The Magellan M2FS spectroscopic survey of high-redshift galaxies: the brightest Lyman-break galaxies at z∼6z \sim 6

We present a study of a sample of 45 spectroscopically confirmed, UV luminous galaxies at $z\sim 6$. They were selected as bright Lyman-break galaxies (LBGs) using deep multi-band optical images in more than 2 deg$^2$ of the sky, and subsequently identified via their strong Ly$\alpha$ emission. The majority of these LBGs span an absolute UV magnitude range from $-22.0$ to $-20.5$ mag with Ly$\alpha$ equivalent width (EW) between $\sim$10 and $\sim$200 \AA, representing the most luminous galaxies at $z\sim 6$ in terms of both UV continuum emission and Ly$\alpha$ line emission. We model the SEDs of 10 LBGs that have deep infrared observations from HST, JWST, and/or Spitzer, and find that they have a wide range of stellar masses and ages. They also have high star-formation rates ranging from a few tens to a few hundreds of Solar mass per year. Five of the LBGs have JWST or HST images and four of them show compact morphology in these images, including one that is roughly consistent with a point source, suggesting that UV luminous galaxies at this redshift are generally compact. The fraction of our photometrically selected LBGs with strong Ly$\alpha$ emission ($\mathrm{EW}>25$ \AA) is about $0.2$, which is consistent with previous results and supports a moderate evolution of the IGM opacity at the end of cosmic reionization. Using deep X-ray images, we do not find evidence of strong AGN activity in these galaxies, but our constraint is loose and we are not able to rule out the possibility of any weak AGN activity.Comment: 19 pages, 11 figures, Accepted for publication in Ap

MetaMath: Bootstrap Your Own Mathematical Questions for Large Language Models

Large language models (LLMs) have pushed the limits of natural language understanding and exhibited excellent problem-solving ability. Despite the great success, most existing open-source LLMs (e.g., LLaMA-2) are still far away from satisfactory for solving mathematical problem due to the complex reasoning procedures. To bridge this gap, we propose MetaMath, a fine-tuned language model that specializes in mathematical reasoning. Specifically, we start by bootstrapping mathematical questions by rewriting the question from multiple perspectives without extra knowledge, which results in a new dataset called MetaMathQA. Then we fine-tune the LLaMA-2 models on MetaMathQA. Experimental results on two popular benchmarks (i.e., GSM8K and MATH) for mathematical reasoning demonstrate that MetaMath outperforms a suite of open-source LLMs by a significant margin. Our MetaMath-7B model achieves 66.4% on GSM8K and 19.4% on MATH, exceeding the state-of-the-art models of the same size by 11.5% and 8.7%. Particularly, MetaMath-70B achieves an accuracy of 82.3% on GSM8K, slightly better than GPT-3.5-Turbo. We release all the MetaMathQA dataset, the MetaMath models with different model sizes and the training code for public use.Comment: Technical Report, Work in Progress. Project Page: https://meta-math.github.io

On the Circular Polarisation of Repeating Fast Radio Bursts

Fast spinning (e.g., sub-second) neutron star with ultra-strong magnetic fields (or so-called magnetar) is one of the promising origins of repeating fast radio bursts (FRBs). Here we discuss circularly polarised emissions produced by propagation effects in the magnetosphere of fast spinning magnetars. We argue that the polarisation-limiting region is well beyond the light cylinder, suggesting that wave mode coupling effects are unlikely to produce strong circular polarisation for fast spinning magnetars. Cyclotron absorption could be significant if the secondary plasma density is high. However, high degrees of circular polarisation can only be produced with large asymmetries in electrons and positrons. We draw attention to the non-detection of circular polarisation in current observations of known repeating FRBs. We suggest that the circular polarisation of FRBs could provide key information on their origins and help distinguish different radiation mechanisms.Comment: ApJ accepte

PSR B0943+10: Mode Switch, Polar Cap Geometry, and Orthogonally Polarized Radiation

As one of the paradigm examples to probe into pulsar magnetospheric dynamics, PSR B0943+10 (J0946+0951) manifests representatively, showing mode switch, orthogonal polarization and subpulse drifting. Both integrated and single pulses are studied with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The mode switch phenomenon of this pulsar is studied using an eigen-mode searching method, based on parameter estimation. A phase space evolution for the pulsar's mode switch shows a strange-attractor-like pattern. The radiative geometry is proposed by fitting polarization position angles with the rotating vector model. The pulsar pulse profile is then mapped to the sparking location on pulsar surface, and the differences between the main pulse's and the precursor component's radiative process may explain the X-ray's synchronization with radio mode switch. Detailed single pulse studies on B0943+10's orthogonally polarized radiation are presented, which may support for certain models of radiative transfer of polarized emission. B0943+10's B and Q modes evolve differently with frequency and with proportions of orthogonal modes, which indicates possible magnetospheric changes during mode switch. An extra component is found in B mode, and it shows distinct polarization and modulation properties compared with main part of B mode pulse component. For Q mode pulse profile, the precursor and the main pulse components are orthogonally polarized, showing that the precursor component radiated farther from the pulsar could be radiated in O-mode (X-mode) if the main pulse originates from low altitude in X-mode (O-mode). The findings could impact significantly on pulsar electrodynamics and the radiative mechanism related.Comment: 27 pages, 28 figures, 2 tables, submitted to Ap

Conditional anonymous remote healthcare data sharing over blockchain

As an important carrier of healthcare data, Electronic Medical Records (EMRs) generated from various sensors, i.e., wearable, implantable, are extremely valuable research materials for artificial intelligence and machine learning. The efficient circulation of EMRs can improve remote medical services and promote the development of the related healthcare industry. However, in traditional centralized data sharing architectures, the balance between privacy and traceability still cannot be well handled. To address the issue that malicious users cannot be locked in the fully anonymous sharing schemes, we propose a trackable anonymous remote healthcare data storing and sharing scheme over decentralized consortium blockchain. Through an “on-chain & off-chain” model, it relieves the massive data storage pressure of medical blockchain. By introducing an improved proxy re-encryption mechanism, the proposed scheme realizes the fine-gained access control of the outsourced data, and can also prevent the collusion between semi-trusted cloud servers and data requestors who try to reveal EMRs without authorization. Compared with the existing schemes, our solution can provide a lower computational overhead in repeated EMRs sharing, resulting in a more efficient overall performance

Atypical radio pulsations from magnetar SGR 1935+2154

Magnetars are neutron stars with extremely strong magnetic fields, frequently powering high-energy activity in X-rays. Pulsed radio emission following some X-ray outbursts have been detected, albeit its physical origin is unclear. It has long been speculated that the origin of magnetars' radio signals is different from those from canonical pulsars, although convincing evidence is still lacking. Five months after magnetar SGR 1935+2154's X-ray outburst and its associated Fast Radio Burst (FRB) 20200428, a radio pulsar phase was discovered. Here we report the discovery of X-ray spectral hardening associated with the emergence of periodic radio pulsations from SGR 1935+2154 and a detailed analysis of the properties of the radio pulses. The complex radio pulse morphology, which contains both narrow-band emission and frequency drifts, has not been seen before in other magnetars, but is similar to those of repeating FRBs - even though the luminosities are many orders of magnitude different. The observations suggest that radio emission originates from the outer magnetosphere of the magnetar, and the surface heating due to the bombardment of inward-going particles from the radio emission region is responsible for the observed X-ray spectral hardening.Comment: 47 pages, 11 figure

A repeating fast radio burst associated with a persistent radio source

The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium1, which is assumed to dominate the total extragalactic dispersion. Although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs2, in at least one case there is evidence for an extreme magneto-ionic local environment3,4 and a compact persistent radio source5. Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific-star-formation rate at a redshift of 0.241 ± 0.001. The estimated host-galaxy dispersion measure of approximately 903−111+72 parsecs per cubic centimetre, which is nearly an order of magnitude higher than the average of FRB host galaxies2,6, far exceeds the dispersion-measure contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications

Spontaneous atomic ordering in MOVPE grown GaAsSb

Spontaneous atomic ordering of semiconductor alloys is of great practical and fundamental interest. Atomic ordering of III-V alloys such as InGaP has been extensively studied experimentally and theoretically. In this thesis, we investigate a little-studied, atomic-ordering phenomenon, the so-called CuAu structure in the III-V material GaAsSb, grown by the technique of metalorganic vapor-phase epitaxy(MOVPE). Despite being first observed in 1986 in this material, there is as yet no detailed microscopic model for its formation mechanism. A key part of the thesis involves the study of surfactant effects on the ordering process in GaAsSb. Surfactants are elements which modify the growth surface without incorporation in the bulk. Nevertheless, they influence the incorporation of the bulk elements. We first explored the surfactant behavior of Bi on GaAs in order to understand how Bi incorporates at the surface and in the bulk in a related III-V material. For GaAs(001), Bi surface layers are stable at temperatures below 500C but rapidly desorb at temperatures of 550C and higher. Bi coverages of over 1ML induce the formation of Bi islands, whose sizes increase with increasing Bi exposure. Bulk incorporation of Bi remains essentially zero at typical MOVPE growth temperatures. In the case of GaAsSb alloys, Bi surfactant was found to induce CuAu ordering, with no measurable Bi incorporation in the bulk. High resolution TEM was used to study the detailed microstructural features for ordered and disordered samples. The domain sizes of the ordered regions are from 5nm to 20nm under all growth conditions. In contrast to orderings in other alloys such as InGaP, CuAu ordering had no observable effect on the bandgap. CuAu ordering in GaAsSb was studied in a function of growth conditions, including Bi surfactant concentration, growth temperature, growth rate, and substrate miscut. All of these experiments confirm that bulk CuAu ordering is a surface driven, rather than bulk process. It is unlikely that the ordering mechanism is similar to the dimer-induced strain models that have been successfully used to explain CuPt ordering in InGaP. We propose a simple model based on alternating incorporation of group V adatoms at step edges

Second harmonic conversion enhancement in 2-D planar photonic crystals

Two dimensional (2D) periodic texture in a thin GaAs semiconductor waveguide is used to dramatically enhance the second harmonic conversion efficiency of infrared laser radiation incident on the structure from above. A square lattice of through-holes was etched into a 140 nm thick layer of GaAs supported on a ~ 1 μm thick alumina cladding layer. The pitch of 770 nm, and hole diameter of 320 nm, were chosen so that fundamental light at a wavelength of ~ 2 μm could resonantly excite the lowest order band of leaky photonic eigenstates characteristic of the strongly textured membrane. The energy of the fundamental and second harmonic are both less than the band gap energy of the GaAs, thus avoiding any linear absorption. Used in this way, the structure acts effectively like a nonlinear cavity, where strong internal fields are generated in the GaAs waveguide core layer when the incident light excites a leaky photonic eigenstate. The 2nd order polarization excited in the GaAs can also be resonant with photonic eigenstates at twice the fundamental frequency and in-plane wavevector. Both model calculations and experimental results (obtained by J. Mondia at the University of Toronto) clearly demonstrate that the second harmonic conversion efficiency is dramatically influenced when the fundamental and/or the second harmonic fields are resonant with photonic bands. Peak enhancements of over 1000 times are observed under the "doubleresonance" condition when both the fundamental and second harmonic fields excite photonic eigenstates, and the enhancement clearly tracks the in-coming and out-going dispersion of the photonic bands.Science, Faculty ofPhysics and Astronomy, Department ofGraduat