An FPGA-based digital-receiver has been developed for a low-frequency imaging radio interferometer, the Murchison Widefield Array (MWA). The MWA, located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia, consists of 128 dual-po
larized aperture-array elements (tiles) operating between 80 and 300,MHz, with a total processed bandwidth of 30.72 MHz for each polarization. Radio-frequency signals from the tiles are amplified and band limited using analog signal conditioning units; sampled and channelized by digital-receivers. The signals from eight tiles are processed by a single digital-receiver, thus requiring 16 digital-receivers for the MWA. The main function of the digital-receivers is to digitize the broad-band signals from each tile, channelize them to form the sky-band, and transport it through optical fibers to a centrally located correlator for further processing. The digital-receiver firmware also implements functions to measure the signal power, perform power equalization across the band, detect interference-like events, and invoke diagnostic modes. The digital-receiver is controlled by high-level programs running on a single-board-computer. This paper presents the digital-receiver design, implementation, current status, and plans for future enhancements.
We present images of C110$alpha$ and H110$alpha$ radio recombination line (RRL) emission at 4.8 GHz and images of H166$alpha$, C166$alpha$ and X166$alpha$ RRL emission at 1.4 GHz, observed toward the starforming region NGC 2024. The 1.4 GHz image wit
h angular resolution $sim$ 70arcsec is obtained using VLA data. The 4.8 GHz image with angular resolution $sim$ 17arcsec is obtained by combining VLA and GBT data. The similarity of the LSR velocity (10.3 kms) of the C110$alpha$ line to that of lines observed from molecular material located at the far side of the HII region suggests that the photo dissociation region (PDR) responsible for C110$alpha$ line emission is at the far side. The LSR velocity of C166$alpha$ is 8.8 kms. This velocity is comparable with the velocity of molecular absorption lines observed from the foreground gas, suggesting that the PDR is at the near side of the HII region. Non-LTE models for carbon line forming regions are presented. Typical properties of the foreground PDR are $T_{PDR} sim 100$ K, $n_e^{PDR} sim 5$ cmthree, $n_H sim 1.7 times 10^4$ cmthree, path length $l sim 0.06$ pc and those of the far side PDR are $T_{PDR} sim$ 200 K, $n_e^{PDR} sim$ 50 cmthree, $n_H sim 1.7 times 10^5$ cmthree, $l sim$ 0.03 pc. Our modeling indicates that the far side PDR is located within the HII region. We estimate magnetic field strength in the foreground PDR to be 60 $mu$G and that in the far side PDR to be 220 $mu$G. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024.
A compact steep spectrum radio source (J0535-0452) is located in the sky coincident with a bright optical rim in the HII region NGC1977. J0535-0452 is observed to be $leq 100$ mas in angular size at 8.44 GHz. The spectrum for the radio source is stee
p and straight with a spectral index of -1.3 between 330 and 8440 MHz. No 2 mu m IR counter part for the source is detected. These characteristics indicate that the source may be either a rare high redshift radio galaxy or a millisecond pulsar (MSP). Here we investigate whether the steep spectrum source is a millisecond pulsar.The optical rim is believed to be the interface between the HII region and the adjacent molecular cloud. If the compact source is a millisecond pulsar, it would have eluded detection in previous pulsar surveys because of the extreme scattering due to the HII region--molecular cloud interface. The limits obtained on the angular broadening along with the distance to the scattering screen are used to estimate the pulse broadening. The pulse broadening is shown to be less than a few msec at frequencies $gtsim$ 5 GHz. We therefore searched for pulsed emission from J0535-0452 at 14.8 and 4.8 GHz with the Green Bank Telescope (GBT). No pulsed emission is detected to 55 and 30 mu Jy level at 4.8 and 14.8 GHz. Based on the parameter space explored by our pulsar search algorithm, we conclude that, if J0535-0452 is a pulsar, then it could only be a binary MSP of orbital period $ltsim$ 5 hrs.
