  We obtained a second Chandra timing measurement of the 3.82 s pulsar CXOU
J171405.7-381031 in the supernova remnant (SNR) CTB 37B, which shows that it is
spinning down rapidly. The average period derivative of (5.88+/-0.08)E-11 over
the 1 year time span corresponds to a dipole magnetic field strength B = 4.8E14
G, well into the magnetar range. The spin-down power E-dot = 4.2E34 erg/s is
among the largest for magnetars, and the corresponding characteristic age Tau =
P/2P-dot = 1030 years is comparable to estimates of the age of the SNR. The
period derivative enables us to recover probable pulsations in an ASCA
observation taken in 1996, which yields a mean characteristic age of 860 years
over the longer 13 year time span. The source is well detected up to 10 keV,
and its composite spectrum is typical of a magnetar. CTB 37B hosts HESS
J1713-381, the first TeV source that is coincident with a magnetar. While the
TeV emission has been attributed to the SNR shell, it is possibly centrally
peaked, and we hypothesize that this particularly young, energetic magnetar may
contribute to the HESS source. We also searched for pulsations from another
source in a HESS SNR, XMMU J173203.3-344518 in HESS J1731-347/G353.6-0.7 but
could not confirm pulsations or long-term flux variability, making it more
likely that this source is a weakly magnetized central compact object.
