Measurement of the Crab Nebula spectrum over three decades in energy with the MAGIC telescopes

TL;DR

This study presents a precise measurement of the Crab Nebula's gamma-ray spectrum from 50 GeV to 30 TeV using MAGIC telescopes, revealing a broad Inverse Compton peak and challenging simple magnetic field models.

Contribution

It provides the most accurate spectral energy distribution of the Crab Nebula over three decades, with a refined peak position and evaluation of theoretical models against new VHE data.

Findings

The Inverse Compton peak is at (53 ± 3 stat + 31 syst - 13 syst) GeV.

A modified log-parabola function better fits the spectrum than a simple log-parabola.

Constant B-field models struggle to reproduce the broad IC peak observed.

Abstract

The MAGIC stereoscopic system collected 69 hours of Crab Nebula data between October 2009 and April 2011. Analysis of this data sample using the latest improvements in the MAGIC stereoscopic software provided an unprecedented precision of spectral and night-by-night light curve determination at gamma rays. We derived a differential spectrum with a single instrument from 50 GeV up to almost 30 TeV with 5 bins per energy decade. At low energies, MAGIC results, combined with Fermi-LAT data, show a flat and broad Inverse Compton peak. The overall fit to the data between 1 GeV and 30 TeV is not well described by a log-parabola function. We find that a modified log-parabola function with an exponent of 2.5 instead of 2 provides a good description of the data ($\chi^2=35/26$). Using systematic uncertainties of red the MAGIC and Fermi-LAT measurements we determine the position of the Inverse Compton peak to be at (53 $\pm$ 3stat + 31syst -13syst) GeV, which is the most precise estimation up to date and is dominated by the systematic effects. There is no hint of the integral flux variability on daily scales at energies above 300 GeV when systematic uncertainties are included in the flux measurement. We consider three state- of-the-art theoretical models to describe the overall spectral energy distribution of the Crab Nebula. The constant B-field model cannot satisfactorily reproduce the VHE spectral measurements presented in this work, having particular difficulty reproducing the broadness of the observed IC peak. Most probably this implies that the assumption of the homogeneity of the magnetic field inside the nebula is incorrect. On the other hand, the time-dependent 1D spectral model provides a good fit of the new VHE results when considering a 80 {\mu}G magnetic field. However, it fails to match the data when including the morphology of the nebula at lower wavelengths.