The 2012 flare of PG 1553+113 seen with H.E.S.S. and Fermi-LAT

TL;DR

This paper reports on the detection of a VHE gamma-ray flare from PG 1553+113 with H.E.S.S., constrains its redshift using Bayesian methods, and sets new limits on Lorentz Invariance Violation effects related to Quantum Gravity.

Contribution

It introduces a novel Bayesian method to estimate the redshift of PG 1553+113 and a new approach to test for Lorentz Invariance Violation using intra-night variability data.

Findings

Detected a gamma-ray flux increase by a factor of 3 during April 26-27, 2012.

Constrained the redshift of PG 1553+113 using Bayesian statistics.

Set lower limits on Quantum Gravity energy scales for LIV effects.

Abstract

Very high energy (VHE, $E>$100 GeV) $\gamma$-ray flaring activity of the high-frequency peaked BL Lac object \pg\ has been detected by the \hess\ telescopes. The flux of the source increased by a factor of 3 during the nights of 2012 April 26 and 27 with respect to the archival measurements with hint of intra-night variability. No counterpart of this event has been detected in the \fla\ data. This pattern is consistent with VHE $\gamma$ ray flaring being caused by the injection of ultrarelativistic particles, emitting $\gamma$ rays at the highest energies. The dataset offers a unique opportunity to constrain the redshift of this source at \bestz\ using a novel method based on Bayesian statistics. The indication of intra-night variability is used to introduce a novel method to probe for a possible Lorentz Invariance Violation (LIV), and to set limits on the energy scale at which Quantum Gravity (QG) effects causing LIV may arise. For the subluminal case, the derived limits are $\textrm{E}_{\rm QG,1}>4.10\times 10^{17}$ GeV and $\textrm{E}_{\rm QG,2}>2.10\times 10^{10}$ GeV for linear and quadratic LIV effects, respectively.