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Pulsars Yield Dark Matter Density in Our Galactic Neighborhood

A new technique to measure the motions of pulsars for the first time enables more precise estimates of local dark matter density.

The pulsar at the center of the Crab Nebula, pictured here in a composite image, spins around every 33 millisecond pulsar, putting it almost but not quite in the millisecond range. Even more rapidly rotating pulsars can be used to measure gravitational acceleration throughout the Milky Way.

X-ray: NASA / CXC / SAO; Optical: NASA / STScI; Infrared: NASA/JPL-Caltech

Astronomers have long used measurements of stellar motion to illuminate the gravitational forces at play in our galaxy. Now, a team of astronomers has pioneered a technique that, for the first time, directly measures the acceleration of solitary pulsars. The result: a map of the distribution of mass — including dark matter — in the Milky Way.

Thomas Donlon II and Sukanya Chakrabarti (both at University of Alabama in Huntsville) and their colleagues look at a particular type of pulsar known as millisecond pulsars (MSPs). These city-size stars rotate at incredible speeds, whipping around in less than 10 milliseconds. They also emit lighthouse-like beams of radiation, which serve as ultra-precise clocks.

As such a pulsar moves through the galaxy, its motion will slightly shift its precise periodic signal. Astronomers can use this signature to measure the pulsar’s acceleration along our line of sight. The acceleration comes from the mass of our galaxy, which exerts gravitational force on the matter in and around it. More mass will lead to faster pulsar accelerations. Pulsar acceleration measurements can therefore indicate how mass is distributed in our Milky Way.

Pulsar model

However, the tick of these clocks is also affected by other factors, such as the very gradual slowing of pulsars’ spins. One way those spins can slow is magnetic braking, in which the pulsar’s intense magnetic field flings particles off of the star, pumping away some of the star’s angular momentum and slowing down its spin rate.

The team, whose newest results were posted on the astronomy arXiv preprint server, previously estimated accelerations of pairs of millisecond pulsars (binary MSPs). In binaries, the mutual orbit isn’t affected by magnetic braking, enabling the researchers to calibrate for the effect.

This animation illustrates how an old pulsar in a binary system can be reactivated — and sped up to a millisecond spin — by accreting gas from its companion star. However, more typically, millisecond pulsars are young objects that haven't yet slowed.

NASA

The effect is more difficult to account for in the case of solitary MSPs, but using the calibrated spindown rates of the binary pulsars, the team was able to estimate the magnetic braking for the solitary MSPs, too.

The new technique effectively doubled the number of pulsars with acceleration measurements, from 28 to 54 MSPs. For the first time, the accelerations of solitary pulsars were obtained from observed qualities alone, providing an entirely new source of data about our galaxy’s mass distribution.

Mapping the Milky Way

“Over the last several years, we have shown that it is now possible to not just estimate, but directly measure the accelerations experienced by stars in our galaxy,” Chakrabarti says. These acceleration measurements map the Milky Way’s structure, capturing a snapshot of its dynamic history.

For example, the Milky Way has been perturbed by interactions between the dwarf galaxies that orbit it. One of the most important is the Large Magellanic Cloud, which pulls some of the mass in the Milky Way’s disk toward itself as it passes by. “This lopsided effect leads to the galaxy wobbling, similar to the way a toddler walks because they're not fully balanced,” Chakrabarti says.

“The changes in galaxies that occur over human timescales are tiny,” Chakrabarti says, “and so measuring these tiny changes requires extreme precision measurements, which have only just become possible.” These tiny accelerations increase by around 1 centimeter per second — around the speed of a caterpillar — every year.

The MSP accelerations also provide a more precise measurement of the distribution of dark matter in the galaxy’s midplane, the region where most of the Milky Way’s mass lies. This local estimate found that the dark matter density would be equivalent to a grain of sand (1 microgram) in a volume the size of Mount Everest.

“I believe that in a few years, as direct acceleration measurements continue to grow,” Chakrabarti says, “they will help us understand the very nature of dark matter.”