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.
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.
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.
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.
Observations of H1426+428 with HEGRA Aharonian, F.; Akhperjanian, A.; Beilicke, M. ...
Astronomy and astrophysics (Berlin),
05/2003, Letnik:
403, Številka:
2
Journal Article
Recenzirano
Odprti dostop
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 $5.3~\sigma$, 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 $\Delta E/E\le 12\%$ 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 effects 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 μm.
The nearby BL Lacertae (BL Lac) object Markarian 421 (Mkn 421) at a red shift $z=0.031$ was observed to undergo strong TeV γ-ray outbursts in the observational periods from December 1999 until May ...2001. The time averaged flux level $F(E>1{\rm \,TeV})$ in the 1999/2000 season was $(1.43\pm0.04)$ $\times$ 10-11 ph cm-2 s-1, whereas in the 2000/2001 season the average integral flux increased to $(4.19\pm0.04)$ $\times$ 10-11 ph cm-2 s-1. Both energy spectra are curved and well fit by a power law with an exponential cut-off energy at $3.6(+0.4-0.3)_{\rm stat}(+0.9-0.8)_{\rm sys}$ TeV. The respective energy spectra averaged over each of the two time periods indicate a spectral hardening for the 2000/2001 spectrum. The photon index changes from $2.39\pm0.09_{\rm stat}$ for 1999/2000 to $2.19\pm0.02_{\rm stat}$ in 2000/2001. The energy spectra derived for different average flux levels ranging from 0.5 to 10 $\times$ 10-11 ph cm-2 s-1 follow a clear correlation of photon index and flux level. Generally, the energy spectra are harder for high flux levels. From January to April 2001 Mkn 421 showed rapid variability (doubling time as short as 20 min), accompanied with a spectral hardening with increasing flux level within individual nights. For two successive nights (MJD 51989-51991, March 21-23, 2001), this correlation of spectral hardness and change in flux has been observed within a few hours. The cut-off energy for the Mkn 421 TeV spectrum remains within the errors constant for the different flux levels and differs by $\Delta E=2.6\pm0.6_{\rm stat}\pm0.6_{\rm sys}$ TeV from the value determined for Mkn 501. This indicates that the observed exponential cut-off in the energy spectrum of Mkn 421 is not solely caused by absorption of multi-TeV photons by pair-production processes with photons of the extragalactic near/mid infrared background radiation.
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