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 $${903}_{-111}^{+72}$$ 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.

Using X-ray monitoring observations with the All-Sky Monitor on board the Rossi X-Ray Timing Explorer and the Burst Alert Telescope on board the Swift, we are able to study the spectral state transitions occurred in about 20 bright persistent and transient black hole and neutron star binaries. We have confirmed that there is a correlation between the X-ray luminosity corresponding to the hard-to-soft transition and the X-ray luminosity of the following soft state. This correlation holds over a luminosity range spanning by 2 orders of magnitude, with no indication of a flux saturation or cutoff. We have also found that the transition luminosity correlates with the rate of increase in the X-ray luminosity during the rising phase of an outburst or flare, implying that the origin of the variation of the transition luminosity is associated with non-stationary accretion in both transient sources and persistent sources. The correlation between the luminosity corresponding to the end of the soft-to-hard transition and the peak luminosity of the preceding soft state is found insignificant. The results suggest that the hysteresis effect of spectral state transitions is primarily driven by non-stationary accretion when the mass accretion rate increases rather than the mass accretion rate decreases. Our results also imply that Galactic X-ray binaries can reach more luminous hard states during outbursts of higher luminosities and of similar rise timescales as those observed. Based on the correlations, we speculate that bright hard state beyond the Eddington luminosity will be observed in Galactic binaries in the next century. We also suggest that some ultra-luminous X-ray sources in nearby galaxies, which stay in the hard states during bright, short flares, harbor stellar-mass compact stars.

Aquila X-1 is a soft X-ray transient source and emits type I X-ray bursts. A spectral state transition was observed with RXTE during its outburst decay in 1997 February and March. Its 10-30 keV and 5-10 keV count rate ratio increased suddenly when its luminosity was between 4 and ergs s Ϫ1 , assuming a 2.5 kpc 35 12 # 10 distance. Spectral fitting with a model composed of a blackbody and a power-law component showed that its blackbody component decreased and that the power-law component became much harder significantly and simultaneously. We interpret this transition to be caused by the centrifugal barrier, more commonly known as the "propeller" effect. We thus infer that the magnetic field strength of the neutron star is around G, if the 8 1 # 10 neutron star spin period is 1.8 ms. Similarly, we infer the neutron star magnetic field strength in another soft Xray transient Cen X-4 is about G. We also propose a unified scheme for spectral state transitions in soft 9 2 # 10 X-ray transients, from soft high state to hard low state and further to quiescent state. With this scheme accretion onto neutron star may take place even during the propeller regime.

Fast radio bursts (FRBs) are highly dispersed radio bursts prevailing in the universe [1][2][3] . The recent detection of FRB 200428 from a Galactic magnetar [4][5][6][7][8] suggested that at least some FRBs originate from magnetars, but it is unclear whether the majority of cosmological FRBs, especially the actively repeating ones, are produced from the magnetar channel. Here we report the detection of 1863 polarised bursts from the repeating source FRB 20201124A 9 during a dedicated radio observational campaign of Five-hundred-meter Aperture Spherical radio Telescope (FAST). The large sample of radio bursts detected in 88 hr over 54 days indicate a significant, irregular, short-time variation of the Faraday rotation measure (RM) of the source during the first 36 days, followed by a constant RM during the later 18 days. Significant circular polarisation up to 75% was observed in a good fraction of bursts. Evidence suggests that some low-level circular polarisation originates from the conversion from linear polarisation during the propagation of the radio waves, but an intrinsic radiation mechanism is required to produce the higher degree of circular polarisation. All of these features provide evidence for a more complicated, dynamically evolving, magnetised immediate environment around this FRB source. Its host galaxy was previously known 10-12 . Our optical observations reveal that it is a Milky-Way-sized, metal-rich, barred-spiral galaxy at redshift z = 0.09795 ± 0.00003, with the FRB source residing in a low stellar density, interarm region

We investigate further a model of the accreting millisecond X-ray pulsars we proposed earlier. In this model, the X-ray-emitting regions of these pulsars are near their spin axes but move. This is to be expected if the magnetic poles of these stars are close to their spin axes, so that accreting gas is channeled there. As the accretion rate and the structure of the inner disk vary, gas is channeled along different field lines to different locations on the stellar surface, causing the X-ray-emitting areas to move. We show that this "nearly aligned moving spot model" can explain many properties of the accreting millisecond X-ray pulsars, including their generally low oscillation amplitudes and nearly sinusoidal waveforms; the variability of their pulse amplitudes, shapes, and phases; the correlations in this variability; and the similarity of the accretion-and nuclear-powered pulse shapes and phases in some. It may also explain why accretion-powered millisecond pulsars are difficult to detect, why some are intermittent, and why all detected so far are transients. This model can be tested by comparing with observations the waveform changes it predicts, including the changes with accretion rate.

We present the X-ray timing results of the new black hole candidate (BHC) MAXI J1535-571 during its 2017 outburst from Hard X-ray Modulation Telescope (Insight -HXMT) observations taken from 2017 September 6 to 23. Following the definitions given by Belloni (2010), we find that the source exhibits state transitions from Low/Hard state (LHS) to Hard Intermediate state (HIMS) and eventually to Soft Intermediate state (SIMS). Quasi-periodic oscillations (QPOs) are found in the intermediate states, which suggest different types of QPOs. With the large effective area of Insight -HXMT at high energies, we are able to present the energy dependence of the QPO amplitude and centroid frequency up to 100 keV which is rarely explored by previous satellites. We also find that the phase lag at the type-C QPOs centroid frequency is negative (soft lags) and strongly correlated with the centroid frequency. By assuming a geometrical origin of type-C QPOs, the source is consistent with being a high inclination system.

We have analyzed Rossi X-ray timing explorer (RXTE) pointed observations of the outbursts of black hole and neutron star soft X-ray transients in which an initial low/hard state or 'island' state, followed by a transition to a softer state, was observed. In three sources, the black hole transient XTE J1550-564, the neutron star transient Aquila X-1 and a quasi-persistent neutron star low mass X-ray binary (LMXB) 4U 1705-44, two such outbursts were found. We find that the flux of the soft X-ray peak, which lags the hard X-ray peak by a few days to several weeks, scales with the flux of the hard X-ray peak. We conclude that we are able to predict the soft X-ray outburst peak flux based on the 'preceding' hard X-ray peak flux, implying an early set up of the outbursts. We also find that the X-ray luminosity corresponding to the peak of the hard X-ray flux, which corresponds to the X-ray luminosity of the start of the hard-tosoft state transition, varies by a factor of about 2. If the accretion geometry early in the outburst rise is composed of two flows (e.g. a hot sub-Keplerian halo flow and a Keplerian disk flow, or an outflow and a Keplerian disk flow), the correlation indicates that the two flows are initially related, probably due to processes in the outer part of the accretion disk. We discuss constraints on a single flow model and a disk-jet model from these observations.

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