arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 739 records · Page 41Linked to original sources

arXiv2001

Microlensing and the Surface Brightness Profile of the Afterglow Image of GRB 000301C

The optical afterglow of Gamma-Ray Burst (GRB) 000301C exhibited a significant, short-timescale deviation from the power-law flux decline expected in the standard synchrotron shock model. Garnavich, Loeb & Stanek found that this deviation was well-fit by an ad hoc model in which a thin ring of emission is microlensed by an intervening star. We revisit the microlensing interpretation of this variability, first by testing whether microlensing of afterglow images with realistic surface brightness profiles (SBPs) can fit the data, and second by directly inverting the observed light curve to obtain a non-parametric measurement of the SBP. We find that microlensing of realistic SBPs can reproduce the observed deviation, provided that the optical emission arises from frequencies above the cooling break. Conversely, if the variability is indeed caused by microlensing, the SBP must be significantly limb-brightened. Specifically, greater than 60% of the flux must originate from the outer 25% of the area of the afterglow image. The latter requirement is satisfied by the best fit theoretical SBP. The underlying optical/infrared afterglow lightcurve is consistent with a model in which a jet is propagating into a uniform medium with the cooling break frequency below the optical band.

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arXiv2001

Cosmological Studies with Gamma-Ray Bursts

Gamma-Ray Burst (GRB) explosions from the first generation of stars offer an exciting opportunity to probe the epoch of reionization. Clues about how and when the intergalactic medium was ionized can be read off their UV emission spectrum. Roughly a percent of all GRBs should be strongly gravitationally lensed by intervening stars. A microlensed lightcurve can be inverted to reconstruct the surface brightness profile of the GRB image on the sky, with micro-arcsecond resolution.

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arXiv2001

Constraining Cosmological Parameters Based on Relative Galaxy Ages

We propose to use relative galaxy ages as a means of constraining cosmological parameters. By measuring the age difference between two ensembles of old galaxies at somewhat different redshifts, one could determine the derivative of redshift with respect to cosmic time, dz/dt. At high redshifts, z=1-2, this measurement would constrain the equation-of-state of the dark energy, while at low redshifts, z< 0.2, it would determine the Hubble constant, H_0. The selected galaxies need to be passively-evolving on a time much longer than their age difference.

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arXiv2001

Chromatic Signatures in the Microlensing of GRB Afterglows

We calculate the radial surface brightness profile of the image of a Gamma-Ray-Burst (GRB) afterglow. The afterglow spectrum consists of several power-law segments separated by breaks. The image profile changes considerably across each of the spectral breaks. It also depends on the density profile of the ambient medium into which the GRB fireball propagates. Gravitational microlensing by an intervening star can resolve the afterglow image. We calculate the predicted magnification history of GRB afterglows as a function of observed frequency and ambient medium properties. We find that intensive monitoring of a microlensed afterglow lightcurve can be used to reconstruct the parameters of the fireball and its environment, and provide constraints on particle acceleration and magnetic field amplification in relativistic blast waves.

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arXiv2001

The Imprint of Lithium Recombination on the Microwave Background Anisotropies

Following Loeb (2001), we explore the imprint of the resonant 6708A line opacity of neutral lithium on the temperature and polarization anisotropies of the cosmic microwave background (CMB) at observed wavelengths of 250-350 micron (0.9-1.2 THz). We show that if lithium recombines in the redshift range of z=400-500 as expected, then the standard CMB anisotropies would be significantly modified in this wavelength band. The modified polarization signal could be comparable to the expected polarization anisotropies of the far-infrared background on sub-degree angular scales (l>100). Detection of the predicted signal can be used to infer the primordial abundance of lithium, and to probe structure in the Universe at z~500.

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arXiv2001

Probing the Universe After Cosmological Recombination Through the Effect of Neutral Lithium on the Microwave Background Anisotropies

I show that neutral lithium couples strongly to the cosmic microwave background (CMB) through its 6708A resonant transition after it recombines at z~500. At observed wavelengths of <335 micron, the CMB anisotropies are significantly altered since the optical depth for resonant scattering by neutral lithium is substantial, of order 0.5. The scattering would suppress the original anisotropies, but will generate strong new anisotropies in the CMB temperature and polarization on sub-degree scales (l>100). Observations at different wavelengths in this spectral regime can probe different thin slices of the early universe. The anisotropy noise contributed by continuum radiation from foreground far--infrared sources could be taken out by subtracting maps at slightly different wavelengths. Detection of the above effects can be used to study structure at z<500 and to constrain the primordial abundance and recombination history of lithium.

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arXiv2001

In the Beginning: The First Sources of Light and the Reionization of the Universe

The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at redshift 30 and reionized most of the hydrogen in the universe by redshift 7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.

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arXiv2001

Resolving the Image of Gamma-Ray Burst Afterglows with Gravitational Microlensing

Microlensing of a gamma-ray burst afterglow by an intervening star can be used to infer the radial structure of the afterglow image. Near the peak of the microlensing event, the outer edge of the image is more highly magnified than its central region, whereas the situation is reversed at later times due to the rapid radial expansion of the image on the sky. Thus, the microlensed afterglow light curve can be inverted to recover the self-similar radial intensity profile of the afterglow image. We calculate the expected errors in the recovered intensity profile as a function of the number of resolution elements, under the assumption that the afterglow and microlensing event parameters are known. For a point-mass lens and uniform source, we derive a simple scaling relation between these parameters and the resultant errors. The afterglow need not be monitored for its entire duration; rather, observations from the peak magnification time t_p of the microlensing event until 7 t_p are sufficient to resolve the majority of the image. Thus, microlensing events can be alerted by relatively infrequent observations of afterglows and then monitored intensively, without significant loss of information about the afterglow intensity profile. The relative intensity profile of about 1% of all afterglows can be measured with 10 resolution elements to an accuracy of 1% in the optical and 10% in the infrared, using 4m-class telescopes. Weak microlensing events with large impact parameters are more common; we estimate that for about 10% of afterglows the image profile may be inverted to a fractional accuracy 20% through frequent optical observations. We also calculate the effects of external shear due to the host galaxy or a binary companion, and contamination by background light from the host galaxy.

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arXiv2001

Generic Spectrum and Ionization Efficiency of a Heavy Initial Mass Function for the First Stars

We calculate the generic spectral signature of an early population of massive stars at high redshifts. For metal-free stars in the mass range of 100-1000 solar masses, we find that the combined spectral luminosity per unit stellar mass is almost independent of the mass distribution of these stars. To zeroth order, the generic spectrum resembles a black-body with an effective temperature of ~10^5 K, making these stars highly efficient at ionizing hydrogen and helium. The production rate of ionizing radiation per stellar mass by stars more massive than 100 solar masses is larger by about an order of magnitude for hydrogen and He I, and by about two orders of magnitude for He II, than the emission from a standard initial mass function. This would result in unusually strong hydrogen and helium recombination lines from the surrounding interstellar medium. It could also alleviate the current difficulty of ionizing the intergalactic medium at z>6 with the cosmic star formation rate inferred at somewhat lower redshifts.

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arXiv2001

Enhanced Microlensing by Stars Around the Black Hole in the Galactic Center

The effect of stars on the lensing properties of the supermassive black hole in the Galactic Center is similar to the effect of planets on microlensing by a star. We show that the dense stellar cluster around SgrA* increases by factors of a few the probability of high-magnification lensing events of a distant background source by the black hole. Conversely, the gravitational shear of the black hole changes and enhances the microlensing properties of the individual stars. The effect is largest when the source image lies near the Einstein radius of the black hole 1.75"+/-0.20" for a source at infinity). We estimate that the probability of observing at least one distant background star which is magnified by a factor >5 in any infrared snapshot of the inner ~ 2" of the Galactic Center is 1% with a K-band detection threshold of 20 mag. The largest source of uncertainty in this estimate is the luminosity function of the background stars. The gravitational shear of the black hole lengthens the duration of high-magnification events near the Einstein radius up to a few months, and introduces a large variety of lightcurve shapes that are different from those of isolated microlenses. Identification of such events by image subtraction can be used to probe the mass function, density and velocity distributions of faint stars near the black hole, which are not detectable otherwise.

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arXiv2001

TeV Neutrinos and GeV Photons from Shock Breakout in Supernovae

We show that as a Type II supernova shock breaks out of its progenitor star, it becomes collisionless and may accelerate protons to energies >10 TeV. Inelastic nuclear collisions of these protons produce a ~1 hr long flash of TeV neutrinos and 10 GeV photons, about 10 hr after the thermal (10 MeV) neutrino burst from the cooling neutron star. A Galactic supernova in a red supergiant star would produce a photon and neutrino flux of ~10^{-4} erg/cm^2 s. A km^2 neutrino detector will detect ~100 muons, thus allowing to constrain both supernova models and neutrino properties.

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arXiv2001

Intergalactic Magnetic Fields from Quasar Outflows

Outflows from quasars inevitably pollute the intergalactic medium (IGM) with magnetic fields. The short-lived activity of a quasar leaves behind an expanding magnetized bubble in the IGM. We model the expansion of the remnant quasar bubbles and calculate their distribution as a function of size and magnetic field strength at different redshifts. We generically find that by a redshift z=3, about 5-20% of the IGM volume is filled by magnetic fields with an energy density >10% of the mean thermal energy density of a photo-ionized IGM (at T=10^4 K). As massive galaxies and X-ray clusters condense out of the magnetized IGM, the adiabatic compression of the magnetic field could result in the field strength observed in these systems without a need for further dynamo amplification. The intergalactic magnetic field could also provide a nonthermal contribution to the pressure of the photo-ionized gas that may account for the claimed discrepancy between the simulated and observed Doppler width distributions of the Ly-alpha forest.

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arXiv2000

Steady-state structure of relativistic collisionless shocks

We explore analytically the structure of relativistic shock and solitary wave solutions in collisionless plasmas. In the wave frame of reference, a cold plasma is flowing from one end and impacting on a low velocity plasma. First we show that under astrophysical conditions, a cold electron-positron plasma is unstable with respect to a two-stream instability in the interface between these regions. The instability heats the inflowing cold plasma rapidly, on a timescale comparable to the inverse of its plasma frequency. We then derive time-independent equations to describe the resulting hot state of the pair plasma, and describe the conditions under which the spatially uniform solution is the unique stable solution for the post shock conditions. We also examine plasmas composed of cold protons and hot electrons, and show that the spatially uniform solution is the unique stable solution there as well. We state the shock jump conditions which connect a cold, electron-proton plasma to a hot electron-proton plasma. The generic feature evident in all of these models is that the plasma's initial, directed kinetic energy gets almost completely converted into heat. The magnetic field plays the role of catalyst which can induce the plasma instability, but our solutions indicate that the macroscopic field only gets amplified by a factor of approximately three in the frame of the shock.

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arXiv2000

What is the Highest Plausible Redshift of Luminous Quasars?

The recent discoveries of luminous quasars at high redshifts imply that black holes more massive than a few billion solar masses have been assembled already when the universe was less than a billion years old. We show that the existence of these black holes is not surprising in popular hierarchical models of structure formation. For example, the black hole needed to power the quasar SDSS 1044-0125 at redshift z=5.8 can arise naturally from the growth of stellar mass seeds forming at z>10, when typical values are assumed for the radiative accretion efficiency and the bolometric accretion luminosity in Eddington units. Nevertheless, SDSS 1044-0125 yields a non-trivial constraint on a combination of these parameters. Extrapolating our model to future surveys, we derive the highest plausible redshift for quasars which are not lensed or beamed, as a function of their apparent magnitude. We find that at a limiting magnitude of K(AB)=20, quasar surveys can yield strong constraints on the growth of supermassive black holes out to z=10.

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arXiv2000

Gravitational Microlensing of Gamma-Ray Burst Afterglows by Single and Binary Stars

We calculate the magnification light curves due to stellar microlensing of gamma-ray burst (GRB) afterglows. A GRB source appears on the sky as a thin ring which expands faster than the speed of light and is maximally magnified as it crosses the lens caustics. While a single star lens produces a single peak in the magnification light curve, binary star lenses may produce multiple peaks. The shape of the magnification light curve provides invaluable information on the surface brightness distribution of the afterglow photosphere on sub micro-arcsecond scales. We find that all afterglows are likely to show variability at the level of a few percent about a year following the explosion, due to stars which are separated by tens of Einstein radii from their line-of-sight.

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arXiv2000

The Reionization of the Universe by the First Stars and Quasars

The first light from stars and quasars ended the ``dark ages'' of the universe and led to the reionization of hydrogen by redshift 7. Current observations are at the threshold of probing this epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.

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arXiv2000

Does the Mass Accretion Rate Depend On the Radius of the Accreting Star?

In some circumstances, the mass accretion rate M^dot onto a compact star may depend not only on external boundary conditions, but also on the radius R of the star. Writing the dependence as a power-law in which M^dot is proportional to R^p, we estimate the power-law index, p, for transient binary systems in a quiescent state. We use the observed luminosities $L$ of accreting neutron stars (R=10^6 cm) in soft X-ray transients and white dwarfs (R=10^9 cm) in similar cataclysmic variables, and estimate M^dot in each system through the relation L=(GM/R)*M^dot, where M is the mass of the star. From the available data we infer that p~0.9+-0.5. This radial dependence is consistent with radiatively inefficient accretion flows that either are convective or lose mass via a wind.

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arXiv2000

Emission from Bow Shocks of Beamed Gamma-Ray Bursts

Beamed gamma-ray burst (GRB) sources produce a bow shock in their gaseous environment. The emitted flux from this bow shock may dominate over the direct emission from the jet for lines of sight which are outside the angular radius of the jet emission, theta. The event rate for these lines of sight is increased by a factor of 260*(theta/5_degrees)^{-2}. For typical GRB parameters, we find that the bow shock emission from a jet with half-angle of about 5 degrees is visible out to tens of Mpc in the radio and hundreds of Mpc in the X-rays. If GRBs are linked to supernovae, studies of peculiar supernovae in the local universe should reveal this non-thermal bow shock emission for weeks to months following the explosion.

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