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1991 June 30 (M5.0)

A GOES M5.0 solar flare occurred on 1991 June 30. The soft X-ray flux began at 2:35 (9328 s), peaked at 3:01 (10868 s), and ended at 3:20 (12045 s) UT. Although originally believed to have been located at S06E19 (heliographic coordinates), the event is now believed to have been associated with AR6703 which, at the time of the flare, was located beyond the east limb of the Sun. The event was detected at >25 keV by the Gamma Ray Burst experiment (Kane et al. 1995) on Ulysses which was located at ~135 degrees east of the Sun-Earth line. Kane et al. classified the event as one of the "giant" flares observed by their instrument. Even though the flare was located beyond the limb, hard X rays and gamma rays up to 100 MeV were also detected by the PHEBUS instrument on Granat in Earth orbit (Vilmer et al. 1999). They found that the spectrum could be reasonably fit with a broken power law from 0.1 to almost 100 MeV and showed no evidence for nuclear line emission. They suggested that this flare is therefore similar to the "electron-dominated" events identified by Rieger and Marschhauser (1990) in that the gamma-ray spectrum was very hard, extended to high energies, and was without detectable nuclear line emission.

OSSE received a BATSE transient trigger at 10537 s UT while OSSE was observing Centaurus A, but, because the source position calculated on-board by BATSE was outside the acceptance window for a solar source, OSSE did not respond except to alter some data collection parameters. The position of the Sun was ~55 degrees off the collimator axis in both the scan and trans-scan directions resulting in strong attenuation of <1 MeV solar photons. However, the flare was intense enough to allow a significant fraction of the >1 MeV gamma-ray emission to penetrate the shields and be detected by the central detectors. Because of the strong low-energy attenuation, the preliminary results presented here are based on an analysis of >0.55 MeV data only.

The gamma-ray emission observed by OSSE began at about 10558 s UT and peaked at about 10586 s UT. The decay was interrupted at 10607 s UT by onset of the CGRO day-night transition which lasted until about 10653 s UT. Count rate time profiles in various energy windows at a temporal resolution of ~8 s are shown in Figure 1. Emission to energies greater than 16 MeV was observed. A 0.55-10 MeV count spectrum for one detector obtained from data accumulated up to the onset of satellite night (10566-10623 s UT) is shown in Figure 2. A fit to the >0.55 MeV data using a model based on the fit to the 1981 April 27 flare data obtained with SMM (Murphy et al. 1990) gave a >1 MeV narrow-line fluence of only 3.5 +- 1.0 photons cm-2. This lack of significant nuclear line emission is consistent with the observation of Vilmer et al. (1999). A fit to the >0.55 MeV data using a single power law gave a power-law index of 2.01 +- 0.03, consistent with the index found by Vilmer et al (1999) for this flare at these energies.

References: Kane, S. R., et al. 1995, ApJ, 446, L47

Murphy, R., Share, G. H., Letaw, J. R., & Forrest, D. J. 1990, Apj, 358, 298

Rieger, E., & Marshhauser, H. 1990, in MAX91/SMM Solar Flares:MAX91 Workshop 3, R. M. Winglee & A. L. Kiplinger (eds.) , p. 68

Vilmer, N. et al. 1999, to be submitted to ApJ

Figure 1

Figure 2

Last revised: 11 Dec 1997