H. Bloemen

The Soft Gamma-Ray Spectral Variability of Cygnus X-1

We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV γ-ray emission between the hard and soft spectral states, using spectra that cover the energy range from 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 γ-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 γ-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband 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 nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.

Emission from 44Ti associated with a previously unknown Galactic supernova

Nearly 400 years have passed since a supernova was last observed directly in the Milky Way (by Kepler, in 1604). Numerous Galactic supernovae are expected to have occurred since then, but only one (Cassiopeia A) may have been seen. The historical record of supernovae is therefore incomplete, as demonstrated by the spatial distribution of young supernova remnants. The discovery, of γ-ray emission from the decay of 44Ti nuclei associated with Cassiopeia A, the youngest known remnant, has revealed a new way to search for the remnants of other relatively recent supernovae (less than ∼1,000 years old). Here we report the discovery of 44Ti line emission from a previously unknown young supernova remnant, in the direction of the Vela remnant. We estimate a distance of ∼200 parsecs and an age of ∼680 years for the remnant, making it the closest young remnant to the Earth. Why it was not recorded historically remains unknown.

A High-Sensitivity Measurement of the MeV Gamma-Ray Spectrum of Cygnus X-1

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 broadband γ-ray spectrum, corresponding to the low X-ray state of Cygnus X-1, extending from 50 keV up to ~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 α = -3.2, with no evidence for a cutoff at high energies. For the 200 keV-5 MeV spectrum, we provide a quantitative description of the underlying electron spectrum, in the context of a hybrid thermal/nonthermal model for the emission. The electron spectrum can be described by a thermal Maxwellian with a temperature of kTe = 86 keV and a nonthermal power-law component with a spectral index of pe = 4.5. The spectral data presented here should provide a useful basis for further theoretical modeling.

Detection of the 67.9 keV and 78.4 ke.V lines associated with the radio-active decay of 44 Ti in Cassiopeia A

We report the detection of the 44Sc nuclear decay lines at 67.9 and 78.4 keV associated with the nuclear decay of 44Ti in Cassiopeia A. The line emission was observed by the Phoswich Detection System instrument on board BeppoSAX, which recently observed the supernova remnant for over 500 ks. The detection of the line emission with a flux of (2.1 ± 0.7) × 10-5 photons cm-2 s-1 in each line (90% confidence) is at the 5 σ significance level, if we can assume that the 12-300 keV continuum is adequately represented by a single power law. However, as the nature of the continuum is not clear, we investigate various other possibilities. A more conservative estimate of the line flux is made by assuming that a power-law continuum is at least a good approximation to the continuum emission for a narrower 30-100 keV energy range. With this limitation, the measured line flux is (1.9 ± 0.9) × 10-5 photons cm-2 s-1, with the detection still at the 3.4 σ significance level. We suggest that together with the Compton Gamma Ray Observatory/COMPTEL measurement of the 44Ca line at 1157 keV of (3.3 ± 0.6) × 10-5 photons cm-2 s-1, a flux for all three lines of (2.5 ± 1.0) × 10-5 photons cm-2 s-1 for Cas A can be adopted. This implies an initial 44Ti mass of (0.8-2.5) × 10-4 M☉.

The Revised COMPTEL Orion Results

COMPTEL observations of the Orion/Monoceros region have shown distinct evidence for excessive 3-7 MeV emission that was attributed to nuclear de-excitation lines from accelerated 12C and 16O nuclei. Unfortunately, we must conclude now that this appears to be a spurious result. This conclusion follows from a better understanding of the instrumental background, from a better exposure of the region, and from an improved analysis method. We show here how the impact of each of these gradually reduces the signal to a less than 3 σ result. The prime underlying cause seems to be 24Na activation in and around the upper COMPTEL detectors. Combining all available data, we now set a 2 σ flux upper limit on the 3-7 MeV emission of Orion of 3 × 10-5 γ cm-2 s-1, to be compared with the previously derived flux of ~10-4 γ cm-2 s-1.

A gamma-ray constraint on the nature of dark matter

If even a small component of the Galactic spheroid consists of the weakly interacting majorana fermions that are cold-dark-matter candidate particles for the Galactic halo, there should be a substantial flux of annihilation gamma rays from a source of about 1-deg extent at the Galactic center. COS B observations already constrain the halo cold-dark-matter (CDM) content entrained in the inner spheroid to be less than about 10 percent. A somewhat weaker constraint applies to the CDM believed to be present in the Galactic disk, but still only about 15 percent can be in such particles. Monochromatic line photons of energy 3-10 GeV are also predicted, and future experiments may be capable of improving these limits. Since both theoretical models of galaxy formation in a CDM-dominated universe and mass models for the rotation curve in the inner Galaxy suggest that a substantial fraction of the spheroid component should be nonluminous and incorporate entrained halo CDM, the hypothesis that the halo CDM consists predominantly of weakly interacting fermions such as photinos or heavy majorana mass neutrinos or higgsinos may already be subject to observational test. 30 references.

The cosmic diffuse gamma-ray background measured with COMPTEL

We report a refined analysis of the cosmic diffuse gamma-ray background (hereafter CDG) in the energy range 0.8–30 MeV with the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory. We have identified all major instrumental-background lines, included the results of a detailed study of the instantaneous instrumental continuum-background characteristics, and used all available COMPTEL data at high galactic latitudes. The new “whole-sky” average CDG spectrum again shows no evidence for an MeV-bump, merges smoothly with the spectra at higher and lower energies, and is consistent with a transition from a softer to a harder component around a few MeV. This spectrum is consistent with previous COMPTEL results. In addition, comparison of the CDG intensity from various regions of the sky allows us to place limits on the large-scale anisotropy of the CDG in selected energy bands. Upper limits on the relative deviations from isotropy consistent with the data at the 95% confidence limit range from abou...

COMPTEL upper limits for the 56Co γ-ray emission from SN1998bu

Supernova 1998bu in the galaxy M 96 was observed by COMPTEL for a total of 88 days starting 17 days after the explosion. We searched for a signal in the 847 keV and 1238 keV lines of radioactive 56 Co from this type Ia supernova. Using several dierent analysis methods, we did not detect SN1998bu. Our measurements should have been sensitive enough to detect 60 Co gamma-rays as predicted from supernova models. Our 2 flux limit is 2:3 10 5 photons cm 2 s 1 ; this would correspond to 0.35 M of ejected 56 Ni, if SN1998bu were at a distance of 11.3 Mpc and transparent to MeV gamma rays for the period of our measurements. We discuss our measurements in the context of common supernova models, and conclude disfavoring a supernova event with large mixing and major parts of the freshly-generated radioactivity in outer layers.

New COMPTEL Results on MeV Gamma Rays from the Orion/Monoceros Region

We present new COMPTEL observations of the OrionyMonoceros region, confirming our previous report of excessive 3‐7 MeV emission which we tentatively attributed to nuclear deexcitation lines at 4.44 and 6.13 MeV fromaccelerated 12 Cand 16 Onuclei.Weshowfirstresultsfromacombinationofalldataobtainedduringthefirst five years of the mission, with emphasis on the extent of the emission region. The 3‐7 MeV radiation is observed at the 9 ssignificance level with aflux of (12.8H1.5) 310 25 gcm 22 s 21 , consistent with our previousfindings. Inaddition,wecanfirmlyconcludenowthatonesinglesourcecannotexplainthemeasuredintensitydistribution: we find that the emission extends over the complex (the Orion A and B and the Mon R2 clouds), but it may actually be due to a few localized source regions. Evidence (4.4 s) for emission from the Mon OB1yOB2 area isappearingaswell.Wepresentapreliminaryspectrum(notfullydeconvolved),whichindicatesthattheemission is widely spread over the 3‐7 MeV range, suggesting a dominantly broad-line origin from energetic C and O nuclei if indeed caused by nuclear lines. The spectrum shows features that might be due to splitting of the C and O lines, but further substantiation should come from forthcoming spectral analyses and new observations. Subject headings: acceleration of particles—cosmic rays—gamma rays: observations— nuclear reactions, nucleosynthesis, abundances

COMPTEL gamma-ray line analysis techniques

This paper presents some of the imaging analysis techniques which are currently used to investigate COMPTEL 1.8 MeV gamma-ray line data. In the first part of this paper, an algorithm is presented which allows the accurate prediction of the background distribution at a specific line energy using measurements at adjacent energy intervals. The second part deals with the different image reconstruction methods which are applied to COMPTEL data, namely the maximum entropy method, the Richardson-Lucy algorithm and Pixon- based image reconstruction. We conclude that after 5 years of experience with COMPTEL, 1.8 MeV gamma-ray line imaging techniques are well established allowing a comprehensive study of cosmic gamma-ray line emission.