Daiki Takahashi

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.

Observational response of MAXI onboard ISS

The current status is reported of the development of Monitor of All-sky X-ray Image and the measurement of its observational response. MAXI is a scanning X-ray camera to be attached to the Japanese Experiment Module of the International Space Station in 2008. MAXI is mainly composed of two kinds of instruments, GSC which is sensitive to the 2 - 30 keV photons, and SSC to the 0.5 - 10 keV ones. As an X-ray all-sky monitor, MAXI has an unprecedented sensitivity of 7 mCrab in one orbit scan, and 1 mCrab in one week. Using the engineering mode of the proportional counter and of the collimator for GSC, the observational response of GSC is extensively measured. The acceptable performances are obtained as a whole for both the collimator and the counter. The engineering models of the other part of MAXI are also constructed and the measurement of their performance is ongoing.

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.

The First Light from MAXI onboard JEM (Kibo)‐EF on ISS

MAXI, the first astronomical payload attached to ISS JEM‐EF, began operation on August 3, 2009 for monitoring all‐sky X‐ray images every ISS orbit (92 min). This paper reports the first results obtained during the 1‐month test observations. All instruments as well as two main X‐ray slit cameras, the GSC and SSC, worked as we expected. The detection limits of MAXI‐GSC are about 25 mCrab and 8 mCrab for one ISS orbit (92 min) and 1‐day observations, respectively. GSC covers about 76% and 96% of the entire sky for respective single‐orbit and 1‐day observations. MAXI has detected a transient X‐ray pulsar A0535+26, an X‐ray burst, and a gamma‐ray burst in the first month of the test observation period.

X‐Ray Bursts Observed by the HETE‐2 Satellite

In the period that the Galactic center region was in the field of view of HETE‐2 satellite from May to September 2001, it detected bursts from well‐known X‐ray bursters (XRBs). More than 130 events were localized with the Wide‐Field X‐ray Monitor (WXM) on HETE‐2 in these four months to the XRBs including X 1728‐34, SAX J1750‐29, Aq1 X‐1, NGC 6624, and GS 1826‐238. Localization accuracy is better than 18 arc‐minutes for most of the events. In this paper, we summarize the XRBs detected by HETE‐2 and show the light curves of the several events.

Observations of X‐ray Bursts by HETE‐2

The scientific instruments aboard HETE‐2 are pointed to the Galactic center region each summer. The WXM has detected more than 407 X‐ray busrts from 17 known X‐ray burst sources such as GS 1826‐238, 4U 1850‐087 and 4U 1820‐30. In particular, 173 events were localized to GS 1826‐238 in 2001, 2002 and 2003.We find that the X‐ray bursts from GS 1826‐238 were produced periodically with recurrence intervals of 4.20, 3.57 and 3.73 hours in 2001, 2002 and 2003 respectively. These recurrence intervals until 2002 are shorter than that observed with Beppo‐SAX/WFC from 1996 to 1998. In contrast, the interval in 2003 is slightly increasing.

Astrometric Calibration and Estimate of the Systematic Error in WXM Localizations Obtained by the Chicago Bayesian Method

WXM gives GRB localizations in instrument coordinates. WXM localizations must be converted to celestial coordinates using spacecraft aspect information obtained by the optical cameras on HETE. We must therefore accurately determine the alignment of the WXM boresight with respect to that of the optical cameras, in order to accurately determine the celestial coordinates of WXM burst locations. We use a seven‐parameter model that treats as free parameters the three Euler angles of a pure rotation, two horizontal shifts of the coded‐aperture masks with respect to the detectors, and the heights of the masks above the two detectors. We determine the alignment by fitting the model to a set of 252 WXM localizations of Sco X‐1 obtained between 23 April and 28 June 2001. We estimate the systematic error in WXM GRB locations by comparing the actual and the calculated locations of Sco X‐1. We find that the systematic error corresponding to a 68.3% confidence region is 1.7′, and the systematic error corresponding to a...