X-ray Spectral Variations in the Youngest Galactic Supernova Remnant
  G1.9+0.3
  The discovery of the youngest Galactic supernova remnant (SNR) G1.9+0.3 has
allowed a look at a stage of SNR evolution never before observed. We analyze
the 50 ks Chandra observation with particular regard to spectral variations.
The very high column density ($N_H \sim 6 \times 10^{22}$ cm$^{-2}$) implies
that dust scattering is important, and we use a simple scattering model in our
spectral analysis. The integrated X-ray spectrum of G1.9+0.3 is well described
by synchrotron emission from a power-law electron distribution with an
exponential cutoff. Using our measured radio flux and including scattering
effects, we find a rolloff frequency of $5.4 (3.0, 10.2) \times 10^{17}$ Hz ($h
\nu_{\rm roll} = 2.2$ keV). Including scattering in a two-region model gives
lower values of \nu_roll by over a factor of 2. Dividing G1.9+0.3 into six
regions, we find a systematic pattern in which spectra are hardest (highest
\nu_roll) in the bright SE and NW limbs of the shell. They steepen as one moves
around the shell or into the interior. The extensions beyond the bright parts
of the shell have the hardest spectra of all. We interpret the results in terms
of dependence of shock acceleration properties on the obliquity angle
$\theta_{\rm Bn}$ between the shock velocity and a fairly uniform upstream
magnetic field. This interpretation probably requires a Type Ia event. If
electron acceleration is limited by synchrotron losses, the spectral variations
require obliquity-dependence of the acceleration rate independent of the
magnetic-field strength.
