Extreme Astrophysical Environments Research Articles

Precision spectroscopy of H13CN using a free-running, all-fiber dual electro-optic frequency comb system.

We demonstrate the precision molecular spectroscopy of H13CN using a free-running, all-fiber dual electro-optic frequency comb system. Successive interferograms, acquired at a rate of Δfrep=1 MHz, were phase-corrected in post-processing, averaged, and normalized to yield the complex transmission spectrum of several transitions within the 2ν1H13CN band centered near λ=1545 nm. With spectral signal-to-noise ratios as high as 326:1 achieved in 2ms of integration time, we report accurate measurements of H13CN transition intensities which will aid in the study of extreme astrophysical environments.

Instabilities and propagation of neutrino magnetohydrodynamic waves in arbitrary direction

In a previous work [Haas et al., Phys. Plasmas 23, 012104 (2016)], a new model was introduced, taking into account the role of the Fermi weak force due to neutrinos coupled to magnetohydrodynamic plasmas. The resulting neutrino-magnetohydrodynamics was investigated in a particular geometry associated with the magnetosonic wave, where the ambient magnetic field and the wavevector are perpendicular. The corresponding fast, short wavelength neutrino beam instability was then obtained in the context of supernova parameters. The present communication generalizes these results, allowing for arbitrary direction of wave propagation, including fast and slow magnetohydrodynamic waves and the intermediate cases of oblique angles. The numerical estimates of the neutrino-plasma instabilities are derived in extreme astrophysical environments where dense neutrino beams exist.

Energy cascade and scaling in supersonic isothermal turbulence

AbstractSupersonic turbulence plays an important role in a number of extreme astrophysical and terrestrial environments, yet its understanding remains rudimentary. We use data from a three-dimensional simulation of supersonic isothermal turbulence to reconstruct an exact fourth-order relation derived analytically from the Navier–Stokes equations (Galtier & Banerjee, Phys. Rev. Lett., vol. 107, 2011, p. 134501). Our analysis supports a Kolmogorov-like inertial energy cascade in supersonic turbulence previously discussed on a phenomenological level. We show that two compressible analogues of the four-fifths law exist describing fifth- and fourth-order correlations, but only the fourth-order relation remains ‘universal’ in a wide range of Mach numbers from incompressible to highly compressible regimes. A new approximate relation valid in the strongly supersonic regime is derived and verified. We also briefly discuss the origin of bottleneck bumps in simulations of compressible turbulence.

Space-station experiment measures arriving positrons with unprecedented precision

The excess positrons above those produced by cosmic-ray collisions may result from dark-matter annihilation or from extreme astrophysical environments.

Neutrinos from Supernovae and Gamma Ray Bursts: Nucleosynthesis and Detection

We discuss the way in which neutrino oscillations, sterile neutrinos and ambient conditions impact the nucleosynthesis in extreme astrophysical environments such as supernovae and gamma ray bursts. We focus first on r-process nucleosynthesis and supernovae and the role of neutrinos. Secondly, we discuss active sterile neutrino oscillations in this environment. We then turn to gamma ray bursts and examine the nucleosynthesis that can be produced by accretion disk winds, and the impact of the neutrinos. Finally we comment on the prospects for detecting such an accretion disk if it were to occur in the Galaxy with existing neutrino detectors.

GRAVITAS: general relativistic astrophysics via timing and spectroscopy

GRAVITAS is an X-ray observatory, designed and optimised to address the ESA Cosmic Vision theme of "Matter under extreme conditions". It was submitted as a response to the call for M3 mission proposals. The concept centres around an X-ray telescope of unprecedented effective area, which will focus radiation emitted from close to the event horizon of black holes or the surface of neutron stars. To reveal the nature and behaviour of matter in the most extreme astrophysical environments, GRAVITAS targets a key feature in the X-ray spectra of compact objects: the iron Kalpha line at ~6.5 keV. The energy, profile, and variability of this emission line, and the properties of the surrounding continuum emission, shaped by General Relativity (GR) effects, provide a unique probe of gravity in its strong field limit. Among its prime targets are hundreds of supermassive black holes in bright Active Galactic Nuclei (AGN), which form the perfect laboratory to help understand the physical processes behind black hole growth. Accretion plays a fundamental role in the shaping of galaxies throughout cosmic time, via the process of feedback. Modest (~sub-arcmin) spatial resolution would deliver the necessary sensitivity to extend high quality X-ray spectroscopy of AGN to cosmologically-relevant distances. Closer to home, ultra-high count rate capabilities and sub-millisecond time resolution enable the study of GR effects and the equation of state of dense matter in the brightest X-ray binaries in our own Galaxy, using multiple probes, such as the broad iron line, the shape of the disk continuum emission, quasi-periodic oscillations, reverberation mapping, and X-ray burst oscillations. Despite its breakthrough capabilities, all enabling technologies for GRAVITAS are already in a high state of readiness. It is based on ultra light-weight X-ray optics and a focal plane detector using silicon technology.

Magnetic Reconnection in Extreme Astrophysical Environments

Magnetic reconnection is a basic plasma process of dramatic rearrangement of magnetic topology, often leading to a violent release of magnetic energy. It is important in magnetic fusion and in space and solar physics --- areas that have so far provided the context for most of reconnection research. Importantly, these environments consist just of electrons and ions and the dissipated energy always stays with the plasma. In contrast, in this paper I introduce a new direction of research, motivated by several important problems in high-energy astrophysics --- reconnection in high energy density (HED) radiative plasmas, where radiation pressure and radiative cooling become dominant factors in the pressure and energy balance. I identify the key processes distinguishing HED reconnection: special-relativistic effects; radiative effects (radiative cooling, radiation pressure, and Compton resistivity); and, at the most extreme end, QED effects, including pair creation. I then discuss the main astrophysical applications --- situations with magnetar-strength fields (exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares and magnetically-powered central engines and jets of GRBs. Here, magnetic energy density is so high that its dissipation heats the plasma to MeV temperatures. Electron-positron pairs are then copiously produced, making the reconnection layer highly collisional and dressing it in a thick pair coat that traps radiation. The pressure is dominated by radiation and pairs. Yet, radiation diffusion across the layer may be faster than the global Alfv\'en transit time; then, radiative cooling governs the thermodynamics and reconnection becomes a radiative transfer problem, greatly affected by the ultra-strong magnetic field. This overall picture is very different from our traditional picture of reconnection and thus represents a new frontier in reconnection research.

AUTOMATED NanoSIMS MEASUREMENTS OF SPINEL STARDUST FROM THE MURRAY METEORITE

We report new O isotopic data on 41 presolar oxide grains, 38 MgAl2O4 (spinel) and 3 Al2O3 from the CM2 meteorite Murray, identified with a recently developed automated measurement system for the NanoSIMS. We have also obtained Mg-Al isotopic results on 29 of the same grains (26 spinel and 3 Al2O3). The majority of the grains have O isotopic compositions typical of most presolar oxides, fall well into the four previously defined groups, and are most likely condensates from either red giant branch or asymptotic giant branch stars. We have also discovered several grains with more unusual O and Mg compositions suggesting formation in extreme astrophysical environments, such as novae and supernovae. One of these grains has massive enrichments in 17O, 25Mg, and 26Mg, which are isotopic signatures indicative of condensation from nova ejecta. Two grains of supernova origin were also discovered: one has a large 18O/16O ratio typical of Group 4 presolar oxides; another grain is substantially enriched in 16O, and also contains radiogenic 44Ca from the decay of 44Ti, a likely condensate from material originating in the O-rich inner zones of a Type II supernova. In addition, several Group 2 presolar spinel grains also have large 25Mg and 26Mg isotopic anomalies that are difficult to explain by standard nucleosynthesis in low-mass stars. Auger elemental spectral analyses were performed on the grains and qualitatively suggest that presolar spinel may not have higher-than- stoichiometric Al/Mg ratios, in contrast to SIMS results obtained here and reported previously.

The evolution of dust in extreme astrophysical environments

Dust is present in almost every astrophysical environment, ranging from circumstellar shells and disks to spiral, elliptical, starburst, and active galaxies, and to pre-galactic objects such as QSO absorption-line and damped Ly α systems. Dust leaves its imprint on interstellar extinction curves, IR spectra, and the elemental depletion patterns in the ISM of galaxies. Understanding the origin and the complex evolutionary cycle of dust is therefore an important goal in astrophysics. In this contribution, we present models to describe the evolutionary history of interstellar dust in a diverse set of astrophysical environments, ranging from normal star-forming galaxies like the Milky Way to high-redshift galaxies undergoing extreme rates of star formation. In particular, we show how the chemical evolution models can explain the correlations of dust abundances with galactic metallicities, and the presence of large amounts of dust in young dusty hyperluminous infrared galaxies in which supernovae are the only source of newly-condensed dust.

Astronomy and astrophysics with neutrinos

Traversing cosmological distances without bending or energy loss, high-energy neutrinos are messengers from extreme astrophysical environments.

FUTURE SUMMARY

We are emerging from a period of consolidation in particle physics. Its great, historic achievement was to establish the Theory of Matter. This Theory will serve as our description of ordinary matter under ordinary conditions — allowing for an extremely liberal definition of "ordinary" — for the foreseeable future. Yet there are many indications, ranging from the numerical to the semimystical, that a new fertile period lies before us. We will discover compelling evidence for the unification of fundamental forces and for new quantum dimensions (low-energy supersymmetry). We will identify new forms of matter, which dominate the mass density of the Universe. We will achieve much better fundamental understanding of the behavior of matter in extreme astrophysical and cosmological environments. Lying beyond these expectations, we can identify deep questions that seem to call for ideas outside our present grasp. And there is still plenty of room for surprises.