Swift monitoring of the central X-ray source in RCW 103

The X-ray source 1E 161348-5055 lies at the centre of the 2-kyr-old supernova remnant RCW 103. Owing to its 24-ks modulation, orders-of-magnitude flux variability over a few months/years and lack of an obvious optical counterpart, 1E 161348-5055 defies assignment to any known class of X-ray sources. Starting from 2006 April, Swift observed 1E 161348-5055 with its X-Ray Telescope for ∼2 ks approximately once per month. During the five years covered, the source has remained in a quiescent state, with an average observed flux of ∼1.7 × 10^-12 erg cm^-2 s^-1 (1-10 keV), ∼20 times lower than the historical maximum attained in its 1999-2000 outburst. The long time-span of the Swift data allows us to obtain an accurate measure of the period of 1E 161348-5055 [P= 24 030.42(2) s] and to derive the first upper limit on its period derivative (? s s^-1 at 3σ).