X-ray hardening preceding the onset of SGR 1935+2154's radio pulsar phase

Magnetars are neutron stars with extremely strong magnetic fields, frequently powering high-energy activity in X-rays. Pulsed radio emission following some X-ray outbursts have been detected (Camilo2006, 2007), albeit its physical origin is unclear.It has long been speculated that the origin of magnetars' radio signals is different from those from canonical pulsars, although convincing evidence is still lacking. Five months after magnetar SGR 1935+2154's X-ray outburst and its associated Fast Radio Burst (FRB) 20200428, a radio pulsar phase was discovered. Here we report the discovery of X-ray spectral hardening associated with the emergence of periodic radio pulsations from SGR 1935+2154 and a detailed analysis of the properties of the radio pulses. The observations suggest that radio emission originates from the outer magnetosphere of the magnetar, and the surface heating due to the bombardment of inward-going particles from the radio emission region is responsible for the observed X-ray spectral hardening.Among the ~30 magnetars known (2014ApJS..212....6O, https://www.physics.mcgill.ca/~pulsar/magnetar/main), only five of them have shown radio pulsations (Swift J1818.0-1607, SGR 1745-2900, PSR J1622-4950, XTE J1810-197, 1E 1547.0-5408). A monitoring campaign of SGR 1935+2154 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) was carried out during the April 2020 outburst. A radio burst was detected on 30 April 2020 21:43:05 (coordinated universal time, UTC), two days after FRB~20200428. We then detected radio pulsations in 464 rotation cycles from SGR 1935+2154 with 563 pulses, making it the sixth member of the rare radio magnetar population. The primary objective of this study is to summarize the properties of the pulsed emission observed during the same epoch in October 2020, based on the radio detections made by FAST and the X-ray detections made by NICER and SWIFT. The second primary goal is to use this comprehensive analysis of combined radio and X-ray detections to address critical pieces of the astrophysical puzzle such as the following.