Jonathan Granot

Evidence for a canonical GRB afterglow light curve in the Swift/XRT data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) detected with the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments. These power law segments are separated by two corresponding break times. On top of this canonical behavior of the early X-ray light curve, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long lasting sporadx activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission: from photons that are radiated at large angles relative to our line of sight. The first break in the light curve takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve. This energy injection increases the energy of the afterglow shock by at least a factor of f greater than or approx. equal to 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.

Evidence for a canonical gamma-ray burst afterglow light curve in the Swift XRT data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) well observed by the Swift X-Ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power-law segments: (1) an initial very steep decay (/t � � with 3P � 1 P5), followed by (2) a very shallow decay (0:5P � 2 P1:0), and finally (3) a somewhat steeper decay (1P � 3 P1:5). These power-law segments are separated by two corresponding break times, tbreak;1 P500 s and 10 3 sPtbreak;2P 10 4 s. On top of this canonical behavior, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long-lasting sporadic activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission, from photons that are radiated at large angles relative to our line of sight. The first break in the light curve (tbreak;1) takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay (� 2) likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve (tbreak;2). This energy injection increases the energy of the afterglow shock by at least a factor of f k4 and augments the already severe requirements for the efficiency of the prompt gamma-ray emission. Subject headingg gamma rays: bursts — radiation mechanisms: nonthermal

The Shape of Spectral Breaks in Gamma-Ray Burst Afterglows

Gamma-ray burst 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 are known to consist of 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 five possible regimes, depending on the ordering of the break frequencies. The flux density is calculated by integrating over all of the contributions to a given photon arrival time from all of 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 typically a factor of a few, and they describe some new regimes that were previously ignored.

A giant γ-ray flare from the magnetar SGR 1806-20

Two classes of rotating neutron stars—soft γ-ray repeaters (SGRs) and anomalous X-ray pulsars—are magnetars, whose X-ray emission is powered by a very strong magnetic field (B ≈ 1015 G). SGRs occasionally become ‘active’, producing many short X-ray bursts. Extremely rarely, an SGR emits a giant flare with a total energy about a thousand times higher than in a typical burst. Here we report that SGR 1806–20 emitted a giant flare on 27 December 2004. The total (isotropic) flare energy is 2 × 1046 erg, which is about a hundred times higher than the other two previously observed giant flares. The energy release probably occurred during a catastrophic reconfiguration of the neutron stars magnetic field. If the event had occurred at a larger distance, but within 40 megaparsecs, it would have resembled a short, hard γ-ray burst, suggesting that flares from extragalactic SGRs may form a subclass of such bursts.1 Los Alamos National Laboratory, Los Alamos, NM, 87545, USA 2 NASA/Goddard Space Flight Center, Greenbelt, MD, 20771, USA 3 NASA/Marshall Space Flight Center, NSSTC, XD-12, 320 Sparkman Dr., Huntsville, AL 35805, USA 4 Department of Physics, Ben Gurion University, POB 653, Beer Sheva 84105, Israel 5 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ, Amster-

The missing link: Merging neutron stars naturally produce jet-like structures and can power short Gamma-Ray Bursts

Short Gamma-Ray Bursts (SGRBs) are among the most luminous explosions in the universe, releasing in less than one second the energy emitted by our Galaxy over one year. Despite decades of observations, the nature of their “central-engine” remains unknown. Considering a generic binary of magnetized neutron stars and solving Einstein equations, we show that their merger results in a rapidly spinning black hole surrounded by a hot and highly magnetized torus. Lasting over 35 ms and much longer than previous simulations, our study reveals that magnetohydrodynamical instabilities amplify an initially turbulent magnetic field of 10 12 G to produce an ordered poloidal field of 10 15 G along the black-hole spin-axis, within a half-opening angle of 30 , which may naturally launch a relativistic jet. The broad consistency of our ab-initio calculations with SGRB observations shows that the merger of magnetized neutron stars can provide the basic physical conditions for the central-engine of SGRBs. Subject headings: Gamma-ray burst: general — black hole physics — stars: neutron — gravitational waves — magnetohydrodynamics (MHD) — methods: numerical

Broadband observations of the naked-eye gamma-ray burst GRB 080319B

Long-duration γ-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and so provide a window into the process of black hole formation from the collapse of massive stars. Previous early optical observations of even the most exceptional GRBs (990123 and 030329) lacked both the temporal resolution to probe the optical flash in detail and the accuracy needed to trace the transition from the prompt emission within the outflow to external shocks caused by interaction with the progenitor environment. Here we report observations of the extraordinarily bright prompt optical and γ-ray emission of GRB 080319B that provide diagnostics within seconds of its formation, followed by broadband observations of the afterglow decay that continued for weeks. We show that the prompt emission stems from a single physical region, implying an extremely relativistic outflow that propagates within the narrow inner core of a two-component jet.

Images and spectra from the interior of a relativistic fireball

The power-law decay of gamma-ray burst (GRB) afterglow can be well described by synchrotron emission from a relativistic spherical blast wave, driven by an expanding fireball. We calculate the spectrum and the light curve expected from an adiabatic blast wave which is described by the Blandford-McKee self-similar solution. These calculations include emission from the whole blast wave and not just from the shock front. We provide numerical corrections that can be used to modify simple analytic estimates of such emission. We find that the expected light curve and spectra are flat near the peak. This rules out the interpretation of the sharp optical peak observed in GRB 970508 as the peak of the light curve. We also calculate the observed image of an afterglow. This image could be resolved in future VLBI observations, and its structure could influence microlensing and scintillation. The observed image is ringlike: brighter near the edge and dimmer at the center. The image depends on the observed frequency. The contrast between the edge and the center increases and the ring becomes narrower at higher frequencies.

A new γ-ray burst classification scheme from GRB 060614

Gamma-ray bursts (GRBs) are known to come in two duration classes, separated at ∼2 s. Long-duration bursts originate from star-forming regions in galaxies, have accompanying supernovae when these are near enough to observe and are probably caused by massive-star collapsars. Recent observations show that short-duration bursts originate in regions within their host galaxies that have lower star-formation rates, consistent with binary neutron star or neutron star–black hole mergers. Moreover, although their hosts are predominantly nearby galaxies, no supernovae have been so far associated with short-duration GRBs. Here we report that the bright, nearby GRB 060614 does not fit into either class. Its ∼102-s duration groups it with long-duration GRBs, while its temporal lag and peak luminosity fall entirely within the short-duration GRB subclass. Moreover, very deep optical observations exclude an accompanying supernova, similar to short-duration GRBs. This combination of a long-duration event without an accompanying supernova poses a challenge to both the collapsar and the merging-neutron-star interpretations and opens the door to a new GRB classification scheme that straddles both long- and short-duration bursts.Gamma ray bursts (GRBs) are known to come in two duration classes, separated at {approx}2 s. Long bursts originate from star forming regions in galaxies, have accompanying supernovae (SNe) when near enough to observe and are likely caused by massive-star collapsars. Recent observations show that short bursts originate in regions within their host galaxies with lower star formation rates, consistent with binary neutron star (NS) or NS - black hole (BH) mergers. Moreover, although their hosts are predominantly nearby galaxies, no SNe have been so far associated with short GRBs. We report here on the bright, nearby GRB 060614 that does not fit in either class. Its {approx}102 s duration groups it with long GRBs, while its temporal lag and peak luminosity fall entirely within the short GRB subclass. Moreover, very deep optical observations exclude an accompanying supernova, similar to short GRBs. This combination of a long duration event without accompanying SN poses a challenge to both a collapsar and merging NS interpretation and opens the door on a new GRB classification scheme that straddles both long and short bursts.

Off-Axis Afterglow Emission from Jetted Gamma-Ray Bursts

We calculate gamma-ray burst (GRB) afterglow light curves from a relativistic jet as seen by observers at various viewing angles, θobs, relative to the jet axis. We describe three increasingly more realistic models and compare the resulting light curves. An observer at θobs θ0 sees a rising light curve at early times, peaking when the jet Lorentz factor is ~1/θobs, and approaching that seen by an on-axis observer, at later times. A strong linear polarization (40%) may occur near the peak in the light curve and slowly decay with time. We show that, if GRB jets have a universal energy, then orphan afterglows are detectable up to a maximum offset angle that is independent of the jet initial aperture and thus at a rate proportional to the true GRB rate. We also discuss the implications of the proposed connection between SN 1998bw and GRB 980425.

Synchrotron Self-Absorption in Gamma-Ray Burst Afterglow

Gamma-ray burst (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, νa, and derive an accurate expression for νa. We show that the image is rather homogeneous for ν < νa, as opposed to the bright ring at the outer edge and the dim center, which appear at higher frequencies. We compare the spectra we obtain to radio observations of GRB 970508. 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: E52 = 0.53, e = 0.59, B = 0.014, n1 = 3.0. These estimates different by up to 2 orders of magnitude from the estimates based on an approximate spectrum.