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 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.

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.

THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5-1 arcmin localization, an energy band extending from several MeV down to 0.3 keV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) followup with a 0.7 m class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift ∼10, signatures of Pop III stars, sources and physics of reionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA).