  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 April 2006, 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
1.7e-12 erg/cm^2/s (1-10 keV), about 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 =
24030.42(2) s] and to derive the first upper limit on its period derivative
(|dP/dt| < 1.6e-9 s/s at 3 sigma).
