J. Prahl

First results on the performance of the HEGRA IACT array

Abstract First results concerning the performance characteristics of the HEGRA IACT array are given based on stereoscopic observations of the Crab Nebula with four telescopes. The system provides a γ-ray energy threshold around 0.5 TeV. The Crab signal demonstrates an angular resolution of about 0.1°. Shape cuts allow to suppress cosmic ray background by almost a factor 100, while maintaining 40% efficiency for γ-rays. The Crab signal is essentially background free. For longer observation times of order 100 h, the system in its present form provides sensitivity to point sources at a level of 3% of the Crab flux. Performance is expected to improve further with the inclusion of the fifth telescope and the implementation of advanced algorithms for shower reconstruction.

An unidentified TeV source in the vicinity of Cygnus OB2

Deep observation (113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air ˇ Cerenkov telescopes has serendipitously revealed a signal positionally inside the core of the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and0:5 north of Cyg X-3. The source centre of gravity is RAJ2000 :2 0 hr 32 m 07 s 9:2 s 2:2 s ,D ecJ2000:+4130 0 30 00 2:0 0 0:4 0. The source is steady, has a post-trial significance of+4.6, indication for extension with radius 5:6 0 at the3 level, and has a dierential power-law flux with hard photon index of 1:9 0:3stat 0:3sys. The integral flux above 1 TeV amounts3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to multi-TeV particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage-ray production via a jet-driven termination shock.

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.

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.

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.

Detection of gamma rays above 1 TeV from the Crab Nebula by the second HEGRA imaging atmospheric Cherenkov telescope at La Palma

Abstract The results of observations of the Crab Nebula by the second HEGRA imaging atmospheric Cherenkov telescope during the period 1994/1995 are presented. The signal, detected at a level of 10 σ , allows one to estimate the flux and the spectral index of the γ -ray energy spectrum above 1 TeV. The analysis, based on the comparison of measurements with comprehensive Monte Carlo simulations, reveals a γ -ray flux F γ (≥ 1 TeV) ⋍ 8 × 10 −12 photons cm −2 s −1 , and indicates a rather steep integral spectrum in the energy region 1–3 TeV with a power-law index α ⋍ 1.6–1.8.

A search for gamma radiation above 24 TeV energy from cosmic point sources

Abstract The HEGRA collaboration has searched for γ-ray emission from 36 potential cosmic point sources with the wide angle air Cerenkov detector AIROBICC which is part of the HEGRA cosmic ray detector complex on La Palma. AIROBICC has a γ-ray energy threshold of around 24 TeV and a mean angular resolution of 0.29 °. None of the sources could be found. Upper limits to their fluxes are given. For the VHE γ-ray emitters, the Crab nebula and Mrk 421, these flux limits are below those extrapolated from lower energy observations. Our lower limit for Mrk 421 might be interpreted either as being due to an energy cut-off in the acceleration mechanism or to the onset of γ-interaction with photons of the universal infrared background.

A search for gamma-ray emission from the Galactic plane in the longitude range between

Using the HEGRA system of imaging atmospheric Cherenkov telescopes, a region of the Galactic plane ( 10 <b< 5 ,3 8 <l< 43) was surveyed for TeV gamma-ray emission, both from point sources and of diuse nature. The region covered includes 15 known pulsars, 6 known supernova remnants (SNR) and one unidentied EGRET source. No evidence for emission from point sources was detected; upper limits are typically below 0.1 Crab units for the flux above 1 TeV. For the diuse gamma-ray flux from the Galactic plane, an upper limit of 6:1 10 15 ph cm 2 s 1 sr 1 MeV 1 was derived under the assumption that the spatial distribution measured by the EGRET instrument extends to the TeV regime. This upper flux limit is a factor of about 1.5 larger than the flux expected from the ensemble of gamma-ray unresolved Galactic cosmic ray sources.

\mathsf{37}^\circ

Tychos supernova remnant (SNR) was observed during 1997 and 1998 with the HEGRA Cerenkov Telescope System in a search for gamma-ray emission at energies above 1 TeV. An analysis of these data, 65 hours in total, resulted in no evidence for TeV gamma-ray emission. The 3 upper limit to the gamma-ray flux (>1 TeV) from Tycho is estimated at 5:7810 13 photons cm 2 s 1 , or 33 milli-Crab. We interpret our upper limit within the framework of the following scenarios: (1) that the observed hard X-ray tail is due to synchrotron emission. A lower limit on the magnetic eld within Tycho may be estimated B 22 G, assuming that the RXTE-detected X-rays were due to synchrotron emission. However, using results from a detailed model of the ASCA emission, a more conservative lower limit B 6 G is derived. (2) The hadronic model of Drury and (3) the more recent time-dependent kinetic theory of Berezhko & Volk. Our upper limit lies within the range of predicted values of both hadronic models, according to uncertainties in physical parameters of Tycho, and shock acceleration details. In the latter case, the model was scaled to suit the parameters of Tycho and re-normalised to account for a simplication of the original model. We nd that we cannot rule out Tycho as a potential contributor at an average level to the Galactic cosmic-ray flux.