The Crab supernova remnant has been observed regularly with the stereoscopic system of five imaging air Cerenkov telescopes that was part of the High Energy Gamma Ray Astronomy (HEGRA) experiment. In ...total, close to 400 hr of useful data have been collected from 1997 to 2002. The differential energy spectrum of the combined data set can be approximated by a power law-type energy spectrum: d Phi /dE = Phi sub(0) (E/TeV) Gamma , Phi sub(0) = (2.83 plus or minus 0.04 sub(stat) plus or minus 0.6 sub(sys)) 10 super(-11) photons cm super(-2) s super(-1) TeV super(-1), and Gamma = -2.62 plus or minus 0.02 sub(stat) plus or minus 0.05 sub(sys). The spectrum extends up to energies of 80 TeV and is well matched by model calculations in the framework of inverse Compton scattering of various seed photons in the nebula, including for the first time a recently detected compact emission region at millimeter wavelengths. The observed indications for a gradual steepening of the energy spectrum in data is expected in the inverse Compton emission model. The average magnetic field in the emitting volume is determined to be 161.6 plus or minus 0.8 sub(stat) plus or minus 18 sub(sys) mu G. The presence of protons in the nebula is not required to explain the observed flux, and upper limits on the injected power of protons are calculated to be as low as 20% of the total spin-down luminosity for bulk Lorentz factors of the wind in the range of 10 super(4)-10 super(6). The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 13" to the west of the pulsar, with a systematic uncertainty of 25". No significant shift in the position with energy is observed. The size of the emission region is constrained to be less than 2' at energies between 1 and 10 TeV. Above 30 TeV the size is constrained to be less than 3'. No indication of pulsed emission has been found, and upper limits in differential bins of energy have been calculated reaching typically 1%-3% of the unpulsed component.
Ground-based atmospheric Cherenkov telescopes are proven to be effective instruments for observations of very high-energy (VHE) γ-radiation from celestial objects. For effective use of such technique ...one needs detailed Monte Carlo simulations of γ-ray- and proton/nuclei-induced air showers in the Earth atmosphere. Here we discuss in detail the algorithms used in the numerical code ALTAI developed particularly for the simulations of Cherenkov light emission from air showers of energy below
50
TeV
. The specific scheme of sampling the charged particle transport in the atmosphere allows the performance of very fast and accurate simulations used for interpretation of the VHE γ-ray observations.
The sensitivity of ground-based imaging atmospheric Cherenkov γ-ray observatories depends critically on the primary particle identification methods which are used to retain photon-initiated events ...and suppress the spurious background produced by cosmic rays. We suggest a new discrimination technique which utilizes differences in the fluctuations of the light intensity in the images of showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events for the proposed VERITAS observatory has been used to evaluate the efficiency of the new technique. Analysis has been performed for both a single VERITAS imaging telescope, and a system of these telescopes. We demonstrate that a discrimination efficiency of ⩾1.5–2.0 can be achieved in addition to traditional background rejection methods based on image shape parameters.
The unidentified TeV source in Cygnus is now confirmed by follow-up observations from 2002 with the HEGRA stereoscopic system of Cherenkov Telescopes. Using all data (1999 to 2002) we confirm this ...new source as steady in flux over the four years of data taking, extended with radius 6.2′ (±$1.2^\prime_{\rm stat}$ ± $0.9^\prime_{\rm sys}$) and exhibiting a hard spectrum with photon index -1.9. It is located in the direction of the dense OB stellar association, Cygnus OB2. Its integral flux above energies $E>1$ TeV amounts to ~5% of the Crab assuming a Gaussian profile for the intrinsic source morphology. There is no obvious counterpart at radio, optical nor X-ray energies, leaving TeV J2032+4130 presently unidentified. Observational parameters of this source are updated here and some astrophysical discussion is provided. Also included are upper limits for a number of other interesting sources in the FoV, including the famous microquasar Cygnus X-3.
Deep observation (~113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Čerenkov 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, and ~$0.5^\circ$ north of Cyg X-3. The source centre of gravity is RA $\alpha_{\rm J2000}$: $20^{\rm hr} 32^{\rm m} 07^{\rm s}\pm 9.2^{\rm s}_{\rm stat} \pm2.2^{\rm s}_{\rm sys}$, Dec $\delta_{\rm J2000}$: $+41^\circ 30^\prime 30^{\prime\prime}\pm 2.0^\prime_{\rm stat} \pm 0.4^\prime_{\rm sys}$. The source is steady, has a post-trial significance of +4.6σ, indication for extension with radius $5.6^\prime$ at the ~$3\sigma$ level, and has a differential power-law flux with hard photon index of $-1.9 \pm0.3_{\rm stat}\pm0.3_{\rm sys}$. The integral flux above 1 TeV amounts ~3% 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 particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage γ-ray production via a jet-driven termination shock.
The technique of imaging atmospheric Cherenkov telescopes (IACTs) has proved to be an effective tool to register cosmic γ-rays in the very high energy region. The high detection rate of the IACT ...technique and its capability to reconstruct accurately the air shower parameters make it attractive to use this technique for the study of the mass composition of cosmic rays (CRs). In this article we suggest a new approach to study the CR mass composition in the energy region from 30 TeV/nucleus up to the “knee” region, i.e. up to a few PeV/nucleus, using an array of imaging telescopes of a special architecture. This array consists of telescopes with a relatively small mirror size (∼10 m
2) separated from each other by large distances (∼500 m) and equipped by multi-channel cameras with a modest pixel size (0.3–0.5°) and a sufficiently large viewing angle (6–7°).
Compared to traditional IACT systems (like HEGRA, HESS or VERITAS) the IACT array considered in this study could provide a very large detection area (several km
2 or more). At the same time, it allows an accurate measurement of the energy of CR-induced air showers (the energy resolution ranges within 25–35%) and an effective separation of air showers created by different nuclei. Particularly, it is possible to enrich air showers belonging to the nucleus group assigned for selection up to ∼90% purity at a detection efficiency of 15–20% of such showers.
232 hours of data were accumulated from 1997 to 1999, using the HEGRA Stereoscopic Cherenkov Telescope System to observe the supernova remnant Cassiopeia A. TeV γ-ray emission was detected at the $5 ...\sigma$ level, and a flux of $(5.8 \pm 1.2_{\mathrm{stat}} \pm 1.2_{\mathrm{syst}}) 10^{-9}\ {ph} {m}^{-2} {s}^{-1}$ above 1 TeV was derived. The spectral distribution is consistent with a power law with a differential spectral index of $-2.5 \pm 0.4_{\mathrm{stat}} \pm 0.1_{\mathrm{syst}}$ between 1 and 10 TeV. As this is the first report of the detection of a TeV γ-ray source on the "centi-Crab"scale, we present the analysis in some detail. Implications for the acceleration of cosmic rays depend on the details of the source modeling. We discuss some important aspects in this paper.
The contributions to the trigger rate of an imaging atmospheric Cherenkov telescope and the optimization of the trigger conditions are discussed. Among the trigger options studied are cameras with ...trigger regions of different sizes, triggers requiring a coincidence of 2, 3 and 4 pixels as well as triggers imposing additional conditions on topology of the trigger pixels. A two-pixel trigger based on neighboring trigger pixels is shown to provide full efficiency, at considerably reduced random trigger rates. Numerical examples of effective areas and rates are given based on the parameters of the new HEGRA Cherenkov telescopes. Especially addressed are trigger conditions for the observation of extended sources with angular size ≈ 1°.
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