VERITAS observations of exceptionally bright TeV flares from LS I +61$^\circ$ 303

TL;DR

VERITAS observed exceptionally bright TeV gamma-ray flares from LS I +61° 303 in 2014, marking the brightest VHE activity from this source, with short, intense flares around its orbital phase.

Contribution

This study reports the first detection of extremely bright and short TeV flares from LS I +61° 303, expanding understanding of its VHE variability.

Findings

Detected peak fluxes above 25% of Crab Nebula flux

Flares occurred around orbital phase ~0.6 (apastron)

Flares were the brightest ever observed from this source

Abstract

The very-high-energy (VHE; E > 100 GeV) gamma-ray experiment, VERITAS, detected exceptionally bright flares from the high-mass X-ray binary LS I +61$^\circ$ 303 during the period October-December 2014. LS I +61$^\circ$ 303 is a known VHE gamma-ray source, the flux from which varies strongly with the orbital period of ~26.5 days. The maximum VHE flux is found around apastron (orbital phase ~0.6) at a level typically corresponding to 10-15% of the Crab Nebula flux (>300 GeV). During these most recent observations, relatively short (day scale), bright TeV flares were observed from the source around apastron in two orbital cycles (October and November). Both cases exhibited peak fluxes above 25% of the Crab Nebula flux (>300 GeV), making these the brightest VHE flares ever detected from this source. In the last orbital cycle observed (December), the source had returned to its historical level of activity. The results of these VERITAS observations from 2014 will be presented.