Spectropolarimetric Properties of Swift J1818.0–1607: A 1.4 s Radio Magnetar

The soft-gamma repeater Swift J1818.0−1607 is only the fifth magnetar found to exhibit pulsed radio emission. Using the Ultra-Wideband Low receiver system of the Parkes radio telescope, we conducted a 3 hr observation of Swift J1818.0−1607. Folding the data at a rotation period of P = 1.363 s, we obtained wideband polarization profiles and flux density measurements covering radio frequencies between 704 and 4032 MHz. After measuring, and then correcting for the pulsar's rotation measure of 1442.0 ± 0.2 rad m−2, we find the radio profile is between 80% and 100% linearly polarized across the wide observing band, with a small amount of depolarization at low frequencies that we ascribe to scatter broadening. We also measure a steep spectral index of $\alpha =-{2.26}_{-0.03}^{+0.02}$ across our large frequency range, a significant deviation from the flat or inverted spectra often associated with radio-loud magnetars. The steep spectrum and temporal rise in flux density bears some resemblance to the behavior of the magnetar-like, rotation-powered pulsar PSR J1119−6127. This leads us to speculate that Swift J1818.0−1607 may represent an additional link between rotation-powered pulsars and magnetars.