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

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 × 10 −13 TeV cm −2 s −1 for the Geminga pulsar and 3.5 × 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.

Pulsar emission in the very high energy regime

“…Deep upper limits have been obtained by VERITAS for this pulsar above 100 GeV [9]. Sixty-three hours of MAGIC data were analysed by [14], but no VHE pulsed emission was detected. Upper limits for the VHE flux above an energy threshold of 166 GeV have also been obtained by VERITAS for the transitional binary pulsar PSR J1023+0038, with 18.1 hours observational time for the radio MSP state and 8.2 hours for the accretion / low-mass X-ray binary state [32].…”

Ground-based gamma-ray astronomy

“…In addition, the slightly lower significance obtained by HAWC in comparison to Milagro, despite a larger dataset, favors a spectrum harder than the Crab although a Crab-like spectrum cannot be ruled out. It should be noted that no detection was reported by MAGIC and VERITAS despite intense observations but such large sources are extremely hard to detect with small field of view IACTs [20,21]. Two analysis techniques are being implemented by VERITAS which will help in the study of spatially extended gamma-ray emission which may emanate from this region.…”