21,352 research outputs found
Reexamination of inflation in noncommutative space-time after Planck results
An inflationary model in the framework of noncommutative space-time may
generate a nontrivial running of the scalar spectral index, but usually induces
a large tensor-to-scalar ratio simultaneously. With the latest observational
data from the Planck mission, we reexamine the inflationary scenarios in a
noncommutative space-time. We find that either the running of the spectral
index is tiny compared with the recent observational result, or the
tensor-to-scalar ratio is too large to allow a sufficient number of -folds.
As examples, we show that the chaotic and power-law inflation models with the
noncommutative effects are not favored by the current Planck data.Comment: 8 pages, 4 figures; version published in Physical Review
Cosmology-Independent Distance Moduli of 42 Gamma-Ray Bursts between Redshift of 1.44 and 6.60
This report is an update and extension of our paper accepted for publication
in ApJ (arXiv:0802.4262). Since objects at the same redshift should have the
same luminosity distance and the distance moduli of type Ia supernovae (SNe Ia)
obtained directly from observations are completely cosmology independent, we
obtain the distance modulus of a gamma-ray burst (GRB) at a given redshift by
interpolating or iterating from the Hubble diagram of SNe Ia. Then we calibrate
five GRB relations without assuming a particular cosmological model, from
different regression methods, and construct the GRB Hubble diagram to constrain
cosmological parameters. Based upon these relations we list the
cosmology-independent distance moduli of 42 GRBs between redshift of 1.44 and
6.60, with the 1- uncertainties of 1-3%.Comment: 6 pages, 2 figures, 3 tables. To appear in the proceedings of "2008
Nanjing GRB conference", Nanjing, 23-27 June 200
Intranight optical variability of radio-quiet BL Lacertae objects
Aims: Intranight variation (or microvariation) is a common phenomenon of
radio-loud BL Lac objects. However, it is not clear whether the recently found
radio-quiet BL Lac objects have the same properties. The occurrence rate of
intranight variation is helpful in distinguishing the mechanism of the
continuum of radio-quiet BL Lac objects.
Methods: We conducted a photometric monitoring of 8 radio-quiet BL Lac
objects by the Xinglong 2.16m and Lijiang 2.4m telescopes. The differential
light curves are calculated between each target and two comparison stars. To
quantify the variation, the significance of variation is examined by a scaled
-test.
Results: No significant variation is found in the 11 sessions of light curves
of 8 radio-quiet BL Lac objects (one galactic source is excluded). The lack of
microvariation in radio-quiet BL Lac objects is consistent with the detection
rate of microvariation in normal radio-quiet AGNs, but much lower than for
radio-loud AGNs. This result indicates that the continua of the radio-quiet BL
Lac objects are not dominated by jets that will induce frequent
microvariations.Comment: Accepted for publication in Astronomy and Astrophysics; 7 pages, 1
figure, 3 table
Functional studies of the ‘GAFTGA’ motif of Escherichia coli Phage Shock Protein F
In the archetypal bacterial transcription, the multi-subunit core RNA polymerase (RNAP) is
specifically bound to the promoter through the sigma factor (σ), forming a closed complex
where DNA remains double-stranded. The promoter DNA is initially spontaneously melted
by the σ factor within the core enzyme and subsequently loaded into the active channel of
the holoenzyme. However, a major alternative transcription paradigm that depends on the
sigma factor σ54 also exists in bacteria and controls pathogenicity, biofilm formation,
bioluminescence, nitrogen fixation and stress responses. σ54 forms inhibitory interactions
with DNA in the closed complex, which can only be alleviated by ATP hydrolysis-driven
remodelling catalysed by bacterial enhancer binding proteins (bEBPs). In this regard, σ54-
dependent transcription can be viewed analogous to the eukaryotic RNA Pol II system. This
study was initiated to advance the understanding of: (i) how the ATP hydrolysis energy is
relayed from the ATPase catalytic site to the closed complex for remodelling, (ii) the
hexameric interface organisation of bEBPs for ATP hydrolysis, and (iii) the role of core RNAP
in σ54-dependent transcription. A newly devised cross-linking technique combined with the
DNA footprinting methods provided new insights of the organisation of each transcription
component. The data gathered from this study updated the current working model for ATPdependent
transcription. In addition, the cross-linking method proved to be an excellent
tool to study protein-protein and nucleo-protein interactions
Parameter constraints in a near-equipartition model with multi-frequency \emph{NuSTAR}, \emph{Swift} and \emph{Fermi-LAT} data from 3C 279
Precise spectra of 3C 279 in the 0.5-70 keV range, obtained during two epochs
of
\emph{Swift} and \emph{NuSTAR} observations, are analyzed using a
near-equipartition model. We apply a one-zone leptonic model with a
three-parameter log-parabola electron energy distribution (EED) to fit the
\emph{Swift} and \emph{NuSTAR} X-ray data, as well as simultaneous optical and
\emph{Fermi}-LAT -ray data. The Markov Chain Monte Carlo (MCMC)
technique is used to search the high-dimensional parameter space and evaluate
the uncertainties on model parameters. We show that the two spectra can be
successfully fit in near-equipartition conditions, defined by the ratio of the
energy density of relativistic electrons to magnetic field
being close to unity. In both spectra, the observed X-rays are dominated by
synchrotron-self Compton photons, and the observed rays are dominated
by Compton scattering of external infrared photons from a surrounding dusty
torus.
Model parameters are well constrained. From the low state to the high state,
both the curvature of the log-parabola width parameter and the synchrotron peak
frequency significantly increase. The derived magnetic fields in the two states
are nearly identical (\ G), but the Doppler factor in the high state is
larger than that in the low state (28 versus 18). We derive that
the gamma-ray emission site takes place outside the broad-line region, at
0.1 pc from the black hole, but within the dusty torus. Implications
for 3C 279 as a source of high-energy cosmic-rays are discussed.Comment: accepted by MNRA
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