Gamma-ray astronomy

Abstract:

The interactions of ultra-high energy cosmic rays (UHECRs) with background photons in extragalactic space generate high-energy neutrinos and photons. Simulating UHECR propagation requires assumptions about physical quantities such as the spectrum of the extragalactic background light (EBL) and photodisintegration cross sections. These assumptions, as well as the approximations used in the codes, may influence the computed predictions both of cosmic-ray spectra and composition, and of cosmogenic neutrino and photon fluxes. Following up on our previous work where we studied the resulting uncertainties on cosmic-ray simulations, here we quantify those on neutrinos and photons, using the Monte Carlo codes CRPropa and SimProp in various source scenarios. We discuss the results in the light of the constraining power of the neutrino and photon spectra on the origin of the UHECRs. We show that cosmogenic neutrino fluxes are more sensitive to the parametrization of the EBL than UHECR spectra, whereas the overall cosmogenic gamma-ray production rates are relatively independent on details of the propagation. We also find large differences between neutrino fluxes predicted by the latest released versions of CRPropa and SimProp, and discuss their causes and possible improvements in future versions of the codes.

Abstract:

Powerful relativistic jets are one of the main ways in which accreting black holes provide kinetic feedback to their surroundings. Jets launched from or redirected by the accretion flow that powers them are expected to be affected by the dynamics of the flow, which for accreting stellar-mass black holes has shown evidence for precession1 due to frame-dragging effects that occur when the black-hole spin axis is misaligned with the orbital plane of its companion star2. Recently, theoretical simulations have suggested that the jets can exert an additional torque on the accretion flow3, although the interplay between the dynamics of the accretion flow and the launching of the jets is not yet understood. Here we report a rapidly changing jet orientation—on a time scale of minutes to hours—in the black-hole X-ray binary V404 Cygni, detected with very-long-baseline interferometry during the peak of its 2015 outburst. We show that this changing jet orientation can be modelled as the Lense–Thirring precession of a vertically extended slim disk that arises from the super-Eddington accretion rate4. Our findings suggest that the dynamics of the precessing inner accretion disk could play a role in either directly launching or redirecting the jets within the inner few hundred gravitational radii. Similar dynamics should be expected in any strongly accreting black hole whose spin is misaligned with the inflowing gas, both affecting the observational characteristics of the jets and distributing the black-hole feedback more uniformly over the surrounding environment5,6.