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

Abstract

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 (HPBW 37 at 74 MHz, and 13 at 324 MHz), 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. We measured total flux densities of 634 Jy and 411 Jy at 74 and 324 MHz, respectively, for W44, and from a careful assessment of published values between 22 and 10 700 MHz derived a global integrated continuum spectral index α = −0.37 ± 0.02. 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 α ∼ −0.5, with two clear exceptions: a short portion of the SNR limb to the southeast, with α 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. No spectral index trace was found indicating any connection between the associated pulsar PSR B1953+0.1 and the surrounding shell, nor between the SNR and the 3EG 1853+0114 γ-ray source proposed to be associated with W44. The comparison of the 324 MHz image with a 4.5 µm 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.

The Density and Mass of Unshocked Ejecta in Cassiopeia a Through Low Frequency Radio Absorption

“…More generally, the seemingly ubiquitous detection of resolved thermal absorption by the 74 MHz legacy VLA against the Galactic background (Nord et al 2006) and toward, discrete non thermal sources (e.g., see Lacey et al 2001;Brogan et al 2005;Castelletti et al 2007) confirms the phenomena will continue to emerge as a powerful tool for low frequency astrophysics.…”

Core collapse supernova remnants with ears

“…8, together with our drawing of the relevant geometrical quantities of the ears. The ears are revealed in the infra-red, but neither in the X-ray (the inner bright region in cyan) nor in the radio (Castelletti et al 2007;Egron et al 2017). W44 contains a central pulsar, PSR B1853+01, which has a PWN that emits radio and X-ray (e.g., Shelton et al 2004).…”

Radio and X-ray study of two multi-shell supernova remnants: Kes 79 and G352.7-0.1

“…Additional observations at 324 MHz were performed in the C and D arrays on December 14, 2002 andFebruary 27, 2003, respectively. Details of the observations and reduction procedures are described in Castelletti et al (2007). Since Kes 79 is far from the phase center of these observations, the images at both frequencies were corrected for the primary beam attenuation.…”