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Gehrels, N.

Publications and source records attributed to Gehrels, N..

At least 181 records · Page 10
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NASA NTRS1990

Observations of gamma-ray line profiles from SN 1987A

Fully resolved gamma-ray line observations from the decay of Co-56 in SN 1987A are presented. Data are from the first two balloon flights of the Gamma-Ray Imaging Spectrometer. On day 433 (after the initial optical sighting) the 847 and 1238 keV lines from Co-56 were observed at 2.3 and 4.3 sigma significance. On day 613, the lines at 847, 1238, and 2599 keV were observed at 4.6, 3.4, and 1.9 sigma, respectively. The combined significance for the three-line complex in both flights is 7.8 sigma. Gaussian profiles yield acceptable least-squares fits to the lines. The line profiles are centered on the red side of the rest energy with typical velocity dispersions of about 3500 km/s FWHM, consistent with an optically thin source, but the line intensities are less than about 30 percent of those produced by the 0.075 solar mass of Co-56 determined from the bolometric light curve.

Tueller, J.↗
NASA NTRS1990

GRIS observations of the Galactic Center

The 511 keV positron annihilation line source in the Galactic Center (GC) region has reappeared after being in a quiescent state since the early 1980's. Observations are reported made by the GRIS balloon instrument showing that the 511 keV line has returned to an intensity level similar to that seen in the 1970's. The line width was resolved for the first time and a measurement of the spatial extent of the emission along the galactic plane was made. Gamma ray imaging spectrometer (GRIS) is a high resolution germanium (Ge) spectrometer with a 17 deg field-of-view. Eleven hours of data were obtained from GC pointings on balloon flights over Australia on 1 May 1988 and again on 29 October 1988. An additional seven hours were obtained on 30 October 1988 from a point in the galactic plane (GP) 25 deg west of the center. Preliminary results for the line fluxes from the GC are 9.8 + or - 1.9 in May and 12.3 + or - 1.6 in October, and from the GP are 2.4 + or - 1.6 (1 sigma statistical errors). The flux for the off-center pointing is significantly lower than that for the GC pointings and indicates that the dominant emission is narrowly concentrated at the center. The line width for the GC pointing in October is 3.6 + or - 0.5 keV, which implies a temperature for the annihilation medium of less than or + 10(exp 5)K. A step in the continuum emission at 511 keV is found in both the GC and GP data. The step may be due to orthopositronium three-photon annihilation for the GC, but is too large relative to the 511 keV line for the GP to be simply explained by positronium.

Gehrels, N.↗
NASA NTRS1989

Resolution of the 1,238-keV gamma-ray line from supernova 1987A

Observations of supernova 1987A from the maiden flight of the Gamma-Ray Imaging Spectrometer (GRIS) are reported. SN1987A was observed for a period of 11.1 hours on May 1, 1988. Line emission at 1238 keV and continuum emission from 60-800 keV were detected. A gaussian line profile gives an acceptable fit to the 1238 keV line. The best-fit parameters are: flux = 8.5(+ 2.3, - 2.2) x 10 to the -4th photons/sq cm/s; peak energy = 1235.4 (+ 2.2, - 2.4) keV; FWHM = 16.3 (+ 6.1, - 5.7) keV. No evidence is found for a supernova-produced red- or blueshift in the 1238 keV line. The measured linewidth is a factor of about two greater than model predictions, although the discrepancy represents only two standard deviations. The line profiles are characteristic of optically thin regions, whereas the intensity implies a mean optical depth of about two. Fragmentation or nonspherical geometry of the supernova shell are possible explanations of the data.

Teegarden, B. J.↗
NASA NTRS1989

Reappearance of the annihilation line source at the Galactic Centre

The results of a new balloon-borne measurements of gamma-ray emission from the Galactic Center using a germanium telescope are reported. The 511-keV line was detected at a flux level of (9.8 + or - 1.9) x 10 to the -4th photons/sq cm/s on May 1, 1988 and (12.3 + or - 1.6) x 10 to the -4th photons/sq cm/s on October 29, 1988. Apparently the compact object reemerged sometime between 1984 and 1988. Flux was also measured from a point in the Galactic Plane 25 deg west of the Galactic Center, and was found to be (2.4 + or - 1.7) x 10 to the -4th photons/sq cm/s.

Leventhal, M.↗
NASA NTRS1989

The Nuclear Astrophysics Explorer

The Nuclear Astrophysics Explorer was proposed in 1986 for NASA's Explorer Concept Study Program by an international collaboration of 25 scientists from nine institutions. The one-year feasibility study began in June 1988. The Nuclear Astrophysics Explorer would obtain high resolution observations of gamma-ray lines, E/Delta E about 1000, at a sensitivity of about 0.000003 ph/sq cm s, in order to study fundamental problems in astrophysics such as nucleosynthesis, supernovae, neutron star and black-hole physics, and particle acceleration and interactions. The instrument would operate from 15 keV to 10 Mev and use a heavily shielded array of nine cooled Ge spectrometers in a very low background configuration. Its 10 deg FWHM field of view would contain a versatile coded mask system which would provide two-dimensional imaging with 4 deg resolution, one-dimensional imaging with 2 deg resolution, and efficiendt measurements of diffuse emission. An unshielded Ge spectrometer would obtain wide-field measurements of transient gamma-ray sources. The earliest possible mission would begin in 1995.

Matteson, J. L.↗
NASA NTRS1988

Hard X-ray and low-energy gamma-ray spectrometers

Basic principles of operation and characteristics of scintillation and semi-conductor detectors used for solar hard X-ray and gamma-ray spectrometers are presented. Scintillation materials such as NaI offer high stopping power for incident gamma rays, modest energy resolution, and relatively simple operation. They are, to date, the most often used detector in solar gamma-ray spectroscopy. The scintillator BGO has higher stopping power than NaI, but poorer energy resolution. The primary advantage of semi-conductor materials such as Ge is their high-energy resolution. Monte-Carlo simulations of the response of NaI and Ge detectors to model solar flare inputs show the benefit of high resoluton for studying spectral lines. No semi-conductor material besides Ge is currently available with adequate combined size and purity to make general-use hard X-ray and gamma-ray detectors for solar studies.

Gehrels, N.↗
NASA NTRS1986

Confidence limits for small numbers of events in astrophysical data

The calculation of limits for small numbers of astronomical counts is based on standard equations derived from Poisson and binomial statistics; although the equations are straightforward, their direct use is cumbersome and involves both table-interpolations and several mathematical operations. Convenient tables and approximate formulae are here presented for confidence limits which are based on such Poisson and binomial statistics. The limits in the tables are given for all confidence levels commonly used in astrophysics.

Gehrels, N.↗
NASA NTRS1985

Optical monitoring of gamma-ray source fields

The three gamma-ray burst source fields GBS1028+46, GBS1205+24, and GBS2252-03 have been monitored for transient optical emission for a combined total of 52 hours. No optical events were seen. The limiting magnitude for the search was M sub V = 15.8 longer and M sub V = 17.0 for 6.0 s or longer.

Gehrels, N.↗
NASA NTRS1985

The Gamma-Ray Imaging Spectrometer (GRIS): A new balloon-borne experiment for gamma-ray line astronomy

High resolution gamma-ray spectroscopy is a relatively new field that holds great promise for further understanding of high energy astrophysical processes. When the high resolution gamma-ray spectrometer (GRSE) was removed from the GRO payload, a balloon program was initiated to permit continued development and improvement of instrumentation in this field, as well as continued scientific observations. The Gamma-Ray Imaging Spectrometer (GRIS) is one of the experiments selected as part of this program. The instrument contains a number of new and innovative features that are expected to produce a significant improvement in source location accuracy and sensitivity over previous balloon and satellite experiments.

Teegarden, B. J.↗
NASA NTRS1985

Instrumental background in balloon-borne gamma-ray spectrometers and techniques for its reduction

Instrumental background in balloon-borne gamma-ray spectrometers is presented. The calculations are based on newly available interaction cross sections and new analytic techniques, and are the most detailed and accurate published to date. Results compare well with measurements made in the 20 keV to 10 MeV energy range by the Goddard Low Energy Gamma-ray Spectrometer (LEGS). The principal components of the continuum background in spectrometers with GE detectors and thick active shields are: (1) elastic neutron scattering of atmospheric neutrons on the Ge nuclei; (2) aperture flux of atmospheric and cosmic gamma rays; (3) beta decays of unstable nuclides produced by nuclear interactions of atmospheric protons and neutrons with Ge nuclei; and (4) shield leakage of atmospheric gamma rays. The improved understanding of these components leads to several recommended techniques for reducing the background.

Gehrels, N.↗
NASA NTRS1984

The gamma-ray spectrum of Centaurus A - A high-resolution observation between 70 keV and 8 MeV

The NASA/Goddard Space Flight Center Low Energy Energy Gamma ray Spectrometer (LEGS) observed the nearby active nucleus galaxy Centaurus A (NGC 5128) during a balloon flight on 1981 November 19. There is no evidence of a break in the spectrum or of any line features. The 1.6 MeV limit is a factor of 8 lower than the 1974 line flux, indicating that, if the 1974 feature was real, and, if it was narrow, then the line intensity decreased significantly between 1974 and 1981. The lack of observed annihilation radiation from Cen A, combined with the temporal variations that are seen in the X-ray and gamma-ray intensities, constrain the size of the emission region to be between 10 to the 13th power and 5 x 10 to the 17th power cm. Previously announced in STAR as N83-35990

Gehrels, N.↗
NASA NTRS1984

The development of a segmented N-type germanium detector, and its application to astronomical gamma-ray spectroscopy

Extensive calculations and simulations have shown that the instrumental background in a coaxial germanium photon detector flown at balloon altitudes or in space, can be substantially reduced by segmenting the outer contact. The contact is divided into horizontal strips around the side of the detector, giving it many characteristics similar to that of a stack of planar detectors. By choosing different segment coincidence requirements in different energy ranges, one can obtain a factor of approx. 2 increase in sensitivity to spectral lines between 40 keV and 1 MeV, compared with an unsegmented detector. The reverse electrode configuration (using n-type germanium), with the p contact outside, is preferred for this application due to its thin dead layer and resistance to radiation damage in space. A small two segment n type detector is being developed to serve as a prototype for larger multisegment devices. Results of this development effort and of detector tests are presented. Previously announced in STAR as N84-13039

Gehrels, N.↗
NASA NTRS1983

The gamma-ray spectrum of Centaurus A: A high-resolution observation between 70 keV and 8 MeV

The NASA/Goddard Space Flight Center Low Energy Gamma ray Spectrometer (LEGS) observed the nearby active nucleus galaxy Centaurus A (NGC 5128) during a balloon flight on 1981 November 19. There is no evidence of a break in the spectrum or of any line features. The 1.6 MeV limit is a factor of 8 lower than the 1974 line flux, indicating that, if the 1974 feature was real, and, if it was narrow, then the line intensity decreased significantly between 1974 and 1981. The lack of observed annihilation radiation from Cen A, combined with the temporal variations that are seen in the X-ray and gamma-ray intensities, constrain the size of the emission region to be between 10 to the 13th power and 5 x 10 to the 17th power cm.

Gehrels, N.↗
NASA NTRS1983

A balloon-borne instrument for high-resolution astrophysical spectroscopy in the 20-8000 keV energy range

The Low Energy Gamma ray Spectrometer (LEGS) is designed to perform fine energy resolution measurements of astrophysical sources. The instrument is configured for a particular balloon flight with either of two sets of high purity germanium detectors. In one configuration, the instrument uses an array of three coaxial detectors (effective volume equal to or approximately 230 cubic cm) inside an NaI (T1) shield and collimator (field of view equal to or approximately 16 deg FWHM) and operates in the 80 to 8000 keV energy range. In the other configuration, three planar detectors (effective area equal to or approximately square cm) surrounded by a combination of passive Fe and active NaI for shielding and collimation (field of view equal to or approximately 5 deg x 10 deg FWHM) are optimized for the 20 to 200 keV energy range. In a typical one day balloon flight, LEGS sensitivity limit (3 sigma) for narrow line features is less than or approximately .0008 ph/cm/s square (coaxial array: 80 to 2000 keV) and less than or approximately .0003 ph/square cm/s (planar array: 50 to 150 keV).

Paciesas, W. S.↗
NASA NTRS1983

Upper Limits to the Annihilation Radiation Luminosity of Centaurus a

A high resolution observation of the active nucleus galaxy Centaurus A (NGC 5128) was made by the GSFC low energy gamma-ray spectrometer (LEGS) during a balloon flight on 1981 November 19. The measured spectrum between 70 and 500 keV is well represented by a power law of the form 1.05 x 10 (-4) (E/100 keV) (-1.59) ph/sq cm /s with no breaks or line features observed. The 98% confidence (2 sigma) flux upper limit for a narrow ( 3 keV) 511-keV positron annihilation line is 9.9 x 10 (-4) ph/ sq cm /s. Using this upper limit, the ratio of the narrow-line annihilation radiation luminosity to the integral or = 511 keV luminosity is estimated to be 0.09 (2 sigma upper limit). This is compared with the measured value for our galactic center in the Fall of 1979 of 0.10 to 0.13, indicating a difference in he emission regions in the nuclei of the two galaxies.

Gehrels, N.↗
NASA NTRS1983

Development of a segmented n-type germanium detector, and its application to astronomical gamma-ray spectroscopy

Extensive calculations and simulations have shown that the instrumental background in a coaxial germanium photon detector flown at balloon altitudes or in space, can be substantially reduced by segmenting the outer contact. The contact is divided into horizontal strips around the side of the detector, giving it many characteristics similar to that of a stack of planar detectors. By choosing different segment coincidence requirements in different energy ranges, one can obtain a factor of approx. 2 increase in sensitivity to spectral lines between 40 keV and 1 MeV, compared with an unsegmented detector. The reverse electrode configuration (using n-type germanium), with the p contact outside, is preferred for this application due to its thin dead layer and resistance to radiation damage in space. A small two segment n type detector is being developed to serve as a prototype for larger multisegment devices. Results of this development effort and of detector tests are presented.

Gehrels, N.↗