arXiv subjects

S. Ohm

Publications and source records attributed to S. Ohm.

47 records · Page 3Linked to original sources

arXiv2013

The dual-mirror Small Size Telescope for the Cherenkov Telescope Array

In this paper, the development of the dual mirror Small Size Telescopes (SST) for the Cherenkov Telescope Array (CTA) is reviewed. Up to 70 SST, with a primary mirror diameter of 4 m, will be produced and installed at the CTA southern site. These will allow investigation of the gamma-ray sky at the highest energies accessible to CTA, in the range from about 1 TeV to 300 TeV. The telescope presented in this contribution is characterized by two major innovations: the use of a dual mirror Schwarzschild-Couder configuration and of an innovative camera using as sensors either multi-anode photomultipliers (MAPM) or silicon photomultipliers (SiPM). The reduced plate-scale of the telescope, achieved with the dual-mirror optics, allows the camera to be compact (40 cm in diameter), and low-cost. The camera, which has about 2000 pixels of size 6x6 mm^2, covers a field of view of 10°. The dual mirror telescopes and their cameras are being developed by three consortia, ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana, Italy/INAF), GATE (Gamma-ray Telescope Elements, France/Paris Observ.) and CHEC (Compact High Energy Camera, universities in UK, US and Japan) which are merging their efforts in order to finalize an end-to-end design that will be constructed for CTA. A number of prototype structures and cameras are being developed in order to investigate various alternative designs. In this contribution, these designs are presented, along with the technological solutions under study.

astro-ph.IM↗

arXiv2013

Progress in Monte Carlo design and optimization of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) will be an instrument covering a wide energy range in very-high-energy (VHE) gamma rays. CTA will include several types of telescopes, in order to optimize the performance over the whole energy range. Both large-scale Monte Carlo (MC) simulations of CTA super-sets (including many different possible CTA layouts as sub-sets) and smaller-scale simulations dedicated to individual aspects were carried out and are on-going. We summarize results of the prior round of large-scale simulations, show where the design has now evolved beyond the conservative assumptions of the prior round and present first results from the on-going new round of MC simulations.

astro-ph.IM↗

arXiv2013

A Compact High Energy Camera for the Cherenkov Telescope Array

The Compact High Energy Camera (CHEC) is a camera-development project involving UK, US, Japanese and Dutch institutes for the dual-mirror Small-Sized Telescopes (SST-2M) of the Cherenkov Telescope Array (CTA). Two CHEC prototypes, based on different photosensors are funded and will be assembled and tested in the UK over the next ~18 months. CHEC is designed to record flashes of Cherenkov light lasting from a few to a hundred nanoseconds, with typical RMS image width and length of ~0.2 x 1.0 degrees, and has a 9 degree field of view. The physical camera geometry is dictated by the telescope optics: a curved focal surface with radius of curvature 1m and diameter ~35cm is required. CHEC is designed to work with both the ASTRI and GATE SST-2M telescope structures and will include an internal LED flasher system for calibration. The first CHEC prototype will be based on multi-anode photomultipliers (MAPMs) and the second on silicon photomultipliers (SiPMs or MPPCs). The first prototype will soon be installed on the ASTRI SST-2M prototype structure on Mt. Etna.

astro-ph.IM↗

arXiv2013

Gamma-ray emission from the Westerlund 1 region

Westerlund 1 (Wd 1) is the most massive stellar cluster in the Galaxy and associated with an extended region of TeV emission. Here we report the results of a search for GeV gamma-ray emission in this region. The analysis is based on ~4.5 years of Fermi-LAT data and reveals significantly extended emission which we model as a Gaussian, resulting in a best-fit sigma of sigma_S = (0.475 +/- 0.05) deg and an offset from Wd 1 of ~1 deg. A partial overlap of the GeV emission with the TeV signal as reported by H.E.S.S. is found. We investigate the spectral and morphological characteristics of the gamma-ray emission and discuss its origin in the context of two distinct scenarios. Acceleration of electrons in a Pulsar Wind Nebula provides a reasonably natural interpretation of the GeV emission, but leaves the TeV emission unexplained. A scenario in which protons are accelerated in or near Wd 1 in supernova explosion(s) and are diffusing away and interacting with molecular material, seems consistent with the observed GeV and TeV emission, but requires a very high energy input in protons, ~10^51 erg, and rather slow diffusion. Observations of Wd 1 with a future gamma-ray detector such as CTA provide a very promising route to fully resolve the origin of the TeV and GeV emission in Wd 1 and provide a deeper understanding of the high-energy (HE) astrophysics of massive stellar clusters.

astro-ph.HE↗

arXiv2013

Supernova remnants interacting with molecular clouds as seen with H.E.S.S

About 30 Galactic supernova remnants (SNRs) are thought to be physically associated with molecular clouds (MCs). These systems are prime \g-ray source candidates as the accelerated particles from shock fronts collide with the surrounding high-density medium thus emitting gamma-rays through hadronic interactions. However only a handful of such interacting SNRs are detected at TeV energies. We report the current status of the High Energy Stereoscopic System (H.E.S.S.) observations towards these SNR-MC systems, with a particular emphasis on the latest results.

astro-ph.GA↗

arXiv2012

Monte Carlo design studies for the Cherenkov Telescope Array

The Cherenkov Telescopes Array (CTA) is planned as the future instrument for very-high-energy (VHE) gamma-ray astronomy with a wide energy range of four orders of magnitude and an improvement in sensitivity compared to current instruments of about an order of magnitude. Monte Carlo simulations are a crucial tool in the design of CTA. The ultimate goal of these simulations is to find the most cost-effective solution for given physics goals and thus sensitivity goals or to find, for a given cost, the solution best suited for different types of targets with CTA. Apart from uncertain component cost estimates, the main problem in this procedure is the dependence on a huge number of configuration parameters, both in specifications of individual telescope types and in the array layout. This is addressed by simulation of a huge array intended as a superset of many different realistic array layouts, and also by simulation of array subsets for different telescope parameters. Different analysis methods -- in use with current installations and extended (or developed specifically) for CTA -- are applied to the simulated data sets for deriving the expected sensitivity of CTA. In this paper we describe the current status of this iterative approach to optimize the CTA design and layout.

astro-ph.IM↗

arXiv2012

High-energy emission from galaxies: the star-formation/gamma-ray connection

The impact of non-thermal processes on the spectral energy distributions of galaxies can be dramatic, but such processes are often neglected in considerations of their structure and evolution. Particle acceleration associated with high mass star formation and AGN activity not only leads to very broad band (radio-γ-ray) emission, but may also produce very significant feedback effects on galaxies and their environment. The recent detections of starburst galaxies at GeV and TeV energies suggest that γ-ray instruments have now reached the critical level of sensitivity to probe the connection between particle acceleration and star-formation in galaxies. In this paper we will try to summarise this recent progress, put it into a multi-wavelength context and also discuss the prospects for more precise and sensitive γ-ray measurements with the upcoming CTA observatory.

astro-ph.HE↗

arXiv2012

Constraints on the non-thermal emission from Eta Carinae's blast wave of 1843

Non-thermal hard X-ray and high-energy (HE; 1 MeV < E < 100 GeV) gamma-ray emission in the direction of Eta Carinae has been recently detected using the INTEGRAL, AGILE and Fermi satellites. This emission has been interpreted either in the framework of particle acceleration in the colliding wind region between the two massive stars or in the very fast moving blast wave which originates in the historical 1843 "Great Eruption". Archival Chandra data has been reanalysed to search for signatures of particle acceleration in Eta Carinae's blast wave. No shell-like structure could be detected in hard X-rays and a limit has been placed on the non-thermal X-ray emission from the shell. The time dependence of the target radiation field of the Homunculus is used to develop a single zone model for the blast wave. Attempting to reconcile the X-ray limit with the HE -ray emission using this model leads to a very hard electron injection spectrum dN/dE ~ E^-Gamma with Gamma < 1.8, harder than the canonical value expected from diffusive shock acceleration.

astro-ph.HE↗

arXiv2012

H.E.S.S. observations of the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is a satellite galaxy of the Milky Way at a distance of approximately 48 kpc. Despite its distance it harbours several interesting targets for TeV gamma-ray observations. The composite supernova remnant N 157B/PSR J05367-6910 was discovered by H.E.S.S. being an emitter of very high energy (VHE) gamma-rays. It is the most distant pulsar wind nebula ever detected in VHE gamma-rays. Another very exciting target is SN 1987A, the remnant of the most recent supernova explosion that occurred in the neighbourhood of the Milky Way. Models for Cosmic Ray acceleration in this remnant predict gamma-ray emission at a level detectable by H.E.S.S. but this has not been detected so far. Fermi/LAT discovered diffuse high energy (HE) gamma-ray emission from the general direction of the massive star forming region 30 Doradus but no clear evidence for emission from either N 157B or SN 1987A has been published. The part of the LMC containing these objects has been observed regularly with the H.E.S.S. telescopes since 2003. With deep observations carried out in 2010 a very good exposure of this part of the sky has been obtained. The current status of the H.E.S.S. LMC observations is reported along with new results on N 157B and SN 1987A.

astro-ph.HE↗

arXiv2011

Discovery of VHE and HE emission from the blazar 1ES 0414+009 with H.E.S.S and Fermi-LAT

The high energy peaked BL Lac (HBL) object 1ES 0414+009 (z=0.287) is a distant very high-energy (VHE, E > 100 GeV) blazars with well-determined redshift. This source was detected with the High Energy Stereoscopic System (H.E.S.S.) between October 2005 and September 2009. It was also detected with the Fermi Large Area Telescope (LAT) in 21 months of data. The combined high energy (HE) and VHE spectra, once corrected for gamma-gamma absorption on the extragalactic background light (EBL), indicate a Compton peak located above few TeV, among the highest in the BL Lac class.

astro-ph.HE↗

arXiv2010

Particle acceleration in the expanding blast wave of Eta Carina's Great Eruption of 1843

Non-thermal hard X-ray and high-energy (HE; 1 MeV < E < 100 GeV) gamma-ray emission in the direction of Eta Carina has been recently detected using the INTEGRAL, AGILE and Fermi satellites. So far this emission has been interpreted in the framework of particle acceleration in the colliding wind region between the two massive stars. However, the existence of a very fast moving blast wave which originates in the historical 1843 "Great Eruption" provides an alternative particle acceleration site in this system. Here we explore an alternate scenario and find that inverse Compton emission from electrons accelerated in the blast wave can naturally explain both the flux and spectral shape of the measured hard X-ray and HE gamma-ray emission. This scenario is further supported by the lack of significant variability in the INTEGRAL and Fermi measured fluxes.

astro-ph.HE↗