The Crab Nebula as a standard candle in very high-energy astrophysics

Abstract

The continuum high-energy gamma-ray emission between 1 GeV and 10 5 GeV from the Crab Nebula has been measured for the first time in overlapping energy bands by the Fermi large-area telescope (Fermi/LAT) below ≈100 GeV and by ground-based imaging air Cherenkov telescopes (IACTs) above ≈60 GeV. To follow up on the phenomenological approach suggested by Hillas et al. (1998), the broad band spectral and spatial measurement (from radio to low-energy gamma-rays <1 GeV) is used to extract the shape of the electron spectrum. While this model per construction provides an excellent description of the data at energies <1 GeV, the predicted inverse Compton component matches the combined Fermi/LAT and IACT measurements remarkably well after including all relevant seed photon fields and fitting the average magnetic field to B = (124 ± 6 (stat.) +15 −6 (sys.)) μG. The close match of the resulting broad band inverse Compton component with the combined Fermi/LAT and IACTs data is used to derive instrument specific energy-calibration factors. These factors can be used to combine data from Fermi/LAT and IACTs without suffering from systematic uncertainties on the common energy scale. As a first application of the cross calibration, we derive an upper limit to the diffuse gamma-ray emission between 250 GeV and 1 TeV based upon the combined measurements of Fermi/LAT and the H.E.S.S. groundbased Cherenkov telescopes. Finally, the predictions of the magneto-hydrodynamic flow model of Kennel & Coroniti (1984) are compared to the measured SED.

Limits on the extragalactic background light in theFermiera

“…They have been estimated to be 31% in optical to near infrared and 32-55% in mid to far infrared wavelengths, mainly from the grid spacing and the uncertainties on the absolute energy scale of ground based VHE instruments which is taken to be 15%. Note that Meyer et al (2010) achieved a cross-calibration using the broadband SED of the Crab Nebula between the Fermi-LAT and IACTs by shifting the IACT measurements by ∼5% in energy. As it turns out, additional uncertainties arise from the phenomenological description of the EBL evolution (<4% for a redshift z = 0.2 and <10% for z = 0.5, Raue & Mazin 2008).…”

Gamma-ray pulsars: A gold mine

“…But E > B can occur in a Figure 12. Spectral energy distribution of the Crab nebula from the radio to very high energy γ rays (from [172]). Also shown are the detected flares above 100 MeV, from [173].…”

Constraints on particle acceleration sites in the Crab nebula from relativistic magnetohydrodynamic simulations

“…These might have been injected in the nebula at very early stages, and then reaccelerated by local turbulence (Olmi et al 2014), rather than being continuously injected at the TS, as is instead the case for the higher energy pairs (X-ray particles). The overall synthetic spectrum from one-zone models of the CN is in fact best reproduced if two distinct particle populations are assumed, with a spectral break around the infrared band (Atoyan & Aharonian 1996;Meyer et al 2010). From a theoretical point of view the two scenarios are very different: if radio particles are injected as part of the outflow we expect a much higher pair multiplicity κ, and a lower wind Lorentz factor γ, (Bucciantini et al 2011) than magnetospheric models are able to predict (Hibschman & Arons 2001;Harding & Muslimov 2011;Timokhin & Arons 2013), while the most commonly accepted values of κ ∼ 10 4 and γ ∼ 10 6 (Kennel & Coroniti 1984) are those pertaining two models in which the radio electrons are a relic population of early evolutionary stages.…”