arXiv subjects

Jonathan Granot

Publications and source records attributed to Jonathan Granot.

At least 127 records · Page 7Linked to original sources

arXiv2003

Neutrinos from Pulsar Wind Bubbles as Precursors to Gamma-Ray Bursts

The supranova model for gamma-ray bursts (GRBs) has recently gained popularity. In this scenario the GRB occurs weeks to years after a supernova explosion, and is located inside a pulsar wind bubble (PWB). High energy protons from the PWB can interact with photons from the rich radiation field inside the PWB or collide with cold protons from the supernova remnant, producing pions which decay into \sim 10-10^3 TeV neutrinos. The predicted neutrino flux from the PWBs that host the GRBs should be easily detectable by planned 1 km^2 detectors.

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arXiv2003

Variability in GRB Afterglows and GRB 021004

We present general analytic expressions for GRB afterglow light curves arising from a variable external density profile and/or a variable energy in the blast wave. The former could arise from a clumpy ISM or a variable stellar wind; The latter could arise from refreshed shocks or from an angular dependent jet structure (patchy shell). Both scenarios would lead to a variable light curve. Our formalism enables us to invert the observed light curve and obtain possible density or energy profiles. The optical afterglow of GRB 021004 was detected 537 seconds AB (after the burst) (Fox et al. 2002). Extensive follow up observations revealed a significant temporal variability. We apply our formalism to the R-band light curve of GRB 021004 and we find that several models provide a good fit to the data. We consider the patchy shell model with $p=2.2$ as the most likely explanation. According to this model our line of sight was towards a "cold spot" that has lead to a relativity low $γ$-ray flux and an initially weak afterglow (while the X-ray afterglow flux after a day was above average). Observations above the cooling frequency, $ν_c$, could provide the best way to distinguish between our different models.

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arXiv2003

The Evolution of a Structured Relativistic Jet and GRB Afterglow Light-Curves

We carry out a numerical hydrodynamical modeling for the evolution of a relativistic collimated outflow, as it interacts with the surrounding medium, and calculate the light-curve resulting from synchrotron emission of the shocked fluid. The hydrodynamic equations are reduced to 1-D by assuming axial symmetry and integrating over the radial profile of the flow, thus considerably reducing the computation time. We present results for a number of different initial jet structures, including several different power-laws and a Gaussian profile for the dependence of the energy per unit solid angle, $ε$, and the Lorentz factor, $Γ$, on the angle from the jet symmetry axis. Our choice of parameters for the various calculations is motivated by the current knowledge of relativistic outflows from gamma-ray bursts and the observed afterglow light-curves. Comparison of the light curves for different jet profiles with GRB afterglow observations provides constraints on the jet structure. One of the main results we find is that the transverse fluid velocity in the comoving frame ($v_t$) and the speed of sideways expansion, for smooth jet profiles, is typically much smaller than the speed of sound ($c_s$) throughout much of the evolution of the jet; $v_t$ approaches $c_s$ when $Γ$ along the jet axis becomes of order a few (for large angular gradient of $ε$, $v_t\sim c_s$ while $Γ$ is still large). This result suggests that the dynamics of relativistic structured jets may be reasonably described by a simple analytic model where $ε$ is independent of time, as long as $Γ$ along the jet-axis is larger than a few.

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arXiv2003

Gamma Ray Bursts in Pulsar Wind Bubbles: Observational implications

We present the main observational features expected for Gamma-Ray Bursts (GRBs) that occur inside pulsar wind bubbles (PWBs). This is the most natural outcome of the supranova model, where initially a supernova (SN) explosion takes place, leaving behind a supra-massive neutron star, which loses its rotational energy over a time t_sd and collapses to a black hole, triggering a GRB. We find that the time delay, t_sd, between the SN and GRB events is the most important parameter that determines the behavior of the system. We consider the afterglow, prompt GRB and PWB emission. Constraints on the model are derived for a spherical PWB, from current afterglow observations and the lack of direct detection of the PWB emission. We find that a simple spherical model cannot account for the X-ray features detected in some afterglows, together with the typical afterglow emission that was observed for the same GRBs. The discrepancies with observations may be reconciled by resorting to a non-spherical geometry, where the PWB is elongated along the polar axis. Finally, we predict that the inverse Compton upscattering of PWB photons by the relativistic electrons of the afterglow (external Compton) should lead to high energy emission during the early afterglow that may explain the GeV photons detected by EGRET for a few GRBs, and should be detectable by future missions such as GLAST.

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arXiv2003

The Jet Angular Profile and the Afterglow Light Curves

We investigate how the angular structure of GRB jets effects the afterglow light curves at different viewing angles, $θ_{v}$, from the jet symmetry axis. A numerical hydrodynamical modeling for the evolution of a relativistic collimated outflow, as it interacts with the surrounding medium, is carried out, and compared to two simple models that make opposite and extreme assumptions for the degree of lateral energy transfer. The Lorentz factor, $Γ$, and kinetic energy per unit solid angle, $ε$, are initially taken to be power laws of the angle $θ$ from the jet axis. We find that the lateral velocity in the comoving frame, $v'_θ$, is typically much smaller than the sound speed, $c_s$, as long as $Γ\gg 1$, and the dynamics of relativistic structured jets may be reasonably described by a simple analytic model where $ε$ is independent of time, as long as $Γ(θ=0)\ga$ a few. We perform a qualitative comparison between the resulting light curves and afterglow observations. This constrains the jet structure, and poses problems for a `universal' jet model, where all GRB jets are assumed to be intrinsically identical, and differ only by our viewing angle, $θ_{v}$.

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arXiv2002

Constraining the Structure of GRB Jets Through the Afterglow Light Curves

We investigate the effect that the structure of GRB jets has on the afterglow light curves for observers located at different viewing angles, $θ_{obs}$, from the jet symmetry axis. The largest uncertainty in the jet dynamics is the degree of lateral energy transfer. Thus, we use two simple models, that make opposite and extreme assumptions for this point, and calculate the light curves for an external density that is either homogeneous, or decreases as the square of the distance from the source. The Lorentz factor and kinetic energy per unit solid angle are initially taken to be power laws of the angle from the jet axis. We perform a qualitative comparison between the resulting light curves and afterglow observations. This constrains the jet structure, and poses problems for a `universal' jet model, where all GRB jets are assumed to be intrinsically identical, and differ only by our viewing angle, $θ_{obs}$.

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arXiv2002

Observational Implications of a Plerionic Environment for GRBs

We consider GRBs that occur inside pulsar wind bubbles (PWBs), in the context of the supranova model, where initially a supernova explosion takes place, leaving behind a supra-massive neutron star (SMNS), which eventually loses its rotational energy and collapses to a black hole, triggering a GRB explosion. The most natural mechanism by which the SMNS can lose its rotational energy is through a strong pulsar type wind, between the supernova and the GRB events, which is expected to create a PWB. We analyze in some detail the observational implications of such a plerionic environment on the afterglow (AG) and prompt GRB emissions, and the prospect for direct detection of the plerion emission. We find that for a simple spherical model, GRBs with iron lines detected in the X-ray AG should not have a detectable radio afterglow, and should have small jet break times and non-relativistic transition times, in disagreement with observations for some of the GRBs with X-ray lines. These discrepancies with the observations may be reconciled by resorting to a non-spherical geometry, where the PWB is elongated along the polar axis. We find that lack of detection of the plerion emission provides interesting constraints on the model parameters. Finally, we predict that the inverse Compton upscattering of the PWB photons by the relativistic electrons of the AG (external Compton, EC) should lead to high energy emission during the early AG that might explain the GeV photons detected by EGRET for a few GRBs, and should be detectable by future missions such as GLAST.

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arXiv2002

Gamma-Ray Bursts in Pulsar Wind Bubbles: Putting the Pieces Together

We present the main observational features expected for GRBs that occur inside pulsar wind bubbles (PWBs). This is the most natural outcome of supranova model where initially a supernova explosion takes place, leaving a supra-massive NS, which loses its rotational energy over a time t_sd and collapses to a BH, triggering a GRB explosion. We find that the time delay t_sd between the supernova and GRB events is the most important parameter that determines the behavior of the system. We consider the afterglow, prompt GRB and direct emission from the PWB. The observational signatures for different t_sd are described and joined together into one coherent framework. Constraints on the model are derived for a spherical PWB, from the lack of detection of the PWB emission and afterglow observations. For very low values of t_sd<1 hr the supranova model reduces to the collapsar model; 0.4<t_sd/1yr<1 is required to produce the iron lines seen in some X-ray afterglows. However, we find that for a spherical PWB, this implies no detectable radio afterglow, a small jet break time and non-relativistic transition time, in disagreement with observations for some of these GRBs. These discrepancies with the observations may be reconciled by resorting to a non-spherical geometry. We find that light element lines, recently detected in a few X-ray afterglows, are expected to dominate over iron lines for small t_sd, while for large t_sd the situation is reversed. Finally, we predict that external Compton scattering of the PWB photons by the afterglow relativistic electrons, should lead to high energy emission during the early afterglow that may explain the GeV photons detected by EGRET in some GRBs, and should be detectable by future missions (GLAST).

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arXiv2002

The Mean Number of Extra Micro-Image Pairs for Macro-Lensed Quasars

When a gravitationally lensed source crosses a caustic, a pair of images is created or destroyed. We calculate the mean number of such pairs of micro-images $ $ for a given macro-image of a gravitationally lensed point source, due to microlensing by the stars of the lensing galaxy. This quantity was calculated by Wambsganss, Witt & Schneider (1992) for the case of zero external shear, $γ=0$, at the location of the macro-image. Since in realistic lens models a non-zero shear is expected to be induced by the lensing galaxy, we extend this calculation to a general value of $γ$. We find a complex behavior of $ $ as a function of $γ$ and the normalized surface mass density in stars $κ_*$. Specifically, we find that at high magnifications, where the average total magnification of the macro-image is $<μ>=|(1-κ_*)^2-γ^2|^{-1}\gg 1$, $ $ becomes correspondingly large, and is proportional to $<μ>$. The ratio $ /<μ>$ is largest near the line $γ=1-κ_*$ where the magnification $<μ>$ becomes infinite, and its maximal value is 0.306. We compare our semi-analytic results for $ $ to the results of numerical simulations and find good agreement. We find that the probability distribution for the number of extra image pairs is reasonably described by a Poisson distribution with a mean value of $ $, and that the width of the macro-image magnification distribution tends to be largest for $ \sim 1$.

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arXiv2002

High Energy Emission from the Prompt Gamma-Ray Burst

We study the synchrotron and synchrotron self-Compton (SSC) emission from internal shocks that are responsible for the prompt gamma-ray emission in GRBs, and consider the relation between these two components, taking into account the high energy (HE) cutoff due to pair production and Thomson scattering. We find that in order for the peak energy of the synchrotron to be E_p\sim 300 keV with a variability time t_v>1 ms, a Lorentz factor Γ<350 is needed, implying no HE emission above \sim 30 MeV and the synchrotron component would dominate at all energies. If we want both E_p\sim 300 keV and prompt HE emission up to 2 GeV, as detected by EGRET for GRB 940217, we need Γ\sim 600 and t_v\sim 0.1 ms, which might be resolved by super AGILE. If such prompt HE emission is common in GRBs, as may be tested by GLAST, then for t_v\gtrsim 1 ms we need Γ\gtrsim 350, which implies E_p\lesssim 100 keV. Therefore if X-ray flashes are GRBs with high values of t_v and Γ, they should produce \gtrsim 1 GeV emission. For an electron power law index p>2, the SSC component dominates the emission above 100 MeV. Future observations by GLAST may help determine the value of p and whether the HE emission is consistent with a single power law (one component--the synchrotron, dominates) or has a break where the \nuF_νslope turns from negative to positive, implying that the SSC component becomes dominant above \sim 100 MeV. The HE emission is expected to show similar variability and time structure to that of the soft gamma-ray emission. Finally, we find that in order to see delayed HE emission from the prompt GRB due to pair production with the cosmic IR background, extremely small intergalactic magnetic fields (\lessim 10^{-22} G) are required.

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arXiv2002

The Detectability of Orphan Afterglows

The realization that GRBs release a constant amount of energy implies that the post jet-break afterglow evolution would be largely universal. For a given redshift all afterglows should be detected up to a fixed observer angle. We estimate the observed magnitude and the implied detectability of orphan afterglows. We show that for reasonable limiting magnitudes ($m_{lim}=25$) orphan afterglows would typically be detected from small ($\sim 10^\circ$) angles away from the GRB jet axis. A detected orphan afterglow would generally correspond to a "near-miss" of the GRB whose jet was pointing just slightly away from us. With our most optimistic parameters we expect that 15 orphan afterglows will be recorded in the SDSS and 35 transients will be recorded in a dedicated 2m class telescope operating full time for a year in an orphan afterglow search. The rate is smaller by a factor of 15 for our "canonical" parameters. We show that for a given facility an optimal survey should be shallower, covering larger area rather than deeper. The limiting magnitude should not be, however, lower than $\sim$ 23rd as in this case more transients from on-axis GRBs will be discovered than orphan afterglows. About 15% of the transients could be discovered with a second exposure of the same area provided that it follows after 3, 4 and 8 days for $m_{lim}=23$, 25 and 27.

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arXiv2002

Gamma-Ray Burst Afterglows in Pulsar-Wind Bubbles

We propose to identify pulsar-wind bubbles (PWBs) as the environment in which the afterglow emission in at least some gamma-ray bursts (GRBs) originates. Such bubbles could naturally account for the high fraction of the internal energy residing in relativistic electrons/positrons ($ε_e$) and the high magnetic-to-internal energy ratio ($ε_B$) that have been inferred in a number of sources. GRBs might occur within PWBs under a number of scenarios: in particular, in the supranova model of GRB formation a prolonged (months to years) period of intense pulsar-type wind from the GRB progenitor precedes the burst. Focusing on this scenario, we construct a simple model of the early-time structure of a plerionic supernova remnant. The model is based on the assumption of an ``equipartition'' upper bound on the electromagnetic- to-thermal pressure ratio in the bubble and takes into account synchrotron- radiation cooling. We derive an expression for the effective hydrogen number density $n_{H,equiv}$ of the shocked pulsar wind in terms of the comoving particle pressure and electromagnetic fields. We show that, for plausible parameter values, $n_{H,equiv}$ spans the range inferred from spectral fits to GRB afterglows and that its radial profile varies within the bubble and may resemble a uniform interstellar medium or a stellar wind. We consider how the standard expressions for the characteristic synchrotron spectrum are modified when the afterglow-emitting shock propagates inside a PWB and demonstrate that the predictions for the empirically inferred values of $ε_e$ and $ε_B$ are compatible with the observations. Finally, we outline a self-consistent interpretation of the X-ray emission features detected in sources like GRB 991216 in the context of the supranova/PWB picture.

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arXiv2001

The Shape of Spectral Breaks in GRB Afterglows

Gamma-Ray Burst (GRB) afterglows are well described by synchrotron emission from relativistic blast waves expanding into an external medium. The blast wave is believed to amplify the magnetic field and accelerate the electrons into a power law distribution of energies promptly behind the shock. These electrons then cool both adiabatically and by emitting synchrotron and inverse Compton radiation. The resulting spectra is known to consist several power law segments, which smoothly join at certain break frequencies. Here, we give a complete description of all possible spectra under those assumptions, and find that there are 5 possible regimes, depending on the ordering of the break frequencies. The flux density is calculated by integrating over the contributions from all the shocked region, using the Blandford McKee solution. This allows us to calculate more accurate expressions for the value of these break frequencies, and describe the shape of the spectral breaks around them. This also provides the shape of breaks in the light curves caused by the passage of a break frequency through the observed band. These new, more exact, estimates are different from more simple calculations by up to a factor of about 70, and describe some new regimes which where previously ignored.

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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

Theory of GRB Afterglow

The most interesting current open question in the theory of GRB afterglow is the propagation of jetted afterglows during the sideway expansion phase. Recent numerical simulations show hydrodynamic behavior that differs from the one suggested by simple analytic models. Still, somewhat surprisingly, the calculated light curves show a `jet break' at about the expected time. These results suggest that the expected rate of orphan optical afterglows should be smaller than previously estimated.

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arXiv2001

Radiative Hydromagnetic Shocks in Relativistic Outflow Sources

We calculate the structure of a relativistic shock wave in which the internal energy of the shocked fluid is radiated away on a time scale much shorter than the characteristic shock propagation time. The shock is assumed to move through a uniformly magnetized, neutral plasma consisting of protons, electrons, and positrons, and allowance is made for the possible production of electron/positron pairs in the shock itself. The radiation mechanism is taken to be synchrotron and inverse-Compton emission (the latter involving both synchrotron-produced and externally supplied seed photons) by the electrons and positrons. We simplify the discussion by considering a shock in which the magnetic field is transverse to the direction of propagation and focus attention on the properties of the radiative zone that forms behind the shock transition. In particular, we investigate the possibility that the compression induced by the cooling of the gas amplifies the magnetic pressure until it reaches (and ultimately exceeds) equipartition with the thermal pressure (which, in turn, limits the overall compression). We show that, if a significant fraction of the postshock thermal energy is deposited in the electron/positron component, then a considerable portion of the emitted radiation will come from regions of strong magnetic field even if the field immediately behind the shock transition is well below equipartition. This field amplification mechanism may be relevant to the production of synchrotron flares in blazars, miniquasars, and gamma-ray burst sources. We consider the latter application in some detail and show that this process may play a role in the prompt $γ$-ray and possibly also the optical ``flash'' and radio ``flare'' emission, but probably not in the afterglow.

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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

Light Curves from an Expanding Relativistic Jet

We perform fully relativistic hydrodynamic simulations of the deceleration and lateral expansion of a relativistic jet as it expands into an ambient medium. The hydrodynamic calculations use a 2D adaptive mesh refinement (AMR) code, which provides adequate resolution of the thin shell of matter behind the shock. We find that the sideways propagation is different than predicted by simple analytic models. The physical conditions at the sides of the jet are found to be significantly different than at the front of the jet, and most of the emission occurs within the initial opening angle of the jet. The light curves, as seen by observers at different viewing angles with respect to the jet axis, are then calculated assuming synchrotron emission. For an observer along the jet axis, we find a sharp achromatic `jet break' in the light curve at frequencies above the typical synchrotron frequency, at $t_{jet}\approx 5.8(E_{52}/n_1)^{1/3}(θ_0/0.2)^{8/3}$ days, while the temporal decay index $α$ ($F_ν\propto t^α$) after the break is steeper than $-p$ ($α=-2.85$ for $p=2.5$). At larger viewing angles $t_{jet}$ increases and the jet break becomes smoother.

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