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1991 December 13 (M2.9)

A GOES M2.9 solar flare occurred on 1991 December 13 at an H-alpha location of N18W55 (heliographic coordinates). The soft X-ray flux began at 03:48 (13680 s), peaked at 03:57 (14220 s) and ended at 05:17 (19020 s) UT (GOES Geophysical Data Web Site). OSSE received a BATSE transient trigger at 13901 s but did not respond because the four OSSE detectors were involved in a complicated chopping procedure to observe the Galactic center. Fortunately, there was always at least one detector pointing at the scan-angle position of the Sun for the duration of the flare, and significant data were recorded.

Significant emmission of up to about 300 keV was observed during a ~165 s time interval (13980-14145 s). Count rate time profiles in various energy windows at a temporal resolution of ~16 s are shown in Figure 1. A representative count spectrum is shown in Figure 2. A broken power law which hardened from an index of about -4.8 below a break energy of ~100 keV to about -3.5 above the break energy fit the data better than a single power law. The total fluence above 50 keV was 441.8 +/- 6.8 photons cm-2.

Figure 1

Figure 2

Last revised: 23 Jul 1999