289 research outputs found
Non-Equilibrium Ionization States of GRB Environments
Iron spectral features are thought to be the best tracer of a progenitor of
gamma-ray bursts (GRBs). The detections of spectral features such as an iron
line and/or a Radiative Recombination edge and Continuum (RRC) were reported in
four X-ray afterglows of GRBs. However their properties were different each
other burst by burst. For example, Chandra observation of GRB 991216 reported
both the strong H-like iron line together with its RRC. On the contrary,
Yoshida et al. (2001) report only a detection of the strong RRC in GRB 970828
with ASCA. Since it is difficult to produce the strong RRC, we have to consider
special condition for the line and/or the RRC forming region. In this paper, we
point out a possibility of a ``non-equilibrium ionization state'' for the line
and the RRC forming region.Comment: 10pages, 2figures. Accepted for ApJL. This is a companion paper by
A.Yoshida et. a
Strict Limit on CPT Violation from Polarization of Gamma-Ray Bursts
We report the strictest observational verification of CPT invariance in the
photon sector, as a result of gamma-ray polarization measurement of distant
gamma-ray bursts (GRBs), which are brightest stellar-size explosions in the
universe. We detected the gamma-ray polarization of three GRBs with high
significance, and the source distances may be constrained by a well-known
luminosity indicator for GRBs. For the Lorentz- and CPT-violating dispersion
relation E_{\pm}^2=p^2 \pm 2\xi p^3/M_{Pl}, where \pm denotes different
circular polarization states of the photon, the parameter \xi is constrained as
|\xi|<O(10^{-15}). Barring precise cancellation between quantum gravity effects
and dark energy effects, the stringent limit on the CPT-violating effect leads
to the expectation that quantum gravity presumably respects the CPT invariance.Comment: 4 pages; accepted for publication in Physical Review Letters;
redshift estimates of GRBs changed (i.e z=0.382 was wrong for GRB 110721A)
and calculations of \xi limit improved from the previous versio
GRB formation rate derived by the Ep-luminosity relation
We estimate a GRB formation rate based on the new relation between the spectral peak energy (Ep) and the peak luminosity. This relation is derived 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
The rapid decline of the prompt emission in Gamma-Ray Bursts
Many gamma ray bursts (GRBs) have been observed with the Burst-Alert and
X-Ray telescopes of the Swift satellite. The successive `pulses' of these GRBs
end with a fast decline and a fast spectral softening, until they are overtaken
by another pulse, or the last pulse's decline is overtaken by a less
rapidly-varying `afterglow'. The fast decline-phase has been attributed, in the
currently-explored standard fireball model of GRBs, to `high-latitude'
synchrotron emission from a collision of two conical shells. This high latitude
emission does not explain the observed spectral softening. In contrast, the
temporal behaviour and the spectral evolution during the fast-decline phase
agree with the predictions of the cannonball model of GRBs.Comment: Four added figures comparing the evolution of the inferred effective
photon spectral index during the fast decline phase of the prompt emission in
14 selected Swift GRBS and the cannonball (CB) model predictio
The Reionization History and Early Metal Enrichment inferred from the Gamma-Ray Burst Rate
Based on the gamma-ray burst (GRB) event rate at redshifts of , 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 Pop III stars in a high redshift universe. As a result, we find
that Pop III stars could form continuously at . Using the
derived Pop III SFR, we attempt to estimate the ultraviolet (UV) photon
emission rate at in which redshift range no observational
information has been hitherto obtained on ionizing radiation intensity. We find
that the UV emissivity at can make a noticeable contribution
to the early reionization. The maximal emissivity is higher than the level
required to keep ionizing the intergalactic matter at .
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.Comment: 12 pages, 2 figures, ApJL accepte
Evidence for a Molecular Cloud Origin for Gamma-Ray Bursts: Implications for the Nature of Star Formation in the Universe
It appears that the majority of rapidly-, well-localized gamma-ray bursts
with undetected, or dark, optical afterglows, or `dark bursts' for short, occur
in clouds of size R > 10L_{49}^{1/2} pc and mass M > 3x10^5L_{49} M_{sun},
where L is the isotropic-equivalent peak luminosity of the optical flash. We
show that clouds of this size and mass cannot be modeled as a gas that is bound
by pressure equilibrium with a warm or hot phase of the interstellar medium
(i.e., a diffuse cloud): Such a cloud would be unstable to gravitational
collapse, resulting in the collapse and fragmentation of the cloud until a
burst of star formation re-establishes pressure equilibrium within the
fragments, and the fragments are bound by self-gravity (i.e., a molecular
cloud). Consequently, dark bursts probably occur in molecular clouds, in which
case dark bursts are probably a byproduct of this burst of star formation if
the molecular cloud formed recently, and/or the result of lingering or latter
generation star formation if the molecular cloud formed some time ago. We then
show that if bursts occur in Galactic-like molecular clouds, the column
densities of which might be universal, the number of dark bursts can be
comparable to the number of bursts with detected optical afterglows: This is
what is observed, which suggests that the bursts with detected optical
afterglows might also occur in molecular clouds. We confirm this by modeling
and constraining the distribution of column densities, measured from absorption
of the X-ray afterglow, of the bursts with detected optical afterglows: We find
that this distribution is consistent with the expectation for bursts that occur
in molecular clouds, and is not consistent with the expectation for bursts that
occur in diffuse clouds. More...Comment: Accepted to The Astrophysical Journal, 22 pages, 6 figures, LaTe
Precursor Plerionic Activity and High Energy Gamma-Ray Emission in the Supranova Model of Gamma-Ray Bursts
The supranova model of gamma-ray bursts (GRBs), in which the GRB event is
preceded by a supernova (SN) explosion by a few months to years, has recently
gained support from Fe line detections in X-ray afterglows. A crucial
ingredient of this model yet to be studied is the fast-rotating pulsar that
should be active during the time interval between the SN and the GRB, driving a
powerful wind and a luminous plerionic nebula. We discuss some observational
consequences of this precursor plerion, which should provide important tests
for the supranova model: 1) the fragmentation of the outlying SN ejecta
material by the plerion and its implications for Fe line emission; and 2) the
effect of inverse Compton cooling and emission in the GRB external shock due to
the plerion radiation field. The plerion-induced inverse Compton emission can
dominate in the GeV-TeV energy range during the afterglow, being detectable by
GLAST from redshifts and distinguishable from self-Compton
emission by its spectrum and light curve. The prospects for direct detection
and identification of the precursor plerion emission are also briefly
considered.Comment: ApJ vol.583, in pres
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
Gamma-Ray Burst Polarimeter - GAP - aboard the Small Solar Power Sail Demonstrator IKAROS
The small solar power sail demonstrator "IKAROS" is a Japanese engineering
verification spacecraft launched by H-IIA rocket on May 21, 2010 at JAXA
Tanegashima Space Center. IKAROS has a huge sail with 20 m in diameter which is
made of thin polyimide membrane. This sail converts the solar
radiation-pressure into the propulsion force of IKAROS and accelerates the
spacecraft. The Gamma-Ray Burst Polarimeter (GAP) aboard IKAROS is the first
polarimeter to observe the gamma-ray polarization of Gamma-Ray Bursts (GRBs)
during the IKAROS cruising phase. GAP is a tinny detector of 3.8 kg in weight
and 17 cm in size with an energy range between 50-300 keV. The GAP detector
also plays a role of the interplanetary network (IPN) to determine the GRB
direction. The detection principle of gamma-ray polarization is the anisotropy
of the Compton scattering. GAP works as the GRB polarimeter with the full
coincidence mode between the central plastic and the surrounding CsI detectors.
GAP is the first instrument, devoted for the observation of gamma-ray
polarization in the astronomical history. In this paper, we present the GAP
detector and its ground and onboard calibrations.Comment: Submitted to Publications of the Astronomical Society of Japan
(PASJ), 23 pages, 14 figure
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