Aims. Results from γ-ray observations by the HESS telescope array in the direction of the young stellar cluster Westerlund 2 are presented. Methods. Stereoscopic imaging of Cherenkov light emission ...of γ-ray induced showers in the atmosphere is used to study the celestial region around the massive Wolf-Rayet (WR) binary WR 20a. Spectral and positional analysis is performed using standard event reconstruction techniques and parameter cuts. Results. The detection of a new γ-ray source is reported from HESS observations in 2006. HESS J1023–575 is found to be coincident with the young stellar cluster Westerlund 2 in the well-known HII complex RCW 49. The source is detected with a statistical significance of more than 9σ, and shows extension beyond a point-like object within the HESS point-spread function. The differential γ-ray spectrum of the emission region is measured over approximately two orders of magnitude in flux. Conclusions.The spatial coincidence between HESS J1023–575 and the young open cluster Westerlund 2, hosting e.g. the massive WR binary WR 20a, requires one to look into a variety of potential models to account for the observed very-high-energy (VHE) γ-ray emission. Considered emission scenarios include emission from the colliding wind zone of WR 20a, collective stellar winds from the extraordinary ensemble of hot and massive stars in the stellar cluster Westerlund 2, diffusive shock acceleration in the wind-blown bubble itself, and supersonic winds breaking out into the interstellar medium (ISM). The observed source extension argues against a single star origin of the observed VHE emission.
Aims. Detection of a γ-ray source above 300 GeV is reported, confirming the unidentified source MGRO J1908+06, discovered by the Milagro collaboration at a median energy of 20 TeV. Methods. The ...source was observed during 27 h as part of the extension of the HESS Galactic plane survey to longitudes >30°. Results. HESS J1908+063 is detected at a significance level of 10.9σ with an integral flux above 1 TeV of (3.76 ± $0.29_{\rm~stat}$± $0.75_{\rm sys}$)$\times$10-12 ph cm-2 s-1, and a spectral photon index Γ = 2.10±$0.07_{\rm~stat}$± 0.2$_{\rm sys}$. The positions and fluxes of HESS J1908+063 and MGRO J1908+06 are in good agreement. Possible counterparts at other wavelengths and the origin of the γ-ray emission are discussed. The nearby unidentified GeV source, GRO J1908+0556 (GeV) which also remains unidentified and the new Fermi pulsar 0FGL J1907.5+0617, may be connected to the TeV source.
Context. Observations of very high-energy γ-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects lead to a ...better understanding of the mechanisms in play. Aims. To investigate the spectral and temporal variability of VHE (>100 GeV) γ-rays of the well-known high-frequency-peaked BL Lac object PKS 2155–304 with the HESS imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 were analyzed. Spectra were derived on time scales ranging from 3 years to 4 min. Light curve variability was studied through doubling timescales and structure functions and compared with red noise process simulations. Results. The source was found to be in a low state from 2005 to 2007, except for a set of exceptional flares that occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 ± 0.09stat ± 0.86syst) × 10-11 cm-2 s-1 above 200 GeV, or approximately $15\%$ of the Crab Nebula, and a power-law photon index of Γ = 3.53 ± 0.06stat ± 0.10syst. During the flares of July 2006, doubling timescales of ~2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behavior at low and high fluxes, which is a new phenomenon in VHE γ-ray emitting blazars. The variability amplitude characterized by the fractional rms Fvar is strongly energy-dependent and is $\propto E^{0.19\pm0.01}$. The light curve rms correlates with the flux. This is the signature of a multiplicative process that can be accounted for as a red noise with a Fourier index of ~2. Conclusions. This unique data set shows evidence of a low-level γ-ray emission state from PKS 2155–304 that possibly has a different origin than the outbursts. The discovery of the light curve lognormal behavior might be an indicator of the origin of aperiodic variability in blazars.
Aims. HESS J1745-303 is an extended, unidentified VHE (very high energy) gamma-ray source discovered using HESS in the Galactic Plane Survey. Since no obvious counterpart has previously been found in ...longer-wavelength data, the processes that power the VHE emission are not well understood. Methods. Combining the latest VHE data with recent XMM-Newton observations and a variety of source catalogs and lower-energy survey data, we attempt to match (from an energetic and positional standpoint) the various parts of the emission of HESS J1745-303 with possible candidates. Results. Though no single counterpart is found to fully explain the VHE emission, we postulate that at least a fraction of the VHE source may be explained by a supernova-remnant/molecular-cloud association and/or a high-spin-down-flux pulsar.
We estimate systematic errors due to K-corrections in standard photometric analyses of high-redshift Type Ia supernovae. Errors due to K-correction occur when the spectral template model underlying ...the light curve fitter poorly represents the actual supernova spectral energy distribution, meaning that the distance modulus cannot be recovered accurately. In order to quantify this effect, synthetic photometry is performed on artificially redshifted spectrophotometric data from 119 low-redshift supernovae from the Nearby Supernova Factory, and the resulting light curves are fit with a conventional light curve fitter. We measure the variation in the standardized magnitude that would be fit for a given supernova if located at a range of redshifts and observed with various filter sets corresponding to current and future supernova surveys. We find significant variation in the measurements of the same supernovae placed at different redshifts regardless of filters used, which causes dispersion greater than ~0.05 mag for measurements of photometry using the Sloan-like filters and a bias that corresponds to a 0.03 shift in w when applied to an outside data set. To test the result of a shift in supernova population or environment at higher redshifts, we repeat our calculations with the addition of a reweighting of the supernovae as a function of redshift and find that this strongly affects the results and would have repercussions for cosmology. We discuss possible methods to reduce the contribution of the K-correction bias and uncertainty.
Observations of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy were carried out with the HESS array of four imaging air Cherenkov telescopes in June 2006. A total of 11
h of high quality data are ...available after data selection. There is no evidence for a very high energy γ-ray signal above the energy threshold at the target position. A 95% CL flux limit of
3.6
×
10
-
12
cm
-
2
s
-
1
above 250
GeV has been derived. Constraints on the velocity-weighted cross-section
〈
σ
v
〉
are calculated in the framework of dark matter particle annihilation using realistic models for the dark matter halo profile of Sagittarius dwarf galaxy. Two different models have been investigated encompassing a large class of halo types. A 95% CL exclusion limit on
〈
σ
v
〉
of the order of
2
×
10
-
25
cm
3
s
-
1
is obtained for a core profile in the 100
GeV–1
TeV neutralino mass range.
Aims. Galaxy clusters are key targets in the search for ultra high energy particle accelerators. The Coma cluster represents one of the best candidates for such a search owing to its high mass, ...proximity, and the established non-thermal radio emission centred on the cluster core. Methods. The HESS (High Energy Stereoscopic System) telescopes observed Coma for ~8 h in a search for γ-ray emission at energies >1 TeV. The large 3.5° FWHM field of view of HESS is ideal for viewing a range of targets at various sizes including the Coma cluster core, the radio-relic (1253+275) and merger/infall (NGC 4839) regions to the southwest, and features greater than $1^\circ$ away. Results. No evidence for point-like nor extended TeV γ-ray emission was found and upper limits to the TeV flux $F(E)$ for $E>1$, >5, and >10 TeV were set for the Coma core and other regions. Converting these limits to an energy flux $E^2F(E)$ the lowest or most constraining is the $E>5$ TeV upper limit for the Coma core (0.2° radius) at ~8% Crab flux units or ${\sim}10^{-13}$ ph cm-2 s-1. Conclusions. The upper limits for the Coma core were compared with a prediction for the γ-ray emission from proton-proton interactions, the level of which ultimately scales with the mass of the Coma cluster. A direct constraint using our most stringent limit for $E>5$ TeV, on the total energy content in non-thermal protons with injection energy spectrum ${\propto} E^{-2.1}$ and spatial distribution following the thermal gas in the cluster, is found to be ~0.2 times the thermal energy, or ${\sim} 10^{62}$ erg. The $E>5$ TeV γ-ray threshold in this case corresponds to cosmic-ray proton energies $\ga$50 TeV. Our upper limits rule out the most optimistic theoretical models for gamma ray emission from clusters and complement radio observations which constrain the cosmic ray content in clusters at significantly lower proton energies, subject to assumptions on the magnetic field strength.
Aims. Previous observations with the HESS telescope array revealed the existence of extended very-high-energy (VHE; E > 100 GeV) γ-ray emission, HESS J1023–575, coincident with the young stellar ...cluster Westerlund 2. At the time of discovery, the origin of the observed emission was not unambiguously identified, and follow-up observations have been performed to further investigate the nature of this γ-ray source. Methods. The Carina region towards the open cluster Westerlund 2 has been re-observed, increasing the total exposure to 45.9 h. The combined dataset includes 33 h of new data and now permits a search for energy-dependent morphology and detailed spectroscopy. Results. A new, hard spectrum VHE γ-ray source, HESS J1026–582, was discovered with a statistical significance of 7σ. It is positionally coincident with the Fermi LAT pulsar PSR J1028–5819. The positional coincidence and radio/γ-ray characteristics of the LAT pulsar favors a scenario where the TeV emission originates from a pulsar wind nebula. The nature of HESS J1023–575 is discussed in light of the deep HESS observations and recent multi-wavelength discoveries, including the Fermi LAT pulsar PSR J1022–5746 and giant molecular clouds in the region. Despite the improved VHE dataset, a clear identification of the object responsible for the VHE emission from HESS J1023–575 is not yet possible, and contribution from the nearby high-energy pulsar and/or the open cluster remains a possibility.
Aims. Very-high-energy (VHE; $\ga$100 GeV) γ-rays are expected from γ-ray bursts (GRBs) in some scenarios. Exploring this photon energy regime is necessary for understanding the energetics and ...properties of GRBs. Methods. GRBs have been one of the prime targets for the HESS experiment, which makes use of four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE γ-rays. Dedicated observations of 32 GRB positions were made in the years 2003–2007 and a search for VHE γ-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. Results from observations of 22 GRB positions are presented. 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 γ-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.
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