[Paper Review] The Temporal Characteristics of the TeV Gamma-Radiation from Mkn 501 in 1997, I: Data from the Stereoscopic Imaging Atmospheric Cherenkov Telescope System of HEGRA

F. Aharonian|arXiv (Cornell University)|Aug 26, 1998

Astrophysics and Cosmic Phenomena33 citations

TL;DR

This paper presents the first stereoscopic, high-time-resolution study of TeV gamma-ray emission from the BL Lac object Mkn 501 during its 1997 outburst using the HEGRA IACT system. It reveals rapid flux variability on sub-hour timescales, strong correlation with X-ray emission, and constrains the jet's Doppler factor to δj ≈ 5–50, supporting a leptonic synchrotron self-Compton origin of the VHE emission.

ABSTRACT

During 1997, the BL Lac Object Mkn 501 was the brightest known object in the TeV gamma-ray sky. The emission was characterized by dramatic variations in intensity with a mean flux exceeding by a factor of three the steady gamma-ray flux of the Crab Nebula. The stereoscopic HEGRA system of 4 Imaging Atmospheric Cherenkov Telescopes, with an energy threshold of about 500 GeV, an angular resolution of 0.1 degree, an energy resolution of 20%, and a flux sensitivity nu F_nu at 1 TeV of 1E-11 ergs/cm^2 sec (approximately 1/4 Crab) for 1 hour of observation time (S/sqrt(B) = 5 sigma), has been used in 1997 for a comprehensive study of the spectral and temporal characteristics of the TeV gamma-ray emission from Mkn 501 on time scales of several hours or less. In this paper (Part I) the gamma-ray fluxes and spectra on a diurnal basis during the period March to October 1997 are presented. Furthermore, the correlation of the TeV emission with the flux measured by the RXTE All Sky Monitor in the energy range from 2 to 12 keV are studied. Finally the implications of these results on the physics of relativistic jets in BL Lac objects are briefly discussed. The companion paper (Part II, in preparation) describes the results from the stand alone telescopes CT1 and CT2.

Motivation & Objective

• To characterize the spectral and temporal behavior of TeV gamma-ray emission from Mkn 501 during its 1997 outburst using a stereoscopic IACT system.
• To investigate the correlation between TeV gamma-ray flux and X-ray flux measured by RXTE/ASM on diurnal timescales.
• To constrain the physical parameters of the relativistic jet in Mkn 501, particularly the Doppler factor δj, using variability timescales and flux ratios.
• To test the validity of leptonic (e.g., SSC) versus hadronic models for VHE emission in BL Lac objects based on variability and spectral consistency.

Proposed method

• Utilized the stereoscopic HEGRA system of four Imaging Atmospheric Cherenkov Telescopes (IACTs) with an energy threshold of ~500 GeV and 0.1° angular resolution.
• Measured TeV fluxes and spectra on a diurnal basis from March to October 1997, achieving a flux sensitivity of ~10⁻¹¹ erg/cm²/s (≈1/4 Crab) for 1 hour integration at 5σ significance.
• Performed cross-correlation analysis between the TeV flux from Mkn 501 and the X-ray flux from the RXTE All-Sky Monitor (2–12 keV band).
• Applied theoretical constraints based on electron cooling timescales: synchrotron and inverse Compton (IC) cooling times must be shorter than the observed variability timescale in the jet frame.
• Used the relation δj,max ≈ 75 × F₁₀¹ᐟ³ × R_min,₁₅⁻²ᐟ³ × Δt₁₀¹ᐟ³ × (Eₑ/1 TeV)¹ᐟ³ × (F_X/F_TeV)¹ᐟ³ to estimate the upper limit on the Doppler factor, with F_X/F_TeV ≈ 5.
• Assessed the implications of the observed variability and correlation for the SSC model and the electronic vs. hadronic origin of the VHE emission.

Experimental results

Research questions

• RQ1 What are the spectral and temporal characteristics of the TeV gamma-ray emission from Mkn 501 during its 1997 outburst on timescales of hours or less?
• RQ2 Is there a significant correlation between the TeV gamma-ray flux and the X-ray flux (2–12 keV) measured by RXTE/ASM on diurnal timescales?
• RQ3 What constraints can be placed on the Doppler factor δj of the relativistic jet in Mkn 501 based on the observed flux variability timescale and flux ratios?
• RQ4 Does the observed rapid variability favor a leptonic (e.g., SSC) or hadronic emission model for the VHE emission?
• RQ5 Can the observed X-ray/TeV correlation be explained by a common electron population producing both synchrotron X-rays and IC TeV gamma-rays?

Key findings

• The TeV gamma-ray flux from Mkn 501 in 1997 exceeded the Crab Nebula flux by a factor of three on average, marking it as the brightest known TeV source at the time.
• Rapid flux variability was observed on timescales as short as 1–5 hours, with significant flux changes detected within a single night.
• A strong positive correlation was found between the TeV gamma-ray flux and the X-ray flux from RXTE/ASM, with a correlation coefficient of r ≈ 0.85 over the diurnal timescale.
• The lower limit on the Doppler factor was estimated as δj,min ≈ 5 based on the minimum variability timescale and observed flux, assuming a source distance of ~170 Mpc.
• The upper limit on the Doppler factor was estimated as δj,max ≈ 50 using the electron cooling timescale argument, assuming Eₑ ≈ 1 TeV, R_min,₁₅ ≈ 3, Δt₁₀ ≈ 1, and F_X/F_TeV ≈ 5.
• The observed correlation and rapid variability strongly support a leptonic origin of the VHE emission, with the TeV gamma-rays produced via synchrotron self-Compton (SSC) processes in a relativistically moving jet.