70 research outputs found
Discovery of the Binary Pulsar PSR B1259-63 in Very-High-Energy Gamma Rays around Periastron with H.E.S.S
We report the discovery of very-high-energy (VHE) gamma-ray emission of the
binary system PSR B1259-63/SS 2883 of a radio pulsar orbiting a massive,
luminous Be star in a highly eccentric orbit. The observations around the 2004
periastron passage of the pulsar were performed with the four 13 m Cherenkov
telescopes of the H.E.S.S. experiment, recently installed in Namibia and in
full operation since December 2003. Between February and June 2004, a gamma-ray
signal from the binary system was detected with a total significance above 13
sigma. The flux was found to vary significantly on timescales of days which
makes PSR B1259-63 the first variable galactic source of VHE gamma-rays
observed so far. Strong emission signals were observed in pre- and
post-periastron phases with a flux minimum around periastron, followed by a
gradual flux decrease in the months after. The measured time-averaged energy
spectrum above a mean threshold energy of 380 GeV can be fitted by a simple
power law F_0(E/1 TeV)^-Gamma with a photon index Gamma =
2.7+-0.2_stat+-0.2_sys and flux normalisation F_0 = (1.3+-0.1_stat+-0.3_sys)
10^-12 TeV^-1 cm^-2 s^-1. This detection of VHE gamma-rays provides unambiguous
evidence for particle acceleration to multi-TeV energies in the binary system.
In combination with coeval observations of the X-ray synchrotron emission by
the RXTE and INTEGRAL instruments, and assuming the VHE gamma-ray emission to
be produced by the inverse Compton mechanism, the magnetic field strength can
be directly estimated to be of the order of 1 G.Comment: 10 pages, 8 figures, accepted in Astronomy and Astrophysics on 2 June
2005, replace: document unchanged, replaced author field in astro-ph entry -
authors are all members of the H.E.S.S. collaboration and three additional
authors (99+3, see document
TeV Gamma-ray Observations of the Crab and Mkn 501 during Moonshine and Twilight
TeV Gamma-ray signals from the Crab Nebula and Mkn 501 were detected with the
HEGRA CT1 imaging Cerenkov telescope during periods when the moon was shining
and during twilight. This was accomplished by lowering the high voltage supply
of the photomutipliers in fixed steps up to 13%. No other adjustments were made
and no filters were used. Laser runs could not establish any non-linearity in
the gain of the individual pixels, and the trigger rate was uniform over the
whole camera. The energy threshol was increased by up to a factor of two,
depending on the amount of HV reduction. In a series of observations lasting
11.7 hours, a signal with a 3.4 sigma significance was detected from the Crab.
During the 1997 multiple flare episode of Mkn 501 a 26 sigma combined excess
has been recorded during 134 hours of observations under various
moonshine/twilight conditions. The results show that this technique can easily
be adapted to increase the exposure of a source, which is important for sources
showing rapid time variability such as AGNs or GRBs. Observations can be made
up to ~20 deg. angular separation from the moon and until the moon is 85%
illuminated (ten to eleven days before and after new moon), as well as during
20 to 40 minutes during twilight, before the commencement of astronomical
darkness.Comment: 16 pages, 5 figures, submitted to Astroparticle Physic
A low level of extragalactic background light as revealed by big gamma-rays from blazars
The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the 'first stars', which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light1. An alternative approach2, 3, 4, 5 is to study the absorption features imprinted on the -ray spectra of distant extragalactic objects by interactions of those photons with the background light photons6. Here we report the discovery of -ray emission from the blazars7 H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies8. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources—in particular from the first stars formed9. This result also indicates that intergalactic space is more transparent to -rays than previously thought
Constraints on axionlike particles with H.E.S.S. from the irregularity of the PKS 2155-304 energy spectrum
Axionlike particles (ALPs) are hypothetical light (sub-eV) bosons predicted in some extensions of the Standard Model of particle physics. In astrophysical environments comprising high-energy gamma rays and turbulent magnetic fields, the existence of ALPs can modify the energy spectrum of the gamma rays for a sufficiently large coupling between ALPs and photons. This modification would take the form of an irregular behavior of the energy spectrum in a limited energy range. Data from the H.E.S.S. observations of the distant BL Lac object PKS 2155-304 (z=0.116) are used to derive upper limits at the 95% C.L. on the strength of the ALP coupling to photons, ggammaa<2.1×10-11GeV-1 for an ALP mass between 15 and 60 neV. The results depend on assumptions on the magnetic field around the source, which are chosen conservatively. The derived constraints apply to both light pseudoscalar and scalar bosons that couple to the electromagnetic fieldFil: Medina, Maria Clementina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto Argentino de Radioastronomia (i); ArgentinaFil: H.E.S. S. collaboration
First detection of VHE γ-rays from SN 1006 by HESS
Aims. Recent theoretical predictions of the lowest very high energy (VHE) luminosity of SN 1006 are only a factor 5 below the previously published HESS upper limit, thus motivating further in-depth observations of this source. Methods. Deep observations at VHE energies (above 100 GeV) were carried out with the high energy stereoscopic system (HESS) of Cherenkov Telescopes from 2003 to 2008. More than 100 h of data have been collected and subjected to an improved analysis procedure. Results. Observations resulted in the detection of VHE γ-rays from SN 1006. The measured γ-ray spectrum is compatible with a power-law, the flux is of the order of 1% of that detected from the Crab Nebula, and is thus consistent with the previously established HESS upper limit. The source exhibits a bipolar morphology, which is strongly correlated with non-thermal X-rays. Conclusions. Because the thickness of the VHE-shell is compatible with emission from a thin rim, particle acceleration in shock waves is likely to be the origin of the γ-ray signal. The measured flux level can be accounted for by inverse Compton emission, but a mixed scenario that includes leptonic and hadronic components and takes into account the ambient matter density inferred from observations also leads to a satisfactory description of the multi-wavelength spectrum
An upper limit on the TeV gamma-ray flux from W Comae
The BL Lac object W Comae was observed by the HEGRA collaboration using the first two Cherenkov telescopes (CT1 and CT2) on seven nights in February 1996. The data were taken in consecutive ON-source runs at zenith angles between 0.5 and 40 and about 23 hours of observations with each telescope were obtained. No significant excess over the expected number of events after all image parameter cuts (as determined from OFF-source observations at the same zenith angles) is observed. From the W Comae data in the zenith angle range 0.5 - 30 and using 84 h/84 h of CT1/CT2 OFF data, we obtain 3#sigma# upper limits to the excess rate of 2.1 h"-"1 for CT1 and 6.3 h"-"1 for CT2. Using simulated gamma showers at primary energies between 0.05 TeV and 30 TeV and at zenith angles of 0 and 30 , the expected rates for the given average zenith angle have been calculated for both telescopes assuming a differential spectral index of 2.7. The 3 #sigma# upper limits on the time averaged integral gamma flux from W Comae for this observation period are F(E> 1.0 TeV) = 1.05 x 10"-"1"1 cm"-"2s"-"1 (from the CT2 data) and F(E> 1.5 TeV) = 0.29 x 10"-"1"1 cm"-"2s"-"1 (from the CT1 data). (orig.)9 refs.Available from TIB Hannover: RR 2916(97-01) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
Search for Dark Matter Annihilations towards the Inner Galactic Halo from 10 Years of Observations with H.E.S.S.
The inner region of the Milky Way halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the annihilations of DM particles using γ-ray observations towards the inner 300 pc of the Milky Way, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic Center (GC) data accumulated by H.E.S.S. over the last 10 years (2004–2014), and does not show any significant γ-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section hσvi. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach hσvi values of 6 × 10−26 cm3 s−1 in the WþW− channel for a DM particle mass of 1.5 TeV, and 2 × 10−26 cm3 s−1 in the τþτ− channel for a 1 TeV mass. For the first time, ground-based γ-ray observations have reached sufficient sensitivity to probe hσvi values expected from the thermal relic density for TeV DM particles
Simultaneous Observations of PKS 2155-304 with HESS, Fermi, RXTE, and Atom: Spectral Energy Distributions and Variability in a Low State
Characterizing the gamma-ray long-term variability of PKS2155 304 with HESS and Fermi-LAT
Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in the high (HE, 100 MeV 200 GeV) γ-ray domain. Over the course of ∼9 yr of H.E.S.S. observations the VHE light curve in the quiescent state is consistent with a log-normal behavior. The VHE variability in this state is well described by flicker noise (power-spectral-density index βVHE = 1.10-0.13 +0.10) on timescales larger than one day. An analysis of ∼5.5 yr of HE Fermi-LAT data gives consistent results (βHE = 1.20-0.23 +0.21), on timescales larger than 10 days) compatible with the VHE findings. The HE and VHE power spectral densities show a scale invariance across the probed time ranges. A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established. These long-term-variability properties are discussed and compared to the red noise behavior (β ∼ 2) seen on shorter timescales during VHE-flaring states. The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes, while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection.EM201
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