Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017

Aims: The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio fluxes. Our aim is to understand the radiative processes by investigating the observed emission and its production mechanism using the High Energy Stereoscopic System (H.E.S.S.) experiment. Methods: We report recent observations of the BL Lac source RGB J0152+017 made in late October and November 2007 with the H.E.S.S. array consisting of four imaging atmospheric Cherenkov telescopes. Contemporaneous observations were made in X-rays by the Swift and RXTE satellites, in the optical band with the ATOM telescope, and in the radio band with the Nancay Radio Telescope. Results: A signal of 173 gamma-ray photons corresponding to a statistical significance of 6.6 sigma was found in the data. The energy spectrum of the source can be described by a powerlaw with a spectral index of 2.95+/-0.36stat+/-0.20syst. The integral flux above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source spectral energy distribution (SED) can be described using a two-component non-thermal synchrotron self-Compton (SSC) leptonic model, except in the optical band, which is dominated by a thermal host galaxy component. The parameters that are found are very close to those found in similar SSC studies in TeV blazars. Conclusions: RGB J0152+017 is discovered as a source of VHE gamma-rays by H.E.S.S. The location of its synchrotron peak, as derived from the SED in Swift data, allows clearly classification it as a high-frequency-peaked BL Lac (HBL).Comment: Accepted for publication in A&A Letters (5 pages, 4 figures

First detection of a VHE gamma-ray spectral maximum from a Cosmic source: H.E.S.S. discovery of the Vela X nebula

The Vela supernova remnant (SNR) is a complex region containing a number of sources of non-thermal radiation. The inner section of this SNR, within 2 degrees of the pulsar PSR B0833-45, has been observed by the H.E.S.S. gamma-ray atmospheric Cherenkov detector in 2004 and 2005. A strong signal is seen from an extended region to the south of the pulsar, within an integration region of radius 0.8 deg. around the position (RA = 08h 35m 00s, dec = -45 deg. 36' J2000.0). The excess coincides with a region of hard X-ray emission seen by the ROSAT and ASCA satellites. The observed energy spectrum of the source between 550 GeV and 65 TeV is well fit by a power law function with photon index = 1.45 +/- 0.09(stat) +/- 0.2(sys) and an exponential cutoff at an energy of 13.8 +/- 2.3(stat) +/- 4.1(sys) TeV. The integral flux above 1 TeV is (1.28 +/- 0.17 (stat) +/- 0.38(sys)) x 10^{-11} cm^{-2} s^{-1}. This result is the first clear measurement of a peak in the spectral energy distribution from a VHE gamma-ray source, likely related to inverse Compton emission. A fit of an Inverse Compton model to the H.E.S.S. spectral energy distribution gives a total energy in non-thermal electrons of ~2 x 10^{45} erg between 5 TeV and 100 TeV, assuming a distance of 290 parsec to the pulsar. The best fit electron power law index is 2.0, with a spectral break at 67 TeV.Comment: 5 pages, 4 figures, accepted for publication in Astronomy and Astrophysics letter

H.E.S.S. observations of gamma-ray bursts in 2003-2007

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.Comment: 9 pages, 4 tables, 3 figure

3.9 day orbital modulation in the TeV gamma-ray flux and spectrum from the X-ray binary LS 5039

New observations of LS 5039, a High Mass X-ray Binary comprising a massive star and compact object, were carried out with the High Energy Stereoscopic System of Cherenkov Telescopes (H.E.S.S.) in 2005 at very high energy (VHE) gamma-ray energies. These observations reveal that its flux and energy spectrum are modulated with the 3.9 day orbital period of the binary system. This is the first time in gamma-ray astronomy that orbital modulation has been observed, and periodicity clearly established using ground-based gamma-ray detectors. The VHE gamma-ray emission is largely confined to half of the orbit, peaking around the inferior conjunction epoch of the compact object. For this epoch, there is also a hardening of the energy spectrum in the energy range between 0.2 TeV and a few TeV. The flux vs. orbital phase profile provides the first clear indication of gamma-ray absorption via pair production within an astrophysical source, a process which is expected to occur if the gamma-ray production site is situated within ~1 AU of the compact object. Moreover the production region size must be not significantly greater than the binary separation (~0.15 AU). Notably, these constraints are also considerably smaller than the collimated outflows or jets (extending out to ~1000 AU) observed in LS 5039. The spectral hardening could arise from variations with phase in the maximum electron energies, and/or the dominant VHE gamma-ray production mechanism.Comment: 8 pages, 8 figures, accepted for publication in Astronomy & Astrophysic

Component Interactions and Electron Transfer in Toluene/o-Xylene Monooxygenase

The multicomponent protein toluene/o-xylene monooxygenase (ToMO) activates molecular oxygen to oxidize aromatic hydrocarbons. Prior to dioxygen activation, two electrons are injected into each of two diiron(III) units of the hydroxylase, a process that involves three redox active proteins: the ToMO hydroxylase (ToMOH), Rieske protein (ToMOC), and an NADH oxidoreductase (ToMOF). In addition to these three proteins, a small regulatory protein is essential for catalysis (ToMOD). Through steady state and pre-steady state kinetics studies, we show that ToMOD attenuates electron transfer from ToMOC to ToMOH in a concentration-dependent manner. At substoichiometric concentrations, ToMOD increases the rate of turnover, which we interpret to be a consequence of opening a pathway for oxygen transport to the catalytic diiron center in ToMOH. Excess ToMOD inhibits steady state catalysis in a manner that depends on ToMOC concentration. Through rapid kinetic assays, we demonstrate that ToMOD attenuates formation of the ToMOC–ToMOH complex. These data, coupled with protein docking studies, support a competitive model in which ToMOD and ToMOC compete for the same binding site on the hydroxylase. These results are discussed in the context of other studies of additional proteins in the superfamily of bacterial multicomponent monooxygenases.National Institute of General Medical Sciences (U.S.) (5-R01-GM032134)United States. National Institutes of Health (T32GM008334

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

Discovery of Very High Energy Gamma-Ray Emission from the BL Lac Object H2356-309 with the H.E.S.S. Cherenkov Telescopes

The extreme synchrotron BL Lac object H2356-309, located at a redshift of z = 0.165, was observed from June to December 2004 with a total exposure of approx. 40 h live-time with the H.E.S.S. (High Energy Stereoscopic System) array of atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for the first time, a strong excess of 453 gamma-rays (10 standard deviations above background) from H2356-309, corresponding to an observed integral flux above 200 GeV of I(>200GeV) = (4.1+-0.5) 10^12 cm^-2.s^-1 (statistical error only). The differential energy spectrum of the source between 200 GeV and 1.3 TeV is well-described by a power law with a normalisation (at 1 TeV) of N_0 = (3.00 +- 0.80_stat +- 0.31_sys) 10^-13 cm^-2.s^-1.TeV^-1 and a photon index of Gamma = 3.09 +- 0.24_stat +- 0.10_sys. H2356-309 is one of the most distant BL Lac objects detected at very-high-energy gamma-rays so far. Results from simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT (radio) are also included and used together with the H.E.S.S. data to constrain a single-zone homogeneous synchrotron self-Compton (SSC) model. This model provides an adequate fit to the H.E.S.S. data when using a reasonable set of model parameters.Comment: 7 pages, 4 figures, accepted for publication in Astronomy and Astrophysics (05/07/2006

Simultaneous observations of PKS 2155-304 with H.E.S.S., Fermi, RXTE and ATOM: spectral energy distributions and variability in a low state

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.Comment: Accepted for publication in the Astrophysical Journa

Discovery of very-high-energy gamma-ray emission from the vicinity of PSR J1913+1011 with H.E.S.S

The H.E.S.S. experiment, an array of four Imaging Atmospheric Cherenkov Telescopes with high sensitivity and large field-of-view, has been used to search for emitters of very-high-energy (VHE, >100 GeV) gamma-rays along the Galactic plane, covering the region 30 deg < l < 60 deg, 280 deg < l < 330 deg, and -3 deg < b < 3 deg. In this continuation of the H.E.S.S. Galactic Plane Scan, a new extended VHE gamma-ray source was discovered at alpha(2000)=19h12m49s, delta(2000)=+10d09'06'' (HESS J1912+101). Its integral flux between 1-10 TeV is ~10% of the Crab Nebula flux in the same energy range. The measured energy spectrum can be described by a power law with a photon index Gamma = 2.7+-0.2(stat)+-0.3(sys). HESS J1912+101 is plausibly associated with the high spin-down luminosity pulsar PSR J1913+1011. We also discuss associations with an as yet unconfirmed SNR candidate proposed from low frequency radio observation and/or with molecular clouds found in 13CO data.Comment: 6 pages, 2 figures; Accepted for publication in A&A on February 20, 200