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Thursday
Jun132013

How far away is that pulsar, exactly?

A team of scientists just used a Hemisphere-sized network of radio telescopes to find out exactly (well, within 0.4%) how far away the binary pulsar J2222-0137 is. With this new distance, which is 15% closer than everybody thought it was, astronomers can figure out what the pulsar's orbiting companion is and are one step closer to detecting gravitational waves.

Spoiler alert: This pulsar is 871.4 light-years from Earth.

But it's not the actual distance that matters. The important part is that astronomers can figure out the distance at all. Here's the story in three acts.

Act 1: PulsarsThis is not an image (Credit: Bill Saxton/NRAO/AUI).

They're weird.

Aside from black holes, they're the weirdest things in the universe. They have more mass than the Sun but are the sizes of earthly cities. In other words, dense. According to a Nature Physics result released today, they are made of nuclear pasta. They spin tens or hundreds of times every second (seriously, imagine DC rotating 716 times in a "one Mississippi," as the fastest known pulsar does) and warp spacetime way more than you do. Once every time they spin, the radiation coming out of their poles is pointed toward Earth, so we see a pulse.

Most pulsars are precise "clocks," hardly slowing down at all. If a pulsar is spinning once per second today, you can bet the farm that it will be spinning once per second tomorrow. As long as the person you're betting against doesn't have a super-atomic stopwatch. The average pulsar will slow down by approximately 10-15 seconds every second, which means that a pulsar that spins every 1 second today will spin once every 0.999999999136 seconds tomorrow. 

But the fast-spinning pulsars -- called millisecond pulsars (MSPs) -- are even more precise. If one rotated every 0.01 second, that number might change to 0.00999999999999136 seconds tomorrow.

These minute changes in period, too, are stable.

Act 2: Pulsars as tools

Because pulsars appear to pulse once for each rotation AND their rotation changes so slowly and measurably, astronomers know when they should see each pulse.

For the fastest and most stable objects, one could almost say they know exactly when they should see each pulse. So when one is off by 0.0000001 second, a dozing graduate student (j/k) sits up and takes notice.Bow down, atomic clocks. You have been beaten. By tiny, dead stars. (Credit: METAS).

Astronomers who are part of the European Pulsar Timing Array (EPTA), the Parkes Pulsar Timing Array (PPTA), and North American Nanohertz Observatory for Gravitational Waves (NANOGrav) are searching for -- you guessed it -- gravitational waves (GWs).

GWs ripple spacetime when objects with mass interact, like when they orbit each other or collide. As a GW passes through the universe, undulating the fabric of the cosmos, it will affect pulsars' pulses. Astronomers, obsessively monitoring a batch of the most reliable pulsars, are watching for tiny deviations in their pulses' arrival times, incidcating they were delayed by GWs. The set of pulsars -- currently there are 40 -pulsars that are worthy -- that astronomers watch closely is called a "pulsar timing array." Having more pulsars and knowing more about each one -- say, its distance from Earth -- increases the array's abilities.

Although scientists have found indirect evidence that gravitational waves are real and not something Einstein just made up, no one has seen them directly. Pulsar-timing astronomers would like to be the first. 

Act 3: Binary pulsar J222-0137

In the research paper headed-up by A. T. Deller of ASTRON, the scientists share new details about J222-0137, which is in a binary system that the Green Bank Telescope (GBT) discovered as part of a large survey.

Objects orbiting around each other radiate gravitational waves into space. They don't look like this unless you squint and turn your head to the right (Credit: NASA).

It

  • is 871.4 light-years away
  • spins every 32.82 milliseconds, or 30.5 times every second
  • will spin 0.00000000000000409536 seconds slower tomorrow
  • has a binary companion that is at least 1.1 times as massive as the Sun. Its nature is still unknown, but since astronomers know its mass and its distance, ID is on the way. If it shows up in optical observations, it's likely a white dwarf. If not, it's likely a fraternal twin neutron star.

They did all of this with precise observations from the National Radio Astronomy Observatory's Very Long Baseline Array, a set of 10 radio telescopes that work together to synthesize a larger radio telescope that has higher resolution than Hubble. By measuring the pulsar's parallax*, or how much it appears to move relative to distant background objects, they made the most accurate distance measurement ever for this kind of object.

Gathering up lots of accurate distance measurements to the timing-array pulsars would help in the race to finally "see" gravitational waves. Unfortunately, the paper says, most of the superfast pulsars are much farther away than J222-0137, and their parallaxes would be harder -- or, for the most part, impossible -- to measure well.

But fear not. Science, like love, will find a way ... to find gravitational waves.

It will, however, take a lot longer if the National Science Foundation (NSF) de-funds both the telescope that discovered this pulsar (and at least 85 of its closest pulsar friends) -- the GBT --- and the VLBA. Both are under threat of divestment, meaning all NSF funding would disappear.

Not good for many science topics, but especially for GW research. Maura McLaughlin, a professor at West Virginia University and an author on this paper, says, "Both the GBT and the VLBA are absolutely essential to detecting gravitational waves with pulsars. The GBT is the most sensitive instrument in the world for pulsar searches, and the VLBA can measure pulsar distances more accurately than any other telescope. Accurate distances to pulsars dramatically increase our ability to characterize gravitational wave sources. The GBT and the VLBA are really a perfect match, and it would be disastrous if we lost one or, god forbid, both of them."

ResearchBlogging.orgDeller, A., Boyles, J., Lorimer, D., Kaspi, V., McLaughlin, M., Ransom, S., Stairs, I., & Stovall, K. (2013). VLBI ASTROMETRY OF PSR J2222-0137: A PULSAR DISTANCE MEASURED TO 0.4% ACCURACY The Astrophysical Journal, 770 (2) DOI: 10.1088/0004-637X/770/2/145

 

 

**I've written about parallax several times. If you're interested in this technique's superpowers, consider these posts:

  1. For a summary of why parallax is important, see "Wink once if you think astronomy is is better because of parallax."
  2. VLBA-observed parallax bested Hubble's distance estimates and solved a mystery.
  3. Parallax also proved the most famous black hole actually is a black hole and not something else weird.
  4. To read an overview of the different ways scientists determine how far away stuff in space is, check out the intro of "Variable Stars: Blinking the way to better distances since 1957."

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