5,688 research outputs found
The bulk Lorentz factors of Fermi-LAT GRBs
The Lorentz factor (LF) of gamma-ray burst (GRB) ejecta may be constrained by
observations of high-energy (HE) spectral attenuation. The recent Fermi-LAT
observations of prompt GeV emission from several bright GRBs have leaded to
conclusions of unexpectedly large LFs, . Here we revisit this
problem with two main concerns. (1) With one-zone assumption where all photons
are assumed to be generated in the same region (radius) and time, we {\em
self-consistently} calculate the optical depth by adopting a
target photon spectrum with HE cutoff. We find that this might be important
when the GRB LF is below a few hundreds. (2) Recent Fermi-LAT observations
suggest that the bulk MeV-range and HE (\ga100 MeV) emission may arise from
different regions. We then consider a two-zone case where HE emission is
generated in much larger radii than that of the MeV-range emission. We find
that the HE emission may be mainly attenuated by MeV-range emission and that
the attenuated HE spectrum does not show an exponential spectral cutoff but a
slight steepening. This suggests that there may be no abrupt cutoff due to
attenuation if relaxing the one-zone assumption. By studying the
spectra of three bright Fermi-LAT GRBs 080916C, 090510 and 090902B, we show
that a bulk LF of\textbf{ }can be consistent with observations
in the two-zone case. Even lower LFs can be obtained in the multi-zone case.Comment: 8 pages, a schematic figure added, the other 5 figures updated, ApJ,
in pres
Implications of Fermi-LAT observations on the origin of IceCube neutrinos
The IceCube (IC) collaboration recently reported the detection of TeV-PeV
extraterrestrial neutrinos whose origin is yet unknown. By the photon-neutrino
connection in and interactions, we use the \fermi-LAT
observations to constrain the origin of the IC detected neutrinos. We find that
Galactic origins, i.e., the diffuse Galactic neutrinos due to cosmic ray (CR)
propagation in the Milky Way, and the neutrinos from the Galactic point
sources, may not produce the IC neutrino flux, thus these neutrinos should be
of extragalactic origin. Moreover, the extragalactic gamma-ray bursts (GRBs)
may not account for the IC neutrino flux, the jets of active galactic nuclei
may not produce the IC neutrino spectrum, but the starburst galaxies (SBGs) may
be promising sources. As suggested by the consistency between the IC detected
neutrino flux and the Waxman-Bahcall bound, GRBs in SBGs may be the sources of
both the ultrahigh energy, eV, CRs and the ~PeV CRs that
produce the IC detected TeV-PeV neutrinos.Comment: JCAP accepted version; 8 pages, 2 figs; discussion on blazar origin
added; conclusion unchange
Fair Credit Scorer through Bayesian Approach
Machine learning currently plays an increasingly important role in people's
lives in areas such as credit scoring, auto-driving, disease diagnosing, and
insurance quoting. However, in many of these areas, machine learning models
have performed unfair behaviors against some sub-populations, such as some
particular groups of race, sex, and age. These unfair behaviors can be on
account of the pre-existing bias in the training dataset due to historical and
social factors. In this paper, we focus on a real-world application of credit
scoring and construct a fair prediction model by introducing latent variables
to remove the correlation between protected attributes, such as sex and age,
with the observable feature inputs, including house and job. For detailed
implementation, we apply Bayesian approaches, including the Markov Chain Monte
Carlo simulation, to estimate our proposed fair model
A Portable and Automatic Biosensing Instrument for Detection of Foodborne Pathogenic Bacteria in Food Samples
Foodborne diseases are a growing public health problem. In recent years, many rapid detection methods have been reported, but most of them are still in lab research and not practical for use in the field. In this study, a portable and automatic biosensing instrument was designed and constructed for separation and detection of target pathogens in food samples using nanobead-based magnetic separation and quantum dots (QDs)-labeled fluorescence measurement. The instrument consisted of a laptop with LabVIEW software, a data acquisition card (DAQ), a fluorescent detector, micro-pumps, stepper motors, and 3D printed tube holders. First, a sample in a syringe was mixed with magnetic nanobead-antibody (MNB-Ab) conjugates and then injected to a low binding reaction tube. After incubation and magnetic separation, target bacterial cells were captured and collected and the solution was pumped out. Then the QD-antibody (QD-Ab) conjugates were pumped into the reaction tube to form the MNB-Ab-cell-Ab-QD complexes that were then collected by magnetic separation and resuspended in PBS buffer solution through air pressure control. Finally, the sample solution was pushed into the detection tube by an air pump and the fluorescence intensity was measured using a fluorescent detector. A virtual instrument (VI) was programmed using LabVIEW software to provide a platform for magnetic separation, fluorescent measurement, data processing, and control. The DAQ was used for data communication. The results showed that the separation efficiency of this instrument was 78.3 ± 3.4% and 60.7 ± 4.2% for E. coli O157:H7 in pure culture and ground beef samples, respectively. The limit of detection was 3.98 × 103 and 6.46 × 104 CFU/mL in pure culture and ground beef samples, respectively. Sample preparation and detection could be finished in 2 hours. The instrument was portable and automatic with great potential to serve as a more effective tool for in-field/on-line detection of foodborne pathogenic bacteria in food products
Secondary-electron radiation accompanying hadronic GeV-TeV gamma-rays from supernova remnants
The synchrotron radiation from secondary electrons and positrons (SEPs)
generated by hadronic interactions in the shock of supernova remnant (SNR)
could be a distinct evidence of cosmic ray (CR) production in SNR shocks. Here
we provide a method where the observed gamma-ray flux from SNRs, created by
pion decays, is directly used to derive the SEP distribution and hence the
synchrotron spectrum. We apply the method to three gamma-ray bright SNRs. In
the young SNR RX J1713.7-3946, if the observed GeV-TeV gamma-rays are of
hadronic origin and the magnetic field in the SNR shock is mG,
the SEPs may produce a spectral bump at eV, exceeding the
predicted synchrotron component of the leptonic model, and a soft spectral tail
at keV, distinct from the hard spectral slope in the leptonic
model. In the middle-aged SNRs IC443 and W44, if the observed gamma-rays are of
hadronic origin, the SEP synchrotron radiation with G can
well account for the observed radio flux and spectral slopes, supporting the
hadronic origin of gamma-rays. Future microwave to far-infrared and hard X-ray
(>100keV) observations are encouraged to constraining the SEP radiation and the
gamma-ray origin in SNRs.Comment: 9 pages, 5 figures and 1 table, MNRAS accepte
- …