Abstract
Abstract The blazar Mrk 501 (z = 0.034) was observed at very-high-energy (VHE, E ≳ 100 GeV) gamma-ray wavelengths during a bright flare on the night of 2014 June 23–24 (MJD 56832) with the H.E.S.S. phase-II array of Cherenkov telescopes. Data taken that night by H.E.S.S. at large zenith angle reveal an exceptional number of gamma-ray photons at multi-TeV energies, with rapid flux variability and an energy coverage extending significantly up to 20 TeV. This data set is used to constrain Lorentz invariance violation (LIV) using two independent channels: a temporal approach considers the possibility of an energy dependence in the arrival time of gamma-rays, whereas a spectral approach considers the possibility of modifications to the interaction of VHE gamma-rays with extragalactic background light (EBL) photons. The non-detection of energy-dependent time delays and the non-observation of deviations between the measured spectrum and that of a supposed power-law intrinsic spectrum with standard EBL attenuation are used independently to derive strong constraints on the energy scale of LIV (E QG) in the subluminal scenario for linear and quadratic perturbations in the dispersion relation of photons. For the case of linear perturbations, the 95% confidence level limits obtained are E QG,1 > 3.6 × 1017 GeV using the temporal approach and E QG,1 > 2.6 × 1019 GeV using the spectral approach. For the case of quadratic perturbations, the limits obtained are E QG,2 > 8.5 × 1010 GeV using the temporal approach and E QG,2 > 7.8 × 1011 GeV using the spectral approach.
Highlights
Blazars are commonly considered to be active galactic nuclei with jets closely aligned with the line of sight to the observer (Urry & Padovani 1995)
The detailed discussion on astrophysical implications of this rapid variability relative to the long-term activity of Markarian 501 (Mrk 501) seen in γ-rays by H.E.S.S. along with First G-APD Cherenkov Telescope (FACT) and Fermi-LAT is left for a dedicated forthcoming paper
The maximum likelihood (ML) method for the extraction of energy-dependent time lags was first proposed in Martínez & Errando (2009) and extensively applied for Lorentz invariance violation (LIV) analyses in H.E.S.S. with the flares of PKS 2155-304 (Abramowski et al 2011) and PG 1553+113 (Abramowski et al 2015)
Summary
Blazars are commonly considered to be active galactic nuclei with jets closely aligned with the line of sight to the observer (Urry & Padovani 1995). A non-infinite value of EQG in Equation (1) would induce non-negligible observational effects It would cause an energydependent velocity of photons in vacuum that in turn would translate into an energy-dependent time delay in the arrival time of γ-rays traveling over astrophysical distances (AmelinoCamelia et al 1998; Ellis & Mavromatos 2013). The constraints on the quadratic term remain several orders of magnitude below the Planck scale and will continue to be a challenge for future studies Both the temporal and spectral LIV effects can be used to put competitive constraints on EQG using VHE γ-ray observations of a blazar flare, given certain conditions on the energy coverage and distance to the source. This configuration (H.E.S.S. phase-II) can be triggered by events detected either by CT5 alone
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