An improved method to search for flares from point sources of ultra-high-energy photons

TL;DR

This paper introduces a novel stacking analysis method combined with photon tagging to detect space-time clustering of ultra-high-energy photon-induced air showers, aiding in identifying cosmic sources and constraining photon fluxes.

Contribution

The paper presents an innovative stacking and photon tagging approach for distinguishing photon events from hadronic background in ultra-high-energy cosmic ray data.

Findings

Effective discrimination between photon and hadron events.

Accurate retrieval of photon event count and flare duration.

Requires only a few events to identify photon flares.

Abstract

Flares produced by certain classes of astrophysical objects may be sources of some ultra-high-energy particles, which, if they are photons, would group into clusters of events correlated in space and time. Identification of such clustering in cosmic-ray data would provide important evidence for possible existence of ultra-high-energy (UHE) photons and could potentially help identify their sources. We present an analysis method to search for space-time clustering of ultra-high-energy extensive air showers, namely the stacking method, which combines a time-clustering algorithm with an unbinned likelihood study. In addition, to enhance the capability to discriminate between signal (photon-initiated events) and background (hadron-initiated) events, we apply a photon tag. This involves using relevant probability distribution functions to classify each event as more likely to be either a photon or a hadron. We demonstrate that the stacking method can effectively distinguish between events initiated by photons and those initiated by hadrons (background). The number of photon events in a data sample, as well as the flare(s) duration can also be retrieved correctly. The stacking method with a photon tag requires only a few events to identify a photon flare. This method can be used to search for the cosmic ray sources and/or improve limits on the fluxes of UHE photons.