THE INTERPLANETARY NETWORK SUPPLEMENT TO THE HETE-2 GAMMA-RAY BURST CATALOG

The Wide Field X-Ray Monitor (WXM) and Soft X-Ray Camera (SXC) on board the High Energy Transient Experiment (HETE-2) mission localized 79 gamma-ray bursts (GRBs) rapidly and precisely between 2001 and 2006 (Atteia et al. 2003; Kawai et al. 2003; Ricker et al. 2003; R. Vanderspek et al. 2011, in preparation; Villasenor et al. 2003). About 1400 more GRBs, however, occurred outside their fields of view and were not detected or localized by them. In some cases these events were detected by the HETE-2 French Gamma-Ray Telescope (FREGATE) at angles up to 180° from the detector axis and identified by on-board and/or ground-based software. These detections were used to initiate searches through the data of the spacecraft comprising the interplanetary network (IPN), and in many cases precise, delayed localizations could be obtained by triangulation, and multiwavelength counterpart searches were initiated. Between 2000 November and 2006 May, when these detections occurred, the IPN contained between four and nine spacecraft. They were, in addition to HETE: Ulysses, in heliocentric orbit at distances between 670 and 3000 lt-s from Earth (Hurley et al. 1992); Konus-Wind, in various orbits up to around 4 lt-s from Earth (Aptekar et al. 1995); BeppoSAX, in low Earth orbit (Frontera et al. 1997; Hurley et al. 2000b); the Near-Earth Asteroid Rendezvous (NEAR) mission, in orbit around the asteroid Eros at distances between 775 and 1060 lt-s from Earth (Trombka et al. 1999); Mars Odyssey, launched in 2001 April and in orbit around Mars starting in 2001 October, up to 1250 lt-s from Earth (Hurley et al. 2006); RHESSI in low Earth orbit (Smith et al. 2002); INTEGRAL, in an eccentric Earth orbit at up to 0.5 lt-s from Earth (Rau et al. 2005); the MESSENGER mission, launched in 2004 August, but commencing full operation only in 2007 (Gold et al. 2001); and Swift (Gehrels et al. 2004) and Suzaku (Takahashi et al. 2007; Yamaoka et al. 2009), both in low Earth orbit. Their timelines are presented in Figure 1. In this paper, we present the localization data obtained by the IPN for these bursts.

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At least two other spacecraft recorded GRB detections during this period, although they were not used for triangulation and therefore were not part of the IPN. The Rossi X-Ray Timing Explorer (RXTE) All Sky Monitor detected and localized some HETE bursts (Smith et al. 1999). It operated in the low-energy X-ray range, where the light curves of gamma-ray bursts differ significantly from the high-energy range where the other IPN instruments operate, and it was not utilized for triangulation. The Defense Meteorological Satellite Program spacecraft detected, but did not localize bursts (Terrell et al. 1996, 1998; Terrell & Klebesadel 2004).

Whenever a gamma-ray burst or a rapid transient event was detected by the HETE onboard or ground-based software, a search was initiated in the data of the IPN spacecraft. For the spacecraft within a few light-seconds of Earth, the search window was centered on the HETE trigger time, and its duration was somewhat greater than the HETE event duration. For the spacecraft at interplanetary distances, the search window was twice the light-travel time to the spacecraft if the event arrival direction was unknown, which was the case for most events. If the arrival direction was known, even coarsely, the search window was defined by calculating the expected arrival time at the spacecraft, and searching in a window around it. In addition to these searches, which were initiated by HETE events, the HETE data were searched whenever an IPN spacecraft detected an event. Of the more than 1400 events detected by up to 7 IPN spacecraft, 226 were detected by HETE; these are listed in Table 1. Table 2 shows the number of events observed by each spacecraft in the IPN, and Table 3 gives the number of bursts that were detected by a total of N spacecraft, where N is between 2 and 9.

When a GRB arrives at two spacecraft with a delay δT, it may be localized to an annulus whose half-angle θ with respect to the vector joining the two spacecraft is given by

$$\begin{equation} \cos \theta =\frac{c \delta T}{D}, \end{equation} \tag{ 1 }$$

where c is the speed of light and D is the distance between the two spacecraft. (This assumes that the burst is a plane wave, i.e., that its distance is much greater than D.) The annulus width dθ is

$$\begin{equation} d \theta =c \sigma (\delta T)/D\sin \theta, \end{equation} \tag{ 2 }$$

where σ(δT) is the uncertainty in the time delay. σ(δT) is generally of the order of 100 ms or more, when both statistical and systematic uncertainties are considered; thus triangulation between two near-Earth spacecraft, for which D is at most ∼40 lt-ms, does not constrain the burst arrival direction. When D is of the order of several light-seconds (e.g., the distance between Konus-Wind  and a near-Earth spacecraft), annuli with widths of several degrees can be obtained; when D is several hundred light-seconds (i.e., an interplanetary spacecraft and a near-Earth spacecraft), annulus widths of the order of arcminutes or less are possible. When two interplanetary spacecraft and a near-Earth spacecraft observe a GRB, a small error box can be obtained. Table 4 gives the number of events observed by 0, 1, and 2 interplanetary spacecraft.

In some cases, no localizations can be obtained that constrain the burst arrival direction significantly, even though the spacecraft separations are several light-seconds. This is due to the fact that one or more of the spacecraft recorded the event with low time resolution.

Note that the Swift Burst Alert Telescope (BAT) observes numerous bursts outside its coded field of view (as indicated by the footnote in Table 1). The Swift data on these events is nevertheless useful for triangulations.

One hundred and fifty-seven bursts could be localized by the method above; Table 5 gives the localization information, and the first column (the date of the GRB) contains a link to a map on the IPN Web site. Triangulation annuli are given in the 8 IPN columns: these are the right ascension (R.A.) and declination of the annulus center α, δ, the annulus radius R, and the uncertainty in the radius δR. One or two annuli are specified. In addition to triangulation annuli, several other types of localization information are included in this catalog. The three SAX columns give the R.A., declination, and radius of the BeppoSAX gamma-ray burst monitor error circle (Frontera et al. 2009). The three HETE columns give the R.A., declination, and radius of the WXM or SXC error circle, whichever is smaller (R. Vanderspek et al. 2011, in preparation). Combining these error circles with the IPN annuli often results in smaller error regions. In one case, GRB040810, the HETE attitude could not be determined precisely, and consequently, the IPN annulus does not intersect the error circle. We have omitted the HETE error circle coordinates for this burst. The two ecliptic columns give the ecliptic latitudes of the bursts, measured northward (positive) from the ecliptic plane toward the north ecliptic pole. These are derived by comparing the count rates of the two Konus-Wind  detectors (Aptekar et al. 1995), and can be considered to be at the 90%–95% confidence level. In some cases, however, systematic uncertainties can result in incorrect estimates. Two cases are noted in Table 5 where the estimated ecliptic latitude and the IPN localization are inconsistent; this is thought to be due to these systematics. Planet blocking is specified by the R.A. and declination of the planet's center and its radius, in the three planet columns. When a spacecraft in low Earth or Mars orbit observes a burst, the planet blocks up to ≈3.7 sr of the sky. This is often useful for deciding which of two annulus intersections is the correct one, or for eliminating portions of a single annulus. Finally, the other column gives the R.A., declination, and radius of any other localization region, which is obtained in one of several ways. For example, error circles can be derived from the intersection of an IPN annulus and a HETE WXM one-dimensional localization. Or they may be from the Swift spacecraft.²⁸ Or they may be derived from planet blocking by a second spacecraft in addition to the data in the planet column. In this case the error circle given is the complement of the planet-blocking circle, that is, a circle whose R.A. is the R.A. of the planet plus 180°, whose declination is the negative of the planet's declination, and whose radius is 180° minus the planet's angular radius. The units of all entries in Table 5 are degrees, and all coordinates are J2000. For some events, no triangulation was possible, but coarse constraints on the burst arrival direction can be derived from planet blocking, ecliptic latitudes, or both. This information is not given here, but information on these events, as well as the ones in this catalog, may be found at the IPN Web site.²⁹ Figures 2 and 3 show examples of IPN localizations. Table 6 gives the localization areas in square degrees for the bursts in Table 5.

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This is the tenth in a continuing series of IPN catalogs (Table 7); the localization data for all of them can be found in electronic form at the IPN website. The IPN is, in effect, a full-time, all-sky monitor, when the duty cycles and viewing constraints of all its instruments are considered. Its threshold for 50% detection efficiency is about $6 \times 10^{-7}\,\rm erg \, cm^{-2}\, or \, 1 \rm \, photon \, cm^{-2}\, s^{-1}$. Over the HETE-2 mission, 226 bursts were detected by HETE-FREGATE and at least one other IPN instrument and 157 of them could be localized to some extent by triangulation. The more precise and/or rapid localizations were announced via 55 GCN Circulars, resulting in multiwavelength counterpart searches. Regardless of precision and speed of the localizations, however, burst data such as these are useful for numerous studies, such as searching for indications of activity from previously unknown soft gamma repeaters, associating supernovae with bursts, or searching for neutrino and gravitational radiation associated with bursts.

Between 2000 and 2006, support for the interplanetary network came from the following sources: JPL Contracts 958056 and 1268385 (Ulysses), MIT Contract SC-R-293291 and NASA NAG5-11451 (HETE), NASA NNX07AH52G (Konus), NASA NAG5-13080 (RHESSI), NASA NAG5-12614 and NNG04GM50G (INTEGRAL), NASA NAG5-11451 and JPL Contract 1282043 (Odyssey), NASA NNG05GF72G (Swift), NASA NAG5-9126 (BeppoSAX), NASA NNX06AI36G (Suzaku), NASA NAG5-9503 (NEAR), and NASA NNX07AR71G (MESSENGER). In Russia, this work was supported by the Federal Space Agency of Russia and RFBR Grant 09-02-00166a. C.G., F.F., and E.M. acknowledge financial support by ASI-INAF contract I/088/06/0.