35 research outputs found
Upper limits on very-high-energy gamma-ray emission from core-collapse supernovae observed with H.E.S.S.
Young core-collapse supernovae with dense-wind progenitors may be able to accelerate cosmic-ray hadrons beyond the knee of the cosmic-ray spectrum, and this may result in measurable gamma-ray emission. We searched for gamma-ray emission from ten supernovae observed with the High Energy Stereoscopic System (H.E.S.S.) within a year of the supernova event. Nine supernovae were observed serendipitously in the H.E.S.S. data collected between December 2003 and December 2014, with exposure times ranging from 1.4 to 53 h. In addition we observed SN 2016adj as a target of opportunity in February 2016 for 13 h. No significant gamma-ray emission has been detected for any of the objects, and upper limits on the >1 TeV gamma-ray flux of the order of 10^-13 cm-2s-1 are established, corresponding to upper limits on the luminosities in the range 2 x 10^39 to 1 x 10^42 erg s-1. These values are used to place model-dependent constraints on the mass-loss rates of the progenitor stars, implying upper limits between 2 x 10^-5 and 2 x 10^-3 Msun yr-1 under reasonable assumptions on the particle acceleration parameters
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
Gravitational Radiation From Cosmological Turbulence
An injection of energy into the early Universe on a given characteristic
length scale will result in turbulent motions of the primordial plasma. We
calculate the stochastic background of gravitational radiation arising from a
period of cosmological turbulence, using a simple model of isotropic
Kolmogoroff turbulence produced in a cosmological phase transition. We also
derive the gravitational radiation generated by magnetic fields arising from a
dynamo operating during the period of turbulence. The resulting gravitational
radiation background has a maximum amplitude comparable to the radiation
background from the collision of bubbles in a first-order phase transition, but
at a lower frequency, while the radiation from the induced magnetic fields is
always subdominant to that from the turbulence itself. We briefly discuss the
detectability of such a signal.Comment: 20 pages. Corrections for an errant factor of 2 in all the gravity
wave characteristic amplitudes. Accepted for publication in Phys. Rev.