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We perform Monte Carlo simulations to study Ep-Eiso correlation in the context of a multiple subjet model (or inhomogeneous jet model) for gamma-ray bursts (GRBs), X-ray--rich GRBs (XRRs), and X-ray flashes (XRFs). For a single subjet, we find that Ep prop. Eiso^0.4 for large viewing angles. For the multiple subjet model in which all the subjets have the same intrinsic properties, off-axis events show Ep prop. Eiso^a with 0.4, Comment: 9 pages, 7 figures; accepted for publication in ApJ

A galactic supernova remnant (SNR) Vela Jr. (RX J0852.0$-$4622, G266.6$-$1.2) shows sharp filamentary structure on the north-western edge of the remnant in the hard X-ray band. The filaments are so smooth and located on the most outer side of the remnant. We measured the averaged scale width of the filaments ($w_u$ and $w_d$) with excellent spatial resolution of {\it Chandra}, which are in the order of the size of the point spread function of {\it Chandra} on the upstream side and 49.5 (36.0--88.8) arcsec on the downstream side, respectively. The spectra of the filaments are very hard and have no line-like structure, and were well reproduced with an absorbed power-law model with $\Gamma = $2.67 (2.55--2.77), or a {\tt SRCUT} model with $\nu_{rolloff}$ = 4.3 (3.4--5.3)$\times 10^{16}$ Hz under the assumption of $p=0.3$. These results imply that the hard X-rays are synchrotron radiation emitted by accelerated electrons, as mentioned previously. Using a correlation between a function ${\cal B} \equiv \nu_{rolloff}/w_d^2$ and the SNR age, we estimated the distance and the age of Vela Jr.: the estimated distance and age are 0.33 (0.26--0.50) kpc and 660 (420--1400) years, respectively. These results are consistent with previous reports, implying that ${\cal B}$--age relation may be a useful tool to estimate the distance and the age of synchrotron X-ray emitting SNRs., Comment: 19 pages, 8 figures, ApJ, in press

We study the distribution of the durations of gamma-ray bursts (GRBs) in the unified model of short and long GRBs recently proposed by Yamazaki, Ioka, and Nakamura. Monte Carlo simulations show clear bimodal distributions, with lognormal-like shapes for both short and long GRBs, in a power-law as well as a Gaussian angular distribution of the subjets. We find that the bimodality comes from the existence of the discrete emission regions (subjets or patchy shells) in the GRB jet. To explain other temporal properties of short and long GRBs, the subjet parameters should depend on the angle in the whole jet., Comment: 5 pages, 8 figures, expanded discussion on the distribution of "on time" durations, references added, accepted for publication in ApJ

Using a simple uniform jet model of prompt emissions of gamma-ray bursts (GRBs), we reproduce the observed peak energy--isotropic energy relation. A Monte Carlo simulation shows that the low-isotropic energy part of the relation is dominated by events viewed from off-axis directions, and the number of the off-axis events is about one-third of the on-axis emissions. We also compute the observed event rates of the GRBs, the X-ray-rich GRBs, and the X-ray flashes detected by HETE-2, and we find that they are similar., Comment: 11 pages, 2 eps figures

金沢大学大学院自然科学研究科物理学, Based on the gamma-ray burst event rate at redshifts of 4 ≤ z ≤ 12, which is assessed by the spectral peak energy-to-luminosity relation recently found by Yonetoku et al., we observationally derive the star formation rate (SFR) for Population III (Pop. III) stars in a high-redshift universe. As a result, we find that Pop. III stars could form continuously at 4 ≤ z ≤ 12. Using the derived Pop. III SFR, we attempt to estimate the ultraviolet (UV) photon emission rate at 7 ≤ z ≤ 12 in which redshift range no observational information has been hitherto obtained on ionizing radiation intensity. We find that the UV emissivity at 7 ≤ z ≤ 12 can make a noticeable contribution to the early reionization. The maximal emissivity is higher than the level required to keep ionizing the intergalactic matter (IGM) at 7 ≤ z ≤ 12. However, if the escape fraction of ionizing photons from Pop. III objects is smaller than 10%, then the IGM can be neutralized at some redshift, which may lead to the double reionization. As for the enrichment, the ejection of all metals synthesized in Pop. III objects is marginally consistent with the IGM metallicity, although the confinement of metals in Pop. III objects can reduce the enrichment significantly. © 2005. The American Astronomical Society. All rights reserved.

Taking into account the recent suggestion that a short gamma-ray burst (GRB) looks like the first 1 sec of a long GRB, we propose that the jet of a GRB consists of multiple sub-jets or sub-shells (i.e., an inhomogeneous jet model). The multiplicity of the sub-jets along a line of sight n_s is an important parameter. If n_s is large (>> 1) the event looks like a long GRB, while if n_s is small (~1) the event looks like a short GRB. If our line of sight is off-axis to any sub-jets, the event looks like an X-ray flash or an X-ray rich GRB. The log-normal distribution of durations of short and long GRBs are also suggested in the same model., 13 pages, 4 eps figures

金沢大学大学院自然科学研究科物理学, We estimate a gamma-ray burst (GRB) formation rate based on the new relation between the spectral peak energy (Ep) and the peak luminosity. The new relation is derived by combining the data of Ep 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 Ep-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).

Using the near-infrared (NIR) observations of the afterglows of the high redshift ($5 \la z \la 25$) gamma-ray bursts (GRBs) which will be detected by the {\it Swift} satellite, we discuss a way to study the cosmic reionization history. In principle the details of the cosmic reionization history are well imprinted in the NIR spectra of the GRB afterglows. However the spectroscopy with a space telescope is required to obtain such an information for a very high redshift ($z\ga 15$) unless the neutral fraction of the high-$z$ universe is less than $10^{-6}$. The broad-band photometry has the higher sensitivity than that of the spectroscopy, so that the NIR photometric follow-up of the GRB afterglows is very promising to examine the cosmic reionization history. A few minutes exposure with a 8-m class ground-based telescope of the afterglow of the high-$z$ GRBs will reveal how many times the reionization occurred in the universe., Comment: 14 pages (emulateapj; 6 figures), ApJ submitted (revised version)