Search

The energy and waiting time distributions are important properties for understanding the physical mechanism of repeating fast radio bursts (FRBs). Recently, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) detected the largest burst sample of FRB 121102, containing 1652 bursts. We use this sample to investigate the energy and waiting time distributions. The energy count distribution $dN/dE$ at the high-energy range ($>10^{38}$ erg) can be fitted with a single power-law function with an index of $-1.86 ^{+0.02}_{-0.02}$, while the distribution at the low-energy range deviates from the power-law function. An interesting result of Li et al. (2021) is that there is an apparent temporal gap between early bursts (occurring before MJD 58740) and late bursts (occurring after MJD 58740). We find the energy distributions of high-energy bursts at different epochs are inconsistent. The power-law index is $-1.70^{+0.03}_{-0.03}$ for early bursts and $-2.60^{+0.15}_{-0.14}$ for late bursts. For bursts observed in a single day, a linear repetition pattern is found. We use the Weibull function to fit the distribution of waiting time of consecutive bursts. The shape parameter $k = 0.72^{+0.01}_{-0.01}$ and the event rate $r = 736.43^{+26.55}_{-28.90}$ day$ ^{-1} $ are derived. If the waiting times with $��_t < 28$ s are excluded, the burst behavior can be described by a Poisson process. The best-fitting values of $k$ are slightly different for low-energy ($E < 1.58\times10^{38}$ erg) and high-energy ($E > 1.58\times10^{38}$ erg) bursts., 20 pages, 1 table, 10 figures, accepted for publication in ApJ Letters

The event rate, energy distribution, and time-domain behaviour of repeating fast radio bursts (FRBs) contains essential information regarding their physical nature and central engine, which are as yet unknown. As the first precisely-localized source, FRB 121102 has been extensively observed and shows non-Poisson clustering of bursts over time and a power-law energy distribution. However, the extent of the energy distribution towards the fainter end was not known. Here we report the detection of 1652 independent bursts with a peak burst rate of 122~hr^{-1}, in 59.5 hours spanning 47 days. A peak in the isotropic equivalent energy distribution is found to be ~4.8 x 10^{37} erg at 1.25~GHz, below which the detection of bursts is suppressed. The burst energy distribution is bimodal, and well characterized by a combination of a log-normal function and a generalized Cauchy function. The large number of bursts in hour-long spans allow sensitive periodicity searches between 1 ms and 1000 s. The non-detection of any periodicity or quasi-periodicity poses challenges for models involving a single rotating compact object. The high burst rate also implies that FRBs must be generated with a high radiative efficiency, disfavoring emission mechanisms with large energy requirements or contrived triggering conditions., 69 pages, 14 figures, 2 tables; Updates for author correction

Magnetars are highly magnetized neutron stars that are characterized by recurrent emission of short-duration bursts in soft gamma-rays/hard X-rays. Recently, FRB 200428 were found to be associated with an X-ray burst from a Galactic magnetar. Two fast radio bursts (FRBs) show mysterious periodic activity. However, whether magnetar X-ray bursts are periodic phenomena is unclear. In this paper, we investigate the period of SGR 1806-20 activity. More than 3000 short bursts observed by different telescopes are collected, including the observation of RXTE, HETE-2, ICE and Konus. We consider the observation windows and divide the data into two sub-samples to alleviate the effect of unevenly sample. The epoch folding and Lomb-Scargle methods are used to derive the period of short bursts. We find a possible period about $ 398.20 \pm 25.45 $ days. While other peaks exist in the periodograms. If the period is real, the connection between short bursts of magnetars and FRBs should be extensively investigated., 14 pages, 4 figures, accepted for publication in ApJ

Accompanied by an X-ray burst, the fast radio burst (FRB) FRB 200428 was recently confirmed as originating from the Galactic magnetar soft gamma repeater (SGR) SGR J1935+2154. Just before and after FRB 200428 was detected, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) had been monitoring SGR J1935+2154 for eight hours. From UTC 2020 April 27 23:55:00 to 2020 April 28 00:50:37, FAST detected no pulsed radio emission from SGR J1935+2154, while Fermi/Gamma-ray Burst Monitor registered 34 bursts in the X/soft $\gamma$-ray band, forming a unique sample of X-ray bursts in the absence of FRBs. After a comprehensive analysis on light curves, time-integrated, and time-resolved spectral properties of these FRB-absent X-ray bursts, we compare this sample with the FRB-associated X-ray burst detected by Insight-HXMT, INTEGRAL, and Konus-Wind. The FRB-associated burst distinguishes itself from other X-ray bursts by its nonthermal spectrum and a higher spectral peak energy, but otherwise is not atypical. We also compare the cumulative energy distribution of our X-ray burst sample with that of first repeating FRB source, FRB 121102, with the calibration of FRB 200428-X-ray burst association. We find a similarity between the two, offering indirect support of the magnetar origin of cosmological FRBs. The event rate density of magnetar bursts is about $\sim 150$ times higher than the FRB event rate density at the energy of FRB 200428. This again suggests that, if all FRBs originate from magnetars, only a small fraction of X-ray bursts are associated with FRBs., Comment: 18 pages, 3 tables, 8 figures. Published in the Astrophysical Journal Letter on January 12, 2021

Fast radio bursts (FRBs) are extragalactic, bright pulses of emission at radio frequency with milliseconds duration. Observationally, FRBs can be divided into two classes, repeating FRBs and non-repeating FRBs. At present, twenty repeating FRBs have been discovered with unknown physical origins. Localization of the first repeating FRB 121102 and discovery of an associated persistent radio source support that FRBs are powered by young millisecond magnetars, which could be formed by core-collapses of massive stars or binary neutron stars mergers. These two formation channels can be distinguished by gravitational waves generated by binary neutron stars mergers. We first calculate the lower limit of the local formation rate of repeating FRBs observed by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) . Then we show that only a small fraction ($6\%$) of repeating FRBs is produced by young magnetars from binary neutron star mergers, basing on the gravitational wave detections by the third observing run (O3) of Advanced LIGO/Virgo gravitational-wave detectors. Therefore, we believe that repeating FRBs are more likely produced by the magnetars newborn from the core-collapses of massive stars rather than the magnetars from the binary neutron stars mergers., 7 pages, 2 figures, accepted for publication in ApJ

Four significant events of rapid $^{14}$C increase have taken place within the past several thousand years. The physical origin of these rapid increases is still a mystery but must be associated with extremely energetic cosmic processes. Pulsars are highly magnetized neutron stars that emit a beam of electromagnetic radiations. Any sudden release of the energy stored in the magnetic multipole field will trigger outbursts similar to the giant flares of magnetars. Here we show that the relativistic outflow from the outbursts of a nearby pulsar interacting with the interstellar medium generates a shock, which accelerates electrons to trillions of electron volts. The high-energy photons from synchrotron emission of the shock interact with Earth's atmosphere, producing the cosmogenic nuclide $^{14}$C, which can cause the rapid $^{14}$C increases discovered in tree rings. These same relativistic electrons can account for a significant fraction of the cosmic electron spectrum in the trillion electron volts energy range, as observed by space-borne satellites. Since these outburst events can significantly affect our environment, monitoring nearby pulsars for such outbursts may be important in the future., 24 pages, 2 figures, accepted for publication in ApJ

In this paper, we investigate the energy function, formation rate and environment of fast radio bursts (FRBs) using Parkes sample and Australian Square Kilometer Array Pathfinder (ASKAP) sample. For the first time, the metallicity effect on the formation rate is considered. If FRBs are produced by the mergers of compact binaries, the formation rate of FRBs should have a time delay relative to cosmic star formation rate (CSFR). We get the time delay is about 3-5 Gyr and the index of differential energy function $\gamma$ ($dN/dE\propto E^{-\gamma}$) is between 1.6 and 2.0 from redshift cumulative distribution. The value of $\gamma$ is similar to that of FRB 121102, which indicates single bursts may share the same physical mechanism with the repeaters. In another case, if the formation rate of FRB is proportional to the SFR without time delay, the index $\gamma$ is about 2.3. In both cases, we find that FRBs may prefer to occur in low-metallicity environment with $ 12 +\log(\rm{O/H}) \simeq 8.40$, which is similar to those of long gamma-ray bursts (GRBs) and hydrogen-poor superluminous supernovae (SLSNe-I)., Comment: 7 pages, 4 figures, 1 table, accepted for publication in MNRAS

Fast radio bursts (FRBs) are bright milliseconds radio transients with large dispersion measures. Recently, FRB 200428 was detected in temporal coincidence with a hard X-ray flare from the Galactic magnetar SGR 1935+2154, which supports that at least some FRBs are from magnetar activity. Interestingly, a portion of X-ray flares from magnetar XTE J1810-197 and the Sun are also accompanied by radio bursts. Many features of Galactic FRB 200428 and cosmological FRBs resemble solar radio bursts. However, a common physical origin among FRBs, magnetar radio pulses and solar radio bursts has not yet been established. Here we report a universal correlation between X-ray luminosity and radio luminosity over twenty orders of magnitude among solar type III radio bursts, XTE J1810-197 and Galactic FRB 200428. This universal correlation reveals that the energetic electrons which produce the X-ray flares and those which cause radio emissions have a common origin, which can give stringent limits on the generation process of radio bursts. Moreover, we find similar occurrence frequency distributions of energy, duration and waiting time for solar radio bursts, SGR 1935+2154 and repeating FRB 121102, which also support the tight correlation and the X-ray flares temporally associated with radio bursts. All of these distributions can be understood by avalanche models of self-organized criticality systems. The universal correlation and statistical similarities indicate that the Galactic FRB 200428 and FRBs seen at cosmological distances can be treated as scaled-up solar radio bursts., 7 pages, 4 figures, 1 table, title changed, the L_X-L_R correlation between FRBs and solar type III radio bursts added, accepted for publication in MNRAS

The millimeter-sized Nd9.5Fe61.5Co10Ti2.5Nb0.5B16−x C x (x = 0–1.25) alloy rods with various compositions were fabricated by direct casting. Nano-sized hard phase Nd2(FeCo)14B, soft phase ɑ-FeCo, and amorphous phase were observed in all alloys. An optimized amount of carbon additions improved the magnetic properties by enhancing the glass forming ability and forming near single domain-sized Nd2(FeCo)14B grains around the rod surface. Various intergranular structures were observed in the alloys with x = 0.25–1. Micromagnetic simulation using the images obtained from the magnetic force microscope and transition electron microscope indicates that the distribution and magnetism of the intergranular phase have an important influence on the magnetic properties and demagnetization process of the alloys. A uniformly distributed nonmagnetic intergranular amorphous phase may enhance the magnetic properties, but the coercivity decreases when the amorphous phase is magnetic. It is important to modify the structure and distribution of the inter-grain amorphous phase in order to achieve high hard magnetic properties in these alloys.

Numerical approach based on the electrical simulation method is proposed to solve a nonlinear transient heat conduction problem with nonlinear boundary for a spherical body. This problem represents a strong nonlinearity in both the governing equation for temperature dependent thermal property and the boundary condition for combined convective and radiative cooling. By analysing the equivalent electrical model using the electrical circuit simulation program HSPICE, transient temperature and heat flux distributions at sphere can be obtained easily and fast. The solutions clearly illustrate the effect of the radiation-conduction parameter Nrc, the Biot number and the linear coefficient of temperature dependent conductivity and heat capacity. On comparing the results with corresponding numerical solutions, the accuracy and efficiency of this computational method is found to be good., {"references":["R. Siegel-J. R. Howell, 2001, Thermal Radiation Heat Transfer, Taylor & Francis, New York, NY 10001, USA","Lall, P. S., and Viskanta, R., 1967, \"Transient Energy Transfer in a Gray Radiating Gas During Expansion,\" Physics of Fluids, 10(1), pp. 98-107.","Viskanta, R., and Merriam, R. L., 1967, \"Shielding of Surfaces in Couette Flow against Radiation by Transpiration of an Absorbing-Emitting Gas,\" International Journal of Heat and Mass Transfer, 10(5), pp. 641-653.","Bayazitoglu, Y., and Suryanarayana, P. V. R., 1989, \"Transient Radiative Heat Transfer from a Sphere Surrounded by a Participating Medium,\" Journal of Heat Transfer, 111(3), pp. 713-718.","Tsai, J. R., and Özişik, M. N., 1987, \"Transient, Combined Conduction and Radiation in an Absorbing, Emitting, and Isotropically Scattering Solid Sphere,\" Journal of Quantitative Spectroscopy and Radiative Transfer, 38(4), pp. 243-251.","Thynell, S. T., 1990, \"Interaction of Conduction and Radiation in Anisotropically Scattering, Spherical Media,\" Journal of Thermophysics and Heat Transfer, 4(3), pp. 299-304.","Trabelsi, H., Sghaier, T., and Sifaoui, M. S., 2005, \"A Theoretical Study of Radiation between Two Concentric Spheres Using a Modified Discrete Ordinates Method Associated with Legendre Transform,\" Journal of Quantitative Spectroscopy and Radiative Transfer, 93(4), pp. 415-428.","A. Haji-Sheikh, E. M. S., 1967, \"Solution of Heat Conduction Problems by Probability Methods,\" Trans. ASME, J. Heat Transfer, 89, pp. 121-131.","Davies, T. W., 1985, \"The Cooling of a Plate by Combined Thermal Radiation and Convection,\" International Communications in Heat and Mass Transfer, 12(4), pp. 405-415.\n[10]\tParang, M., Crocker, D. S., and Haynes, B. D., 1990, \"Perturbation Solution for Spherical and Cylindrical Solidification by Combined Convective and Radiative Cooling,\" International Journal of Heat and Fluid Flow, 11(2), pp. 142-148.\n[11]\tSundén, B., 1986, \"Transient Heat Conduction in a Composite Slab by a Time-Varying Incident Heat Flux Combined with Convective and Radiative Cooling,\" International Communications in Heat and Mass Transfer, 13(5), pp. 515-522.\n[12]\tKessler, G., 2009, \"Steady State and Transient Temperature Profiles in a Multishell Spherical System Heated Internally by Reactor-Grade Plutonium,\" Nuclear Engineering and Design, 239(11), pp. 2430-2443.\n[13]\tSu, J., 2004, \"Improved Lumped Models for Transient Radiative Cooling of a Spherical Body,\" International Communications in Heat and Mass Transfer, 31(1), pp. 85-94.\n[14]\tLiao, S., Su, J., and Chwang, A. T., 2006, \"Series Solutions for a Nonlinear Model of Combined Convective and Radiative Cooling of a Spherical Body,\" International Journal of Heat and Mass Transfer, 49(15-16), pp. 2437-2445.\n[15]\tEl-Nahhas, A., 2009, \"Analytic Approximations for a Strongly Nonlinear Problem of Combined Convective and Radiative Cooling of a Spherical Body,\" Journal of Heat Transfer, 131(11), pp. 111703-6.\n[16]\tV. Paschkis, H. D. B., 1942, \"A Method for Determining Unsteady-State Heat Transfer by Means of an Electrical Analogy,\" Transactions ASME, 46, pp. 7-15.\n[17]\tJordán, J. Z., 2006, \"Network Method to Study the Transient Heat Transfer Problem in a Vertical Channel with Viscous Dissipation,\" International Communications in Heat and Mass Transfer, 33(9), pp. 1079-1087.\n[18]\tGonzález Fernández, C. F., Alhama, F., López Sánchez, J. F., and Horno, J., 1998, \"Application of the Network Method to Heat Conduction Processes with Polynomial and Potential-Exponentially Varying Thermal Properties,\" Numerical Heat Transfer; Part A: Applications, 33(5), pp. 549-559.\n[19]\tZueco, J., Eguía, P., Granada, E., Míguez, J. L., and Bég, O. A., 2010, \"An Electrical Network for the Numerical Solution of Transient Mhd Couette Flow of a Dusty Fluid: Effects of Variable Properties and Hall Current,\" International Communications in Heat and Mass Transfer, 37(10), pp. 1432-1439.\n[20]\tNagel, L. W., 1973, Spice, a Computer Program to Simulate Semiconductor Circueqits, University of California."]}

MnBi magnets with a high content of low temperature phase (LTP) and excellent magnetic properties were prepared by spark plasma sintering (SPS) using ball milling powders as precursors without magnetic purification. A complicated intergranular phase, which contains Mn phase, Bi phase, MnO phase, and even amorphous phase in MnBi magnets, was characterized and reported systematically. It was found that the formation of intergranular phase which was contributed by ball milling precursors and sintering mechanism, jointly, had important influence on the magnetic properties. The appropriate content of intergranular phase was beneficial in improving the coercivity due to the strong magnetic isolation effects. The optimum magnetic properties with Mr=26.0 emu/g, Hci= 7.11 kOe and (BH)max=1.53 MGOe at room temperature, and a maximum value Hci= 25.37 kOe at 550 K can be obtained. Strongly favorable magnetic properties make SPSed MnBi magnets an attractive candidate material for small permanent magnets used in high-temperature applications.

Inspired by superior flight performance of natural flight masters like birds and insects and based on the ventilating flaps that can be opened and closed by the changing air pressure around the wing, a new flapping wing type has been proposed. It is known that the net lift force generated by a solid wing in a flapping cycle is nearly zero. However, for the case of the ventilated wing, results for the net lift force are positive which is due to the effect created by the “ventilation” in reducing negative lift force during the upstroke. The presence of moving flaps can serve as the variable in which, through careful control of the areas, a correlation with the decrease in negative lift can be generated. The corresponding aerodynamic characteristics have been investigated numerically by using different flapping frequencies and forward flight speeds. Published version

The aerodynamic characteristics for the vee-tail aircraft configuration have been investigated numerically at low Reynolds number (Re < 106). Three types of vee-tails have been adopted to be compared with the conventional tail type (with horizontal and vertical tails). The results show that the effect created by the vee-tail can greatly affect the aerodynamic characteristics of the whole aircraft. If the dihedral angle of the vee-tail is too large, it can directly affect the flight stability. The main reason for the longitudinal moment curve “moving up” at high dihedral angle can be attributed to the serious asymmetric downwash effect. Based on these, the force variation and the corresponding aerodynamic characteristics for three different sizes of aileron with different deflection angles have also been obtained computationally and analyzed in detail.

The aerodynamic characteristics between the canard and wing of the Canard-forward swept wing aircraft configurations have been investigated numerically at low Reynolds number. The variation of the aerodynamic characteristics at different canard positions is the focus of the present investigation. The aerodynamic interference and the mutual coupling effect between the canard and wing will have great influences on the lift, drag, and sideslip characteristics of the whole aircraft. The canard-generated vortex can induce a favorable interference onto the main wing, controlling the onset of the boundary layer separation from the leading edge. At small angles of attack (α20 deg they are not only related to the orientation of the canard (forward or backward), but also the features of the vortices generated above the canard and the wing, including their strength and location.

The aerodynamic characteristics of propeller-wing interaction for the rocket launched UAV have been investigated numerically by means of sliding mesh technology. The corresponding forces and moments have been collected for axial wing placements ranging from 0.056 to 0.5 D and varied rotating speeds. The slipstream generated by the rotating propeller has little effects on the lift characteristics of the whole UAV. The drag can be seen to remain unchanged as the wing's location moves progressively closer to the propeller until 0.056 D away from the propeller, where a nearly 20% increase occurred sharply. The propeller position has a negligible effect on the overall thrust and torque of the propeller. The efficiency affected by the installation angle of the propeller blade has also been analyzed. Based on the pressure cloud and streamlines, the vortices generated by propeller, propeller-wing interaction, and wing tip have also been captured and analyzed.

The hinge moment characteristics for ONERA-M6 wing with aileron configuration have been investigated numerically based on the different gaps and deflecting angles. The results show that the effects on the wing made by the deflecting aileron are notable. Comparing with the nonaileron case, the chordwise pressure coefficient distribution for the wing with aileron has shown the totally different trends. The small gap can force the air flow through and form the extremely strong spraying flow. It can directly destroy the previously formed leading edge vortex (LEV). Due to the presence of the positive deflecting angle, the trailing edge vortex (TEV) will begin to generate at the trailing edge of the aileron. The induced secondary LEV will be mixed with the developing TEVs and form the stronger TEVs at the downstream position. Comparing with the subsonic flow, the curve for the supersonic flow has shown a good linear. The corresponding hinge moments are also extremely sensitive to the changing angle of attack, and the slope of curves is also bigger than that of the subsonic flow. The bigger gap and deflecting angle can result in the curve of hinge moment bending upward at high angle of attack. The corresponding pressure cloud and streamlines have also been obtained computationally and analyzed in detail.

Animal slurry is a source of environmental pollutants including NH3, CO2 and H2S. Animal slurry stored in-barn is characterized by the frequent occurrence of surface liquid disturbances. One type of disturbance occurs when new slurry is produced by the animals, which falls into the pit, and alters the equilibrium of the slurry surface and the gas emission patterns. However, information about emissions during and after disturbances is very limited. In this study, an evaluation of the effect of spatial and temporally distributed slurry addition on the total emissions of NH3, CO2 and H2S is provided. The emissions of NH3 and CO2 remained rather constant under steady conditions during storage. Ammonia emissions showed a decrease shortly after performing each addition, followed by an increase, while carbon dioxide emissions showed an immediate increase followed by a sharp decrease. Slurry frequently disturbed by slurry addition caused a fluctuating NH3 and CO2 emission patterns. Hydrogen sulphide emissions were much higher during disturbances.

The mechanical characteristics of the long-chain double-stranded DNA (dsDNA) molecule under the axial stretching and lateral unzipping are studied by the clustered atomistic-continuum method (CACM). The CACM consisted of the clustered atom method (CAM) and the atomistic-continuum method (ACM). The CAM treats the specific atomic group as the superatom, and the ACM describes the chemical binding energies between (super)atoms by virtual elements. The Newtonian based model of the dsDNA includes the superatoms of the backbones∕base pairs and the virtual elements of the stacking energies∕hydrogen bonds. The effective properties of the superatoms are numerically extracted from the single-stranded DNA experiments. Good agreements were achieved between the dsDNA numerical results and the single molecular experimental results. Via the simulation of stretching dsDNA, the mechanical responses, including the twisting of the backbone and variation of the elastic deformation energy and stacking energy, can be elucidated. Moreover, the predictive capability of the dsDNA CACM model is then examined. Furthermore, the dsDNA model with sequential information is subjected to the unzipping loading, and the in silico results reveal that the sliding of the backbones and the sequential dependent mechanical responses.