37 research outputs found
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
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
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
Prospects in space-based Gamma-Ray Astronomy
With the unequalled INTEGRAL observatory, ESA has provided a unique tool to
the astronomical community that has made Europe the world leader in the field
of gamma-ray astronomy. INTEGRAL provides an unprecedented survey of the soft
gamma-ray sky, revealing hundreds of sources of different kinds, new classes of
objects, extraordinary views of antimatter annihilation in our Galaxy, and
fingerprints of recent nucleosynthesis processes. While INTEGRAL provides the
longly awaited global overview over the soft gamma-ray sky, there is a growing
need to perform deeper, more focused investigations of gamma-ray sources,
comparable to the step that has been taken in X-rays by going from the ROSAT
survey satellite to the more focused XMM-Newton observatory. Technological
advances in the past years in the domain of gamma-ray focusing using Laue
diffraction techniques have paved the way towards a future European gamma-ray
mission, that will outreach past missions by large factors in sensitivity and
angular resolution. Such a future Gamma-Ray Imager will allow to study particle
acceleration processes and explosion physics in unprecedented depth, providing
essential clues on the intimate nature of the most violent and most energetic
processes in the Universe.Comment: 8 pages, 7 figures, to be published in the Proceedings of the 39th
ESLAB Symposiu
Novae Ejecta as Colliding Shells
Following on our initial absorption-line analysis of fifteen novae spectra we
present additional evidence for the existence of two distinct components of
novae ejecta having different origins. As argued in Paper I one component is
the rapidly expanding gas ejected from the outer layers of the white dwarf by
the outburst. The second component is pre-existing outer, more slowly expanding
circumbinary gas that represents ejecta from the secondary star or accretion
disk. We present measurements of the emission-line widths that show them to be
significantly narrower than the broad P Cygni profiles that immediately precede
them. The emission profiles of novae in the nebular phase are distinctly
rectangular, i.e., strongly suggestive of emission from a relatively thin,
roughly spherical shell. We thus interpret novae spectral evolution in terms of
the collision between the two components of ejecta, which converts the early
absorption spectrum to an emission-line spectrum within weeks of the outburst.
The narrow emission widths require the outer circumbinary gas to be much more
massive than the white dwarf ejecta, thereby slowing the latter's expansion
upon collision. The presence of a large reservoir of circumbinary gas at the
time of outburst is suggestive that novae outbursts may sometime be triggered
by collapse of gas onto the white dwarf, as occurs for dwarf novae, rather than
steady mass transfer through the inner Lagrangian point.Comment: 12 pages, 3 figures; Revised manuscript; Accepted for publication in
Astrophysics & Space Scienc