High-cadence observations and variable spin behaviour of magnetar Swift
  J1818.0-1607 after its outburst
  We report on multi-frequency radio observations of the new magnetar Swift
J1818.0-1607, following it for more than one month with high cadence. The
observations commenced less than 35 hours after its registered first outburst.
We obtained timing, polarisation and spectral information. Swift J1818.0-1607
has an unusually steep spectrum for a radio emitting magnetar and also has a
relatively narrow and simple pulse profile. The position angle swing of the
polarisation is flat over the pulse profile, possibly suggesting that our
line-of-sight grazes the edge of the emission beam. This may also explain the
steep spectrum. The spin evolution shows large variation in the spin-down rate,
associated with four distinct timing events over the course of our
observations. Those events may be related to the appearance and disappearance
of a second pulse component. The first timing event coincides with our actual
observations, while we did not detect significant changes in the emission
properties which could reveal further magnetospheric changes. Characteristic
ages inferred from the timing measurements over the course of months vary by
nearly an order of magnitude. A longer-term spin-down measurement over
approximately 100 days suggests an characteristic age of about 500 years,
larger than previously reported. Though Swift J1818.0-1607 could still be one
of the youngest neutron stars (and magnetars) detected so far, we caution using
the characteristic age as a true-age indicator given the caveats behind its
calculation.
