[Paper Review] Detection of Gamma-Rays above 1.5 TeV from Mkn 501

S. M. Bradbury, T. Deckers+2 more|arXiv (Cornell University)|Dec 5, 1996

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper reports the first detection of TeV gamma-rays from the blazar Mkn 501 above 1.5 TeV using the HEGRA Cherenkov telescope array. With a 5.2σ significance and an integral flux of 2.3 × 10⁻¹² cm⁻²s⁻¹ above 1.5 TeV, the results confirm Mkn 501 as a bright TeV source, with a spectral index of 2.6 ± 0.5, comparable to the Crab Nebula but with a flux 2.2–3.6 times lower.

ABSTRACT

A detection of TeV $γ$-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 $γ$-ray events are obtained from the 147 h dataset. The statistical significance of the excess is 5.2 $σ$. 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 $2.3 (\pm0.4)_{Stat} (+1.5-0.6)_{Syst} imes 10^{-12}$ 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.

Motivation & Objective

• To detect very high-energy gamma-ray emission from the blazar Mkn 501 above 1.5 TeV.
• To measure the spectral properties and flux of Mkn 501 in the TeV energy range.
• To compare the TeV emission of Mkn 501 with the Crab Nebula, used as a standard astrophysical calibrator.
• To validate the performance of the upgraded HEGRA CT1 telescope after camera replacement.

Proposed method

• Observations were conducted using the first HEGRA Cherenkov telescope (CT1) between March and August 1996.
• Image analysis of Cherenkov light patterns from air showers was used to identify gamma-ray candidates.
• A 147-hour dataset was analyzed to extract excess events above the 1.5 TeV threshold.
• Statistical significance of the excess was calculated using standard methods, yielding 5.2σ.
• Spectral index and integral flux were derived assuming a power-law model for the gamma-ray spectrum.
• The Crab Nebula was used as a reference source to calibrate the instrument response and compare flux levels.

Experimental results

Research questions

• RQ1 Is Mkn 501 detectable as a source of TeV gamma-rays above 1.5 TeV?
• RQ2 What is the spectral shape and flux level of Mkn 501 in the TeV band?
• RQ3 How does the TeV flux of Mkn 501 compare to that of the Crab Nebula?
• RQ4 What is the statistical significance of the observed gamma-ray excess above background?
• RQ5 Does the HEGRA CT1 telescope system perform reliably after the 127-pixel camera upgrade?

Key findings

• A total of 351 excess gamma-ray candidate events were detected above the 1.5 TeV threshold with a statistical significance of 5.2σ.
• The average excess gamma-ray rate was measured at 2.4 ± 0.5 h⁻¹ above 1.5 TeV.
• The differential spectral index of Mkn 501 was determined to be 2.6 ± 0.5 for a power-law spectrum.
• The time-averaged integral flux above 1.5 TeV was found to be 2.3 × 10⁻¹² cm⁻²s⁻¹ with 17% statistical and 30% systematic uncertainty.
• The flux from Mkn 501 was found to be between 2.2 and 3.6 times lower than that of the Crab Nebula in the same energy range.
• The spectral shape of Mkn 501 is consistent with that of the Crab Nebula above 1.5 TeV, indicating similar emission mechanisms at high energies.