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A fast radio burst (FRB) was recently detected to be associated with a hard X-ray burst from the Galactic magnetar SGR 1935+2154. Scenarios involving magnetars for FRBs are hence highly favored. In this work, we suggest that the impact between an asteroid and a magnetar could explain such a detection. According to our calculations, an asteroid of mass $10^{20}$ g will be disrupted at a distance of $7 \times 10^9$ cm when approaching the magnetar. The accreted material will flow along the magnetic field lines from the Alfv\'en radius $\sim 10^7$ cm. After falling onto the magnetar's surface, an instant accretion column will be formed, producing a Comptonized X-ray burst and an FRB in the magnetosphere. We show that all the observational features of FRB 200428 could be interpreted self-consistently in this scenario. We predict quasi-periodic oscillations in this specific X-ray burst, which can serve as an independent observational test., Comment: 5 pages, 1 figure. References updated, accepted by ApJL

The broadband afterglow of GW170817/GRB 170817A is believed to be from an off-axis structured jet. The central engine of a gamma-ray burst usually launches a pair of outflows that move oppositely, it is reasonable to consider the emission from a double-sided structured jets, with a near-jet moving toward us and a counter-jet moving away from us. Assuming that the two branches of the jet have the same physical parameters, we have calculated their radio emission. It is found that the counter-jet component will emerge in the radio light curves $\sim$ 2500 days post-merger.It typically leads to a plateau in the light curve, thus could be hopefully revealed by high accuracy radio observations. We have also considered the possibilities that both branches have different parameters, and found that if some of the physical parameters of the counter-jet are evaluated more favorably than those of the near-jet, then the counter-jet emission will be enhanced and will clearly show up as an obvious plateau or even a rebrightening. For example, when the circum-burst medium encountered by the counter-jet is assumed to be 100 times denser than that of the near-jet, a remarkable radio plateau will appear at $\sim$ 600 days. However, in X-ray bands, the counter-jet component is generally too faint to be discerned. It is argued that the late radio observations of GW170817/GRB 170817A can help determine the key parameters and diagnose the environment of the event., Comment: 7 pages, 3 figures, accepted for publication in the Astrophysical Journal (ApJ)

Currently, FRB 121102 is the only fast radio burst source that was observed to give out bursts repeatedly. It shows a high repeating rate, with more than one hundred bursts being spotted, but with no obvious periodicity in the activities. Thanks to its repetition, the source was well localized with a subarcsecond accuracy, leading to a redshift measurement of about 0.2. FRB 121102 is a unique source that can help us understand the enigmatic nature of fast radio bursts. In this study, we analyze the characteristics of the waiting times between bursts from FRB 121102. It is found that there is a clear bimodal distribution for the waiting times. While most waiting times cluster at several hundred seconds, a small portion of the waiting times are strikingly in the range of 2--40 millisecond. More interestingly, it is found that the waiting time does not correlate with the burst intensity, either for the preceding burst or for the subsequent burst. It strongly indicates that the repeating bursts should be generated by some external mechanisms, but not internal mechanisms. As a result, the models involving collisions between small bodies and neutron stars could be competitive mechanisms for this interesting source., Comment: 11 pages, 3 figures, accepted for publication in International Journal of Cosmology, Astronomy and Astrophysics

Fast radio bursts (FRBs) are bright radio pulses from the sky with millisecond durations and Jansky-level flux densities. Their origins are still largely uncertain. Here we suggest a new model for FRBs. We argue that the collision of a white dwarf with a black hole can give birth to a transient accretion disk, from which powerful episodic magnetic blobs will be launched. The collision between two consecutive magnetic blobs can result in a catastrophic magnetic reconnection, which releases a large amount of free magnetic energy and forms a forward shock. The shock propagates through the cold magnetized plasma within the blob in the collision region, radiating through synchrotron maser mechanism, which is responsible for a non-repeating FRB signal. Our calculations show that the theoretical energetics, radiation frequency, duration timescale, and event rate can be well consistent with the observational characteristic of FRBs., Comment: 8 pages, 1 figure, final version, RAA (Research in Astronomy and Astrophysics), 2018, 18, 61

Recent follow-up observations of the binary neutron star (NS) merging event GW170817/SGRB 170817A reveal that its X-ray/optical/radio emissions are brightening continuously up to $\sim 100$ days post-merger. This late-time brightening is unexpected from the kilonova model or the off-axis top-hat jet model for gamma-ray burst afterglows. In this paper, by assuming that the merger remnant is a long-lived NS, we propose that the interaction between an electron-positron-pair ($e^+e^-$) wind from the central NS and the jet could produce a long-lived reverse shock, from which a new emission component would rise and can interpret current observations well. The magnetic-field-induced ellipticity of the NS is taken to be $4 \times 10^{-5}$ in our modeling, so that the braking of the NS is mainly through the gravitational wave (GW) radiation rather than the magnetic dipole radiation, and the emission luminosity at early times would not exceed the observational limits. In our scenario, since the peak time of the brightening is roughly equal to the spin-down time scale of the NS, the accurate peak time may help constrain the ellipticity of the remnant NS. We suggest that radio polarization observations of the brightening would help to distinguish our scenario from other scenarios. Future observations on a large sample of short gamma-ray burst afterglows or detections of GW signals from merger remnants would test our scenario., Comment: 7 pages, 1 figure, accepted by ApJL, submitted on January 25, 2018

The brightness of the multi-wavelength afterglow of GRB 170817A is increasing unexpectedly even $\sim$ 160 days after the associated gravitational burst. Here we suggest that the brightening can be caused by a late-time energy injection process. We use an empirical expression to mimic the evolution of the injection luminosity, which is consisted of a power-law rising phase and a power-law decreasing phase. It is found that the power-law indices of the two phases are $0.92$ and $-2.8$, respectively, with the peak time of the injection being $\sim 110$ days. The energy injection could be due to some kind of accretion, with the total accreted mass being $\sim 0.006 M_\odot$. However, normal fall-back accretion, which usually lasts for a much shorter period, cannot provide a natural explanation. Our best-fit decaying index of $-2.8$ is also at odds with the expected value of $-5/3$ for normal fall-back accretion. Noting that the expansion velocities of the kilonova components associated with GW170817 is $0.1-0.3\,c$, we argue that there should also be some ejecta with correspondingly lower velocities during the coalescence of the double neutron star system. They are bound by the gravitational well of the remnant central compact object and might be accreted at a timescale of about 100 days, providing a reasonable explanation for the energy injection. Detailed studies on the long-lasting brightening of GRB 170817A thus may provide useful information on the matter ejection during the merger process of binary neutron stars., Comment: 7 pages, 3 figures, accepted for publication in The Astrophysical Journal Letter

Considering the contribution of the emission from the host galaxies of gamma-ray bursts (GRBs) to the radio afterglows, we investigate the effect of host galaxies on observations statistically. For the three types of events, e.g. low-luminosity, standard and high-luminosity GRBs, it is found that a tight correlation exists between the ratio of the radio flux (RRF) of host galaxy to the total radio peak emission and the observational frequency. Especially, toward lower frequencies, the contribution from the host increases significantly. The correlation can be used to get a useful estimate for the radio brightness of those host galaxies which only have very limited radio afterglow data. Using this prediction, we re-considered the theoretical radio afterglow light curves for four kinds of events, i.e. high-luminosity, low-luminosity, standard and failed GRBs, taking into account the contribution from the host galaxies and aiming at exploring the detectability of these events by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Lying at a typical redshift of $z=1$, most of the events can be detected by FAST easily. For the less fierce low-luminosity GRBs, their radio afterglows are not strong enough to exceed the sensitivity limit of FAST at such distances. However, since a large number of low luminosity bursts actually happen very near to us, it is expected that FAST will still be able to detect many of them., Comment: 10 pages, 5 figures and 4 tables; accepted for publication in MNRAS

Using the generic hydrodynamic model of Gamma-Ray Burst (GRB) afterglows, we calculate the radio afterglow light curves of low luminosity, high luminosity, failed and standard GRBs in different observational bands of FAST's energy window. The GRBs are assumed to be located at different distances from us. Our results rank the detectability of GRBs in descending order as high luminosity, standard, failed and low luminosity GRBs. We predict that almost all types of radio afterglows except that of low luminosity GRBs could be observed by the large radio telescope as long as the domains of time and frequency are appropriate. It is important to note that FAST can detect relatively weak radio afterglows at a higher frequency of 2.5 GHz for very high redshift up to z=15 or even more. Radio afterglows of low luminosity GRBs can be detected only after the completion of the 2nd phase of FAST. FAST is expected to significantly expand the sample of GRB radio afterglows in the near future., Comment: 16 pages, 8 figures (2 new figs added), 1 table; accepted by Research in Astronomy and Astrophysics