H. Krawczynski

The optical system of the H.E.S.S. imaging atmospheric Cherenkov telescopes. Part I: layout and components of the system

Abstract H.E.S.S.––the high energy stereoscopic system––is a new system of large imaging atmospheric Cherenkov telescopes, with about 100 m2 mirror area for each of four telescopes, and photomultiplier cameras with a large field of view (5°) and small pixels (0.16°). The dish and reflector are designed to provide good imaging properties over the full field of view, combined with mechanical stability. The paper describes the design criteria and specifications of the system, and the individual components––dish, mirrors, and Winston cones––as well as their characteristics. The optical performance of the telescope as a whole is the subject of a companion paper.

The Energy Spectrum of TeV Gamma Rays from the Crab Nebula as Measured by the HEGRA System of Imaging Air Cerenkov Telescopes

The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray Astronomy) stereoscopic system of imaging air Cerenkov telescopes (IACTs) for a total of ~200 hr during two observational campaigns: from 1997 September to 1998 March and from 1998 August to 1999 April. The recent detailed studies of system performance give an energy threshold and an energy resolution for γ-rays of 500 GeV and ~18%, respectively. The Crab energy spectrum was measured with the HEGRA IACT system in a very broad energy range up to 20 TeV, using observations at zenith angles up to 65°. The Crab data can be fitted in the energy range from 1 to 20 TeV by a simple power law, which yields dJγ/dE = (2.79 ± 0.02 ± 0.5) × 10-7(E/1 TeV)-2.59±0.03±0.05 photons m-2 s-1 TeV-1. The Crab Nebula energy spectrum, as measured with the HEGRA IACT system, agrees within 15% in the absolute scale and within 0.1 units in the power-law index with the latest measurements by the Whipple, CANGAROO, and CAT groups, consistent within the statistical and systematic errors quoted by the experiments. The pure power-law spectrum of TeV γ-rays from the Crab Nebula constrains the physics parameters of the nebula environment as well as the models of photon emission.

Complex Spectral Variability from Intensive Multiwavelength Monitoring of Markarian 421 in 1998

We conducted a multifrequency campaign for the TeV blazar Markarian 421 in 1998 April. The campaign started from a pronounced high-amplitude flare recorded by BeppoSAX and Whipple; the ASCA observation started 3 days later. In the X-ray data, we detected multiple flares, occurring on timescales of about 1 day. ASCA data clearly reveal spectral variability. The comparison of the data from ASCA, the Extreme Ultraviolet Explorer, and the Rossi X-Ray Timing Explorer indicates that the variability amplitudes in the low-energy synchrotron component are larger at higher photon energies. In TeV γ-rays, large intraday variations—which were correlated with the X-ray flux—were observed when results from three Cerenkov telescopes were combined. The rms variability of TeV γ-rays was similar to that observed in hard X-rays, above 10 keV. The X-ray light curve reveals flares that are almost symmetric for most cases, implying that the dominant timescale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of 7 days indicates that the characteristic timescale is ~0.5 days. The analysis of ASCA light curves in various energy bands appears to show both soft (positive) and hard (negative) lags. These may not be real, as systematic effects could also produce these lags, which are all much smaller than an orbit. If the lags of both signs are real, these imply that the particle acceleration and X-ray cooling timescales are similar.

SIMULTANEOUS X-RAY AND TeV GAMMA-RAY OBSERVATION OF THE TeV BLAZAR MARKARIAN 421 DURING 2000 FEBRUARY AND MAY

We present the results of simultaneous observations of the TeV blazar Markarian 421 at X-ray and TeV gamma-ray energies with the Rossi X-Ray T iming Explorer and the stereoscopic Cerenkov telescope system of the High Energy Gamma-Ray Astronomy (HEGRA) experiment, respectively. The source was monitored from 2000 February 2 to 16 and from 2000 May 3 to 8. In both energy bands several —ares with very rapid —ux variability were observed. In the X-ray band, the —ux increased and decreased with e-folding times as short as about 5 hr. The 3¨20 keV photon index varied between values of 2.2 and 2.9. For —ve pointings the data show statistically signi—cant evidence for spectral curvature. The photon index varied substantially on very short timescales: on 2000 February 11 it hardened within 1.6 hr by *! \ 0.18, and on February 14 it softened within 1.6 hr by *! \ 0.2. The TeV observations of February 7/8 showed statistically signi—cant evidence for substantial TeV —ux variability on a 30 minute timescale. The TeV energy spectrum averaged over all the observations of the campaign shows a similar steep slope as in earlier HEGRA observations: TeV)~! with dN/dE \ N 0 (E/1 N 0 \ (25 ^ 1 stat ) ] 10~12 photons cm~2 s~1 TeV~1 and Within statistical errors no evidence for a curvature of ! \ 2.94 ^ 0.06 stat . the TeV energy spectrum is found. We show the results of modeling the data with a time-dependent homogeneous synchrotron self-Compton model. The X-ray and TeV gamma-ray emission strengths and energy spectra together with the rapid —ux variability strongly suggest that the emission volume is approaching the observer with a Doppler factor of 50 or higher. The diUerent —ux variability timescales observed at X-rays and TeV gamma rays indicate that a more detailed analysis will require inhomogeneous models with several emission zones. Subject headings: BL Lacertae objects: individual (Markarian 421) ¨ galaxies: jets ¨ gamma rays: observations

Correlated Intense X-Ray and TeV Activity of Markarian 501 in 1998 June

We present exactly simultaneous X-ray and TeV monitoring with RXTE and HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial period of very low flux at both wavelengths, the source underwent a remarkable flare in the TeV and X-ray energy bands, lasting for about 6 days and with a larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV flare, rapid TeV flux variability on subhour timescales is found. Large spectral variations are observed at X-rays, with the 3-20 keV photon index of a pure power-law continuum flattening from ? = 2.3 to ? = 1.8 on a timescale of 2-3 days. This implies that during the maximum of the TeV activity the synchrotron peak shifted to energies 50 keV, a behavior similar to that observed during the longer lasting, more intense flare in 1997 April. The TeV spectrum during the flare is described by a power law with photon index ? = 1.9 and an exponential cutoff at ~4 TeV; an indication for spectral softening during the flare decay is observed in the TeV hardness ratios. Our results generally support a scenario in which the TeV photons are emitted via inverse Compton scattering of ambient seed photons by the same electron population responsible for the synchrotron X-rays. The simultaneous spectral energy distributions can be fit with a one-zone synchrotron self-Compton model assuming a substantial increase of the magnetic field and the electron energy by factors of 3 and 10, respectively.

Comparison of techniques to reconstruct VHE gamma-ray showers from multiple stereoscopic Cherenkov images

Abstract For air showers observed simultaneously by more than two imaging atmospheric Cherenkov telescopes, the shower geometry is overconstrained by the images and image information should be combined taking into account the quality of the images. Different algorithms are discussed and tested experimentally using data obtained from observations of Mkn 501 with the HEGRA IACT system. Most of these algorithms provide an estimate of the accuracy of the reconstruction of shower geometry on an event-by-event basis, allowing, e.g., to select higher-quality subsamples for precision measurements.

Observations of H1426+428 with HEGRA - Observations in 2002 and reanalysis of 1999&2000 data

The HEGRA system of imaging air Cherenkov telescopes has been used to observe the BL Lac object H1426+428 (z= 0:129) for 217.5 hours in 2002. In this data set alone, the source is detected at a confidence level of 5:3, confirming this object as a TeV source. The overall flux level during the observations in 2002 is found to be a factor of 2.5 lower than during the previous observations by HEGRA in 1999 and 2000. A new spectral analysis has been carried out, improving the signal-to-noise ratio at the expense of a slightly increased systematic uncertainty and reducing the relative energy resolution to E=E 12% over a wide range of energies. The new method has also been applied to the previously published data set taken in 1999 and 2000, confirming the earlier claim of a flattening of the energy spectrum between 1 and 5 TeV. The data set taken in 2002 shows again a signal at energies above 1 TeV. We combine the energy spectra as determined by the CAT and VERITAS groups with our reanalyzed result of the 1999 and 2000 data set and apply a correction to account for eects of absorption of high energy photons on extragalactic background light in the optical to mid infrared band. The shape of the inferred source spectrum is mostly sensitive to the characteristics of the extragalactic background light between wavelengths of 1 and 15 m.

Observations of 54 Active Galactic Nuclei with the HEGRA system of Cherenkov telescopes

A sample of 54 selected Active Galactic Nuclei (AGN) has been observed with the HEGRA stereoscopic system of Cherenkov Telescopes between 1996 and 2002 in the TeV energy regime. The observations were motivated by the positive results obtained for Mkn 421 and Mkn 501. The distances of the selected objects vary over a large range of redshifts between z = 0.004 and z = 0.7. Among the observed AGN are the now-established TeV-emitting BL Lac type objects H 1426+428 and 1ES 1959+650. Furthermore the BL Lac object 1ES 2344+514 and the radio galaxy M 87 show evidence for a signal on a4 σ level. The observation of 1ES 2344+514 together with the Whipple results firmly establishes this AGN as a TeV source. Several objects (PKS 2155-304, BL Lacertae, 3C 066A) that have been claimed as TeV γ-ray emitters by other groups are included in this data sample but could not be confirmed using data analysed here. The upper limits from several AGN included in this analysis are compared with predictions in the frame-work of SSC models.

The TeV Energy Spectrum of Markarian 501 Measured with the Stereoscopic Telescope System of HEGRA during 1998 and 1999

During 1997, the BL Lac object Mrk 501 went into an extraordinary state of high X-ray and TeV gamma-ray activity, lasting more than 6 months. In this paper we report on the TeV emission characteristics of the source in the subsequent years of 1998 and 1999 as measured with the stereoscopic Cherenkov telescope system of the High-Energy Gamma-Ray Astronomy (HEGRA; La Palma, Canary Islands). Our observations reveal a 1998-1999 mean emission level at 1 TeV of of the flux of the Crab Nebula, a factor of 10 lower than during the year of 1997. A data set of 122 observation hours with the HEGRA telescope system makes it possible to assess for the first time the Mrk 501 TeV energy spectrum for a mean flux level substantially below that of the Crab Nebula with reasonable statistical accuracy. Excluding the data of a strong flare, we find evidence that the 1998-1999 low-flux spectrum is substantially softer (by 0.44 ± 0.1stat in spectral index) than the 1997 time-averaged spectrum. The 500 GeV to 10 TeV energy spectrum can well be described by a power-law model with exponential cutoff: dN/dE ∝ E-α exp(-E/E0), with α = 2.31 ± 0.22stat and E0 = 5.1()stat TeV. Within statistical accuracy, a pure power-law model also gives an acceptable fit to the data: dN/dE ∝ E-Γ, with Γ = 2.76 ± 0.08stat. After presenting the 1998-1999 TeV characteristics of the source, we discuss the implications of the results.

Reanalysis of the high energy cutoff of the 1997 Mkn 501 TeV energy spectrum

Data taken with the HEGRA system of imaging atmospheric Cherenkov telescopes during the 1997 flares of Markarian 501 (Mkn 501) are reanalyzed using an algorithm providing improved energy resolution. A resolution of 10% to 12% is obtained by accounting for the variation of the Cherenkov light yield with the height of the shower maximum in the atmosphere. The improved energy resolution is particularly relevant for the study of the high-energy cutoff in the spectrum, which might be caused by interactions with the intergalactic infrared background radiation. The reanalysis presented here confirms the results obtained in the previous analysis, but hints a steeper slope of the spectrum in the region around 20 TeV.