A&A 471, 537–549 (2007) Astronomy DOI: 10.1051/0004-6361:20077062 & c ESO 2007 Astrophysics The low-frequency radio emission and spectrum of the extended SNR W44: new VLA observations at 74 and 324 MHz G. Castelletti1,, G. Dubner1,, C. Brogan2, and N. E. Kassim3 1 Instituto de Astronomía y Física del Espacio (IAFE), CC 67, Suc. 28, 1428 Buenos Aires, Argentina e-mail: [email protected] 2 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 3 Remote Sensing Division, Code 7213, Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington DC, USA Received 5 January 2007 / Accepted 1 March 2007 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 µmIRimage 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. Key words. ISM: individual objects: W44 – ISM: supernova remnants – radio continuum: ISM 1. Introduction processes taking place either within SNRs (e.g. shock acceler- ation), in their immediate surrounding, or in the ISM along the The interaction of supernova remnant (SNR) shocks with inho- ff line of sight (e.g. thermal absorption, Lacey et al. 2001; Brogan mogeneous interstellar gas strongly a ects the spatial and tem- et al. 2005). Based on accurate spectral continuum index studies poral evolution of SNRs. The density distribution of the ambi- it is possible to constrain shock acceleration theories that predict ent circumstellar material (CSM) and interstellar medium (ISM) subtle variations in both spatially resolved (Anderson & Rudnick can influence the formation of a variety of SNR morphologies, 1993) and integrated spectra (Reynolds & Ellison 1992). In ad- including specific sub-classes of SNRs such as “thermal-X-ray dition, the information provided by low frequency observations composites” (also called mixed-morphology SNRs, Rho & Petre of SNRs is a powerful complement to higher frequency radio, 1998), or remnants with bilateral symmetry (Gaensler 1998). X-ray, and infrared (IR) data to study the complex processes as- In this respect, sensitive high-resolution radio observations of sociated with SNR shocks (Brogan et al. 2005). SNRs are particularly useful for studying the physics of shock W44 (G34.7−0.4) is a bright radio SNR (S 1 GHz ∼ 230 Jy) waves traveling through nonuniform environments in the ISM. ◦ On the other hand, continuum radio spectra made accu- with an asymmetric morphology, about 0.5 in size, and located rate by extension to long wavelengths provide sensitive and of- in a complex region of the inner Galactic plane rich in both ther- ten unique means of distinguishing between different physical mal and nonthermal sources. W44 is an archetype of the “mixed- morphology” class of remnants, characterized by a highly fil- Post-Doc Fellow of CONICET, Argentina. amentary radio shell and centrally concentrated thermal X-ray Member of the Carrera del Investigador Científico of CONICET, emission. Morphological studies of this remnant in the optical Argentina. band reveal Balmer-dominated, [NII], and [SII] filaments from Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20077062 538 G. Castelletti et al.: Low-frequency observations of SNR W44 radiative shocks, with some concordance with the radio emission Table 1. Observational summary. (Rho et al. 1994; Giacani et al. 1997). This SNR is also of interest because it constitutes one of the Observing VLA Bandwidth Integration few demonstrated cases of a SNR-molecular cloud interacting dates Config. (MHz) time (h) system. Seta et al. (1998) observed six giant molecular clouds 74 MHz Parameters that appear to be partially surrounding the remnant. Higher spa- 2003 Aug. 31 A 1.562 7.5 tial resolution 12CO observations of the full extent of W44 pre- 2003 Sep. 1 A 1.562 2.6 sented by Seta et al. (2004) revealed that some of these clouds (at 2002 Jun. 15 B 1.562 6.0 − 324 MHz Parameters v ∼ 48 km s 1) are physically interacting with the remnant on LSR 2003 Aug. 31 A 3.125 7.5 its southeastern and western sides. These observations also show ∼ 2003 Sep. 1 A 3.125 2.6 that the shocked molecular gas is confined to compact ( 1.5 pc) 2002 Jun. 15 B 3.125 6.0 cores located adjacent to bright radio filaments or knots. The 2002 Dec. 14 C 3.125 4.0 physical interaction of the blast wave with the clumpy ISM is 2003 Mar. 16 D 3.125 6.0 also supported by the detection of bright OH (1720 MHz) masers (at LSR velocities between ∼43 and ∼47 km s−1,Hoffman et al. 2005, and references therein), and IR cooling lines from H2 (Reach et al. 2006). 2. Observations and data reduction The associated pulsar PSR B1853+01 is located inside the W44 shell, about 9 south of the geometric center of the rem- We have performed low frequency observations of the SNR W44 nant. A characteristic age of about 2 × 104 years has been esti- at 74 and 324 MHz using multiple-configurations of the VLA. mated for this pulsar (Wolszczan et al. 1991). Frail et al. (1996) Table 1 summarizes various observing details. Data at both fre- and Petre et al. (2002) report the detection of a radio and X-ray quencies were obtained in spectral-line mode to enable radio- nebula powered by an associated pulsar wind. frequency interference (RFI) excision and to mitigate bandwidth smearing. The observations at 74 MHz and 324 MHz were per- It has also been proposed that the EGRET source formed with 64 and 32 channels, respectively. A model image of 3EG 1856+0114 is associated with W44 (Esposito et al. 1996). Cygnus A was used to correct the bandpass2. The absolute am- The 95% confidence contour for this γ-ray source coincides with plitude scale was set using Cygnus A at 74 MHz and 3C 48 at the southeastern sector of the remnant (Thompson et al. 1996). 324 MHz. Also, for the 324 MHz data, regular observations of de Jager & Mastichiadis (1997) explained the observed GeV γ- the secondary calibrator 1822−096 were used for initial phase rays in terms of relativistic bremsstrahlung and inverse Compton and amplitude calibration. At both frequencies, the data from scattering produced by a power-law distribution of relativistic each array were fully reduced and imaged separately using the electrons injected by the pulsar PSR B1853+01. While this con- NRAO Astronomical Image Processing Software (AIPS) pack- stitutes a possible explanation, a significant part of the γ-ray age. After initial calibration the data at 74 and 324 MHz were flux observed may also be due to SNR shock-cloud interactions averaged down to fourteen and eight channel spectral resolution (Fatuzzo & Melia 2005). No γ-ray radiation at TeV energies was data cubes, respectively, before imaging. detected towards W44 using either the Whipple (Leslard 1995) ffi or CANGAROO (Buckley et al. 1998) telescopes, suggesting a Two additional di culties associated with low frequency spectral cutoff between GeV and TeV energies (Rowell et al.