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Recent high-energy missions have allowed keeping watch over blazars in flaring states, which provide deep insights into the engine powered by supermassive black holes. However, having a quasar caught in a very bright flaring state is not easy requiring long surveys. Therefore, the observation of such flaring events represents a goldmine for theoretical studies. Such a flaring event was captured by the INTEGRAL mission in June 2015 while performing its (as of today) deepest extragalactic survey when it caught the prominent blazar 3C~279 in its brightest flare ever recorded at gamma-ray energies. The flare was simultaneously recorded by the Fermi gamma-ray mission, by the Swift mission, by the INTEGRAL mission and by observations ranging from UV, through optical to the near-IR bands. The derived snapshot of this broad spectral energy distribution of the flare has been modeled in the context of a one-zone radiation transfer leptonic and lepto-hadronic models constraining the single emission components. The derived parameters of both models challenge the physical conditions in the jet. However, very recently published very-high-energy (VHE) data at TeV energies are very close to our lepto-hadronic model., ICRC 2017 conference proceeding

Blazars radiate from radio through gamma-ray frequencies and thereby make ideal targets for multifrequency studies. Such studies allow the properties of the emitting jet to be constrained. 3C 279 is among the most notable blazars and therefore subject to extensive multifrequency campaigns. We report the results of a campaign ranging from near-IR to gamma-ray energies that targeted an outburst of 3C 279 in 2015 June. The campaign pivots around the detection in only 50 ks by INTEGRAL, whose IBIS/ISGRI data pin down the high-energy component of the spectral energy distribution (SED) between Swift-XRT data and Fermi-LAT data. The overall SED from near-IR to gamma rays can be well represented by either a leptonic or a lepto-hadronic radiation transfer model. Even though the data are equally well represented by the two models, their inferred parameters challenge the physical conditions in the jet. In fact, the leptonic model requires parameters with a magnetic field far below equipartition with the relativistic particle energy density. In contrast, equipartition may be achieved with the lepto-hadronic model, although this implies an extreme total jet power close to the Eddington luminosity.

To probe further the possible nature of the unidentified source IGR J17098-3628, we have carried out a detailed analysis of its long-term time variability as monitored by RXTE/ASM, and of its hard X-ray properties as observed by INTEGRAL. INTEGRAL has monitored this sky region over years and significantly detected IGR J17098-3628 only when the source was in this dubbed active state. In particular, at $\ge$ 20 keV, IBIS/ISGRI caught an outburst in March 2005, lasting for $\sim$5 days with detection significance of 73$��$ (20-40 keV) and with the emission at $< $200 keV. The ASM observations reveal that the soft X-ray lightcurve shows a similar outburst to that detected by INTEGRAL, however the peak of the soft X-ray lightcurve either lags, or is preceded by, the hard X-ray ($>$20 keV) outburst by $\sim$2 days. This resembles the behavior of X-ray novae like XN 1124-683, hence it further suggests a LMXB nature for IGR J17098-3628. While the quality of the ASM data prevents us from drawing any definite conclusions, these discoveries are important clues that, coupled with future observations, will help to resolve the as yet unknown nature of IGR J17098-3628., 15 pages, 7 figure, accepted in PASJ

We report on an analysis of X- and $\gamma$-ray observations of PKS 1830-211, based on the long-term campaigns carried out by \emph{INTEGRAL} and COMPTEL. The \emph{INTEGRAL} data currently available present a $33\sigma$ significance detection in the 20-100 keV band, while the COMPTEL 6-years data provide a $5.2\sigma$ significance detection in the 1-3 MeV energy band. At hard X-rays, \emph{INTEGRAL} and supplementary \emph{SWIFT} observations show flux variability on timescales of months. At $\gamma$-rays, the source shows persistent emission over years. The hard X-ray spectrum is well represented by a power-law model, with $\Gamma \sim 1.3$ in the 20-250 keV band. This photon index is well consistent with the previous report of $\Gamma \sim 1.3$ obtained at $E > 3.5$ keV from the best fit of \emph{XMM-Newton} data with a broken power law model. The joint \emph{XMM-Newton}/\emph{INTEGRAL} spectrum presented here is then fit with a broken power-law model and the parameters are refined compared to the previous. The results show the photon index changes from $\sim 1.0$ to $\sim 1.3$ at a break energy $\sim 4$ keV. At MeV energies, the spectrum softens to $\Gamma \sim 2.2$. These results, together with the EGRET measurement at $E \ge 100$ MeV, constitute a broad-band spectrum containing the peak of the power output at MeV energies, similar to most high-luminosity $\gamma$-ray blazars. The measured spectral characterstics are then discussed in the framework of the gravitational lens effects., Comment: accepted for ApJ

We report first evidence for a new unidentified and variable MeV source, located near the galactic plane at (l,b)∼(284.5°, 2.5°). The source, GRO J1036-55, is found at a significance level of ∼5.6σ by COMPTEL in its 3–10 MeV band. The energy spectrum indicates a spectral maximum at 3–4.3 MeV with a steep slope at higher energies. Since the COMPTEL 3–4.3 MeV data contain contamination by an instrumental background line, we performed several consistency checks, which all are consistent with an astrophysical nature of this emission feature.

INTEGRAL and RXTE performed three simultaneous observations of the nearby radio galaxy Cen A in 2003 March, 2004 January, and 2004 February with the goals of investigating the geometry and emission processes via the spectral/temporal variability of the X-ray/low energy gamma ray flux, and intercalibration of the INTEGRAL instruments with respect to those on RXTE. When combined with earlier archival RXTE results, we find the power law continuum flux and the line-of-sight column depth varied independently, and the iron line flux was essentially unchanging. Taking X-ray spectral measurements from satellite missions since 1970 into account, we discover a variability in the column depth between 1.0x10^23 cm^-2 and 1.5x10^23 cm^-2, and suggest that variations in the edge of a warped accretion disk viewed nearly edge-on might be the cause., Comment: 48 pages, 14 figures, 10 tables, accepted for publication by The Astrophysical Journal

The COMPTEL experiment on CGRO observed the gamma-ray sky at energies from 0.75 MeV to 30 MeV between April 1991 and June 2000. COMPTEL detected many gamma-ray sources, among them an unidentified one labeled GRO J1823-12, which is positionally consistent with the prominent high-mass X-ray binary LS 5039. Because LS 5039 was established as gamma-ray emitter during recent years, whose gamma-radiation radiation is modulated along its binary orbit, we reanalysed the COMPTEL data of GRO J1823-12 including an orbital resolved analysis. We find a significant MeV source, showing evidence for a modulated MeV flux corresponding to the orbital period of LS 5039 of about 3.9 days. We show that its MeV emission is stronger at the orbital part around the inferior conjuction than at the part of the superior conjunction, being in phase with X-rays and TeV gamma-rays, however being in anti-phase with GeV gamma-rays. We conclude that the COMPTEL source GRO J1823-12 is the counterpart of the microquasar candidate LS 5039, at least for the majority of its MeV emission. The COMPTEL fluxes, put into multifrequency perspective, provide new constraints on the modelling of the high-energy emission of LS 5039., Comment: accepted by Astronomy & Astrophysics; 11 pages, 9 figures

We present results from the spectral analysis of 29 gamma-ray bursts measured by the COMPTEL Telescope instrument (0.75–30 MeV) during the period from April 1991 through May 1995. The time-averaged spectra of these events are consistent with a simple power law model with spectral index in the range 1.5–3.5, whereas simple thermal models are statistically inconsistent with the burst sample. We find good agreement between burst spectra measured simultaneously by BATSE, COMPTEL and EGRET, which typically show a spectral transition or “break” in the BATSE energy range around a few hundred keV followed by simple power law emission extending to hundreds of MeV. Observations of rapid intensity variations up to a few MeV in the stronger bursts are used to investigate the importance of photon-photon opacity and to place limits on the source distance and bulk Lorentz factor.

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.

In its first three years of operation, the COMPTEL instrument on theCompton Gamma-Ray Observatory has measured the locations (mean accuracy ∼1°) and spectra (0.75-30 MeV) of 18 gamma-ray bursts and continues to observe new events at a rate of ∼1/month. With good angular resolution and sensitivity at MeV energies, the growing COMPTEL burst catalog is an important new piece of evidence in the on-going GRB mystery. The COMPTEL burst locations are consistent with an isotropic distribution of sources, yet the spatial coincidence of two of the bursts indicates the possibility of repetition. The COMPTEL burst spectra are in most cases consistent with a single power law model with spectral index in the range 2–3. However, two bursts show evidence of a spectral break in the MeV range. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby (within the Galaxy) or the gamma-ray emission is relativistically beamed. We present an overview of analysis results obtained from the COMPTEL burst catalog concentrating on the search for burst repetition and the implications of highly variable MeV emission.

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.

著者人数: 93名, Accepted: 2012-05-30, 資料番号: SA1004165000

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.

Summary.Pulsar measurements performed by the experiment COMPTEL, aboard the Compton Gamma Ray Observatory, are described. The main results refer to the Crab and Vela pulsars whose pulse shape characteristics are given in some detail and light curves are compared with those above 50 MeV, as observed by the COS-B satellite. No other gamma-ray pulsars have been detected to date by COMPTEL, the upper limit on the pulsed signal from Geminga being compatible with indications by other experiments.

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.

The COMPTEL experiment on GRO images 0.7 – 30 MeV celestial gamma-radiation that falls within its 1 steradian field of view. During the first fifteen months in orbit, preliminary localizations from BATSE triggers indicated that about 1 in 6 cosmic events could have fallen within COMPTEL's field of view. We summarize work on the brightest of these gamma-ray bursts and present new position constraints for GRB 911118 and GRB 920622.

We present the results of a multifrequency campaign from radio to hard X-ray energies on the blazar 3C 279 during an optical high-state in January 2006. We give the observational results (multifrequency light curves and spectra) and compile an SED. This complements an SED from an optical low-state in June 2003. Surprisingly the two SEDs differ only in their high-energy synchrotron emission (near-IR - UV), while the low-energy inverse-Compton emission (X- to Gamma-rays) remained unchanged. By interpreting with a steady-state leptonic emission model, the variability among the SED can be reproduced by a change solely of the low-energy cutoff of the relativistic electron distribution. In an internal shock model for blazar emission, such a change could e.g. achieved through a varying relative Lorentz factor of colliding shells producing internal shocks in the jet., Comment: 11 pages, 9 figures, A&A accepted

The primary scientific goal of the GRIPS mission is to revolutionize our understanding of the early universe using gamma-ray bursts. We propose a new generation gamma-ray observatory capable of unprecedented spectroscopy over a wide range of gamma-ray energies (200 keV-50 MeV) and of polarimetry (200-1000 keV). The gamma-ray sensitivity to nuclear absorption features enables the measurement of column densities as high as 10(28)cm (-aEuro parts per thousand 2). Secondary goals achievable by this mission include direct measurements of all types of supernova interiors through gamma-rays from radioactive decays, nuclear astrophysics with massive stars and novae, and studies of particle acceleration near compact stars, interstellar shocks, and clusters of galaxies.

A ToO observation of the TeV-emitting blazar Mrk 421 with INTEGRAL was triggered in June 2006 by an increase of the RXTE count rate to more than 30 mCrab. The source was then observed with all INTEGRAL instruments with the exception of the spectrometer SPI for a total exposure of 829 ks. During this time several outbursts were observed by IBIS and JEM-X. Multiwavelength observations were immediately triggered and the source was observed at radio, optical and X-ray wavelengths up to TeV energies. The data obtained during these observations are analysed with respect to spectral evolution and correlated variability. Preliminary results of the analysis are presented in this poster., Comment: 2 pages, 3 figures; 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 COMPTEL unidentified source GRO J 1411-64 was observed by INTEGRAL and XMM-Newton in 2005. The Circinus Galaxy is the only source detected within the 4$\sigma$ location error of GRO J1411-64, but in here excluded as the possible counterpart. At soft X-rays, 22 reliable and statistically significant sources (likelihood $> 10$) were extracted and analyzed from XMM-Newton data. Only one of these sources, XMMU J141255.6 -635932, is spectrally compatible with GRO J1411-64 although the fact the soft X-ray observations do not cover the full extent of the COMPTEL source position uncertainty make an association hard to quantify and thus risky. At the best location of the source, detections at hard X-rays show only upper limits, which, together with MeV results obtained by COMPTEL suggest the existence of a peak in power output located somewhere between 300-700 keV for the so-called low state. Such a spectrum resembles those in blazars or microquasars, and might suggest at work by a similar scenario. However, an analysis using a microquasar model consisting on a magnetized conical jet filled with relativistic electrons, shows that it is hard to comply with all observational constrains. This fact and the non-detection at hard X-rays introduce an a-posteriori question mark upon the physical reality of this source, what is discussed here., Comment: Accepted for publication in Astrophysics and Space Science, as proceedings of "The Multi-Messenger Approach to High-Energy Gamma-Ray Sources", Barcelona, July 4-7, 2006, J. M. Paredes, O. Reimer, and D. F. Torres, editors

Aims. We investigate the nature of the COMPTEL unidentified source GRO J1411-64. Methods. The source was observed by INTEGRAL. Its central part was also observed by XMM-Newton. Results. The data analysis shows no hint for new detections at hard X-rays. The upper limits in flux herein presented constrain the energy spectrum of whatever was producing GRO J1411-64, imposing, in the framework of earlier COMPTEL observations, the existence of a peak in power output located somewhere between 300–700 keV for the so-called low state. The Circinus Galaxy is the only source detected within the 4σ location error of GRO J1411-64, but can be safely excluded as the possible counterpart: the extrapolation of the energy spectrum is well below the one for GRO J1411-64 at MeV energies. 22 reliable and statistically significant sources (likelihood > 10) were extracted and analyzed from XMM-Newton data. Only one of these sources, XMMU J141255.6-635932, is spectrally compatible with GRO J1411-64 although the fact the soft X-ray observations do not cover the full extent of the COMPTEL source position uncertainty make an association hard to quantify and thus risky. Conclusions. The unique peak of the power output at high energies (hard X-rays and gamma-rays) resembles that found in the SED seen in blazars or microquasars, and might suggest that a similar scenario is at work. However, an analysis using a microquasar model consisting on a magnetized conical jet filled with relativistic electrons which radiate through synchrotron and inverse Compton scattering with star, disk, corona and synchrotron photons shows that it is hard to comply with all observational constrains. This fact and the non-detection at hard X-rays introduce an a-posteriori question mark upon the physical reality of this source, which is discussed in some detail Fil: Torres, Diego F.. Institut de Ciències de l’Espai; España Fil: Zhang, S.. Institute of High Energy Physics. Laboratory for Particle Astrophysics; China Fil: Reimer, O.. Stanford University; Estados Unidos Fil: Barcons, X.. Instituto de Física de Cantabria; España Fil: Corral, A.. Instituto de Física de Cantabria; España Fil: Bosch Ramon, Valentí. Universidad de Barcelona; España Fil: Paredes, Josep Maria. Universidad de Barcelona; España Fil: Romero, Gustavo Esteban. Provincia de Buenos Aires. Gobernación. Comision de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomia; Argentina Fil: Qu, J. L.. Institute of High Energy Physics. Laboratory for Particle Astrophysics; China Fil: Collmar, W.. Max Planck Institut für extraterrestrische Physik; Alemania Fil: Schönfelder, V.. Max Planck Institut für extraterrestrische Physik; Alemania Fil: Butt, Y.. Harvard-Smithsonian Center for Astrophysics; Estados Unidos

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

We report the detection of an unidentified gamma-ray source near the Galactic plane by the COMPTEL experiment aboard the Compton Gamma-Ray Observatory. The source is detected at a significance level of ~ 7.2 sigma in the energy range 1-3 MeV and at ~ 4.6 sigma in the lower 0.75-1 MeV band in the time period March to July 1995. At energies above 3 MeV are only marginal hints or upper limits obtained. The MeV spectrum has a soft shape. Strong flux variability is found within one year at energies below 3 MeV. Possible counterparts of galactic and extragalactic nature are discussed., Comment: 7 pages including 5 figures, accepted for publication in A&A

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)

X-ray astronomy, for which measurements above the Earth’s atmosphere are necessary as in γ-ray astronomy, is also a relatively new astronomical field, which was opened roughly 40 years ago. A rocket flight in 1962, carrying an X-ray detector above the Earth’s atmosphere, resulted in two important discoveries: the detection of an extrasolar X-ray source, and the detection of a bright diffuse X-ray sky, in contrast to a dark optical one (Giacconi et al. 1962). Roughly five years after the discovery of this X-ray source in the constellation Scorpius, which was subsequently called Scorpius X-1 (Sco X-1), it was suggested that this source is a mass-accreting compact object in a binary system. It took another five years, however, until this picture was experimentally proven. Optical spectroscopy of the supergiant HD 226868, the optical counterpart of Cygnus X-1 (the first X-ray source detected in the constellation Cygnus), revealed a sinusoidal 5.6 day variation in radial velocity (Webster and Murdin 1972), which was a convincing sign of orbital motion in a binary system. This discovery immediately revealed a surprise. The mass function derived from the orbital period and amplitude in radial velocity, together with a canonical mass estimate of ≥ 15 M⊙ for the supergiant, implied a mass of the compact object of more than 3 M⊙, the maximal mass limit of a neutron star, and therefore strongly suggested that the compact object was a black hole. So, surprisingly, the first X-ray source with a proven binary nature most likely contains a black hole as the compact object.

During the CGRO mission, the Galactic center region was well exposed by COMPTEL. There is evidence for a distinct excess near the Galactic center direction in the COMPTEL energy range 1–30 MeV. The analysis for point sources is however complicated by the intense emission from the Galactic ridge. We use data from the full mission to investigate this source, present a multiwavelength spectrum and discuss various possible counterparts, including the microquasar source 1E1740.7-2942.

Light curves of 3C 279 are presented in optical (R-band), X-rays (RXTE/PCA), and gamma rays (CGRO/EGRET) for 1999 Jan-Feb and 2000 Jan-Mar. During both of those epochs the gamma-ray levels were high, and all three observed bands demonstrated substantial variation, on time scales as short as one day. Correlation analyses provided no consistent pattern, although a rather significant optical/gamma-ray correlation was seen in 1999, with a gamma-ray lag of ~2.5 days, and there are other suggestions of correlations in the light curves. For comparison, correlation analysis is also presented for the gamma-ray and X-ray light curves during the large gamma ray flare in 1996 Feb and the two gamma-bright weeks leading up to it; the correlation at that time was strong, with a gamma-ray/X-ray offset of no more than 1 day., Comment: 20 pages, including 7 figures; accepted by The Astrophysical Journal

We report the main MeV properties (detections, light curves, spectra) of the Virgo blazars 3C 273 and 3C 279 which were derived from a consistent analysis of all COMPTEL Virgo observations between 1991 and 1997.

We have analyzed the two blazars of 3C 454.3 and CTA 102 using all available COMPTEL data from 1991 to 1999. In the 10-30 MeV band, emission from the general direction of the sources is found at the 4$\sigma$-level, being consistent with contributions from both sources. Below 10 MeV only 3C 454.3 is significantly detected, with the strongest evidence (5.6 $\sigma$) in the 3-10 MeV band. Significant flux variability is not observed for both sources, while a low emission is seen most of the years in the 3-10 MeV light curve for 3C 454.3. Its time-averaged MeV spectrum suggests a power maximum between 3 to 10 MeV., Comment: 5 pages including 8 figures, to appear in the proceedings of the GAMMA 2001 High Energy Astrophysics Symposium (Baltimore, April 4-6, 2001)

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

During analysis of sources below the threshold of the third EGRET catalog, we have discovered a source, named GRO J1400-3956 based on the best position, with a remarkably steep spectrum. Archival analysis of COMPTEL data shows that the spectrum must have a strong turn-over in the energy range between COMPTEL and EGRET. The EGRET data show some evidence of time variability, suggesting an AGN, but the spectral change of slope is larger than that seen for most gamma-ray blazars. The sharp cutoff resembles the high-energy spectral breaks seen in some gamma-ray pulsars. There have as yet been no OSSE observations of this source.

CGRO-COMPTEL measures gamma-ray burst positions, time-histories and spectra in the 0.1–30 MeV energy range, in both imaging “telescope” and single detector “burst spectroscopy” mode. GRB 990123, one of the most recent bright bursts seen by COMPTEL, was caught in the optical while the gamma-ray emission was ongoing. The burst spectral shape can be characterized by a peak in ν−Fν just below 1 MeV and a power-law tail above (photon index∼−2.4,) and flattening below. There is also spectral evolution by downward movement of the peak and/or softening of the power laws. We present light-curves, time resolved spectra and an image map for this burst.

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...

In the course of the mission we have gradually developed an analysis method that separates in an iterative manner the celestial emission and the (a priori unknown) instrumental background. It has become our standard analysis tool for point sources. We illustrate here that this method is widely applicable now. It provides mutually consistent sets of model-fitting parameters (spectra) and sky maps, both for continuum and line studies. Because of the wide applicability, it has been possible to make various cross-checks while building up confidence in this procedure.

Energetic particles accelerated in supernova remnants provide a variety of continuum emission signatures over the entire electromagnetic spectrum. The measured hard X-ray continuum emission from the Cas A supernova remnant indicates in fact the presence of high-energy electrons and makes it an interesting candidate for study at MeV energies. We have analyzed COMPTEL data with respect to continuum emission in addition to the previously detected 44Ti line. The results provide useful constraints on models of electron shock acceleration in this object.

The imaging COMPTEL telescope has accumulated 0.1–30 MeV spectra, time-histories, and positions of more than forty γ-ray bursts within its ∼3 sr field of view in the eight years since its launch. CGRO-COMPTEL measures in both imaging “telescope” and single detector “burst spectroscopy” mode. In an ongoing collaboration with BACODINE/GCN, bursts are imaged automatically, with localizations relayed to a global network of multiwavelength observers in near real time (∼10 minutes). We have updated our burst search procedure in two ways: 1) using more sensitive search algorithms; and 2) using data from more detectors. The first are double change-point algorithms. With these we can find regions of significant excess flux with no assumptions on the wide range of burst time-scales (e.g., rise-times or decay-times) or intensities, and only one adjustable parameter (the time-averaged count-rate of the detectors). This makes it simpler to combine information on burst time-histories from the larger effective area (but...