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This is the first report on spectral measurements from 50 GeV for the Crab nebula and from 70 GeV for the blazar Markarian 421. Both sources were detected by the CELESTE experiment in 1999-2000. Here we present the energy measurement and spectral reconstruction methods. The energy resolution is shown to smoothly vary with energy, with a mean value of ∼20% above the detection threshold. Preliminary spectra are presented and compared with those obtained quasi-simultaneously above 300 GeV by the CAT imaging telescope.

Context. Located in the French Pyrenees, CELESTE was the first ground-based γ-ray telescope with an energy threshold below 100 GeV. It acquired data from 1999 to 2004, and allowed flux measurements of the Crab nebula and the blazars Mrk 421 and Mrk 501. Aims. We search for Galactic diffuse γ-ray emission, which is most significant around the Galactic plane, for b = [−5 ◦ , +5 ◦ ]. Methods. By using the significant data set available for the Crab nebula, we selected Crab OFF-source data at various Galactic latitudes, in order to analyse the diffuse emission. Selection criteria were applied to the sky position, atmospheric conditions, and detector stability. Results. We obtained 108 mn of data in the Galactic anticentre region, providing the first upper limits of around 100 GeV to the diffuse γ-ray emission with atmospheric Cherenkov detectors. These limits are φ UL (E > 140 GeV) = 9.4 × 10 −3 m −2 s −1 sr −1 and φ UL (E > 120 GeV) = 1.2 × 10 −2 m −2 s −1 sr −1 .

X. Barcons1, F. J. Carrera1, M. T. Ceballos1, M. J. Page2, J. Bussons-Gordo1, A. Corral1, J. Ebrero1, S. Mateos1,3, J. A. Tedds3, M. G. Watson3, D. Baskill3, M. Birkinshaw4, T. Boller5, N. Borisov6, M. Bremer4, G. E. Bromage7, H. Brunner5, A. Caccianiga8, C. S. Crawford9, M. S. Cropper2, R. Della Ceca8, P. Derry3, A. C. Fabian9, P. Guillout10, Y. Hashimoto5, G. Hasinger5, B. J. M. Hassall7, G. Lamer11, N. S. Loaring2,12, T. Maccacaro8, K. O. Mason2, R. G. McMahon9, L. Mirioni10, J. P. D. Mittaz2, C. Motch10, I. Negueruela10,13, J. P. Osborne3, F. Panessa1, I. Pérez-Fournon14, J. P. Pye3, T. P. Roberts3,15, S. Rosen2,3, N. Schartel16, N. Schurch3,15, A. Schwope11, P. Severgnini8, R. Sharp9,17, G. C. Stewart3, G. Szokoly5, A. Ullán1,18, M. J. Ward3,15, R. S. Warwick3, P. J. Wheatley3,19, N. A. Webb20, D. Worrall4, W. Yuan9,21, and H. Ziaeepour2

We have measured the gamma-ray fluxes of the blazars Mrk 421 and Mrk 501 in the energy range between 50 and 350 GeV ( 1.2 to 8.3 x 10(25) Hz). The detector, called CELESTE, used first 40, then 53 heliostats of the former solar facility "Themis" in the French Pyrenees to collect Cherenkov light generated in atmospheric particle cascades. The signal from Mrk 421 is often strong. We compare its flux with previously published multi-wavelength studies and infer that we are straddling the high energy peak of the spectral energy distribution. The signal from Mrk 501 in 2000 was weak ( 3.4 sigma). We obtain an upper limit on the flux from 1ES 1426+428 of less than half that of the Crab flux near 100 GeV. The data analysis and understanding of systematic biases have improved compared to previous work, increasing the detector's sensitivity.

Very high energy gamma-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy gamma-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of "dark" nucleonic cosmic ray sources.

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been detected with high significance (~45 sigma) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of 3.32 +/- 0.06. An improved chi-square per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2 sigma). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.

The serendipitous discovery of an unidentified extended TeV gamma-ray source close to the galactic plane named HESSJ1303-631 at a significance of 21 standard deviations is reported. The observations were performed between February and June 2004 with the HESS stereoscopic system of Cherenkov telescopes in Namibia. HESS J1303-631 was discovered roughly 0.6 degrees north of the binary system PSRB1259-63/SS2883, the target object of the initial observation campaign which was also detected at TeV energies in the same field of view. HESS J1303-631 is extended with a width of an assumed intrinsic Gaussian emission profile of sigma = (0.16 +/- 0.02)degrees. and the integral flux above 380 GeV is compatible with constant emission over the entire observational period of (17 +/- 3)% of the Crab Nebula flux. The measured energy spectrum can be described by a power-law dN/dE similar to E-Gamma with a photon index of G = 2.44 +/- 0.05(stat) +/- 0.2(syst). Up to now, no counterpart at other wavelengths is identified. Various possible TeV production scenarios are discussed.

X-ray binaries are composed of a normal star in orbit around a neutron star or stellar-mass black hole. Radio and x-ray observations have led to the presumption that some x-ray binaries called microquasars behave as scaled-down active galactic nuclei. Microquasars have resolved radio emission that is thought to arise from a relativistic outflow akin to active galactic nuclei jets, in which particles can be accelerated to large energies. Very high energy γ-rays produced by the interactions of these particles have been observed from several active galactic nuclei. Using the High Energy Stereoscopic System, we find evidence for gamma-ray emission of >100 gigaelectron volts from a candidate microquasar, LS 5039, showing that particles are also accelerated to very high energies in these systems.

The Supernova Remnant MSH 15-52 has been observed in very high energy (VHE) gamma-rays using the H.E.S.S. 4-telescope array located in Namibia. A gamma-ray signal is detected at the 25 sigma level during an exposure of 22.1 hours live time. The image reveals an elliptically shaped emission region around the pulsar PSR B1509-58, with semi-major axis 6' in the NW-SE direction and semi-minor axis 2' approximately. This morphology coincides with the diffuse pulsar wind nebula as observed at X-ray energies by ROSAT. The overall energy spectrum from 280 GeV up to 40 TeV can be fitted by a power law with photon index Gamma = 2.27 +/- 0.03(stat.) +/- 0.20(syst.). The detected emission can be plausibly explained by inverse Compton scattering of accelerated relativistic electrons with soft photons.

Very high energy (> 100 GeV) gamma-ray emission has been detected for the first time from the composite supernova remnant G0.9+0.1 using the H.E.S.S. instrument. The source is detected with a significance of 13 sigma, and a photon flux above 200 GeV of (5.7+/-0.7 stat +/- 1.2 sys) * 10^-12 cm^-2 s^-1, making it one of the weakest sources ever detected at TeV energies. The photon spectrum is compatible with a power law (dN/dE \propto E^-Gamma) with photon index Gamma = 2.40 +/- 0.11 stat +/- 0.20 sys. The gamma-ray emission appears to originate in the plerionic core of the remnant, rather than the shell, and can be plausibly explained as inverse Compton scattering of relativistic electrons.

The BL Lacertae (BL Lac) object 1ES 2344+514 (1ES 2344), at a redshift of 0.044, was discovered as a source of very high energy (VHE) gamma rays by theWhipple collaboration in 1995 (Catanese et al.). This detection was recently confirmed by the HEGRA collaboration (Tluczykont et al.). As is typical for high-frequency–peaked blazars, the VHE gamma-ray emission is highly variable. On the night of 1995 December 20, a gamma-ray flare of 5.3 significance was detected, the brightest outburst from this object to date. The emission region is compatible with a point source. The spectrum between 0.8 and 12.6 TeV can be described by a power law,

Observations of the shell-type supernova remnant SN1006 have been carried out with the HESS system of Cherenkov telescopes during 2003 (18.2 h with two operating telescopes) and 2004 (6.3 h with all four telescopes). No evidence for TeV $\gamma$-ray emission from any compact or extended region associated with the remnant is seen and resulting upper limits at the 99.9% confidence level are up to a factor 10 lower than previously-published fluxes from CANGAROO. For SN1006 at its current epoch of evolution we define limits for a number of important global parameters. Upper limits on the $\gamma$-ray luminosity (for E = 0.26 to 10 TeV, distance d = 2 kpc) of $L_\gamma 25~\mu$G results when considering the synchrotron/inverse-Compton framework for the observed X-ray flux and $\gamma$-ray upper limits.

The region around PSR B1706 - 44 has been observed with the HESS imaging atmospheric Cherenkov telescopes in 2003. No evidence for gamma-ray emission in the TeV range was found at the pulsar position or at the radio arc which corresponds to the supernova remnant G343.1 - 2.3. The 99% confidence level flux upper limit at the pulsar position is F-ul( E > 350 GeV) = 1.4 x 10(-12) s(-1) cm(-2) assuming a power law (dN/dE proportional to E-Gamma) with photon index of Gamma = 2.5 and F-ul( E > 500 GeV) = 1.3 x 10(-12) s(-1) cm(-2) without an assumption on the spectral shape. The reported upper limits correspond to 8% of the flux from an earlier detection by the CANGAROO experiment.

A significant fraction of the energy density of the interstellar medium is in the form of high-energy charged particles (cosmic rays)1. The origin of these particles remains uncertain. Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically2,3, unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7 - 3946 (G347.3 - 0.5), which was discovered by ROSAT4 in the X-ray spectrum and later claimed as a source of high-energy γ-rays5,6 of TeV energies (1 TeV = 1012 eV). We present a TeV γ-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that very-high-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks.

We summarise the status of the CELESTE high-energy γ-ray observatory as of April 2003 with an emphasis on the changes introduced since the summer of 2001 in detector configuration, atmospheric monitoring and observing strategy. We report newer upper limits to the pulsed emission from the Crab and PSR B1951+32 pulsars and preliminary spectra above 50 GeV for the Crab Nebula and above 70 GeV for Markarian 421. We close with an outlook of our plans until the summer of 2004.

The energy spectrum of Markarian 421 in flaring states has been measured from 0.3 to 10 TeV using both small and large zenith angle observations with the Whipple Observatory 10 m imaging telescope. The large zenith angle technique is useful for extending spectra to high energies, and the extraction of spectra with this technique is discussed. The resulting spectrum of Markarian 421 is fitted reasonably well by a simple power law: J(E)=E−2.54 ± 0.03 ± 0.10 photons m-1 s-1 TeV-1, where the first set of errors is statistical and the second set is systematic. This is in contrast to our recently reported spectrum of Markarian 501, which over a similar energy range has substantial curvature. The differences in TeV energy spectra of gamma-ray blazars reflect both the physics of the gamma-ray production mechanism and possibly differential absorption effects at the source or in the intergalactic medium. Since Markarian 421 and Markarian 501 have almost the same redshift (0.031 and 0.033, respectively), the difference in their energy spectra must be intrinsic to the sources and not due to intergalactic absorption, assuming the intergalactic infrared background is uniform.

The BL Lacertae object Markarian 501 was identified as a source of γ-ray emission at the Whipple Observatory in 1995 March. Here we present a flux variability analysis on several timescales of the 233 hr data set accumulated over 213 nights (from March 1995 to July 1998) with the Whipple Observatory 10 m atmospheric Cerenkov imaging telescope. In 1995, with the exception of a single night, the flux from Markarian 501 was constant on daily and monthly timescales and had an average flux of only 10% that of the Crab Nebula, making it the weakest very high energy source detected to date. In 1996, the average flux was approximately twice the 1995 flux and showed significant month-to-month variability. No significant day-scale variations were detected. The average γ-ray flux above ~350 GeV in the 1997 observing season rose to 1.4 times that of the Crab Nebula—14 times the 1995 discovery level—allowing a search for variability on timescales shorter than 1 day. Significant hour-scale variability was present in the 1997 data, with the shortest, observed on MJD 50,607, having a doubling time of ~2 hr. In 1998 the average emission level decreased considerably from that of 1997 (to ~20% of the Crab Nebula flux), but two significant flaring events were observed. Thus the emission from Markarian 501 shows large amplitude and rapid flux variability at very high energies, as does Markarian 421. It also shows large mean flux level variations on year-to-year timescales, behavior that has not been seen from Markarian 421 so far.

The detection of the AGN Markarian 421 by the Whipple Observatory γ-ray telescope and subsequent discovery of a TeV flare from this same source initiated the new field of extragalactic “TeV astronomy”. The Whipple telescope has now observed γ-ray emission at E >300 GeV from the BL Lac objects Mrk 421, Mrk 501 and 1ES2344+514. Here we give a review of the status of Whipple observations including measurements of rapid (sub hour-scale) γ-ray variability at very high energies; observations of correlated variability in γ-ray, X-ray, UV and optical wavebands; and measurements of the γ-ray spectra of AGNs. These results are used to derive constraints on the physical conditions in the blazar jets, the emission mechanism and the level of the diffuse extragalactic IR radiation.