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Future Goals for Gamma-Ray Spectroscopy

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Future Goals for Gamma-Ray Spectroscopy FUTURE GOALS FOR -RAY SPECTROSCOPY P.von BALLMOOS Centre d'Etude Spatiale des Rayonnements, Toulouse, France Abstract. Gamma-ray lines are the ngerprints of nuclear transitions, carrying the mem- ory of high energy pro cesses in the universe. Setting out from what is presently known ab out line emission in gamma-ray astronomy, requirements for future telescop es are out- lined. The inventory of observed line features shows that sources with a wide range of angular and sp ectral extenthave to b e handled: the scienti c ob jectives for gamma-ray sp ectroscopy are spanning from compact ob jects as broad class annihilators, over long- lived galactic radioisotop es with hotsp ots in the degree-range to the extremely extended + galactic disk and bulge emission of the narrowe e line. The instrumental categories which can b e identi ed in the energy range of nuclear astro- physics have their origins in the di erent concepts of light itself: geometrical optics is the base of mo dulating ap erture systems - these metho ds will continue to yield adequate p er- formances in the near future. Beyond this, fo cusing telescop es and Compton telescop es, based on wave- and quantum- optics resp ectively,may b e capable to further push the limits of resolution and sensitivity. Key words: gamma-ray astronomy { instrumentation 1. Intro duction With the Compton Gamma-Ray Observatory and the GRANAT/SIGMA telescop e in orbit, high energy astrophysics has for the rst time ever the opp ortunity to study all the facets of celestial gamma-rays side by side. In the energy range of nuclear transitions, the gamma-ray sky has b een mapp ed on various angular scales and a large numb er of new gamma-ray sources has b een discovered see reviews by Gehrels et al. 1994 and Paul 1995. Mayb e the most imp ortant scienti c p otential of the present generation high ener- gy telescop es is their extremely broad common energy coverage. Together with the op erating X-ray and high energy gamma-ray telescop es, a quasi- continuous coverage has op ened the p ossibility for multi-wavelength studies of continuum sp ectra spanning from the keV- to the GeV-range. Figure 1 indicates that the present area once might b e called the \golden age of gamma-ray astronomy": never b efore has the high-energy sky b een exam- ined so thoroughly and over such a broad energy range. For many of the high energy sources, multi-wavelength studies may b e the only way that leads to an understanding of their complex source mechanisms. A mo del case is the sp ectrum of the quasar 3C273 Lichti et al. 1995 that has b een observed - partly simultaneously - from radio to gamma-ray energies. Nevertheless, the gamma-ray telescop es on the Compton Gamma Ray Observatory and on GRANAT also have raised new astrophysical questions and highlighted those which remain unanswered. The future goals of gamma-ray astronomy must b e de ned in this context. The progress of SIGMA, BATSE, OSSE and 2 P.von BALLMOOS Fig. 1. The \golden age of gamma-ray astronomy" ? Never b efore the high-energy sky has b een examined so thoroughly and over such a broad energy range. COMPTEL is based primarily on skymaps, excellent timing analysis, and mo derate to fair sp ectral resolution. The observations have revealed certain asp ects of the morphology of celestial gamma-ray emitters, yet, the physical pro cesses at work are often only p o orly understo o d. Frequently, the observed sp ectra do not suciently constrain the emission mechanisms : explaining a relatively simple, featureless continuum with a complex multiparameter mo del can b e ambiguous, moreover, di erent comp onents may blend into one another, each of them can dep end on various physical parameters in the emitting region. In manyways, the present situation resembles the situation of optical astronomy in the b eginning of the last century: Back then, the available observational data mainly consisted in images, starcounts, variability's, and colors. Astrophysics was b orn in 1859 when G. Kirchhof and R. Bunsen develop ed the sp ectral analysis and explained the Frauenhofer-lines in the sp ectrum of the sun. The exploration of atomic and molecular lines has since turned out to b e the most p owerful to ol for the study of the physical conditions in celestial sources. Up to to day, only little advantage has b een taken from the fundamental astrophysical information contained in gamma-ray lines. The reason for this is the mo dest energy resolution of the existing instruments. High resolution Spectro_ExAstr_95.tex - Date: May 16, 1995 Time: 22:50 FUTURE GOALS FOR -RAY SPECTROSCOPY 3 sp ectroscopy will therefore b e one of the ma jor goals of the next generation of space b orne gamma-ray telescop es. The scienti c p otential of nuclear gamma-ray astronomy is outlined in section 2. In sections 3 instruments for sp ectroscopy in the low and medi- um gamma-raychannel are presented: mo dulating ap erture systems, Comp- ton telescop es and di raction lens telescop es. The three techniques actually re ect our current p erception of light itself - they are based on the principles of geometrical optics, quantum optics and wave optics, resp ectively. 2. The promise of nuclear gamma-ray astronomy Ultraviolet, visible and infrared-sp ectroscopy has long b ecome an extreme- ly valuable technique in astronomy. While optical lines re ect structural changes in the electron shell of atoms, caused by collisions with energies of 3 the order of 10 eV T 10 K, transition b etween discrete nuclear energy 7 9 levels imply MeV energies T 10 to 10 K, corresp onding to the bind- ing energy of nucleons. Collision energies of this order are characteristic for the temp eratures inside stars, particles accelerated by electromagnetic elds in solar ares, or interactions of cosmic ray particles with the interstellar medium. Gamma-ray lines are the ngerprints of nuclear transitions, car- rying the memory of high energy pro cesses in the universe. High resolution gamma-ray sp ectroscopy is a unique to ol to identify the presence of exited nuclei, quantitatively determine their abundance's, and provide insightinto the physical conditions of the source regions temp erature, density, gravita- tional or cosmological energy shifts. The scienti c p otential of gamma-ray sp ectroscopy includes b etter under- standing of the chemical evolution of the Galaxy by mapping the sites of recentnucleosynthetic activity novae, SN, massive stars. The observation of sup ernovae in the galaxies of our lo cal cluster and closeby sup erclusters Virgo cluster will verify our mo dels of nucleosynthesis in massive stars. Sp ectral features in solar ares, cyclotron lines in gamma-ray bursts and pulsars are among the further challenges of gamma-ray sp ectroscopy. An inventory of observed gamma-ray lines is presented in table I, it re ects the current status of nuclear gamma-ray astronomy. Note that most of the knowledge on gamma-ray lines is based on observations made with scintillation detectors that typically feature energy resolutions of 10. Nev- ertheless, these elementary sp ectroscopic measurements already indicate the tremendous p otential of gamma-ray line emission. Performance requirements for future sp ectroscopy missions b ecome conspicuous when the measured/- anticipated line uxes are compared to the exp ected angular scales. Figure 2 indicates that emissions with a wide range of angular and sp ectral extent are exp ected, varying in intensityby several orders of magnitude. The sci- enti c ob jectives for gamma-ray sp ectroscopy are spanning from compact Spectro_ExAstr_95.tex - Date: May 16, 1995 Time: 22:50 4 P.von BALLMOOS TABLE I Observed celestial gamma-ray line features Physical Pro cess Energy Source Flux Instrument, 2 1 [keV] ph cm s Detector Nuclear deexcitation 56 Fep,p', 847 Solar ares 0.05 SMM, NaI 24 Mg p,p', 1369 Solar ares 0.08 SMM, NaI 20 Ne p,p', 1634 Solar ares 0.1 SMM, NaI 28 Si p,p', 1779 Solar ares 0.08 SMM, NaI 20 12Cp,p', 4439 Solar ares 0.1 SMM, NaI 5 4439 Orion Comp. 510 COMPTEL, sc 16 Op,p', 6129 Solar ares 0.1 SMM, NaI 5 6129 Orion Comp. 510 COMPTEL, sc Radioactive decay 56 56 3 CoEC, Fe 847, 1238 SN 1987A 10 b Ge det's and 2598 var. scint's, 5 b COMPTEL, sc 847,1238 SN 1991T 510 57 57 4 CoEC, Fe 122, 136 SN 1987A 10 OSSE, NaI-CsI 44 44 + 5 TiEC Sc 1157 Cas A SNR 710 COMPTEL, sc 26 + 26 4 Al Mg 1809 gal. plane 410 COMPTEL, sc 5 1809 Vela SNR 1-610 COMPTEL, sc + e e Annihilation 3 511 Gal. bulge 1.710 OSSE, NaI-CsI 4 511 Gal. disk 4.510 OSSE, NaI-CsI 2 480120 a 1E 1740-29 c 1.310 SIGMA, NaI 511 Solar Flares 0.1 SMM, NaI scint. 3 47918 a Nova Muscae 6.310 SIGMA, NaI 400-500 a Bursts ? c 70 various scint. 4 44010 a Crab PSR ? c 310 FIGARO, NaI Neutron Capture 1 2 Hn, H 2223 Solar ares 1 SMM, NaI 56 57 2 Fen, Fe 5947 a 6/10/1974 tr. 1.5 10 SMM, NaI Cyclotron Lines 3 20-58 Hercules X-1 310 various scint. legend: sc liquid scintillator/NaI scintillator; a Redshifted line; b Maximum emission, c detection uncertain, the feature has not b een con rmed yet sources as broad class annihilators, over long-lived galactic radioisotop es with hotsp ots in the degree-range to the extremely extended galactic disk + and bulge emission of the narrowe e line.
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