  We analyze a deep, 100 ksec long, Chandra ACIS observation of NGC2264. We
detect 420 sources, 85% of which are associated with known optical and NIR
counterparts. More than 90% of these counterparts are NGC2264 members, thus
significantly increasing the known low mass cluster population by about 100
objects. Among the sources without counterpart, ~50% are likely members,
several of which we expect to be previously unknown protostellar objects. We
confirm several previous findings on the X-ray activity of low mass PMS stars:
X-ray luminosity is related to stellar mass, although with a large scatter;
L_X/L_bol is close to but almost invariably below the saturation level, 1e-3. A
comparison between CTTS and WTTS shows several differences: CTTS are on average
less active than WTTS; their emission may also be more time variable and is on
average slightly harder. However, we find evidence in some CTTS of extremely
cool, ~0.1-0.2 keV, plasma which we speculate is heated by accretion shocks. We
conclude that activity in low mass PMS stars, while generally similar to that
of saturated MS stars, may be affected by mass accretion in several ways:
accretion is likely responsible for very soft X-ray emission directly produced
in the accretion shock; it may reduce the average energy output of solar-like
coronae, at the same time making them hotter and more dynamic.
