Measurement of the Multi‐TeV Gamma‐Ray Flare Spectra of Markarian 421 and Markarian 501

Abstract

The energy spectrum of Markarian 421 in flaring states has been measured from 0.3 to 10 TeV using both small zenith angle and large zenith angle observations with the Whipple Observatory 10 m imaging telescope. The large zenith angle technique is useful for extending spectra to high energies, and the extraction of spectra with this technique is discussed. The resulting spectrum of Markarian 421 is fit reasonably well by a simple power-law:where the first set of errors is statistical and the second systematic. This is in contrast to our recently reported spectrum of Markarian 501 which, over a similar energy range, has substantial curvature. The differences in TeV energy spectra of gamma-ray blazars reflect both the physics of the gamma-ray production mechanism and possibly differential absorption effects at the source or in the intergalactic medium. Since Markarian 421 and Markarian 501 have almost the same redshift (0.031 and 0.033 respectively), the difference in their energy spectra must be intrinsic to the sources and not due to intergalactic absorption, assuming the intergalactic infrared background is uniform.Subject headings: BL Lacertae objects: individual (Markarian 421, Markarian 501) -gamma rays: energy spectrum

VHE γ-ray spectral properties of the blazars Mrk 501 and Mrk 421 from CAT observations in 1997 and 1998

“…This agrees with a recent result of the HEGRA group concerning the 1997-98 period [2]. It is however in contrast with the former behaviour of the source at the time of the 1995-96 flaring period, as observed by the Whipple group, who found γ = 2.54 ± 0.03 stat ± 0.10 sys [6]. Thus, the higher value of the differential spectral index found in this work could come from a correlation between the intensity level and the spectral hardness, like that observed for Mrk 501 in 1997 (see [4] and above).…”

“…1b), shows a significant curvature which is now well confirmed by different ground-based experiments [1,6]. The peak γ-energy is found to lie just above the CAT threshold, and it seems to shift towards higher energies as the flux increases.…”

Temporal and spectral gamma-ray properties of Mkn 421 above 250 GeV from CAT observations between 1996 and 2000

“…Whereas the latter has shown a γ-ray peak lying above the CAT threshold, Mkn 421 exhibited a power-law spectrum in 1998, indicating that current imaging Cherenkov detectors cover the end part of its spectral energy distribution. However, the 2000 time-averaged spectrum shows some indication of curvature, which in fact has been also marginally observed by the Whipple Observatory at the time of the 1995-96 flaring periods (Krennrich et al 1999a). …”

“…The latter parameterization, previously used by the Whipple group for the study of Mkn 421 and Mkn 501 (Krennrich et al 1999a), corresponds to a parabolic law in a log(νF (ν)) vs. log(ν) representation, where νF (ν) ≡ E 2 dφ dE and E = hν. The relevance of H pl with respect to H cs is estimated from the likelihood ratio of the two hypotheses, which is defined as λ = 2 log L cs L pl : it behaves (asymptotically) like a χ 2 with one degree of freedom and permits the search for possible spectral curvature.…”

Hadronic correlated flares from Mrk 501

“…Simultaneously, the Whipple and HEGRA Cherenkov telescopes observed correlated flares in the TeV band [2]. Integrating over a larger time window, both found that the high energy emission during this period has extended on average up to at least 20 TeV [3], but with a significant curvature consistent with an exponential cutoff at ∼ 5 TeV. The most common way to explain this emission is the synchrotron-self Compton (SSC) model, which naturally expects correlated variability because both the X-ray and the TeV component are radiated by the same population of particles (electrons).…”