The BATSE Rapid Burst Response System

R. M. Kippen,tex2html_wrap_inline154 V. Connaughton,tex2html_wrap_inline156 G. N. Pendleton,tex2html_wrap_inline154
S. D. Barthelmy,tex2html_wrap_inline160 P. Woods,tex2html_wrap_inline154 M. S. Briggs,tex2html_wrap_inline154 G. J. Fishman,tex2html_wrap_inline166
C. Kouveliotou,tex2html_wrap_inline168 C. R. Robinson,tex2html_wrap_inline168 and C. A. Meegantex2html_wrap_inline166
tex2html_wrap_inline154University of Alabama in Huntsville, AL 35899
tex2html_wrap_inline156NAS/NRC NASA/Marshall Space Flight Center, Huntsville, AL 35812
tex2html_wrap_inline160USRA NASA/Goddard Space Flight Center, Greenbelt, MD 20771
tex2html_wrap_inline166NASA/Marshall Space Flight Center, Huntsville, AL 35812
tex2html_wrap_inline168USRA NASA/Marshall Space Flight Center, Huntsville, AL 35812


The recently developed BATSE Rapid Burst Response (RBR) system provides gamma-ray burst locations accurate to tex2html_wrap_inline1842tex2html_wrap_inline186-3tex2html_wrap_inline186 for several bursts per month within tex2html_wrap_inline18415-35 minutes of onset. Combined with wide-field or scanning instruments, this system offers the chance of detecting prompt x-ray/optical/radio afterglow emission from many bursts. We discuss the operation, performance and status of the BATSE RBR system as well as results from coordinated multi-wavelength follow-up observations.


The combination of rapid and accurate gamma-ray burst (GRB) locations has long been recognized as the fundamental key to solving the burst counterpart problem. Unfortunately, quick and precise measurement of GRB locations is a difficult technical challenge. For the past four years the BACODINE system has been attempting to solve this problem by providing roughly-determined (accurate to tex2html_wrap_inline1846tex2html_wrap_inline186-12tex2html_wrap_inline186; 1tex2html_wrap_inline198) BATSE burst locations in near real-time (typically < 10 seconds) to a small network of dedicated wide-field optical/radio counterpart observers [1]. The BACODINE system sacrifices location accuracy in favor of speed through its use of small amounts of data, automated background estimation and a simplified burst location algorithm. Unfortunately, BACODINE locations have proved to be too imprecise to allow the sensitive observations required to detect faint GRB afterglows.

To facilitate more sensitive counterpart searches on somewhat longer time-scales, we have developed the BATSE Rapid Burst Response (RBR) system. BATSE RBR takes a different approach from BACODINE--better location accuracy at the cost of longer delay. This is accomplished using a larger sample of BATSE data, interactive background estimation, and a much more comprehensive burst location algorithm. The system operates in parallel with normal BACODINE processing and the locations are publicly distributed via the GRB Coordinates Network (GCN [1]). The BATSE RBR approach, favoring precise locations over real-time delay, has significant merit given that the recently discovered GRB afterglows are faint, but can last from several hours, to days [2, 3, 4, 5]. Below, we discuss the system's operation and performance.

BATSE RBR Operation

The BATSE RBR system is a combination of the existing real-time BACODINE system and the burst processing software of the BATSE instrument team. By using aspects of both, we are able to provide BATSE GRB locations as accurate as possible, with minimal delay. The system operates as follows.

Upon recognition of a transient event trigger, up to 500 s of buffered, real-time BATSE telemetry are accumulated and then automatically downloaded by the BACODINE computer at Goddard Space Flight Center to the BATSE team headquarters at Marshall Space Flight Center. Concurrently, on-duty BATSE team scientists are alerted via e-mail and alphanumeric pagers (24 hours), and begin to manually reduce the data as soon as they arrive. BATSE Large Area Detector (LAD) count rate data, with 1024 ms temporal resolution, in four discriminator energy channels (DISCLA), are interactively used to estimate source and background intervals that optimize the signal-to-noise ratio. Interactive processing also allows the efficient rejection of non-GRB events, and events with large location errors. The background-subtracted count rates in each BATSE LAD are used as input to the LOCBURST [6] burst location algorithm. The LOCBURST algorithm has a considerable history of development over the course of the BATSE mission. It uses detailed models of the detector response and atmospheric scattering to determine the GRB location that best fits the observed count rates. Upon completion, the burst location and its statistical uncertainty are transferred to the BACODINE computer, from which they are distributed to the GCN network of multiwavelength observers. A special GCN burst alert notice type ``LOCBURST'' distinguishes the RBR locations from BACODINE locations. Under normal conditions, the entire process requires only 15-35 minutes from trigger to completion.

To reduce the burden on duty scientists, only the brightest bursts are processed. The current threshold for RBR notification results in tex2html_wrap_inline1842 LOCBURST alerts per week. Weaker bursts are not a significant loss since they have larger location errors.

Figure 1: BATSE RBR location errors. (a) Cumulative distributions of 73 BATSE/IPN offset angles compared with best-fit RBR two-component systematic error model. (b) Total error histograms (referenced to the left axis) for the same 73 bursts using the best-fit systematic error model. Smooth curves (referenced to right axis) show the relation between total error and statistical error in this model.

Location Errors

The BATSE RBR system uses the most current version of LOCBURST--the same algorithm that is employed for the newest (since June 1997) burst locations in the ``current'' BATSE GRB catalog [7]. This version of LOCBURST differs from that used for earlier catalogs in that it incorporates a more accurate energy calibration of the LAD discriminator. The BATSE RBR locations also differ from cataloged results because RBR cannot use the full BATSE dataset (most burst data are stored in memory and are thus not available in the real-time telemetry at the time of a burst). We have therefore independently assessed the BATSE RBR location errors.

As a comparison sample, we use 73 bursts (detected between Feb. 1995 and Sept. 1997) that have been independently located to single annuli through BATSE/Ulysses Interplanetary Network (IPN) timing analysis [8]. While only 7 of these bursts were processed in ``real-time'' with the RBR system, they are typical, in brightness, of the majority of RBR events. Since the IPN annuli are precisely determined (tex2html_wrap_inline2080.1tex2html_wrap_inline186 in width), the shortest angular separation tex2html_wrap_inline212 between a BATSE location and an IPN annulus yields an estimate of the total BATSE location error (a combination of the measured statistical error and an unknown systematic contribution). Figure 1a shows the cumulative distribution of tex2html_wrap_inline212 for locations from RBR, the official BATSE catalog, and BACODINE. The RBR locations are statistically indistinguishable from the official catalog results, and a factor of tex2html_wrap_inline1843.5 more accurate than BACODINE.

The BATSE team has recently significantly improved the characterization of systematic burst location errors from a single Gaussian distribution (tex2html_wrap_inline218), to the weighted sum of two Gaussians, with differing widths [9]. A maximum-likelihood fit of this improved ``core-plus-tail'' model to the 73 RBR/IPN bursts is shown in Figure 1a. The best fit model, which is statistically consistent with fits to larger data sets, has a 1.9tex2html_wrap_inline186 systematic error ``core'' component with 78% probability and a 5.3tex2html_wrap_inline186 ``tail'' component with 22% probability. Using these parameters, we can compute the total error of a burst, given its measured statistical error radius. The cumulative distribution of total errors for the 73 test bursts is shown in Figure 1b, which indicates that tex2html_wrap_inline18485% of the RBR bursts have total location errors in the range 2tex2html_wrap_inline186-3tex2html_wrap_inline186 (68% confidence level). Also shown in this figure is the relation between statistical and total error. For example, RBR bursts with statistical errors tex2html_wrap_inline230 have 68% confidence total errors tex2html_wrap_inline232.

Figure 2: Equatorial coordinate (J2000) sky maps of the five BATSE RBR events scanned by RXTE-PCA. Each map shows the RBR localization (solid lines; 50%, 68%, and 90% confidence), the PCA scan region (dashed lines), and the preliminary IPN annulus (dotted lines [11]). For GRB 970807, COMPTEL location contours (1,2,3tex2html_wrap_inline198 [12]) are shown and for GRB 970616, the PCA transient x-ray source location is shown (boxed X).

Performance & RXTE Follow-Up

The BATSE RBR system has been operational since late April 1997-- providing burst location alerts at the rate of tex2html_wrap_inline1842 per week. Wide-field, ground-based telescopes have observed some of the locations within several hours, but no optical afterglow counterparts have been discovered (see these proceedings).

The most promising use of BATSE RBR locations lies in a coordinated effort with the Rossi X-ray Timing Explorer's Proportional Counter Array instrument (RXTE-PCA). In this effort, RXTE is re-pointed and the PCA (field-of-view tex2html_wrap_inline1841tex2html_wrap_inline186) is used to scan the most precise RBR localizations for fading x-ray counterparts (2-10 keV) within tex2html_wrap_inline1843 hours [10]. This has been done successfully for five GRBs. The BATSE RBR localizations of these events (derived from the systematic error model discussed above) are shown in Figure 2.

The nominal PCA GRB follow-up strategy is to scan a tex2html_wrap_inline254-6tex2html_wrap_inline186 region, centered on the RBR location, in a 40-60 minute observation. This is sufficient to detect 0.5 mCrab sources with a significance >3tex2html_wrap_inline198. In one case (GRB 970807), a smaller scan region was used due to poor observing efficiency and a scheduling mistake. The nominal PCA scan area covers most of the typical RBR 68% confidence region, but this region will not always contain the GRB source (e.g., GRB 970925, which has an unusually large location error). In the case of GRB 970616, a previously unknown transient x-ray source was detected by the PCA in the RBR location scan. The 0.5 mCrab source was localized by the PCA to tex2html_wrap_inline18418tex2html_wrap_inline264 and the location was later found to be consistent with IPN triangulation. A large-scale search for afterglow emission at other wavelengths was undertaken--resulting in several potentially interesting x-ray/optical sources. It is still uncertain which, if any, is the counterpart to GRB 970616.

The BATSE/RXTE effort will continue to search for x-ray afterglows from the brightest (best localized) bursts at the rate of tex2html_wrap_inline1841 per month. In addition, several new optical instruments show the promise of being able to scan RBR error boxes with improved sensitivity within a few hours.


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Pendleton G. N., Briggs, M. S., & Meegan, C. A., in AIP Conf. Proc. 307, Gamma-Ray Bursts, eds. C. Kouveliotou, M. S. Briggs, & G. J. Fishman, 877 (1996).
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The BATSE Rapid Burst Response System

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