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

We report the detections of the anticorrelated soft and hard X-rays, and the time lags of $hectosecond from the neutron star low-mass X-ray binary Cyg X-2, a well-known Z-type luminous source. Both anticorrelation and the positive correlation were detected during the low-intensity states, while only the latter showed up during highintensity states. Comparing with the lower part of normal branch and flaring branch, we find that more observations located on the horizontal and the upper normal branches are accompanied with anticorrelation, implying the occurrence of the anticorrelation when there is a low mass accretion rate. So far anticorrelated hard lags of 1000 s timescale are only reported from Galactic black hole candidates in their hard states. Here we provide the first evidence that a similar feature can also be established in neutron star systems such as Cyg X-2. Finally, the possible origins of the observed time lags are discussed under current LMXB models.

We study the evolution of the temporal properties of MAXIJ1820+070 during the 2018 outburst in its hard state from MJD58,190 to 58,289 with Insight-HXMT in a broad energy band 1-150 keV. We find different behaviors of the hardness ratio, the fractional rms and time lag before and after MJD58,257, suggesting a transition occurred around this point. The observed time lags between the soft photons in the 1-5 keV band and the hard photons in higher energy bands, up to 150 keV, are frequency-dependent: the time lags in the low-frequency range, 2-10mHz, are both soft and hard lags with a timescale of dozens of seconds but without a clear trend along the outburst; the time lags in the high-frequency range, 1-10Hz, are only hard lags with a timescale of tens of milliseconds; they first increase until around MJD58,257 and decrease after this date. The high-frequency time lags are significantly correlated to the photon index derived from the fit to the quasi-simultaneous NICER spectrum in the 1-10 keV band. This result is qualitatively consistent with a model in which the high-frequency time lags are produced by Comptonization in a jet.Unified Astronomy Thesaurus concepts: Black holes (162); Compact objects (288); Low-mass x-ray binary stars (939)