565 research outputs found
GeV Emission from Collisional Magnetized Gamma Ray Bursts
Magnetic fields may play a dominant role in gamma-ray bursts, and recent
observations by the Fermi satellite indicate that GeV radiation, when detected,
arrives delayed by seconds from the onset of the MeV component. Motivated by
this, we discuss a magnetically dominated jet model where both magnetic
dissipation and nuclear collisions are important. We show that, for parameters
typical of the observed bursts, such a model involving a realistic jet
structure can reproduce the general features of the MeV and a separate GeV
radiation component, including the time delay between the two. The model also
predicts a multi-GeV neutrino component.Comment: ApJ(Lett.), in pres
On the High Energy Emission of the Short GRB 090510
Long-lived high-energy (>100MeV) emission, a common feature of most Fermi-LAT
detected gamma-ray burst, is detected up to \sim 10^2 s in the short GRB
090510. We study the origin of this long-lived high-energy emission, using
broad-band observations including X-ray and optical data. We confirm that the
late > 100 MeV, X-ray and optical emission can be naturally explained via
synchrotron emission from an adiabatic forward shock propagating into a
homogeneous ambient medium with low number density. The Klein-Nishina effects
are found to be significant, and effects due to jet spreading and magnetic
field amplification in the shock appear to be required. Under the constraints
from the low-energy observations, the adiabatic forward shock synchrotron
emission is consistent with the later-time (t>2s) high-energy emission, but
falls below the early-time (t < 2s) high energy emission. Thus we argue that an
extra high energy component is needed at early times. A standard reverse shock
origin is found to be inconsistent with this extra component. Therefore, we
attribute the early part of the high-energy emission (t< 2s) to the prompt
component, and the long-lived high energy emission (t>2s) to the adiabatic
forward shock synchrotron afterglow radiation. This avoids the requirement for
an extremely high initial Lorentz factor.Comment: 29 pages, 2 figures; Accepted for publication in Ap
Prospects for Detecting Gamma-Ray Bursts at Very High Energies with the Cherenkov Telescope Array
We discuss the prospects for the detection of gamma-ray bursts (GRBs) by the
Cherenkov Telescope Array (CTA), the next generation, ground-based facility of
imaging atmospheric Cherenkov telescopes (IACTs) operating above a few tens of
GeV. By virtue of its fast slewing capabilities, the lower energy threshold
compared to current IACTs, and the much larger effective area compared to
satellite instruments, CTA can measure the spectra and variability of GRBs with
excellent photon statistics at multi-GeV energies. Employing a model of the GRB
population whose properties are broadly consistent with observations by the
Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) onboard Fermi, we
simulate follow-up observations of GRBs with the Large Size Telescopes (LSTs),
the component of CTA with the fastest slew speed and the best sensitivity at
energies below a few hundred GeV. For our fiducial assumptions, we foresee that
the LSTs can detect ~0.1 GRBs per year during the prompt phase and ~0.5 per
year in the afterglow phase, considering only one array site and both GBM and
the Space-based multi-band astronomical Variable Object Monitor (SVOM) as the
alert instruments. The detection rates can be enhanced by a factor of about 5
and 6 for the prompt emission and the afterglow, respectively, assuming two
array sites with the same sensitivity and that the GBM localization error can
be reduced to less than 1 deg. The expected distribution of redshift and photon
counts are presented, showing that despite the modest event rate, hundreds or
more multi-GeV photons can be anticipated from a single burst once they are
detected. We also study how the detection rate depends on the intrinsic GRB
properties and the delay time between the burst trigger and the follow-up
observation.Comment: 15 pages, 9 figures, accepted for publication in MNRA
Single- and Two-Component GRB Spectra in the Fermi GBM-LAT Energy Range
Most Fermi GRB spectra appear as either a broken power law extending to GeV
energies or as a broken power with a separate GeV power law component. Here we
show that such spectra can be understood in terms of magnetically dominated
relativistic jets where a dissipative photosphere produces the prompt MeV
emission, which is extended into the GeV range by inverse Compton scattering in
the external shock, with possible contributions from a reverse shock as well.
The bulk Lorentz factors required in these models are in the range of 300-600,
and the MeV-GeV time delays arise naturally. In some cases an optical flash and
a sub-dominant thermal component are also present.Comment: Accepted for publication in ApJ, 31 pages, 4 figure
A Photosphere-Internal Shock Model of Gamma-Ray Bursts: Case Studies of Fermi/LAT Bursts
Radially inhomogeneous gamma-ray burst (GRB) jets release variable
photospheric emission and can have internal shocks occurring above the
photosphere. We generically formulate a photospheric emission model of GRBs
including Compton up-scattered photospheric (UP) emission off the electrons
(and positrons) in the internal shocks, and find that the photospheric emission
may correspond to the traditional (Band) component at <~1 MeV and the UP
emission to the high-energy emission observed by Fermi/LAT for some GRBs at >~
10 MeV. The two components can be separate in the spectrum in some cases or can
mimic a smooth broad Band spectrum in other cases. We apply our formulation to
the well-studied long and short LAT GRBs, GRB 080916C, GRB 090902B, and GRB
090510, and typically find reasonable parameters for fitting the time-binned
spectra, although fine tuning of several parameters is required. The observed
delays of the high-energy emission with respect to the MeV emission which are
large compared to the variability times are unlikely to be due to simple
kinematic effects of a non-evolving jet. These delays may be attributed to the
temporal evolution of the physical parameters of the jet, and thus the delay
timescales could provide a potential tool for investigating the structures of
GRB jets themselves and their progenitors. The difference of the delay
timescales of long and short GRBs inferred from the Fermi data might be due to
the differences in the progenitors of long and short GRBs. Some other
properties and consequences of this model are discussed, including temporal
correlations among the prompt optical, the soft X-ray, and the distinct
high-energy component as well as the Band component.Comment: 21 pages, 7 figures, 2 tables; accepted for publication in MNRA
Localization of the succinate receptor in the distal nephron and its signaling in polarized MDCK cells
When the succinate receptor (SUCNR1) is activated in the afferent arterioles of the glomerulus it increases renin release and induces hypertension. To study its location in other nephron segments and its role in kidney function, we performed immunohistochemical analysis and found that SUCNR1 is located in the luminal membrane of macula densa cells of the juxtaglomerular apparatus in close proximity to renin-producing granular cells, the cortical thick ascending limb, and cortical and inner medullary collecting duct cells. In order to study its signaling, SUCNR1 was stably expressed in Madin-Darby Canine Kidney (MDCK) cells, where it localized to the apical membrane. Activation of the cells by succinate caused Gq and Gi-mediated intracellular calcium mobilization, transient phosphorylation of extracellular regulated kinase (ERK)1/2 and the release of arachidonic acid along with prostaglandins E2 and I2. Signaling was desensitized without receptor internalization but rapidly resensitized upon succinate removal. Immunohistochemical evidence of phosphorylated ERK1/2 was found in cortical collecting duct cells of wild type but not SUCNR1 knockout streptozotocin-induced diabetic mice, indicating in vivo relevance. Since urinary succinate concentrations in health and disease are in the activation range of the SUCNR1, this receptor can sense succinate in the luminal fluid. Our study suggests that changes in the luminal succinate concentration may regulate several aspects of renal function
A Comprehensive Analysis of Fermi Gamma-Ray Burst Data. I. Spectral Components and Their Possible Physical Origins of LAT/GBM GRBs
We present a systematic analysis of the spectral and temporal properties of
17 GRBs co-detected by GBM and LAT on board the Fermi satellite by May 2010. We
performed a time-resolved spectral analysis of all the bursts with the finest
temporal resolution allowed by statistics, in order to avoid temporal smearing
of different spectral components. We found that the time-resolved spectra of 14
out of 17 GRBs are best modeled with the Band function over the entire Fermi
spectral range, which may suggest a common origin for emissions detected by LAT
and GBM. GRB 090902B and GRB 090510 require the superposition between an MeV
component and an extra power law component, with the former having a sharp
cutoff above E_p. For GRB 090902B, this MeV component becomes progressively
narrower as the time bin gets smaller, and can be fit with a Planck function as
the time bin becomes small enough. In general, we speculate that
phenomenologically there may be three elemental spectral components : (I) a
Band-function component (e.g. in GRB 080916C) that extends in a wide energy
range and does not narrow with reducing time bins, which may be of the
non-thermal origin; (II) a quasi-thermal component (e.g. in GRB 090902B) with
the spectra progressively narrowing with reducing time bins; and (III) another
non-thermal power law component extending to high energies. The spectra of
different bursts may be decomposed into one or more of these elemental
components. We compare this sample with the BATSE sample and investigate some
correlations among spectral parameters. We discuss the physical implications of
the data analysis results for GRB prompt emission, including jet compositions
(matter-dominated vs. Poynting-flux-dominated outflow), emission sites
(internal shock, external shock or photosphere), as well as radiation
mechanisms (synchrotron, synchrotron self-Compton, or thermal Compton
upscattering).Comment: 61 pages, 25 figures, 3 tables. 2011 ApJ in pres
Modeling the broadband emission of Fermi/LAT GRB 090902B
GRB 090902B, detected by Fermi Large Array Telescope (Fermi/LAT), shows
extend high-energy emission (>100 MeV) up to 10^3 s after the burst, which
decays with time in a power-law as t^{-1.5}. It has been also observed by
several follow-up low-energy instruments, including an early optical detection
around 5000 s after the burst. The optical emission at early time decays faster
than t^{-1.6}, which has been suspected to originate from the reverse shock. We
here explore the models that can possibly explain the the broadband afterglow
emission of GRB 090902B. We find that the reverse shock model for the early
optical emission would overpredict the radio afterglow flux that is
inconsistent with observations. A partially radiative blast wave model, which
though is able to produce a sufficiently steep decay slope, can not explain the
broadband data of GRB 090902B. The two-component jet model, which consists of a
narrow and bright jet component in the core and a surrounding wider and less
energetic jet component, is shown to be able to explain the broadband afterglow
data, including the LAT high-energy data after ~50 s and low-energy (radio,
optical and X-ray) afterglow data. The early-time high-energy emission detected
by LAT before ~50 s is likely due to internal origin as that of the sub-MeV
emission. The highest energy (33 GeV) photon of GRB090902B detected at 80 s can
be marginally accommodated within the forward shock emission under the
optimistic condition that electrons are accelerated by the Bohm diffusive
shock.Comment: Accepted for publication in ApJ (minor changes, references updated),
11 pages (emulateapj style), 4 figure
Spectral-Temporal Simulations of Internal Dissipation Models of Gamma-Ray Bursts
We present calculations of the time evolution of the prompt spectra of
gamma-ray burst models involving generic internal dissipation regions,
including internal shocks, either by itself or in the presence of an external
photon source such as a photosphere. The method uses a newly developed
time-dependent code involving synchrotron emission and absorption, inverse
Compton scattering and pair formation. The models reproduce the typical
observed Band spectra and their generic time evolution, including the
appearance of an extra keV-GeV component, whose delay in simple SSC models,
however, is only partially able to explain the several seconds observed GeV
delays. On the other hand, models involving both a photosphere and an internal
dissipation region at a larger radius produce both an extra GeV component and
time delays which are in the range of the observations.Comment: 37 pages, 15 figures, accepted for publication in The Astrophysical
Journa
Scanning electrochemical microscopy at thermal sprayed anti-corrosion coatings: effect of thermal spraying on heterogeneous electron transfer kinetics
The effect of thermal spraying on the electrochemical activity of an anti-corrosion superalloy was studied quantitatively using scanning electrochemical microscopy (SECM). The superalloy used was Inconel 625 (a Ni base superalloy) and thin coatings of the alloy were formed on mild steel using high velocity oxy-fuel (HVOF) thermal spraying. The kinetics of electron transfer (ET) across the Inconel 625 coating/electrolyte interface were studied using SECM using ferrocenemethanol as the redox mediator. For comparison, the kinetics of ET across stainless steel/electrolyte and bulk wrought Inconel 625/electrolyte interfaces were also studied using SECM. The standard heterogeneous ET rate constant, k°, for ferrocenemethanol reduction at stainless steel was 1.0 ± 0.5 × 10−3 cm s−1, compared to 2.6 ± 1.8 × 10−2 cm s−1 at the wrought Inconel 625 surface. However, at the HVOF-sprayed Inconel 625 surface, the kinetics of ET varied across the surface and k° for ferrocenemethanol reduction ranged between ∼2.2 × 10−4 cm s−1 and ∼2.6 × 10−3 cm s−1. These results clearly demonstrate that SECM can be used to quantify the effect of thermal spraying on the electrochemical properties of Inconel 625 and that thermal spraying results in an electrochemically-heterogeneous surface
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