Detection of the Geminga pulsar with MAGIC hints at a power-law tail emission beyond 15 GeV

TL;DR

This paper reports the first detection of pulsed gamma-ray emission from the Geminga pulsar above 15 GeV, revealing a power-law tail that extends beyond previous observations and suggests a transition in emission mechanisms.

Contribution

The study provides the first detection of Geminga pulsar emission above 15 GeV with MAGIC, demonstrating a power-law tail and constraining the spectral cutoff, advancing understanding of pulsar high-energy emission.

Findings

Detection of pulsed emission up to 75 GeV with MAGIC

Spectral extension as a power law with index 5.62

No sub-exponential cutoff at 3.6 sigma significance

Abstract

We report the detection of pulsed gamma-ray emission from the Geminga pulsar (PSR J0633+1746) between $15\,$GeV and $75\,$GeV. This is the first time a middle-aged pulsar has been detected up to these energies. Observations were carried out with the MAGIC telescopes between 2017 and 2019 using the low-energy threshold Sum-Trigger-II system. After quality selection cuts, $\sim 80\,$hours of observational data were used for this analysis. To compare with the emission at lower energies below the sensitivity range of MAGIC, $11$ years of Fermi-LAT data above $100\,$MeV were also analysed. From the two pulses per rotation seen by Fermi-LAT, only the second one, P2, is detected in the MAGIC energy range, with a significance of $6.3\,\sigma$. The spectrum measured by MAGIC is well-represented by a simple power law of spectral index $\Gamma= 5.62\pm0.54$, which smoothly extends the Fermi-LAT spectrum. A joint fit to MAGIC and Fermi-LAT data rules out the existence of a sub-exponential cut-off in the combined energy range at the $3.6\,\sigma$ significance level. The power-law tail emission detected by MAGIC is interpreted as the transition from curvature radiation to Inverse Compton Scattering of particles accelerated in the northern outer gap.