Browsing by Author "Moss, Michael Jamieson"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Gamma ray observations of black hole candidates nova Ophiuchus 1993 and nova Velorum 1993
    (1997) Moss, Michael Jamieson; Haymes, Robert C.
    Results of spectral analysis and time series analysis of the transient source x-ray nova Velorum 1993 (GRS 1009-45) and x-ray nova Ophicuhus 1993 (GRS 1716-249) are presented. These data were accumulated using the OSSE (Oriented Scintillation Spectrometer Experiment) low-energy gamma ray telescope on the Compton Gamma Ray Observatory. Spectral observations of the two x-ray novae showed the presence of gamma ray emission to greater than 100 keV, which is thought to be a signature of a binary system with an accreting black hole. Observations of GRS 1009-45 shows that it was in a gamma-ray soft state during the one-day OSSE observation, which occurred during the exponential decay phase of the outburst. The spectrum of GRS 2716-249 acquired during the plateau stage of outburst indicated a hard state, while spectra acquired during the sawtooth outbursts suggest that a hard to soft transition occurs as the decay progresses. The 40-200 keV pre-outburst luminosity of GRS 1716-249 is shown to be two orders of magnitude less than that estimated during the plateau-stage observation. The only significant power density spectrum was that acquired for GRS 1716-249 during the plateau stage of outburst. It is consistent with the sum of two QPO peaks at 0.128 and 0.32 Hz and a red noise ($\nu\sp{-1}$) component. This power spectrum is consistent with other black hole candidate power spectra for which the source is in the low state. Power spectra for GRS 1009-45 and GRS 1716-249 during the peak of a sawtooth outburst are also presented. They are consistent with band-limited white noise. Corresponding autocorrelations are shown for all observations.
  • Thumbnail Image
    Item
    Implementation of techniques for background reduction in low-energy gamma ray telescopes
    (1995) Moss, Michael Jamieson
    Prometheus I, a low-energy ($\sim$0.06-12.0 MeV), balloon-borne gamma ray telescope has been developed and successfully flown. It consists of a central NaI(Tl) detector that is segmented into an array of 9 x 9 crystals and an active anticoincidence shield of thick plastic scintillator. Implemented on Prometheus are several background reduction techniques which allow it to be 10 times more sensitive to aperture gamma rays than previous generations of low-energy gamma ray telescopes. First, a $\beta$ ray rejection method is used. Next, a low-Z shield minimizes cosmic ray activation. Also layers of Li$\sp6$ absorb slow neutrons, thereby reducing background caused from inelastic neutron scattering and neutron capture. Finally the use of lightweight, low-Z construction materials minimizes locally produced background. Semi-analytical calculations and preliminary in-flight count rate data confirm the usefulness of these techniques.