Current Issue : October - December Volume : 2018 Issue Number : 4 Articles : 5 Articles
Active galactic nuclei (AGN) produce the highest intrinsic luminosities in the Universe\nfrom within a compact region. The central engine is thought to be powered by accretion onto a\nsupermassive black hole. A fraction of this huge release of energy influences the evolution of the\nhost galaxy, and in particular, star formation. Thus, AGN are key astronomical sources not only\nbecause they play an important role in the evolution of the Universe, but also because they constitute\na laboratory for extreme physics. However, these objects are under the resolution limit of current\ntelescopes. Polarimetry is a unique technique capable of providing us with information on physical\nAGN structures. The incoming new era of X-ray polarimetry will give us the opportunity to explore\nthe geometry and physical processes taking place in the innermost regions of the accretion disc.\nHere we exploit this future powerful tool in the particular case of changing-look AGN, which are key\nfor understanding the complexity of AGN physics....
SPHiNX is a proposed satellite-borne gamma-ray burst polarimeter operating in the energy\nrange 50ââ?¬â??500 keV. The mission aims to probe the fundamental mechanism responsible for gamma-ray\nburst prompt emission through polarisation measurements. Optimising the signal-to-background\nratio for SPHiNX is an important task during the design phase. The Geant4 Monte Carlo toolkit is\nused in this work. From the simulation, the total background outside the South Atlantic Anomaly\n(SAA) is about 323 counts/s, which is dominated by the cosmic X-ray background and albedo gamma\nrays, which contribute âË?¼60% and âË?¼35% of the total background, respectively. The background from\nalbedo neutrons and primary and secondary cosmic rays is negligible. The delayed background\ninduced by the SAA-trapped protons is about 190 counts/s when SPHiNX operates in orbit for one\nyear. The resulting total background level of âË?¼513 counts/s allows the polarisation of âË?¼50 GRBs with\nminimum detectable polarisation less than 30% to be determined during the two-year mission lifetime...
The origin of sub-TeV gamma rays detected by Fermi-LAT from the Fermi bubbles at the\nGalactic center is still unknown. In a hadronic model, acceleration of protons and/or nuclei and\ntheir subsequent interactions with gas in the bubble volume can produce observed gamma rays.\nSuch interactions naturally produce high-energy neutrinos, and a detection of those can discriminate\nbetween a hadronic and a leptonic origin of gamma rays. Additional constraints on the Fermi bubbles\ngamma-ray flux in the TeV range from recent HAWC observations restrict hadronic model parameters,\nwhich in turn disfavor Fermi bubbles as the origin of a large fraction of neutrino events detected by\nIceCube along the bubble directions. We revisit our hadronic model and discuss future constraints on\nparameters from observations in very high-energy gamma rays and neutrinos....
We review the concept of the slow motion problem in General relativity. We discuss how the\nunderstanding of this process may imprint influence on the explanation of astrophysical problems....
The new generation of X-ray polarisation detectors, the gas pixel detectors, which will be\nemployed by the future space missions IXPE and eXTP, allows for spatially resolved X-ray polarisation\nstudies. This will be of particular interest for X-ray synchrotron emission from extended sources like\nyoung supernova remnants and pulsar wind nebulae. Here we report on employing a polarisation\nstatistic that can be used to makes maps in the Stokes I, Q, and U parameters, a method that\nwe expand by correcting for the energy-dependent instrumental modulation factor, using optimal\nweighting of the signal. In order to explore the types of Stokes maps that can be obtained, we present\na Monte Carlo simulation program called xpolim, with which different polarisation weighting schemes\nare explored. We illustrate its use with simulations of polarisation maps of young supernova remnants,\nafter having described the general science case for polarisation studies of supernova remnants, and its\nconnection to magnetic-field turbulence. We use xpolim simulations to show that in general deep,\n~2 Ms observations are needed to recover polarisation signals, in particular for Cas A, for which in\nthe polarisation fraction may be as low as 5%....
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