Multiwavelength View of the Close-by GRB 190829A Sheds Light on Gamma-Ray Burst Physics

Abstract

We monitored the position of the close-by (about 370 Mpc) gamma-ray burst GRB 190829A, which originated from a massive star collapse, through very long baseline interferometry (VLBI) observations with the European VLBI Network and the Very Long Baseline Array, carrying out a total of nine observations between 9 and 117 days after the gamma-ray burst at 5 and 15 GHz, with a typical resolution of a few milliarcseconds. From a state-of-the art analysis of these data, we obtained valuable limits on the source size and expansion rate. The limits are in agreement with the size evolution entailed by a detailed modeling of the multiwavelength light curves with a forward-plus-reverse shock model, which agrees with the observations across almost 18 orders of magnitude in frequency (including the HESS data at TeV photon energies) and more than 4 orders of magnitude in time. Thanks to the multiwavelength, high-cadence coverage of the afterglow, inherent degeneracies in the afterglow model are broken to a large extent, allowing us to capture some unique physical insights; we find a low prompt emission efficiency of ≲10−3, a low fraction of relativistic electrons in the forward shock downstream χ e < 13% (90% credible level), and a rapid decay of the magnetic field in the reverse shock downstream after the shock crossing. While our model assumes an on-axis jet, our VLBI astrometry is not sufficiently tight as to exclude any off-axis viewing angle, but we can exclude the line of sight to have been more than ∼2° away from the border of the gamma-ray-producing region based on compactness arguments.

H.E.S.S. Follow-up Observations of GRB 221009A

“…potentially the brightest ever detected GRB, highlight the distinct character of these two bursts, both in terms of their nonthermal particle acceleration and emission properties. As discussed in Abdalla et al (2021; see also Huang et al 2022b;Salafia et al 2022), an accurate reproduction of themultiwa-velengthobservations of GRB 190829A is challenging within a single-zone SSC framework. Other theoretical models put forward to account for the multiwavelength measurements of GRB 190829A, including external Compton (Zhang et al 2021) or two-zone (Khangulyan et al 2023) models serve to highlight the necessity for high-quality spectral and temporal data of GRBs and their afterglows at all available wavelengths to understand the underlying physical mechanisms at play.…”

The Structure of Gamma Ray Burst Jets

“…The light curve of the reverse shock emission in the Optical (where the peak of the synchrotron spectrum at t pk,RS is expected to lie for "standard" parameters, [234]) and X-rays are expected to display a rapid rise and decay before and after the peak, therefore appearing as a flare. In the radio, the expected decay is slower (as the synchrotron peak moves rapidly to lower frequencies after the peak), with possible late-time bumps [235], even though this critically depends on how rapidly the shock-generated magnetic field decays after the reverse shock has disappeared [236]. The emission as seen by a far off-axis observer may be instead dominated by material moving at a different angle, or more generally consist of a comparable amount of radiation from a broader portion of the jet, resulting in a delayed and smoother light curve (see [229] for examples in short GRBs).…”

“…Most often, the fraction is set to χ e = 1, despite the theoretical expectation being χ e ∼ few × 10 −2 [233,252]. Yet, in some GRB afterglows with broad-band, highcadence datasets, χ e < 1 has been shown to provide a substantially better fit to the data, e.g., [236,253]. χ e of the total shocked ISM electrons, and their energy density is assumed to be a fraction e of the internal energy in the shock downstream (which in a strong shock depends only on the shock velocity/Lorentz factor and on the adiabatic index, being set by shock-jump conditions [254]): the minimum Lorentz factor γ m is entirely determined once the shock Lorentz factor Γ and the χ e , e and p parameters are given.…”

Extragalactic Millimeter Transients in the Era of Next-generation CMB Surveys

“…We scale the RS cooling frequency, and characteristic synchrotron frequency, and RS peak flux to that of the FS at t dec using the relations in Kobayashi & Zhang (2003), using p = 2.5 and fiducial values for the microphysical parameters, ò e = 0.1 and ò B = 0.01, as for the FS. For simplicity, we assume equal electron acceleration efficiency in both RS and FS regions and no additional fireball magnetization (ò e,RS /ò e,FS = 1 and ò B,RS /ò B,FS = 1) or magnetic field decay (which may suppress the RS; e.g., Salafia et al 2022). RS synchrotron self-absorption is not included as it does not have a strong effect on the millimeterband light curves for these parameters.…”