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HETE Burst Alerts

One of the key features of the HETE satellite is its ability to calculate precise localizations of GRBs on board within seconds of burst onset, and then to transmit the burst localizations to the ground as soon as they have been calculated. The HETE-2 satellite utilizes a low-rate VHF transmitter to continuously broadcast the burst information; on the ground, an array of listen-only burst alert stations (BAS) receive the data and transmit them to the MIT Control Center. Once received at MIT, burst information are immediately relayed to the GRB Coordinate Distribution Network (GCN) at the Goddard Space Flight Center for distribution to interested ground observers.

On this page, we describe

A recent modification to the ground burst location selection processes is described in the imaging quality section .

Burst information downlinked by HETE

Information about a GRB will come to the ground in two ways:

Burst analysis procedures

The analysis of HETE burst data occurs in several different stages on several different fronts:

What HETE sends to the GCN...

...in real time

The current criteria for the real-time distribution of GCN Notices are as follows:

This method of distribution of GCN Notices results in a few common situations:

...and after Ground Analysis

Ground analysis of a burst begins as soon as the full burst data reach MIT after a Primary Ground Station contact (from a few minutes to over an hour after the burst, depending on where in its orbit HETE was at the time of the burst). Automated software performs standard analyses of the downlinked data, and a human is notified to make the final decisions. A followup GCN Notice, of type HETE_Gnd_Analysis , will be distributed under the following circumstances:

In general, if there is a position in a real-time GCN Notice, it should be considered accurate. If there is no position in a real-time GCN Notice, a position may be forthcoming within an hour or so of the original Notice. If a burst position was distributed and it is wrong because of software or operator error, a HETE_Gnd_Analysis message will be distributed; if no position was distributed, no HETE_Gnd_Analysis message will be sent.

HETE-2 Burst Detection

The HETE mission has three instruments on board which can generate burst triggers: Fregate, the WXM, and the SXC. Fregate data are examined in two broad energy channels: 5-80 keV and 30-400 keV. The WXM data are from 2-30 keV, and the SXC data are from 1.5 to 12 keV. There are three processors, running in parallel, examining these data for signatures of a burst.

When a burst is detected, the real-time spacecraft notification will distribute

HETE-2 Burst Imaging

Once a burst has been detected on board, all other flight instrument computers are notified: the WXM and SXC begin their attempts to image the X-rays from the burst, and the optical system, which has immediate knowledge of the spacecraft attitude and aspect, delivers the spacecraft orientation to the the mission operations center at MIT via the real-time VHF link and the BAS network.

Both the WXM and SXC search data from seconds before the burst trigger to minutes after, looking for the image of the burst. The WXM software matches the shadow pattern on the detector with template patterns, looking for a best fit; the SXC looks for peaks in the cross-correlation map. In either case, if a significant position is found in either instrument, its location is sent to the ground in real time. Once the positions and their significances are received on the ground, the RA and declination of the image are calculated and, if the significances are high enough, transmitted to the GCN. At present, SXC positions are not sent to the GCN automatically, but rather only after ground analysis.

Once a burst position has been determined on board and sent to the ground, the on-board software continues to look for better images using shadow patterns collected during different time intervals. If a new burst position is calculated which has a higher signficance than the preceding one(s) , that position will be sent to the ground via the VHF.

Determining the quality of HETE real-time positions

After extensive analysis of GRBs and XRBs detected by HETE, we have determined a method of verifying with >90% certainty that a real-time position is correct. This method involves measurement of the "lightcurve signal-to-noise" and the "image signal-to-noise" of a burst in each of the X and Y modules of the WXM. (Real-time burst positions as calculated by the SXC flight software are currently not distributed to the GCN).

We have determined that bursts that are well-localized by the WXM have image S/N and lightcurve S/N >3.0. We have also found that those which do not have image and lightcurve S/N >3.0 are moderately well imaged by the flight software if the total image S/N (the quadrature sum of the X and Y image S/N) is >3.7 and the measured incident angle of the burst is <30 degrees in both X and Y modules.

As a result, HETE positions distributed in real time from the spacecraft are in one of two categories:

  1. Category I: The image and lightcurve S/N all exceed 3.0, so the position is distributed with a 90% error radius of 12-14 arcminutes.
  2. Category II: Not all of the image and lightcurve S/N exceeds 3.0, but the quadrature sum of the image S/N from the WXM X and Y detectors is > 3.7, and neither the X nor the Y incident angle exceeds 30 degrees, so the position is distributed with a 90% error radius of 30 arcminutes.

We are confident that, with this scheme in place, the burst coordinates as calculated by the flight software will be correct >90% of the time.

However, the HETE GCN interface has been modified recently (4/2002) to accommodate those observers who would like to make their own estimate of the quality of a real-time burst localization. Now included in the GCN message are

The higher the image and lightcurve S/N, the more reliable the localization will be. Low values of image and lightcurve S/N are typically associated with events localized at the edge of the instrument FOV.

The only exceptions to this rule has been seen during particle events, where both the image and lightcurve S/N values can been extremely high. These events were quite frequent during solar maximum, but have become less so since the beginning of 2002. To determine whether a trigger was caused by a particle event,

Since this solar maximum cycle is now on the decline, particle events are going to be very rare, so each real-time location distributed by HETE should be considered valid at the level of significance measured by the error circle radius.