  Magnetars are young, rotating neutron stars that possess larger magnetic
fields ($B$ $\approx$ $10^{13}$-$10^{15}$ G) and longer rotational periods ($P$
$\approx$ 1-12 s) than ordinary pulsars. In contrast to rotation-powered
pulsars, magnetar emission is thought to be fueled by the evolution and decay
of their powerful magnetic fields. They display highly variable radio and X-ray
emission, but the processes responsible for this behavior remain a mystery. We
report the discovery of bright, persistent individual X-ray pulses from XTE
J1810-197, a transient radio magnetar, using the Neutron star Interior
Composition Explorer (NICER) following its recent radio reactivation. Similar
behavior has only been previously observed from a magnetar during short time
periods following a giant flare. However, the X-ray pulses presented here were
detected outside of a flaring state. They are less energetic and display
temporal structure that differs from the impulsive X-ray events previously
observed from the magnetar class, such as giant flares and short X-ray bursts.
Our high frequency radio observations of the magnetar, carried out
simultaneously with the X-ray observations, demonstrate that the relative
alignment between the X-ray and radio pulses varies on rotational timescales.
No correlation was found between the amplitudes or temporal structure of the
X-ray and radio pulses. The magnetar's 8.3 GHz radio pulses displayed frequency
structure, which was not observed in the pulses detected simultaneously at 31.9
GHz. Many of the radio pulses were also not detected simultaneously at both
frequencies, which indicates that the underlying emission mechanism producing
these pulses is not broadband. We find that the radio pulses from XTE J1810-197
share similar characteristics to radio bursts detected from fast radio burst
(FRB) sources, some of which are now thought to be produced by active
magnetars.
