The exceptional flare of Mrk 501 in 2014 combined observations with H.E.S.S. and FACT

Abstract

Abstract. The BL Lac type object Mrk 501 was observed at very high energies (E > 100 GeV) in 2014 with the upgraded H.E.S.S. (High Energy Stereoscopic System) phase 2 array. The data collected with the central 28 m telescope allow for a broader energy range extending to lower energies when compared to the one obtained with the four small telescopes alone. A strong flaring event with a flux level comparable to the 1997 historical maximum has been detected as a consequence of target of opportunity observations triggered by alerts from the FACT collaboration. The First G-APD Cherenkov Telescope (FACT) is continuously monitoring bright blazars at TeV energies providing important pre-and post-flare information. For the first time, the data and lightcurves from H.E.S.S. and FACT are compared. These contemporaneous observations allow for a better characterization of the source emission. In a multiwavelength context, more precise correlation studies between VHE and lower energies are possible thanks to the dense sampling of the FACT observations. The hard intrinsic spectrum detected by H.E.S.S. during the flare allows the derivation of strong constraints on the scale of Lorentz invariance violation via the non-detection of EBL opacity modifications and from time-of-flight studies.

Physics of the saturation of particle acceleration in relativistic magnetic reconnection

“…The intrinsic power law spectra of Fermi-LAT blazars have hard power law indices in the range νF ν ∝ ν −0.65 − ν −0.15 (Singal et al 2012). Flaring galaxies have quiescent spectra consistent with these values, but the flaring spectra are typically harder (Abramowski et al 2012;Albert et al 2008;Cologna et al 2017). Thus, most TeV flares are consistent with being produced in reconnection at low σ, or even at moderate σ in extraordinary cases.…”

Improved limits on Lorentz invariance violation from astrophysical gamma-ray sources

“…Only the two leading orders n = 1, 2 are considered hereafter. Best current limits at 2σ confidence level for subluminal signatures of LIV in high-energy gamma rays are E (1) LIV = 9.27 × 10 28 eV [41] and E (2) LIV = 8.7 × 10 20 eV [36]. On their way to Earth, TeV gamma rays interact with the EBL photons creating pairs: γ + γ EBL → e + + e − [42].…”

“…We show the limits for most conservative scenario based on the LIV-favored case. Figure 4 compares the LIV energy scale limits presented in this work with the best limits in the literature: (a) the best limits from spectral analysis of a single TeV source imposed by the Markarian 501 measurements from HESS and FACT [36,37], (b) the best limits from spectral analysis of multiple TeV sources [31], and (c) the best limits from time delay analysis of gamma-ray bursts (GRB) imposed by the GRB090510 measurements from MAGIC [41]. The limits imposed in this work are at least 3 times better than the ones presented in previous works.…”

“…Astroparticles have proven to be a sensitive probe for LIV and its signatures in the photon sector have been searched through arrival time delay, photon splitting, spontaneous emission, shift in the pair production energy threshold and many others effects . In particular, the strongest limits for subluminal signatures of LIV based on the propagation of high-energy gamma rays have been imposed using the energy spectra of TeV gamma rays sources [31,36,37] and the time delay of TeV gamma-rays emitted by gamma-ray bursts (GRBs) [41].…”

“…The modulation of the spectrum is considerably different if the interactions in the propagation are taken to be LI or LIV. Previous works have shown how to extract the effect of the propagation from the measured energy spectrum allowing us to identify the assumption (LI or LIV) which best describes the data [31,36,37]. These analyses have been limited mainly by (a) poor knowledge of the extra-Galactic background light (EBL), (b) large uncertainties in the intrinsic energy spectra functional form, (c) scarce data and (d) not fully optimized analysis procedures.…”