The low frequency radio emission and spectrum of the extended SNR W44: new VLA observations at 74 and 324 MHz

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.