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We present new Very Large Array (VLA) radio images at 74 and 324 MHz of theSNR W44. The VLA images, obtained with unprecedented angular resolution andsensitivity for such low frequencies have been used in combination withexisting 1442 MHz radio data, Spitzer IR data, and ROSAT and Chandra X-ray datato investigate morphological and spectral properties of this SNR. The spatiallyresolved spectral index study revealed that the bright filaments, both aroundand across the SNR, have a straight spectrum between 74 and 1442 MHz, withalpha ~ -0.5, with two clear exceptions: a short portion of the SNR limb to thesoutheast, with alpha varying between 0 and +0.4 and a bright arc to the westwhere the spectrum breaks around 300 MHz and looks concave down. We concludethat at the shell and along the internal filaments, the electrons responsiblefor the synchrotron emission were accelerated at the shock according to asimple diffusive shock model; the positive spectrum corresponds to a locationwhere the SN shock is running into a molecular cloud and where the line ofsight intersects the photo dissociation region of an HII region and a youngstellar object is present. The curved spectrum on the westernmost bright arc isexplained as the consequence of strong post-shock densities and enhancedmagnetic fields after the interaction of the SN shock with a collindantmolecular cloud.Comment: After language edited, 16 pages, 12 figures (3 in color). Figures  degraded to reduce file size. Accepted 01/03/2007 for publicaion in A&

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