{
  "bibcode": "2005GCN..2936....1B",
  "body": "S. Boggs (UCB), K. Hurley (UCB), D.M. Smith (UCSC), R.P. Lin (UCB),\nG. Hurford (UCB), W. Hajdas (PSI), C. Wigger (PSI)\n\nRHESSI observed both the precursor and the giant flare from SGR 1806-20\nin their entirety, staring at 21:28:03.44 UT and 21:30:26.65 UT \n2004-12-27 respectively. The SGR was 5 degrees from RHESSI's pointing \naxis which was directed toward the Sun. This placed the SGR outside the \nnormal imaging FOV of the instrument. During the main peak of the flare \nthe RHESSI spectroscopy detectors were saturated for ~0.5s after the \nrise, but observed the decay of the main peak and the 400-s long \noscillatory component.\n\nWhile the main RHESSI spectroscopy detectors (9 segmented germanium\ndetectors operating from 3 keV to 15 MeV) were saturated during the\npeak, the RHESSI particle detector (used for detecting SAA passages)\nwas able to measure the incident flux with 0.125-s time resolution\nin two energy channels determined by thresholds in the electronics:\n>65 keV, and >650 keV. These data indicate significant emission above\n650 keV for ~0.25 s, during the giant flare, and softening of the giant\npeak during its evolution. In addition, even though the RHESSI \nspectroscopy detectors are saturated during the giant peak, we can use\nthe reset rates of the preamplifiers to constrain the rise and fall \ntimes of the giant peak to <1 ms and ~65 ms respectively.\n\nThe particle detector data and the spectroscopy detector reset rates\nallow us to set conservative lower limits to the total fluence of the\nprimary giant peak: >0.1 erg/cm^2 and >0.3 erg/cm^2 respectively. This\nfluence is >1-2 orders of magnitude higher than the 1998 flare of\nSGR 1900+14, which had a fluence of 7e-3 erg/cm^2 (Hurley et al.,\nNature 397, 41, 1999). Assuming a distance of ~15 kpc for SGR 1806-20\n(Corbel & Eikenberry, A&A 419, 191,2004) and isotropic emission, we\nderive a lower limit on the total hard X-ray/gamma-ray energy released\nin the giant peak to be >8e45 erg. We note that given BATSE's \nsensitivity of <1e-8 erg/cm^2, this type of giant flare would have been \ndetectable by BATSE (as a short, hard GRB) out to >80 Mpc.\n\nWhen they came out of saturation during the giant peak, the RHESSI\nspectroscopy detectors were measuring a peak count rate of\n~280,000 cnt/s. After the giant peak, RHESSI recorded a series of 51 \npulsations with a period of 7.56 s, similar to the INTEGRAL, KONUS, and \nSwift-BAT observations (Borkowski et al., GCN 2920; Mazets et al., GCN \n2922; Palmer et al., GCN #2925). The pulse profile shows evidence for \nboth spectral variations throughout the pulse, and evolution of the \npulse shapes throughout the decay. The average 20-100 keV pulse profile \nshows 3-4 peaks in its structure. During this decay phase, the average \n20-100 keV flux is well modeled by the trapped fireball model of \nThompson & Duncan (ApJ 561, 980, 2001), with an evaporation time \nt_evap=382 s, and index a=0.606, where flux ~ (1-t/t_evap)^(a/1-a). \nPreliminary lightcurves show indications of significant spectral \nsoftening during the 400-s oscillatory decay.\n\nRHESSI observed the precursor during 21:28:03.44-21:28:04.49 UT, with a \npeak count rate in the spectroscopy detectors of ~30,000 cnt/s, and \n~25,000 counts total. The profile is square, as reported by Swift-BAT \n(Palmer et al., GCN #2925), with emission extending up to 150 keV. We \nsee a rise time for the precursor <50 ms, and a fall time <100 ms.\n\nResults of this analysis will be posted as they come available at:\nhttp://www.ssl.berkeley.edu/ipn3/041227",
  "circularId": 2936,
  "createdOn": 1104951251000,
  "email": "boggs@ssl.berkeley.edu",
  "subject": "SGR 1806-20, RHESSI observations of the 041227 giant flare",
  "submitter": "Steven E. Boggs at UCB/SSL  <boggs@ssl.berkeley.edu>"
}