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We report the observations of FRB 20220912A using the Five-hundred-meter Aperture Spherical radio Telescope. We conducted 17 observations totaling 8.67 hr and detected a total of 1076 bursts with an event rate up to 390 hr−1. The cumulative energy distribution can be well described using a broken power-law function with the lower- and higher-energy slopes of −0.38 ± 0.02 and −2.07 ± 0.07, respectively. We also report the L-band (1–1.5 GHz) spectral index of the synthetic spectrum of FRB 20220912A bursts, which is −2.6 ± 0.21. The average rotation measure value of the bursts from FRB 20220912A is −0.08 ± 5.39 rad m−2, close to 0 rad m−2 and was relatively stable over 2 months. Most bursts have nearly 100% linear polarization. About 45% of the bursts have circular polarization with Signal-to-Noise ratio > 3, and the highest circular polarization degree can reach 70%. Our observations suggest that FRB 20220912A is located in a relatively clean local environment with complex circular polarization characteristics. These various behaviors imply that the mechanism of circular polarization of FRBs likely originates from an intrinsic radiation mechanism, such as coherent curvature radiation or inverse Compton scattering inside the magnetosphere of the FRB engine source (e.g., a magnetar).

The nondetection of periodicity related to rotation challenges magnetar models for fast radio bursts (FRBs) with FRB emission from close to the magnetar surface. Moreover, a bimodal distribution of the burst waiting times is widely observed in hyperactive FRBs, a significant deviation from the exponential distribution expected from stationary Poisson processes. By combining the epidemic-type aftershock sequence earthquake model and the rotating vector model involving the rotation of the magnetar and orientations of the spin and magnetic axes, we find that starquake events modulated by the rotation of FRB-emitting magnetar can explain the bimodal distribution of FRB waiting times, as well as the nondetection of periodicity in hyperactive repeating FRBs. We analyze data from multiple FRB sources, demonstrating that differences in waiting time distributions, and to some extent, observed energies can be explained by varying parameters related to geometric properties of the magnetar FRB emission and starquake dynamics. Our results show that the assumption that all FRBs are repeaters is compatible with our model. Notably, we find that hyperactive repeaters tend to have small magnetic inclination angles in order to hide their periodicity. We also show that our model can reproduce the waiting time distribution of a pulsar phase of the galactic magnetar SGR J1935+2154 with a larger inclination angle than the hyperactive repeaters, which could explain the detection of spin period and the relatively low observed energy for FRBs from the magnetar. The spin periods of hyperactive repeaters are not well constrained, but most likely fall in the valley region between the two peaks of the waiting time distributions.

We report the observations of a repeating FRB 20230607A for 15.6 hr spanning 16 months using the Five-hundred-meter Aperture Spherical Radio Telescope with the detection of 565 bursts. We present three bright bursts with detailed temporal/spectral structures. We also report that one burst carries a narrow component with a width of only 0.3 ms, which is surrounded by broader components. This suggests that repeaters can make both narrow and broad components in one burst. With the narrow spike, we precisely measure the dispersion measure of 362.85 ± 0.15 pc cm−3 and the Faraday rotation measures (RMs) of −12249.0 ± 1.5 rad m−2. We also analyze the statistical distribution of the burst parameters, including waiting times, temporal widths, central frequencies and frequency widths, fluences and energies, all showing typical distributions of known active repeaters. In particular, most bursts show narrow spectra with Δν/ν0 = 0.125 ± 0.001. This fact, together with the narrow 0.3 ms spike, strongly suggests a magnetospheric origin of the FRB emission. Based on a predicted correlation between RM and the luminosity of a persistent radio source (PRS) by Yang et al., we predict that the PRS should have a specific luminosity of the order of 1029 erg s−1 Hz−1 and encourage a search for such a PRS.

Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients of extragalactic origin. Some of them repeat, while others apparently do not. Investigations of periodic activity in repeating FRB have been conducted to probe their origins. While periodicity in the burst rate has been reported, studies of periodicities in other properties, such as dispersion measure and rotation measure (RM), are sparse. FRB 20220529 was monitored by the Five-hundred-meter Aperture Spherical radio Telescope for nearly 3 yr, providing an opportunity to investigate periodicity in its observed properties. Here we report a possible period of ∼200 days in the RM evolution, with a significance of 4.1σ estimated via the Lomb–Scargle algorithm and 3.1σ with the phase-folding method. Periodicity in the burst rate was also investigated. It may indicate that the FRB progenitor is in a binary system, which is consistent with the significant RM increase and prompt recovery of this FRB on a week timescale. Other scenarios, such as a system with an intermediate-mass black hole, are also explored.

We present the interstellar scintillation analysis of fast radio burst (FRB) 20220912A during its extremely active episode in 2022 using data from the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). We detect a scintillation arc in the FRB’s secondary spectrum, which describes the power in terms of the scattered FRB signals’ time delay and Doppler shift. The arc indicates that the scintillation is caused by a highly localized region. Our analysis favors a Milky Way origin of the ionized interstellar medium (IISM) for the localized scattering medium but cannot rule out a host galaxy origin. We present our method for detecting the scintillation arc, which can be applied generally to sources with irregularly spaced bursts or pulses. These methods could help shed light on the complex interstellar environment surrounding the FRBs and in our Galaxy.

The nature of irregularly spaced pulses of rotating radio transients (RRATs) complicates interstellar scintillation studies. In this Letter, we report the primary scintillation parameters of a sample of RRATs using pairwise correlations of pulse spectra. Moreover, from the measured scintillation velocities, we constrain their transverse velocities. We also find a reduced modulation index, m = 0.13 ± 0.01, for RRAT J1538+2345. Several possible explanations are discussed. Furthermore, the single-pulse-based interstellar scintillation technique is applicable to other pulsar populations, including nulling pulsars and those with short scintillation timescales, and fast radio bursts.

The physical origin of fast radio bursts (FRBs) remains uncertain. Although multiwavelength observations have been widely conducted, only Galactic FRB 20200428D is associated with an X-ray burst from the magnetar SGR J1935+2154. Here we present multiwavelength follow-up observations of the nearby bright FRB 20250316A, including the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Einstein Probe (EP) X-ray mission, Chandra X-ray Observatory, Wide Field Survey Telescope (WFST), and Space Variable Objects Monitor/Visible Telescope (SVOM/VT). The 13.08 hr FAST follow-up campaign without pulse detection requires an energy distribution flatter than those of well-known repeating FRBs, suggesting that this burst is likely a one-off event. A prompt EP follow-up and multiepoch observational campaign totaling >100 ks led to the detection of an X-ray source within the angular resolution of its Follow-up X-ray Telescope (FXT; 10″). A subsequent Chandra observation revealed this source to be offset by 7″ from the FRB position and established a 0.5–10 keV flux upper limit of 7.6 × 10−15 erg cm−2 s−1 at the FRB position, corresponding to ∼1039 erg s−1 at the 40 Mpc distance of the host galaxy NGC 4141. These results set one of the most stringent limits on X-ray emission from a nonrepeating FRB, disfavoring ultraluminous X-ray sources as counterparts of apparently one-off FRBs and offering critical insights into afterglow models. Our study suggests that an arcsecond localization of both the FRB and its potential X-ray counterpart is essential for exploring the X-ray counterpart of an FRB.

Limited information is available related to soil organic carbon (SOC), nitrogen (N), and their associated fractions, especially in diversified cropping sequences with a combination of tillage systems. Therefore, a field study was conducted to evaluate the effects of cropping sequences and tillage systems on SOC and N and associated fractions. The experiment was comprised of two factors, i.e., (i) tillage systems: no tillage (NT) and rotary tillage (RT), and (ii) cropping sequences: wheat-soybean-wheat-maize (WSWM); wheat-maize-wheat-soybean (WMWS); wheat-soybean-wheat-soybean (WS); and wheat-maize-wheat-maize (WM). Tillage systems influenced the distribution of SOC and N and their associated fractions mainly at topsoil depth rather than deep soil, while cropping sequences affected SOC and N and their associated fractions differently in the whole soil sampling depth (0–50 cm). The results showed that NT had significantly higher SOC concentrations than RT at the 0–10- (17% higher) and 20–30-cm (19% higher) soil layers. Similarly, NT had 17% significantly higher N contents than RT at the 0–10-cm soil layer, but RT had 21% significantly higher N accumulation at the 10–20-cm soil layer. The particulate organic carbon (POC) was highest in WM and lowest in WS cropping sequence at 0–10-cm soil depth, while tillage did not affect POC distribution at 0–30-cm soil depth. Similarly, particulate organic nitrogen (PON) was significantly higher in soybean-included cropping sequences only at 0–10-cm soil depth. Some other fractions, such as dissolved organic carbon (DOC) and dissolved organic nitrogen (DON), were higher in soybean-included cropping sequences at 0–30- and 0–20-cm soil depths respectively. Mineral-associated organic carbon (MAOC) also increased by 28% and 34% ( p  < 0.05) under NT compared to RT at the 0–10- and 10–20-cm soil layers, respectively. In the case of cropping sequence comparison, WSWM had 30% higher SOC at the 10–20-cm soil layer than the other three cropping sequences. Notably, legume-included cropping sequences (WSWM, WMWS, WS) significantly increased N contents by 9%, 15%, and 22% and mineral-associated organic nitrogen (MAON) by 12%, 15%, and 17.5%, respectively, compared to the WM cropping sequence at the 0–10-cm soil layer. SOC and TN and their fractions were redistributed by tillage and cropping sequences at 20–50-cm soil layers. However, SOC stock was only affected by tillage systems (NT had 10% higher than RT) rather than cropping sequences. But WMWS and WS cropping sequences had 11% and 10% significantly higher N stock than WSWM and WM sequences, respectively. Overall, our findings suggested that NT especially with soybean could be a suitable practice to sequester SOC and N in the North China Plain.

We present the interstellar scintillation analysis of fast radio burst (FRB) 20220912A during its extremely active episode in 2022 using data from the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). We detect a scintillation arc in the FRB's secondary spectrum, which describes the power in terms of the scattered FRB signals' time delay and Doppler shift. The arc indicates that the scintillation is caused by a highly localized region of the ionized interstellar medium (IISM). Our analysis favors a Milky Way origin for the localized scattering medium but cannot rule out a host galaxy origin. We present our method for detecting the scintillation arc, which can be applied generally to sources with irregularly spaced bursts or pulses. These methods could help shed light on the complex interstellar environment surrounding the FRBs and in our Galaxy.

We report the properties of more than 800 bursts detected from the repeating fast radio burst (FRB) source FRB 20201124A with the Five-hundred-meter Aperture Spherical radio telescope (FAST) during an extremely active episode on UTC September 25th-28th, 2021 in a series of four papers. In this fourth paper of the series, we present a systematic search of the spin period and linear acceleration of the source object from both 996 individual pulse peaks and the dedispersed time series. No credible spin period was found from this data set. We rule out the presence of significant periodicity in the range between 1 ms to 100 s with a pulse duty cycle \\(< 0.49\\pm0.08\\) (when the profile is defined by a von-Mises function, not a boxcar function) and linear acceleration up to \\(300\\) m s\\(^{-2}\\) in each of the four one-hour observing sessions, and up to \\(0.6\\) m s\\(^{-2}\\) in all 4 days. These searches contest theoretical scenarios involving a 1 ms to 100 s isolated magnetar/pulsar with surface magnetic field \\(<10^{15}\\) G and a small duty cycle (such as in a polar-cap emission mode) or a pulsar with a companion star or black hole up to 100 M\\(_{\\rm \\odot}\\) and \\(P_b>10\\) hours. We also perform a periodicity search of the fine structures and identify 53 unrelated millisecond-timescale \"periods\" in multi-components with the highest significance of 3.9 \\(\\sigma\\). The \"periods\" recovered from the fine structures are neither consistent nor harmonically related. Thus they are not likely to come from a spin period. We caution against claiming spin periodicity with significance below \\(\\sim\\) 4 \\(\\sigma\\) with multi-components from one-off FRBs. We discuss the implications of our results and the possible connections between FRB multi-components and pulsar micro-structures.