W. R. Webber

The Imaging Compton Telescope Comptel on the Gamma Ray Observatory

This instrument is based on a newly established concept of ¿-ray detection in the very difficult 1-30 MeV range. It employs the unique feature of a two-step interaction of the y-ray: a Compton scattering collision in a first detector followed by an interaction in a second detector element. COMPTEL has been designed to perform a very sensitive survey of the y-ray sky. Extreme care has been taken to minimize background so that the detection limits of COMPTEL will be dominated by source counting statistics. It combines a wide field of view (about 1 steradian) with a good angular resolution. The design criteria of COMPTEL and the perforrmance of a Science Model are described.

The rigidity dependence of forbush decreases observed at the Earth

The rigidity dependence of the large Forbush decreases occurring on July 23, 1981, July 11, 1982, and February 6, 1986, has been determined using neutron monitor and IMP spacecraft data which cover the energy range from about 50 MeV to 30 GeV. The contribution of solar flare protons to the lower-energy data from the IMP cosmic ray telescopes was carefully removed. We found that the rigidity dependences of the magnitudes of the July 1981, July 1982, and February 1986 Forbush decreases for P ≥ 2 GV were given by exp (−1/P0.75), exp (−1/P0.6), and exp (−1/P1.0), respectively. For 0.5 ≤ P ≤ 2 GV the magnitude of the Forbush decreases in July 1981 and July 1982 was rigidity independent. The February 1986 event also appeared to be rigidity independent below ∼1 GV. The characteristic recovery time of these Forbush decreases was found to be not strongly rigidity dependent. These results on the rigidity dependence of Forbush decreases for 0.5 < P < 20 GV are discussed in the context of proposed models.

Forbush decreases and interplanetary magnetic field disturbances : association with magnetic clouds

Magnetic clouds have been proposed as a mechanism to produce Forbush decreases in the cosmic radiation. We have examined the temporal association of magnetic clouds and Forbush decreases and find practically no association of the main phase of the Forbush decrease with the arrival of a magnetic cloud. On the other hand, Forbush decreases generally follow the strong interplanetary shocks which sometimes precede magnetic clouds. The main phase of the cosmic ray decrease occurs 2–5 hours after the shock and during the passage of the region in which the magnetic field is disturbed. It appears that a Forbush decrease is more likely to occur following a shock in which the magnetic field and plasma parameters are strongly enhanced. These results indicate that the decrease of the cosmic ray intensity may be produced by the smaller diffusion coefficient in the region behind the shock. The sweeping effect of the enhanced magnetic field associated with the fast shock also probably contributes to the rapid depression of the cosmic ray intensity seen in some decreases.

Differences in the maximum intensities and the intensity‐time profiles of cosmic rays in alternate solar magnetic field polarities

We have found that there is a significant difference in the intensity-time profiles of the galactic cosmic rays with energies from 100 MeV to 50 GeV around the solar activity minima in the five alternate solar magnetic field polarities from 1954 to 2000. This difference in the appearance of the two halves of the 22-year solar magnetic cycle, peaked and flat-topped, supports the role of drifts near the cosmic ray intensity maxima. The intensity of cosmic rays at the solar activity minima also depends on the solar magnetic field polarity, and it is less at neutron monitor energies when the solar magnetic field polarity is positive than when it is negative. At energies of ∼500 MeV and lower the opposite is the case: The values of the intensity maxima are less when the solar magnetic field polarity is negative. This intensity difference arises from the role of drifts in the cosmic ray propagation, together with possibly a change in the value of the diffusion coefficient in the two polarity cycles, that produces changes in the shape of the proton and helium energy spectra. The rigidity dependence of the ratios of the maximum intensities of both protons and helium nuclei below 1 and 2 GV, respectively, in alternate solar magnetic cycles (e.g., 1987/1977) is given by P(0.45±0.05), but the ratios are shifted by a factor of 2 in rigidity. This shift does not appear to be predicted by current modulation theories that include cosmic ray drift effects.

Comptel measurements of solar flare neutrons

Abstract The ability to measure solar flare neutrons by the COMPTEL γ-ray telescope on-board the Compton Gamma Ray Observatory has been demonstrated during the observations of the powerful solar flares of June 1991 from Active Region 6659. We present intensity-time profiles and count rate spectra from the impulsive X10 flare on 1991 June 19 and the prolonged X12+ flare on 1991 June 15. The emission of neutrons during the 9 June flare is coincident in time with the impulsive γ-ray emission, while the neutron emission during the 15 June flare takes place from the time of the X-ray maximum and for at least 30 minutes thereafter. The detected neutrons can also be used to image the Sun in heavy particles, the first time a celestial object has been imaged in radiation other than electromagnetic radiation.

Observations of gamma radiation between 0. 4 MeV and 7 MeV at balloon altitudes using a Compton telescope

Balloon-borne measurements of the atmospheric and diffuse gamma-ray flux in the energy range 0.4-7.0 MeV with a Compton telescope, which included pulse-shape discrimination of the first scattering detector and a time-of-flight system between the first and second detector elements, are reported. Comparison of the diffuse cosmic gamma-ray flux to the atmospheric gamma rays indicates that 0.2-5.0 MeV is the optimum energy range for measurements made at the top of the earths atmosphere. The measured total atmospheric gamma-ray flux between zero and 40 deg has an energy spectrum that agrees with the calculations of Ling (1975). Observations indicate that the ratio of the diffuse to atmospheric gamma ray fluxes at 3.5 g/sq cm is a maximum, about 1.0, between 0.7 and 3.0 MeV.

COMPTEL as a Solar Gamma Ray and Neutron Detector

The imaging Compton telescope COMPTEL on the Gamma Ray Observatory has unusual spectroscopic capabilities for measuring solar γ-ray and neutron emissions. Flares can be observed above the 800 keV γ-ray threshold of the telescope. The telescope energy range extends to 30 MeV with high time resolution burst spectra available from 0.1 to 10 MeV. Strong Compton tail suppression facilitates improved spectral analysis of solar flare γ-ray emissions. In addition, the high signal-to-noise ratio for neutron detection and measurement provides new neutron spectroscopic capabilities. For example, a flare similar to that of 1982 June 3 will yield spectroscopic data on > 1500 individual neutrons, enough to construct an unambiguous spectrum in the energy range of 20 to 150 MeV. Details of the instrument response to solar γ-rays and neutrons are presented.

COMPTEL measurements of the gamma-ray burst GRB 930131

On 1993 January 31 at 1857:12 Universal Time (UT), the Imaging Compton Telescope COMPTEL onboard the Compton Gamma Ray Observatory (CGRO) detected the cosmic gamma-ray burst GRB 930131. COMPTELs MeV imaging capability was employed to locate the source to better than 2 deg (1 sigma error radius) within 7 hr of the event, initiating a world-wide search for an optical and radio counterpart. The maximum likelihood position of the burst from the COMPTEL data is alpha(sub 2000) = 12h 18m, delta(sub 2000) = -9 deg 42 min, consistent with independent CGRO-Burst and Transient Source Experiment (CGRO-BATSE) and Energetic Gamma Ray Experiment Telescope (EGRET) locations as well as with the triangulation annulus constructed using BATSE and Ulysses timing data. The combined COMPTEL and EGRET burst data yield a better estimate of the burst location: alpha(sub 2000) = 12h 18m and delta(sub 2000) = -10 deg 21 min, with a 1 sigma error radius of 32 min. In COMPTELs energy range, this burst was short, consisting of two separate spikes occurring within a approximately 1 s interval with a low intensity tail for approximately 1 s after the second spike. No statistically significant flux is present for a 30 s period after the main part of the burst. This is consistent with the EGRET data. The COMPTEL telescope events indicate a hard, power-law emission extending to beyond 10 MeV with a spectral index of -1.8 +/- 0.4. The rapid fluctuations and high intensities of the gamma-ray flux greater than 10 MeV place the burst object no farther than 250 pc if the burst emission is not beamed.

Neutron and gamma‐ray measurements of the solar flare of 1991 June 9

The COMPTEL Imaging Compton Telescope on‐board the Compton Gamma Ray Observatory measured significant neutron and γ‐ray fluxes from the solar flare of 9 June 1991. The γ‐ray flux had an integrated intensity (≳1 MeV) of ∼30 cm−2, extending in time from 0136 UT to 0143 UT, while the time of energetic neutron emission extended approximately 10 minutes longer, indicating either extended proton acceleration to high energies or trapping and precipitation of energetic protons. The production of neutrons without accompanying γ‐rays in the proper proportion indicates a significant hardening of the precipitating proton spectrum through either the trapping or extended acceleration process.

Gamma-ray burst detection capabilities of comptel

Abstract The imaging gamma-ray telescope COMPTEL, capable of detecting gamma rays in the 1 to 30 MeV range, is one of four experiments onboard NASAs Gamma-Ray Observatory GRO. Besides its primary objectives COMPTEL will contribute to the understanding of cosmic gamma-ray bursts. Summarising, COMPTEL localises bursts (S (E > 1 MeV) ≥ 2.10 −6 erg/cm 2 ) within 1 sr FOV to better than 1° at medium gamma-ray energies, measures continuum energy spectra in the range 0.1 MeV to 20 MeV with fluence S ≥ 6.9 10 −7 erg/cm 2 (5 σ , E≥100 keV), measures gamma-ray lines with detector resolution 9.6% (at 0.5 MeV) and 7.0% (at 1.5 MeV) and determines time histories of both gamma-ray line and continuum emission with t ≥ 0.1 sec resolution.