M. Matsuoka

High Energy Transient Explorer 2 Observations of the Extremely Soft X-Ray Flash XRF 020903

We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-Ray Monitor/French Gamma Telescope observations of the X-ray flash XRF 020903. This event was extremely soft: the ratio log(SX/Sγ) = 0.7, where SX and Sγ are the fluences in the 2-30 and 30-400 keV energy bands, is the most extreme value observed so far by HETE-2. In addition, the spectrum has an observed peak energy of E < 5.0 keV (99.7% probability upper limit), and no photons were detected above ~10 keV. The burst is shorter at higher energies, which is similar to the behavior of long gamma-ray bursts (GRBs). We consider the possibility that the burst lies at very high redshift and that the low value of E is due to the cosmological redshift, and show that this is very unlikely. We find that the properties of XRF 020903 are consistent with the relation between the fluences S(7-30xa0keV) and S(30-400xa0keV), found by Barraud et al. for GRBs and X-ray-rich GRBs, and are consistent with the extension by a decade of the hardness-intensity correlation found by the same authors. Assuming that XRF 020903 lies at a redshift z = 0.25, as implied by the host galaxy of the candidate optical and radio afterglows of this burst, we find that the properties of XRF 020903 are consistent with an extension by a factor ~300 of the relation between the isotropic-equivalent energy Eiso and the peak Epeak of the νFν spectrum (in the source frame of the burst) found by Amati et al. for GRBs. The results presented in this paper therefore provide evidence that X-ray flashes (XRFs), X-ray-rich GRBs, and GRBs form a continuum and are a single phenomenon. The results also impose strong constraints on models of XRFs and X-ray-rich GRBs.

Scientific highlights of the HETE-2 mission

Abstract The High Energy Explorer Satellite 2 (HETE-2) mission has been highly productive. It has observed more than 250 γ-ray bursts (GRBs) so far. It is currently localizing 25–30 GRBs per year, and has localized 43 GRBs to date. Twenty-one of these localizations have led to the detection of X-ray, optical, or radio afterglows, and as of now, 11 of the bursts with afterglows have redshift determinations. HETE-2 has also observed more than 45 bursts from soft γ-ray repeaters, and more than 700 X-ray bursts. HETE-2 has confirmed the connection between GRBs and Type Ic supernovae, a singular achievement and certainly one of the scientific highlights of the mission so far. It has provided evidence that the isotropic-equivalent energies and luminosities of GRBs may be correlated with redshift; such a correlation would imply that GRBs and their progenitors evolve strongly with redshift. Both of these results have profound implications for the nature of GRB progenitors and for the use of GRBs as a probe of cosmology and the early universe. HETE-2 has placed severe constraints on any X-ray or optical afterglow of a short GRB. It has made it possible to explore the previously unknown behavior of optical afterglows at very early times, and has opened up the era of high-resolution spectroscopy of GRB optical afterglows. It is also solving the mystery of “optically dark” GRBs, and revealing the nature of X-ray flashes.

HETE-2 localization and observation of the bright, X-ray-rich gamma-ray burst GRB 021211

A bright, X-ray-rich gamma-ray burst (GRB) was detected by the French Gamma Telescope (FREGATE) and localized with the Wide Field X-ray Monitor (WXM) and Soft X-ray Camera (SXC) instruments on the High Energy Transient Explorer 2 satellite (HETE-2) at 11:18:34.03 UT (40714.03 SOD) on 2002 December 11. The WXM flight software localized the burst to a 14 radius; this was relayed to the astronomical community 22 s after the start of the burst. Ground analysis of WXM and SXC data provided refined localizations; the latter can be described as a circle with a radius of 2 centered at R.A. 08h09m00s, decl. 06°4420 (J2000.0). GRB 021211 consists of a single, FRED-like pulse with a duration t90 ≈ 2.3 s at high energies (85-400 keV), which increases to t90 ≈ 8.5 s at low energies (2-10 keV). The peak photon number and photon energy fluxes in the 2-400 keV band are (34.0 ± 1.8) photons cm-2 s-1 and (1.68 ± 0.11) × 10-6 ergs cm-2 s-1, respectively. The energy fluences in the 2-30 and 30-400 keV energy bands are SX = (1.36 ± 0.05) × 10-6 ergs cm-2 and Sγ = (2.17 ± 0.15) × 10-6 ergs cm-2, respectively. Thus, GRB 021211 is an X-ray-rich GRB (SX/Sγ = 0.63 > 0.32). The average spectrum of the burst is well fitted by a Band function (low-energy power-law index α = -0.805; high-energy power-law index β = -2.37; and energy of the peak of the spectrum in νFν, E = 46.8 keV). The near-real-time optical follow-up of GRB 021211 made possible by HETE-2 led to the detection of an optical afterglow for what otherwise would quite likely have been classified as an optically dark GRB, since the optical transient faded rapidly (from R < 14 to R ≈ 19) within the first 20 minutes, and was fainter than R ≈ 23 within 24 hr after the burst. GRB 021211 demonstrates that some fraction of burst afterglows are optically dark because their optical afterglows at times greater than 1 hr after the burst are very faint, and previously have often escaped detection. Such bursts are optically dim rather than truly optically dark. GRB 021211 also shows that even such optically dim bursts can have very bright optical afterglows at times less than 20 minutes after the burst.

Design and Performance of the Wide-Field X-Ray Monitor on Board the High-Energy Transient Explorer 2

The Wide-field X-ray Monitor (WXM) is one of the scientific instruments carried on the High Energy Transient Explorer 2 (HETE-2) satellite launched on 2000 October 9. HETE-2 is an international mission consisting of a small satellite dedicated to provide broad-band observations and accurate localizations of gamma-ray bursts (GRBs). A unique feature of this mission is its capability to determine and transmit GRB coordinates in almost real-time through the burst alert network. The WXM consists of three elements: four identical Xe-filled one-dimensional positionsensitive proportional counters, two sets of one-dimensional coded apertures, and the main electronics. The WXM counters are sensitive to X-rays between 2keV and 25keV within a field-of-view of about 1.5sr, with a total detector area of about 350cm 2 . The in-flight triggering and localization capability can produce a real-time GRB location of several to 30arcmin accuracy, with a limiting sensitivityof 10 −7 ergcm −2 . In this report, the details of the mechanical structure, electronics, on-board software, ground and in-flight calibration, and in-flight performance of the WXM are discussed.

Performance of the wide-field x-ray monitor on board the High-Energy Transient Explorer 2

The Wide-field X-ray Monitor is one of the scientific instruments carried on the High Energy Transient Explorer 2 (HETE-2) satellite planned to be launched in May, 2000 (on the present schedule in February, 2000). HETE-2 is an international mission of a small satellite dedicated to provide broad band observations and accurate localizations of gamma-ray bursts (GRBs). The first HETE satellite was lost due to a Pegasus XL rocket mishap on November 4, 1996. The HETE-2 has been developed on basically the same concept except that the UV cameras were replaced with the Soft X-ray Camera. A unique feature of this mission is its capability of determination and transmission of GRB coordinates in near real time through a network of primary and secondary ground stations.

In‐Orbit Performance of WXM (Wide‐Field X‐Ray Monitor)

The Wide‐field X‐ray Monitor (WXM) is one of the three main scientific instruments on HETE‐2, and is designed to measure the light curves, spectra, and locations of gamma‐ray bursts (GRBs) and other transients in the energy range of 2–25 keV. It consists of Xe‐filled 1‐D position‐sensitive proportional counters equipped with two 1‐D coded apertures in orthogonal directions with a field of view of 40° × 40°. The sophisticated onboard processing allows the localization of GRBs in real time with ∼ 10′ accuracy based on the alerts from FREGATE, the gamma‐ray detector. The WXM also triggers on its own count time history with a flexible algorithm and can localize X‐ray events on various time scales. We present the design and basic characteristics of the detectors, the handling of the data, the in‐flight performance, and some of the early observations.

In-orbit performance of wide-field x-ray monitor on HETE-2

The Wide-field X-ray Monitor (WXM) is one of the scientific instruments carried on the High Energy Transient Explorer 2 (HETE-2) satellite launched in October 2000. The WXM consists of three elements: (1) four identical Xe-filled one-dimensional position-sensitive proportional counters, two in the spacecraft X-direction and two in the Y-direction, (2) two sets of one-dimensional coded apertures orthogonally mounted above the counters in the X and Y-direction, and (3) the main electronics that processes analog signals from the counters. The WXM counters are sensitive to X-rays between 2 keV and 25 keV within a field-of-view of about 1.5 sr with a total detector area of about 350 cm2. The combination of the apertures and the counters provides GRB locations with accuracy ~10 arcmin. The counters and electronics are developed and fabricated by RIKEN, and the apertures and on-board software are designed and provided by Los Alamos National Laboratory. The WXM plays a major roll in the GRB localization and its spectroscopy in the energy range between 2 keV and 25 keV. During the first year of observations, a number of steady X-ray sources as well as high-energy transients were detected with the WXM. Observing Crab nebula and Sco X-1, we have calibrated the detector alignment between the WXM and the optical camera system with 2 arcmin accuracy. As of 29 July 2002, nineteen GRBs have been localized with the WXM in the 18 months of stable operations. Twelve of them were reported to the GCN within a delay of 10 hours, and 4 optical transients were identified by ground based telescopes. The energy response of the detectors has also been calibrated using the Crab spectrum. We report the in-orbit performance of the WXM instrument during the first 18 months.

HETE-2 Observations of Gamma-Ray Bursts and Their Follow-Ups

The High Energy Transient Explorer 2 (HETE-2), launched in October 2000, is currently localizing gamma-ray bursts (GRBs) at a rate of ∼ 20 yr - 1 , many in real time. As of September 2003, HETE-2 had localized 47 GRBs; 16 localizations had led to the detection of an X-ray, optical, or radio afterglows. The prompt position notification of HETE-2 enabled probing the nature of so-called dark bursts for which no optical afterglows were found despite of accurate localizations. In some cases, the optical afterglow was found to be intrinsically faint, and its flux declined rapidly. In another case, the optical emission was likely to be extinguished by the dust in the vicinity of the GRB source. The bright afterglows of GRB021004 and GRB030329 were observed in unprecedented details by telescopes around the world. Strong evidence for the association of long GRBs with the core-collapse super-novae was found. HETE-2 has localized almost as many X-ray rich GRBs as classical GRBs. The nature of the X-ray rich GRBs and X-ray flashes have been studied systematically with HETE-2, and they are found to have many properties in common with the classical GRBs, suggesting that they are a single phenomenon.

High-energy transient explorer-2

We describe the scientific goals, the spacecraft and instrumentation, and the operation of the High-Energy Transient Explorer-2 (HETE-2), which is currently scheduled to be launched in early 2000.