Constraints on Very High Energy Emission from GRB 130427A

TL;DR

This paper investigates the high-energy gamma-ray emission from GRB 130427A, combining space-based observations with ground-based follow-up to constrain emission models, despite no detection by VERITAS.

Contribution

It provides the first combined analysis of Fermi-LAT and VERITAS data to constrain high-energy emission mechanisms in GRB 130427A.

Findings

VERITAS did not detect VHE gamma rays from GRB 130427A.

Upper limits constrain synchrotron self-Compton emission models.

High-energy emission may be an extension of synchrotron spectrum.

Abstract

Prompt emission from the very fluent and nearby (z=0.34) gamma-ray burst GRB 130427A was detected by several orbiting telescopes and by ground-based, wide-field-of-view optical transient monitors. Apart from the intensity and proximity of this GRB, it is exceptional due to the extremely long-lived high-energy (100 MeV to 100 GeV) gamma-ray emission, which was detected by the Large Area Telescope on the Fermi Gamma-ray Space Telescope for ~70 ks after the initial burst. The persistent, hard-spectrum, high-energy emission suggests that the highest-energy gamma rays may have been produced via synchrotron self-Compton processes though there is also evidence that the high-energy emission may instead be an extension of the synchrotron spectrum. VERITAS, a ground-based imaging atmospheric Cherenkov telescope array, began follow-up observations of GRB 130427A ~71 ks (~20 hr) after the onset of the burst. The GRB was not detected with VERITAS; however, the high elevation of the observations, coupled with the low redshift of the GRB, make VERITAS a very sensitive probe of the emission from GRB 130427A for E > 100 GeV. The non-detection and consequent upper limit derived place constraints on the synchrotron self-Compton model of high-energy gamma-ray emission from this burst.