arXiv subjects

S. Funk

Publications and source records attributed to S. Funk.

204 records · Page 12Linked to original sources

arXiv2007

XMM-Newton observations reveal the X-ray counterpart of the very-high-energy gamma-ray source HESSJ1640-465

We present X-ray observations of the as of yet unidentified very high-energy (VHE) gamma-ray source HESSJ1640-465 with the aim of establishing a counterpart of this source in the keV energy range, and identifying the mechanism responsible for the VHE emission. The 21.8 ksec XMM-Newton observation of HESSJ1640-465 in September 2005 represents a significant improvement in sensitivity and angular resolution over previous ASCA studies in this region. These new data show a hard-spectrum X-ray emitting object at the centroid of the H.E.S.S. source, within the shell of the radio Supernova Remnant (SNR) G338.3-0.0. This object is consistent with the position and flux previously measured by both ASCA and Swift-XRT but is now shown to be significantly extended. We argue that this object is very likely the counterpart to HESSJ1640-465 and that both objects may represent the Pulsar Wind Nebula of an as of yet undiscovered pulsar associated with G338.3-0.0.

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arXiv2006

XMM-Newton observations of HESS J1813-178 reveal a composite Supernova remnant

We present X-ray and 12CO(J=1-0) observations of the very-high-energy (VHE) gamma-ray source HESS J1813-178 with the aim of understanding the origin of the gamma-ray emission. Using this dataset we are able to undertake spectral and morphological studies of the X-ray emission from this object with greater precision than previous studies. NANTEN 12CO(J=1-0) data are used to search for correlations of the gamma-ray emission with molecular clouds which could act as target material for gamma-ray production in a hadronic scenario. The NANTEN 12CO(J=1-0) observations show a giant molecular cloud of mass 2.5 10^5 M$_{\sun}$ at a distance of 4 kpc in the vicinity of HESS J1813-178. Even though there is no direct positional coincidence, this giant cloud might have influenced the evolution of the gamma-ray source and its surroundings. The X-ray data show a highly absorbed non-thermal X-ray emitting object coincident with the previously known ASCA source AX J1813-178 showing a compact core and an extended tail towards the north-east, located in the centre of the radio shell-type Supernova remnant (SNR) G12.82-0.2. This central object shows morphological and spectral resemblance to a Pulsar Wind Nebula (PWN) and we therefore consider that the object is very likely to be a composite SNR. We discuss the scenario in which the gamma-rays originate in the shell of the SNR and the one in which they originate in the central object. We demonstrate, that in order to connect the core X-ray emission to the VHE gamma-ray emission electrons have to be accelerated to energies of at least 1 PeV.

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arXiv2006

Background Modelling in Very-High-Energy gamma-ray Astronomy

Ground based Cherenkov telescope systems measure astrophysical gamma-ray emission against a background of cosmic-ray induced air showers. The subtraction of this background is a major challenge for the extraction of spectra and morphology of gamma-ray sources. The unprecedented sensitivity of the new generation of ground based very-high-energy gamma-ray experiments such as H.E.S.S. has lead to the discovery of many previously unknown extended sources. The analysis of such sources requires a range of different background modelling techniques. Here we describe some of the techniques that have been applied to data from the H.E.S.S. instrument and compare their performance. Each background model is introduced and discussed in terms of suitability for image generation or spectral analysis and possible caveats are mentioned. We show that there is not a single multi-purpose model, different models are appropriate for different tasks. To keep systematic uncertainties under control it is important to apply several models to the same data set and compare the results.

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arXiv2006

Precision Measurement of Optical Pulsation using a Cherenkov Telescope

During 2003, a camera designed to measure the optical pulsations of pulsars was installed on a telescope of the H.E.S.S. array. The array is designed for gamma-ray astronomy in the ~100 GeV - 100 TeV energy regime. The aims of this exercise were two-fold: to prove the pulsar timing capabilities of H.E.S.S. on all relevant time-scales, and to explore the possibility of performing sensitive optical pulsar measurements using the ~100 m^2 mirror of a Cherenkov telescope. Measurements of the Crab pulsar with this instrument demonstrate an order of magnitude sensitivity improvement over previous attempts using Cherenkov telescopes. Here we describe the design and performance of the system and discuss design considerations for future instruments of this type.

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arXiv2005

HESS J1825-137: A pulsar wind nebula associated with PSR B1823-13?

HESS J1825-137 was detected with a significance of 8.1 $σ$ in the Galactic Plane survey conducted with the H.E.S.S. instrument in 2004. Both HESS J1825-137 and the X-ray pulsar wind nebula G18.0--0.7 (associated with the Vela-like pulsar PSR B1823-13) are offset south of the pulsar, which may be the result of the SNR expanding into an inhomogeneous medium. The TeV size ($\sim 35$ pc, for a distance of 4 kpc) is $\sim 6$ times larger than the X-ray size, which may be the result of propagation effects as a result of the longer lifetime of TeV emitting electrons, compared to the relatively short lifetime of keV synchrotron emitting electrons. The TeV photon spectral index of $\sim 2.4$ can also be related to the extended PWN X-ray synchrotron photon index of $\sim 2.3$, if this spectrum is dominated by synchrotron cooling. The anomalously large size of the pulsar wind nebula can be explained if the pulsar was born with a relatively large initial spindown power and braking index $n\sim 2$, provided that the SNR expanded into the hot ISM with relatively low density ($\sim 0.003$ cm$^{-3}$).

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arXiv2004

The Trigger System of the H.E.S.S. Telescope Array

H.E.S.S. -- The High Energy Stereoscopic System-- is a new system of large atmospheric Cherenkov telescopes for GeV/TeV Gamma-ray astronomy. This paper describes the trigger system of H.E.S.S. with emphasis on the multi-telescope array level trigger. The system trigger requires the simultaneous detection of air-showers by several telescopes at the hardware level. This requirement allows a suppression of background events which in turn leads to a lower system energy threshold for the detection of Gamma-rays. The implementation of the H.E.S.S. trigger system is presented along with data taken to characterise its performance.

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