Detection of parabolic arc helps locate pulsar scattering screen
Millisecond pulsars are precise clocks provided by nature. By measuring pulse arrival times of many such pulsars distributed over the sky – a pulsar timing array (PTA) – astronomers are hoping to detect long-period gravitational waves, as produced in super-massive black hole mergers. This requires extremely high timing precisions: every possible source of error must be accounted for, including those arising from propagation effects in the interstellar medium (ISM). The ISM effects are strongest at the low frequencies that the Murchison Widefield Array (MWA) operates at.
A Curtin-based CAASTRO team led by Associate Investigator Dr Ramesh Bhat undertook MWA observations of PSR J0437–4715, a high-priority pulsar for the Parkes PTA experiment. The observations were made at a frequency of 192 MHz and exploited a brand new capability that they had recently developed for high time resolution science with the MWA: a tied-array beam processing pipeline which coherently combines voltage signals from all 128 antenna tiles. These signals were calibrated for the complex gains of individual tiles, as well as their polarimetric responses, yielding a ten-fold increase in sensitivity.
The turbulent ISM has denser regions that act as ‘scattering screens’ and produce thousands of images of the same pulsar, all at once. To an observer, interference between the scattered radiation – or scintillation – appears as brightening and dimming of the pulsar, the radio analogue of twinkling stars. This interference pattern can be captured by creating the ‘secondary spectrum’ (a two-dimensional Fourier transform of the dynamic spectrum that records the pulsar’s intensity as a function of time). The research team found faint, parabolic arc-like features in the secondary spectrum of PSR J0437–4715. These ‘scintillation arcs’ result from interference between the pulsar’s bright central core image and a ‘halo’ of images produced by the scattering screen. The curvature of the arcs critically depends on the distance to the scattering screen.
Adopting a technique based on Hough transform, which involves repeatedly fitting for the curve while summing the power along the curve segments, the researchers identified the curve with maximum power as the best fit. Based on this measurement, together with the distance and space velocity of PSR J0437-4715 from timing observations, they determined that the scattering screen lies at a distance of 115± 2 parsecs from Earth. This was independently verified using data from CSIRO’s Parkes telescope at a higher frequency, 732 MHz. The inferred location matches well with the estimated distance to the edge of the Local Bubble: a large, elongated cavity in the local ISM in which our Solar System resides.
The work demonstrates the MWA’s potential to play an important role in studying scattering delays in the signals of timing-array pulsars and the effect of the turbulent ISM in timing-array observations.
Ramesh Bhat, Steve Ord, Steven Tremblay, Sammy McSweeney and Steven Tingay in ApJ (2016) “Scintillation arcs in low-frequency observations of the timing-array millisecond pulsar PSR J0437−4715”