Showing papers by "Roger Blandford published in 2023"
21 Jan 2023
TL;DR: In this article , the authors present a wind model in which interstellar gas flows along a non-rotating, expanding tube with a changing speed and cross-sectional area, and the highest-energy reaccelerated particles propagate upstream against the wind and can contribute to the PeV-EeV range of the spectrum.
Abstract: The origin of cosmic rays above the knee in the spectrum is an unsolved problem. We present a wind model in which interstellar gas flows along a non-rotating, expanding flux tube with a changing speed and cross-sectional area. Cosmic rays from Galactic sources, such as supernova remnants, which are coupled to the plasma via Alfv´en waves, provide the main pressure source for driving this outflow. These cosmic rays are then subject to diffusive shock reacceleration at the Galactic wind termination shock, which is located at a distance ∼ 200 kpc. Some of the highest-energy reaccelerated particles propagate upstream against the wind and can contribute to the PeV–EeV range of the spectrum. We analyze the conditions under which efficient reacceleration can occur and find that rigidities ∼ 10–40 PV can be obtained and that the termination shock may account for half of the proton spectrum measured in IceCube/IceTop experiment. The highest-energy particles that escape downstream from our termination shock, and similar shocks surrounding most galaxies, can be further accelerated by intergalactic shock fronts.
1 citations
20 Mar 2023
TL;DR: In this article , the authors examined a carefully selected sample of 54 CSOs, and confirmed that there are two unrelated classes: an edge-dimmed, low-luminosity class (CSO 1), and an edgebrightened, high-lighted, CSO 2.
Abstract: Compact Symmetric Objects (CSOs) form a distinct class of jetted active galactic nuclei (jetted-AGN). We examine a carefully selected sample of 54 CSOs, and confirm that there are two unrelated classes: an edge-dimmed, low-luminosity class (CSO 1), and an edge-brightened, high-luminosity class (CSO 2). Using statistically significant blind tests, we show that CSO 2s themselves consist of two morphologically distinct classes: CSO 2.0, having prominent hot-spots at the leading edges of narrow jets and/or narrow lobes; and CSO 2.2, without prominent hot-spots, and with broad jets and/or lobes. An intermediate class, CSO 2.1, exhibits mixed properties. The four classes occupy different, overlapping, portions of the luminosity-size plane, with the sizes of largest CSOs being $\sim 500$pc. We advance the hypothesis that CSO 2.0s are young and evolve through CSO 2.1s into CSO 2.2s, which are old (up to $\sim 5000$ yr). Thus CSOs do not evolve into larger types of jetted-AGN, but spend their whole life cycle as CSOs. The radio emission region energies in the CSO 2s we have studied range from $\sim 10^{-4}\, M_\odot {c}^2$ to $\sim 7 \, M_\odot {c}^2$. We show that the transient nature of CSO 2s, and their birthrate, can be explained through ignition in the tidal disruption events of giant stars. We also consider possibilities for tapping the spin energy of the supermassive black hole, and tapping the energy of the accretion disk, in a manner similar to, but not the same as, that which occurs in dwarf novae. Our results demonstrate conclusively that CSOs constitute a large family of AGN in which we have thus far studied only the brightest. More comprehensive radio studies, with higher sensitivity, resolution, and dynamic range, will revolutionize our understanding of AGN and the central engines that power them.
TL;DR: In this article , the authors summarize the general observational constraints on their composition, spectrum and isotropy which indicate that nuclei heavier than single protons dominate their spectra above ˜ 5 EeV, that they are strongly suppressed above energies ˜ 50 eeV and that the only significant departure from isotropy is a dipole.
Abstract: Understanding the acceleration of Ultra High Energy Cosmic Rays is one of the great challenges of contemporary astrophysics. In this short review, we summarize the general observational constraints on their composition, spectrum and isotropy which indicate that nuclei heavier than single protons dominate their spectra above ˜ 5 EeV, that they are strongly suppressed above energies ˜ 50 EeV, and that the only significant departure from isotropy is a dipole. Constraints based upon photopion and photodisintegration losses allow their ranges and luminosity density to be estimated. Three general classes of source model are discussed - magnetospheric models (including neutron stars and black holes), jet models (including Gamma Ray Bursts, Active Galactic Nuclei and Tidal Disruption Events) and Diffusive Shock Acceleration models (involving large accretion shocks around rich clusters of galaxies). The value of constructing larger and more capable arrays to measure individual masses at the highest energies and probably identifying their sources is emphasized.