1,684 research outputs found
Curvature Effect of a Non-Power-Law Spectrum and Spectral Evolution of GRB X-Ray Tails
The apparent spectral evolution observed in the steep decay phase of many GRB
early afterglows raises a great concern of the high-latitude "curvature effect"
interpretation of this phase. However, previous curvature effect models only
invoked a simple power law spectrum upon the cessation of the prompt internal
emission. We investigate a model that invokes the "curvature effect" of a more
general non-power-law spectrum and test this model with the Swift/XRT data of
some GRBs. We show that one can reproduce both the observed lightcurve and the
apparent spectral evolution of several GRBs using a model invoking a power-law
spectrum with an exponential cut off. GRB 050814 is presented as an example.Comment: 12 pages, 4 figures, 1 table, . Accepted for publication in ApJ
Letter
A New Classification Method for Gamma-Ray Bursts
Recent Swift observations suggest that the traditional long vs. short GRB
classification scheme does not always associate GRBs to the two physically
motivated model types, i.e. Type II (massive star origin) vs. Type I (compact
star origin). We propose a new phenomenological classification method of GRBs
by introducing a new parameter epsilon=E_{gamma, iso,52}/E^{5/3}_{p,z,2}, where
E_{\gamma,iso} is the isotropic gamma-ray energy (in units of 10^{52} erg), and
E_{p,z} is the cosmic rest frame spectral peak energy (in units of 100 keV).
For those short GRBs with "extended emission", both quantities are defined for
the short/hard spike only. With the current complete sample of GRBs with
redshift and E_p measurements, the epsilon parameter shows a clear bimodal
distribution with a separation at epsilon ~ 0.03. The high-epsilon region
encloses the typical long GRBs with high-luminosity, some high-z
"rest-frame-short" GRBs (such as GRB 090423 and GRB 080913), as well as some
high-z short GRBs (such as GRB 090426). All these GRBs have been claimed to be
of the Type II origin based on other observational properties in the
literature. All the GRBs that are argued to be of the Type I origin are found
to be clustered in the low-epsilon region. They can be separated from some
nearby low-luminosity long GRBs (in 3sigma) by an additional T_{90} criterion,
i.e. T_{90,z}<~ 5 s in the Swift/BAT band. We suggest that this new
classification scheme can better match the physically-motivated Type II/I
classification scheme.Comment: 7 pages, including 4 figures and 1 Table, minor revision, accepted
for publication in Ap
A Comprehensive Study of Gamma-Ray Burst Optical Emission: I. Flares and Early Shallow Decay Component
Well-sampled optical lightcurves of 146 gamma-ray bursts (GRBs) are compiled
from the literature. By empirical fitting we identify eight possible emission
components and summarize the results in a "synthetic" lightcurve. Both optical
flare and early shallow-decay components are likely related to long-term
central engine activities. We focus on their statistical properties in this
paper. Twenty-four optical flares are obtained from 19 GRBs. The isotropic
R-band energy is smaller than 1% of . The relation between
isotropic luminosities of the flares and gamma-rays follows . Later flares tend to be
wider and dimmer, i.e., and . The detection
probability of the optical flares is much smaller than that of X-ray flares. An
optical shallow decay segment is observed in 39 GRBs. The relation between the
break time and break luminosity is a power-law, with an index of , similar to that derived from X-ray flares. The X-ray and optical breaks
are usually chromatic, but a tentative correlation is found. We suggest that
similar to the prompt optical emission that tracks -rays, the optical
flares are also related to the erratic behavior of the central engine. The
shallow decay component is likely related to a long-lasting spinning-down
central engine or piling up of flare materials onto the blastwave. Mixing of
different emission components may be the reason of the diverse chromatic
afterglow behaviors.Comment: 43 pages, 13 figures, 3 tables, accepted for publication in Ap
Constraining GRB Initial Lorentz Factor with the Afterglow Onset Feature and Discovery of a Tight Gamma_0-E_iso Correlation
The onset of GRB afterglow is characterized by a smooth bump in the early
afterglow lightcurve. We make an extensive search for such a feature. Twenty
optically selected GRBs and 12 X-ray selected GRBs are found, among which 17
optically selected GRBs and 2 X-ray-selected GRBs have redshift measurements.
We fit the lightcurves with a smooth broken power-law and measure the temporal
characteristic timescales of the bumps at FWHM. Strong mutual correlations
among these timescales are found, and a dimmer and broader bump tends to peak
at a later peak time. The ratio of rising to decaying timescales is almost
universal among bursts, but the ratio of the rising time to the peak time
varies from 0.3~1. The E_iso is tightly correlated with the peak luminosity and
the peak time of the bump in the burst frame. Assuming that the bumps signal
the deceleration of the GRB fireballs in a constant density medium, we
calculate the initial Lorentz factor (Gamma_0) and the deceleration radius
(R_dec) of the GRBs in the optical-selected sample. It is found that Gamma_0
are typically a few hundreds, and the typical deceleration radius is
R_dec~10^{17} cm. More intriguingly, a tight correlation between the Gamma_0
and E_iso is found, namely Gamma_0 ~ 195 E_iso, 52}^{0.27} (satisfied for both
the optical and X-ray z-known samples). It is helpful to understand GRB
physics, and may serve as an indicator of Gamma_0. We find that the early
bright X-rays are usually dominated by a different component from the external
shock emission, but occasionally (for one case) an achromatic deceleration
feature is observed. Components in X-rays would contribute to the diversity of
the observed X-ray lightcurves (abridge).Comment: 15 pages, including 4 tables and 7 figures, Submitted to Ap
Coulomb excitation of Ni at safe energies
The value in Ni has been measured using Coulomb
excitation at safe energies. The Ni radioactive beam was
post-accelerated at the ISOLDE facility (CERN) to 2.9 MeV/u. The emitted
rays were detected by the MINIBALL detector array. A kinematic
particle reconstruction was performed in order to increase the measured c.m.
angular range of the excitation cross section. The obtained value of
2.8 10 efm is in good agreement with the value
measured at intermediate energy Coulomb excitation, confirming the low
transition probability.Comment: 4 pages, 5 figure
Selection Effects on the Observed Redshift Dependence of GRB Jet Opening Angles
Apparent redshift dependence of the jet opening angles () of
gamma-ray bursts (GRBs) is observed from current GRB sample. We investigate
whether this dependence can be explained with instrumental selection effects
and observational biases by a bootstrapping method. Assuming that (1) the GRB
rate follows the star formation history and the cosmic metallicity history and
(2) the intrinsic distributions of the jet-corrected luminosity () and are a Gaussian or a power-law function, we
generate a mock {\em Swift}/BAT sample by considering various instrumental
selection effects, including the flux threshold and the trigger probability of
BAT, the probabilities of a GRB jet pointing to the instrument solid angle and
the probability of redshift measurement. Our results well reproduce the
observed dependence. We find that in case of
good consistency between the mock and
observed samples can be obtained, indicating that both and
are degenerate for a flux-limited sample. The parameter set
gives the best consistency for the current {\em Swift} GRB sample.
Considering the beaming effect, the derived intrinsic local GRB rate
accordingly is Gpc yr, inferring that of Type Ib/c SNe may be accompanied by a GRB.Comment: 25pages, 7 figures. ApJ in pres
Simulations on High-z Long Gamma-Ray Burst Rate
Since the launch of Swift satellite, the detections of high-z (z>4) long
gamma-ray bursts (LGRBs) have been rapidly growing, even approaching the very
early Universe (the record holder currently is z=8.3). The observed high-z LGRB
rate shows significant excess over that estimated from the star formation
history. We investigate what may be responsible for this high productivity of
GRBs at high-z through Monte Carlo simulations, with effective Swif/BAT trigger
and redshift detection probabilities based on current Swift/BAT sample and
CGRO/BATSE LGRB sample. We compare our simulations to the Swift observations
via log N-log P, peak luminosity (L) and redshift distributions. In the case
that LGRB rate is purely proportional to the star formation rate (SFR), our
simulations poorly reproduce the LGRB rate at z>4, although the simulated log
N-log P distribution is in good agreement with the observed one. Assuming that
the excess of high-z GRB rate is due to the cosmic metallicity evolution or
unknown LGRB rate increase parameterized as (1+z)^delta, we find that although
the two scenarios alone can improve the consistency between our simulations and
observations, incorporation of them gives much better consistency. We get
0.2<epsilon<0.6 and delta<0.6, where epsilon is the metallicity threshold for
the production of LGRBs. The best consistency is obtained with a parameter set
(epsilon, delta)=(~0.4, ~0.4), and BAT might trigger a few LGRBs at z~14. With
increasing detections of GRBs at z>4 (~15% of GRBs in current Swift LGRB sample
based on our simulations), a window for very early Universe is opening by Swift
and up-coming SVOM missions.Comment: 9 pages, including 8 figures and 1 table, one more figure added.
Accepted for publication in MNRA
Synergies for Improving Oil Palm Production and Forest Conservation in Floodplain Landscapes
Lowland tropical forests are increasingly threatened with conversion to oil palm as global demand and high profit drives crop expansion throughout the world’s tropical regions. Yet, landscapes are not homogeneous and regional constraints dictate land suitability for this crop. We conducted a regional study to investigate spatial and economic components of forest conversion to oil palm within a tropical floodplain in the Lower Kinabatangan, Sabah, Malaysian Borneo. The Kinabatangan ecosystem harbours significant biodiversity with globally threatened species but has suffered forest loss and fragmentation. We mapped the oil palm and forested landscapes (using object-based-image analysis, classification and regression tree analysis and on-screen digitising of high-resolution imagery) and undertook economic modelling. Within the study region (520,269 ha), 250,617 ha is cultivated with oil palm with 77% having high Net-Present-Value (NPV) estimates (637/ha?yr); but 20.5% is under-producing. In fact 6.3% (15,810 ha) of oil palm is commercially redundant (with negative NPV of -65/ha?yr) due to palm mortality from flood inundation. These areas would have been important riparian or flooded forest types. Moreover, 30,173 ha of unprotected forest remain and despite its value for connectivity and biodiversity 64% is allocated for future oil palm. However, we estimate that at minimum 54% of these forests are unsuitable for this crop due to inundation events. If conversion to oil palm occurs, we predict a further 16,207 ha will become commercially redundant. This means that over 32,000 ha of forest within the floodplain would have been converted for little or no financial gain yet with significant cost to the ecosystem. Our findings have globally relevant implications for similar floodplain landscapes undergoing forest transformation to agriculture such as oil palm. Understanding landscape level constraints to this crop, and transferring these into policy and practice, may provide conservation and economic opportunities within these seemingly high opportunity cost landscapes
Low-Luminosity Gamma-Ray Bursts as a Distinct GRB Population:A Firmer Case from Multiple Criteria Constraints
The intriguing observations of Swift/BAT X-ray flash XRF 060218 and the
BATSE-BeppoSAX gamma-ray burst GRB 980425, both with much lower luminosity and
redshift compared to other observed bursts, naturally lead to the question of
how these low-luminosity (LL) bursts are related to high-luminosity (HL)
bursts. Incorporating the constraints from both the flux-limited samples
observed with CGRO/BATSE and Swift/BAT and the redshift-known GRB sample, we
investigate the luminosity function for both LL- and HL-GRBs through
simulations. Our multiple criteria, including the log N - log P distributions
from the flux-limited GRB sample, the redshift and luminosity distributions of
the redshift-known sample, and the detection ratio of HL- and LL- GRBs with
Swift/BAT, provide a set of stringent constraints to the luminosity function.
Assuming that the GRB rate follows the star formation rate, our simulations
show that a simple power law or a broken power law model of luminosity function
fail to reproduce the observations, and a new component is required. This
component can be modeled with a broken power, which is characterized by a sharp
increase of the burst number at around L < 10^47 erg s^-1}. The lack of
detection of moderate-luminosity GRBs at redshift ~0.3 indicates that this
feature is not due to observational biases. The inferred local rate, rho_0, of
LL-GRBs from our model is ~ 200 Gpc^-3 yr^-1 at ~ 10^47 erg s^-1, much larger
than that of HL-GRBs. These results imply that LL-GRBs could be a separate GRB
population from HL-GRBs. The recent discovery of a local X-ray transient
080109/SN 2008D would strengthen our conclusion, if the observed non-thermal
emission has a similar origin as the prompt emission of most GRBs and XRFs.Comment: 22 pages, 9 figures, 3 tables; MNRAS, in press; Updated analysis and
figure
Decomposition cross-correlation for analysis of collagen matrix deformation by single smooth muscle cells
Microvascular remodeling is known to depend on cellular interactions with matrix tissue. However, it is difficult to study the role of specific cells or matrix elements in an in vivo setting. The aim of this study is to develop an automated technique that can be employed to obtain and analyze local collagen matrix remodeling by single smooth muscle cells. We combined a motorized microscopic setup and time-lapse video microscopy with a new cross-correlation based image analysis algorithm to enable automated recording of cell-induced matrix reorganization. This method rendered 60–90 single cell studies per experiment, for which collagen deformation over time could be automatically derived. Thus, the current setup offers a tool to systematically study different components active in matrix remodeling
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