Search for VHE gamma-ray emission from Geminga pulsar and nebula with the MAGIC telescopes

TL;DR

This study used MAGIC telescopes and Fermi-LAT data to search for VHE gamma-ray emission from Geminga but found no detection, setting the most stringent upper limits and analyzing spectral properties.

Contribution

First deep VHE gamma-ray observations of Geminga with MAGIC, establishing the most restrictive upper limits and analyzing the spectral cutoff with Fermi-LAT data.

Findings

No significant VHE gamma-ray detection from Geminga.

Fermi-LAT data favor a sub-exponential cutoff over an exponential cutoff.

Extrapolated spectra are below MAGIC upper limits, making detection challenging.

Abstract

The Geminga pulsar, one of the brighest gamma-ray sources, is a promising candidate for emission of very-high-energy (VHE > 100 GeV) pulsed gamma rays. Also, detection of a large nebula have been claimed by water Cherenkov instruments. We performed deep observations of Geminga with the MAGIC telescopes, yielding 63 hours of good-quality data, and searched for emission from the pulsar and pulsar wind nebula. We did not find any significant detection, and derived 95% confidence level upper limits. The resulting upper limits of 5.3 x 10^{-13} TeV cm^{-2} s^{-1} for the Geminga pulsar and 3.5 x 10^{-12} TeV cm^{-2} s^{-1} for the surrounding nebula at 50 GeV are the most constraining ones obtained so far at VHE. To complement the VHE observations, we also analyzed 5 years of Fermi-LAT data from Geminga, finding that the sub-exponential cut-off is preferred over the exponential cut-off that has been typically used in the literature. We also find that, above 10 GeV, the gamma-ray spectra from Geminga can be described with a power law with index softer than 5. The extrapolation of the power-law Fermi-LAT pulsed spectra to VHE goes well below the MAGIC upper limits, indicating that the detection of pulsed emission from Geminga with the current generation of Cherenkov telescopes is very difficult.