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e-ASTROGAM is a concept for a breakthrough observatory space mission carrying a gamma-ray telescope dedicated to the study of the non-thermal Universe in the photon energy range from 0.15 MeV to 3 GeV. The lower energy limit can be pushed down to energies as low as 30 keV for gamma-ray burst detection with the calorimeter. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with remarkable polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous and current generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will be a major player of the multiwavelength, multimessenger time-domain astronomy of the 2030s, and provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LISA, LIGO, Virgo, KAGRA, the Einstein Telescope and the Cosmic Explorer, IceCube-Gen2 and KM3NeT, SKA, ALMA, JWST, E-ELT, LSST, Athena, and the Cherenkov Telescope Array., Comment: 15 pages, 7 figures. Submitted to the proceedings of the conference SPIE Astronomical Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray. v2: corrections of authors' affiliations

We present spectropolarimetric observations of the nearby Type Ia SN 2014J in M82 over six epochs: +0, +7, +23, +51, +77, +109, and +111 days with respect to B-band maximum. The strong continuum polarization, which is constant with time, shows a wavelength dependence unlike that produced by linear dichroism in Milky Way dust. The observed polarization may be due entirely to interstellar dust or include a circumstellar scattering component. We find that the polarization angle aligns with the magnetic field of the host galaxy, arguing for an interstellar origin. Additionally, we confirm a peak in polarization at short wavelengths that would imply $R_V < 2 $ along the light of sight, in agreement with earlier polarization measurements. For illustrative purposes, we include a two component fit to the continuum polarization of our +51 day epoch that combines a circumstellar scattering component with interstellar dust where scattering can account for over half of the polarization at $4000$ \AA. Upon removal of the interstellar polarization signal, SN 2014J exhibits very low levels of continuum polarization. Asymmetries in the distribution of elements within the ejecta are visible through moderate levels of time-variable polarization in accordance with the Si II 6355 \AA absorption line. At maximum light, the line polarization reaches $\sim0.6$% and decreases to $\sim0.4\%$ one week later. This feature also forms a loop on the $q_{RSP}$-$u_{RSP}$ plane illustrating that the ion does not have an axisymmetric distribution. The observed polarization properties suggest the explosion geometry of SN 2014J is generally spheroidal with a clumpy distribution of silicon., Comment: 33 pages, 10 figures, Accepted to ApJ

Novae are now firmly established as a high-energy (>100 MeV) gamma-ray source class by the Fermi Large Area Telescope (LAT). In symbiotic binary systems such as V407 Cyg 2010, there is a firm theoretical framework for the production of shock-accelerated particles in the nova ejecta from interactions with the dense wind of the red giant companion. Yet, the high-energy gamma-ray emission detected in classical novae involving less evolved stellar companions cannot be explained in the same way and could instead be produced in internal shocks in the ejecta. We summarize the Fermi-LAT gamma-ray observations of novae, highlighting the main properties that will guide further studies. Additionally, we report on the soft gamma-ray (~0.1 MeV) continuum detection of the oxygen-neon type classical nova V382 Vel 1999 with the OSSE detector aboard the Compton Gamma Ray Observatory in light of its Fermi-era analog, V959 Mon 2012., 9 pages, 5 figures. The 34th International Cosmic Ray Conference, 30 July - 6 August 2015 (The Hague, The Netherlands), PoS(ICRC2015) 880

We present multiple spectropolarimetric observations of the nearby Type Ia supernova (SN) 2011fe in M101, obtained before, during, and after the time of maximum apparent visual brightness. The excellent time coverage of our spectropolarimetry has allowed better monitoring of the evolution of polarization features than is typical, which has allowed us new insight into the nature of normal SNe Ia. SN 2011fe exhibits time-dependent polarization in both the continuum and strong absorption lines. At early epochs, red wavelengths exhibit a degree of continuum polarization of up to 0.4\%, likely indicative of a mild asymmetry in the electron-scattering photosphere. This behavior is more common in sub-luminous SNe Ia than in normal events, such as SN2011fe. The degree of polarization across a collection of absorption lines varies dramatically from epoch to epoch. During the earliest epoch a $\lambda$4600-5000 \AA\ complex of absorption lines shows enhanced polarization at a different position angle than the continuum. We explore the origin of these features, presenting a few possible interpretations, without arriving at a single favored ion. During two epochs near maximum, the dominant polarization feature is associated with the Si{\sc ii} $\lambda$6355 \AA\ absorption line. This is common for SNeIa, but for SN2011fe the polarization of this feature increases after maximum light, whereas for other SNeIa, that polarization feature was strongest before maximum light., Comment: 15 pages, accepted by ApJ

Observations of newly synthesized radioactive nuclei have provided a wealth of information about the extensively observed supernova 1987A. We add another such study, searching Chandra X-ray spectra of SN 1987A for evidence of 5.9 keV Mn Kα line emission due to the decay of radioactive 55Fe. This isotope is synthesized in explosive silicon burning and in normal freezeout of nuclear statistical equilibrium, mostly as 55Co, in the inner ejecta of core-collapse supernovae. The X-rays from the decay in these regions escape only after 10-20 yr of expansion, depending on the extent of mixing of core material outward in mass. Otherwise generally successful models of SN 1987A predict fluxes at 5.9 keV detectable by Chandra. However, in 400 ks of Chandra ACIS data, we find no evidence for this line emission, with the mean line flux ≤3 × 10-7 cm-2 s-1. We consider the implications for atomic mass A = 55 nucleosynthesis and for the kinematics and mixing of SN 1987A.

We present late-epoch optical photometry (BVRI) of seven normal/super-luminous Type Ia supernovae: SN 2000E, SN 2000ce, SN 2000cx, SN 2001C, SN 2001V, SN 2001bg, SN 2001dp. The photometry of these objects was obtained using a template subtraction method to eliminate galaxy light contamination during aperture photometry. We show the optical light curves of these supernovae out to epochs of up to ~640 days after the explosion of the supernova. We show a linear decline in these data during the epoch of 200-500 days after explosion with the decline rate in the B,V,& R bands equal to about 1.4 mag/100 days, but the decline rate of the I-band is much shallower at 0.94 mag/100 days., 33 pages, 11 figures, Accepted for publication in The Astronomical Journal

Observations of newly synthesized radioactive nuclei can provide a wealth of information about nucleosynthesis and dynamics of supernovae. This has been realized first for the extensively observed SN 1987A, through both direct (γ-ray line) and indirect (light curve power and infrared line) studies. We would like to extend this into the soft X-ray band, with the possibility of direct detection of electron capture decay of several radioactive isotopes in supernovae, supernova remnants, and the interstellar medium. Here, we discuss several possibilities and then present the results of searching Chandra X-ray spectra of SN 1987A for evidence of 5.9 keV Mn K α line emission due to the decay of radioactive 55 Fe. This isotope is produced directly and as short-lived parent 55 Co in the innermost ejected regions of core collapse supernovae, and is the source of 55 Mn in nature. The escape of X-rays from the decay of 55 Fe depends on the mixing of core material into the envelope. Despite expectation of detectable line flux, we find only an upper limit near 3 × 10 −7 cm −2 s −1 .

Radioactive 44 Ti is of great interest as a diagnostic of the core collapse supernova explosion mechanism and as the source of stable 44 Ca in our Galaxy. Its decay gamma-ray lines have been measured several times in the supernova remnant Cassiopeia A, with some inconsistencies among the measurements. This isotope’s decay ( 44 Ti → 44 Sc) also produces X-ray fluorescence emission following the electron capture, which might provide an alternate method of direct measurement of the mass of 44 Ti ejected, and the location of the 44 Ti within the remnant. Here we search for the Sc K-α emission in Chandra ACIS observations of Cassiopeia A. This instrument has sufficient effective area to detect the expected line flux, but because of the large continuum flux, we thus far have obtained only upper limits on the line emission. In two Chandra observations the 2 σ upper limit for the 4.09 keV line flux from the entire remnant is ∼3.8 × 10 −5 γ /cm 2 s. We also study more limited parts of the remnant, including apparently iron-rich regions of CasA, and discuss possible complications and limitations of this method.

The use of Type Ia supernovae as cosmological tools has reinforced the need to better understand these objects and their light curves. The light curves of Type Ia supernovae are powered by the nuclear decay of $^{56}Ni \to ^{56}Co \to ^{56}Fe$. The late time light curves can provide insight into the behavior of the decay products and their effect of the shape of the curves. We present the optical light curves of six "normal" Type Ia supernovae, obtained at late times with template image subtraction, and the fits of these light curves to supernova energy deposition models., Comment: Proceedings of Astronomy with Radioactivities V Conference

Despite their great intrinsic interest and extensive use as tools in other areas of astronomy, fundamental questions remain as to the nature of supernovae of all types. The radioactivity produced by nuclear fusion reactions in the explosions provides several excellent diagnostics, observable through gamma-ray emission. This great potential is largely unrealized because the fluxes from events at common distances are very small. Instrument sensitivity is paramount, and gamma-ray optics offer an excellent possibility for greatly increasing collecting area, without a large increase in background rates, and reaching the required sensitivities. Here we outline some of the major open questions about supernovae and discuss how gamma-ray spectroscopy can address them, including instrument requirements for a future focusing gamma-ray telescope.

Radioactive nuclei, especially those created in SN explosion, have long been suggested to be important contributors of galactic positrons. In this paper we describe the findings of three independent OSSE/SMM/TGRS studies of positron annihilation radiation, demonstrating that the three studies are largely in agreement as to the distribution of galactic annihilation radiation. We then assess the predicted yields and distributions of SN-synthesized radionuclei, determining that they are marginally compatible with the findings of the annihilation radiation studies.

We discuss the prospects for direct measurements of nucleosynthesis yields of electron capture decay radioactive nuclei from supernovae, by detecting the characteristic K X-rays of the daughters. Several isotopes in galactic supernova remnants and SN 1987A appear to be detectable now or in the near future. Precise measurements of these nuclear yields, mostly achievable no other way, would greatly constrain nucleosynthesis theory.

We present the discovery of a light echo from SN 2007af, a normal Type Ia supernova (SN Ia) in NGC 5584. Hubble Space Telescope (HST) images taken three years post explosion reveal two separate echoes; an outer echo and extended central region, which we propose as an unresolved inner echo. Multiple images were obtained in the F160W, F350LP, F555W, and F814W using the Wide Field Camera 3. If the outer echo is produced by an interstellar dust sheet perpendicular to the line of sight, it is located ~800 pc in front of the SN. The dust for the inner echo is 0.45 pc < d < 90 pc away from the SN. The inner echo color is consistent with typical interstellar dust wavelength-dependent scattering cross-sections, while the outer echo color does not match the predictions. Both dust sheets, if in the foreground, are optically thin for scattering, with the outer echo sheet thickness consistent with the inferred extinction from peak brightness. Whether the inner echo is from interstellar or circumstellar dust is ambiguous. Overall, the echo characteristics are quite similar to previously observed SN Ia echoes., 15 pages, 6 figures, 7 tables

Among the many new nuclei synthesized in supernova explosions are unstable proton-rich isotopes. Many of these decay by electron capture, leaving atomic K-shell electron vacancies. The characteristic X-rays emitted when these vacancies are filled can escape the supernova ejecta for sufficiently long-lived isotopes and can possibly be detected by the current generation of X-ray spectrometers. We discuss several candidate isotopes produced in both core-collapse and thermonuclear supernovae. Among the most promising are 55Fe, 44Ti, 59Ni, and 53Mn, which trace various types of explosive silicon burning. These offer the possibility of direct study of individual supernova nucleosynthesis and dynamics, and of the global structure of the Galaxy in multiple stellar populations.

Gamma-ray spectroscopy holds great promise for probing nucleosynthesis in individual stellar explosions via short-lived radioactivity, and for measuring current global Galactic nucleosynthesis with longer-lived radioactivity. The former case was realized first for a Type II supernova when both 56 Co and 57 Co were detected in SN 1987A. These provide unprecedented constraints on models of core-collapse explosions. In the second category, 26 Al in the Galaxy is now well observed and comes from massive stars, either before or during core collapse explosions, or from relatively massive AGB stars. We currently try to use these observations to constrain massive star evolution, supernova nucleosynthesis, and the Galactic supernova rate. Type Ia supernovae, thought to be entirely thermonuclear events, have not yet been definitively detected in γ-rays (although there is a hint of a signal from SN 1991T.) Ultimately, γ-ray measurements will confirm or disprove their thermonuclear character, probe the nuclear burning conditions, and help evaluate their contributions to Galactic nucleosynthesis. Positron annihilation radiation is also well-observed from the Galactic plane and bulge. All types of supernovae are plausibly significant contributors via their ejected positron emitters. By sorting out the contributions of the many possible sources, we can use these observations to constrain supernova nucleosynthesis, and we might obtain important clues to the progenitors of thermonuclear supernovae. All supernova types might contribute to a diffuse glow of 60 Fe γ-ray lines. Current limits are very close to expected emission levels. Remnants of any type of age less that a few centuries might be detectable as individual spots of 44 Ti γ-ray line emission. It is quite surprising that the brightest such spot is apparently the Cas A remnant, a relatively old one. That younger ones are not detected provides constraints on the combination of nucleosynthesis yields and supernova rates. Here we review the status of observations of γ-ray lines from these sources, discuss their implications, and briefly outline prospects for the future.

DUAL will study the origin and evolution of the elements and explores new frontiers of physics: extreme energies that drive powerful stellar explosions and accelerate particles to macroscopic energies; extreme densities that modify the laws of physics around the most compact objects known; and extreme fields that influence matter in a way that is unknown on Earth. The variability of these extreme objects requires continuous all-sky coverage, while detailed study demands an improvement in sensitivity over previous technologies by at least an order of magnitude. The DUAL payload is composed of an All-Sky Compton Imager (ASCI), and two optical modules, the Laue-Lens Optic (LLO) and the Coded-Mask Optic (CMO). The ASCI serves dual roles simultaneously, both as an optimal focal-plane sensor for deep observations with the optical modules and as a sensitive true all-sky telescope in its own right for all-sky surveys and monitoring. While the optical modules are located on the main satellite, the All-Sky Compton Imager is situated on a deployable structure at a distance of 30 m from the satellite. This configuration not only permits to maintain the less massive payload at the focal distance, it also greatly reduces the spacecraft-induced detector background, and, above all it provides ASCI with a continuous all-sky exposure.

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 Galaxy’s 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 Galaxy’s gravitational well. INTEGRAL has a unique potential to shed light on the various aspects of positron astrophysics, through its capability for imaging spectroscopy.

Satellite, rocket, and balloon borne instruments have obtained a wealth of high-energy data from SN 1987A. These have added significantly to our understanding of Type II supernovae. Besides the exciting direct detection of just-synthesized radioactivity via γ-ray line observations, the light curves of the lines and the extremely hard continuum which results from them have provided clues to the structure of the newly disrupted star. Mysteries still remain. We do not yet understand how the radioactive matter reached low depths in the envelope at such early times. We have not yet identified the mechanism which produced the softest observed X-ray emission. We do not know how much of other radioactive species were produced. This could help us begin to understand in detail the nuclear burning and mass ejection processes. We have not yet seen the signature of the compact remnant of the collapse. At this point we have great hope that some of these questions will be answered with the state-of-the-art experiments already flying and scheduled for launch soon.

The Solar Maximum Mission satellite's Gamma Ray Spectrometer observed Earth's atmosphere for most of the period 1980-1989. Its 28deg orbit ensured that a range of geomagnetic latitudes (geomagnetic cutoff rigidities) was sampled. We measured the variation with time and rigidity of albedo gamma-ray lines at 1.6 MeV, 2.3 MeV and 4.4 MeV which are diagnostic of Galactic cosmic radiation penetrating the cutoff and of the secondary neutrons produced in the atmosphere. We found that the gamma-ray line intensities varied inversely with solar activity and cutoff rigidity, as expected. The line ratio (1.6 MeV + 2.3 MeV)/4.4 MeV was remarkably constant (close to 0.39) at all times and rigidities; the former two lines are produced by 5-10 MeV secondary neutrons causing excitation and de-excitation of 14N, while the latter is produced by more energetic (>20 MeV) neutrons inducing spallation. We infer that the shape of the secondary neutron energy spectrum is virtually constant everywhere and at all times. We also measured the intensity of the 0.511 MeV electron-positron annihilation line. This line too varies with solar cycle and cutoff rigidity, but its fall-off from low to high rigidity is less marked than that of the nuclear lines. This results from the energy dependences of the cross sections for positron production and for the hadronic processes which which produce secondary neutrons., 11 pages, 12 figs., J. Geophys. Res. in press

Positron annihilation radiation from the Galaxy has been observed by the OSSE, SMM and TGRS instruments. Improved spectral modeling of OSSE observations has allowed studies of the distribution of both positron annihilation radiation components, the narrow line emission at 511 keV and the positronium continuum emission. The results derived for each individual annihilation component are then compared with each other. These comparisons reveal approximate agreement between the distribution of these two emissions. In certain regions of the sky (notably in the vicinity of the previously reported positive latitude enhancement), the distribution of the emissions differ. We discuss these differences and the methods currently being employed to understand whether the differences are physical or a systematic error in the present analysis., Comment: 5 pages, to appear in the proceedings of the Gamma 2001 Symposium (Baltimore, April 2001)

Given the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer’s (GRS) nine years of exposure and large field of view, its data contain a tremendously significant signal from the isotropic cosmic γ-ray background (CGB) in the energy range 0.3–8.0 MeV. We have extracted this signal by modeling its modulation by the Earth’s motion through the GRS field of view, along with several other background components, such as from the SAA and Earth Albedo γ rays. We can quantify the success of the technique and evaluate possible systematic errors because we have many independent measurements of the CGB, which should be constant in time, and because all known other background components have narrow lines that should not be present in the CGB. We thus obtain the definitive measurement of the CGB in this energy range to date. We compare the CGB spectrum with line emission from iron production in thermonuclear supernovae, and conclude that some other source(s) probably dominate the 1 MeV region.

A “positron phase” exists in type Ia supernovae, during which the energy deposition into the SN ejecta is dominated by the slowing of 56Co→56Fe β+ decay positrons. Comparisons of model-generated bolometric light curves with V and B band photometry during this phase suggests that the magnetic field is either too weak to confine positrons, or is radially combed. In each case, positrons escape the ejecta leading to a deficit of energy deposition. Ramifications of this escape upon other studies of SN Ia emission is discussed.

44Ti is one of the few radioactive nuclei produced abundantly in supernovae (SNe). Because its life time is comparable to the typical time between supernovae in the Galaxy, γ-ray lines from 44Ti provide a powerful tool for the search for optically obscured supernovae that occurred during the last few 100 years. We present our analysis of COMPTEL data (Cycle 1–6), comparing γ-ray line images and flux distributions with Monte Carlo simulations of SNe in the Galaxy. Assuming standard SNe distribution models and theoretical estimates of the 44Ti yields, we constrain the Galactic supernova rate. We compare these constraints with those obtained from the historical record covering the last millennium. Relating the current supernova rate to the observed 44Ti solar abundance through chemical evolution models, agreement can be obtained only if there is a special type of supernova (e.g., helium detonations) that produces large amounts of 44Ti, but with a very low event rate.

We generate bolometric light curves for a variety of type Ia supernova models at late times, simulating gamma-ray and positron transport for various assumptions about the magnetic field and ionization of the ejecta. These calculated light curve shapes are compared with light curves of specific supernovae for which there have been adequate late observations. %The selection of models is generally not based upon the %ability to fit the late observations, but rather because the %model has been demonstrated by other authors to approximate the spectra %and early light curves of that specific SN. From these comparisons we draw two conclusions: whether a suggested model is an acceptable approximation of a particular event, and, given that it is, the magnetic field characteristics and degree of ionization that are most consistent with the observed light curve shape. For the ten SNe included in this study, five strongly suggest $^{56}$Co positron escape as would be permitted by a weak or radially-combed magnetic field. Of the remaining five SNe, none clearly show the upturned light curve expected for positron trapping in a strong, tangled magnetic field. Chandrasekhar mass models can explain normally, sub-, and super- luminous supernova light curves; sub-Chandrasekhar mass models have difficulties with sub- (and potentially normally) luminous SNe. An estimate of the galactic positron production rate from type Ia SNe is compared with gamma-ray observations of Galactic 511 keV annihilation radiation. Additionally, we emphasize the importance of correctly treating the positron transport for calculations of spectra, or any properties, of type Ia SNe at late epochs ($\geq$ 200 d)., Comment: 82 pages including 25 figures

By combining OSSE, SMM and TGRS observations of the galactic center region, Purcell et al. (1997) and Cheng et al. (1997) produced the first maps of galactic positron annihilation. That data-set has been augmented with additional data, both recent and archival, and re-analyzed to improve the spectral fitting. The improved spectral fitting has enabled the first maps of positronium continuum emission and the most extensive maps of 511 keV line emission. Bulge and disk combinations have been compared with the 511 keV line data, demonstrating that extended bulges are favored over a GC point source for every disk model tested. This result is independent of whether OSSE-only, OSSE/SMM, or OSSE/SMM/TGRS data-sets are used. The estimated bulge to disk ratio (and to a lesser extent the total flux) is shown to be dependent upon the assumption of bulge shape. A positive latitude enhancement is shown to have an effect upon the B/D ratio, but this effect is secondary to the choice of bulge shape., Comment: 10 pages, to appear in "Proceedings of the 5th COMPTON Symposium"

XENA is a new Compton telescope concept, designed to image about 50% of the gamma-ray sky with a sensitivity that would significantly surpass CGRO/COMPTEL's multi-year sensitivity with a 2 weeks balloon flight from the Southern Hemisphere. The detector, based on liquid xenon time-projection chambers, is optimized for approximately 0.3 - 10 MeV and combines high efficiency within a 3 sr field-of-view with approximately 1 degree(s) angular resolution and excellent background reduction capability. XENA's primary scientific goal is the discovery and mapping of 60Fe radioactivity from the Galaxy, which is pivotal for understanding nucleosynthesis. XENA will detect 60Fe even if current predictions are 7X overestimated. At 1.8 MeV, XENA's sensitivity (6 10-6 cm-2 s-1) will significantly refine the COMPTEL 26Al mapping along the Southern Milky Way. Also, XENA would be the first instrument capable to decide whether the 3 - 7 MeV excess seen in Orion is indeed due to nuclear lines from 12C and 16O, and it could discover the predicted lower-energy lines. The scanned sky area includes many continuum (gamma) -ray sources as well, such as pulsars and numerous (gamma) -ray AGNs. Secondary scientific objectives include also supernova remnants, gamma-ray bursts, and solar flares.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report the results of extracting the hard X-ray continuum spectrum of Cas A SNR from RXTE/PCA Target of Opportunity (TOO) observations and CGRO/OSSE observations. The data can rule out the single thermal bremsstrahlung model for Cas A continuum between 2 and 150 keV. The single power law model gives a mediocre fit (~5%) to the data with a power-law index, $\Gamma$ = 2.94$\pm$0.02. A model with two component (bremsstrahlung + bremsstrahlung or bremsstrahlung + power law) gives a good fit. The power law index is quite constrained suggesting that this continuum might not be the X-ray thermal bremmstrahlung from accelerated MeV electrons at shock fronts (Asvarov et al. 1989) which would have $\Gamma\simeq$2.26. With several SNRs detected by ASCA showing a hard power-law nonthermal X-ray continuum, we expect a similar situation for Cas A SNR which has $\Gamma$=2.98$\pm$0.09. We discuss the implication of the hardest nonthermal X-rays detected from Cas A to the synchrotron radiation model., Comment: 5 pages, 2 postscript figures, 1 postscript table, latex uses epsfig.sty, aipproc.sty, to appear in the proc. of the 4th COMPTON symposium, held in Williamsburg, VA, April 27-30, 1997. Minor correction for SN1006's power-law index in Table 2

26Al emission at 1.809 MeV has been measured by several different instruments. The constraints set by each are very different, due to differing sky exposures and background estimation methods. We report on a first combined analysis of data from the COMPTEL, OSSE, and SMM instruments. The COMPTEL 1.8 MeV image as deconvolved with the Maximum-Entropy algorithm was tested for its flux normalization and its fine structure in the inner Galaxy, using data from SMM and OSSE. We find consistency of the OSSE data with COMPTEL’s image structure, in particular the choice of the ‘stopping point’ of the iterative deconvolution method. The SMM data suggest substantially more total flux than that given by the COMPTEL map. We discuss possible explanations, in particular large-scale emission outside the plane of the Galaxy, and outline future work on combined data from the three instruments.

We summarize CGRO observations of supernovae, including SN 1987A, SN 1991T, and SN 1993J, and classical novae, and their impact on current understanding of those phenomena. Many important fundamental questions about these objects remain, some of which might still be addressed by future CGRO observations or by data already in hand. We briefly sketch some of these prospects.

At times later than 200 days, Type Ia SN light curves are dominated by the kinetic energy deposition from positrons created in the decay of $^{56}$Co. In this paper, the transport of positrons after emission are simulated, for deflagration and delayed detonation models, assuming various configurations of the magnetic field. We find the light curve due to positron kinetic energy has a different shape for a radially combed magnetic field than it does for a tangled field that traps positrons. The radial field light curves fit the observed light curves better than do the trapping field light curves for the four SNe used in this study. The radial field permits a much larger fraction of the positrons to escape, perhaps enough to explain a large percentage of the 511 keV annihilation radiation from the Galactic plane observed by OSSE., 5 pages, 5 figures, latex uses epsfig.sty aipproc.sty, to appear in Proceedings of the 4th Compton Symposium held in Williamsburg, Va., April 27-30,1997

Burst locations with an arc second telescope (BLAST) is a new mission concept being studied for NASA's medium explorer (MIDEX) mission opportunities. The principal scientific objectives of the BLAST mission are (1) to localize gamma- ray burst (GRB) positions to arcsec accuracy; (2) to search for enhancements in the rate of GRBs toward M31; and (3) to conduct the most sensitive sky survey to date of x-ray sources in the 7 - 200 keV regime. These objectives are achieved using a large array of position-sensitive scintillation detectors with a total area of 17,000 cm2. This array is combined with a large field of view telescope (greater than 1 steradian) comprising two separate imaging systems. A coded aperture telescope provides arcminute source localization. For low energy x-rays (less than 50 keV), the aperture is also defined by phase modulation grids with provide complementary arcsecond information. The grid system consists of two aperture planes with 'checker board' patterns of slightly different pitch. The beating between the two grid pitches casts a broad interference pattern on the detector plane. Determining the phase of this interference pattern in both coordinates gives the location of a point source source in the sky, with aliased positions at approximately 1 arcmin spacing. The arcmin ambiguity is resolved by the coded aperture image. BLAST has a sensitivity to bursts of 0.03 photons cm-2 s-1, almost ten times more sensitive than BATSE. We expect to position 20 bursts per year to better than 2 arcsec accuracy and 35 bursts per year to better than 5 arcsec. BLAST will provide an all sky survey in hard x-rays with a sensitivity of 0.2 milliCrab at low energies.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on ~120 days of observations of Cygnus X-1 with OSSE onboard the Compton Observatory. Emission is detected in the range 50 keV to 1 MeV and we find evidence for a continuum of hard X-ray spectra over a broad range of fluxes rather than the existence of distinct flux states. Comparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average spectrum in the OSSE energy band. Although we have measured a new minimum in the hard X-ray flux from the source, no evidence was found for either a broad 1 MeV feature or a narrow 511 keV line previously reported in association with a low flux state. Upper limits on such emission features are an order of magnitude lower than earlier reported detections. The 5.6-day periodicity of the source measured at optical wavelengths was not detected with a sensitivity to the rms modulation fraction of 5% in the 60-140 keV energy band.

We present a space mission concept for a low energy gamma-ray telescope, ATHENA, which is under investigation as the next major advance in gamma-ray spectroscopy following the current COMPTON Gamma Ray Observatory and the planned INTEGRAL missions. The instrument covers the nuclear line emission energy domain with dramatically improved sensitivity and spectral resolution. The baseline configuration combines a high resolution Compton telescope constructed from Ge planar strip detectors for the 0.3 - 10 MeV energy region with a coded-aperture system for the 10 - 200 keV domain. The Ge Compton telescope provides a broad field of view with exceptional spectral and imaging resolution. The requirements, capabilities and simulations of ATHENA are discussed.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The interstellar medium contains a substantial amount of radioactive 26Al, which is well known from its γ‐ray line emission, and probably holds a similar quantity of 60Fe, which has yet to be seen in γ‐rays. The interstellar abundances of these place significant constraints on the current global Galactic nucleosynthesis rate and therefore on models of Galactic chemical evolution. We review the γ‐ray observations to date and their implications. The nucleus 44Ti might have been detected in the supernova remnant Cas A, but surprisingly has not been detected in any younger remnants. These observations offer considerable insight into supernova explosion dynamics and constrain the very recent Galactic supernova rate. Both 26Al and 60Fe, along with other radioactivities too rare to be detected anytime soon in γ‐rays, are also known to have existed in significant quantities in the forming solar system. It is natural then to compare those abundances inferred for the presolar nebula with expected interstellar abund...

A major objective of the Oriented Scintillation Spectrometer Experiment (OSSE) on NASA’s COMPTON Observatory is the observation of gamma ray line emission from a variety of astronomical objects. Gamma ray lines are of particular interest because they provide unique information on the production, relative abundances, acceleration and interactions of the nuclear component of matter. In this paper, we present some of the initial results obtained by OSSE on observations of line gamma-ray emission from solar flares, the galactic center region, and from supernovae.

Transient line features in the energy range from 400 keV to ≳1 MeV have been reported from several sources over the past two decades. These lines have been explained in terms of electron‐positron annihilation occurring close to black holes (Cygnus X‐1 and 1E 1740.7–2942) or neutron stars (the Crab). We searched in the data obtained by the SMM Gamma Ray Spectrometer during 1981–1989 for evidence of these lines, using the method of Harris et al. (1993). We have not found any convincing evidence of the occurrence of any transient line. We present upper limits on transient fluxes from these sources in the broad continuum 0.6–7 MeV energy band, and in the following previously‐reported lines; narrow lines from the Crab in the range 400–600 keV on time‐scales ∼1 d; a broad 1 MeV line from the Galactic Center region and Cyg X‐1, on time‐scales ∼12 d; and a broadened, redshifted 511 keV line from the Galactic Center region, on time‐scales ∼1 d and longer.

The significance of supernovae for the Compton GRO has lain primarily in the large amount of radioactivity that they eject. This activity and its daughters find their way into the interstellar medium and into other arenas of astrophysics. Direct measures of the quantity of such activity carries profound information that is not available from observations of stable nuclei. CGRO has had both success and disappointment: success at detecting or placing important limits to radiation from three of the brightest extragalactic supernovae of recent decades, disappointment that those three lay at the ‘‘plate limit’’ for OSSE and COMPTEL, and that nature has not smiled even wider upon us with a Galactic Type II or a Type Ia within 10 Mpc. While we review this situation we call attention to some exciting puzzles about supernovae and radioactive production within them. These puzzles place the CGRO mission into the larger framework of the science of cosmic radioactivity.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the COMPTON Gamma Ray Observatory has undertaken comprehensive observations of astrophysical sources during the eighteen‐month Phase 1 of the emision. These include investigations of many galactic sources, including binary X‐ray sources, pulsars, several transient X‐ray sources observed as Targets‐of‐Opportunity, and Nova Cygni 1992. Multiple observations of the galactic center region were undertaken to map the diffuse galactic emission and search for point sources. An overview of the galactic source observations and some preliminary results are presented.

The Oriented Scintillation Spectrometer Experiment on the Compton Gamma Ray Observatory has carried out spectral and timing observations of Galactic black hole candidates GX 339−4 and Cgy X‐1. GX 339−4 was observed as a target of opportunity in 1991 September, in response to the outburst reported by BATSE and SIGMA. The source was detected from 50 to 400 keV, at a level relative to the Crab of ∼30%. During a follow‐up observation made in 1991 November, the intensity of the source below 100 keV had dropped by nearly two orders of magnitude, and its was no longer detected above 100 keV. The observations of Cyg X‐1 were made during three different observing periods between 1991 June and November. The OSSE time‐averaged spectrum of Cyg X‐1 is about 10 times brighter than that of GX 339−4, but remarkably similar in shape over the energy range in which both are detected. No significant emission is seen above about 1 MeV, and there is no evidence for any bumps or narrow lines near 0.511 MeV. The spectra of both sources are described reasonably well by a Comptonization model

We investigate γ‐ray transport in spherically expanding supernova ejecta in the presence of density inhomogeneities. We compare the emergent photon spectra for the clumpy model with those predicted by the uniform density model and by an effective model with reduced opacity. We find that the γ‐ray line fluxes in a model with uniformly distributed clumps can be calculated analytically by the reduced opacity method of Bowyer and Field. However, the Comptonized x‐ray continuum significantly differs from the predictions of the reduced opacity model if the optical depth is large. The differences gradually disappear as the optical depth of the ejecta decreases.

The Oriented Scintillation Spectrometer Experiment (OSSE) on board the Gamma Ray Observatory (GRO) spacecraft has a wide variety of operating modes, each of which produces its own type of data. Each operating mode has various adjustable parameters that effect the availability and interpretation of the various data types. OSSE, its modes and data products, have been described in detail elsewhere [1].

Supernovae have long been considered important sites of nucleosynthesis of many nuclides found in nature, including some thought to be ejected in the form of radioactive parents. These unstable progenitors can reveal themselves through their power input to the ejecta [l], or more directly through their emission of signature γ-ray line photons [2]. Supernova 1987A has yielded an awesome quantity of data which has for the most part confirmed theories of Type II supernovae, as is demonstrated by many articles in these Proceedings. Among the important observations was the the precise agreement between the decline of the “bolometric” light curve and the 113 day exponential decay of 56Co [3,4], implying the production of its radioactive parent 56Ni by the explosion. The 56Co nucleus was further implicated by observations of large abundances of the element cobalt in infrared lines [5,6]. The direct observation of 7-ray lines from 56Co decay (e.g., [7]) laid to rest any doubts about its production in the explosion. It is the detection of these γ-ray lines by the γ-ray spectrometer (GRS) on the Solar Maximum Mission (SMM) satellite we discuss here. We also describe how the time evolution of the fluxes in these lines can be used to probe the distribution of the 56Co within the ejecta. These γ-ray light curves show that the radioactivity is found over a large range of optical depths in the ejected matter, a conclusion consistent with many other observations discussed in this volume.

There is now a wealth of evidence supporting the idea that classical novae are thermonuclear runaways on white dwarfs. Explosive hydrogen burning produces substantial quantities of unstable nuclei. These nuclei emit γ‐rays and β+’s which might be detectable to present and future γ‐ray spectrometers. Here we describe the production and observability of several unstable nuclei, review the observations made to date, evaluate the prospects for future observations, and discuss the importance of detection of γ‐rays for improving our understanding of the nova phenomenon.

We report on ~120 days of observations of Cygnus X-1 with OSSE onboard the Compton Observatory. Emission is detected in the range 50 keV to 1 MeV and we find evidence for a continuum of hard X-ray flux levels rather than the existence of distinct flux states. Comparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average spectrum in the OSSE energy band. Although we have measured a new minimum in the hard X-ray flux from the source, no evidence was found for either a broad 1 MeV feature or a narrow 511 keV line previously reported in association with a low flux state. Upper limits on such emission features are an order of magnitude lower than earlier reported detections. The 5.6-day periodicity of the source measured at optical wavelengths was not detected with a sensitivity to the rms modulation fraction of 5% in the 60-140 keV energy band.

We have searched for 1980-1988 Solar Maximum Mission gamma-ray spectrometer data for transient emission on timescales from hours to approximately 12 days of broad gamma-ray lines at energies approximately 400 keV, which were reported by the High Energy Astronomy Observatory (HEAO) 1 and SIGMA experiments from two sources lying toward the Galactic center. The lines have been interpreted as the product of the annihilation of positrons in pair plasmas surrounding the black hole candidate 1E 1740.7-2942 and the X-ray binary 1H 1822-371. Our results from a combined exposure of approximately 1.5 x 10(exp 7)s provide no convincing evidence for transient emission of this line on any timescale between approximately 9 hr and approximately 1 yr. Our 3 sigma upper limits on the line flux during approximately 12 day intervals are characteristically 4.8 x 10(exp -3) photon/sq cm/s, while for approximately 1 day intervals our 3 sigma upper limits are characteristically 4.9 x 10(exp -3) photon/sq cm/s. These results imply a duty cycle of less than 1.3% for the transient line measured from 1H 1822-371 during a approximately 3 week interval in 1977 by HEAO 1, and a duty cycle of less than or = 0.8% for the transient line detected in 1990 and 1992 from 1E 1740.7-2942 on approximately 1 day timescales by SIGMA.

Gamma-ray spectroscopy holds great promise for probing nucleosynthesis in individual supernova explosions via short-lived radioactivity, and for measuring current global Galactic supernova nucleosynthesis with longer-lived radioactivity. It was somewhat surprising that the former case was realized first for a Type II supernova, when both Co-56 and Co-57 were detected in SN 1987A. These provide unprecedented constraints on models of Type II explosions and nucleosynthesis. Live Al-26 in the Galaxy might come from Type II supernovae, and if it is eventually shown to be so, can constrain massive star evolution, supernova nucleosynthesis, and the Galactic Type II supernova rate. Type Ia supernovae, thought to be thermonuclear explosions, have not yet been detected in gamma-rays. This is somewhat surprising given current models and recent Co-56 detection attempts. Ultimately, gamma-ray measurements can confirm their thermonuclear nature, probe the nuclear burning conditions, and help evaluate their contributions to Galactic nucleosynthesis. Type Ib/c supernovae are poorly understood. Whether they are core collapse or thermonuclear events might be ultimately settled by gamma-ray observations. Depending on details of the nuclear processing, any of these supernova types might contribute to a detectable diffuse glow of Fe-60 gamma-ray lines. Previous attempts at detection have come very close to expected emission levels. Remnants of any type of age less that a few centuries might be detectable as individual spots of Ti-44 gamma-ray line emission. It is in fact quite surprising that previous surveys have not discovered such spots, and the constraints on the combination of nucleosynthesis yields and supernova rates are very interesting. All of these interesting limits and possibilities mean that the next mission, International Gamma-Ray Astrophysics Laboratory (INTEGRAL), if it has sufficient sensitivity, is very likely to lead to the realization of much of the great potential of gamma-ray spectroscopy for understanding supernovae.

We have search spectra obtained by the Solar Maximum Mission Gamma-Ray Spectrometer during 1981-1988 for evidence of transient gamma-ray lines from the Crab Nebula which have been reported by previous experiments at energies 400-460 keV and 539 keV. We find no evidence for significant emission in any of these lines on time scales between aproximately 1 day and approximately 1 yr. Our 3 sigma upper limits on the transient flux during 1 d intervals are approximately equal to 2.2 x 10(exp -3) photons/sq cm/s for narrow lines at any energy, and approximately equal to 2.9 x 10(exp -3) photons/sq cm/s for the 539 keV line if it is as broad as 42 keV Full Width at Half Maximum (FWHM). We also searched our data during the approximately 5 hr period on 1981 June 6 during which Owens, Myers, & Thompson (1985) reported a strong line at 405 keV. We detected no line down to a 3 upper sigma limit of 3.3 x 10(exp -3) photons/sq cm/s in disagreement with the flux 7.2 +/- 2.1 x 10(exp -3) photos/sq cm/s measured by Owens et al.