  In the highly non-equilibrium conditions of laser induced spin dynamics
magnetic moments can only be obtained from the spectral information, most
commonly from the spectroscopy of semi-core states using the so-called x-ray
magnetic circular dichroism (XMCD) sum rules. The validity of the these sum
rules in tracking femtosecond spin dynamics remains, however, an open question.
Employing the time dependent extension of density functional theory (TD-DFT) we
compare spectroscopically obtained moments with those directly calculated from
the TD-DFT densities. We find that for experimentally typical pump pulses these
two very distinct routes to the spin moment are, for Co and Ni, in excellent
agreement, validating the experimental approach. However, for short and intense
pulses or high fluence pulses of long duration the XMCD sum rules fail, with
errors exceeding 50\%. This failure persists only during the pulse and occurs
when the pump pulse excites charge out of the $d$-band and into $sp$-character
bands, invalidating the semi-core to $d$-state transitions assumed by the XMCD
sum rules.
