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High energy astrophysics

Adding the products of big bang nucleosynthesis and the ejecta of all stellar sources in their right proportions and on the relevant timescales causes the galactic evolution of the elements. Crucial quantities are the predicted ejecta of the individual objects (stellar winds, type II supernovae, novae, possibly X-ray bursts, type Ia supernovae, neutron star mergers, etc.) and the statistical occurrence of the individual contributors [18]. Radionuclides with half-lives ranging from roughly 100 days to the age of the Galaxy, like ²²Na, ²⁶Al, ⁴⁴Ti, ^56,57Co, ¹⁸²Hf, ¹²⁹I, ²³²Th, ^235,238U, ²⁴⁴Pu, being found in expanding explosion remnants, the interstellar medium or interstellar grains and meteorites, can provide information on the composition in individual events, the integral over galactic evolution or late additions to the forming solar system.

    Gamma-ray line astronomy became a privileged tool for the study of stellar nucleosynthesis in the past ten years. The proximity of SN1987A and the launch of NASA's CGRO with the most powerful instruments ever operative played a major role in this rapid progress [19]. Among these, the COMPTEL telescope provides for the first time extensive possibilities for spatially resolved gamma-ray spectroscopy at MeV energies. This allowed to confront supernova models with observations (⁵⁶Co and ⁵⁷Co in SN1987A, ⁵⁶Co in SN1991T and ⁴⁴Ti in CasA) and to locate the sites of large scale nucleosynthetic activity in the Galaxy through the profile of the ²⁶Al emission. Hidden supernovae in dense regions may be uncovered through their ⁴⁴Ti radioactivity in the 1.157 MeV line, thus constraining the Galactic supernova rate. The increased sensitivity of ESA's INTEGRAL (to be launched in 2001) will allow to explore those issues further and tackle several other related topics: ²²Na from galactic novae, a chance closeby explosion (as with SN1987A) would make ⁵⁶Co and ⁵⁷Co observable, the detection of diffuse ⁶⁰Fe $\gamma$-ray lines at 1.2 and 1.3 MeV is expected from type II supernova models predicting a yield between 0.25-0.35 of the corresponding ²⁶Al yield.

Figure: Intensities of the 1.8 MeV gamma transition following ²⁶Al decay in galactic co-ordinates, ranging from 10^-7 (black) to $1.6\times 10^{-3}$ photons cm^-2 s^-1 sr^-1 (bright yellow).

 

Gamma-ray observations by satellite experiments provide also unique opportunities to trace accelerated nuclei. Low-energy ( $\lower.5ex\hbox{$\; \buildrel < \over \sim \;$ }100$ MeV/nucleon) accelerated particles, interacting (scattering) with ambient matter, produce excited nuclei. Gamma-ray lines from energetic nuclei can be distinguished from those of ambient nuclei because they are Doppler broadened and can even contain line splitting features due to non-isotropic emission in the rest frame of the nucleus. Until recently, gamma-ray lines from accelerated particle interactions had been observed only from solar flares, e.g. ¹²C (4.44 MeV), ¹⁶O (6.13 MeV), ²⁰Ne, ²⁴Mg, ²⁸Si, ³²S and ⁵⁶Fe in the 1-2 MeV range. COMPTEL has now detected emission in the 3-7 MeV range from the Orion complex, which can be identified as de-excitations of ¹²C and ¹⁶O at 4.44 and 6.13 MeV. This surprise indicates that a large flux of low-energy accelerated particles is present in the Orion region, most likely enriched in C and O. The interpretation of spectral features with line splitting is severely hampered by the current lack of sufficient laboratory measurements, which are largely limited to the 4.44 MeV line of ¹²C, but even here far from complete.

    The appearance of the high-energy (>100 MeV) gamma-ray sky is dominated by diffuse emission from the Galactic disk, originating from $\pi^\circ$-decay following the interaction of high-energy ( $\lower.5ex\hbox{$\; \buildrel > \over \sim \;$ }1$ GeV/nucleon) protons and nuclei with the interstellar gas. This emission (well mapped by the ESA COS-B satellite and the EGRET telescope aboard the CGRO) has provided the most direct means of studying the large-scale distribution of energetic particles in the Galaxy. Inverse Compton scattering of high energy electrons is another important contributor to the gamma-ray spectrum. At lower gamma-ray energies (about 1-100 MeV) electron bremsstrahlung dominates the gamma-ray continuum emission.

    Energetic particles, i.e. cosmic rays (CR) that penetrate the solar system from deep space, leaves many open questions: (i) What/where are the sources, and (ii) what is their composition (electrons, protons, neutrons, alpha particles, heavier nuclei, their antiparticles, gammas, neutrinos, or possibly particles not producible in current accelerators)? The galactic cosmic ray composition in the 10 MeV to 1 GeV range has been analysed in detail by the ESA-NASA Ulysses mission. Up to energies of some hundred TeV of particle energy it is possible to measure chemical abundances of cosmic rays directly on satellites or balloons. Nuclear spallation changes the original (primary) composition and is the dominant process for the rare isotopes of Li, Be, B in the interstellar medium, requiring better spallation cross sections near the particle thresholds [20]. There exists still controversy about the chemical composition above $\sim 10^{15}$ eV. The spectrum of cosmic rays extends to energies beyond 10²⁰ eV and obeys a power law with a differential coefficient ranging between -2.7 (below $5\times 10^{15}$ eV) and -3.1 (up to $3\times 10^{18}$ eV), where it flattens out again.

Figure: Composition of cosmic rays in comparison to solar system abundances (normalised to C). Spallation enhances the abundances of nuclei with low solar system values. The effect is energy dependent.

 

It has been proposed that cosmic ray particles can be attributed to three main sites of origin and acceleration, (i) supernova shocks in the interstellar medium, (ii) supernova shocks in the stellar wind of the progenitor star, and (iii) powerful radio galaxies with jets [21]. New CGRO(EGRET) data from supernova remnants indicate that CR's follow a different power law ( $\propto E^{-2.4}$) at their source of acceleration. In general, the search for the sources of charged CR's is hampered by magnetic fields which destroy directional information, confining low energy cosmic rays to the Galaxy and leading to a shift from galactic to extragalactic origin with increasing energy, expected at 10¹⁸-10¹⁹ eV. Therefore, more energetic particles should provide a better information of their origin. However, their interaction with CMB photons limits the mean free path. The handful of observed events point towards areas within 50 Mpc, where few potential acceleration regions are known. The cut-off at high energies due the interaction with the CMB has been estimated at $5\times 10^{19}$ eV (Greisen-Zatsepin-Kuzmin cut-off). The highest energies observed are at $3\times 10^{20}$ eV (FLY'S EYE, AGASA, HAVERAH PARK, YAKUTSK). An alternative explanation for these particles might be that they are decay products of topological defects from the early universe. The study of these >10²⁰ eV events is therefore of great importance for universal particle models and the grand unification of the four forces.

    Long-lived neutral particles like gammas (<10^-4 of the total flux) and neutrinos (flux fraction unknown) are not affected by magnetic fields. They reveal their origin more easily. Since 1989 high significance observations (5 - 35 $\sigma$) have identified 7 sources of high(est) energy gamma-rays in the range of 300 GeV up to  50 TeV [22]. The recent progress came by perfecting "Air Cerenkov" detectors. Interestingly, among the 7 sources, there are 3 extragalactic ones, the Active Galactic Nuclei (AGN's) Mkn 421, Mkn 501 (WHIPPLE, HEGRA, CAT) and an Einstein source 1ES 2344+519 (WHIPPLE). The most recent discovery was episodic gamma-emission from the galactic micro-quasar GRS 1915. Three out of the seven sources show strong flaring, changing on timescales as short as hours. Thus, accelerations must take place in volumes of only a few tenths of light hours in contrast to model expectations. The gamma-production in all 7 sources can be explained by electron acceleration and inverse Compton scattering on low energy photons, with hadronic cascading being a viable competitor. A completely different aspect of high energy gamma-astronomy is to make use of gammas to probe the presently unquantified infrared (IR) background (IRB). It might be possible that the IRB contains up to a few % of the CMB photon energy budget. Similarly to the CMB, the IRB is interacting with energetic gamma-rays. Therefore, the universe is not fully transparent to gammas in certain energy bands (at around 10¹⁵ eV the "gamma-visibility" extends barely to the centre of our Galaxy). Detailed mapping of the gamma-spectra of AGN's at different redshifts will permit to map the distance at which gamma-rays reach us undisturbed. The IRB radiation is related to galaxy formation and thus deeply related to the question of dark matter. Another long-lived neutral particle, the neutrino, can also be used to trace high energy sources. The weak interaction allows neutrinos to pass the IRB and CMB unabsorbed, but very large detector volumes are needed for their observation.

Figure: Very high energy gamma-ray distribution, indicating point sources in and outside the Milky Way (pulsars, X-ray binaries, cataclysmic variables, a microquasar - stellar mass black hole - and [extragalactic] active galactic nuclei with massive black holes).