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Register a new userA Census of Southern Pulsars at 185 MHz
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The Murchison Widefield Array (MWA), and its recently-developed Voltage Capture System (VCS), facilitates extending the low-frequency range of pulsar observations at high-time and -frequency resolution in the Southern Hemisphere, providing further information about pulsars and the ISM. We present the results of an initial time-resolved census of known pulsars using the MWA. To significantly reduce the processing load, we incoherently sum the detected powers from the 128 MWA tiles, which yields ~10% of the attainable sensitivity of the coherent sum. This preserves the large field-of-view (~450 deg2 at 185 MHz), allowing multiple pulsars to be observed simultaneously. We developed a WIde-field Pulsar Pipeline (WIPP) that processes the data from each observation and automatically folds every known pulsar located within the beam. We have detected 50 pulsars to date, 6 of which are millisecond pulsars. This is consistent with our expectation, given the telescope sensitivity and the sky coverage of the processed data (~17,000 deg2). For ten pulsars, we present the lowest-frequency detections published. For a subset of the pulsars, we present multi-frequency pulse profiles by combining our data with published profiles from other telescopes. Since the MWA is a low-frequency precursor to the Square Kilometre Array (SKA), we use our census results to forecast that a survey using Phase 1 of SKA-Low (SKA1-Low) can potentially detect around 9400 pulsars.
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We present the results from the low-frequency (40--78 MHz) extension of the first LOFAR pulsar census of non-recycled pulsars. We have used the Low-Band Antennas of the LOFAR core stations to observe 87 pulsars out of 158 that have been detected previously with the High-Band Antennas. Forty-three pulsars have been detected and we present here their flux densities and flux-calibrated profiles. Seventeen of these pulsars have not been, to our knowledge, detected before at such low frequencies. We re-calculate the spectral indices using the new low-frequency flux density measurements from the LOFAR census and discuss the prospects of studying pulsars at the very low frequencies with the current and upcoming facilities, such as NenuFAR.
We report the detection of 48 millisecond pulsars (MSPs) out of 75 observed thus far using the LOFAR in the frequency range 110-188 MHz. We have also detected three MSPs out of nine observed in the frequency range 38-77 MHz. This is the largest sample of MSPs ever observed at these low frequencies, and half of the detected MSPs were observed for the first time at frequencies below 200 MHz. We present the average pulse profiles of the detected MSPs, their effective pulse widths, and flux densities and compare these with higher observing frequencies. The flux-calibrated, multifrequency LOFAR pulse profiles are publicly available via the EPN Database of Pulsar Profiles. We also present average values of dispersion measures (DM) and discuss DM and profile variations. About 35% of the MSPs show strong narrow profiles, another 25% exhibit scattered profiles, and the rest are only weakly detected. A qualitative comparison of LOFAR profiles with those at higher radio frequencies shows constant separation between profile components. Similarly, the profile widths are consistent with those observed at higher frequencies, unless scattering dominates at the lowest frequencies. This is very different from what is observed for normal pulsars and suggests a compact emission region in the MSP magnetosphere. The amplitude ratio of the profile components, on the other hand, can dramatically change towards low frequencies, often with the trailing component becoming dominant. As previously demonstrated this can be caused by aberration and retardation. This data set enables high-precision studies of pulse profile evolution with frequency, dispersion, Faraday rotation, and scattering in the interstellar medium. Characterising and correcting these systematic effects may improve pulsar-timing precision at higher observing frequencies, where pulsar timing array projects aim to directly detect gravitational waves.
We present 154 MHz Murchison Widefield Array imaging observations and variability information for a sample of pulsars. Over the declination range $-80^{circ} < {delta} < 10^{circ}$ we detect 17 known pulsars with mean flux density greater than 0.3 Jy. We explore the variability properties of this sample on timescales of minutes to years. For three of these pulsars, PSR J0953+0755, PSR J0437-4715 and PSR J0630-2834 we observe interstellar scintillation and variability on timescales of greater than 2 minutes. One further pulsar, PSR J0034-0721, showed significant variability, the physical origins of which are difficult to determine. The dynamic spectra for PSR J0953+0755 and PSR J0437-4715 show discrete time and frequency structure consistent with diffractive interstellar scintillation and we present the scintillation bandwidth and timescales from these observations. The remaining pulsars within our sample were statistically non-variable. We also explore the spectral properties of this sample and find spectral curvature in pulsars PSR J0835-4510, PSR J1752-2806 and PSR J0437-4715.
Polarimetric studies of pulsars at low radio frequencies provide important observational insights into the pulsar emission mechanism and beam models, and probe the properties of the magneto-ionic interstellar medium (ISM). Aperture arrays are the main form of next-generation low-frequency telescopes, including the Murchison Widefield Array (MWA). These require a distinctly different approach to data processing (e.g. calibration and beamforming) compared to traditional dish antennas. As the second paper of this series, we present a verification of the MWAs pulsar polarimetry capability, using two bright southern pulsars, PSRs J0742-2822 and J1752-2806. Our observations simultaneously cover multiple frequencies (76-313 MHz) and were taken at multiple zenith angles during a single night for each pulsar. We show that the MWA can be reliably calibrated for zenith angles < 45 degree and frequencies < 270 MHz. We present the polarimetric profiles for PSRs J0742-2822 and J1752-2806 at frequencies lower than 300 MHz for the first time, along with an analysis of the linear polarisation degree and pulse profile evolution with frequency. For PSR J0742-2822, the measured degree of linear polarisation shows a rapid decrease at low frequencies, in contrast with the generally expected trend, which can be attributed to depolarisation effects from small-scale, turbulent, magneto-ionic ISM components. This effect has not been widely explored for pulsars in general, and will be further investigated in future work.
We present a broadband map of polarized diffuse emission at 167-198 MHz developed from data from the Murchison Widefield Array (MWA). The map is designed to improve visibility simulation and precision calibration for 21 cm Epoch of Reionization (EoR) experiments. It covers a large swath - 11,000 sq. deg. - of the Southern Hemisphere sky in all four Stokes parameters and captures emission on angular scales of 1 to 9 degrees. The band-averaged diffuse structure is predominantly unpolarized but has significant linearly polarized structure near RA = 0 h. We evaluate the accuracy of the map by combining it with the GLEAM catalog and simulating an observation from the MWA, demonstrating that the accuracy of the short baselines (6.1-50 wavelengths) now approaches the accuracy of the longer baselines typically used for EoR calibration. We discuss how to use the map for visibility simulation for a variety of interferometric arrays. The map has potential to improve calibration accuracy for experiments such as the Hydrogen Epoch of Reionization Array (HERA) and the forthcoming Square Kilometre Array (SKA) as well as the MWA.
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