Efficiency Crisis of Swift Gamma-Ray Bursts with Shallow X-ray Afterglows: Prior Activity or Time-Dependent Microphysics?

Most X-ray afterglows of gamma-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase t^{-1/2} in the first few hours. This is not predicted by the standard afterglow model and needs an explanation. We discuss that the shallow decay requires an unreasonably high gamma-ray efficiency, >75-90%, within current models, which is difficult to produce by internal shocks. Such a crisis may be avoided if a weak relativistic explosion occurs ~10^3-10^6 s prior to the main burst or if the microphysical parameter of the electron energy increases during the shallow decay, \epsilon_e ~ t^{1/2}. The former explanation predicts a very long precursor, while both prefer dim optical flashes from the reverse shock, as was recently reported. We also calculate the multi-wavelength afterglows and compare them with observations. No optical break at the end of the shallow X-ray decay indicates a preference for the time-dependent microphysics model with additionally decaying magnetic fields, \epsilon_B ~ t^{-0.6}.Comment: 7 pages, 1 figure, accepted for publication in A&

Radio Afterglows of Gamma-Ray Bursts and Hypernovae at High Redshift, and their Potential for 21-cm Absorption Studies

We investigate the radio afterglows of gamma-ray bursts (GRBs) and hypernovae (HNe) at high redshifts and quantify their detectability, as well as their potential usefulness for 21 cm absorption line studies of the intergalactic medium (IGM) and intervening structures. We examine several sets of source and environment model parameters that are physically plausible at high redshifts. The radio afterglows of GRBs would be detectable out to z ~ 30, while the energetic HNe could be detectable out to z ~ 20 even by the current Very Large Array (VLA). We find that the 21 cm absorption line due to the diffuse neutral IGM is difficult to detect even by the proposed Square Kilometer Array (SKA), except for highly energetic sources. We also find that the 21 cm line due to collapsed gas clouds with high optical depth may be detected on rare occasions.Comment: 36 pages, 12 figures, 1 table, accepted for publication in Ap

Risk factors for intrahepatic cholangiocarcinoma: a possible role of hepatitis B virus

There are several established risk factors for intrahepatic cholangiocarcinoma (ICC), namely primary sclerosing cholangitis, fibropolycystic liver disease, parasitic infection, intrahepatic biliary stones and chemical carcinogen exposure. However, the majority of patients with ICC do not have any of these risk factors. Therefore, identification of other risk factors is warranted for the prevention and early detection of ICC. We evaluated the risk factors for ICC in a large-scale cohort study in the province of Osaka, Japan. This retrospective cohort study included 154,814 apparently healthy individual blood donors, aged 40–64 years at the time of blood donation in the period 1991–1993. The average observation period was 7.6 years, resulting in 1.25 million person-years of observation. Incident ICC cases were identified by linking the blood-donor database to the records in the population-based cancer registry for the province. There were 11 incident ICC cases during follow-up, with an incidence rate of 0.88 per 100 000 person-years. Compared with subjects aged 40–49 years, the subjects aged 50–54 years and 55–59 years had a significantly higher risk for ICC (hazard ratio [HR] = 5.90; 95%CI:1.08–32.31 and 11.07; 95%CI:1.98–61.79, respectively). Compared with those with ALT level of 19 Karmen Units (KU) or less, subjects with ALT level of 40 KU or higher had a significantly higher risk for ICC (HR: 8.30; 95%CI:1.47–46.83). Compared with those who tested negative for both HBsAg and anti-HCV, those who tested HBsAg-positive had a significantly higher risk for ICC (HR: 8.56; 95%CI: 1.33–55.20). Our results suggest that HBV infection and liver inflammation are independently associated with ICC development. These findings need to be verified by further large cohort studies

GRB Formation Rates inferred from the Spectral Peak Energy - Luminosity Relation

We investigate the GRB formation rate based on the relation between the spectral peak energy ($E_{p}$) and the isotropic luminosity. The $E_{p}$--luminosity relation covers the energy range of 50 -- 2000 keV and the luminosity range of $10^{50}$--$10^{54} erg/s, respectively. We find that the relation is considerably tighter than similar relations suggested previously. Using$E_{p}$--luminosity relation, we estimate the luminosity and the redshift of 684 GRBs with the unknown distances and derive the GRB formation rate as a function of the redshift. For$0 \le z \le 2$, the GRB formation rate is well correlated with the star formation rate while it increases monotonously from$z\sim 2$out to$z \sim 12$. This behavior is consistent with the results of previous works using the lag--luminosity relation or the variability--luminosity relation.Comment: 13 pages, 4 figures, Submitted to ApJ

Circular Polarization from Gamma-ray Burst Afterglows

We investigate the circular polarization (CP) from Gamma-Ray Burst (GRB) afterglows. We show that a tangled magnetic field cannot generate CP without an ordered magnetic field because there is always an oppositely directed field, so that no handedness exists. This implies the observation of CP could be a useful probe of an ordered field, which carries valuable information on the GRB central engine. By solving the transfer equation of polarized radiation, we find that the CP reaches 1% at radio frequencies and 0.01% at optical for the forward shock, and 10-1% at radio and 0.1-0.01% at optical for the reverse shock.Comment: 12 pages, 3 figure

Simulating Cherenkov Telescope Array observation of RX J1713.7-3946

We perform simulations of Cherenkov Telescope Array (CTA) observations of a young supernova remnant RX J1713.7-3946. This target is not only one of the brightest sources ever discovered in very high-energy gamma rays but also well observed in other wavebands. In X-rays, the emission is dominated by synchrotron radiation, which links directly to the existence of high-energy electrons. Radio observations of CO and HI gas have revealed a highly inhomogeneous medium surrounding the SNR, such as clumpy molecular clouds. Therefore gamma rays from hadronic interactions are naturally expected. However, the spectrum in GeV energy range measured by Fermi/LAT indicates more typical of leptonic emission from accelerated electrons. Despite lots of multi-wavelength information, the competing interpretations have led to much uncertainty in the quest of unraveling the true origin of the gamma-ray emission from RX~J1713.7--3946. CTA will achieve highest performance ever in sensitivity, angular resolution, and energy resolution. We estimate CTA capability to examine the emission mechanisms of the gamma rays through simulated spatial distribution, spectra, and their time variation.Comment: 8 pages, 3 figures. In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions at arXiv:1508.0589

POET: POlarimeters for Energetic Transients

POET (Polarimeters for Energetic Transients) is a Small Explorer mission concept proposed to NASA in January 2008. The principal scientific goal of POET is to measure GRB polarization between 2 and 500 keV. The payload consists of two wide FoV instruments: a Low Energy Polarimeter (LEP) capable of polarization measurements in the energy range from 2-15 keV and a high energy polarimeter (Gamma-Ray Polarimeter Experiment -- GRAPE) that will measure polarization in the 60-500 keV energy range. Spectra will be measured from 2 keV up to 1 MeV. The POET spacecraft provides a zenith-pointed platform for maximizing the exposure to deep space. Spacecraft rotation will provide a means of effectively dealing with systematics in the polarization response. POET will provide sufficient sensitivity and sky coverage to measure statistically significant polarization for up to 100 GRBs in a two-year mission. Polarization data will also be obtained for solar flares, pulsars and other sources of astronomical interest

Cosmological Constraints from calibrated Yonetoku and Amati relation implies Fundamental plane of Gamma-ray bursts

We consider two empirical relations using data only from the prompt emission of Gamma-Ray Bursts (GRBs), peak energy ($E_p$) - peak luminosity ($L_p$) relation (so called Yonetoku relation) and $E_p$-isotropic energy ($E_{\rm iso}$) relation (so called Amati relation). We first suggest the independence of the two relations although they have been considered similar and dependent. From this viewpoint, we compare constraints on cosmological parameters, $\Omega_m$ and $\Omega_{\Lambda}$, from the Yonetoku and Amati relations calibrated by low-redshift GRBs with $z < 1.8$. We found that they are different in 1-$\sigma$ level, although they are still consistent in 2-$\sigma$ level. This and the fact that both Amati and Yonetoku relations have systematic errors larger than statistical errors suggest the existence of a hidden parameter of GRBs. We introduce the luminosity time $T_L$ defined by $T_L\equiv E_{\rm iso}/L_p$ as a hidden parameter to obtain a generalized Yonetoku relation as $(L_p/{10^{52} \rm{erg s^{-1}}}) = 10^{-3.88\pm0.09}(E_p/{\rm{keV}})^{1.84\pm0.04} (T_L/{\rm{s}})^{-0.34\pm0.04}$. The new relation has much smaller systematic error, 30%, and can be regarded as "Fundamental plane" of GRBs. We show a possible radiation model for this new relation. Finally we apply the new relation for high-redshift GRBs with $1.8 < z < 5.6$ to obtain $(\Omega_m,\Omega_{\Lambda}) = (0.16^{+0.04}_{-0.06},1.20^{+0.03}_{-0.09})$, which is consistent with the concordance cosmological model within 2-$\sigma$ level.Comment: 5 pages, 6 figures, published in JCA