2014ApJ...781L..17R

We study the outburst of the newly discovered X-ray transient 3XMM J185246.6+003317, re-analyzing all available XMM-Newton observations of the source to perform a phase-coherent timing analysis, and derive updated values of the period and period derivative. We find the source rotating at P = 11.55871346(6) s (90% confidence level; at epoch MJD 54728.7) but no evidence for a period derivative in the seven months of outburst decay spanned by the observations. This translates to a 3σ upper limit for the period derivative of {dot}P < 1.4 x 10^–13 s/s, which, assuming the classical magneto-dipolar braking model, gives a limit on the dipolar magnetic field of B_dip< 4.1x10¹³ G. The X-ray outburst and spectral characteristics of 3XMM J185246.6+003317 confirm its identification as a magnetar, but the magnetic field upper limit we derive defines it as the third "low-B" magnetar discovered in the past 3 yr, after SGR 0418+5729 and Swift J1822.3-1606. We have also obtained an upper limit to the quiescent luminosity (<4 x 10³³ erg/s), in line with the expectations for an old magnetar. The discovery of this new low field magnetar reaffirms the prediction of about one outburst per year from the hidden population of aged magnetars.