Twinkling of the nearest and brightest millisecond pulsar
Originally conceived as an imaging telescope, the Murchison Widefield Array (MWA) has now been equipped with a high time resolution data recorder to enable time domain science applications such as pulsars and fast radio bursts. By taking advantage of this new capability, CAASTRO Associate Investigator Dr Ramesh Bhat led the first pulsar science project with the MWA. The team’s observation of the milisecond pulsar J0437-4715 enabled them to study the pulsars’s twinkling (i.e. scintillation), leading to new insights into the location of turbulent interstellar plasma that is causing it.
Discovered in 1993 by the 64-metre CSIRO Parkes radio telescope, this nearest and brightest millisecond pulsar spins at an astonishing rate of 174 times per second and is located at a distance of about 500 light years (150 parsecs). It is an extremely important object for high-precision timing applications such as pulsar timing array experiments that aim to detect nanoHertz (light-year wavelength) gravitational waves and high-precision pulsar polarimetry and astrometry. The MWA observations enabled the lowest-frequency scintillation studies ever made of this top-ranked timing-array pulsar, revealing the observational signatures that arise from diffractive and refractive scintillation effects caused as pulsar signals travel through the interstellar medium.
A radio analogue of stellar twinkling, interstellar scintillation is a powerful tool that enables us to glean a wealth of information relating to the microstructure of turbulent interstellar plasma. In pulsar observations, it can manifest as rapid and deep modulations of pulse intensity in time and frequency and can be used to determine the location and distribution of interstellar plasma along the pulsar’s line of sight. Their analysis suggests much of the plasma that is causing scintillation of the pulsar is located at a distance of ~80-120 parsecs from the Sun, in stark disagreement with a recent claim that the screen is closer (~10 parsecs). Their inferred location also matches well with the predicted distance to the edge of the Local Hot Bubble – a large, tenuous, X-ray emitting cavity that envelopes the Solar neighbourhood, and extends out to ~10-50 parsecs in the Galactic plane and ~100-300 parsecs perpendicular to the plane.
This early science from commissioning observations vividly demonstrates the MWA’s potential for pulsar astronomy. A major boost in sensitivity is imminent with the implementation of a tied-array beam mode that coherently combines the tile powers to reach the full array sensitivity. Dr Bhat and his team will then undertake a low-frequency census of pulsars in the southern hemisphere including millisecond pulsars that are part of pulsar timing arrays (PTAs). Routine observations of such pulsars are particularly valuable for tracking interstellar space weather along their lines of sight, offering the potential of increasing the detection sensitivity of PTAs to gravitational waves.
N. D. Ramesh Bhat, S.M. Ord, S.E. Tremblay, S.J. Tingay et al. in ApJL (2014) “The low-frequency characteristics of PSR J0437-4715 observed with the Murchison Widefield Array”