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The Calorimeter (CAL) of the GLAST Large Area Telescope (LAT) is designed to measure the energy of cosmic gamma rays. The CAL is comprised of a segmented, hodoscopic array of CsI(Tl) scintillating crystals totaling 8.3 radiation lengths in depth. This design allows the CAL to image the development of gamma ray showers and reconstruct their incident energy with greater accuracy, and it makes the CAL a powerful tool in background rejection. The performance of the sixteen CAL Modules has remained stable from subsystem environmental testing through LAT integration and environmental testing. In combination with simulations, this test program has demonstrated that the CAL meets its design requirements.

We report on our analysis of a 274 ks observation of the Vela Pulsar with the Rossi X-Ray Timing Explorer (RXTE). The double-peaked, pulsed emission at 2–30 keV, which we had previously detected during a 93 ks observation, is confirmed with much improved statistics. There is now clear evidence, both in the spectrum and the light curve, that the emission in the RXTE band is a blend of two separate components. The spectrum of the harder component connects smoothly with the OSSE, COMPTEL and EGRET spectra and the peaks in the light curve are in phase coincidence with those of the high-energy light curve. The spectrum of the softer component is consistent with an extrapolation to the pulsed optical flux and the soft component of the second RXTE peak is in phase coincidence with the second optical peak. In addition, we see a peak in the 2–8 keV RXTE light curve at the radio peak phase.

We report on results of an observation with the Rossi X-Ray Timing Explorer of PSR1706-44, to search for pulsed X-ray emission. PSR1706-44 is a radio and high-energy gamma-ray pulsar (detected by EGRET), but no pulsed emission has been detected in the X-ray band. Since most of the other known gamma-ray pulsars emit pulsed X-rays, it is expected that PSR1706-44 would also be an X-ray pulsar. However, ROSAT detected a source at the pulsar position, but did not detect pulsations, giving a pulsed fraction upper limit of 18%. We report here results of a search for modulation at the pulsar period from our ≈90K second RXTE observation (out of the total approved time of 147 K second), carried out in November 1996 during a low state of the nearby X-ray binary 4U1705-44.

Spectral observations of the 4 June 1991 solar flare obtained with the Oriented Scintillation Spectrometer Experiment (OSSE) on board the Compton Gamma Ray Observatory (CGRO) are used to determine the interaction rate of the accelerated protons by fitting two‐minute spectra with a narrow nuclear‐deexcitation line template. The lines are produced primarily when accelerated protons interact with ambient solar material and thus reveal the proton interaction rate. The OSSE observations provide the only direct measurements of this emission during the intense portion of the 4 June flare. A subset of the data is then used to determine individual fluxes of 8 narrow γ‐ray lines. These preliminary fluxes are compared to line fluxes determined for 19 flares obtained with the Solar Maximum Mission Gamma Ray Spectrometer (SMM/GRS). Ambient elements with low first‐ionization potentials (FIP) appear to be enhanced relative to those with higher FIP (as compared to the photosphere) similar to the enhancement found previou...

In its first two years of operation, the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) observed 26 active galaxies classified as Seyferts. These objects were selected for their historical X‐ray characteristics as identified by HEAO‐1, EXOSAT and Ginga and include three of the gamma‐ray emitting AGN detected prior to the launch of CGRO. OSSE detects approximately half of these objects at a significance of 5σ or greater. The OSSE observations in the 50 keV–500 keV range indicate spectra which, on average, are significantly softer than the typical photon power law index, α∼1.7, observed in x‐rays. In the case of NGC 4151, the brightest Seyfert 1 detected by OSSE, a distinctly thermal spectrum is observed during both 1991 and 1993 observations. An average Seyfert AGN spectrum is formed by a weighted summation of the individual observations excluding the brightest detections. This average photon spectrum from 26 observations of 15 AGNs is well described by a ...

We have continued our program to search for fast gamma ray pulsars (P

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory has observed four of the active galactic nuclei detected by EGRET (Fichtel et al. 1993) at photon energies above 100 MeV; namely, 3C 273, 3C 279, MRK 421 and PKS 0528+134. These sources show blazar properties at other wavelengths. Only 3C 273 and 3C 279 were detected with high significance in the OSSE energy range, 0.05–10 MeV. However, not all of the OSSE observations were contemporaenous with the EGRET detections. The OSSE exposures varied from 10 weeks on 3C 273 (in 8 separate viewing periods) to 3 weeks on PKS 0528+134. When combined with contemporaneous COMPTEL observations of these sources, we find strong evidence for spectral softening between the hard X‐ray and medium energy gamma‐ray bands in 3 of the 4 sources. Constraints provided by the OSSE observations on models of blazar emissions are discussed.

OSSE observed the hard X‐ray transient source GRO J0422+32 (XN Per 92) between 11 Aug and 17 Sep 1992. High time resolution data were obtained over several energy bands in the ∼35–600 keV range with timing resolutions of 8 ms and 125 μs. Power spectra at energies below 175 keV show substantial low‐frequency red noise with a shoulder at a few 10−2 Hz, peaked noise with characteristic frequency near 0.2 Hz, and a second shoulder at a few Hz. The peaked noise profile is broad and asymmetric, with a sharp low‐frequency edge and a high‐frequency tail; thus the physical process responsible for the peaked noise appears to have a well‐defined maximum timescale. The characteristic frequencies of the shoulders and the peak are independent of energy and source intensity. In addition, there is evidence for peaked components near 0.04 Hz and 0.1 Hz with variable intensity from day to day.

The OSSE observations of the starburst galaxies NGC 253 and M82 can be used to obtain upper limits to the Type Ia and Ib supernova rates in these galaxies. Monte Carlo simulations of randomly occurring supernova events in NGC 253 and M82 were performed to evaluate the significance of our upper limits to the 0.847 and 1.238 MeV 56Co gamma‐ray line fluxes on the supernova rates from these two galaxies. We obtain a 3σ upper limit of 1 SN yr−1 for Type Ia supernovae in these galaxies. A set of observations with a particular sequence is suggested for nearby starburst galaxies in order to maximize the chances of supernova gamma‐ray line detection from these sources.

We present preliminary results on the X‐ray emission of discrete sources in the Galactic Center region from OSSE observations made between July, 1991 and September, 1992. These observations were designed to investigate the distribution of galactic emission from the central region of our galaxy. The approach consists of scans through the Galactic Center with different position angles of the OSSE collimators’ rectangular (3.8°×11.4°) field‐of‐view. The sources 1E 1740‐2942, GRS 1758‐258, GX 354+0, and GX 1+4, have all been observed in transient, intense states at various times during this period. A sample of these observations is presented here.

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 OSSE detector on boar dthe Compton Gamma Ray Observatory observed the Vela Pulsar (PSR 0833‐45) during August–September 1991 and April–May 1992. Pulsed emission was detected at the 4‐5 σ level in the 0.06–0.57 MeV band in the sum of the two observing periods, as well as in each individual observation at lower significance. There is no significant variability observed. Light curves have a peak structure similar to that observed at higher energies. The spectrum is hard at lower energies and appears to require a break in the 0.5–2 MeV regions.

We report results of a two‐week observation of NGC 4151 with the OSSE instrument. The source had a very soft spectrum which falls off exponentially with an e‐folding energy of 39.2±1.5 keV in the energy range 65–800 keV, and had an intensity at 100 keV of 2.33±0.05×10−2 photons cm−2 sec−1 MeV−1. Simple pair cascade models in a compact source, which have been suggested for Seyfert galaxies, cannot explain the steepness and shape of the spectrum above 100 keV. We suggest pair loading and reacceleration as the mechanism responsible for the observed X‐ray and gamma‐ray spectra and the previously reported luminosity—spectral index correlation between 2–20 keV.

Gamma‐ray observations of the nearby starburst galaxy NGC 253 over the energy range 0.06–10 MeV have been obtained with the OSSE spectrometer. The source was detected up to 200 keV with a total significance of 4.2σ. When attributed to NGC 253 this corresponds to an estimated luminosity of 3×1040 ergs s−1. The spectrum is fit by a power law of photon index ∼2.5. A search for 56Ni→56Co→56Fe supernova gamma‐ray lines yielded no significant detection: the 3σ upper limits at 0.158, 0.847 and 1.238 MeV are 4×10−5, 8×10−5 and 9×10−5 ph cm−2 s−1, respectively. We find that inverse Compton scattering is insufficient to explain the observed continuum radiation. Bremsstrahlung and discrete sources may account for the flux. We also consider the possibility that the detected emission may result from low energy continuum from scattered gamma‐ray lines produced by a very recent Type Ia or Ib supernova outburst in NGC 253.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) provides solar‐flare observation capabilities that represent a significant improvement over previous experiments. In this paper, we discuss the OSSE solar observation modes, give an overview of the solar‐flare observations to date, and discuss in some detail the observations obtained during the June 1991 CGRO solar Target of Opportunity. We show evidence for time structure of flare hard X‐ray (≳100 keV) emission as short as tens of msec. Several nuclear deexcitation lines along with emission ≳10 MeV were seen from the four GOES X10+ flares observed by OSSE. Neutrons were observed from two of the flares. Preliminary results from a search of the OSSE flare database for nuclear emission from weak flares are presented and compared to results from a similar search of the Solar Maximum Mission Gamma‐Ray Spectrometer database.

We present a summary of OSSE observations of galaxies and quasars that have been carried out during the Phase 1 all‐sky survey by the Compton Observatory. The OSSE instrument has detected continuum emission from several Seyfert galaxies and quasars. Seyfert 1 galaxies make up the majority of the detections, typically at energies below 300 keV, with the measured spectra generally compatible with power‐law continuum models with photon spectral indices around −2, or with thermal emission models with temperatures around 50 keV. The quasars generally have harder spectral indices than the Seyfert galaxies.With the exception of Centaurus A and NGC 4151, there is little evidence of significant flux variability in the OSSE data sets for most of the Seyfert galaxies observed. In some cases, the OSSE detections are at flux levels significantly below those reported for previous observations.While the analysis of the complete set of Phase 1 OSSE observations of active galaxies is still in progress, the OSSE data will clearly provide a major new database for the examination and testing of models of high‐energy emission from active galactic nuclei.We present a summary of OSSE observations of galaxies and quasars that have been carried out during the Phase 1 all‐sky survey by the Compton Observatory. The OSSE instrument has detected continuum emission from several Seyfert galaxies and quasars. Seyfert 1 galaxies make up the majority of the detections, typically at energies below 300 keV, with the measured spectra generally compatible with power‐law continuum models with photon spectral indices around −2, or with thermal emission models with temperatures around 50 keV. The quasars generally have harder spectral indices than the Seyfert galaxies.With the exception of Centaurus A and NGC 4151, there is little evidence of significant flux variability in the OSSE data sets for most of the Seyfert galaxies observed. In some cases, the OSSE detections are at flux levels significantly below those reported for previous observations.While the analysis of the complete set of Phase 1 OSSE observations of active galaxies is still in progress, the OSSE data will ...

The Oriented Scintillation spectrometer Experiment (OSSE) on the Compton Gamma‐Ray Observatory has performed numerous observations of the galactic plane and galactic center region to measure the distribution of galactic 511 keV positron annihilation radiation and to search for time variability of the emission. These observations show conclusive evidence for a narrow 511 keV line and positronium continuum. For the first OSSE galactic center observation, the fitted 511 keV line flux was (2.3±0.3)×10−4 γ cm−2 s−1 and the positronium continuum flux was (8.8±0.7)×10−4 γ cm−2 s−1, corresponding to a positronium fraction (0.96±0.04). The quoted uncertainty in the positronium flux does not include the effect of the underlying continuum model on the fitted positronium flux. No significant time variability of the line flux has been observed; the 3σ upper limit to daily variations from the mean is 3×10−4 γ cm−2 s−1. The galactic distribution of the 511 keV line emission is found to be most consistent with a 2‐compon...

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

The Oriented Scintillation Spectrometer Experiment detected over 80 cosmic γ‐ray bursts from mid‐April to late November, 1991. Data at 16 ms resolution were accumulated from its large‐area NaI shields in response to BATSE triggers. Rates in excess of 5000 cts/16 ms were observed from the 1991 May 3 burst. Bursts on June 1 and August 7 were observed in the spectrometer’s aperture. Preliminary information on both the spectrum and location of the source of the June 1 burst are provided. Plans for future observations and analyses of bursts are discussed.

Low‐energy gamma‐ray detectors for the early 21st century will require much improved sensitivity, energy resolution and angular resolution. Superconducting devices offer some potential for meeting these requirements. In particular, we are investigating Superconducting Transition Detectors (STD’s), in which the energy loss and location of a particle interaction are derived by sensing the transitions between the normal and superconducting states in a uniform array of small spherical granules. Such devices could prove especially advantageous for Compton telescopes and coded‐aperture detectors.

Preliminary identifications of instrumental and atmospheric background lines detected by the gamma‐ray spectrometer on NASA’s Solar Maximum Mission satellite (SMM) are presented. The long‐term and stable operation of this experiment has provided data of high quality for use in this analysis. Methods are described for identifying radioactive isotopes which use their different decay times. Temporal evolution of the features are revealed by spectral comparisons, subtractions, and fits. An understanding of these temporal variations has enabled the data to be used for detecting celestial gamma‐ray sources.