Detection of Gamma-Rays above 1.5 TeV from Mkn 501

TL;DR

This paper reports the first detection of gamma-rays above 1.5 TeV from Mkn 501 using the HEGRA Cherenkov telescope, providing spectral and flux measurements that suggest similarities to the Crab Nebula's spectrum.

Contribution

First detection of >1.5 TeV gamma-rays from Mkn 501 with HEGRA, including spectral index and flux measurements, advancing TeV gamma-ray astronomy.

Findings

Detected 351 excess gamma-ray events with 5.2 sigma significance.

Measured an average excess rate of 2.4 gamma-rays per hour above 1.5 TeV.

Found Mkn 501's spectrum similar to the Crab Nebula's above 1.5 TeV.

Abstract

A detection of TeV $\gamma$-rays from Mkn 501 is reported, based on observations made between March and August 1996 with the first HEGRA Cherenkov telescope (CT1). From the image analysis, 351 excess candidate $\gamma$-ray events are obtained from the 147 h dataset. The statistical significance of the excess is 5.2 $\sigma$. The average excess rate is $2.4 \pm 0.5$ h$^{-1}$ above the $\approx$ 1.5 TeV threshold of CT1. Under the assumption that the spectrum of Mkn 501 follows a power law we find a differential spectral index of 2.6$\pm0.5$ and obtain a time-averaged integral flux above 1.5 TeV of $2.3 (\pm0.4)_{Stat} (+1.5-0.6)_{Syst} \times 10^{-12}$ cm$^{-2}$s$^{-1}$. Comparison with our near contemporary observations of the Crab Nebula, used as a standard candle to test CT1 after upgrading to a 127 pixel camera, indicates that Mkn 501 has a spectrum similar to that of the Crab Nebula above 1.5 TeV. The integral flux above 1.5 TeV from Mkn 501 is found to have been between 2.2 and 3.6 times smaller than that from the Crab Nebula. HEGRA is the second experiment to have detected Mkn 501 in the TeV range.