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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 20 orders of magnitude among solar type III radio bursts, XTE J1810−197 and Galactic FRB 200428. This universal correlation reveals that the energetic electrons that produce the X-ray flares and those that 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. [ABSTRACT FROM AUTHOR]

Type I X-ray bursts in a low-mass X-ray binary are caused by unstable nuclear burning of accreted materials. Semi-analytical and numerical studies of unstable nuclear burning have successfully reproduced the partial properties of this kind of burst. However, some other properties (e.g. the waiting time) are not well explained. In this paper, we find that the probability distributions of fluence, peak count, rise time, duration and waiting time can be described as power-law-like distributions. This indicates that type I X-ray bursts may be governed by a self-organized criticality (SOC) process. The power-law index of the waiting time distribution (WTD) is around -1, which is not predicted by any current waiting time model.We propose a physical burst rate model, in which the mean occurrence rate is inversely proportional to time: λ ∝ t-1. In this case, the WTD is explained well by a non-stationary Poisson process within the SOC theory. In this theory, the burst size is also predicted to follow a power-law distribution, which requires that the emission area covers only part of the neutron star surface. Furthermore, we find that the WTDs of some astrophysical phenomena can also be described by similar occurrence rate models. [ABSTRACT FROM AUTHOR]

We present photometry of the intermediate polar FO Aquarii (FO Aqr) obtained as part of the K2 mission using the Kepler space telescope. The amplitude spectrum of the data confirms the orbital period of 4.8508(4) h, and the shape of the light curve is consistent with the outer edge of the accretion disc being eclipsed when folded on this period. The average flux of FO Aqr changed during the observations, suggesting a change in the mass accretion rate. There is no evidence in the amplitude spectrum of a longer period that would suggest disc precession. The amplitude spectrum also shows the white dwarf spin period of 1254.3401(4) s, the beat period of 1351.329(2) s, and 31 other spin and orbital harmonics. The detected period is longer than the last reported period of 1254.284(16) s, suggesting that FO Aqr is now spinning down, and has a positive Ṗ. There is no detectable variation in the spin period over the course of the K2 observations, but the phase of the spin cycle is correlated with the system brightness. We also find that the amplitude of the beat signal is correlated with the system brightness. [ABSTRACT FROM AUTHOR]

The shock model has successfully explained the observed behaviours of afterglows from long gamma-ray bursts (GRBs). Here, we use it to investigate the so-called early afterglows from short GRBs, which arise from blast waves that are not decelerated considerably by their surrounding medium. We consider a nearby medium loaded with pairs. First, the temporal behaviours show a soft-to-hard spectral evolution, from optical to hard X-ray, and then a usual hard-to-soft evolution after the blast waves begin to decelerate. The light curves show variability and consist of two peaks. The first peak, owing to the pair effect, can be observed in the X-ray, although too faint and too short in the optical. The second peak will be detected easily by Swift. We show that detections of the double-peak structure in the light curves of early afterglows are very helpful in determining all the shock parameters of short GRBs, including both the parameters of the relativistic source and the surroundings. Besides, from the requirement that the forward-shock emission in short GRBs should be below the Burst and Transient Source Experiment (BATSE) detection threshold, we give a strong constraint on the shock model parameters. In particular, the initial Lorentz factor of the source is limited to be no more than , and the ambient medium density n is inferred to be low, . [ABSTRACT FROM AUTHOR]