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Register a new userGamma-ray emission from novae related to positron annihilation: constraints on its observability posed by new experimental nuclear data
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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.
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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.
SPI on INTEGRAL has provided spectra and a map of the sky in the emission from annihilations of positrons in the interstellar medium of our Galaxy. From high-resolution spectra we learned that a warm, partially-ionized medium is the site where the observed gamma-rays originate. The gamma-ray emission map shows a major puzzle for broader astrophysics topics, as it is dominated by a bright and extended apparently spherical emission region centered in the Galaxys center. Only recently has the disk of the Galaxy been detected with SPI. This may be regarded as confirmation of earlier expectations that positrons should arise predominantly from sources of nucleosynthesis distributed throughout the plane of the Galaxy, which produce proton-rich unstable isotopes. But there are other plausible sources of positrons, among them pulsars and accreting binaries such as microquasars. SPI results may be interpreted also as hints that these are more significant as positron sources on the Galactic scale than thought before, in the plane and therefore also in the bulge of the Galaxy. This is part of the attempt to understand the surprisingly-bright emission from the central region in the Galaxy, which otherwise also could be interpreted as a first rather direct detection of dark matter annihilations in the Galaxys gravitational well. INTEGRAL has a unique potential to shed light on the various aspects of positron astrophysics, through its capability for imaging spectroscopy.
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)
We consider the indirect detection of dark matter that is captured in the Sun and subsequently annihilates to long lived dark mediators. If these mediators escape the Sun before decaying, they can produce striking gamma ray signals, either via the decay of the mediators directly to photons, or via bremsstrahlung and hadronization of the mediator decay products. Using recent measurements from the HAWC Observatory, we determine model-independent limits on heavy dark matter that are orders of magnitude more powerful than direct detection experiments, for both spin-dependent and spin-independent scattering. We also consider a well-motivated model in which fermionic dark matter annihilates to dark photons. For such a realistic scenario, the strength of the solar gamma ray constraints are reduced, compared to the idealistic case, due to the fact that the dark matter capture cross section and mediator lifetime are related. Nonetheless, solar gamma ray constraints enable us to exclude a previously unconstrained region of dark photon parameter space.
The Milagro gamma-ray observatory employs a water Cherenkov detector to observe extensive air showers produced by high energy particles interacting in the Earths atmosphere. Milagro has a wide field of view and high duty cycle, monitoring the northern sky almost continuously in the 100 GeV to 100 TeV energy range. Milagro is, thus, uniquely capable of searching for very high-energy emission from gamma-ray bursts (GRBs) during the prompt emission phase. Detection of >100 GeV counterparts would place powerful constraints on GRB mechanisms. Twenty-five satellite-triggered GRBs occurred within the field of view of Milagro between January 2000 and December 2001. We have searched for counterparts to these GRBs and found no significant emission from any of the burst positions. Due to the absorption of high-energy gamma rays by the extragalactic background light, detections are only expected to be possible for redshifts less than ~0.5. Three of the GRBs studied have measured redshifts. GRB 010921 has a redshift low enough (0.45) to allow an upper limit on the fluence to place an observational constraint on potential GRB models.
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