8,431 research outputs found
Afterglow upper limits for four short duration, hard spectrum gamma-ray bursts
We present interplanetary network localization, spectral, and time history
information for four short-duration, hard spectrum gamma-ray bursts, GRB000607,
001025B, 001204, and 010119. All of these events were followed up with
sensitive radio and optical observations (the first and only such bursts to be
followed up in the radio to date), but no detections were made, demonstrating
that the short bursts do not have anomalously intense afterglows. We discuss
the upper limits, and show that the lack of observable counterparts is
consistent both with the hypothesis that the afterglow behavior of the short
bursts is like that of the long duration bursts, many of which similarly have
no detectable afterglows, as well as with the hypothesis that the short bursts
have no detectable afterglows at all. Small number statistics do not allow a
clear choice between these alternatives, but given the present detection rates
of various missions, we show that progress can be expected in the near future.Comment: 19 pages, 4 figures; Revised version, accepted by the Astrophysical
Journa
Entanglement entropy of two disjoint intervals in c=1 theories
We study the scaling of the Renyi entanglement entropy of two disjoint blocks
of critical lattice models described by conformal field theories with central
charge c=1. We provide the analytic conformal field theory result for the
second order Renyi entropy for a free boson compactified on an orbifold
describing the scaling limit of the Ashkin-Teller (AT) model on the self-dual
line. We have checked this prediction in cluster Monte Carlo simulations of the
classical two dimensional AT model. We have also performed extensive numerical
simulations of the anisotropic Heisenberg quantum spin-chain with tree-tensor
network techniques that allowed to obtain the reduced density matrices of
disjoint blocks of the spin-chain and to check the correctness of the
predictions for Renyi and entanglement entropies from conformal field theory.
In order to match these predictions, we have extrapolated the numerical results
by properly taking into account the corrections induced by the finite length of
the blocks to the leading scaling behavior.Comment: 37 pages, 23 figure
A photometric redshift of for the \agile GRB 080514B
Aims: The AGILE gamma-ray burst GRB 080514B is the first burst with detected
emission above 30 MeV and an optical afterglow. However, no spectroscopic
redshift for this burst is known.
Methods: We compiled ground-based photometric optical/NIR and millimeter data
from several observatories, including the multi-channel imager GROND, as well
as ultraviolet \swift UVOT and X-ray XRT observations. The spectral energy
distribution of the optical/NIR afterglow shows a sharp drop in the \swift UVOT
UV filters that can be utilized for the estimation of a redshift.
Results: Fitting the SED from the \swift UVOT band to the band, we
estimate a photometric redshift of , consistent with the
pseudo redshift reported by Pelangeon & Atteia (2008) based on the gamma-ray
data.
Conclusions: The afterglow properties of GRB 080514B do not differ from those
exhibited by the global sample of long bursts, supporting the view that
afterglow properties are basically independent of prompt emission properties.Comment: submitted to A&A letter
The very red afterglow of GRB 000418 - further evidence for dust extinction in a GRB host galaxy
We report near-infrared and optical follow-up observations of the afterglow
of the Gamma-Ray Burst 000418 starting 2.5 days after the occurrence of the
burst and extending over nearly seven weeks. GRB 000418 represents the second
case for which the afterglow was initially identified by observations in the
near-infrared. During the first 10 days its R-band afterglow was well
characterized by a single power-law decay with a slope of 0.86. However, at
later times the temporal evolution of the afterglow flattens with respect to a
simple power-law decay. Attributing this to an underlying host galaxy we find
its magnitude to be R=23.9 and an intrinsic afterglow decay slope of 1.22. The
afterglow was very red with R-K=4 mag. The observations can be explained by an
adiabatic, spherical fireball solution and a heavy reddening due to dust
extinction in the host galaxy. This supports the picture that (long) bursts are
associated with events in star-forming regions.Comment: Accepted for publication in The Astrophysical Journal. 12 pages;
citations & references updated; minor textual change
Cryo-EM structure of the dimeric Rhodobacter sphaeroides RC-LH1 core complex at 2.9â Ă : the structural basis for dimerisation
The dimeric reaction centre light-harvesting 1 (RC-LH1) core complex of Rhodobacter sphaeroides converts absorbed light energy to a charge separation, and then it reduces a quinone electron and proton acceptor to a quinol. The angle between the two monomers imposes a bent configuration on the dimer complex, which exerts a major influence on the curvature of the membrane vesicles, known as chromatophores, where the light-driven photosynthetic reactions take place. To investigate the dimerisation interface between two RC-LH1 monomers, we determined the cryogenic electron microscopy structure of the dimeric complex at 2.9â
Ă
resolution. The structure shows that each monomer consists of a central RC partly enclosed by a 14-subunit LH1 ring held in an open state by PufX and protein-Y polypeptides, thus enabling quinones to enter and leave the complex. Two monomers are brought together through N-terminal interactions between PufX polypeptides on the cytoplasmic side of the complex, augmented by two novel transmembrane polypeptides, designated protein-Z, that bind to the outer faces of the two central LH1 ÎČ polypeptides. The precise fit at the dimer interface, enabled by PufX and protein-Z, by C-terminal interactions between opposing LH1 αÎČ subunits, and by a series of interactions with a bound sulfoquinovosyl diacylglycerol lipid, bring together each monomer creating an S-shaped array of 28 bacteriochlorophylls. The seamless join between the two sets of LH1 bacteriochlorophylls provides a path for excitation energy absorbed by one half of the complex to migrate across the dimer interface to the other half
The bright optical afterglow of the long GRB 001007
We present optical follow up observations of the long GRB 001007 between 6.14
hours and ~468 days after the event. An unusually bright optical afterglow (OA)
was seen to decline following a steep power law decay with index alpha = -2.03
+/- 0.11, possibly indicating a break in the light curve at t - to < 3.5 days,
as found in other bursts. Upper limits imposed by the LOTIS alerting system
6.14 hours after the gamma ray event provide tentative (1.2 sigma) evidence for
a break in the optical light curve. The spectral index beta of the OA yields
-1.24 +/- 0.57. These values may be explained both by several fireball jet
models and by the cannonball model. Fireball spherical expansion models are not
favoured. Late epoch deep imaging revealed the presence of a complex host
galaxy system, composed of at least two objects located 1.2" (1.7 sigma) and
1.9" (2.7 sigma) from the afterglow position.Comment: 7 pages, 5 figures, published in A&
The 2.4 Ă cryo-EM structure of a heptameric light-harvesting 2 complex reveals two carotenoid energy transfer pathways
We report the 2.4 Ă
ngström resolution structure of the light-harvesting 2 (LH2) complex from Marichromatium (Mch.) purpuratum determined by cryogenic electron microscopy. The structure contains a heptameric ring that is unique among all known LH2 structures, explaining the unusual spectroscopic properties of this bacterial antenna complex. We identify two sets of distinct carotenoids in the structure and describe a network of energy transfer pathways from the carotenoids to bacteriochlorophyll a molecules. The geometry imposed by the heptameric ring controls the resonant coupling of the long-wavelength energy absorption band. Together, these details reveal key aspects of the assembly and oligomeric form of purple bacterial LH2 complexes that were previously inaccessible by any technique
The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy
Observations of the extraordinarily bright optical afterglow (OA) of GRB
991208 started 2.1 d after the event. The flux decay constant of the OA in the
R-band is -2.30 +/- 0.07 up to 5 d, which is very likely due to the jet effect,
and after that it is followed by a much steeper decay with constant -3.2 +/-
0.2, the fastest one ever seen in a GRB OA. A negative detection in several
all-sky films taken simultaneously to the event implies either a previous
additional break prior to 2 d after the occurrence of the GRB (as expected from
the jet effect). The existence of a second break might indicate a steepening in
the electron spectrum or the superposition of two events. Once the afterglow
emission vanished, contribution of a bright underlying SN is found, but the
light curve is not sufficiently well sampled to rule out a dust echo
explanation. Our determination of z = 0.706 indicates that GRB 991208 is at 3.7
Gpc, implying an isotropic energy release of 1.15 x 10E53 erg which may be
relaxed by beaming by a factor > 100. Precise astrometry indicates that the GRB
coincides within 0.2" with the host galaxy, thus given support to a massive
star origin. The absolute magnitude is M_B = -18.2, well below the knee of the
galaxy luminosity function and we derive a star-forming rate of 11.5 +/- 7.1
Mo/yr. The quasi-simultaneous broad-band photometric spectral energy
distribution of the afterglow is determined 3.5 day after the burst (Dec 12.0)
implying a cooling frequency below the optical band, i.e. supporting a jet
model with p = -2.30 as the index of the power-law electron distribution.Comment: Accepted for publication in Astronomy and Astrophysics, 9 pages, 6
figures (Fig. 3 and Fig. 4 have been updated
GRB 040403: a faint X-ray rich Gamma-ray Burst discovered by INTEGRAL
GRB 040403 is one of the faintest gamma-ray bursts for which a rapid and accurate localization has been obtained. Here we report on the gamma-ray properties of this burst, based on observations with the IBIS instrument aboard INTEGRAL, and the results of searches for its optical afterglow. The steep spectrum (power law photon index =1.9 in the 20â200 keV range) implies that GRB 040403 is most likely an X-ray rich burst. Our optical limit of R \u3e24.2 at 16.5 h after the burst, indicates a rather faint afterglow, similar to those seen in other relatively soft and faint bursts
Mechanism of completion of peptidyltransferase centre assembly in eukaryotes.
During their final maturation in the cytoplasm, pre-60S ribosomal particles are converted to translation-competent large ribosomal subunits. Here, we present the mechanism of peptidyltransferase centre (PTC) completion that explains how integration of the last ribosomal proteins is coupled to release of the nuclear export adaptor Nmd3. Single-particle cryo-EM reveals that eL40 recruitment stabilises helix 89 to form the uL16 binding site. The loading of uL16 unhooks helix 38 from Nmd3 to adopt its mature conformation. In turn, partial retraction of the L1 stalk is coupled to a conformational switch in Nmd3 that allows the uL16 P-site loop to fully accommodate into the PTC where it competes with Nmd3 for an overlapping binding site (base A2971). Our data reveal how the central functional site of the ribosome is sculpted and suggest how the formation of translation-competent 60S subunits is disrupted in leukaemia-associated ribosomopathies.Bloodwise, MRC, Wellcome Trus
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