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 \sim 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 \sim 1.7 \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } ( 1–10 keV ) , \sim 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 ( | \dot { P } | < 1.6 \times 10 ^ { -9 } s s ^ { -1 } at 3 \sigma ) .