arXiv subjects

Jonathan Granot

Publications and source records attributed to Jonathan Granot.

131 records · Page 8Linked to original sources

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.

astro-ph↗

arXiv1999

Hydrodynamics and Radiation from a Relativistic Expanding Jet with Applications to GRB Afterglow

We describe fully relativistic three dimensional calculations of the slowing down and spreading of a relativistic jet by an external medium like the ISM. We calculate the synchrotron spectra and light curves using the conditions determined by the hydrodynamic calculations. Preliminary results with a moderate resolution are presented here. Higher resolution calculations are in progress.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗