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2000 February 05 (X1.2)

A GOES X1.2 solar flare occurred on 2000 February 5 at an H-alpha location of N26E52 (heliographic coordinates). The soft X-ray flux began at 19:17 (69420 s), peaked at 19:28 (70080 s), and ended at 19:31 (70260 s) UT. OSSE received a BATSE transient trigger at 69973 s UT while OSSE was observing the Galactic plane, but because of recent modifications to the BATSE on-board triggering criteria the event was not identified as solar. The position of the Sun was ~40 degrees off the collimator axis resulting in strong attenuation of <1 MeV solar photons. The flare did not produce particularly strong gamma-ray emission and only a weak signal was detected by the central detectors through the shields. The flare was also observed by COMPTEL.

The detected gamma-ray emission began at about 70036 s UT and lasted until about 70101 s UT. Emission to >1 MeV was observed. Count rate time profiles in various energy windows at a temporal resolution of ~16 s are shown in Figure 1. A count spectrum obtained from summing data from all four detectors over the interval 70036-70101 s UT is shown in Figure 2.

We did not attempt a detailed fit to the data because the detection was so weak. The low-energy counts seen in the spectrum of Figure 2 are due mostly to degraded photons from higher-energy gamma rays interacting in the shields and collimator rather than flare emission at those energies. A fit for the 2.223 MeV neutron-capture line showed no significant emission with a 2-sigma upper limit of 2.2 photons cm-2. The estimated total >1 MeV emission was 11 +/- 5 photons cm-2.

The 2.223 MeV line-to->1 MeV fluence ratio varies considerably from flare to flare depending on the flare's accelerated proton-to-electron ratio and location on the Sun (due to limb attenuation of the 2.223 MeV line). For the large flare of 1991 June 4 (located at N30E70) which had a strong nuclear component, this ratio was about 0.3. Based on the measured >1 MeV fluence of the February 5 flare, we would expect a 2.223 MeV line fluence of 3.3 +/- 1.2 photons cm-2 if the flare were similar to the June 4 flare. A measured fluence significantly less than this would imply little or no nuclear emission. The measured upper limit to the line fluence cannot rule out nuclear emission from the February 5 flare.

Figure 1

Figure 2

Last revised: 11 Dec 1997