arXiv subjects

Jonathan Granot

Publications and source records attributed to Jonathan Granot.

At least 109 records · Page 6Linked to original sources

arXiv2005

A compact binary merger model for the short, hard GRB 050509b

The first X-ray afterglow for a short (~30 ms), hard gamma-ray burst was detected by Swift on 9 May 2005 (GRB 050509b). No optical or radio counterpart was identified in follow-up observations. The tentative association of the GRB with a nearby giant elliptical galaxy at redshift z=0.2248 would imply a total energy release E(gamma, isotropic) ~3 x 10**48 erg, and that the progenitor had traveled several tens of kpc from its point of origin, in agreement with expectations linking these events to the final merger of compact binaries driven by gravitational wave emission. We model the dynamical merger of such a system and the time-dependent evolution of the accretion tori thus created. The resulting energetics, variability, and expected durations are consistent with GRB 050509b originating from the tidal disruption of a neutron star by a stellar mass black hole, or of the merger of two neutron stars followed by prompt gravitational collapse of the massive remnant. We discuss how the available gamma-ray and X-ray data provide a probe for the nature of the relativistic ejecta and the surrounding medium.

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arXiv2005

An Off-Axis Model for GRB 031203

The low luminosity radio emission of the unusually faint GRB 031203 has been argued to support the idea of a class of intrinsically sub-energetic gamma-ray bursts (GRBs), currently comprising two members. While low energy GRBs probably exist, we show that the collective prompt and multiwavelength observations of the afterglow of GRB 031203 do not necessarily require a sub-energetic nature for that event. In fact, the data are more consistent with a typical, powerful GRB seen at an angle of about twice the opening angle of the central jet. The (redshift corrected) peak energy, E_p, of GRB 031203 then becomes ~ 2 MeV, similar to many other GRBs.

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arXiv2005

Afterglow Light Curves from Impulsive Relativistic Jets with an Unconventional Structure

The jet structure in gamma-ray burst (GRB) sources is still largely an open question. The leading models invoke either (i) a roughly uniform jet with sharp edges, or (ii) a jet with a narrow core and wide wings where the energy per solid angle drops as a power law with the angle $θ$ from the jet symmetry axis. Recently, a two component jet model has also been considered, with a narrow uniform jet of initial Lorentz factor $Γ_0 \gtrsim 100$ surrounded by a wider uniform jet with $Γ_0 \sim 10-30$. Some models predict more exotic jet profiles, such as a thin uniform ring (i.e. the outflow is bounded by two concentric cones of half opening angle $θ_c$ and $θ_c + Δθ$, with $Δθ\ll θ_c$) or a fan (a thin outflow with $Δθ\ll 1$ along the rotational equator, $θ_c = π/2 + Δθ/2$). In this paper we calculate the expected afterglow light curves from such jet structures, using a simple formalism that is developed here for this purpose, and could also have other applications. These light curves are qualitatively compared to observations of GRB afterglows. It is shown that the two component jet model cannot produce sharp features in the afterglow model due to the deceleration of the wide jet or the narrow jet becoming visible at lines of sight outside of it. We find that a "ring" shaped jet or a "fan" shaped jet produce a jet break in the afterglow light curve that is too shallow compared to observations, where the change in the temporal decay index across the jet break is about half of that for a uniform conical jet. For a "ring" jet, the jet break is divided into two distinct and smaller breaks, the first occurring when $γΔθ\sim 1-2$ and the second when $γθ_c \sim 1/2$.

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arXiv2005

Two-Component Jet Models of Gamma-Ray Burst Sources

Recent observational and theoretical studies have raised the possibility that the collimated outflows in gamma-ray burst (GRB) sources have two distinct components: a narrow (opening half-angle $θ_{\rm n}$), highly relativistic (initial Lorentz factor $η_\rmn \gtrsim 10^2$) outflow, from which the $γ$-ray emission originates, and a wider ($θ_{\rm w} \lesssim 3 θ_{\rm n}$), moderately relativistic ($η_{\rm w}\sim 10$) surrounding flow. Using a simple synchrotron emission model, we calculate the R-band afterglow lightcurves expected in this scenario and derive algebraic expressions for the flux ratios of the emission from the two jet components at the main transition times in the lightcurve. We apply this model to GRB sources, for explaining the structure of afterglows and source energetics, as well as to X-ray flash sources, which we interpret as GRB jets viewed at an angle $θ_{\rm obs} > θ_{\rm n}$. Finally, we argue that a neutron-rich hydromagnetic outflow may naturally give rise to repeated brightening episodes in the afterglow lightcurve as observed in GRB 021004 and GRB 030329.

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arXiv2005

Radio Flares and the Magnetic Field Structure in GRB Outflows

The magnetic field structure in GRB outflows is of great interest as it can provide valuable clues that can help pin down the mechanism responsible for the acceleration and collimation of GRB jets. The most promising way of probing it is through polarization measurements of the synchrotron emission from the GRB ejecta, which includes the gamma-ray emission and the reverse shock emission. Measuring polarization in gamma-rays with current instruments is extremely difficult: so far there is only one claim of detection (in GRB 021206) which despite the favorable conditions remains very controversial. The emission from the reverse shock that propagates into the ejecta as it is decelerated by the ambient medium peaks in the optical after tens of seconds (the `optical flash') and dominates the optical emission up to about ten minutes after the GRB. Unfortunately, no polarization measurements of this optical emission have been made to date. However, after the reverse shock finishes crossing the shell of GRB ejecta, the shocked ejecta cools adiabatically and radiates at lower and lower frequencies, peaking in the radio after $\sim 1$ day (the `radio flare'). We use VLA data of radio flares from GRBs to constrain the polarization of this emission. We find only upper limits for both linear and circular polarization. Our best limits are for GRB 991216, for which we find $3σ$ upper limits on the linear and circular polarization of 7% and 9%, respectively. These limits provide interesting constraints on existing GRB models. Specifically, our results are hard to reconcile with a predominantly ordered toroidal magnetic field in the GRB outflow together with a `structured' jet, where the energy per solid angle drops as the inverse square of the angle from the jet axis, that is expected in models where the outflow is Poynting flux dominated.

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arXiv2005

Afterglow Observations Shed New Light on the Nature of X-ray Flashes

X-ray flashes (XRFs) and X-ray rich gamma-ray bursts (XRGRBs) share many observational characteristics with long duration GRBs, but the reason for which their prompt emission peaks at lower photon energies, $E_p$, is still under debate. Although many different models have been invoked in order to explain the lower $E_p$ values, their implications for the afterglow emission were not considered in most cases, mainly because observations of XRF afterglows have become available only recently. Here we examine the predictions of the various XRF models for the afterglow emission, and test them against the observations of XRF 030723 and XRGRB 041006, the events with the best monitored afterglow light curves in their respective class. We show that most existing XRF models are hard to reconcile with the observed afterglow light curves, which are very flat at early times. Such light curves are, however, naturally produced by a roughly uniform jet with relatively sharp edges that is viewed off-axis (i.e. from outside of the jet aperture). This type of model self consistently accommodates both the observed prompt emission and the afterglow light curves of XRGRB 041006 and XRF 030723, implying viewing angles $θ_{obs}$ from the jet axis of $(θ_{obs}-θ_0)\sim 0.15θ_0$ and $\sim θ_0$, respectively, where $θ_0\sim 3$ deg is the jet half-opening angle. This suggests that GRBs, XRGRBs and XRFs are intrinsically similar relativistic jets viewed from different angles, corresponding to $γ(θ_{obs}-θ_0)$ of less than 1, between 1 and a few, and more than a few, respectively, where $γ$ is the Lorentz factor. Future observations with Swift could help test this unification scheme in which GRBs, XRGRBs and XRFs share the same basic physics and differ only by their orientation relative to our line of sight.

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arXiv2005

Constraining the Structure of GRB Jets Through the log(N)-log(S) Distribution

A general formalism is developed for calculating the luminosity function and the expected number $N$ of observed GRBs above a peak photon flux $S$ for any GRB jet structure. This new formalism directly provides the true GRB rate without the need for a `correction factor'. We apply it to the uniform jet (UJ) and universal structured jet (USJ) models for the structure of GRB jets and perform fits to the observed log(N)-log(S) distribution from the GUSBAD catalog which contains 2204 BATSE bursts. A core angle $θ_c$ and an outer edge at $θ_{max}$ are introduced for the structured jet, and a finite range of half-opening angles $θ_{min}\leqθ_j\leqθ_{max}$ is assumed for the uniform jets. The efficiency $ε_γ$ for producing gamma-rays, and the energy per solid angle $ε$ in the jet are allowed to vary with $θ_j$ (the viewing angle $θ_{obs}$) in the UJ (USJ) model, $ε_γ\proptoθ^{-b}$ and $ε\proptoθ^{-a}$. We find that a single power-law luminosity function provides a good fit to the data. Such a luminosity function arises naturally in the USJ model, while in the UJ model it implies a power-law probability distribution for $θ_j$, $P(θ_j)\proptoθ_j^{-q}$. The value of $q$ cannot be directly determined from the fit to the observed log(N)-log(S) distribution, and an additional assumption on the value of $a$ or $b$ is required. Alternatively, an independent estimate of the true GRB rate would enable one to determine $a$, $b$ and $q$. The implied values of $θ_c$ (or $θ_{min}$) and $θ_{max}$ are close to the current observational limits. The true GRB rate for the USJ model is found to be $R_{GRB}(z=0)=0.86^{+0.14}_{-0.05} Gpc^{-3} yr^{-1}$.

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arXiv2004

Implications of the Measured Image Size for the Radio Afterglow of GRB 030329

We use data on the image size of the radio afterglow of GRB 030329 (Taylor et al. 2004) to constrain the physical parameters of this explosion. Together with the observed broad band spectrum, this data over-constrains the physical parameters, thus enabling to test different GRB jet models for consistency. We consider two extreme models for the lateral spreading of the jet: model 1 with relativistic expansion in the local rest frame, and model 2 with little lateral expansion as long as the jet is highly relativistic. We find that both models are consistent with the data for a uniform external medium, while for a stellar wind environment model 1 is consistent with the data but model 2 is disfavored by the data. Our derivations can be used to place tighter constraints on the dynamics and structure of GRB jets in future afterglows, following a denser monitoring campaign for the temporal evolution of their image size.

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arXiv2004

High Energy Emission from the Double Pulsar system J0737-3039

We discuss the effects of particle acceleration at the bow shocks expected in the binary pulsar system J0737-3039, due to the wind from pulsar A interacting with both the interstellar medium (ISM), and with the magnetosphere of pulsar B. In this model, we find that the likeliest source for the X-rays observed by Chandra is the emission from the shocked wind of pulsar A as it interacts with the ISM. In this case, for favorable model parameter values, better statistics might help Chandra marginally resolve the source. A consequence of the model is a power law high energy spectrum extending up to $\lesssim$ 60 keV, at a level of $\sim 2 \times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$.

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arXiv2004

The Case for a Misaligned Relativistic Jet from SN 2001em

SN 2001em, identified as a Type Ic supernova, has recently been detected in the radio and X-rays, $\gtrsim$ 2 yr after the explosion. The high luminosities at such late times might arise from a relativistic jet viewed substantially off-axis that becomes visible only when it turns mildly relativistic and its emission is no longer strongly beamed away from us. Alternatively, the emission might originate from the interaction of the SN shell with the circumstellar medium. We find that the latter scenario is hard to reconcile with the observed rapid rise in the radio flux and optically thin spectrum, $F_ν\proptoν^{-0.36\pm 0.16}t^{1.9\pm 0.4}$, while these features arise naturally from a misaligned relativistic jet. The high X-ray luminosity provides an independent and more robust constraint -- it requires $\sim 10^{51}$ erg in mildly relativistic ejecta. The source should therefore currently have a large angular size ($\sim$ 2 mas) which could be resolved in the radio with VLBA. It is also expected to be bipolar and is thus likely to exhibit a large degree of linear polarization ($\sim 10%-20%$). The presence of a relativistic outflow in SN 2001em would have interesting implications. It would suggest that several percent of SNe Ib/c produce mildly relativistic jets, with an initial Lorentz factor $Γ_0\gtrsim 2$, while the fraction that produce GRB jets (with $Γ_0\gtrsim 100$) is $\sim 100$ times smaller. This could considerably increase the expected number of transients similar to orphan GRB afterglows in the radio, and to a lesser extent in the optical and X-rays, if there is a continuous distribution in $Γ_0$. Furthermore, this may give further credence to the idea that core collapse SNe, and in particular SNe Type Ib/c, are triggered by bipolar jets.

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arXiv2004

Testing the Predictions of the Universal Structured GRB Jet Model

The two leading models for the structure of GRB jets are the uniform jet model and the universal structured jet (USJ) model. In the latter, all GRB jets are intrinsically identical and the energy per solid angle drops as the inverse square of the angle from the jet axis. The simplicity of the USJ model gives it a strong predictive power, including a specific prediction for the observed GRB distribution as a function of both the redshift $z$ and the viewing angle $θ$. We show that the current sample of GRBs with known $z$ and estimated $θ$ does not agree with the predictions of the USJ model. This can be best seen for a relatively narrow range in $z$, in which the USJ model predicts that most GRBs should be near the upper end of the observed range in $θ$, while in the observed sample most GRBs are near the lower end of that range. Since the current sample is very inhomogeneous (i.e. involves many different detectors), it should be taken with care and cannot be used to rule out the USJ model. Nevertheless, this sample strongly disfavors the USJ model. Comparing the prediction for the observed GRB distribution both in $θ$ and in $z$, with a larger and more homogeneous GRB sample, like the one expected from Swift, would either clearly rule out the USJ model, or alternatively, provide a strong support for it. The test presented here is general, and can be used to test any model that predicts both a luminosity function and a luminosity-angle relation.

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arXiv2003

Some Recent Peculiarities of the Early Afterglow

We consider some recent developments in GRB/afterglow observations: (i) the appearance of a very hard prompt component in GRB 941017, and (ii) variability in the early afterglow light curves of GRB 021004 and GRB 030329. We show that these observations fit nicely within the internal-external shocks model. The observed variability indicates that the activity of the inner engine is more complicated than was thought earlier and that it involves patchy shells and refreshed shocks. We refute the claims of Berger et al. and of Sheth et al. that the radio and mm observations of GRB 030329 are inconsistent with refreshed shocks.

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arXiv2003

Explaining the High Energy Spectral Component in GRB 941017

The gamma-ray burst (GRB) of October 17, 1994 (941017), showed a distinct high energy spectral component extending from $\lesssim$ a few MeV to $\gtrsim$ 200 MeV, in addition to the typical GRB emission which peaked at $\lesssim$ a few hundred keV. The high energy component carried at least $\sim$ 3 times more energy than the lower energy component. It displayed an almost constant flux with a rather hard spectrum ($F_ν\propto ν^{-α}$ with $α\sim 0$) from $\lesssim$ 20 s into the burst up to $\sim$ 200 s, while the duration of the GRB, where 90% of the energy in the lower energy component was emitted, was only 77 s. Such a high energy component was seen in only one out of $\sim$ 30 GRBs in which a similar component could have been detected, and thus appears to be quite rare. We examine possible explanations for this high energy spectral component and find that most models fail. The only emission region that provides the right temporal behavior is the reverse shock that goes into the GRB ejecta as it is decelerated by the ambient medium, or possibly the very early forward shock while the reverse shock is still going on. The best candidate for the emission mechanism is synchrotron self-Compton emission from the reverse shock. Even in this model the most natural spectral slope is only marginally consistent with the observed value, and some degree of fine tuning is required in order to improve the agreement. This might suggest that an additional or alternative emission mechanism is at work here. A prediction of this interpretation is that such a high energy component should be accompanied by a bright optical transient, similar to the one observed in GRB 990123.

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arXiv2003

The Most Probable Cause for the High Gamma-Ray Polarization in GRB 021206

The exciting detection of a very high degree of linear polarization, $P=(80\pm 20)%$, in the prompt gamma-ray emission of GRB 021206, supports synchrotron emission as being the dominant radiation mechanism. There were also claims that this implies a magnetic field ordered on large scales within the ejecta, that must therefore be produced at the source, which in turn was used as an argument in favor magnetic fields playing an active role in the production of GRB jets. However, an alternative explanation was suggested which also works with a magnetic field that is generated in the internal shocks and does not originate at the source: a very narrow jet, of opening angle $θ_j\sim 1/γ$, where $γ\gtrsim 100$ is the Lorentz factor during the GRB, viewed at $θ_j<θ_{obs}\lesssimθ_j+1/γ$. We calculate $P$ for these two scenarios, and find that it is significantly easier to produce $P\gtrsim 50%$ with an ordered field. More specifically, we obtain $P\sim(43-61)%$ for an ordered transverse magnetic field, $B_{ord}$, whereas a random field within the plane of the shock, $B_\perp$, produces $P\lesssim(38-54)%$ for a single pulse in the GRB light curve, but the integrated emission over many pulses (as measured in GRB 021206) is a factor of $\sim 2$ lower. A magnetic field normal to the shock front, $B_\parallel$, produces $P\sim(35-62)%$ for the emission integrated over many pulses. However, polarization measurements from GRB afterglows suggest a more isotropic shock-produced field configuration that would reduce $P$ by a factor $\sim 2-3$. Therefore, an ordered magnetic field, $B_{ord}$, that originates at the source, can produce the observed polarization most naturally, while $B_\parallel$ is less likely, and $B_\perp$ is the least likely of the above.

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arXiv2003

Radio Imaging of GRB Jets in Nearby Supernovae

We calculate the time evolution of the flux, apparent size, and image centroid motion of gamma-ray burst (GRB) radio jets, and show that they can be resolved by the VLBA at distances of hundreds of Mpc. We find that GRB 030329 which showed spectroscopic evidence for an associated Type Ic supernova (SN) at a distance of $\approx$ 800 Mpc might just be resolvable by VLBA after several months. The prospects are much better for jets that are oriented sideways in similar SNe with no GRB counterpart; in particular, the motion of the flux centroid in such jets can be detected by VLBA up to $z\sim 1$ even when the jet cannot be resolved. If most GRBs are accompanied by a Type Ib/c SN, then there should be a few SN/GRB jets per year within a distance $\lesssim$ 200 Mpc, and most of them would be oriented sideways with no gamma-ray or X-ray precursor. Detection of these jets can be used to calibrate the fraction of all core collapse SNe that produce relativistic outflows and determine the local GRB rate. Overall, the rate of Type Ib/c SNe that do not produce a GRB at all, but rather make relativistic radio jets with an initial Lorentz factor of a few, may be larger by up to 2 orders of magnitude than the rate of those that produce GRBs.

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arXiv2003

Linear Polarization in Gamma-Ray Bursts: The Case for an Ordered Magnetic Field

Linear polarization at the level of $\sim 1-3%$ has by now been measured in several gamma-ray burst afterglows. Whereas the degree of polarization, $P$, was found to vary in some sources, the position angle, $θ_p$, was roughly constant in all cases. Until now, the polarization has been commonly attributed to synchrotron radiation from a jet with a tangled magnetic field that is viewed somewhat off axis. However, this model predicts either a peak in $P$ or a $90^\circ$ change in $θ_p$ around the ``jet break'' time in the lightcurve, for which there has so far been no observational confirmation. We propose an alternative interpretation, wherein the polarization is attributed, at least in part, to a large-scale, ordered magnetic field in the ambient medium. The ordered component may dominate the polarization even if the total emissivity is dominated by a tangled field generated by postshock turbulence. In this picture, $θ_p$ is roughly constant because of the uniformity of the field, whereas $P$ varies as a result of changes in the ratio of the ordered-to-random mean-squared field amplitudes. We point out that variable afterglow light curves should be accompanied by a variable polarization. The radiation from the original ejecta, which includes the prompt gamma-ray emission and the emission from the reverse shock (the `optical flash' and `radio flare'), could potentially exhibit a high degree of polarization (up to $\sim 60%$) induced by an ordered transverse magnetic field advected from the central source.

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arXiv2003

The Variable Light Curve of GRB 030329: The Case for Refreshed Shocks

GRB 030329 is unique in many aspects. It has a very low redshift for a GRB, $z=0.1685$, and is therefore very bright and easy to monitor, making it the most well studied afterglow to date. It shows a supernova bump in the light curve, with a spectrum very similar to SN 1998bw, thus establishing with much better confidence the connection between GRBs and core collapse SNe. There are also two important physical characteristics that make this burst especially interesting, aside from its remarkably low redshift. First, unlike most GRB afterglows, the light curve of GRB 030329 shows a very large variability a few days after the burst. These fluctuations show a roughly constact amplitude, and a constant duration $Δt$, while $Δt/t$ decreases with time $t$. Second, its $γ$-ray energy output and X-ray luminosity at $10 $hr are a factor of $\sim 20$ and $\sim 30$, respectively, below the average value around which most GRBs are narrowly clustered. We consider several interpretations for the variability in the light curve, in the context of different physical mechanisms, and find that the most likely cause is refreshed shocks, i.e. slow shells that are ejected from the source and catch up with the afterglow shock at late times. In GRB 030329 this happens after the jet break, which implies an approximately constant duration $Δt$ of the bumps, in agreement with the observations. This interpretation also explains the anomalously low initial energy of this burst, as the total energy of the afterglow shock is increased by a factor of $\sim 10$ due to the refreshed shocks, thus bringing the total energy output close to the average value for all GRBs.

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arXiv2003

Neutrinos from Gamma-Ray Bursts in Pulsar Wind Bubbles: \sim 10^{16} eV

The supranova model for Gamma-Ray Bursts (GRBs) is becoming increasingly more popular. In this scenario the GRB occurs weeks to years after a supernova explosion, and is located inside a pulsar wind bubble (PWB). Protons accelerated in the internal shocks that emit the GRB may interact with the external PWB photons producing pions which decay into \sim 10^{16} eV neutrinos. A km^2 neutrino detector would observe several events per year correlated with the GRBs.

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