The effect of pair cascades on the high-energy spectral cut-off in gamma-ray bursts

Gill, Ramandeep; Granot, Jonathan

doi:10.1093/mnrasl/slx199

Cited by 25 publications

(27 citation statements)

References 33 publications

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“…In case the structure of the jet can be modeled as having sharp edges, then θ c = θ j , where θ j is the half-opening angle of the jet. In these GRBs, compactness arguments (e.g., Lithwick & Sari 2001;Gill & Granot 2018b) dictate that Γ > 100, otherwise opacity due to γγ-annihilation (i.e., γγ → e − + e + ) would prevent the radiation of gamma- ray photons. Modeling of the radio/optical/X-rays afterglow emission from GRB 170817A, both using semi-analytical models (e.g., Gill & Granot 2018a;Lamb & Kobayashi 2018;Troja et al 2018), as well as numerical simulations (e.g., Granot et al 2018;Lazzati et al 2018;Margutti et al 2018;Kathirgamaraju et al 2019) of structured jets, has revealed that the line-of-sight is significantly off-axis, with 20 • θ obs 28 • and θ c ∼ 5 • .…”

Section: Collapse Time From Em Delaymentioning

confidence: 99%

When Did the Remnant of GW170817 Collapse to a Black Hole?

Gill

Nathanail

Rezzolla

2019

ApJ

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121

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The main hard pulse of prompt gamma-ray emission in GRB 170817A had a duration of ∼ 0.5 s and its onset was delayed with respect to the gravitational-wave chirp signal by t del ≈ 1.74 s. Detailed follow-up of the subsequent broadband kilonova emission revealed a two-component ejecta -a lanthanide-poor ejecta with mass M ej,blue ≈ 0.025 M that powered the early but rapidly fading blue emission and a lanthanide-rich ejecta with mass M ej,red ≈ 0.04 M that powered the longer lasting redder emission. Both the prompt gamma-ray onset delay and the existence of the blue ejecta with modest electron fraction, 0.2 Y e 0.3, can be explained if the collapse to a black hole was delayed by the formation of a hypermassive neutron star (HMNS). Here, we determine the survival time of the merger remnant by combining two different constraints, namely, the time needed to produce the requisite blue-ejecta mass and that necessary for the relativistic jet to bore its way out of the expanding ejecta. In this way, we determine that the remnant of GW170817 must have collapsed to a black hole after t coll = 0.98 +0.31 −0.26 s. We also discuss how future detections and the delays between the gravitational and electromagnetic emissions can be used to constrain the properties of the merged object.

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Section: Collapse Time From Em Delaymentioning

confidence: 99%

When Did the Remnant of GW170817 Collapse to a Black Hole?

Gill

Nathanail

Rezzolla

2019

ApJ

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121

111

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“…However, the recent short GRB170817A associated with the NS-NS merger gravitational wave event GW170817 provides stricter and more robust constraints on the value of b. Detailed theoretical modeling (Gill & Granot 2018) together with the very elaborate afterglow observations from this event, and in particular the detection of super-luminal motion of the radio flux centroid with an apparent velocity of β app = 4.1 ± 0.5 (Mooley et al 2018), clearly imply that the late time afterglow emission arises primarily from near the energetic narrow core of a relativistic jet viewed from well outside of its core. The jet structure and viewing angle implied by these observations result in clear predictions for the afterglow linear polarization (Gill & Granot 2018).…”

Section: Likely Origin and Configuration Of The Magnetic Fieldmentioning

confidence: 99%

Linear polarization in gamma-ray burst prompt emission

Gill

Granot

Kumar

2019

Monthly Notices of the Royal Astronomical Society

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Despite being hard to measure, GRB prompt γ-ray emission polarization is a valuable probe of the dominant emission mechanism and the GRB outflow's composition and angular structure. During the prompt emission the GRB outflow is ultra-relativistic with Lorentz factors Γ 1. We describe in detail the linear polarization properties of various emission mechanisms: synchrotron radiation from different magnetic field structures (ordered: toroidal B tor or radial B , and random: normal to the radial direction B ⊥ ), Compton drag, and photospheric emission. We calculate the polarization for different GRB jet angular structures (e.g. top-hat, Gaussian, power-law) and viewing angles θ obs . Synchrotron with B ⊥ can produce large polarizations, up to 25% Π 45%, for a top-hat jet but only for lines of sight just outside (θ obs −θ j ∼ 1/Γ) the jet's sharp edge at θ = θ j . The same also holds for Compton drag, albeit with a slightly higher overall Π. Moreover, we demonstrate how Γ-variations during the GRB or smoother jet edges (on angular scales 0.5/Γ) would significantly reduce Π. We construct a semi-analytic model for non-dissipative photospheric emission from structured jets. Such emission can produce up to Π 15% with reasonably high fluences, but this requires steep gradients in Γ(θ). A polarization of 50% Π 65% can robustly be produced only by synchrotron emission from a transverse magnetic field ordered on angles 1/Γ around our line of sight (like a global toroidal field, B tor , for 1/Γ < θ obs < θ j ). Therefore, such a model would be strongly favored even by a single secure measurement within this range. We find that such a model would also be favored if Π 20% is measured in most GRBs within a large enough sample, by deriving the polarization distribution for our different emission and jet models.

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“…However, it is only true in the case when the effect of pair production is ignored. In a fully time-dependant model this could be 1.5 -2 times lower (Gill & Granot 2018).…”

Section: Joint Gbm-lat Analysismentioning

confidence: 96%

MAGICal GRB 190114C: Implications of cutoff in the spectrum at sub-$GeV$ energies

Chand,

Pal,

Banerjee

et al. 2019

Preprint

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GRB 190114C is a famous Gamma-Ray Burst (GRB) due to its detection at sub-T eV energies by MAGIC, seen at redshift z = 0.42. This burst is one of the brightest GRB detected by Fermi . We present a detailed analysis of GRB 190114C prompt emission, using the two Fermi detectors: GBM and LAT. The LAT low energy events (LLE) data is also considered. A joint GBM-LAT analysis reveals a sub-GeV spectral cutoff. A similar high energy cutoff was reported in GRB 160509A and GRB 100724B earlier, and a handful of other sources. The cutoff can be explained by the intrinsic opacity due to pair production within the emitting region. Such morphology in these GRBs suggests that they belong to one specific class having a similar source of the radiation mechanism. GRB 190114C shows a transition from non-thermal to a quasi-thermal-like spectrum along with radiation due to external shock. From spectrum analysis and Lorentz factor evolution from the trigger time to late emission, considering the fact that sub-T eV photons are detected in MAGIC, we are able to draw an emission mechanism picture, where the prompt emission spectrum is more consistent with spectrum via photospheric dissipation with presence of external shock emission simultaneously.

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The effect of pair cascades on the high-energy spectral cut-off in gamma-ray bursts

Cited by 25 publications

References 33 publications

When Did the Remnant of GW170817 Collapse to a Black Hole?

When Did the Remnant of GW170817 Collapse to a Black Hole?

Linear polarization in gamma-ray burst prompt emission

MAGICal GRB 190114C: Implications of cutoff in the spectrum at sub-$GeV$ energies

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