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It is thought that millisecond pulsars with white dwarf companions are born from X-ray binaries. The majority of known systems have been studied uniquely in the radio domain, which limits our understanding of such systems. We present here the X-ray observations of the millisecond pulsar PSR J0218+4232 and the two faint millisecond pulsars PSR J0751+1807 and PSR J1012+5307, which we discuss in conjunction with radio observations. We confirm the previously detected X-ray pulsations of PSR J0218+4232 and we show that its folded lightcurve is strongly dependent on energy. We present evidence to suggest that the broad band X-ray spectrum for this pulsar may not be a simple power law, but that there is some evidence for an excess of soft thermal emission over the power law spectrum, in particular from the strongest pulse, in support of a heated polar cap model for this pulsar. We also present the X-ray spectra of the two faint millisecond pulsars as well as some evidence to suggest that both of these millisecond pulsars show pulsations in the X-ray band. We then discuss the implied nature of the magnetic field configuration as a means of discriminating between competing magnetic field evolution theories in millisecond pulsars. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

FREGATE, the gamma-ray detector of HETE-2 is entirely dedicated to the study of GRBs. Its main characteristic is its broad energy range, from 7 keV to 400 keV. This energy range can be further extended down to 2 keV using the data from the WXM, the X-ray detector of HETE-2. Such a large energy range allows studies of the prompt emission of GRBs. determining with a high precision their spectral parameters. Moreover, because this energy range is at low energies, the sample of GRBs detected by both FREGATE and WXM contains a significant fraction of X-Ray Rich GRBs and X-Ray Flashes. We present here the distributions of the spectral parameters mesured for the time integrated spectra of 50 GRBs. We put emphasis on the distribution of the low energy spectral index α. Because FREGATE and WXM detected all classes of GRBs, we also discuss the connection between GRBs, X-Ray Rich GRBs and X-Ray Flashes. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We report HETE-2 WXM and FREGATE observations of two X-ray flashes (XRFs), XRF010213 and XRF020903. The signal is only seen in < 25 keV and < 10 keV for XRF010213 and XRF020903 respectively. Both events show a double-peak structure in their light curves. The durations of the bursts are > 10 seconds, and this feature is similar to that of the “long” GRBs. According to the time-averaged spectral analysis using both WXM and FREGATE data, the fluence ratio of 2–30 keV to 30–400 keV energy band is 11.4 and 5.6 for XRF010213 and XRF020903 respectively. The Epeak energy in the Band function is < 10 keV. They are likely to belong to the same class as the X-ray flash events detected with GINGA and BeppoSAX. In this paper, we will present the detail study of the prompt emission of XRF010213 and XRF020903, and compare with the characteristics of classic GRBs. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present the in-flight calibration results of the detector response matrices of the X-ray instrument, Wide-field X-ray Monitor (WXM), on board the HETE-2 satellite. WXM consists of four one-dimensional position-sensitive proportional counters and two sets of one-dimensional coded aperture. Its energy range is 2–25 keV. The detector response matrix (DRM) of WXM is calculated numerically using the ground measurements for each anode wire. It is then calibrated with the observation of the Crab nebula in December 2001 and January 2002. The spectral parameters and the normalization of the spectra are investigated at various incident angles, and compared with the known values in literature. With the current DRM, the derived spectral parameters are mostly consistent with the values in literature, expect for 10–20% uncertainty in normalization for individual anode wires, and at large incidence angles. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

After a long period of quiescence, the soft gamma repeater SGR1900+14 was suddenly reactivated on April 2001. On July 2[sup nd] 2001, a bright flare emitted by this source triggerred the WXM and FREGATE instruments onboard the HETE-2 satellite. Unlike typical short (∼ 0.1 s) and spiky SGRs recurrent bursts, this event features a 4.1 s long main peak, with a sharp rise (∼ 50 ms) and a slower cutoff (∼ 250 ms). This main peak is followed by a ∼ 2 sec decreasing tail. We found no evidence of any precursor or any extended 'afterglow' tail to this burst. We present the preliminary spectral fits of the total emission of this flare as observed by the FREGATE instrument between 7 and 150 keV. The best fit is obtained with a model consisting of two blackbody components of temperatures 4.15 kev and 10.4 keV. A thermal bremsstrahlung can not be fitted to this spectrum. We compare these features and the burst energetics with the other strong or giant flares from SGR1900+14. [ABSTRACT FROM AUTHOR]

The gamma-ray detector of HETE-2, called FREGATE, has been designed to detect gamma-ray bursts in the energy range 6-400 keV. Its main task is to alert the other instruments of the occurrence of a gamma-ray burst (GRB) and to provide the spectral coverage of the GRB prompt emission in hard X-rays and soft gammarays. FREGATE was switched on on October 16, 2000, one week after the successful launch of HETE-2, and has been continuously working since then. We describe here the main characteristics of the instrument, its in-flight performance and we briefly discuss the first GRB observations. [ABSTRACT FROM AUTHOR]

The FREGATE gamma ray detector of HETE-2 is sensitive to photons between 6 and 400 keV. This sensitivity range, extended towards low energies, allows us to explore the emission of GRBs in hard X-rays. We fit the spectra of 23 GRBs with Band's spectral function in order to derive the distribution of their peak energies (E-peak). This distribution is then compared with the E-peak distributions measured by BATSE and GINGA. [ABSTRACT FROM AUTHOR]

Using Rossi X-ray Timing Explorer (RXTE) observations, we have discovered both low (5-80 Hz, LF) and high (200–1300 Hz, HF) frequency Quasi Periodic Oscillations (QPOs) from the Low Mass X-ray Binary (LMXB) 4U1915-05. We find that the frequencies of the LF and HF QPOs positively correlate, suggesting that both QPOs are produced by related mechanisms. Their properties also depend upon the spectral state of the X-ray source. In this paper, we study the QPOs as a function of the source count rate and its position in the color-color diagram. We conclude that the timing behavior of the source better correlates with the latter parameter. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Supernova remnants (SNR) can be broadly classified into two types: plerionic or plasma-filled SNR, like the Crab Nebula, and shell-type SNR, like Cassiopeia A (Cas A). VHE observations of the Crab Nebula and Cas A made with the CAT atmospheric Cherenkov imaging telescope are used to constrain models for production of gamma rays in SNR. For both plerionic and shell-type SNR, these observations serve primarily to impose a limit on the magnetic field in the region where the particles are accelerated. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

During the first year of operation, the FREGATE instrument onboard HETE-2 has detected a large number of Gamma-Ray Bursts in the 6-400 kev energy range. In this paper, the sensitivity of FREGATE is computed in various energy ranges using several parameters such as the observed background, the angle of arrival of the photons on the detector, and the trigger criteria. From the current list of GRB candidates and with the duty cycle of FREGATE, we estimate the instrument burst detection rate within the field of view. The sensitivity and the burst rate are in good agreement with preflight simulations and with the results of previous experiments. [ABSTRACT FROM AUTHOR]