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

TL;DR

This paper reports the discovery of X-ray spectral hardening linked to the emergence of radio pulsations from magnetar SGR 1935+2154, suggesting a connection between surface heating and radio emission origin.

Contribution

It provides the first evidence of X-ray spectral hardening preceding the radio pulsar phase in a magnetar, indicating a different emission mechanism from canonical pulsars.

Findings

X-ray spectral hardening observed before radio pulsations

Radio emission likely originates from the outer magnetosphere

Surface heating caused by inward particles explains spectral changes

Abstract

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 (\citealt{Camilo2006,camilo2007a}), 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.