Calculation of Burst Epeak from Fregate Data

The HETE team is now supplying spectral information for localized bursts within minutes of reception of the full burst data set using an automated processing script. This automatic spectral fit is derived from triggered data from the Fregate instrument for bursts which are localized by the WXM. The processing takes about 5 minutes and starts as soon as priority data reaches one of the three Primary HETE Ground stations. Typically this occurs within about one hour after the burst trigger but can occur within a range from 10 minutes to several hours depending on the location of HETE in orbit and the status of the three primary grond stations. The goal is to provide this information to the full community as soon as possible.

The processing uses the Fregate data summed over all four detectors. The first step is to use a wavelet based analysis of the 6-400 keV temporal light curve to estimate the duration of the burst and set foreground and background start and stop times. These times are used to generate a PHA histogram for input into XSPEC, assuming a standard gain for the detectors. The location of the burst within the field of view of Fregate is determined from the WXM data. XSPEC runs using the closest calibrated response file that is computed every 2 degrees off-axis from 0 to 65 degrees. The spectral fit of a "cutoffpl" (cutoff power law) is calculated. From the determination of E0 (cutoff energy) and alpha (power law number index) we estimate the value of Epeak and the fluences in pre-determined energy ranges.

The overall quality of any given fit should be checked by visual inspection of the XSPEC nu F_n nu plot of both the raw data and the overlayed best model. First check that the model matches the data. This type of plot was selected so that the peak of the data or the model marks the value of Epeak. If the model does not fit well then it is possible to visually estimate the Epeak as the maximum of the plotted data (if Epeak is within the energy range of FREGATE).

The attached plot shows a comparison of the determination of Epeak from 21 older HETE GRBs that have published values of Epeak (Barraud et al 2003). The abcissa is the value of Epeak computed by the new automated script. The ordinate shows the value of Epeak as determined previously by manual analysis. In general, there is an excellent correspondance between the values of Epeak computed by both methods. The differences show that Epeak is determined to approximately plus or minus 20 per cent. Epeak is not well determined by either method for values less that 40 keV or greater than 300 keV. However, results outside the spectral range of Fregate (40-300 keV) are still usefully constrained by Fregate data.

The final fitted model yields the estimated values of duration, Epeak, and 25-100 keV fluence.

It is anticipated that these rapid spectral fits will be followed up by more careful manual fits within several hours of the burst, and that these results will also be posted to the public website.