Gamma-ray signatures of classical novae

The role of classical novae as potential gamma-ray emitters is reviewed, on the basis of theoretical models of the gamma-ray emission from different nova types. The interpretation of the up to now negative results of the gamma-ray observations of novae, as well as the prospects for detectability with future instruments (specially onboard INTEGRAL) are also discussed.Comment: 10 pages, 7 figures. Invited talk at the "Gamma 2001" Symposium, April 4-6, 2001, Baltimore, Maryland. To be published by AI

Prospects for detectability of classical novae with INTEGRAL

Classical novae are potential gamma-ray emitters, both in lines and in a continuum. Continuum emission (at energies between 20-30 and 511 keV) and line emission at 511 keV are related to positron annihilation and its Comptonization in the expanding shell; 18F is the main responsible of positron production. The lines at 478 and 1275 keV have their origin in the decay of the radioactive nuclei 7Be and 22Na. Updated models of nova explosions have been adopted for the computation of the gamma-ray emission. New yields of some radioactive isotopes directly translate into new detectability distances of classical novae with INTEGRAL.Comment: Contributed paper at the 4th INTEGRAL Workshop, 4-8 September 2000, Alicante (Spain). To be published in the ESA-SP series: 4 pages, 4 figure

Gamma-ray emission from novae related to positron annihilation: constraints on its observability posed by new experimental nuclear data

Classical novae emit gamma-ray radiation at 511 keV and below, with a cut-off at around (20-30) keV, related to positron annihilation and its Comptonization in the expanding envelope. This emission has been elusive up to now, because it occurs at epochs well before the maximum in optical luminosity, but it could be detected by some sensitive intrument on board a satellite, provided that the nova is close enough and that it is observed at the right moment. The detection of this emission, which is a challenge for the now available and for the future gamma-ray instruments, would shed light into the physical processes occurring in the early phases of the explosion, which are invisible in other lower energy ranges. A good prediction of the emitted fluxes and of the corresponding detectability distances with different instruments relies critically on a good knowledge of reaction rates relevant to f18 destruction, which have been subject to a strong revision after recent nuclear spectroscopy measurements. With respect to previous results, smaller ejected masses of f18 are predicted, leading to smaller emitted fluxes in the (20-511) keV range and shorter detectability distances.Comment: 9 pages, 2 figures, accepted for publication in Astrophys. J. Letter

Gamma-ray emission of classical novae and its detectability by INTEGRAL

A lot of information concerning the mechanism of nova explosions will be extracted from the possible future observations with INTEGRAL. In order to be prepared for this task, we are performing detailed models of the gamma-ray emission of classical novae, for a wide range of possible initial conditions. Spectra at different epochs after the explosion and light curves for the different lines (511, 478 and 1275 keV) and the continuum are presented, as well as the detectability distances with INTEGRAL spectrometer SPI. New results related to 18F synthesis related to very recent data of nuclear physics are advanced as preliminary.Comment: 4 pages, 2 figures, to appear in "3rd INTEGRAL Workshop: The Extreme Universe", Taormina (Italy

Simplified treatment of the radiative transfer problem in expanding envelopes

In this paper we study the application of a simplified method to solve the dynamic radiative transfer problem in expanding envelopes. The method, which requires a computational effort similar to that of the diffusion approximation, is based on the use of a generalization of the Eddington closure relationship allowing the inclusion of scattering and relativistic corrections to order (v/c). We apply this method to the calculation of light curves of type Ia supernovae, showing that it gives much more accurate results than the diffusion approximation, and that the latter is seriously in error when applied to determine emergent flux and its spectral distribution.Comment: 18 pages (LaTeX, prepared with AAS macros) + 6 figures in PostScript, Accepted by the Astrophysical Journa

The prompt gamma-ray emission of novae

Classical novae are potential gamma-ray emitters, because of the disintegration of some radioactive nuclei synthesized during the explosion. Some short-lived isotopes (such as 13N and 18F), as well as the medium-lived 22Na, decay emitting positrons, which annihilate with electrons and thus are responsible for the prompt emission of gamma-rays from novae. This emission consists of a 511 keV line plus a continuum between 20 and 511 keV, and is released before the maximum in visual luminosity, i.e., before the discovery of the nova. The main characteristics of this prompt emission, together with the related uncertainties (both of nuclear and hydrodynamical origin, with a particular emphasis on the influence of the envelope properties) and prospects for detectability are analyzed in this paper.Comment: 6 pages, 3 figures (6 PS files), to appear in New Astronomy Reviews (Proceedings of the Ringberg Workshop "Astronomy with Radioactivities III"