Switches between accretion structures during flares in 4U 1901+03

We report on our analysis of the 2019 outburst of the X-ray accreting pulsar 4U 1901+03 observed with Insight-HXMT and NICER. Both spectra and pulse profiles evolve significantly in the decaying phase of the outburst. Dozens of flares are observed throughout the outburst. They are more frequent and brighter at the outburst peak. We find that the flares, which have a duration from tens to hundreds of seconds, are generally brighter than the persistent emission by a factor of -1.5. The pulse-profile shape during the flares can be significantly different from that of the persistent emission. In particular, a phase shift is clearly observed in many cases. We interpret these findings as direct evidence of changes of the pulsed beam pattern, due to transitions between the sub- and supercritical accretion regimes on a short time-scale. We also observe that at comparable luminosities the flares’ pulse profiles are rather similar to those of the persistent emission. This indicates that the accretion on the polar cap of the neutron star is mainly determined by the luminosity, i.e. the mass accretion rate

X-ray reprocessing in accreting pulsar GX 301-2 observed with Insight-HXMT

We investigate the absorption and emission features in observations of GX 301-2 detected with Insight-HXMT/LE in 2017-2019. At different orbital phases, we found prominent Fe Kα, Kβ, and Ni Kα lines, as well as Compton shoulders and Fe K-shell absorption edges. These features are due to the X-ray reprocessing caused by the interaction between the radiation from the source and surrounding accretion material. According to the ratio of iron lines (Kα and Kβ), we infer the accretion material is in a low ionization state. We find an orbital-dependent local absorption column density, which has a large value and strong variability around the periastron. We explain its variability as a result of inhomogeneities of the accretion environment and/or instabilities of accretion processes. In addition, the variable local column density is correlated with the equivalent width of the iron Kα lines throughout the orbit, which suggests that the accretion material near the neutron star is spherically distributed

Identification of a non-thermal X-ray burst with the Galactic magnetar SGR J1935+2154 and a fast radio burst using Insight-HXMT

Abstract Fast radio bursts (FRBs) are short pulses observed in radio band from cosmological distances, some of which emit repeating bursts. The physical origins of these mysterious events have been subject to wide speculations and heated debates. One class of models invoke soft gamma-ray repeaters (SGRs), or magnetars, as the sources of FRBs. Magnetars are rotating neutron stars with extremely strong magnetic field and can sporadically emit bursts from X-ray (keV) to soft gamma-ray (sub-MeV) with duration from 10􀀀2 s to 102 s. However, even though some bright radio bursts have been observed from some magnetars, no FRB-like events had been detected to be associated with any magnetar burst, including one giant flare, and no radio burst has been associated with any X-ray event from any magnetar. Therefore, there is still no observational evidence for magnetar-FRB association up to today. Recently, a pair of FRB-like bursts (FRB~200428 hereafter) separated by 30 milliseconds (ms) were detected from the general direction of the Galactic magnetar SGR~J1935+2154. Here we report the detection of a non-thermal X-ray burst in the 1--250\,keV energy band with the Insight-HXMT satellite, which we identify as emitted from SGR~J1935+2154. The burst showed two hard peaks with a separation of ms, consistent with the separation between the two bursts in FRB~200428. The delay time between the double radio and X-ray peaks is 8:57s, fully consistent with the dispersion delay of FRB~200428. We thus identify the non-thermal X-ray burst is associated with FRB~200428 whose high energy counterpart is the two hard peaks in X-ray. Our results suggest that the non-thermal X-ray burst and FRB~200428 share the same physical origin in an explosive event from SGR~J1935+2154.</jats:p