39,120 research outputs found
Brief comment: Dicke Superradiance and Superfluorescence Find Application for Remote Sensing in Air
This letter briefly introduces the concepts of Dicke superradiance (SR) and
superfluorescence (SF), their difference to amplified spontaneous emission
(ASE), and the hints for identifying them in experiment. As a typical example
it analyzes the latest observations by Dogariu et al. (Science 331, 442, 2011),
and clarifies that it is SR. It also highlights the revealed potential
significant application of SR and SF for remote sensing in air.Comment: 1.5 pages, 1 illustration figure, manuscript finished in March 2011,
declined by two journal
Selection and Estimation Optimality in High Dimensions with the TWIN Penalty
We introduce a novel class of variable selection penalties called TWIN, which
provides sensible data-adaptive penalization. Under a linear sparsity regime
and random Gaussian designs we show that penalties in the TWIN class have a
high probability of selecting the correct model and furthermore result in
minimax optimal estimators. The general shape of penalty functions in the TWIN
class is the key ingredient to its desirable properties and results in improved
theoretical and empirical performance over existing penalties. In this work we
introduce two examples of TWIN penalties that admit simple and efficient
coordinate descent algorithms, making TWIN practical in large data settings. We
demonstrate in challenging and realistic simulation settings with high
correlations between active and inactive variables that TWIN has high power in
variable selection while controlling the number of false discoveries,
outperforming standard penalties
Constraining Parameters in Pulsar Models of Repeating FRB 121102 with High-Energy Follow-up Observations
Recently, a precise (sub-arcsecond) localization of the repeating fast radio
burst (FRB) 121102 has led to the discovery of persistent radio and optical
counterparts, the identification of a host dwarf galaxy at a redshift of
, and several campaigns of searches for higher-frequency counterparts,
which gave only upper limits on the emission flux. Although the origin of FRBs
remains unknown, most of the existing theoretical models are associated with
pulsars, or more specifically, magnetars. In this paper, we explore persistent
high-energy emission from a rapidly rotating highly magnetized pulsar
associated with FRB 121102 if internal gradual magnetic dissipation occurs in
the pulsar wind. We find that the efficiency of converting the spin-down
luminosity to the high-energy (e.g., X-ray) luminosity is generally much
smaller than unity, even for a millisecond magnetar. This provides an
explanation for the non-detection of high-energy counterparts to FRB 121102. We
further constrain the spin period and surface magnetic field strength of the
pulsar with the current high-energy observations. In addition, we compare our
results with the constraints given by the other methods in previous works and
would expect to apply our new method to some other open issues in the future.Comment: 6 pages, 5 figures, ApJ in press, minor changes due to proof
correction
Neutrino Emission in Jet Propagation Process
Relativistic jets are universal in long-duration gamma-ray burst (GRB)
models. Before breaking out, they must propagate in the progenitor envelope
along with a forward shock and a reverse shock forming at the jet head. Both
electrons and protons will be accelerated by the shocks. High energy neutrinos
could be produced by these protons interacting with stellar materials and
electron-radiating photons. The jet will probably be collimated, which may have
a strong effect on the final neutrino flux. Under the assumption of a power-law
stellar-envelope density profile with an index
, we calculate this neutrino emission flux by these shocks for
low-luminosity GRBs (LL-GRBs) and ultra-long GRBs (UL-GRBs) in different
collimation regimes, using the jet propagation framework developed by
\citet{bro11}. We find that LL-GRBs and UL-GRBs are capable for detectable high
energy neutrinos up to , and obtain the final neutrino
spectrum. Besides, we conclude that larger corresponds to greater
neutrino flux at high energy end () and higher maximum neutrino
energy as well. However, such differences are so small that it is not promising
for us to distinguish from observations, given the energy resolution we have
now.Comment: 21 pages, 4 figures, accepted for publication in Ap
TeV-PeV Neutrino Oscillation of Low-luminosity Gamma-ray Bursts
There is a sign that long-duration gamma-ray bursts (GRBs) originate from the
core collapse of massive stars. During a jet puncturing through the progenitor
envelope, high energy neutrinos can be produced by the reverse shock formed at
the jet head. It is suggested that low-luminosity GRBs (LL-GRBs) are possible
candidates of this high energy neutrino precursor up to .
Before leaving the progenitor, these high energy neutrinos must oscillate from
one flavor to another with matter effect in the envelope. Under the assumption
of a power-law stellar envelope density profile with
an index , we study the properties of neutrino
oscillation. We find that adiabatic conversion is violated for these neutrinos
so we do certain calibration of level crossing effect. The resonance condition
is reached for different energies at different radii. We notice that the
effective mixing angles in matter for neutrinos are close to zero
so the transition probabilities from one flavor to another are almost invariant
for neutrinos. We plot all the transition probabilities versus
energy of neutrinos from the birth place to the surface of the
progenitor. With an initial flavor ratio
, we plot how the
flavor ratio evolves with energy and distance when neutrinos are still in the
envelope, and further get the ratio when they reach the Earth. For
neutrinos, the ratio is always
on Earth. In
addition, we discuss the dependence of the flavor ratio on energy and
and get a pretty good result. This dependence may provide a promising probe of
the progenitor structure.Comment: 18 pages, 7 figures, accepted for publication in Ap
Production of the Extreme-Ultraviolet Late Phase of an X Class Flare in a Three-Stage Magnetic Reconnection Process
We report observations of an X class flare on 2011 September 6 by the
instruments onboard the Solar Dynamics Observatory (SDO). The flare occurs in a
complex active region with multiple polarities. The Extreme-Ultraviolet (EUV)
Variability Experiment (EVE) observations in the warm coronal emission reveal
three enhancements, of which the third one corresponds to an EUV late phase.
The three enhancements have a one-to-one correspondence to the three stages in
flare evolution identified by the spatially-resolved Atmospheric Imaging
Assembly (AIA) observations, which are characterized by a flux rope eruption, a
moderate filament ejection, and the appearance of EUV late phase loops,
respectively. The EUV late phase loops are spatially and morphologically
distinct from the main flare loops. Multi-channel analysis suggests the
presence of a continuous but fragmented energy injection during the EUV late
phase resulting in the warm corona nature of the late phase loops. Based on
these observational facts, We propose a three-stage magnetic reconnection
scenario to explain the flare evolution. Reconnections in different stages
involve different magnetic fields but show a casual relationship between them.
The EUV late phase loops are mainly produced by the least energetic magnetic
reconnection in the last stage.Comment: 6 pages, 4 figures, 1 table. Accepted for Publication in ApJ
Two-dimensional Fourier-transform Spectroscopy of Potassium Vapor
Optical two-dimensional Fourier-transformed (2DFT) spectroscopy is used to
study the coherent optical response of potassium vapor in a thin transmission
cell. Rephasing and non-rephasing spectra of the D1 and D2 transitions are
obtained and compared to numerical simulations. Calculations using the optical
Bloch equations gives very good agreement with the experimental peak strengths
and line shapes. Non-radiative Raman-like coherences are isolated using a
different 2DFT projection. Density-dependent measurements show distortion of
2DFT spectra due to pulse propagation effects
Determining the locations visited by GPS users: a clustering approach
The aim of our research is to use the GPS log captured over 2 days from a PDA and try to extract locations the user have visited. For this research, we have logged GPS data over two days when the user was moving at least 2 miles per hour. To achieve this we cluster the information using data mining clustering software and then analyse the results to see which locations represent a place the user has spent his time
Can Gamma-Ray Bursts Be Used to Measure Cosmology? A Further Analysis
Three different methods of measuring cosmology with gamma-ray bursts (GRBs)
have been proposed since a relation between the -ray energy of a GRB jet and the peak energy of the spectrum in the
burst frame was reported by Ghirlanda and coauthors. In Method I, to calculate
the probability for a favored cosmology, only the contribution of the
relation that is already best fitted for this cosmology is
considered. We apply this method to a sample of 17 GRBs, and obtain the mass
density () for a flat CDM
universe. In Method II, to calculate the probability for some certain
cosmology, contributions of all the possible relations that are
best fitted for their corresponding cosmologies are taken into account. With
this method, we find a constraint on the mass density () for a flat universe. In Method III, to obtain the probability for some
cosmology, contributions of all the possible relations
associated with their unequal weights are considered. With this method, we
obtain an inspiring constraint on the mass density () for a flat universe, and a for the
concordance model of . (abridged)Comment: 26 pages including 6 figures, different methods compared, accepted
for publication in Ap
Constraining Gamma-Ray Emission from Luminous Infrared Galaxies with Fermi-LAT; Tentative Detection of Arp 220
Star-forming galaxies produce gamma-rays primarily via pion production,
resulting from inelastic collisions between cosmic ray protons and the
interstellar medium (ISM). The dense ISM and high star formation rates of
luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) imply that
they should be strong gamma-ray emitters, but so far only two LIRGs have been
detected. Theoretical models for their emission depend on the unknown fraction
of cosmic ray protons that escape these galaxies before interacting. We analyze
Fermi-LAT data for 82 of the brightest IRAS LIRGs and ULIRGs. We examine each
system individually and carry out a stacking analysis to constrain their
gamma-ray fluxes. We report the detection of the nearest ULIRG Arp 220
(~4.6sigma). We observe a gamma-ray flux (0.8--100 GeV) of 2.4e-10 phot cm^-2
s^-1 with photon index of 2.23 (8.2e10^41 ergs s^-1 at 77 Mpc) We also derive
upper limits for the stacked LIRGs and ULIRGs. The gamma-ray luminosity of
Arp~220 and the stacked upper limits agree with calorimetric predictions for
dense star-forming galaxies. With the detection of Arp 220, we extend the
gamma-ray--IR luminosity correlation to the high luminosity regime with
log(L_[0.1-100 GeV]) = 1.25log(L_[8-1000]) + 26.7 as well as the
gamma-ray--radio continuum luminosity correlation with log(L_[0.1-100 GeV]) =
1.22log(L_[1.4 GHz]) + 13.3. The current survey of Fermi-LAT is on the verge of
detecting more LIRGs/ULIRGs in the local universe, and we expect even more
detections with deeper Fermi-LAT observations or the next generation of
gamma-ray detectors.Comment: 14 single column pages, 4 figures, accepted by ApJ
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