  We present new Very Large Array (VLA) radio images at 74 and 324 MHz of the
SNR W44. The VLA images, obtained with unprecedented angular resolution and
sensitivity for such low frequencies have been used in combination with
existing 1442 MHz radio data, Spitzer IR data, and ROSAT and Chandra X-ray data
to investigate morphological and spectral properties of this SNR. The spatially
resolved spectral index study revealed that the bright filaments, both around
and across the SNR, have a straight spectrum between 74 and 1442 MHz, with
alpha ~ -0.5, with two clear exceptions: a short portion of the SNR limb to the
southeast, with alpha varying between 0 and +0.4 and a bright arc to the west
where the spectrum breaks around 300 MHz and looks concave down. We conclude
that at the shell and along the internal filaments, the electrons responsible
for the synchrotron emission were accelerated at the shock according to a
simple diffusive shock model; the positive spectrum corresponds to a location
where the SN shock is running into a molecular cloud and where the line of
sight intersects the photo dissociation region of an HII region and a young
stellar object is present. The curved spectrum on the westernmost bright arc is
explained as the consequence of strong post-shock densities and enhanced
magnetic fields after the interaction of the SN shock with a collindant
molecular cloud.
