A detailed X-ray and radio study of the supernova remnant W44

Abstract

We analyze EXOSAT and Einstein Observatory X-ray spectra, an Einstein X-ray image, and a detailed VLA radio map of the SNR W44. The diffuse soft X-ray emission, which is only observed from the interior of the SNR, is shown to be largely of thermal origin. The absence of soft X-ray emission from the shell can be explained by the measured line-of-sight absorption, and provides an opportunity to study the SNR interior without confusion by bright shell emission. A deprojection of the interior X-ray surface brightness measured with the Einstein IPC is performed to derive density profiles which are relatively insensitive to the assumptions made. The density is found to decrease with radius by a factor of 3–4 between the center and a fractional radius of 0.8. The interior density profiles are completely inconsistent with those predicted by the Sedov solution for expansion into a uniform medium, but can be largely explained in terms of expansion into a cloudy interstellar medium in which most of the mass resides in a cool dense state. Shocked and compressed clouds of gas of size ≈ 0.4 to 8 pc in this state may be responsible for the numerous bright knots and filaments of radio emission revealed for the first time in our VLA map. A radio synchrotron nebula associated with the pulsar PSR 1853 + 01 may have been detected within the SNR. This proposed nebula is unusual because its position is offset from the pulsar, and in a direction consistent with motion of the pulsar away from the SNR centre.