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, $\Gamma>10^3$. 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 $\gamma\gamma$ 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 $\gamma\gamma$ 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{ $\Gamma\sim600$ }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 $pp$ and $p\gamma$ 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, $>10^{19}$eV, CRs and the $1-100$~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 $B\gtrsim 0.5$mG, the SEPs may produce a spectral bump at $10^{-5}-10^{-2}$eV, exceeding the predicted synchrotron component of the leptonic model, and a soft spectral tail at $\gtrsim 100$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 $B\sim 400 - 500 \mu$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