  During a pointed 2018 NuSTAR observation, we detected a flare with a 2.2 hour
duration from the magnetar 1RXS J170849.0$-$400910. The flare, which rose in
$\sim25$ seconds to a maximum flux 6 times larger than the persistent emission,
is highly pulsed with an rms pulsed fraction of $53\%$. The pulse profile shape
consists of two peaks separated by half a rotational cycle, with a peak flux
ratio of $\sim$2. The flare spectrum is thermal with an average temperature of
2.1 keV. Phase resolved spectroscopy show that the two peaks possess the same
temperature, but differ in size. These observational results along with simple
light curve modeling indicate that two identical antipodal spots, likely the
magnetic poles, are heated simultaneously at the onset of the flare and for its
full duration. Hence, the origin of the flare has to be connected to the global
dipolar structure of the magnetar. This might best be achieved externally, via
twists to closed magnetospheric dipolar field lines seeding bombardment of
polar footpoint locales with energetic pairs. Approximately 1.86 hours
following the onset of the flare, a short burst with its own 3-minute thermal
tail occurred. The burst tail is also pulsating at the spin period of the
source and phase-aligned with the flare profile, implying an intimate
connection between the two phenomena. The burst may have been caused by a
magnetic reconnection event in the same twisted dipolar field lines anchored to
the surface hot spots, with subsequent return currents supplying extra heat to
these polar caps.
