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1999 August 28 (X1.1)

A GOES X1.1 solar flare occurred on 1999 August 28 at an H-alpha location of S26W14 (heliographic coordinates). The soft X-ray flux began at 17:52 (64320 s), peaked at 18:05 (65100 s) and ended at 18:18 (65880 s) UT (GOES Geophysical Data Web Site). OSSE received a BATSE transient trigger at 64534 s and the four OSSE detectors slewed to the scan-angle position of the Sun. The front two detectors then initiated +/- 4.5 degree on-source-off-source viewing strategies while the rear detectors remained pointed at the Sun. The Sun was located ~10.5 degrees off the OSSE scan plane. The scan angle position of the detectors was such that the rear detectors were significantly shadowed by those in front so we use here only data from the two front detectors.

Significant emission up to a few hundred keV was observed during an ~270 s time interval (64571-64840 s). Count rate time profiles in various energy windows at a temporal resolution of ~8 s are shown in Figure 1. A representative count spectrum is shown in Figure 2. A power law fit the data reasonably well. There was some evidence for a hardening of the power law above ~500 keV but no clear evidence for nuclear lines. The total fluence above 50 keV was 4350 +/- 50 photons cm-2.

Figure 1

Figure 2

Last revised: 11 Dec 1997