The aromatic Universe

The rich molecular structures of polycyclic aromatic hydrocarbons — essentially planar flakes of fused benzene rings — and their fullerene cousins are revealed through their vibrational and electronic spectra.

In this artistic impression, large polycyclic aromatic hydrocarbon (PAH) molecules exposed to the strong radiation field of a star first lose all their peripheral hydrogen atoms (white atoms, top right) and are transformed into small graphene flakes whose fragile, dangling carbon rings at the corners break off. That degradation is then followed by the loss of carbon atoms. The loss creates pentagons (red) in the dehydrogenated PAH molecule, which bends the structure out of the plane and culminates in the formation of a C60 fullerene

A. Candian, J. Zhen, A.G.G.M. Tielens
Over the past 20 years, ground- and space-based observations have revealed that the universe is filled with molecules. Astronomers have identified nearly 200 types of molecules in the interstellar medium (ISM) of our galaxy and in the atmospheres of planets; for the full list, see http://www.astrochymist.org. Molecules are abundant and pervasive, and they control the temperature of interstellar gas. Not surprisingly, they directly influence such key macroscopic processes as star formation and the evolution of galaxies.
read more at https://arxiv.org/ftp/arxiv/papers/1908/1908.05918.pdf

Black Hole as Extreme Particle Accelerator

Life of the jet set. This simulation follows along in a “co-moving” reference frame with a fixed set of particles as they are blasted out of an active galactic nucleus (AGN). The magnetic field lines they experience change as they move from a smoother region (left) to a region with a kink instability (right).  [Credit: E. P. Alves et al., Phys. Rev. Lett. (2018)]

Efficient Nonthermal Particle Acceleration by the Kink Instability in Relativistic Jets

E. Paulo Alves, Jonathan Zrake, Frederico Fiuza
Relativistic magnetized jets from active galaxies are among the most powerful cosmic accelerators, but their particle acceleration mechanisms remain a mystery. We present a new acceleration mechanism associated with the development of the helical kink instability in relativistic jets, which leads to the efficient conversion of the jet’s magnetic energy into nonthermal particles. Large-scale three-dimensional ab initio simulations reveal that the formation of highly tangled magnetic fields and a large-scale inductive electric field throughout the kink-unstable region promotes rapid energization of the particles. The energy distribution of the accelerated particles develops a well-defined power-law tail extending to the radiation-reaction limited energy in the case of leptons, and to the confinement energy of the jet in the case of ions. When applied to the conditions of well-studied bright knots in jets from active galaxies, this mechanism can account for the spectrum of synchrotron and inverse Compton radiating particles, and offers a viable means of accelerating ultra-high-energy cosmic rays to 1020 eV.

Read more at https://physics.aps.org/articles/v11/130 and https://arxiv.org/pdf/1810.05154.pdf

Introduction to neutrino astronomy

neutrino flux

Energy dependence of the neutrino fluxes produced by the different nuclear processes in the Sun

Andrea Gallo Rosso, Carlo Mascaretti, Andrea Palladino, Francesco Vissani
This writeup is an introduction to neutrino astronomy, addressed to astronomers and written by astroparticle physicists. While the focus is on achievements and goals in neutrino astronomy, rather than on the aspects connected to particle physics, we will introduce the particle physics concepts needed to appreciate those aspects that depend on the peculiarity of the neutrinos. The detailed layout is as follows: In Sect.~1, we introduce the neutrinos, examine their interactions, and present neutrino detectors and telescopes. In Sect.~2, we discuss solar neutrinos, that have been detected and are matter of intense (theoretical and experimental) studies. In Sect.~3, we focus on supernova neutrinos, that inform us on a very dramatic astrophysical event and can tell us a lot on the phenomenon of gravitational collapse. In Sect.~4, we discuss the highest energy neutrinos, a very recent and lively research field. In Sect.~5, we review the phenomenon of neutrino oscillations and assess its relevance for neutrino astronomy. Finally, we offer a brief overall assessment and a summary in Sect.~6. The material is selected – i.e., not all achievements are reviewed – and furthermore it is kept to an introductory level, but efforts are made to highlight current research issues. In order to help the beginner, we prefer to limit the list of references, opting whenever possible for review works and books.

Read more at https://arxiv.org/pdf/1806.06339.pdf

Aside

Gravitational Waves: A New Astronomy


Luc Blanchet
Contemporary astronomy is undergoing a revolution, perhaps even more important than that which took place with the advent of radioastronomy in the 1960s, and then the opening of the sky to observations in the other electromagnetic wavelengths. The gravitational wave detectors of the LIGO/Virgo collaboration have observed since 2015 the signals emitted during the collision and merger of binary systems of massive black holes at a large astronomical distance. This major discovery opens the way to the new astronomy of gravitational waves, drastically different from the traditional astronomy based on electromagnetic waves. More recently, in 2017, the detection of gravitational waves emitted by the inspiral and merger of a binary system of neutron stars has been followed by electromagnetic signals observed by the γ and X satellites, and by optical telescopes. A harvest of discoveries has been possible thanks to the multi-messenger astronomy, which combines the information from the gravitational wave with that from electromagnetic waves. Another important aspect of the new gravitational astronomy concerns fundamental physics, with the tests of general relativity and alternative theories of gravitation, as well as the standard model of cosmology.
Read more at https://arxiv.org/pdf/1805.08563.pdf

Aside

Lets Talk About Black Hole Singularities

Abraham Loeb
Does the collision of black hole singularities imprint an observable quantum signature on the resulting gravitational wave signal?

The singularities at the centers of astrophysical black holes mark the breakdown of Einstein’s theory of gravity, General Relativity. They represent the only breakdown sites accessible to experimentalists, since the other known singularity, the Big Bang,is believed to be invisible due to the vast expansion that occurred afterwards during cosmic inflation…
Read more https://arxiv.org/ftp/arxiv/papers/1805/1805.05865.pdf

A short walk through the physics of neutron stars

A schematic cross section of a neutron star illustrating the various regions discussed in the text. The different regions shown are not drawn on scale.

Isaac Vidana
In this work we shortly review several aspects of the physics of neutron stars. After the introduction we present a brief historical overview of the idea of neutron stars as well as of the theoretical and observational developments that followed it from the mid 1930s to the present. Then, we review few aspects of their observation discussing, in particular, the different types of telescopes that are used, the many astrophysical manifestations of these objects, and several observables such as masses, radii or gravitational waves. Finally, we briefly summarize some of theoretical issues like their composition, structure equations, equation of state, and neutrino emission and cooling.
read more at https://arxiv.org/pdf/1805.00837.pdf