519 research outputs found
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 40â64 years at the time of blood donation in the period 1991â1993. 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 40â49 years, the subjects aged 50â54 years and 55â59 years had a significantly higher risk for ICC (hazard ratio [HR] = 5.90; 95%CI:1.08â32.31 and 11.07; 95%CI:1.98â61.79, 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:1.47â46.83). 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: 1.33â55.20). 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 () and the isotropic luminosity. The
--luminosity relation covers the energy range of 50 -- 2000 keV and the
luminosity range of --E_{p}0 \le z \le 2z\sim 2z \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 () - peak luminosity ()
relation (so called Yonetoku relation) and -isotropic energy () 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,
and , from the Yonetoku and Amati relations
calibrated by low-redshift GRBs with . We found that they are
different in 1- level, although they are still consistent in 2-
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 defined by as a hidden parameter to obtain a generalized Yonetoku
relation as . 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 to obtain
, which is consistent with the
concordance cosmological model within 2- level.Comment: 5 pages, 6 figures, published in JCA
- âŠ