  We report results from 13 years of Chandra monitoring of nonthermal X-ray
emission from the youngest Galactic supernova remnant G1.9+0.3, the only
remnant known to be increasing in brightness. We confirm the
spatially-integrated flux increase rate of $(1.2 \pm 0.2)$% yr$^{-1}$ between 1
and 7 keV, but find large spatial variations, from decreases of $-3$% yr$^{-1}$
to increases of 7% yr$^{-1}$, over length scales as small as $10''$ or smaller.
We observe relatively little change in spectral slope, though one region shows
significant hardening (photon index $\Delta \Gamma \sim 0.4$) as it brightens
by 1% yr$^{-1}$. Such rates of change can be accommodated by any of several
explanations, including steady evolution of the blast wave, expansion or
compression of discrete plasma blobs, strong magnetic turbulence, or variations
in magnetic-field aspect angle. Our results do not constrain the mean
magnetic-field strength, but a self-consistent picture of the spatially
averaged rate of increase can be produced in which the maximum energies of
accelerated particles are limited by the remnant age (applying both to
electrons and to ions) to about 20 TeV, and the remnant-averaged magnetic field
strength is about 30 $\mu$G. The deceleration parameter $m$ (average shock
radius varying as $t^m$) is about 0.7, consistent with estimates from overall
expansion dynamics, and confirming an explosion date of about 1900 CE.
Shock-efficiency factors $\epsilon_e$ and $\epsilon_B$ (fractions of shock
energy in relativistic electrons and magnetic field) are 0.003 and 0.0002 in
this picture. However, the large range of rates of brightness change indicates
that such a global model is oversimplified. Temporal variations of photon
index, expected to be small but measurable with longer time baselines, can
discriminate among possible models.
