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1991 November 15 (X1.5)

A GOES X1.5 solar flare occurred on 1991 November 15 at an H-alpha location of S13W19 (heliographic coordinates). The soft X-ray flux began at 22:33 (81180 s), peaked at 22:39 (81540 s), and ended at 22:54 (82440 s) UT (GOES Geophysical Data Web Site). OSSE received a BATSE transient trigger at 81278 s UT while observing Eta Carinae. Because the Sun was so far off the scan plane, OSSE was not programmed to respond. The start of gamma-ray emission occurred after OSSE slewed to its secondary source (the Galactic Center) at 81374 s UT. All four detectors were performing +/- 12 degree on/off-source pointing of the Galactic Center. The position of the Sun was off the collimator axis by ~10 degrees in the scan direction and ~30 degrees in the trans-scan direction resulting in strong attenuation of <1 MeV solar photons.

OSSE observed strong gamma-ray emission up to ~10 MeV, lasting about 65 s from (81422-81488 s UT). Weak emission continued until ~81668 s UT. Count rate time profiles in various energy windows at a temporal resolution of ~16 s are shown in Figure 1. A count spectrum for one detector obtained from data summed over the entire emission interval is shown in Figure 2. A 2.223 MeV neutron-capture line and evidence for narrow nuclear lines was seen. The spectrum was fit with a power law, a neutron-capture line, and models representing the narrow and broad nuclear gamma-ray lines. The best-fitting power law spectral index was 2.98 +- 0.71 and the 1 MeV fluence was 16.7 +/- 7.2 photons cm-2. The fluence in the 2.223 MeV neutron-capture line was 7.7 +/- 1.9 photons cm-2 and the fluence in >1 MeV narrow nuclear lines was 7.1 +/- 1.9 photons cm-2.

Figure 1

Figure 2

Last revised: 2 Sep 1998