  We study the temporal evolution of coronal loops using data from the Solar
X-ray Imager (SXI) on board of GOES-12. This instrument allows us to follow in
detail the full lifetime of coronal loops. The observed light curves suggest
three somewhat distinct evolutionary phases: rise, main, and decay. The
durations and characteristic timescales of these phases are much longer than a
cooling time and indicate that the loop-averaged heating rate increases slowly,
reaches a maintenance level, and then decreases slowly. This suggests that a
single heating mechanism operates for the entire lifetime of the loop. For
monolithic loops, the loop-averaged heating rate is the intrinsic energy
release rate of the heating mechanism. For loops that are bundles of
impulsively heated strands, it is an indication of the frequency of occurrence
of individual heating events, or nanoflares. We show that the timescale of the
loop-averaged heating rate is proportional to the timescale of the observed
intensity variation. The ratios of the radiative to conductive cooling times in
the loops are somewhat less than 1, putting them intermediate between the
values measured previously for hotter and cooler loops. Our results provide
further support for the existence of a trend suggesting that all loops are
heated by the same mechanism, or that different mechanisms have fundamental
similarities (e.g., are all impulsive or are all steady with similar rates of
heating).
