Particle Acceleration at Relativistic Shocks in Extragalactic Systems

Diffusive shock acceleration (DSA) at relativistic shocks is expected to be an important acceleration mechanism in a variety of astrophysical objects including extragalactic jets in active galactic nuclei and gamma ray bursts. These sources remain strong and interesting candidate sites for the generation of ultra-high energy cosmic rays. In this paper, key predictions of DSA at relativistic shocks that are salient to the issue of cosmic ray ion and electron production are outlined. Results from a Monte Carlo simulation of such diffusive acceleration in test-particle, relativistic, oblique, MHD shocks are presented. Simulation output is described for both large angle and small angle scattering scenarios, and a variety of shock obliquities including superluminal regimes when the de Hoffman-Teller frame does not exist. The distribution function power-law indices compare favorably with results from other techniques. They are found to depend sensitively on the mean magnetic field orientation in the shock, and the nature of MHD turbulence that propagates along fields in shock environs. An interesting regime of flat spectrum generation is addressed, providing evidence for its origin being due to shock drift acceleration. The impact of these theoretical results on gamma-ray burst and blazar science is outlined. Specifically, Fermi gamma-ray observations of these cosmic sources are already providing significant constraints on important environmental quantities for relativistic shocks, namely the frequency of scattering and the level of field turbulence.Comment: 11 pages, 6 figures, to appear in Proc. of the 8th International Astrophysics Conference "Shock Waves in Space and Astrophysical Environments" (2010), eds. X. Ao, R. Burrows and G. P. Zank (AIP Conf. Proc., New York

Strong Water Absorption in the Dayside Emission Spectrum of the Planet HD 189733b

Recent observations of the extrasolar planet HD 189733b did not reveal the presence of water in the emission spectrum of the planet. Yet models of such 'Hot Jupiter' planets predict an abundance of atmospheric water vapour. Validating and constraining these models is crucial for understanding the physics and chemistry of planetary atmospheres in extreme environments. Indications of the presence of water in the atmosphere of HD 189733b have recently been found in transmission spectra, where the planet's atmosphere selectively absorbs the light of the parent star, and in broadband photometry. Here we report on the detection of strong water absorption in a high signal-to-noise, mid-infrared emission spectrum of the planet itself. We find both a strong downturn in the flux ratio below 10 microns and discrete spectral features that are characteristic of strong absorption by water vapour. The differences between these and previous observations are significant and admit the possibility that predicted planetary-scale dynamical weather structures might alter the emission spectrum over time. Models that match the observed spectrum and the broadband photometry suggest that heat distribution from the dayside to the night side is weak. Reconciling this with the high night side temperature will require a better understanding of atmospheric circulation or possible additional energy sources.Comment: 11 pages, 1 figure, published in Natur

An Improved Red Spectrum of the Methane or T-dwarf SDSS 1624+0029: Role of the Alkali Metals

A Keck~II low resolution spectrum shortward of ome-micron is presented for SDSS 1624+0029, the first field methane or T dwarf discovered in the Sloan Digital Sky Survey. Significant flux is detected down to the spectrum's short wavelength limit of 6200\AA. The spectrum exhibits a broad absorption feature centered at 7700\AA, which we interpret as the K~I 7665/7699 resonance doublet. The observed flux declines shortward of 7000\AA, due most likely to the red wing of the Na~I doublet. Both Cs~I doublet lines are detected more strongly than in an earlier red spectrum. Neither Li~I absorption nor H$\alpha$ emission are detected. An exploratory model fit to the spectrum suggests that the shape of the red spectrum can be primarily accounted for by the broad wings of the K~I and Na~I doublets. This behavior is consistent with the argument proffered by Burrows, Marley and Sharp that strong alkali absorption is principally responsible for depressing T dwarf spectra shortward of 1$\mu$m. In particular, there seems no compelling reason at this time to introduce dust or an additional opacity source in the atmosphere of the SDSS object. The width of the K~I and strengths of the Cs~I lines also indicate that the Sloan object is warmer than Gl~229B.Comment: accepted March 3, 2000 for Ap.J. Letters, LaTeX, 2 figure

Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales

We investigate the rates of production and thermalization of $\nu_\mu$ and $\nu_\tau$ neutrinos at temperatures and densities relevant to core-collapse supernovae and protoneutron stars. Included are contributions from electron scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and nucleon scattering. For the scattering processes, in order to incorporate the full scattering kinematics at arbitrary degeneracy, the structure function formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is employed. Furthermore, we derive formulae for the total and differential rates of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in asymmetric matter. We find that electron scattering dominates nucleon scattering as a thermalization process at low neutrino energies ($\epsilon_\nu\lesssim 10$ MeV), but that nucleon scattering is always faster than or comparable to electron scattering above $\epsilon_\nu\simeq10$ MeV. In addition, for $\rho\gtrsim 10^{13}$ g cm$^{-3}$, $T\lesssim14$ MeV, and neutrino energies $\lesssim60$ MeV, nucleon-nucleon bremsstrahlung always dominates electron-positron annihilation as a production mechanism for $\nu_\mu$ and $\nu_\tau$ neutrinos.Comment: 29 pages, LaTeX (RevTeX), 13 figures, submitted to Phys. Rev. C. Also to be found at anonymous ftp site http://www.astrophysics.arizona.edu; cd to pub/thompso

Impact of ocean warming on sustainable fisheries management informs the Ecosystem Approach to Fisheries

Acknowledgements Serpetti N., Heymans J.J., and Burrows M.T. were funded by the Natural Environment Research Council and Department for Environment, Food and Rural Affairs under the Marine Ecosystems Research Programme (MERP) (grant No. NE/L003279/1). Baudron A. and Fernandes, P.G. were founded by Horizon 2020 European research projects MareFrame (grant No. 613571) and ClimeFish (grant No. 677039). Payne, B.L. was founded by the Natural Environment Research Council and Department for Environment under the ‘Velocity of Climate Change’ (grant No. NE/J024082/1).Peer reviewedPublisher PD

Nuclear Effects on Bremsstrahlung Neutrino Rates of Astrophysical Interest

We calculate in this work the rates for the neutrino pair production by nucleon-nucleon bremsstrahlung taking into account the full contribution from a nuclear one-pion-exchange potential. It is shown that if the temperatures are low enough ($T \leq 20 MeV$), the integration over the nuclear part can be done for the general case, ranging from the completely degenerate (D) to the non-degenerate (ND) regime. We find that the inclusion of the full nuclear contribution enhances the neutrino pair production by $nn$ and $pp$ bremsstrahlung by a factor of about two in both the D and ND limits when compared with previous calculations. This result may be relevant for the physical conditions of interest in the semitransparent regions near the neutrinosphere in type II supernovae, cooling of neutron stars and other astrophysical situations.Comment: 11 pages, no figures, LaTex file. submitted to PR

Theoretical Radii of Transiting Giant Planets: The Case of OGLE-TR-56b

We calculate radius versus age trajectories for the photometrically-selected transiting extrasolar giant planet, OGLE-TR-56b, and find agreement between theory and observation, without introducing an ad hoc extra source of heat in its core. The fact that the radius of HD209458b seems larger than the radii of the recently discovered OGLE family of extremely close-in transiting planets suggests that HD209458b is anomalous. Nevertheless, our good fit to OGLE-TR-56b bolsters the notion that the generic dependence of transit radii on stellar irradiation, mass, and age is, to within error bars, now quantitatively understood.Comment: 11 pages, 1 figure, submitted to the Astrophysical Journa

Spacelab energetic ion mass spectrometer

Basic design criteria are given for an ion mass spectrometer for use in studying magnetospheric ion populations. The proposed instrument is composed of an electrostatic analyzer followed by a magnetic spectrometer and simultaneously measures the energy per unit and mass per unit charge of the ion species. An electromagnet is used for momentum analysis to extend the operational energy range over a much wider domain than is possible with the permanent magnets used in previous flights. The energetic ion source regions, ion energization mechanisms, field line tracing, coordinated investigations, and orbit considerations are discussed and operations of the momentum analyzer and of the electrostatic energy analyzer are examined

Testing the standard fireball model of GRBs using late X-ray afterglows measured by Swift

We show that all X-ray decay curves of GRBs measured by Swift can be fitted using one or two components both of which have exactly the same functional form comprised of an early falling exponential phase followed by a power law decay. The 1st component contains the prompt gamma-ray emission and the initial X-ray decay. The 2nd component appears later, has a much longer duration and is present for ~80% of GRBs. It most likely arises from the external shock which eventually develops into the X-ray afterglow. In the remaining ~20% of GRBs the initial X-ray decay of the 1st component fades more slowly than the 2nd and dominates at late times to form an afterglow but it is not clear what the origin of this emission is. The temporal decay parameters and gamma/X-ray spectral indices derived for 107 GRBs are compared to the expectations of the standard fireball model including a search for possible "jet breaks". For ~50% of GRBs the observed afterglow is in accord with the model but for the rest the temporal and spectral indices do not conform to the expected closure relations and are suggestive of continued, late, energy injection. We identify a few possible jet breaks but there are many examples where such breaks are predicted but are absent. The time, T_a, at which the exponential phase of the 2nd component changes to a final powerlaw decay afterglow is correlated with the peak of the gamma-ray spectrum, E_peak. This is analogous to the Ghirlanda relation, indicating that this time is in some way related to optically observed break times measured for pre-Swift bursts.Comment: submitted to Ap