Blazar Alerts with the HAWC Online Flare Monitor

TL;DR

The paper presents the HAWC Observatory's online flare monitor that detects rapid gamma-ray flares from extragalactic sources, enabling prompt alerts and expanding data on extreme flaring events in the VHE band.

Contribution

Introduction of an automated online flare monitor for HAWC that detects and alerts on high-energy gamma-ray flares in real-time.

Findings

Sensitivity to 1-hour flares at 10 times Crab flux

Detection of several 10 Crab level flares in the past decade

High duty cycle enables extensive monitoring of extreme flares

Abstract

The High Altitude Water Cherenkov (HAWC) Observatory monitors the gamma-ray sky in the 100 GeV to 100 TeV energy range with > 95% uptime and unprecedented sensitivity for a survey instrument. The HAWC Collaboration has implemented an online flare monitor that detects episodes of rapid flaring activity from extragalactic very high energy (VHE) sources in the declination band from -26 to 64 degrees. This allows timely alerts to be sent to multiwavelength instruments without human intervention. The preliminary configuration of the online flare monitor achieves sensitivity to flares of at least 1 hour duration that attain an average flux of 10 times that of the Crab Nebula. While flares of this magnitude are not common, several flares reaching the level of 10 Crab have been observed in the VHE band within the past decade. With its survey capabilities and high duty cycle, HAWC will expand the observational data set on these particularly extreme flares. We characterize the sensitivity of the online flare monitor and outline plans for its upcoming deployment.