We present results of a search for TeV γ-ray emission from the microquasar SS-433 and the surrounding region covering a ~$8^\circ$ $\times$ $8^\circ$ field of view. Analysis of data taken with the ...HEGRA stereoscopic system of imaging atmospheric Čerenkov imaging telescopes reveals no evidence of steady or variable emission from any position. Observation times of over 100 h have been achieved in central parts of the field of view. We set 99% confidence level upper limits to a number of a-priori-chosen objects of interest, including SS-443 and its interaction regions, 32 pulsars, 3 supernova remnants and the GeV source GeV J1907+0537. Our upper limit of 3.2% Crab flux (for energies $E>0.8$ TeV) for the eastern-lobe region e3 of SS-433 permits, after comparison with X-ray fluxes, a lower limit of $B\geq19~\mu$G on the post-shocked magnetic field in this region. An ensemble upper limit at 0.3% Crab flux ($E>0.7$ TeV) from a subset (11) of the 32 pulsars implies a maximum of 4.5% of the spin-down pulsar power is available for TeV γ-ray production. For one of the SNR in our FoV, 3C 396, recent Chandra observations suggest that a central pulsar-driven wind nebula may be the source of X-ray emission. Our upper limit implies that a maximum of 0.1% of the spin-down power from the central source of 3C 396 would be available for TeV γ-rays.
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, the Andromeda galaxy (M 31) was surveyed for TeV gamma ray emission. Given the large field of view of the HEGRA telescopes, three ...pointings were sufficient to cover all of M 31, including also M 32 and NGC 205. No indications for point sources of TeV gamma rays were found. Upper limits are given at a level of a few percent of the Crab flux. A specific search for monoenergetic gamma-ray lines from annihilation of supersymmetric dark matter particles accumulating near the center of M 31 resulted in flux limits in the 10-13 cm-2 s-1 range, well above the predicted MSSM flux levels except for models with pronounced dark-matter spikes or strongly enhanced annihilation rates.
A search for potential point sources of very high energy gamma rays has been carried out on the data taken simultaneously by the HEGRA AIROBICC and Scintillator arrays from August 1994 to March 2000. ...The list of sought sources includes supernova remnants, pulsars, AGNs and binary systems. The energy threshold is around 15 TeV. For the Crab Nebula, a modest excess of 2.5 standard deviations above the cosmic ray background has been observed. Flux upper limits (at $90\%$ c.l.) of around 1.3 times the flux of the Crab Nebula are obtained, in average, for the candidate sources. A different search procedure has been used for an all-sky search which yields absolute flux upper limits between 4 and 9 crabs depending on declination, in the band from $\delta = 0$ to $\delta = 60^\circ$.
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
Odprti dostop
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).
The time profiles of Cherenkov images of cosmic-ray showers and of γ-rays showers are investigated, using data gathered with the HEGRA system of imaging atmospheric Cherenkov telescopes during the ...1997 outbursts of Mrk 501. Photon arrival times are shown to vary across the shower images. The dominant feature is a time gradient along the major axis of the images. The gradient varies with the distance between the telescope and the shower core, and is maximal for large distances. The time profiles of cosmic-ray showers and of γ-ray showers differ in a characteristic fashion. The main features of the time profiles can be understood in terms of simple geometrical models. Use of the timing information towards improved shower reconstruction and cosmic-ray supperssion is discussed.
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