Morphology of the Fe feature in the fast flare of 1-Feb-2003 00:39

This page describes the beginning of a rough study of the behavior of the Fe line complex in the RHESSI observations. The data let us determine the peak energy of the Fe response and its brightness relative to the neighboring continuum. The dynamic spectrum below (five-minute time range, 3-18 keV energy range) shows that the Fe feature is a strong one in this flare:

To do the spectral characterization I have taken the RHESSI front-segment counts at 1/3-keV binning in 2-sec integrations, as described in this code (these integrations omit G2 and G8; should have omitted others, see below). The data reduction is then in this code, which starts by breaking the low energies up into three bands:

  • F3: 2.83 - 5.50 keV
  • Fe: 5.50 - 7.50 keV
  • F8: 7.50 - 10.17 keV.

    The line flux in the Fe band is determined by making a 3rd-degree polynomial fit using the F3 and F8 ranges and subtracting that from the total count in the Fe band. We thus have two count-rate intervals that can be used to determine a continuum temperature and emission measure, plus an Fe flux. The residual spectrum (total minus continuum fit) shows the strong Fe feature, which we then fit with a Gaussian.

    The expectation is that the Fe feature will vary in intensity and location in a physically interesting manner. For example, the centroid energy should would be about (6.4, 6.7, 6.9) keV as the excitation of iron ranges over (cold, He-like, H-like), and the presence of non-thermal excitation would be reflected by a discrepancy between the line location and the F8/F3 temperature. Of course, this simplifies the situation a great deal, but it is a beginning.

    The plots below show the variation of broad-band fluxes (F3, feature, F8) and the feature central energy.

    How does the Fe feature look in the individual front segments? These plots show fits for each of G1,..., G9, and the Gaussian parameters resulting for the fits to the line feature were as follows:

    Detector (1-9)PeakEnergySigmaComment
    G1 7113 6.48 0.37
    G3 6537 6.47 0.34Good resolution, low amplitude!
    G4 10565 6.48 0.34Good resolution, high amplitude
    G5 6241 6.45 0.56Not so good
    G6 8308 6.48 0.41
    G7 882 6.60 0.98Known to be bad
    G8 6806 6.49 0.43
    G9 8575 6.48 0.42

    How to explain the G3 result of poor amplitude, but good resolution? The main contributor to the Gaussian amplitude would just be the spreading via detector noise. Note that we are looking at a flare and there will be an error due to modulation, so this another possible place to look. However it may not be likely, because this is an extremely fast event and therefore likely to be compact in scale.

    Note: Richard Schwartz has explained the discrepancy noted above by calculating the coarse modulation. As shown here this is severe for G3.

    H. Hudson, 19-Mar-03