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Register a new userTransient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from Novae. I. Limits on the Positron Annihilation Line in Five Individual Novae
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The Transient Gamma Ray Spectrometer (TGRS) on board the WIND spacecraft has spent most of the interval 1995-1997 in a high-altitude orbit where gamma-ray backgrounds are low. Its high-resolution Ge spectrometer is thus able to detect weak lines which are slightly offset from stronger background features. One such line is predicted from nucleosynthesis in classical novae, where beta-decays on a time-scale of a few hours in an expanding envelope produce positrons that annihilate to generate a line which is blueshifted by a few keV away from the background annihilation line at 511 keV. The broad TGRS field of view contained five known Galactic novae during 1995 January - 1997 June, and we have searched the spectra taken around the times of these events for the blueshifted nova annihilation line. Although no definite detections were made, the method is shown to be sensitive enough to detect novae occurring on ONeMg-rich white dwarfs out to about 2.5 kpc.
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Classical novae are among the most frequent transient events in the Milky Way, and key agents of ongoing nucleosynthesis. Despite their large numbers, they have never been observed in soft $gamma$-ray emission. Measurements of their $gamma$-ray signatures would provide both, insights on explosion mechanism as well as nucleosynthesis products. Our goal is to constrain the ejecta masses of $mathrm{^7Be}$ and $mathrm{^{22}Na}$ from classical novae through their $gamma$-ray line emissions at 478 and 1275 keV. We extract posterior distributions on the line fluxes from archival data of the INTEGRAL/SPI spectrometer telescope. We then use a Bayesian hierarchical model to link individual objects and diffuse emission and infer ejecta masses from the whole population of classical novae in the Galaxy. Individual novae are too dim to be detectable in soft $gamma$-rays, and the upper bounds on their flux and ejecta mass uncertainties cover several orders of magnitude. Within the framework of our hierarchical model, we can, nevertheless, infer tight upper bounds on the $mathrm{^{22}Na}$ ejecta masses, given all uncertainties from individual objects as well as diffuse emission, of $<2.0 times 10^{-7},mathrm{M_{odot}}$ (99.85th percentile). In the context of ONe nucleosynthesis, the $mathrm{^{22}Na}$ bounds are consistent with theoretical expectations, and exclude that most ONe novae happen on white dwarfs with masses around $1.35,mathrm{M_{odot}}$. The upper bounds from $mathrm{^{7}Be}$ are uninformative. From the combined ejecta mass estimate of $mathrm{^{22}Na}$ and its $beta^+$-decay, we infer a positron production rate of $<5.5 times 10^{42},mathrm{e^+,s^{-1}}$, which would make at most 10% of the total annihilation rate in the Milky Way.
The good energy resolution (3--4 keV FWHM) of the Transient Gamma Ray Spectrometer (TGRS) on board the WIND spacecraft makes it sensitive to Doppler-shifted outbursts of 511 keV electron-positron annihilation radiation, the reason being that the Doppler shift causes the cosmic line to be slightly offset from a strong instrumental background 511 keV line at rest, which is ubiquitous in space environments. Such a cosmic line (blueshifted) is predicted to arise in classical novae due to the annihilation of positrons from $beta$-decay on a timescale of a few hours in an expanding envelope. A further advantage of TGRS - its broad field of view, containing the entire southern ecliptic hemisphere - has enabled us to make a virtually complete and unbiased 3-year search for classical novae at distances up to ~1 kpc. We present negative results of this search, and estimate its implications for the highly-uncertain Galactic classical nova rate and for future space missions.
Gamma-ray emission at energies >100MeV has been detected from nine novae using the Fermi-LAT, and it can be explained by particle acceleration at shocks in these systems. Eight out of these nine objects are classical novae in which interaction of the ejecta with a tenuous circumbinary material is not expected to generate detectable gamma-ray emission. We examine whether particle acceleration at internal shocks can account for the gamma-ray emission from these novae. The shocks result from the interaction of a fast wind radiatively-driven by nuclear burning on the white dwarf with material ejected in the initial runaway stage of the nova outburst. We present a one-dimensional model for the dynamics of a forward and reverse shock system in a nova ejecta, and for the associated time-dependent particle acceleration and high-energy gamma-ray emission. Non-thermal proton and electron spectra are calculated by solving a time-dependent transport equation for particle injection, acceleration, losses, and escape from the shock region. The predicted emission is compared to LAT observations of V407 Cyg, V1324 Sco, V959 Mon, V339 Del, V1369 Cen, and V5668 Sgr. The 100MeV gamma-ray emission arises predominantly from particles accelerated up to ~100GeV at the reverse shock and undergoing hadronic interactions in the dense cooling layer downstream of the shock. The internal shock model can account for the gamma-ray emission of the novae detected by Fermi-LAT, including the main features in the observations of the recent gamma-ray nova ASASSN-16ma. Gamma-ray observations hold potential for probing the mechanism of mass ejection in novae, but should be combined to diagnostics of the thermal emission at lower energies to be more constraining. (abridged version)
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.
The TGRS experiment on board the Wind spacecraft has many advantages as a sky monitor --- broad field of view (~2 pi) centered on the south ecliptic pole), long life (1994-present), and stable low background and continuous coverage due to Winds high altitude high eccentricity orbit. The Ge detector has sufficient energy resolution (3-4 keV at 511 keV) to resolve a cosmic positron annihilation line from the strong background annihilation line from beta-decays induced by cosmic ray impacts on the instrument, if the cosmic line is Doppler-shifted by this amount. Such lines (blueshifted) are predicted from nucleosynthesis in classical novae. We have searched the entire TGRS database for 1995-1997 for this line, with negative results. In principle such a search could yield an unbiased upper limit on the highly-uncertain Galactic nova rate. We carefully examined the times around the known nova events during this period, also with negative results. The upper limit on the nova line flux in a 6-hr interval is typically <3.8 E-3 photon/(cm2 s) at 4.6 sigma. We performed the same analysis for times around the outburst of Nova Vel 1999, obtaining a worse limit due to recent degradation of the detector response caused by cosmic ray induced damage.
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