Tycho's supernova remnant (SNR) was observed during 1997 and 1998 with the HEGRA Čerenkov 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 3σ upper limit to the gamma-ray flux (>1 TeV) from Tycho is estimated at $5.78\times 10^{-13}$ 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 field within Tycho may be estimated $B\geq22$ μG, 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 $B\geq6$ μG is derived. (2) The hadronic model of Drury and (3) the more recent time-dependent kinetic theory of Berezhko & Völk. 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 simplification of the original model. We find that we cannot rule out Tycho as a potential contributor at an average level to the Galactic cosmic-ray flux.
Search for a TeV gamma-ray halo of Mkn 501 Aharonian, F. A.; Akhperjanian, A. G.; Barrio, J. A. ...
Astronomy and astrophysics (Berlin),
02/2001, Letnik:
366, Številka:
3
Journal Article
Recenzirano
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For distant extragalactic sources of gamma-rays in the PeV (1015 eV) energy range, interactions of the gamma rays with intergalactic diffuse radiation fields will initiate a pair cascade. Depending ...on the magnetic fields in the vicinity of the source, the cascade can either result in an isotropic halo around an initially beamed source, or remain more or less collimated. Data recorded by the HEGRA system of imaging atmospheric Cherenkov telescopes are used to derive limits on the halo flux from the AGN Mrk 501. This is achieved by comparing the angular distribution of TeV gamma-rays during the 1997 burst phase -where direct photons should dominate -with the distribution during the 1998/99 quiescent state, where a steady-state halo contribution should be most pronounced. The results depend on the assumptions concerning the angular distribution of the halo; limits on the halo flux within $0.5^\circ$ to $1^\circ$ from the source range between 0.1% and 1% of the peak burst flux.
The array of 5 imaging atmospheric Čerenkov telescopes (IACTs) deployed at La Palma (Canary Islands), and operated by the HEGRA (High Energy Gamma Ray Astronomy) collaboration, was used for ...observations of the Monoceros Loop SNR region for a total of about 120 hrs and 20 hrs in ON-source and OFF-source mode, respectively. The giant molecular cloud Rosette Nebula appears in the sky region, close to the south-east part of the SNR rim. Using the HEGRA system of IACTs of rather large field of view (4.3 degree in diameter), we have mapped the extended sky region of $3^\circ \times 3^\circ$ associated with the Monoceros SNR/Rosette Nebula, which is centered towards the hard spectrum X-ray point source SAX J0635+533. The EGRET unidentified source of diffuse γ-ray emission (3EG J0634+0521) observed in the energy range between 100 MeV–10 GeV, was effectively in the field of view of our present observations. Also, the GeV source GeV J0633+0645 was within the available field of view. The performance of the IACTs array reveals an energy threshold of 500 GeV and an angular resolution of $0.1^\circ$ for γ-rays. In what follows, we present the result of the data analysis and its physical interpretation.
Using data from the HEGRA air shower array, taken in the period from April 1998 to March 2000, upper limits on the ratio
I
γ/
I
CR of the diffuse photon flux
I
γ to the hadronic cosmic ray flux
I
CR ...are determined for the energy region 20–100 TeV. The analysis uses a gamma–hadron discrimination which is based on differences in the development of photon- and hadron-induced air showers after the shower maximum. A method which is sensitive only to the non-isotropic component of the diffuse photon flux yields an upper limit of
I
γ/
I
CR (at 54 TeV) <2.0×10
−3 (at the 90% confidence level) for a sky region near the inner galaxy (20°< galactic longitude <60° and |galactic latitude |<5°). A method which is sensitive to both the isotropic and the non-isotropic component yields global upper limits of
I
γ/
I
CR (at 31 TeV) <1.2×10
−2 and
I
γ/
I
CR (at 53 TeV) <1.4×10
−2 (at the 90% confidence level).
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, one quarter of the Galactic plane ($-2^\circ < l < 85^\circ$) was surveyed for TeV gamma-ray emission from point sources and ...moderately extended sources (Ø$\;\le0.8^\circ$). The region covered includes 86 known pulsars (PSR), 63 known supernova remnants (SNR) and nine GeV sources, representing a significant fraction of the known populations. No evidence for emission of TeV gamma radiation was detected, and upper limits range from 0.15 Crab units up to several Crab units, depending on the observation time and zenith angles covered. The ensemble sums over selected SNR and pulsar subsamples and over the GeV-sources yield no indication of emission from these potential sources. The upper limit for the SNR population is 6.7% of the Crab flux and for the pulsar ensemble is 3.6% of the Crab flux.
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