Evolutionary implications of a magnetar interpretation for GLEAM-X J162759.5-523504.3

The radio pulsar GLEAM-X J162759.5-523504.3 has an extremely long spin period ($P=1091.17s$), and yet seemingly continues to spin down rapidly ($\dot{P}<1.2\times {10}^{-9}s{s}^{-1}$). The magnetic field strength that is implied, if the source is a neutron star undergoing magnetic dipole braking, could exceed ${10}^{16}G$. This object may therefore be the most magnetised neutron star observed to date. In this paper, a critical analysis of a magnetar interpretation for the source is provided. (i) A minimum polar magnetic field strength of $B\sim 5\times {10}^{15}G$ appears to be necessary for the star to activate as a radio pulsar, based on conventional `death valley’ assumptions. (ii) Back-extrapolation from magnetic braking and Hall-plastic-Ohm decay suggests that a large angular momentum reservoir was available at birth to support intense field amplification. (iii) The observational absence of X-rays constrains the star’s field strength and age, as the competition between heating from field decay and Urca cooling implies a surface luminosity as a function of time. If the object is an isolated, young ($\sim 10kyr$) magnetar with a present-day field strength of $B\gtrsim {10}^{16}G$, the upper limit ($\approx {10}^{30}erg{s}^{-1}$) set on its thermal luminosity suggests it is cooling via a direct Urca mechanism.