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RXTE Detects Extremely Bright Burst from SGR 1900+14 RXTE

On Aug. 27th, 1998, RXTE detected a huge burst of background events while observing AM Her as part of a proposal by Dr. Andrew Beardmore of Keele University (Marshall et al., IAU Circular #7005). The burst, however, was due to the Soft Gamma-ray Repeater SGR 1900+14, which is more than 40 degrees away. The events detected with the RXTE PCA were caused by high-energy X-rays passing through the PCA collimators and other material before interacting in the detectors. Despite this intervening material, the PCA count rate was so high that, apparently for the first time during the mission, the PCA stopped recording "good" events because the dead time reached nearly 100%. Good events are those for which only a single anode chain is triggered.

Figure 1 shows the PCA light curve of all good events, uncorrected for the large dead time effects. The burst began at about 10h22m05.618s, and the good event rate rapidly increased for at least 12 ms to a rate of about 100,000 cps. As the burst continued to brighten, the background rate became so high that the good event rate plummeted. Two intervals are marked for which the dead time for good events appears to be nearly 100%. Amazingly, this huge response was caused by a source that was not in the PCA field-of-view.

The burst was seen with experiments on several satellites (Cline et al., IAU Circular #7002; Hurley et al., IAU Circular #7004; Feroci et al., IAU Circular #7005). Prelminary analysis of the relative arrival times of this burst at RXTE, Global Geo-Science Wind, Near Earth Asteroid Rendezvous, and Ulysses were used to determine its position. Figure 2 shows the three annuli involving RXTE. The position of SGR 1900+14 is marked with an "S" and lies within all three annuli. This confirms the identification of the burst with SGR 1900+14. Smaller annuli should be possible with further analysis.

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