Very high energy γ-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 γ-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.
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 gamma-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.
Aims.We present results from deep γ-ray observations of the Galactic pulsar wind nebula HESS J1825–137 performed with the HESS array. Methods. Detailed morphological and spatially resolved spectral ...studies reveal the very high-energy (VHE) γ-ray aspects of this object with unprecedented precision. Results. We confirm previous results obtained in a survey of the Galactic Plane in 2004. The γ-ray emission extends asymmetrically to the south and south-west of the energetic pulsar PSR J1826–1334, that is thought to power the pulsar wind nebula. The differential γ-ray spectrum of the whole emission region is measured over more than two orders of magnitude, from 270 GeV to 35 TeV, and shows indications for a deviation from a pure power law. Spectra have also been determined for spatially separated regions of HESS J1825–137. The photon indices from a power-law fit in the different regions show a softening of the spectrum with increasing distance from the pulsar and therefore an energy dependent morphology. Conclusions. This is the first time that an energy dependent morphology has been detected in the VHE γ-ray regime. The VHE γ-ray emission of HESS J1825–137 is phenomenologically discussed in the scenario where the γ-rays are produced by VHE electrons via Inverse Compton scattering. The high γ-ray luminosity of the source cannot be explained on the basis of constant spin-down power of the pulsar and requires higher injection power in past.
Context. The blazar 1ES 1101-232 was observed with the High Energy Stereoscopic System (HESS) of Atmospheric Cherenkov Telescopes (ACT) in 2004 and 2005, for a live time of 43 h. VHE ( E > 10 ...super(11) eV) \gamma-rays were detected for the first time from this object. Aims. VHE observations of blazars are used to investigate the inner parts of the blazar jets, and also to study the extragalactic background light (EBL) in the near-infrared band. Methods. Observations in 2005 were conducted in a multiwavelength campaign, together with the RXTE satellite and optical observations. In 2004, simultaneous observations with XMM-Newton were obtained. Results. 1ES 1101-232 was detected with HESS with an excess of 649 photons, at a significance of 10\sigma. The measured VHE \gamma-ray flux amounts to {\rm d}N/{\rm d}E = (5.63 \pm 0.89) \times 10 super(-13) (E/{\rm TeV}) - (2.94\pm0.20) } cm super(-2) s super(-1) TeV super(-1), above a spectral energy threshold of 225 GeV. No significant variation of the VHE \gamma-ray flux on any time scale was found. 1ES 1101-232 exhibits a very hard spectrum, and at a redshift of z = 0.186, is the blazar with the highest confirmed redshift detected in VHE \gamma-rays so far. Conclusions. The data allow the construction of truly simultaneous spectral energy distributions of the source, from the optical to the VHE band. Using an EBL model with \nu F_{\nu} = 14 nWm super(-2) sr super(-1) at 1.5 similar to \mum as presented in Aharonian et al. (2006a) suggests an intrinsic VHE power output peak of the source at above 3 TeV.
Aims.Our aim is to study the production mechanism for very-high-energy (VHE; >100 GeV) γ-rays in distant active galactic nuclei (AGN) and use the observed VHE spectrum to derive limits on the ...Extragalactic Background Light (EBL). We also want to determine physical quantities through the modeling of the object's broad-band spectral energy distribution (SED). Methods.VHE observations (~25 h live time) of the BL Lac 1ES 0347-121 (redshift $z = 0.188$) were conducted with the High Energy Stereoscopic System (HESS) between August and December 2006. Contemporaneous X-ray and UV/optical observations from the SWIFT satellite are used to interpret the SED of the source in terms of a synchrotron self Compton (SSC) model. Results.An excess of 327 events, corresponding to a statistical significance of 10.1 standard deviations, is detected from 1ES 0347-121. Its photon spectrum, ranging from ~250 GeV to ~3 TeV, is well described by a power law with a photon index of $\Gamma = 3.10 \pm 0.23_{\mathrm{stat}} \pm 0.10_{\mathrm{sys}}$. The integral flux above 250 GeV corresponds to ~2% of the flux of the Crab Nebula above the same threshold. No VHE flux variability is detected within the data set. Conclusions.Constraints on the EBL density at optical to near-infrared wavelengths derived from the photon spectrum of 1ES 0347-121 are close to the strongest limits derived previously. The strong EBL limits confirm earlier findings, that the EBL density in the near-infrared is close to the lower limits from source counts. This implies that the universe is more transparent to VHE γ-rays than previously believed. An SSC model provides a reasonable description of the contemporaneous SED.
The rapidly varying (~10 min timescale) non-thermal X-ray emission observed from Sgr A$^{\star}$ implies that particle acceleration is occuring close to the event horizon of the supermassive black ...hole. The TeV γ-ray source HESS J1745-290 is coincident with Sgr A$^{\star}$ and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint HESS/Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of ≈9 increase in the X-ray flux of Sgr A$^{\star}$, no evidence is found for an increase in the TeV γ-ray flux from this region. We find that an increase in the γ-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the γ-ray emission is produced within ~1014 cm (~100$\,R_{\rm S}$) of the supermassive black hole.
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.
The Sculptor and Carina dwarf spheroidal galaxies were observed with the H.E.S.S. Cherenkov telescope array between January 2008 and December 2009. The data sets consist of a total of 11.8
h and 14.8
...h of high quality data, respectively. No gamma-ray signal was detected at the nominal positions of these galaxies above 220
GeV and 320
GeV, respectively. Upper limits on the gamma-ray fluxes at 95% CL assuming two forms for the spectral energy distribution (a power law shape and one derived from dark matter annihilation) are obtained at the level of 10
−13–10
−12
cm
−2
s
−1 in the TeV range. Constraints on the velocity weighted dark matter particle annihilation cross section for both Sculptor and Carina dwarf galaxies range from 〈
σv〉
∼
10
−21
cm
3
s
−1 down to 〈
σv〉
∼
10
−22
cm
3
s
−1 depending on the dark matter halo model used. Possible enhancements of the gamma-ray flux are studied: the Sommerfeld effect, which is found to exclude some dark matter particle masses, the internal Bremsstrahlung and clumps in the dark-matter halo distributions.
We report the detection of TeV γ-rays from the shell-type supernova remnant RX J0852.0-4622 with data of 3.2 h of live time recorded with HESS in February 2004. An excess of ($700 \pm 60$) events ...from the whole remnant with a significance of 12 σ was found. The observed emission region is clearly extended with a radius of the order of $1\degr$ and the spatial distribution of the signal correlates with X-ray observations. The spectrum in the energy range between 500 GeV and 15 TeV is well described by a power law with a photon index of $\Gamma = 2.1 \pm 0.1_{{\rm stat}} \pm 0.2_{{\rm syst}}$ and a differential flux at 1 TeV of $\varphi_{1\,{\rm TeV}} = (2.1 \pm 0.2_{{\rm stat}} \pm 0.6_{{\rm syst}}) \times 10^{-11}~\mbox{cm}^{-2}\,\mbox{s}^{-1}\,\mbox{TeV}^{-1}$. The integral flux above 1 TeV was measured to be $\Phi(E>1\,\mbox{TeV}) = (1.9 \pm 0.3_{{\rm stat}} \pm 0.6_{{\rm syst}}) \times 10^{-11}~\mbox{cm}^{-2}\,\mbox{s}^{-1}$, which is at the level of the flux of the Crab nebula at these energies. More data are needed to draw firm conclusions on the magnetic field in the remnant and the type of the particle population creating the TeV γ-rays.
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.
PDF
