arXiv subjects

Re'em Sari

Publications and source records attributed to Re'em Sari.

At least 145 records · Page 8Linked to original sources

arXiv2001

The prompt energy release of gamma-ray bursts using a cosmological k-correction

The fluences of gamma-ray bursts (GRBs) are measured with a variety of instruments in different detector energy ranges. A detailed comparison of the implied energy releases of the GRB sample requires, then, an accurate accounting of this diversity in fluence measurements which properly corrects for the redshifting of GRB spectra. Here, we develop a methodology to ``k-correct'' the implied prompt energy release of a GRB to a fixed co-moving bandpass. This allows us to homogenize the prompt energy release of 17 cosmological GRBs (using published redshifts, fluences, and spectra) to two common co-moving bandpasses: 20-2000 keV and 0.1 keV-10 MeV (``bolometric''). While the overall distribution of GRB energy releases does not change significantly by using a k-correction, we show that uncorrected energy estimates systematically undercounts the bolometric energy by ~5% to 600%, depending on the particular GRB. We find that the median bolometric isotropic-equivalent prompt energy release is 2.2 x 10^{53} erg with an r.m.s. scatter of 0.80 dex. The typical estimated uncertainty on a given k-corrected energy measurement is ~20%.

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arXiv2000

Ultra Efficient Internal Shocks

Gamma-ray bursts are believed to originate from internal shocks which arise in an irregular relativistic wind. The process has been thought to be inefficient, converting only a few percent of the kinetic energy into gamma-rays. We define ultra efficient internal shocks as those in which the fraction of emitted energy is larger than the fraction of energy given to the radiating electrons at each collision. We show that such a scenario is possible and even plausible. In our model, colliding shells which do not emit all their internal energy are reflected from each other and it causes subsequent collisions, allowing more energy to be emitted. As an example, we obtain about 60% overall efficiency even if the fraction of energy that goes to electrons is $ε_e=0.1$ provided that the shells' Lorentz factor varies between 10 and $10^4$. The numerical temporal profile reflects well the activity of the source which ejects the shells, though numerous collisions take place in this model.

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arXiv2000

On The Synchrotron Self-Compton Emission from Relativistic Shocks and Its Implications for Gamma-Ray Burst Afterglows

We consider the effects of inverse Compton scattering of synchrotron photons from relativistic electrons in GRB afterglows. We compute the spectrum of the inverse Compton emission and find that it can dominate the total cooling rate of the afterglow for several months or even years after the initial explosion. We demonstrate that the presence of strong inverse Compton cooling can be deduced from the effect it has on the time-evolution of the cooling break in the synchrotron spectral component, and therefore on the optical and X-ray afterglow lightcurves. We then show how the physical interpretation of the observed characteristics of the synchrotron spectrum must be modified to take into consideration this extra source of cooling, and give a revised prescription for computing physical parameters characterizing the expanding shock wave from the observed quantities. We find that for a given set of observables (synchrotron break frequencies and fluxes) there is either no consistent physical interpretation or two of them. Finally we discuss the prospects of directly detecting the inverse Compton emission with Chandra. We argue that such a detection is possible for GRBs exploding in a reasonably dense (n>1 cm^-3) medium.

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arXiv2000

The Synchrotron Spectrum of Fast Cooling Electrons Revisited

We discuss the spectrum arising from synchrotron emission by fast cooling (FC) electrons, when fresh electrons are continually accelerated by a strong blast wave, into a power law distribution of energies. The FC spectrum was so far described by four power law segments divided by three break frequencies $ν_{sa} < ν_{c} < ν_{m}$. This is valid for a homogeneous electron distribution. However, hot electrons are located right after the shock, while most electrons are farther down stream and have cooled. This spatial distribution changes the optically thick part of the spectrum, introducing a new break frequency, $ν_{ac} < ν_{sa}$, and a new spectral slope, $F_{ν}\propto ν^{11/8}$ for $ν_{ac} < ν< ν_{sa}$. The familiar $F_{ν}\propto ν^{2}$ holds only for $ν< ν_{ac}$. This ordering of the break frequencies is relevant for typical gamma-ray burst (GRB) afterglows in an ISM environment. Other possibilities arise for internal shocks or afterglows in dense circumstellar winds. We discuss possible implications of this spectrum for GRBs and their afterglows, in the context of the internal-external shock model. Observations of $F_ν \propto ν^{11/8}$ would enable us to probe scales much smaller than the typical size of the system, and constrain the amount of turbulent mixing behind the shock.

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arXiv2000

Impulsive and Varying Injection in GRB Afterglows

The standard model of Gamma-Ray Bursts afterglows is based on synchrotron radiation from a blast wave produced when the relativistic ejecta encounters the surrounding medium. We reanalyze the refreshed shock scenario, in which slower material catches up with the decelerating ejecta and reenergizes it. This energization can be done either continuously or in discrete episodes. We show that such scenario has two important implications. First there is an additional component coming from the reverse shock that goes into the energizing ejecta. This persists for as long as the re-energization itself, which could extend for up to days or longer. We find that during this time the overall spectral peak is found at the characteristic frequency of the reverse shock. Second, if the injection is continuous, the dynamics will be different from that in constant energy evolution, and will cause a slower decline of the observed fluxes. A simple test of the continuously refreshed scenario is that it predicts a spectral maximum in the far IR or mm range after a few days.

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arXiv2000

Beaming and Jets in GRBs

The origin of GRBs have been a mystery for almost 30 years. The afterglow observed in the last few years enabled redshift determination for a handful of bursts, and the cosmological origin is now firmly established. Though the distance scale is settled, there still remains orders of magnitude uncertainty in their rate and in the total energy that is released in the explosion due to the possibility that the emission is not spherical but jet-like. Contrary to the GRB itself, the afterglow can be measured up to months and even years after the burst, and it can provide crucial information on the geometry of the ejecta. We review the theory of afterglow from jets and discuss the evidence that at least some of the bursts are not spherical. We discuss the prospects of polarization measurements, and show that this is a powerful tool in constraining the geometry of the explosion.

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arXiv1999

Optical Flashes and Radio Flares in GRBs afterglow: Numerical Study

The strong optical flash observed by ROTSE, as well as the radio flare associated with GRB 990123 can be attributed to the emission of the fireball ejecta, initially heated by the reverse shock. We numerically study the evolution of an adiabatic relativistic fireball interacting with an ambient uniform medium, both in the initial energy transfer stage and in its late evolution. It is shown that the Blandford-McKee solution adequately describes the evolution of the shocked shell quite early on and for as long as the fireball material has relativistic temperatures. In the case where the reverse shock is only mildly relativistic the shocked shell becomes cold almost immediately and the evolution deviates from the Blandford-McKee solution. We derive analytical expressions for the ejecta evolution in its cold regime. This solution gives a good approximation to the numerical results. We estimate the radiation from the fireball ejecta using the numerical hydrodynamic evolution in both cases: cold and hot shells. Surprisingly, we find that both evolutions give rather similar light curves, decaying approximately as $T^{-2}$ in the optical and peaking at about one day in the radio, even though the hydrodynamics is different.

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arXiv1999

Linear Polarization and Proper Motion in The Afterglow of Beamed GRBs

We investigate the polarization and proper motion expected in a beamed Gamma-Ray Burst's ejecta. We find that even if the magnetic field has well defined orientation relative to the direction of motion of the shock, the polarization is not likely to exceed 20%. Taking into account the dynamics of beamed ejecta we find that the polarization rises and decays with peak around the jet break time (when the Lorentz factor of the flow is comparable to the initial opening angle of the jet). Interestingly, we find that when the offset of the observer from the center of the beam is large enough, the polarization as function of time has three peaks, and the polarization direction of the middle peak is rotated by 90 degrees relative to the two other peaks. We also show that some proper motion is expected, peaking around the jet break time. Detection of both proper motion and the direction of polarization can determine which component of the magnetic field is the dominant.

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arXiv1999

Jets in GRBs

In several GRBs afterglows, rapid temporal decay is observed which is inconsistent with spherical (isotropic) blast-wave models. In particular, GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, with t^{-2.05\pm 0.04} in optical as well as in X-rays. We show that such temporal decay is more consistent with the evolution of a jet after it slows down and spreads laterally, for which t^{-p} decay is expected (where p is the index of the electron energy distribution). Such a beaming model would relax the energy requirements on some of the more extreme GRBs by a factor of several hundreds. It is likely that a large fraction of the weak (or no) afterglow observations are also due to the common occurrence of beaming in GRBs, and that their jets have already transitioned to the spreading phase before the first afterglow observations were made. With this interpretation, a universal value of p~2.5 is consistent with all data.

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arXiv1999

GRB990123, The Optical Flash and The Fireball Model

We compare the ongoing observations of the remarkable burst GRB990123, the mother of all bursts, with the predictions of the afterglow theory. We show that the observations agree with the recent prediction that a reverse shock propagating into the ejecta would produce a very strong prompt optical flash. This reverse shock has also produced the 8.46GHz radio signal, observed after one day. The forward shock, which propagates into the ISM is the origin of the classical afterglow. It has produced the prompt X-ray signal as well as the late optical and IR emission. It would most likely produce a radio emission within the next few weeks. The observations suggest that the initial Lorentz factor of the ejecta was $\sim 200$. Within factors of order unity, this crude model explains all current observations of GRB990123.

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arXiv1999

Predictions for The Very Early Afterglow and The Optical Flash

According to the internal-external shocks model for $γ$-ray bursts (GRBs), the GRB is produced by internal shocks within a relativistic flow while the afterglow is produced by external shocks with the ISM. We explore the early afterglow emission. For short GRBs the peak of the afterglow will be delayed, typically, by few dozens of seconds after the burst. For long GRBs the early afterglow emission will overlap the GRB signal. We calculate the expected spectrum and the light curves of the early afterglow in the optical, X-ray and $γ$-ray bands. These characteristics provide a way to discriminate between late internal shocks emission (part of the GRB) and the early afterglow signal. If such a delayed emission, with the characteristics of the early afterglow, will be detected it can be used both to prove the internal shock scenario as producing the GRB, as well as to measure the initial Lorentz factor of the relativistic flow. The reverse shock, at its peak, contains energy which is comparable to that of the GRB itself, but has a much lower temperature than that of the forward shock so it radiates at considerably lower frequencies. The reverse shock dominates the early optical emission, and an optical flash brighter than 15th magnitude, is expected together with the forward shock peak at x-rays or $γ$-rays. If this optical flash is not observed, strong limitations can be put on the baryonic contents of the relativistic shell deriving the GRBs, leading to a magnetically dominated energy density.

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arXiv1999

The Early Afterglows

We calculate the expected spectrum and light curves of the early afterglow. For short GRBs the peak of the afterglow will be delayed, typically, by a few dozens of seconds after the burst. The x-ray and gamma-ray characteristics of this delayed emission provide a way to discriminate between late internal shocks emission (part of the GRB) and the early afterglow signal. Detection of this delayed emission will prove the internal shock scenario as producing the GRB, and will pinpoint the initial Lorentz factor gamma_0. In the optical band, the dominant emission arises from the reverse shock. This shock, carries an energy comparable to that of the forward shock. It radiates this energy at much lower frequencies, producing a short optical flash of 15th magnitude or brighter.

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arXiv1999

Images, Light Curves and Spectra of GRB Afterglow

We calculate the light curve and spectra near the peak and the self absorption break, for an adiabatic blast wave described by the Blandford-McKee solution, considering the emission from the whole region behind the shock front. The expected light curve and spectra are flat near the peak. This rules out the interpretation of the sharp peak observed in the optical afterglow of GRB970508 as the expected peak of the light curve. The observed image of an afterglow is calculated for a broad range of frequencies. We show that for frequencies below the self absorption frequency the image is rather homogeneous, as opposed to the bright ring at the outer edge and dim center, which appear at higher frequencies. We fit the observed spectra of GRB970508 to the detailed theory and obtain estimates of the physical parameters of this burst.

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arXiv1998

Synchrotron Self Absorption in GRB Afterglow

GRB afterglow is reasonably described by synchrotron emission from relativistic blast waves at cosmological distances. We perform detailed calculations taking into account the effect of synchrotron self absorption. We consider emission from the whole region behind the shock front, and use the Blandford McKee self similar solution to describe the fluid behind the shock. We calculate the spectra and the observed image of a GRB afterglow near the self absorption frequency $ν_a$ and derive an accurate expression for $ν_a$. We show that the image is rather homogeneous for $ν<ν_a$, as opposed to the bright ring at the outer edge and dim center, which appear at higher frequencies. We compare the spectra we obtain to radio observations of GRB970508. We combine the calculations of the spectra near the self absorption frequency with other parts of the spectra and obtain revised estimates for the physical parameters of the burst: $E_{52}=0.53$, $ε_e=0.57$, $ε_B=0.0082$, $n_1=5.3$. These estimates are different by up to two orders of magnitude than the estimates based on an approximate spectrum.

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arXiv1998

Images and Spectra From the Interior of a Relativistic Fireball

The detection of an afterglow, following a gamma-ray burst (GRB), can be described reasonably well by synchrotron emission from a relativistic spherical expanding blast wave, driven by an expanding fireball. We perform detailed calculations considering the emission from the whole region behind the shock front. We use the Blandford McKee self similar adiabatic solution to describe the fluid behind the shock. Using this detailed model, we derive expressions for the peak flux, and the peak frequency at a given observed time. These expressions provide important numerical corrections to previous, more simplified models. We calculate the observed light curve and spectra for several magnetic field models. We show that both the light curve and the spectra are flat near the peak. This rules out the interpretation of the optical peak of GRB970508 as the peak of the light curve, predicted by the existing fireball models. We calculate the observed image of a GRB afterglow. The observed image is bright near the edge and dimmer at the center, thus creating a ring. The contrast between the edge and the center is larger at high frequencies and the width of the ring is smaller.

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arXiv1998

Fluid dynamics of partially radiative blast waves

We derive a self similar solution for the propagation of an extreme relativistic (or Newtonian) radiative spherical blast wave into a surrounding cold medium. The solution is obtained under the assumption that the radiation process is fast, it takes place only in the vicinity of the shock and that it radiates away a fixed fraction of the energy generated by the shock. In the Newtonian regime these solutions generalize the Sedov-Taylor adiabatic solution and the pressure-driven fully radiative solution. In the extreme relativistic case these solutions generalize the Blandford-McKee adiabatic solution. They provide a new fully radiative extreme relativistic solution which is different from the Blandford-McKee fully radiative relativistic solution. This new solution develops a hot interior which causes it to cool faster than previous estimates. We find that the energy of the blast wave behaves as a power law of the location of the shock. The power law index depends on the fraction of the energy emitted by the shock. We obtain an analytic solution for the interior of the blast wave. These new solutions might be applicable to the study of GRB afterglow or SNRs.

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arXiv1998

Hydrodynamics of a Relativistic Fireball: the Complete Evolution

We study numerically the evolution of an adiabatic relativistic fireball expanding into a cold uniform medium. We follow the stages of initial free expansion and acceleration, coasting and then deceleration and slowing down to a non-relativistic velocity. We compare the numerical results with simplified analytical estimates. We show that the relativistic self similar Blandford-McKee solution describes well the relativistic deceleration epoch. It is an excellent approximation throughout the relativistic deceleration stage, down to $γ\sim 5$, and a reasonable approximation even down to $γ\sim 2$ though the solution is rigorous only for $γ\gg 1$. We examine the transition into the Blandford-McKee solution, and the transition from the solution to the non-relativistic self similar Sedov-Taylor solution. These simulations demonstrate the attractive nature of the Blandford-McKee solution and its stability to radial perturbations.

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arXiv1997

Spectra and Light Curves of Gamma-Ray Burst Afterglows

The recently discovered GRB afterglow is believed to be described reasonably well by synchrotron emission from a slowing down relativistic shell that collides with an external medium. To compare theoretical models with afterglow observations we calculate here the broad band spectrum and corresponding light curve of synchrotron radiation from a power-law distribution of electrons in an expanding relativistic shock. Both the spectrum and the light curve consist of several power-law segments. The light curve is constructed under two limiting models for the hydrodynamical evolution of the shock: fully adiabatic and fully radiative. We compare the results with observations of $γ$-ray burst afterglows.

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