Photon acceleration in variable ultra-relativistic outflows and high-energy spectra of Gamma-Ray Bursts

MeV seed photons produced in shocks in a variable ultra-relativistic outflow gain energy by the Fermi mechanism, because the photons Compton scatter off relativistically colliding shells. The Fermi-modified high-energy photon spectrum has a non-universal slope and a universal cutoff. A significant increase in the total radiative efficiency is possible. In some gamma ray bursts, most of the power might be emitted at the high-energy cutoff for this mechanism, which would be close to 100 MeV for outflows with a mean bulk Lorentz factor of 100.Comment: 8 pages, submitted to ApJ

Representations of Time Coordinates in FITS

In a series of three previous papers, formulation and specifics of the representation of World Coordinate Transformations in FITS data have been presented. This fourth paper deals with encoding time. Time on all scales and precisions known in astronomical datasets is to be described in an unambiguous, complete, and self-consistent manner. Employing the well--established World Coordinate System (WCS) framework, and maintaining compatibility with the FITS conventions that are currently in use to specify time, the standard is extended to describe rigorously the time coordinate. World coordinate functions are defined for temporal axes sampled linearly and as specified by a lookup table. The resulting standard is consistent with the existing FITS WCS standards and specifies a metadata set that achieves the aims enunciated above.Comment: FITS WCS Paper IV: Time. 27 pages, 11 table

Rapidly Converging Activity Expansions For Representing The Thermodynamic Properties Of Fluid Systems: Gases, Non-Electrolyte Solutions, Weak And Strong Electrolyte Solutions

For dilute gases and non-electrolyte solutions in the McMillan–Mayer standard state, an activity expansion due to Mayer has great advantages over the normal concentration expansion (virial equation) for strongly associating species. For weakly interacting systems, both approaches are suitable. The activity expansion eliminates the need to differentiate between strong “chemical” interactions and weak “physical” interactions since the same equation is used in each situation. The equation has been modified to represent electrolyte solutions in the McMillan–Mayer standard state by requiring that it be consistent with the Debye–Hückel and higher order limiting laws for strong electrolytes and that it be equivalent to a chemical association model for weak electrolytes. The result is a compact equation which contains no arbitrary ion-size parameters and which does not require the classification of an electrolyte as strong or weak. For 2:2 electrolytes, the equation gives a very good fit to the anomalous low concentration region. For practical thermodynamic calculations, similar equations for molal activity coefficients are proposed; good fits of the data are obtained

Freezing Points Of Aqueous Alcohols: Free Energy Of Interaction Of The CHOH, CH₂, CONH And C[double bond]C Functional Groups In Dilute Aqueous Solutions

The freezing temperatures of dilute aqueous solutions of methanol, ethanol, 2-propanol, butanol, t-butanol, cyclohexanol and ethylene glycol were measured over the concentration range 0.1 to 1 mol kg–1. Osmotic coefficients at 0°C were calculated. The limiting pairwise interaction coefficients of the alcohols, plus a variety of polyhydroxy compounds and carbohydrates, were calculated at 25°C from the available data and then correlated using the additivity principle of Savage and Wood. This correlation approximates effective free energies of CH2 and CHOH group interactions with themselves and with each other. Literature data were used to estimate interactions between CONH and C[double bond]C groups. The CONH—CONH interaction appears to be large, consistent with a strong stabilizing effect of these on native protein structures. The CH2…CH2 interaction also indicates attractive forces between these groups. The present model for the hydrophobic interaction is most appropriate for small molecular interactions whereas previous treatments are best for situations involving site binding. The CHOH…CHOH and CH2…CONH interactions are small, while the CHOH…CH2 free energy of interaction is positive, due either to volume exclusion or net repulsive forces. The entropy change associated with the CH2…CH2 interaction is large and positive as expected and is not completely compensated by a corresponding enthalpy change. The entropy change associated with the CONH…CONH interaction indicates that few degrees of freedom are involved, which is consistent with the formation of a strong hydrogen bond. The correlation can be used to estimate thermodynamic properties of dilute non-electrolyte solutions and can also predict the effect of solutes on the solubility of solids and gases

Naturalness vs. Predictability: A Key Debate in Controlled Languages

Abstract. In this paper we describe two quite different philosophies used in developing controlled languages (CLs): A "naturalist " approach, in which CL interpretation is treated as a simpler form of full natural language processing; and a "formalist " approach, in which the CL interpretation is “deterministic” (context insensitive) and the CL is viewed more as an English-like formal specification language. Despite the philosophical and practical differences, we suggest that a synthesis can be made in which a deterministic core is embedded in a naturalist CL, and illustrate this with our own controlled language CPL. In the second part of this paper we present a fictitious debate between an ardent “naturalist ” and an ardent “formalist”, each arguing their respective positions, to illustrate the benefits and tradeoffs of these different philosophies in an accessible way. Part I: The Naturalist vs. Formalist Debate

The observable effects of a photospheric component on GRB's and XRF's prompt emission spectrum

A thermal radiative component is likely to accompany the first stages of the prompt emission of Gamma-ray bursts (GRB's) and X-ray flashes (XRF's). We analyze the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. We consider both the internal shock model and a 'slow heating' model as possible dissipation mechanisms. For comparable energy densities in the thermal and the leptonic component, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum (\nu F_\nu \propto \nu^0) above the thermal peak at ~10-100 keV and below 10-100 MeV, for a wide range of optical depths 0.03 <~ \tau_{\gamma e} <~ 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. At lower energies steep slopes are expected, while above 100 MeV the spectrum depends on the details of the dissipation process. For higher values of the optical depth, a Wien peak is formed at 100 keV - 1 MeV, and no higher energy component exists. For any value of \tau_{\gamma e}, the number of pairs produced does not exceed the baryon related electrons by a factor larger than a few. We conclude that dissipation near the thermal photosphere can naturally explain both the steep slopes observed at low energies and a flat spectrum above 10 keV, thus providing an alternative scenario to the optically thin synchrotron - SSC model.Comment: Discussion added on the results of Baring & Braby (2004); Accepted for publication in Ap.