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1998 August 18 (X4.9)

A GOES X4.9 solar flare occurred on 1998 August 18 at an H-alpha location of N33E87 (heliographic coordinates). The soft X-ray flux began at 22:13 (79980 s), peaked at 22:16 (80160 s), and ended at 23:50 (85800 s) UT (GOES Geophysical Data Web Site). The peak of the flare occurred during satellite night. CGRO emerged into daylight at 22:36 (81360) UT (approximately 1200 seconds after the peak of the flare) and received a BATSE solar trigger. The four detectors slewed to the Sun and began a standard 2-min on-off chopping strategy. Observations continued until satellite night at 23:36 (84960 s) UT. A TDRSS communication satellite transition resulted in missing data near the middle of the orbit (82826 - 83450 s UT).

OSSE observed gamma-ray emission continuing at this late phase of the flare. Count rate time profiles in various energy windows at a temporal resolution of ~16 s are shown in Figure 1. Significant emission up to about 1 MeV was observed during the entire orbit. A representative count spectrum accumulated from 81450 to 84973 s UT (1507 s of accumulation due to data gap) is shown in Figure 2. The total fluence above 50 and 300 keV was 1830 +/- 21 and 70 +/- 4 photons cm-2, respectively. (We note that these are only partial fluences due to occultation and missing data.) The best fitting power law spectral index for the electron bremmsstrahlung was 2.82 +/- 0.01. There was no evidence for nuclear line emission.

Figure 1

Figure 2

Last revised: 2 Sep 1998