGRET) observations of blazars have revealed strong, variable gamma-ray fluxes with no signatures of gamma-ray absorption by pair production. This radiation probably originates from the inner parts of relativistic jets which are aimed nearly toward us. On sub-parsec scales, the jet will be pervaded by radiation from the broad-line region, as well as by photons from the central continuum source (some of which will be scattered by thermal plasma). In a frame moving with the relativistic outflow, the energy of this ambient radiation would be enhanced. This radiation would be Comptonized by both cold and relativistic electrons in the jet, yielding (in the observers frame) a collimated beam of X-rays and gamma rays. On the assumption that this process dominates self-Comptonization of synchrotron radiation, we develop a self-consistent model for variable gamma-ray emission, involving a single population of relativistic electrons accelerated by a disturbance in the jet. The spectral break between the X-ray and gamma-ray band, observed in 3C 279 and deduced for other blazars, results from inefficient radiative cooling of lower energy electrons. The existence of such a break strongly favors a model involving Comptonization of an external radiation field over a synchrotron self-Compton model. We derive constraints on such model parameters as the location and speed of the source, its dimensions and internal physical parameters, the maximum photon energies produced in the source, and the density and distribution of ambient radiation. Finally, we discuss how observations might discriminate between our model and alternative ones invoking Comptonization of ambient radiation.

Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources

The history of the transition from a neutral intergalactic medium (IGM) to one that is almost fully ionized can reveal the character of cosmological ionizing sources. We study the evolution of the volume filling factors of H II and He III regions in a clumpy IGM and discuss the implications for rival reionization scenarios of the rapid decline observed at z3 in the space density of optical and radio-loud quasars and of the large population of star-forming galaxies recently discovered at the same epoch. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of photons emitted above 1 ryd in one recombination time equals the mean number of hydrogen atoms. If stellar sources are responsible for keeping the IGM ionized at z=5, the rate of star formation at this epoch must be comparable or greater than the one inferred from optical observations of galaxies at z≈3 and the mean metallicity per baryon in the universe 0.002 solar. An early generation of stars in dark matter halos with circular velocities, vcirc≈50 km s-1, possibly one of the main sources of UV photons at high z, could be detectable with the Next Generation Space Telescope. Models in which the quasar emissivity declines rapidly at z3 predict a late He II reionization epoch, a feature that could explain the recent detection of patchy He II Lyα at z=2.9 by Reimers et al. and the abrupt change observed by Songaila at about the same epoch of the Si IV /C IV ratio, but appear unable to provide the required number of hydrogen-ionizing photons at z≈5.

How small were the first cosmological objects

The minimum mass that a virialized gas cloud must have in order to be able to cool in a Hubble time is computed, using a detailed treatment of the chemistry of molecular hydrogen. With a simple model for halo profiles, we reduce the problem to that of numerically integrating a system of chemical equations. The results agree well with numerically expensive three-dimensional simulations, and our approach has the advantage of being able to explore large regions of parameter space rapidly. The minimum baryonic mass Mb is found to be strongly redshift dependent, dropping from 106 M☉ at z ~ 15 to 5 × 103 M☉ at z ~ 100 as molecular cooling becomes effective. For z 100, Mb rises again, as cosmic microwave background photons inhibit H2 formation through the H- channel. Finally, for z 200, the H -->+2 channel for H2 formation becomes effective, driving Mb down toward Mb ~ 103 M☉. With a standard cold dark matter power spectrum with σ8 = 0.7, this implies that a fraction 10-3 of all baryons may have formed luminous objects by z = 30, which could be sufficient to reheat the universe.

21 Centimeter 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 that 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. We find that bright quasars with only a small fraction of the observed comoving density at z ~ 4 will suffice to preheat the entire universe at z 6. We also show that, in a cold dark matter dominated cosmology, the thermal bremsstrahlung radiation associated with collapsing galactic mass halos (1010-1011 M?) may warm the IGM to ~100 K by z ~ 7. Alternatively, the equivalent of ~10% of the star formation rate density in the local universe, whether in isolated pregalactic stars, dwarf, or normal galaxies, would be capable of heating the entire IGM to a temperature above that of the CBR by Ly? scattering in a small fraction of the Hubble time at z ~ 6. In the presence of a sufficiently strong ambient flux of Ly? 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-1. Broad beam observations would reveal corresponding angular fluctuations in the sky intensity with ?T/T 10-3 on scales ? ~ 1?. This scale is set either by the thermalization distance from a QSO within which Ly? pumping determines the spin temperature of the IGM or by the quasar lifetime. 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. A next generation facility like the Square Kilometer Array Interferometer could effectively open much of the universe to a direct study of the reheating epoch and possibly probe the transition from a neutral universe to one that is fully ionized.

Reionization of the Inhomogeneous Universe

A model of the density distribution in the intergalactic medium (IGM), motivated by that found in numerical simulations, is used to demonstrate the effect of a clumpy IGM and discrete sources on the reionization of the universe. In an inhomogeneous universe reionization occurs outside-in, starting in voids and gradually penetrating into overdense regions. Reionization should not be sudden but gradual, with a continuous rise of the photon mean free path over a fair fraction of the Hubble time as the emissivity increases. We show that a hydrogen Gunn-Peterson trough should be present at z 6 unless the emissivity increases with redshift at z > 4. However, the epoch of overlap of cosmological H II regions could have occurred at a higher redshift if sources of low luminosity reionized the IGM; the Gunn-Peterson trough at z ~ 6 would then appear because even the most underdense voids have a large enough neutral fraction in ionization equilibrium to be optically thick to Lyα photons. Cosmological H II regions near the epoch of overlap can produce gaps of transmitted flux only if luminous quasars contributed to the reionization, producing large H II regions. Despite the clumpiness of the matter distribution, recombinations do not increase the required emissivity of ionizing photons by a large factor during the reionization of hydrogen because the high-density gas is not ionized until a late time. We show that the He II reionization was most likely delayed relative to the hydrogen reionization but was probably complete by z ~ 3 (the redshift where observations are available). The reported large optical depth fluctuations of He II are not necessarily due to an incomplete He II reionization but can arise from a combination of IGM density fluctuations and variations in the intensity of the He II ionizing background due to luminous QSOs.

The Radiative Feedback of the First Cosmological Objects

In hierarchical models of structure formation, an early cosmic UV background (UVB) is produced by the small K) halos that collapse before reionization. The UVB at energies below 13.6 eV sup- (T vir ( 104 presses the formation of stars or black holes inside small halos by photodissociating their only cooling agent, molecular We self-consistently compute the buildup of the early UVB in Press-Schechter H 2 . models, coupled with photodissociation both in the intergalactic medium (IGM) and inside virialized H 2 halos. We —nd that the intergalactic has a negligible eUect on the UVB, both because its initial H 2 optical depth is small and because it is photodissociated at an early stage. If the UV sources in ((0.1) the —rst collapsed halos are stars, then their UV —ux suppresses further star formation inside small halos. This results in a pause in the buildup of the UVB, and reionization is delayed until larger halos (T vir Z 104 K) collapse. If the small halos host miniquasars with hard spectra extending to D1 keV, then their X-rays balance the eUects of the UVB, the negative feedback does not occur, and reionization could be caused by the small halos. Subject headings: cosmology: theorydiUuse radiationearly universegalaxies: formation ¨ molecular processesradiative transfer

DISSIPATIVE PHOTOSPHERE MODELS OF GAMMA-RAY BURSTS AND X-RAY FLASHES

We consider dissipative effects occurring in the optically thick inner parts of the relativistic outflows producing gamma-ray bursts and X-ray flashes, emphasizing in particular the Comptonization of the thermal radiation flux that is advected from the base of the outflow. Such dissipative effects—e.g., from magnetic reconnection, neutron decay, or shocks would boost the energy density of the thermal radiation. The dissipation can lead to pair production, in which case the pairs create an effective photosphere farther out than the usual baryonic one. In a slow dissipation scenario, pair creation can be suppressed, and the effects are most important when dissipation occurs below the baryonic photosphere. In both cases an increased photospheric luminosity is obtained. We suggest that the spectral peak in gamma-ray bursts is essentially due to the Comptonized thermal component from the photosphere, where the comoving optical depth in the outflow falls to unity. Typical peak photon energies range between those of classical bursts and X-ray flashes. The relationship between the observed photon peak energy and the luminosity depends on the details of the dissipation, but under plausible assumptions can resemble the observed correlations.

Shocked by GRB-970228: The Afterglow of a cosmological fireball

The location accuracy of the BeppoSAX Wide Field Cameras and acute ground-based follow-up have led to the detection of a decaying afterglow in X-rays and optical light following the classical gamma-ray burst GRB 970228. The afterglow in X-rays and optical light fades as a power law at all wavelengths. This behaviour was predicted for a relativistic blast wave that radiates its energy when it decelerates by ploughing into the surrounding medium. Because the afterglow has continued with unchanged behaviour for more than a month, its total energy must be of order 1051 erg, placing it firmly at a redshift of order 1. Further tests of the model are discussed, some of which can be performed with available data, and implications for future observing strategies are pointed out. We discuss how the afterglow can provide a probe for the nature of the burst sources.

Steep Slopes and Preferred Breaks in Gamma-Ray Burst Spectra: The Role of Photospheres and Comptonization

The role of a photospheric component and of pair breakdown is examined in the internal shock model of gamma-ray bursts. We discuss some of the mechanisms by which they would produce anomalously steep low energy slopes, X-ray excesses and preferred energy breaks. Sub-relativistic comptonization should dominate in high comoving luminosity bursts with high baryon load, wh ile synchrotron radiation dominates the power law component in bursts which have lower comoving luminosity or have moderate to low baryon loads. A photosphere leading to steep low energy spectral slopes should be promin ent the lowest baryon load cases. Subject headings: Gamma-rays: Bursts Radiation MechanismsCosmology: Mis cellaneousThe role of a photospheric component and of pair breakdown is examined in the internal shock model of gamma-ray bursts. We discuss some of the mechanisms by which they would produce anomalously steep low-energy slopes, X-ray excesses and preferred energy breaks. Subrelativistic Comptonization should dominate in high comoving luminosity bursts with high baryon load, while synchrotron radiation dominates the power-law component in bursts which have lower comoving luminosity or have moderate to low baryon loads. A photosphere leading to steep low-energy spectral slopes should be prominent in the lowest baryon load cases.