arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 775 records · Page 43Linked to original sources

arXiv1998

Measuring the Virial Temperature of Galactic Halos Through Electron Scattering of Quasar Emission Lines

Semi-analytic models of galaxy formation postulate the existence of virialized gaseous halos around galaxies at high redshifts. A small fraction of the light emitted by any high-redshift quasar is therefore expected to scatter off the free electrons in the halo of its host galaxy. The broadening of the scattered emission lines of the quasar can be used to measure the temperature of these electrons. For gas in virial equilibrium, the velocity width of the scattered line profile is larger by a factor of (m_p/m_e)^{1/2}=43 than the velocity dispersion of the host galaxy and reaches >10,000 km/s for the massive galaxies and groups in which bright quasars reside. In these systems, the scattered width exceeds the intrinsic width of the quasar lines and hence can be used to measure the virial temperature of the quasar host. The high degree of polarization of the scattered radiation can help filter out the extended scattered light from the central emission by the quasar and its host galaxy. The signal-to-noise ratio of the spectral broadening can be enhanced by matching the full spectrum of the scattered radiation to a template of the unscattered quasar spectrum. Although the central fuzz around low-redshift quasars is dominated by starlight, the fuzz around high-redshift quasars might be dominated by scattering before galaxies have converted most of their gas reservoirs into stars.

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arXiv1998

A Sub-Relativistic Shock Model for the Radio Emission of SN1998bw

SN1998bw is the most luminous radio supernova ever observed. Previous discussions argued that its exceptional radio luminosity, 4e38 erg/s, must originate from a highly relativistic shock which is fully decoupled from the supernova ejecta. Here we present an alternative model in which the radio emission originates from a sub-relativistic shock with a velocity of 0.3c, generated in the surrounding gas by the expanding ejecta. In this model, thermal electrons heated by the shock to a relativistic temperature of 60 MeV emit synchrotron self-absorbed radiation in the post-shock magnetic field. This model provides an excellent fit to the observed spectra provided that the thermal electrons are in equipartition with the ions behind the shock. The required magnetic field is much weaker than its equipartition value and could have been carried out by the progenitor's wind prior to the supernova explosion. According to this model, the radio emission from SN1998bw is unrelated to the highly relativistic blast wave that produced the gamma-ray burst GRB980425.

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arXiv1998

Determining the Redshift of Reionization From the Spectra of High-Redshift Sources

The redshift at which the universe was reionized is currently unknown. We examine the optimal strategy for extracting this redshift, z_{reion}, from the spectra of early sources. For a source located at a redshift z_s beyond but close to reionization, 1<[(1+z_s)/(1+z_{reion})]<(32/27), the Gunn-Peterson trough splits into disjoint Lyman-alpha, beta, and possibly higher Lyman series troughs, with some transmitted flux in between these troughs. We show that although the transmitted flux is suppressed considerably by the dense Lyman-alpha forest after reionization, it is still detectable for sufficiently bright sources and can be used to infer the reionization redshift. The Next Generation Space Telescope will reach the spectroscopic sensitivity required for the detection of such sources.

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arXiv1998

The First Stars and Quasars

The first star clusters and quasars resulted directly from the growth of linear density fluctuations in the early Universe. Since they emerged from a well-defined set of initial conditions, the interplay between observational data from the Next Generation Space Telescope (NGST) and theoretical modeling will advance the current understanding of star and quasar formation. The first objects had a substantial impact on the thermal and chemical state of the rest of the Universe as they reionized the intergalactic medium and enriched it with metals.

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arXiv1998

Are HI Supershells the Remnants of Gamma-Ray Bursts?

Gamma-Ray Bursts (GRBs) are thought to originate at cosmological distances from the most powerful explosions in the Universe. If GRBs are not beamed then the distribution of their number as a function of Gamma-ray flux implies that they occur once per (0.3-40) million years per bright galaxy and that they deposit >10^{53} ergs into their surrounding interstellar medium. The blast wave generated by a GRB explosion would be washed out by interstellar turbulence only after tens of millions of years when it finally slows down to a velocity of 10 km/s. This rather long lifetime implies that there could be up to several tens of active GRB remnants in each galaxy at any given time. For many years, radio observations have revealed the enigmatic presence of expanding neutral-hydrogen (HI) supershells of kpc radius in the Milky Way and in other nearby galaxies. The properties of some supershells cannot be easily explained in terms of conventional sources such as stellar winds or supernova explosions. However, the inferred energy and frequency of the explosions required to produce most of the observed supershells agree with the above GRB parameters. More careful observations and analysis might reveal which fraction of these supershells are GRB remnants. We show that if this link is established, the data on HI supershells can be used to constrain the energy output, the rate per galaxy, the beaming factor, and the environment of GRB sources in the Universe.

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arXiv1998

Constraints from the Hubble Deep Field on High Redshift Quasar Models

High resolution, deep imaging surveys are instrumental in setting constraints on semi-analytical structure formation models in Cold Dark Matter (CDM) cosmologies. We show here that the lack of unresolved B-band ``dropouts'' with V > 25 mag in the Hubble Deep Field (HDF) appears to be inconsistent with the expected number of quasars if massive black holes form with a constant universal efficiency in all CDM halos. To reconcile the models with the data, a mechanism is needed that suppresses the formation of quasars in halos with circular velocities v(circ) < 50-75 km/s. This feedback naturally arises due to the photoionization heating of the gas by the UV background. We consider several alternative effects that would help reduce the quasar number counts, and find that these can not alone account for the observed lack of detections. If reddening by dust can be neglected at early epochs, consistency with the optical data also requires that the luminous extent of dwarf galaxies at high redshifts be larger than a few percent of the virial radii of their dark matter halos, in order not to overpredict the number of point-like B-band dropouts. Future deep observations in the J and H bands with NICMOS might reveal several z > 5 objects per field or provide even stronger constraints on the models than existing B, V, and I data.

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arXiv1998

X-Ray Absorption by the Hot Intergalactic Medium

The current census of observed baryons in the local Universe is still missing a significant fraction of them according to standard Big-Bang nucleosynthesis. Numerical simulations predict that most of the missing baryons are in a hot intergalactic medium, which is difficult to observe through its X-ray emission or Sunyaev-Zel'dovich effect. We show that the next generation of X-ray satellites will be able to detect this gas through the X-ray absorption lines imprinted by its highly-ionized metals on the spectrum of a background quasar. For the metallicity typically found in intracluster gas, up to 70% of the baryons produce O VIII absorption lines with an equivalent width >0.1eV. The spectrum of any high redshift quasar is expected to show several such lines per unit redshift due to intervening gaseous halos of galaxy groups. These lines will be detectable at a signal-to-noise ratio >5 after a day of integration with the future Constellation-X telescope for any of the several hundred brightest quasars across the sky.

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arXiv1998

The Expected Rate of Gamma-Ray Burst Afterglows In Supernova Searches

We predict the rate at which Gamma-Ray Burst (GRB) afterglows should be detected in supernova searches as a function of limiting flux. Although GRB afterglows are rarer than supernovae, they are detectable at greater distances because of their higher intrinsic luminosity. Assuming that GRBs trace the cosmic star formation history and that every GRB gives rise to a bright afterglow, we find that the average detection rate of supernovae and afterglows should be comparable at limiting magnitudes brighter than K=18. The actual rate of afterglows is expected to be somewhat lower since only a fraction of all gamma-ray selected GRBs were observed to have associated afterglows. However, the rate could also be higher if the initial gamma-ray emission from GRB sources is more beamed than their late afterglow emission. Hence, current and future supernova searches can place strong constraints on the afterglow appearance fraction and the initial beaming angle of GRB sources.

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arXiv1998

Gamma-Ray Bursts From Neutron Star Phase Transitions

The phase-transition induced collapse of a neutron star to a more compact configuration (typically a ``strange'' star) and the subsequent core bounce is often invoked as a model for gamma-ray bursts. We present the results of numerical simulations of this kind of event using realistic neutrino physics and a high density equation of state. The nature of the collapse itself is represented by the arbitrary motion of a piston deep within the star, but if any shock is to develop, the transition, or at least its final stages, must occur in less than a sonic time. Fine surface zoning is employed to adequately represent the acceleration of the shock to relativistic speeds and to determine the amount and energy of the ejecta. We find that these explosions are far too baryon-rich (ejected Mass > 0.01 solar masses) and have much too low an energy to explain gamma-ray bursts. The total energy of the ejecta having relativistic lorentz factors > 40 is less than 10^46 erg even in our most optimistic models (deep bounce, no neutrino losses or photodisintegration). However, the total energy of all the ejecta, mostly mildly relativistic, is roughly 10^51 erg and, if they occur, these events might be observed. They would also contribute to Galactic nucleosynthesis, especially the r-process, even though the most energetic layers are composed of helium and nucleons, not heavy elements.

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arXiv1997

Gamma Ray Bursts from Baryon Decay in Neutron Stars

The standard unbroken electroweak theory is known to erase baryon number. The baryon number symmetry can be restored in the core of a neutron star as its density diverges via gravitational instability due to a binary merger event. We argue that for certain double Higgs models with discrete symmetries, this process may result in an expanding self-sustained burning front which would convert the entire neutron matter into radiation. This process would release ~10^{54} ergs in electromagnetic radiation over ~10^{-4} sec, with negligible baryonic contamination. The resulting fireball would have all the properties necessary to produce a gamma-ray burst as a result of its interaction with ambient interstellar gas. The subsequent Higgs decay would produce a millisecond burst of ~10^{52} ergs in ~100 GeV neutrinos which might be observable. The above mechanism may have also caused electroweak baryogenesis in the early universe, giving rise to the observed matter-antimatter asymmetry today.

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arXiv1997

Effects of Disks on Gravitational Lensing by Spiral Galaxies

Gravitational lensing of a quasar by a spiral galaxy should often be accompanied by damped Lyman-alpha absorption and dust extinction due to the intervening gaseous disk. In nearly edge-on configurations, the surface mass density of the gas and stars in the disk could by itself split the quasar image and contribute significantly to the overall lensing cross section. We calculate the lensing probability of a disk+halo mass model for spiral galaxies, including cosmic evolution of the lens parameters. A considerable fraction of the lens systems contains two images with sub-arcsecond separation, straddling a nearly edge-on disk. Because of that, extinction by dust together with observational selection effects (involving a minimum separation and a maximum flux ratio for the lensed images), suppress the detection efficiency of spiral lenses in optical wavebands by at least an order of magnitude. The missing lenses could be recovered in radio surveys. In modifying the statistics of damped Lyman-alpha absorbers, the effect of extinction dominates over the magnification bias due to lensing.

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arXiv1997

Identifying the Environment and Redshift of GRB Afterglows from the Time-Dependence of Their Absorption Spectra

The discovery of Gamma-Ray Burst (GRB) afterglows revealed a new class of variable sources at optical and radio wavelengths. At present, the environment and precise redshift of the detected afterglows are still unknown. We show that if a GRB source resides in a compact (<100pc) gas-rich environment, the afterglow spectrum will show time-dependent absorption features due to the gradual ionization of the surrounding medium by the afterglow radiation. Detection of this time-dependence can be used to constrain the size and density of the surrounding gaseous system. For example, the MgII absorption line detected in GRB970508 should have weakened considerably during the first month if the absorption occurred in a gas cloud of size <100pc around the source. The time-dependent HI or metal absorption features provide a precise determination of the GRB redshift.

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arXiv1997

Spectral Implications of Variability in GRB Fireballs

Cosmological $γ$-ray bursts originate from relativistic winds. Temporal fluctuations in the wind velocity can give rise to internal shocks which dissipate a significant fraction of the wind kinetic energy. Part of the energy dissipated is transferred to the electrons through Fermi acceleration. If the post shock fluid is strongly magnetized, the relativistic electrons cool initially through synchrotron emission, and later through Compton scattering. The upsacttered radiation triggers a cascade of $e^{+}e^{-}$-pairs. We compute the final spectrum for a wide range of parameter values for the emission region. We show that the spectral diversity observed by BATSE can be naturally explained by emission from internal shocks, which are associated with the observed source variability.

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arXiv1997

Emission Spectra from Internal Shocks in Gamma-Ray-Burst Sources

Unsteady activity of gamma-ray burst sources leads to internal shocks in their emergent relativistic wind. We study the emission spectra from such shocks, assuming that they produce a power-law distribution of relativistic electrons and posses strong magnetic fields. The synchrotron radiation emitted by the accelerated electrons is Compton up-scattered multiple times by the same electrons. A substantial component of the scattered photons acquires high energies and produces e+e- pairs. The pairs transfer back their kinetic energy to the radiation through Compton scattering. The generic spectral signature from pair creation and multiple Compton scattering is highly sensitive to the radius at which the shock dissipation takes place and to the Lorentz factor of the wind. The entire emission spectrum extends over a wide range of photon energies, from the optical regime up to TeV energies. For reasonable values of the wind parameters, the calculated spectrum is found to be in good agreement with the burst spectra observed by BATSE.

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arXiv1997

Observational Signatures of the First Quasars

We study the observational signatures of a potential population of low-luminosity quasars at high redshifts in a LambdaCDM cosmology. We derive the evolution of the quasar luminosity function at fainter luminosities and higher redshifts than currently detected, based on three assumptions: (1) the formation of dark-matter halos follows the Press-Schechter theory, (2) the ratio of central black hole mass to halo mass is the same for all halos, and (3) the light-curve of quasars, in Eddington units, is universal. We show that a universal light-curve provides an excellent fit to the observed quasar luminosity function at redshifts 2.6 < z < 4.5. By extrapolating the evolution of this luminosity function to higher redshifts (4.5 < z < 20), we find that the associated early population of low-luminosity quasars reionizes the universe around redshift z=12. The reprocessing of the UV light of these quasars by dust from early type II supernovae, distorts the microwave background spectrum by a Compton y-parameter, y = 1e-5, comparable to the upper limit set by COBE. The Next Generation Space Telescope could detect tens of quasars from redshifts z > 10 per square arcminute, with its proposed 1nJy sensitivity at 1-3.5 microns. Absorption spectra of several such quasars would reveal the reionization history of the universe.

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arXiv1997

Signatures of Stellar Reionization of the Universe

The high ionization level and universal metallicity (1% solar) of the intergalactic gas at redshifts z<5 implies that nonlinear structure had started to form in the universe at earlier times than we currently probe. In Cold Dark Matter (CDM) cosmologies, the first generation of baryonic objects emerges at redshifts z=10-50. Here we examine the observable consequences of the possibility that an early generation of stars reionized the universe and resulted in the observed metallicity of the Lyman-alpha forest. Forthcoming microwave anisotropy experiments will be sensitive to the damping of anisotropies caused by scattering off free electrons from the reionization epoch. For a large range of CDM models with a Scalo stellar mass function, we find that reionization occurs at a redshift z>10 and damps the amplitude of anisotropies on angular scales <10 degrees by a detectable amount, of order 10-25%. However, reionization is substantially delayed if the initial stellar mass function transformed most of the baryons into low mass stars. In this case, the mass fraction of pre-galactic stars could be constrained from the statistics of microlensing events in galactic halos or along lines of sight to quasars. Deep infrared imaging with future space telescopes (such as SIRTF or the Next Generation Space Telescope) will be able to detect bright star clusters at z>5. The cumulative Bremsstrahlung emission from these star clusters yields a measurable distortion to the spectrum of the microwave background.

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arXiv1997

Microlensing of $γ$-Ray Burst Afterglows

The afterglow of a cosmological Gamma-Ray Burst (GRB) should appear on the sky as a narrow emission ring of radius 3x10^{16}cm(t/day)^{5/8} which expands faster than light. After a day, the ring radius is comparable to the Einstein radius of a solar mass lens at a cosmological distance. Thus, microlensing by an intervening star can modify significantly the lightcurve and polarization signal from a GRB afterglow. We show that the achromatic amplification signal of the afterglow flux can be used to determine the impact parameter and expansion rate of the source in units of the Einstein radius of the lens, and probe the superluminal nature of the expansion. If the synchrotron emission from the afterglow photosphere originates from a set of coherent magnetic field patches, microlensing would induce polarization variability due to the transient magnification of the patches behind the lens. The microlensing interpretation of the flux and polarization data can be confirmed by a parallax experiment which would probe the amplification peak at different times. The fraction of microlensed afterglows can be used to calibrate the density parameter of stellar-mass objects in the Universe.

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arXiv1997

Microlensing of an Elliptical Source by a Point Mass

We present an efficient method for computing lightcurves of an elliptical source which is microlensed by a point mass. The amplification of an extended source involves a two-dimensional integral over its surface brightness distribution. We show that for a general surface brightness profile with an elliptical symmetry, this integral can be reduced to one dimension. We derive analytical results for the entire lightcurve in the limit of low (e.g. planetary) mass lenses, and for the wings of all microlensing lightcurves in general. In both cases, the lightcurve carries information about deviations of the source from elliptical symmetry, e.g. due to spots. The method is used to find the amplification of a circular red giant photosphere and an inclined accretion disk. We demonstrate that microlensing of an emission line from a disk can be used to infer the disk velocity structure and surface brightness profile.

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