Aims.Search for Very High Energy γ-ray emission in the Kookaburra complex through observations with the HESS array. Methods.Stereoscopic imaging of Cherenkov light emission of the γ-ray showers in ...the atmosphere is used for the reconstruction and selection of the events to search for γ-ray signals. Their spectrum is derived by a forward-folding maximum likelihood fit. Results.Two extended γ-ray sources with an angular (68%) radius of $3.3{-}3.4$´ are discovered at high (>13σ) statistical significance: HESS J1420-607 and HESS J1418-609. They exhibit a flux above 1 TeV of ($2.97 \pm 0.18_{\rm stat} \pm 0.60_{\rm sys}) \times 10^{-12}$ and ($2.17 \pm 0.17_{\rm stat} \pm 0.43_{\rm sys}) \times 10^{-12}$ cm-2 s-1, respectively, and similar hard photon indices ~2.2. Multi-wavelength comparisons show spatial coincidence with the wings of the Kookaburra. Two pulsar wind nebulæ candidates, K3/PSR J1420-6048 and the Rabbit, lie on the edge of the HESS sources. Conclusions. The two new sources confirm the non-thermal nature of at least parts of the two radio wings which overlap with the γ-ray emission and establish their connection with the two X-ray pulsar wind nebulæ candidates. Given the large point spread function of EGRET, the unidentified source(s) 3EG J1420-6038/GeV J1417-6100 could possibly be related to either or both HESS sources. The most likely explanation for the Very High Energy γ-rays discovered by HESS is inverse Compton emission of accelerated electrons on the Cosmic Microwave Background near the two candidate pulsar wind nebulæ, K3/PSR J1420-6048 and the Rabbit. Two scenarios which could lead to the observed large (~10 pc) offset-nebula type morphologies are briefly discussed.
The high-frequency peaked BL Lac PG 1553+113 was observed in 2005 with the HESS stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. Using the HESS standard analysis, an excess ...was measured at the 4.0σ level in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5σ excesses. The observed integral flux above 200 GeV is $(4.8\pm1.3_{\rm stat}\pm1.0_{\rm syst})\times10^{-12}$ cm-2 s-1, and shows no evidence for variability. The measured energy spectrum is characterized by a very soft power law (photon index of $\Gamma=4.0\pm0.6$). Although the redshift of PG 1553+113 is unknown, there are strong indications that it is greater than $z=0.25$ and possibly larger than $z=0.78$. The observed spectrum is interpreted in the context of VHE γ-ray absorption by the Extragalactic Background Light, and is used to place an upper limit on the redshift of $z<0.74$.
Context.We present the discovery of two very-high-energy γ-ray sources in an ongoing systematic search for emission above 100 GeV from pulsar wind nebulae in survey data from the HESS telescope ...array. Aims.Imaging Atmospheric Cherenkov Telescopes are ideal tools for searching for extended emission from pulsar wind nebulae in the very-high-energy regime. HESS, with its large field of view of 5° and high sensitivity, gives new prospects for the search for these objects. Methods.An ongoing systematic search for very-high-energy emission from energetic pulsars over the region of the Galactic plane between $-60^\circ < l < 30^\circ$, $-2^\circ < b < 2^\circ$ is performed. For the resulting candidates, the standard HESS analysis was applied and a search for multi-wavelength counterparts was performed. Results. We present the discovery of two new candidate γ-ray pulsar wind nebulae, HESS J1718-385 and HESS J1809-193. Conclusions.HESS has proven to be a suitable instrument for pulsar wind nebula searches.
The extreme synchrotron BL Lac object H 2356-309, located at a redshift of z = 0.165, was observed from June to December 2004 with a total exposure of approx40 h live-time with the HESS (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 H 2356-309, corresponding to an observed integral flux above 200 GeV of I (>200 GeV) = (4.1 pm 0.5) times 10 super(-12) cm super(-2) s super(-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 sub(0) = (4.1 pm 0.5) times 10 super(-13) cm super(-2) s super(-1) TeV super(-1) and a photon index of Gamma = 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 HESS data to constrain a single-zone homogeneous synchrotron self- Compton (SSC) model. This model provides an adequate fit to the HESS data when using a reasonable set of model parameters.
Ground-based gamma astronomy is based on the detection of the Cherenkov photons. Aerosols and high altitude clouds enhance photon scattering and then disturb the ground detection of Cherenkov ...showers. We present Monte Carlo simulations for the propagation of gamma and proton showers (energy in the range 50 GeV–2 TeV) in inhomogeneous atmosphere with aerosols and clouds of weak optical depth. The results indicate clearly that atmospheric scattering reduces significantly the Cherenkov signal for low-energy showers (50–500 GeV for gamma induced showers). A phenomenological relation is proposed for the estimation of the ground atmospheric attenuation of the Cherenkov signal.
The serendipitous discovery of an unidentified extended TeV γ-ray source close to the galactic plane named HESS J1303-631 at a significance of 21 standard deviations is reported. The observations ...were performed between February and June 2004 with the HESS stereoscopic system of Cherenkov telescopes in Namibia. HESS J1303-631 was discovered roughly $0.6\degr$ north of the binary system PSR B1259-63/SS 2883, the target object of the initial observation campaign which was also detected at TeV energies in the same field of view. HESS J1303-631 is extended with a width of an assumed intrinsic Gaussian emission profile of $\sigma = (0.16 \pm 0.02)\degr$ and the integral flux above $380\,\mathrm{GeV}$ is compatible with constant emission over the entire observational period of $(17 \pm 3)$% of the Crab Nebula flux. The measured energy spectrum can be described by a power-law $\mathrm{d}N/\mathrm{d}E \sim E^{-\Gamma}$ with a photon index of $\Gamma = 2.44 \pm 0.05_{\mathrm{stat}} \pm 0.2_{\mathrm{syst}}$. Up to now, no counterpart at other wavelengths is identified. Various possible TeV production scenarios are discussed.
Mkn 421 was observed during a high flux state for nine nights in April and May 2004 with the fully operational High Energy Stereoscopic System (HESS) in Namibia. The observations were carried out at ...zenith angles of 60°–65°, which result in an average energy threshold of 1.5 TeV and a collection area reaching 2 km2 at 10 TeV. Roughly 7000 photons from Mkn 421 were accumulated with an average gamma-ray rate of 8 photons/min. The overall significance of the detection exceeds 100 standard deviations. The light-curve of integrated fluxes above 2 TeV shows changes of the diurnal flux up to a factor of 4.3. For nights of high flux, intra-night variability is detected with a decay time of less than 1 h. The time averaged energy spectrum is curved and is well described by a power-law with a photon index $\Gamma=2.1\pm0.1_{\rm stat}\pm0.3_{\rm sys}$ and an exponential cutoff at $E_{\rm c}=3.1(+0.5\,-0.4)_{\rm stat}\pm0.9_{\rm sys}$ TeV and an average integral flux above 2 TeV of 3 Crab flux units. Significant variations of the spectral shape are detected with a spectral hardening as the flux increases. Contemporaneous multi-wavelength observations at lower energies (X-rays and gamma-rays above ≈300 GeV) indicate smaller relative variability amplitudes than seen above 2 TeV during high flux state observed in April 2004.
Observations of selected AGN with HESS Aharonian, F.; Akhperjanian, A. G.; Bazer-Bachi, A. R. ...
Astronomy and astrophysics (Berlin),
10/2005, Letnik:
441, Številka:
2
Journal Article
Recenzirano
Odprti dostop
A sample of selected active galactic nuclei (AGN) was observed in 2003 and 2004 with the High Energy Stereoscopic System (HESS), an array of imaging atmospheric-Cherenkov telescopes in Namibia. The ...redshifts of these candidate very-high-energy (VHE, >100 GeV) γ-ray emitters range from $z=0.00183$ to $z=0.333$. Significant detections were already reported for some of these objects, such as PKS 2155-304 and Markarian 421. Marginal evidence (3.1σ) for a signal is found from large-zenith-angle observations of Markarian 501, corresponding to an integral flux of I(>1.65 TeV) = (1.5±0.6$_{\mathrm{stat}}$±0.3$_{\mathrm{syst}}$) $\times$ 10-12 cm-2 s-1 or ~15% of the Crab Nebula flux. Integral flux upper limits for 19 other AGN, based on exposures of ~1 to ~8 h live time, and with average energy thresholds between 160 GeV and 610 GeV, range from 0.4% to 5.1% of the Crab Nebula flux. All the upper limits are the most constraining ever reported for these objects.
Context. In some galaxy clusters, powerful active galactic nuclei (AGN) have blown bubbles with cluster scale extent into the ambient medium. The main pressure support of these bubbles is not known ...to date, but cosmic rays are a viable possibility. For such a scenario copious gamma-ray emission is expected as a tracer of cosmic rays from these systems. Aims. Hydra A, the closest galaxy cluster hosting a cluster scale AGN outburst, located at a redshift of 0.0538, is investigated for being a gamma-ray emitter with the High Energy Stereoscopic System (H.E.S.S.) array and the Fermi Large Area Telescope (Fermi-LAT). Methods. Data obtained in 20.2 h of dedicated H.E.S.S. observations and 38 months of Fermi-LAT data, gathered by its usual all-sky scanning mode, have been analyzed to search for a gamma-ray signal. Results. No signal has been found in either data set. Upper limits on the gamma-ray flux are derived and are compared to models. These are the first limits on gamma-ray emission ever presented for galaxy clusters hosting cluster scale AGN outbursts. Conclusions. The non-detection of Hydra A in gamma-rays has important implications on the particle populations and physical conditions inside the bubbles in this system. For the case of bubbles mainly supported by hadronic cosmic rays, the most favorable scenario, which involves full mixing between cosmic rays and embedding medium, can be excluded. However, hadronic cosmic rays still remain a viable pressure support agent to sustain the bubbles against the thermal pressure of the ambient medium. The largest population of highly-energetic electrons, which are relevant for inverse-Compton gamma-ray production is found in the youngest inner lobes of Hydra A. The limit on the inverse-Compton gamma-ray flux excludes a magnetic field below half of the equipartition value of 16 μG in the inner lobes.
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