Ref HAL: in2p3-00392048_v1
Ref Arxiv: 0904.0361
DOI: 10.1051/0004-6361/200811564
Ref. & Cit.: NASA ADS
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22 citations
Abstract:

Observations of the globular cluster 47 Tucanae (NGC 104), which contains at least 23 millisecond pulsars, were performed with the H.E.S.S. telescope system. The observations lead to an upper limit of F(E>800 GeV) < 6.7e-13 / cm^2 s on the integral gamma-ray photon flux from 47 Tucanae. Considering millisecond pulsars as the unique potential source of gamma-rays in the globular cluster, constraints based on emission models are derived: on the magnetic field in the average pulsar nebula and on the conversion efficiency of spin-down power to gamma-ray photons or to relativistic leptons.

Comments: 5 pages, 2 figures, accepted for publication in A&A

Ref HAL: in2p3-00378403_v1
Ref Arxiv: 0903.2924
DOI: 10.1088/0004-637X/696/2/L150
Ref. & Cit.: NASA ADS
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109 citations
Abstract:

We report on the first simultaneous observations that cover the optical, X-ray, and high energy gamma-ray bands of the BL Lac object PKS 2155-304. The gamma-ray bands were observed for 11 days, between 25 August and 6 September 2008, jointly with the Fermi Gamma-ray Space Telescope and the H.E.S.S. atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectral energy distribution with the new generation of gamma-ray telescopes. The ATOM telescope and the RXTE and Swift observatories provided optical and X-ray coverage of the low-energy component over the same time period. The object was close to the lowest archival X-ray and Very High Energy state, whereas the optical flux was much higher. The light curves show relatively little (~30%$) variability overall when compared to past flaring episodes, but we find a clear optical/VHE correlation and evidence for a correlation of the X-rays with the high energy spectral index. Contrary to previous observations in the flaring state, we do not find any correlation between the X-ray and VHE components. Although synchrotron self-Compton models are often invoked to explain the SEDs of BL Lac objects, the most common versions of these models are at odds with the correlated variability we find in the various bands for PKS 2155-304.

Comments: Accepted for publication in the Astrophysical Journal

Ref HAL: in2p3-00367126_v1
Ref Arxiv: 0903.1582
DOI: 10.1088/0004-637X/695/1/L40
Ref. & Cit.: NASA ADS
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163 citations
Abstract:

We report the discovery of faint very high energy (VHE, E>100 GeV) gamma-ray emission from the radio galaxy Centaurus A in observations performed with the H.E.S.S. experiment, an imaging atmospheric Cherenkov telescope array consisting of four telescopes located in Namibia. Centaurus A has been observed for more than 120 h. A signal with a statistical significance of 5.0 sigma is detected from the region including the radio core and the inner kpc jets. The integral flux above an energy threshold of ~250 GeV is measured to be ~0.8 % of the flux of the Crab Nebula (apparent luminosity: L(>250 GeV)~2.6x10^39 erg s^-1, adopting a distance of 3.8 Mpc. The spectrum can be described by a power law with a photon index of 2.7 +/- 0.5_stat +/- 0.2_sys. No significant flux variability is detected in the data set. However, the low flux only allows detection of variability on the timescale of days to flux increments above a factor of ~15-20 (3 sigma and 4 sigma, respectively). The discovery of VHE gamma-ray emission from Centaurus A reveals particle acceleration in the source to >TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters.

Comments: 5 pages, 4 figures, accepted for publication in ApJ Letters

Ref HAL: in2p3-00354648_v1
Ref Arxiv: 0901.2187
DOI: 10.1051/0004-6361:200811072
Ref. & Cit.: NASA ADS
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38 citations
Abstract:

Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray bursts (GRBs) in some scenarios. Exploring this photon energy regime is necessary for understanding the energetics and properties of GRBs. GRBs have been one of the prime targets for the H.E.S.S. experiment, which makes use of four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays. Dedicated observations of 32 GRB positions were made in the years 2003-2007 and a search for VHE gamma-ray counterparts of these GRBs was made. Depending on the visibility and observing conditions, the observations mostly start minutes to hours after the burst and typically last two hours. Results from observations of 22 GRB positions are presented and evidence of a VHE signal was found neither in observations of any individual GRBs, nor from stacking data from subsets of GRBs with higher expected VHE flux according to a model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from the GRB positions were derived. For those GRBs with measured redshifts, differential upper limits at the energy threshold after correcting for absorption due to extra-galactic background light are also presented.

Comments: 9 pages, 4 tables, 3 figures

Ref HAL: hal-00489473_v1
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Abstract:

difficulties arise in the process of Fermi acceleration at relativistic shocks because they are generically in the superluminal regime and they strongly compress turbulence downstream. Under reasonable conditions usually considered for the turbulent spectrum upstream, cosmic rays do not undergo the scattering that would allow them to make several Fermi cycles. An intense short scale turbulence must be excited in order that the Fermi process can be operative. In this paper, we show the requirement for the turbulence spectrum and give a solution to this issue in the form of the generation of intense compressive waves at short scale upstream. We indicate how the magnetic turbulence compression downstream should not be an obstacle for getting a significant return probability. Thus we conclude that, despite the necessity of some further analysis, the Fermi process at relativistic shock is still promising.

Ref HAL: hal-00430342_v1
Ref Arxiv: 0802.2661
Ref. & Cit.: NASA ADS
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Abstract:

Extreme objects such as X-ray binaries, AGN, or $\gamma$-ray bursters harborhigh energy plasmas whose properties are not well understood yet. Not only arethey responsible of the hard X- and $\gamma$-ray emission we observe but alsothey have a strong influence on the main dynamics and energetics of theseobjects themselves. Here we present a new kinetic code that solves theevolution equations for particles and photons around compact objects. Itproduces spectra that will be compared with observations from Simbol-X toconstrain the radiation and acceleration processes in these objects.

Comments: Poster at the international workshop 'Simbol-X: the hard X-ray universe in focus' held in Bologna, 14-16 May 2007. To appear in Memorie della SAIt, 2 pages, 1 figure

Ref HAL: hal-00430340_v1
Ref Arxiv: 0807.3459
DOI: 10.1111/j.1365-2966.2008.14219.x
Ref. & Cit.: NASA ADS
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46 citations
Abstract:

Fermi acceleration can take place at ultra-relativistic shock waves if theupstream or downstream magnetic field has been remodeled so that most of themagnetic power lies on short spatial scales. The relevant conditions underwhich Fermi acceleration become efficient in the presence of both a coherentand a short scale turbulent magnetic field are addressed. Within the MHDapproximation, this paper then studies the amplification of a pre-existingmagnetic field through the streaming of cosmic rays upstream of a relativisticshock wave. The magnetic field is assumed to be perpendicular in the shockfront frame, as generally expected in the limit of large shock Lorentz factor.In the MHD regime, compressive instabilities seeded by the net cosmic-raycharge in the shock precursor (as seen in the shock front frame) develop on theshortest spatial scales but saturate at a moderate level deltaB/B ~ 1, which isnot sufficient for Fermi acceleration. As we argue, it is possible that otherinstabilities outside the MHD range provide enough amplification to allowsuccessful Fermi acceleration.