Person: Wang, Xiawei
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Wang
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Xiawei
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Wang, Xiawei
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Publication Contribution of quasar-driven outflows to the extragalactic gamma-ray background(Springer Science and Business Media LLC, 2016-07-25) Wang, Xiawei; Loeb, AbrahamThe origin of the extragalactic γ-ray background permeating throughout the Universe remains a mystery forty years after its discovery. The extrapolated population of blazars can account for only half of the background radiation at the energy range of ~ 0.1-10 GeV. Here we show that quasar-driven outflows generate relativistic protons that produce the missing component of the extragalactic γ-ray background and naturally match its spectral fingerprint, with a generic break above ~ 1 GeV. The associated γ-ray sources are too faint to be detected individually, explaining why they had not been identified so far. However, future radio observations may image their shock fronts directly. Our best fit to the Fermi-LAT observations of extragalactic γ-ray background spectrum provides constraints on the outflow parameters that agree with observations of these outflows and theoretical predictions.Publication Cumulative neutrino background from quasar-driven outflows(IOP Publishing, 2016-12-06) Wang, Xiawei; Loeb, AbrahamQuasar-driven outflows naturally account for the missing component of the extragalactic γ-ray background through neutral pion production in interactions between protons accelerated by the forward outflow shock and interstellar protons. We study the simultaneous neutrino emission by the same protons. We adopt outflow parameters that best fit the extragalactic γ-ray background data and derive a cumulative neutrino background of ∼10−7GeVcm−2s−1sr−1 at neutrino energies Eν≳10 TeV, which naturally explains the most recent IceCube data without tuning any free parameters. The link between the γ-ray and neutrino emission from quasar outflows can be used to constrain the high-energy physics of strong shocks at cosmological distances.Publication Detecting stars at the galactic centre via synchrotron emission(Oxford University Press (OUP), 2015) Ginsburg, Idan; Wang, Xiawei; Loeb, Abraham; Cohen, OferStars orbiting within 1$\arcsec$ of the supermassive black hole in the Galactic Centre, Sgr A*, are notoriously difficult to detect due to obscuration by gas and dust. We show that some stars orbiting this region may be detectable via synchrotron emission. In such instances, a bow shock forms around the star and accelerates the electrons. We calculate that around the 10 GHz band (radio) and at 1014 Hz (infrared) the luminosity of a star orbiting the black hole is comparable to the luminosity of Sgr A*. The strength of the synchrotron emission depends on a number of factors including the star's orbital velocity. Thus, the ideal time to observe the synchrotron flux is when the star is at pericenter. The star S2 will be $\sim 0.015\arcsec$ from Sgr A* in 2018, and is an excellent target to test our predictions.Publication Ultrahigh energy cosmic rays from nonrelativistic quasar outflows(American Physical Society (APS), 2017-03-07) Wang, Xiawei; Loeb, AbrahamIt has been suggested that non-relativistic outflows from quasars can naturally account for the missing component of the extragalactic γ-ray background and explain the cumulative neutrino background through pion decay in collisions between protons accelerated by the outflow shock and interstellar protons. Here we show that the same quasar outflows are capable of accelerating protons to energies of ∼1020 eV during the early phase of their propagation. The overall quasar population is expected to produce a cumulative ultra high energy cosmic ray flux of ∼10−7GeVcm−2s−1sr−1 at ECR≳1018 eV. The spectral shape and amplitude is consistent with recent observations for outflow parameters constrained to fit secondary γ-rays and neutrinos without any additional parameter tuning. This indicates that quasar outflows simultaneously account for all three messengers at their observed levels.