The recently detected gamma-ray emission from Starburst galaxies is most commonly considered to be diffuse emission arising from strong interactions of accelerated cosmic rays. Mannheim et al. (2012), however, have argued that a population of individ
ual pulsar-wind nebulae (PWNe) could be responsible for the detected TeV emission. Here we show that the Starburst environment plays a critical role in the TeV emission from Starburst PWNe, and perform the first detailed calculations for this scenario. Our approach is based on the measured star-formation rates in the Starburst nuclei of NGC 253 and M 82, assumed pulsar birth periods and a simple model for the injection of non-thermal particles. The two-zone model applied here takes into account the high far-infrared radiation field, and different densities and magnetic fields in the PWNe and the Starburst regions, as well as particle escape. We confirm that PWNe can make a significant contribution to the TeV fluxes, provided that the injection spectrum of particles is rather hard and that the average pulsar birth period is rather short (~35 ms). The PWN contribution should lead to a distinct spectral feature which can be probed by future instruments such as CTA.
In this paper the current status of gamma-ray observations of starburst galaxies from hundreds of MeV up to TeV energies with space-based instruments and ground-based Imaging Atmospheric Cherenkov Telescopes (IACTs) is summarised. The properties of t
he high-energy (HE; 100 MeV < E < 100 GeV) and very-high-energy (VHE; E > 100 GeV) emission of the archetypical starburst galaxies M 82 and NGC 253 are discussed and put into context with the HE gamma-ray emission detected from other galaxies that show enhanced star-formation activity such as NGC 4945 and NGC 1068. Finally, prospects to study the star-formation - gamma-ray emission connection from Galactic systems to entire galaxies with the forthcoming Cherenkov Telescope Array (CTA) are outlined.
The nature of the first unidentified VHE gamma-ray source with significant angular offset from the Galactic plane of 3.5 degrees, HESS J1507-622, is explored. Fermi-LAT data in the high-energy (HE, 100 MeV < E < 100 GeV) gamma-ray range collected ove
r 34 month are used to describe the spectral energy distribution (SED) of the source. HESS J1507-622 is detected in the Fermi energy range and its spectrum is best described by a power law in energy with Gamma=1.7 +/- 0.1 stat +/- 0.2_sys and integral flux between (0.3-300) GeV of F = (2.0 +/-0.5_stat +/- 1.0_sys) x 10^-9 cm^-2 s^-1. With the available data it is not possible to discriminate between a hadronic and a leptonic scenario for HESS J1507-622. The location and compactness of the source indicate a considerable physical offset from the Galactic plane for this object. In case of a multiple-kpc distance, this challenges a pulsar wind nebula (PWN) origin for HESS J1507-622 since the time of travel for a pulsar born in the Galactic disk to reach such a location would exceed the inverse Compton (IC) cooling time of electrons that are energetic enough to produce VHE gamma-rays. However, an origin of this gamma-ray source connected to a pulsar that was born off the Galactic plane in the explosion of a hypervelocity star cannot be excluded. The nature of HESS J1507-622 is still unknown to date, and a PWN scenario cannot be ruled out in general. On the contrary HESS J1507-622 could be the first discovered representative of a population of spatially extended VHE gamma-ray emitters with HE gamma-ray counterpart that are located at considerable offsets from the Galactic plane. Future surveys in the VHE gamma-ray range are necessary to probe the presence or absence of such a source population.(abridged)
In recent years, Imaging Atmospheric Cherenkov Telescopes (IACTs) have discovered a rich diversity of very high energy (VHE, > 100 GeV) gamma-ray emitters in the sky. These instruments image Cherenkov light emitted by gamma-ray induced particle casca
des in the atmosphere. Background from the much more numerous cosmic-ray cascades is efficiently reduced by considering the shape of the shower images, and the capability to reduce this background is one of the key aspects that determine the sensitivity of a IACT. In this work we apply a tree classification method to data from the High Energy Stereoscopic System (H.E.S.S.). We show the stability of the method and its capabilities to yield an improved background reduction compared to the H.E.S.S. Standard Analysis.
