Context :

Aims : 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 continuum properties of this SNR .

Methods : The observations were carried out with the VLA simultaneously at 74 and 324 MHz in the A and B configurations and at 324 MHz in the C and D configurations .
The radio continuum spectral index distribution was derived through direct comparison of the combined data at 74 , 324 , and 1442 MHz .
In addition , to isolate and identify different spectral components , tomographic spectral analysis was performed .

Results : 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 \sim - 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 becomes 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 the line of sight intersects the photo dissociation region of an HII region and a young stellar object is present .
Such spectral inversion is a classic signature of thermal absorption , either from ionized gas in the postshock region , from the HII region itself , or both .
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 coincident molecular cloud .
The comparison of the 324 MHz image with a 4.5 \mu m IR image obtained with Spitzer underscored an impressive correspondence between emission both to the north and west sides of W44 , while the comparison with ROSAT and Chandra images confirm that the synchrotron radio emission surrounds the thermal X-ray radiation .

Conclusions :