Soft ␥-ray repeaters (SGRs) emit multiple, brief (ϳ0.1-s), intense outbursts of low-energy ␥-rays. They are extremely rare 1 -three 2-4 are known in our Galaxy and one 5 in the Large Magellanic Cloud. Two SGRs are associated 5-7 with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806 ؊ 20, with a period of 7.47 s and a spindown rate of 2:6 ؋ 10 ؊ 3 s yr ؊ 1 . We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are ϳ1,500 years and 8 ؋ 10 14 gauss, respectively. Our observations demonstrate the existence of 'magnetars' , neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions 8,9 that SGR bursts are caused by neutron-star 'crustquakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millennium-a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars.SGR1806 Ϫ 20 became extremely active between October 1996 and November 1997, when over 40 intense bursts and numerous weaker ones were detected 10 with the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO). We observed SGR1806 Ϫ 20 with the Rossi X-Ray Timing Explorer (RXTE) five times between 5 and 18 November 1996, starting five days after the first triggered burst detection with BATSE. (Information on the archival data from RXTE/PCA and ASCA is available at http://heasarc.gsfc.nasa.gov.) During these observations 11 , the source emitted series of outbursts in a 'bunching' mode, never seen before. The intensity of the outbursts, as well as the 'bunching' mode, varied significantly: mini-outbursts were interlaced with very intense ones and the rate of bursts varied from bunch to bunch (S. Dieters et al., manuscript in preparation).We made a period search of the data after excluding all bursts from the time series. The data were then energy-selected for 2-24 keV X-rays, background subtracted and binned at 0.5-s resolution. The resulting light curve was searched for periodicities between 0.03 and 1 Hz, by calculating a fast-Fourier-transform power spectrum (Fig. 1). The peaks in the spectrum are centred on the fundamental frequency of 0.13375 Hz (period of 7.47655 s) and its first harmonic at 0.26750 Hz. We find no significant power in any other frequency in the searched range. The probability that we detect a signal at the fundamental frequency this strong by chance coincidence is 1 ϫ 10 Ϫ 13 (taking into account the number of trials, 1:9 ϫ 10 6 , and the probability per trial, 5 ϫ 10 Ϫ 20 ).To determine the fundamental period, all data sets were then corrected to the Solar System barycentre and separately folded at the longest detected period of 7.47655 s, and sub-harmonics thereof. These sub-harmonic folds showed...

We estimate a gamma-ray burst (GRB) formation rate based on the new relation between the spectral peak energy (E p ) and the peak luminosity. The new relation is derived by combining the data of E p and the peak luminosities by BeppoSAX and BATSE, and it looks considerably tighter and more reliable than the relations suggested by the previous works. Using the new E p -luminosity relation, we estimate redshifts of the 689 GRBs without known distances in the BATSE catalog and derive a GRB formation rate as a function of the redshift. For the redshift range of 0 z 2, the GRB formation rate increases and is well correlated with the star formation rate, while it keeps constant toward z $ 12. We also discuss the luminosity function and the redshift dependence of the intrinsic luminosity (luminosity evolution).

We report on the discovery of two emission features observed in the x-ray spectrum of the afterglow of the gamma-ray burst (GRB) of 16 December 1999 by the Chandra X-ray Observatory. These features are identified with the Ly α line and the narrow recombination continuum by hydrogenic ions of iron at a redshift z = 1.00 ± 0.02, providing an unambiguous measurement of the distance of a GRB. Line width and intensity imply that the progenitor of the GRB was a massive star system that ejected, before the GRB event, a quantity of iron ∼0.01 of the mass of the sun at a velocity ∼0.1 of the speed of light, probably by a supernova explosion.

We report the polarization measurement in prompt γ-ray emission of GRB 100826A with the Gamma-Ray Burst Polarimeter (GAP) aboard the small solar power sail demonstrator IKAROS. We detected the firm change of polarization angle (PA) during the prompt emission with 99.9 % (3.5 σ) confidence level, and the average polarization degree (Π) of 27 ± 11 % with 99.4 % (2.9 σ) confidence level. Here the quoted errors are given at 1 σ confidence level for two parameters of interest. The systematic errors have been carefully included in this analysis, unlike any previous reports. Such a high Π can be obtained in several emission models of gamma-ray bursts (GRBs), including synchrotron and photospheric models. However, it is difficult to explain the observed significant change of PA within the framework of axisymmetric jet as considered in many theoretical works. The non-axisymmetric (e.g., patchy) structures of the magnetic fields and/or brightness inside the relativistic jet are therefore required within the observable angular scale of ∼ Γ −1 . Our observation strongly indicates that the polarization measurement is a powerful tool to constrain the GRB production mechanism, and more theoretical works are needed to discuss the data in more details.

High-sensitivity wide-band X-ray spectroscopy is the key feature of the Suzaku X-ray observatory, launched on 2005 July 10. This paper summarizes the spacecraft, in-orbit performance, operations, and data processing that are related to observations. The scientific instruments, the high-throughput X-ray telescopes, X-ray CCD cameras, non-imaging hard X-ray detector are also described.

No abstract

The massive flare of 27 December 2004 from the soft gamma-ray repeater SGR 1806-20, a possible magnetar, saturated almost all gamma-ray detectors, meaning that the profile of the pulse was poorly characterized. An accurate profile is essential to determine physically what was happening at the source. Here we report the unsaturated gamma-ray profile for the first 600 ms of the flare, with a time resolution of 5.48 ms. The peak of the profile (of the order of 10(7) photons cm(-2) s(-1)) was reached approximately 50 ms after the onset of the flare, and was then followed by a gradual decrease with superposed oscillatory modulations possibly representing repeated energy injections with approximately 60-ms intervals. The implied total energy is comparable to the stored magnetic energy in a magnetar (approximately 10(47) erg) based on the dipole magnetic field intensity (approximately 10(15) G), suggesting either that the energy release mechanism was extremely efficient or that the interior magnetic field is much stronger than the external dipole field.

We collected and reanalyzed about 200 GRB data of prompt-emission with known redshift observed until the end of 2009, and selected 101 GRBs that were well-observed to have good spectral parameters in order to determine the spectral peak energy ($E_{\rm p}$), 1-second peak luminosity ($L_{\rm p}$) and isotropic energy ($E_{\rm iso}$). Using our newly constructed database with 101 GRBs, we first revised the $E_{\rm p}$–$L_{\rm p}$ and $E_{\rm p}$–$E_{\rm iso}$ correlations. The correlation coefficients of the revised correlations were 0.889 for 99 degrees of freedom for the $E_{\rm p}$–$L_{\rm p}$ correlation and 0.867 for 96 degrees of freedom for the $E_{\rm p}$–$E_{\rm iso}$ correlation. These values correspond to a chance probability of 2.18 $\times$ 10$^{-35}$ and 4.27 $\times$ 10$^{-31}$, respectively. It is a very important issue whether these tight correlations are an intrinsic property of GRBs, or are caused by some selection effect of observations. In this paper, we examine how the truncation of the detector sensitivity affects the correlations, and conclude they are surely intrinsic properties of GRBs. Next we investigate origins of the dispersion of the correlations by studying their brightness and redshift dependence. Here, the brightness (flux or fluence) dependence would be regarded as being an estimator of the bias due to the detector threshold. We found a weak fluence-dependence in the $E_{\rm p}$–$E_{\rm iso}$ correlations and a redshift dependence in the $E_{\rm p}$–$L_{\rm p}$ correlation both at the 2$\ \sigma$ statistical level. These two effects may contribute to the dispersion of the correlations, which is larger than the statistical uncertainty. We discuss a possible reason of these dependences and give a future prospect to improve the correlations.