How astronomers are using pulsars to observe evidence of dark matter

These pulsars — neutron stars which rotate and emit lighthouse-like beams of radio waves that rapidly sweep through space — were used to identify mysterious hidden masses.

Pulsars earned their nickname because they send out electromagnetic radiation at very regular intervals, ranging from milliseconds to seconds, making them extremely accurate timekeepers.

“Science has developed very precise methods to measure time,” said the astronomer behind the research, Professor John LoSecco, of the University of Notre Dame, who is presenting his findings at this week’s National Astronomy Meeting at the University of Hull.

“On Earth we have atomic clocks and in space we have pulsars.

“While gravitation has been known to slow down light for more than a century, there have been very few applications so far.”

Professor LoSecco observed variations and delays in pulsar timings, indicating that the radio beams are travelling around an unseen concentration of mass somewhere between the pulsar and the telescope.

He believes these invisible masses are candidates for dark matter objects.

Professor LoSecco studied delays in the arrival times of radio pulses, which normally have nanosecond accuracy. He searched along the path of radio pulses within the PPTA2 survey data release from the Parkes Pulsar Timing Array.

This ongoing project produces precise measurements of pulse arrival times using data from seven different radio telescopes: Effelsberg, Nançay, Westerbork, Green Bank, Arecibo, Parkes and Lovell, the latter in Cheshire.

The pulses have a cadence of approximately three weeks in three observing bands.

Deviations in the arrival times due to dark matter have a well-defined shape and a size proportional to its mass.

Light passing near regions of dark matter will be slowed by its presence. A search of the precision data from 65 ‘millisecond pulsars’ has revealed around a dozen incidents that appear to be interactions with dark matter.

Professor LoSecco said: “We take advantage of the fact that the Earth is moving, the Sun is moving, the pulsar is moving, and even the dark matter is moving.

“We observe the deviations in the arrival time caused by the change in distance between the mass we are observing and the line of sight to our ‘clock’ pulsar.”

A mass the size of the Sun can produce a delay of about 10 microseconds. The observations Professor LoSecco made have resolutions of the order of nanoseconds, 10,000 times smaller.

“One of the findings suggests a distortion of about 20 per cent of the mass of the Sun,” Professor LoSecco said. “This object could be a candidate for dark matter.”

He also confirmed that a side effect of this research is that it improves the pulsar timing data sample. This precision sample has been collected to look for evidence of low-frequency gravitational radiation.

Dark matter objects add ‘noise’ to that data, so identifying and removing them will clean the samples of some variability, eliminating such noise during other searches for gravitational radiation.

“The true nature of dark matter is a mystery,” said Professor LoSecco. “This research sheds new light on the nature of dark matter and its distribution in the Milky Way and may also improve the accuracy of the precision pulsar data.”