K. Egberts

Fermi-LAT upper limits on gamma-ray emission from colliding wind binaries

Context. Colliding wind binaries (CWBs) are thought to give rise to a plethora of physical processes including acceleration and interaction of relativistic particles. Observation of synchrotron radiation in the radio band confirms there is a relativistic electron population in CWBs. Accordingly, CWBs have been suspected sources of high-energy -ray emission since the COS-B era. Theoretical models exist that characterize the underlying physical processes leading to particle acceleration and quantitatively predict the non-thermal energy emission observable at Earth. Aims. We strive to find evidence of -ray emission from a sample of seven CWB systems: WR 11, WR 70, WR 125, WR 137, WR 140, WR 146, and WR 147. Theoretical modelling identified these systems as the most favourable candidates for emitting -rays. We make a comparison with existing -ray flux predictions and investigate possible constraints. Methods. We used 24 months of data from the Large Area Telescope (LAT) on-board the Fermi Gamma Ray Space Telescope to perform a dedicated likelihood analysis of CWBs in the LAT energy range. Results. We find no evidence of -ray emission from any of the studied CWB systems and determine corresponding flux upper limits. For some CWBs the interplay of orbital and stellar parameters renders the Fermi -LAT data not sensitive enough to constrain the parameter space of the emission models. In the cases of WR140 and WR147, the Fermi -LAT upper limits appear to rule out some model predictions entirely and constrain theoretical models over a significant parameter space. A comparison of our findings to the CWB Car is made.

Physics and parameters in Galactic CR transport models

There are several codes available that numerically model the propagation of Galactic cosmic rays in our Galaxy. Each of these tackle this problem in different ways and concentrate on different aspects of the propagation. For the modelling of γ-rays detectable at Earth, however, GALPROP appears to be the most widely used numerical model. The reason for this is due to the fact that it considers a huge variety of effects that affect the propagation of the particles and γ-ray production. This wealth of included physics results in two principle caveats: On the one hand results depend on a vast amount of static parameters, therefore inviting to fit the data by tuning - occasionally resulting in degenerate physical conditions. On the other hand often physical effects are masked by these parameters. We will scrutinise the implementation of different physical aspects in the current GALPROP framework. We contrast this by presenting own developments and give some specific examples.

First Results of Eta Carinae Observations with H.E.S.S. II

Eta Carinae is a colliding-wind binary that shows non-thermal emission from hard X-rays to high-energy gamma rays. The gamma-ray spectrum exhibits two spectral components, where the high-energy component extends up to 300 GeV. Previous observations of Eta Carinae with the High Energy Stereoscopic System (H.E.S.S.) resulted in upper limits. With the addition of the large 28-m central telescope to the H.E.S.S. array in 2012 the lower bound on the energy range has significantly been reduced. This lowers the energy threshold of the analysis compared to the published results due to the improved instruments’ sensitivity at energies below 400 GeV. Eta Carinae has been regularly observed in the following years with H.E.S.S. II. Here we report on the first results of Eta Carinae observations, which also cover the periastron passage in 2014 - the time of maximum gamma-ray emission seen in GeV gamma-rays with Ferm-LAT.

Discovery of gamma-ray emission from the extragalactic pulsar wind nebula N157B with the High Energy Stereoscopic System

We present the significant detection of the first extragalactic pulsar wind nebula (PWN) detected in gamma rays, N157B, located in the large Magellanic Cloud (LMC). Pulsars with high spin-down luminosity are found to power energised nebulae that emit gamma rays up to energies of several tens of TeV. N157B is associated with PSRJ0537-6910, which is the pulsar with the highest known spin-down luminosity. The High Energy Stereoscopic System telescope array observed this nebula on a yearly basis from 2004 to 2009 with a dead-time corrected exposure of 46 h. The gamma-ray spectrum between 600 GeV and 12 TeV is well-described by a pure power-law with a photon index of 2.8 \pm 0.2(stat) \pm 0.3(syst) and a normalisation at 1 TeV of (8.2 \pm 0.8(stat) \pm 2.5(syst)) \times 10^-13 cm^-2s^-1TeV^-1. A leptonic multi-wavelength model shows that an energy of about 4 \times 10^49erg is stored in electrons and positrons. The apparent efficiency, which is the ratio of the TeV gamma-ray luminosity to the pulsars spindown luminosity, 0.08% \pm 0.01%, is comparable to those of PWNe found in the Milky Way. The detection of a PWN at such a large distance is possible due to the pulsars favourable spin-down luminosity and a bright infrared photon-field serving as an inverse-Compton-scattering target for accelerated leptons. By applying a calorimetric technique to these observations, the pulsars birth period is estimated to be shorter than 10 ms.

Fermi LAT upper limits on gamma-ray emission from colliding wind binaries

Colliding wind binaries (CWBs) are thought to give rise to a plethora of physical processes including acceleration and interaction of relativistic particles. Consequently, CWBs have been considered as putative gamma-ray emitters since the COS-B era. On the basis of 24 months of Fermi-LAT data, seven CWB systems are analyzed: WR 11, WR 70, WR 125, WR 137, WR 140, WR 146 and WR 147. Specific theoretical predictions for the gamma-ray emission of CWBs identify these systems as the most promising candidates among known WR-binaries due to their respective stellar and orbital parameters. We find no evidence of gamma-ray emission from any of the studied CWB systems and flux upper limits are determined. In some of the studied CWBs the interplay of orbital and stellar parameters render the Fermi data as not sufficiently sensitive to constrain parameters of the emission model. In others large uncertainties in input parameters of the gamma-ray emission model yield a large variance among model parameters that can be a...