Search

Due to its peculiar appearance in the optical, its proximity, and its importance for the understanding of active galaxies and their active galactic nuclei (AGN), Centaurus A has been observed frequently within the last 150 years in all accessible wavelength bands. Thus a wealth of data exist over a wide range in frequencies (wavelengths, energies), which have been compiled in the ``NASA Extragalactic Database" (NED). A multifrequency combination of all those results allows to establish the important spectral energy distributions (SEDs) of this variable AGN in different emission states, and due to its nearness and the improving spatial resolution of new generation telescopes in all wavelength regimes, an attempt could be made to derive SEDs of different parts of the spatially resolved inner region of this AGN. The difficulties and problems to do this are described in this review.

The next large NASA mission in the field of γ-ray astronomy is the Gamma-Ray Large Area Space Telescope (GLAST), which is scheduled for a launch end of 2007. This satellite consists of the main instrument LAT (Large-Area Telescope) which is sensitive in the energy range between 10 MeV and 300$>>300 GeV, and a secondary instrument, the Gamma-Ray Burst Monitor (GBM), sensitive from 10 keV to 30 MeV. This omnidirectional monitor is an important instrument for γ-ray burst (GRB) science with GLAST, as it provides the link between the majority of the γ-ray busts emitting at lower energies and the high-energy events of γ-ray bursts and other transients. It will also serve as a trigger to increase the detection rate of γ-ray burst with the LAT. The GBM will provide real-time burst locations over a wide field-of-view (FoV) with sufficient accuracy to repoint the whole GLAST spacecraft. Time-resolved spectra of bursts recorded with LAT and the burst monitor will allow the investigation of the relation between the keV and the MeV–GeV emission from GRBs over seven decades in energy and will provide new insights into the physics of GRBs in general. In addition, the excellent localization of GRBs by the LAT will stimulate follow-up observations at other wavelengths which may yield clues about the nature of the burst sources.

Multiwavelength observations of polars are essential for developing the big picture of these systems, particularly to gain understanding of the relevant accretion-induced heating and cooling processes. Eclipsing polars are prime targets for such studies since different radiation processes can be disentangled by observations with high-time resolution. We present a preliminary combined analysis of space-based observations (XMM-Newton, ROSAT, HST) with ground-based high-speed photometry (MCCP, OPTIMA, ULTRACAM) of DP Leo, HU Aqr and UZ For. We determine the location and extent of different emission components and find secular and short-term changes in the accretion geometries. We find displaced optical and X-ray emission regions in DP Leo and HU Aqr as well as mini-bursts and accretion arcs of variable size in HU Aqr. We report marked changes in the X-ray eclipse length of UZ For between high and low states.

The instrumental line background of the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory is due to the activation and/or decay of many isotopes. The major components of this background can be attributed to eight individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl. The identification of instrumental lines with specific isotopes is based on the line energies as well as on the variation of the event rate with time, cosmic-ray intensity, and deposited radiation dose during passages through the South-Atlantic Anomaly. The characteristic variation of the event rate due to a specific isotope depends on its life-time, orbital parameters such as the altitude of the satellite above Earth, and the solar cycle. A detailed understanding of the background contributions from instrumental lines is crucial at MeV energies for measuring the cosmic diffuse gamma-ray background and for observing gamma-ray line emission in the interstellar medium or from supernovae and their remnants. Procedures to determine the event rate from each background isotope are described, and their average activity in spacecraft materials over the first seven years of the mission is estimated., Comment: accepted for publication in A&A, 22 pages, 21 figures

Centaurus A (Cen A, NGC 5128) is the nearest active galaxy and, notably, the viewing angle with respect to the jet axis is very large (> 70°). A first contemporaneous OSSE, COMPTEL, and EGRET spectrum obtained in October 1991 covers an energy range from 50 keV up to 1 GeV. This γ-ray broad-band spectrum was taken when Cen A was in an intermediate emission state as defined by the BATSE X-ray light-curve. The first simultaneous multiwavelength spectrum from radio to γ-rays was measured in July 1995 when Cen A was in a low emission state (the prevailing state for the last 7 years). The different spatial and temporal resolution in the different frequency regimes produces problems in the construction and interpretation of the multiwavelength spectra. These are addressed in this paper. The detection of emission > 1 MeV makes the inclusion of such high-energy emission into models for the spectral energy distribution mandatory.

On four occasions, twice in 1991 (near solar maximum) and twice in 1994 (near solar minimum), one COMPTEL D1 detector module was used as an omnidirectional detector to measure the high-energy (> 12.8 MeV) neutron flux near an altitude of 450 km. The D1 modules are cylindrical, with radius 13.8 cm and depth 8 cm, and are filled with liquid scintillator (NE213A). The combined flux measurements can be fit reasonably well by a product of the Mt. Washington neutron monitor rate, a linear function in the spacecraft geocenter zenith angle, and an exponential function of the vertical geomagnetic cutoff rigidity in which the coefficient of the rigidity is a linear function of the neutron monitor rate. When pointed at the nadir, the flux is consistent with that expected from the atmospheric neutron albedo alone. When pointed at the zenith the flux is reduced by a factor of about 0.54. Thus the production of secondary neutrons in the massive (16000 kg) Compton Gamma-Ray Observatory spacecraft is negligible. Rather, the mass of the spacecraft provides shielding from the earth albedo.

Since its launch in April of 1991, the COMPTEL experiment on the Compton Gamma-Ray Observatory has surveyed the entire sky in the energy range of 0.75–30.0 MeV. Here we survey the latest results obtained from the COMPTEL data with respect to galactic black hole candidates. The most prominent such object, Cygnus X-1, is also one of the brightest sources observed by COMPTEL . In addition, we report on the progress of a full survey of the sky, searching for evidence of MeV emission from other black hole candidates.

COMPTEL on board CGRO has observed a very strong (S[> 0.3 MeV] = 2.03 × 10−4 erg cm−2), complex, and long lasting (162 s) gamma-ray burst on February 17, 1994 (GRB 940217). Temporal fluctuations occur on timescales as short as 100 ms. Hard-to-soft spectral evolution has been observed during the burst emission and also within individual peaks. The photon spectra obtained within the 6 peaks can be modelled by single power law spectra and by broken power laws with break energies at around 1 MeV. The best-fit power law slopes vary between 1.1 and 3.5 throughout the event. The burst is located at [α 2000,δ 2000] = [29.5°, 3.8°] with a 3σ error radius of 0.9°. COMPTEL does not detect any significant “post-burst” emission (as reported by EGRET) at low energies (< 30 MeV), and our upper limits are marginally consistent with the EGRET detections. Using high energy spectral and temporal information, distance limits to GRB 940217 have been derived.

The sky region containing the active radio-galaxy Centaurus A has repeatedly been observed with the (COMPTEL) instrument onboard the Compton Gamma Ray Observatory (CGRO). The nine observation periods during the CGRO phases I and II in which Cen A was in the field of view of COMPTEL are spread over 18 months in the years 1991 to 1993. The energy range 0.75 to 30 MeV is covered. Clear evidence for a source with emission up to several MeV is seen from a region coinciding with the position of Cen A. The spectra change significantly over approximately 6 months between the two observation phases. A possible source confusion with the nearby gamma-ray source MS1312.1-4221 is discussed.

We present extremely high time resolution observations of the visual flare star binary UV Cet obtained with the Optical Pulsar Timing Analyzer (OPTIMA) at the 1.3 m telescope at Skinakas Observatory (SKO) in Crete, Greece. OPTIMA is a fiber-fed optical instrument that uses Single Photon Avalanche Diodes to measure the arrival times of individual optical photons. The time resolution of the observations presented here was 4 μ s, allowing to resolve the typical millisecond variability time scales associated with stellar flares. We report the detection of very short impulsive bursts in the blue band with well resolved rise and decay time scales of about 2 s. The overall energetics put these flares at the lower end of the known flare distribution of UV Cet.

During two 14-day periods in January 1992 and March 1992, several AGN which have been in the wide field of view of COMPTEL and EGRET at that time were monitored optically for variations in intensity and polarization. We report on first results of this correlated optical and CGRO observations.

During the first part of the COMPTON Gamma Ray Observatory sky survey, COMPTEL has detected the quasars 3C273 and 3C279 and the radio galaxy Centaurus A. This paper summarizes the preliminary findings and gives an upper limit on the MeV flux of the Seyfert galaxy NGC4151.

The COMPTEL experiment on the Compton Gamma-Ray Observatory is designed to image celestial gamma radiation in the energy range from 0.75–30 MeV within a field of view of 1 steradian. It can locate stronger point sources with an accuracy better than 0.5° and is capable of mapping diffuse emission as well. The Galactic-center region was observed by COMPTEL for several 2-week periods in 1991/1992. These observations show evidence for 1.8 MeV line emission along the Galactic disk (attributed to radioactive 26 Al), extending over at least 40 degrees in longitude.

Classical T Tauri stars (CTTS) are young, late-type objects, that still accrete matter from a circumstellar disk. Analytical treatments and numerical simulations predict instabilities of the accretion shock on the stellar surface. We search for variability on timescales below a few minutes in the CTTS TW Hya and AA Tau. TW Hya was observed with SALTICAM on the Southern African Large Telescope (SALT) in narrow-band filters around the Balmer jump. The observations were performed in slit mode, which provides a time resolution of about 0.1 s. For AA Tau we obtained observations with OPTIMA, a single photon-counting device with even better time resolution. Small-scale variability typically lasts a few seconds, however, no significant periodicity is detected. We place a 99 % confidence upper limit on the pulsed fraction of the lightcurves. The relative amplitude is below 0.001 for TW Hya in the frequency range 0.02-3 Hz in the 340 nm filter and 0.1-3 Hz in the 380 nm filter. The corresponding value for AA Tau is an amplitude of 0.005 for 0.02-50 Hz. The relevant timescales indicate that shock instabilites should not be seen directly in our optical and UV observations, but the predicted oscialltions would induce observable variations in the reddening. We discuss how the magnetic field could stabilise the accretion shock., 8 pages, 9 figures, accepted by A&A

The Gamma-Ray Burst Monitor (GBM) will significantly augment the science return from the Fermi Observatory in the study of Gamma-Ray Bursts (GRBs). The primary objective of GBM is to extend the energy range over which bursts are observed downward from the energy range of the Large Area Telescope (LAT) on Fermi into the hard X-ray range where extensive previous data exist. A secondary objective is to compute burst locations on-board to allow re-orientiong the spacecraft so that the LAT can observe delayed emission from bright bursts. GBM uses an array of twelve sodium iodide scintillators and two bismuth germanate scintillators to detect gamma rays from ~8 keV to ~40 MeV over the full unocculted sky. The on-board trigger threshold is ~0.7 photons/cm2/s (50-300 keV, 1 s peak). GBM generates on-board triggers for ~250 GRBs per year., 36 pages, 18 figures, to be published in Astrophysical Journal

The GLAST Burst Monitor (GBM) will enhance LAT observations of GRBs by extending the spectral coverage from the LAT threshold down to 8 keV, and will provide a trigger for re-orienting the spacecraft to observe delayed emission from selected bursts outside the LAT field of view. GBM consists of twelve NaI scintillation detectors operating in the 8 keV to 1 MeV energy range and two BGO scintillation detectors operating in the 150 keV to 30 MeV energy range. Detector resolution, effective area, and angular response have been determined by calibrations. Analyses indicate that the on-board burst threshold will be 0.7 photons cm-2s-1 and the on-board burst localization accuracy will typically be better than 8deg.

The high‐speed photometer ‘OPTIMA’, short for Optical Pulsar Timing Analyzer, was used to record optical (450–950 nm) light curves of selected cataclysmic variables of type AM Her (Polars) with sub‐second temporal resolution. The objects RX J0704+62, RX J0953+14, and EK UMa were re‐observed several years after their discovery to provide up to date light curves with good statistics. The eclipsing binary DP Leo, which had been observed during 20 years previous to the OPTIMA measurements, was shown to have an orbital ephemeris with a quadratic term. For HU Aqr we have collected data over the years 1999 to 2007. HU Aqr evolved in luminosity from a high state in 2000 to a low state that persists since 2001. We detected variability in HU Aqr in the form of optical flashes with timescales of seconds.

A high-speed photo-polarimeter, “OPTIMA” short for Optical Pulsar Timing Analyzer, has been designed and developed in the group for gamma-ray astronomy of the Max-Planck-Institut fur Extraterrestrische Physik. This sensitive, portable detector is used to observe optical emissions of sources that radiate mainly at X- and gamma-ray energies, like pulsars and other highly variable compact sources. The single photon counting instrument is based on fiber fed avalanche photodiodes (APDs), a GPS timing receiver,a CCD camera for target acquisition and a stand-alone data acquisition and control system. Several configurations are available: for photometry a hexagonal bundle with seven channels and one fiber offset for sky background monitoring; for polarimetry a rotating polarization filter in front of the photometer or a newly developed 4-channel double Wollaston system; and for coarse spectroscopy a 4-colour prism spectrograph.

The GLAST Burst Monitor (GBM) is designed to provide wide field of view observations of gamma‐ray bursts and other fast transient sources in the energy range 10 keV to 30 MeV. The GBM is composed of several unshielded and uncollimated scintillation detectors (twelve NaI and two BGO) that are widely dispersed about the GLAST spacecraft. As a result, reconstructing source locations, energy spectra, and temporal properties from GBM data requires detailed knowledge of the detectors’ response to both direct radiation as well as that scattered from the spacecraft and Earth’s atmosphere. This full GBM instrument response will be captured in the form of a response function database that is derived from computer modeling and simulation. The simulation system is based on the GEANT4 Monte Carlo radiation transport simulation toolset, and is being extensively validated against calibrated experimental GBM data. We discuss the architecture of the GBM simulation and modeling system and describe how its products will be used for analysis of observed GBM data. Companion papers describe the status of validating the system.

The GLAST Burst Monitor (GBM) will augment the capabilities of GLAST for the detection of cosmic gamma-ray bursts by extending the energy range (20 MeV to > 300 GeV) of the Large Area Telescope (LAT) towards lower energies by 2 BGO-detectors (150 keV to 30 MeV) and 12 NaI(Tl) detectors (10 keV to 1 MeV). The physical detector response of the GBM instrument for GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on-ground calibration measurements, performed extensively with the individual detectors at the MPE in 2005. All flight and spare detectors were irradiated with calibrated radioactive sources in the laboratory (from 14 keV to 4.43 MeV). The energy/channel-relations, the dependences of energy resolution and effective areas on the energy and the angular responses were measured. Due to the low number of emission lines of radioactive sources below 100 keV, calibration measurements in the energy range from 10 keV to 60 keV were performed with the X-ray radiometry working group of the Physikalisch-Technische Bundesanstalt (PTB) at the BESSY synchrotron radiation facility, Berlin., 2 pages, 1 figure; to appear in the Proc. of the First Int. GLAST Symp. (Stanford, Feb. 5-8, 2007), eds. S.Ritz, P.F.Michelson, and C.Meegan, AIP Conf. Proc

The Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006, comprises a Large Area Telescope (LAT) and a GLAST Burst Monitor (GBM). The LAT is a pair telescope with unprecedented sensitivity in the 20 MeV to 300 GeV energy range. The GLAST Burst Monitor consists of an array of NaI and BGO scintillation detectors operating in the 10 keV to 25 MeV range and covering a wide field of view. The GBM will enhance LAT observations of GRBs by extending the spectral coverage into the range of current GRB databases, and will provide a trigger for repointing the spacecraft to observe delayed emission from bursts outside the LAT field of view.

The GLAST Burst Monitor (GBM) comprises 12 NaI and 2 BGO detectors dispersed about the GLAST spacecraft. The GBM instrument simulation software must generate an accurate response function database for all detectors in their flight configuration to optimize the mission science return. Before science analysis codes use the response database, we must confirm that our simulation codes and models can reproduce laboratory observations. To validate the simulation effort, Monte Carlo results are compared to calibrated laboratory measurements collected with a variety of radiation sources.

The GLAST Burst Monitor (GBM) consists of 12 NaI detectors and 2 BGO detectors mounted on two sides of the spacecraft. This provides detection capability for energies between 10 keV and 30 MeV, and sensitivity to all directions in the sky. Extensive measurements using radioactive sources have been performed on the isolated detectors. Simulations of these measurements using the GEANT4 Monte Carlo radiation transport simulations toolset have also been preformed. For gamma‐ray burst the spacecraft itself will have a significant impact on the detector response, therefore the final stage of validations involves measuring the detector response to radioactive sources after the spacecraft is fully assembled, and validating the results using simulation. Details of these simulations will be given in this paper.

The GLAST Burst Monitor (GBM) is the secondary instrument on NASA's next Gamma-ray mission GLAST. It will enhance the capabilities of GLAST by locating and detecting cosmic gamma-ray bursts at lower energies by the use of 12 NaI detectors (energy range 10 keV to 1 MeV) and 2 BGO-detectors (energy range 150 keV to 30 MeV). GBM was built in a close collaboration between the MPE and the Marshall Space Flight Center (MSFC). The angular and energy response of each GBM detector has been calibrated using various radioactive sources at different incidence angles relative to the detector in a laboratory environment at the MPE in 2005. To facilitate the understanding of the reconstruction of the detector response, a detailed simulation of the whole laboratory environment and the setup of the calibration source were performed. A modified version of the CERN GEANT 4 simulation software (provided by collaborators at the Los Alamos National Laboratory) was used.

The onboard Data Processing Unit (DPU), designed and built by Southwest Research Institute, performs the high‐speed data acquisition for GBM. The analog signals from each of the 14 detectors are digitized by high‐speed multichannel analog data acquisition architecture. The streaming digital values resulting from a periodic (period of 104.2 ns) sampling of the analog signal by the individual ADCs are fed to a Field‐Programmable Gate Array (FPGA). Real‐time Digital Signal Processing (DSP) algorithms within the FPGA implement functions like filtering, thresholding, time delay and pulse height measurement. The spectral data with a 12‐bit resolution are formatted according to the commandable look‐up‐table (LUT) and then sent to the High‐Speed Science‐Date Bus (HSSDB, speed=1.5 MB/s) to be telemetered to ground. The DSP offers a novel feature of a commandable & constant event deadtime. The ADC non‐linearities have been calibrated so that the spectral data can be corrected during analysis. The best temporal resolution is 2 μs for the pre‐burst & post‐trigger time‐tagged events (TTE) data. The time resolution of the binned data types is commandable from 64 msec to 1.024 s for the CTIME data (8 channel spectral resolution) and 1.024 to 32.768 s for the CSPEC data (128 channel spectral resolution). The pulse pile‐up effects have been studied by Monte Carlo simulations. For a typical GRB, the possible shift in the Epeak value at high‐count rates (∼100 kHz) is ∼1% while the change in the single power‐law index could be up to 5%.

We report on how the the GLAST Burst Monitor (GBM) Flight Software will detect gamma‐ray rate increases, a process known as “triggering”, and on how the Flight Software will locate and classify the causes of the triggers.

One of the scientific goals of the Large‐Area Telescope (LAT) on GLAST is the study of gamma‐ray bursts (GRBs) in the energy range from ∼20 MeV to ∼300 GeV. In order to extend the energy measurement towards lower energies a secondary instrument, the GLAST Burst Monitor (GBM), will measure GRBs from ∼10 keV to ∼30 MeV and will therefore allow the investigation of the relation between the keV and the MeV‐GeV emission from GRBs over more than six energy decades. These unprecedented measurements will furthermore permit the exploration of the unknown aspects of the high‐energy burst emission. The status of the GBM project approximately one year before launch is reported here.

The understanding of the instrumental response of the GLAST Burst Monitor BGO detectors at energies above the energy range which is accessible by common laboratory radiation sources (< 4.43 MeV), is important, especially for the later cross-calibration with the LAT response in the overlap region between ~ 20 MeV to 30 MeV. In November 2006 the high-energy calibration of the GBM-BGO spare detector was performed at the small Van-de-Graaff accelerator at SLAC. High-energy gamma-rays from excited 8Be* (14.6 MeV and 17.5 MeV) and 16O* (6.1 MeV) were generated through (p,gamma)-reactions by irradiating a LiF-target. For the calibration at lower energies radioactive sources were used. The results, including spectra, the energy/channel-relation and the dependence of energy resolution are presented., 2 pages, 1 figure; to appear in the Proc. of the First Int. GLAST Symp. (Stanford, Feb. 5-8, 2007), eds. S.Ritz, P.F.Michelson, and C.Meegan, AIP Conf. Proc

The Crab nebula and its pulsar have been observed for about 3 hours with the high-speed photo-polarimeter OPTIMA in January 2002 at the Calar Alto 3.5m telescope. The Crab pulsar intensity and polarization are determined at all phases of rotation with higher statistical accuracy than ever. Therefore, we were able to separate the so-called 'off-pulse' phase emission (with an intensity of about 1.2% compared to the main peak, assumed to be present at all phases) from the pulsed emission and show the 'net' polarization of the pulsed structures. Recent theoretical results indicate that the measured optical polarization of the Crab pulsar is similar to expectations from a two-pole caustic emission model or a striped pulsar wind model., AIP Conference Proceedings "Astrophysical Sources of High Energy Particles and Radiation", eds. T. Bulik et al. (NY:AIP), Volume 801, 2005, pp. 306-311

The next large NASA mission in the field of gamma-ray astronomy, GLAST, is scheduled for launch in 2007. Aside from the main instrument LAT (Large-Area Telescope), a gamma-ray telescope for the energy range between 20 MeV and > 100 GeV, a secondary instrument, the GLAST burst monitor (GBM), is foreseen. With this monitor one of the key scientific objectives of the mission, the determination of the high-energy behaviour of gamma-ray bursts and transients can be ensured. Its task is to increase the detection rate of gamma-ray bursts for the LAT and to extend the energy range to lower energies (from ~10 keV to \~30 MeV). It will provide real-time burst locations over a wide FoV with sufficient accuracy to allow repointing the GLAST spacecraft. Time-resolved spectra of many bursts recorded with LAT and the burst monitor will allow the investigation of the relation between the keV and the MeV-GeV emission from GRBs over unprecedented seven decades of energy. This will help to advance our understanding of the mechanisms by which gamma-rays are generated in gamma-ray bursts., 8 pages, 7 figures, to appear in SPIE conference proceedings vol 5488, "UV-Gamma Ray Space Telescope Systems," Glasgow UK, 21-24 June 2004

The radio galaxy Centaurus A was observed by the BeppoSAX satellite five times from 1997 to 2000. From July 6 1999 to August 17 1999, the source was also simultaneously pointed by COMPTEL on-board of the satellite CGRO. Centaurus A has a complex spectrum with multiple extended components and a strongly absorbed (N_H\sim 10^{23}$ cm${-2}) nucleus well fitted by a power law (\Gamma \sim 1.8) which bends at high energies. When the BeppoSAX and COMPTEL observations are combined together, an exponential cutoff with e-folding energy \sim 1000 keV gives an adequate description of the spectral steepening. A complex feature in emission at 6-7 keV is resolved into two Fe components, one narrow cold line and an ionized line centred at 6.8 keV. Significant variations have been observed in the iron feature, with the less prominent ionized line seemingly being the only one responsible for them: its variations do not appear to correlate with the strength of the continuum. The high energy cutoff and the Fe feature suggest the presence of an accretion flow in the Centaurus A nucleus. However the absence of a significant reflection, the narrowness of the cold line as well as the lack of correlation between the continuum and 6.8 keV line variations disfavour a standard cold/ionized thin disk (at least in the inner regions). A more plausible configuration might be a hot thick optically thin accretion flow surrounded by material with different opacities. Finally, we note that high energy break observed by BeppoSAX and COMPTEL could be also reasonably explained by Inverse Compton radiation from a jet. If this is the case, a structured jet with outer slow layers surrounding a beamed inner region is necessary to explain the strong Fe feature observed by BeppoSAX., Comment: 21 pages, 8 figures, accepted for publication in ApJ

A high-speed photometer, "OPTIMA" short for Optical Pulsar Timing Analyzer, has been designed as a sensitive, portable detector to observe optical pulsars and other highly variable sources. The detector contains eight fiber fed avalanche photodiode single photon counters, a GPS timing receiver, a CCD camera for target acquisition and a computerized control unit. The central fibers are configured as a hexagonal bundle around the target fiber, while one fiber is located at a distance of ~1' as a monitor for the night sky background. Recently a rotating polarization filter and a 4-color prism spectrograph have been added to the system as optional equipment. Since January 1999 OPTIMA has been used on different telescopes to measure detailed lightcurves and polarization of the Crab Pulsar, in a search for optical emission from the Geminga pulsar, and for the timing of cataclysmic variables and X-ray transients.

We have used observations of Cygnus X-1 from the Compton Gamma-Ray Observatory (CGRO) and BeppoSAX to study the variation in the MeV gamma-ray emission between the hard and soft spectral states, using spectra that cover the energy range 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder gamma-ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft gamma-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/non-thermal Comptonization models to fit broad band spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of ~4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a non-thermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region., 19 pages, 9 figures, accepted for publication in ApJ. Final version includes expanded discussion section

We have used observations from CGRO to study the variation in the MeV emission of Cygnus X-1 between its low and high X-ray states. These data provide a measurement of the spectral variability above 1 MeV. The high state MeV spectrum is found to be much harder than that of the low state MeV spectrum. In particular, the power-law emission seen at hard X-ray energies in the high state spectrum (with a photon spectral index of 2.6) is found to extend out to at least 5 MeV, with no evidence for any cutoff. Here we present the data and describe our efforts to model both the low state and high state spectra using a hybrid thermal/nonthermal model in which the emission results from the Comptonization of an electron population that consists of both a thermal and nonthermal component., 5 pages, submitted to the proceedings of the GAMMA 2001 High Energy Astrophysics Symposium (Baltimore, April 4-6, 2001)

Due to its proximity and its importance for the understanding of active galaxies and their active nuclei (AGN), Centaurus A has been observed frequently within the last 150 years in all accessible wavelength bands. Thus a wealth of data exists which has been compiled into the "NASA Extragalactic Database" (NED). Missing from this compilation are to date almost completely the results of recent high energy observations as e.g. obtained by the Compton Gamma Ray Observatory (CGRO). A combination of those recent high energy results with all other observations in the NED enables us for the first time to establish the important spectral energy distributions (SEDs) of this closest AGN in different emission states. The combined data have been analyzed to produce SEDs which are from contemporaneous data and in addition an attempt was made to derive SEDs which are spatially resolved, i.e. SEDs from observations which can resolve nucleus and jet in Cen A are treated separately., Comment: To appear in the proceedings of the "Gamma 2001" confernce; v3:one reference corrected, acknowledgement added; v4: error in plots (EGRET data) corrected

The Compton Gamma-Ray Observatory (CGRO) has observed the Cygnus region on several occasions since its launch in 1991. The data collected by the COMPTEL experiment on CGRO represent the most sensitive observations to date of Cygnus X-1 in the 0.75-30 MeV range. A spectrum accumulated by COMPTEL over 10 weeks of observation time shows significant evidence for emission extending out to several MeV. We have combined these data with contemporaneous data from both BATSE and OSSE to produce a broad-band gamma-ray spectrum, corresponding to the low X-ray state of Cygnus X-1, extending from 50 keV up to approximately 5 MeV. Although there is no evidence for any broad line-like emissions in the MeV region, these data further confirm the presence of a hard tail at energies above several hundred keV. In particular, the spectrum at MeV energies can be described as a power-law with a photon spectral index of $\alpha$ = -3.2, with no evidence for a cutoff at high energies. For the 200 keV to 5 MeV spectrum, we provide a quantitative description of the underlying electron spectrum, in the context of a hybrid thermal/non-thermal model for the emission. The electron spectrum can be described by a thermal Maxwellian with a temperature of $kT_e$ = 86 keV and a non-thermal power-law component with a spectral index of $p_e$ = 4.5. The spectral data presented here should provide a useful basis for further theoretical modeling., Comment: 28 pages, 6 figures, accepted for publication in ApJ. Considerably revised from original submission

We describe our program to provide cumulative flux limits in the COMPTEL energy range for a large sample of high-energy active galactic nuclei (AGN) of general interest. The First COMPTEL Source Catalogue [1,2] will contain cumulative two-sigma upper limits to the time-averaged MeV-emission measured with COMPTEL for 142 AGN and other unidentified gamma-ray sources detected at high Galactic latitudes (|b|>10°). These limits were derived using composite COMPTEL all-sky maximum-likelihood maps for the 4.5-year period covering Phases 1 through 4 of the CGRO mission (1991–1995). The composite all-sky maps were produced from standard-processing COMPTEL datasets for individual CGRO viewing periods, in four standard energy bins spanning the sensitive range of COMPTEL (0.75–1, 1–3, 3–10, and 10–30 MeV). From these maps we have extracted statistical likelihoods, significances of potential source detections, and associated fluxes, errors, or upper limits, for an extensive list of target objects. In the choice of can...

Emission in the MeV band from the region containing the bright unidentified EGRET source GRO J1837+59 and the steep-spectrum EGRET blazar QSO 1739+522 was first reported by COMPTEL following an observation in November 1992. During this observation the emission was consistent with a single point source, designated GRO J1753+57. However, the location of GRO J1753+57 was not consistent with either GRO J1837+59 or QSO 1739+522 and identifying its counterpart in other wavelengths proved difficult. Moreover, subsequent observations suggested that the source could not in fact be described as a single point source and explanations involving extended or multiple sources were invoked. We present an analysis of recent observations of this region, which confirm that the emission cannot arise from a single source, but can be modelled as a combination of emission from both GRO J1837+59 and QSO 1739+522. The spectrum and time variability derived for these two sources are discussed and compared to observations in neighbo...

Centaurus A is the nearest active radio galaxy and thus it is the object of many detailed studies. A set of dedicated web based Centaurus A pages has been established in an effort to collect and present all available information and make it available “by a mouse click” at the URL: “http://www.gamma.mpe-garching.mpg.de/∼hcs/Cen-A/.”

In previous work, we have used data from the first three years of the CGRO mission to assemble a broad-band $\gamma$-ray spectrum of the galactic black hole candidate Cygnus X-1. Contemporaneous data from the COMPTEL, OSSE and BATSE experiments on CGRO were selected on the basis of the hard X-ray flux (45--140 keV) as measured by BATSE. This provided a spectrum of Cygnus X-1 in its canonical low X-ray state (as measured at energies below 10 keV), covering the energy range from 50 keV to 5 MeV. Here we report on a comparison of this spectrum to a COMPTEL-OSSE spectrum collected during a high X-ray state of Cygnus X-1 (May, 1996). These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux {\it decreases} during the high X-ray state, the flux at energies above 1 MeV {\it increases}, resulting in a significantly harder high energy spectrum. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft $\gamma$-ray energies. These data extend this picture, for the first time, to energies above 1 MeV., Comment: 5 pages, 4 figures, to be published in AIP Conf. Proc., "The Fifth Compton Symposium"

Observations with Compton Gamma Ray Observatory have provided key insights to the high energy emission mechanisms in Active Galactic Nuclei (AGN) and clearly separate them into a class of Blazars, with apparent non-thermal jet emissions often peaking in the EGRET energy band, and into the Seyfert class with emissions more closely related to the accretion disk presumably energizing the nuclear black hole. OSSE measurements indicate that the high energy emission from Seyfert AGN has a high energy cutoff below ∼100–200 keV and no Seyferts have been detected by COMPTEL or EGRET. Here, we review the X-ray and γ-ray observations of Seyferts and Radio Galaxies and the broad band emission models that describe the observed spectral characteristics. A comparison of Seyfert 1, Seyfert 2, and radio galaxies can be made in the context of the AGN unification models.

The COMPTEL experiment on the Compton Gamma-Ray Observatory (CGRO) has observed the Cygnus region on several occasions since launch. These data represent the most sensitive observations to date of Cygnus X-1 in the 0.75–30 MeV range. The spectrum shows significant evidence for emission extending out to several MeV. These data alone suggest a need to modify the thermal Comptonization models or to incorporate some type of non-thermal emission mechanism. Here we report on the results of an analysis of selected COMPTEL data collected during the first three years of the CGRO mission. These data are then compared with contemporaneous data from both BATSE-EBOP and OSSE. Given a lack of consistency between the OSSE and BATSE-EBOP spectra, it is difficult to draw firm conclusions regarding the exact shape of the spectrum near 1 MeV. A few general conclusions can, however, be drawn from these data.

During the CGRO observations in the years 1991 to 1995, the region on the sky including the active radio galaxy Centaurus A, was in the wide field-of-view of COMPTEL and EGRET in 15 pointings of various durations. During some of these pointings, also OSSE made simultaneous observations of Cen A. BATSE, the fourth instrument on CGRO, continuously monitors this source. The analysis of the COMPTEL data obtained during these 15 pointings shows emission consistent with the position of Cen A, and spectra in two different emission states have been measured. The COMPTEL spectra in the energy range 0.75–30 MeV are consistent with the published Centaurus A spectra in the energy range 0.05–4 MeV from the OSSE instrument. For the higher emission state, a moderate steepening of the spectrum above 30 MeV is required to fit the EGRET data point. Thus, a spectrum of Cen A over 4 decades of energy is available for theoretical interpretation.

Using the COMPTEL data of the first 5 years of the CGRO mission we have derived the average pulsed spectrum of the Crab Pulsar, as well as phase-resolved spectra of the pulsed emission of the Crab Pulsar. The spectra in the COMPTEL energy range (0.75 to 30 MeV) are compared to those in the neighboring energy bands. The pulsed flux has been examined for its stability. Pulsed lightcurves have been derived for different energy intervals. Preliminary results of this analysis are presented.

We present first results of a systematic search through COMPTEL all-sky maps for evidence of MeV gamma-ray emission from active galaxies. All-sky maximum-likelihood and flux maps have been produced from publicly-available COMPTEL datasets for individual CGRO viewing periods for the 4.5-year period covering Phases 1 to 4 of the CGRO mission (1991–1995), in four standard energy bins spanning the sensitive range of COMPTEL (0.8–30 MeV). From these all-sky maps we have extracted statistical likelihoods of source emission and corresponding fluxes or upper limits for ∼150 candidate objects (primarily active galaxies) lying at high Galactic latitudes |b|>10°. Included in these results is a compilation of COMPTEL fluxes and/or limits for the unidentified high-latitude sources of the the Second EGRET Catalog. Our scientific objective is to derive the global properties of high-energy AGN in the medium-energy gamma-ray regime, and to provide cumulative fluxes or limits in the COMPTEL energy range for all high-energy...

The blazer‐type active galactic nuclei (AGNs) CTA 102 (QSO 2230+114) and 3C 454.3 (QSO 2251+158), located about 7° apart on the sky, were observed by the Compton Gamma Ray Observatory at four epochs in 1992. Both were detected by EGRET and here we present the COMPTEL observations of the two quasars. These observations clearly indicate that the power‐law spectra measured by EGRET above ∼50 MeV show a flattening at lower MeV energies. A comparison with observations at other wavelengths shows that the power spectra of CTA 102 and 3C 454.3 peak at MeV energies. This behavior appears to be a common feature of AGNs radiating at gamma‐ray energies.

The imaging Compton Telescope (COMPTEL) is sensitive in the energy range of 0.75 to 30 MeV. COMPTEL observed the Crab several times during the Compton Gamma Ray Observatory (CGRO) sky survey and CGRO Phase II. Both the Crab pulsar and nebula are detected over the entire COMPTEL energy range. The phase-averaged energy spectra of the Crab Pulsar and Nebula are presented. The combined observations provide sufficient statistics for a phase-resolved analysis of the Crab pulsar spectrum.

The recent nucleosynthetic activity in the Galaxy can be traced through the 1.809 MeV gamma‐ray line from radioactive 26Al with its decay time of one million years. We review here the 1.809 MeV measurements and potential 26Al sources with emphasis on recent findings from the COMPTEL survey of the Galactic plane. This survey shows highly structured emission over a wide longitude range with a marked asymmetry relative to the Galactic Centre. It suggests that the emission originates in rather localized regions, not necessarily concentrated in the inner disk of the Galaxy. Smooth intensity distributions as expected for a nova origin are very hard to reconcile with the data. In fact, no Galaxy‐wide source distribution resembles the observations. The intermittent nature of both massive‐star formation and 26Al line emission, each on a time scale of a few million years, probably determines to a large extent the structured appearance of the 1.809 MeV sky. Nearby sources of 26Al, presumably core‐collapse supernovae...