Date: February 11, 2010

Title: Nature’s Most Precise Clocks May Make ‘Galactic GPS’ Possible

Podcaster: Kevin John and Paul Ray

Organization: Sonoma State NASA E/PO: http://epo.sonoma.edu
US Naval Research Laboratory: http://www.nrl.navy.mil/

Description: NASA’s Fermi Gamma-ray Space Telescope has recently aided radio astronomers in the discovery of 17 new millisecond pulsars. These high energy pulsars may one day be used as part of a larger network of pulsars which could help astronomers in the detection of gravitational waves. Dr. Paul Ray of the U.S. Naval Research Laboratory joins interviewer Kevin John to answer questions regarding these recent discoveries.

Bio: Paul Ray is an Astrophysicist in the Space Science Division at the Naval Research Laboratory. He received a Ph.D. in Physics in 1995 from Caltech and an A.B. in Physics from the University of California, Berkeley. His research interests include radio, X-ray, and gamma-ray observations of neutron stars and black holes.

Kevin John is a physics graduate from Sonoma State University who currently works in SSU’s NASA-funded Education and Public Outreach group. SSU E/PO support several different high-energy astrophysics missions including the Fermi Gamma-ray Space Telescope, which launched into earth-orbit in June 2008.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Daniel Herron on behalf of his wife, Misty. Happy Birthday, Snuggs! This is for all those nights you joined me under the stars!

Transcript:

John: Radio astronomers have uncovered 17 millisecond pulsars in our galaxy by studying unknown high-energy sources detected by NASA’s Fermi Gamma-ray Space Telescope. The astronomers made the discovery in less than three months. Such a jump in the pace of locating these hard-to-find objects holds the promise of using them as a kind of “galactic GPS” to detect gravitational waves passing near Earth.

From Sonoma State University I’m Kevin John and joining us from US Naval’s Research Labs is Dr. Paul Ray. Our first question, what are millisecond pulsars?

Ray: Well, first let me remind you what pulsars are. You probably know that pulsars are neutron stars that are the remnants of massive star explosions that are left behind and they contain the entire mass of our sun maybe even one and a half times the mass of our sun compressed down into a volume the size of a city–only about ten kilometers or a little bit more than that in radius. Pulsars, as they collapse that core of that star as it is collapsing, of course, spins faster and faster and pulsars are born spinning tens of times per second. And that rapid rotation powers these radio waves that we see from most pulsars pulsing on and off as that kind of lighthouse beam of radio waves sweeps past the earth and blinks. Now of course they are powered by this rotation and they are powering these emissions and so they spin down and slow down as they lose energy and those pulsars over the course of millions of years slow down and eventually sputter out when their rotation is no longer strong enough to power that radio emission. But a few lucky pulsars actually have a binary companion along with them and when that companion normal star evolves enough to start dumping out matter onto the pulsar it can cause the pulsar to be reborn by spinning it back up to very high speeds and in some cases they can be spun up to frequencies spinning at hundreds of times per second. The fastest known are over six hundred times per second and so those we call the millisecond pulsars those are then reborn once they stop taking matter from their companion. Their companion turns into a white dwarf or another type of degenerate star and they are reborn as radio pulsars again spinning hundreds of times per second and they have much lower magnetic fields and slow down much more slowly and live for a very very long time.

(2:24)

John: What makes these new millisecond pulsars so exciting?

Ray: Well, it’s a huge increase in the number of millisecond pulsars that we’ve had and there are all sorts of exciting applications or uses for them to tell us about basic physics. From a lot of these binary pulsars we can learn about the masses of neutron stars and that leads towards the equation of state for nuclear matter which is a really exciting problem. Several of these pulsars are black widow systems where wind from the pulsar is blowing away the companion and might eventually completely eradicate the companion leaving behind an isolated millisecond pulsar. Those systems tell us a lot about stellar evolution and the formation of millisecond pulsars. And finally, some of these pulsars we hope to be extremely good clocks that we could add to an array of millisecond pulsars being timed all over the sky that can hopefully make a direct detection of gravitational waves that are predicted by general relativity.

(3:26)

John: What methods were used to detect millisecond pulsars before Fermi was launched?

Ray: Well, these millisecond pulsars I mentioned they can be very old and so they have had a long time to move throughout our galaxy. They are born and can have some velocity when they are born and so they’re spread out over a quite inflated population in our galaxy–they are not just concentrated right in the disk. They are also rather faint so the ones we see tend to be close by. And the fact that they are spread out and close by means that they are spread out over all of the sky when you view them from earth and so there is not any one place that is preferential to look for them at least the ones that are part of our galaxy. And so most millisecond pulsars that have been then found require very large sky surveys where you survey regions of the sky with big telescopes. And they are a really massive undertaking because it takes many hours of telescope time to survey the sky and then many many hours of computer time–much more than the survey time–to analyze that data looking for millisecond pulsars because they are extremely fast and very difficult to detect. And so it is with Fermi pointing us the way to these millisecond pulsars that has been a real boon in making us be able to efficiently discover new millisecond pulsars with much less time and effort.

(4:39)

John: Why has Fermi been so successful at finding these pulsars that have proven so illusive in the past?

Ray: Well, it seems to me that Fermi has reached a sensitivity threshold where is can see gamma rays for many of these millisecond pulsars. The previous generation of gamma ray telescopes did not quite get to the sensitivity level where it saw many millisecond pulsars as point sources throughout the sky but Fermi seems to have kind of crossed that threshold. But it can only see them as point sources not as pulsating sources themselves because there are just too few photons for the pulsations to be discovered by looking blindly at the gamma ray data, but it does see them as point sources and can direct us towards them.

(5:18)

John: How did scientists know these high energy sources were pulsars if they did not pulse?

Ray: There are only a few known classes of gamma ray emitters. The biggest class that Fermi sees the most are Active Galactic Nuclei called blazars and really the next biggest class is pulsars so many gamma ray sources in the sky we expect to be pulsars. And thus we look at the list of gamma ray sources that Fermi sees on the sky and take the ones that are unidentified that we don’t have a good counterpart for. Then we can try to classify them either as likely pulsars or likely blazars based on their spectra or variability characteristics. Blazars are characterized by being extremely variable–they rise up and down on time scales of days and months and years. Whereas pulsars are extremely stable in terms of their just overall gamma ray brightness not including their blinking on and off, of course, but over the long term they have extremely low variability and so by looking at sources that we don’t know yet what they are and looking whether their variability and spectra look like pulsars that we rate them as good targets with the radio telescope.

(6:25)

John: What are gravitational waves and why have they proven so hard to detect?

Ray: Well, gravitational waves are ripples in the fabric of space-time that are formed by the acceleration of very large masses. If you try to detect gravitational waves using an array of pulsars, as I described, you will be sensitive to gravitational waves that have periods of months to years and the kinds of sources that have those kinds of periods would be pairs of super massive black holes whipping around each other at the center of galaxies far far away. And although these gravitational waves are huge for gravitational waves they are very distant and gravitational waves have very little affect that is sensible on earth so it is not like you can see a gravitational wave coming by–it doesn’t affect you. It just causes a stretch and a squeeze in the fabric of space-time as it passes by and so they are extremely difficult to detect at earth.

(7:19)

John: How would astronomers observe a gravitational wave passing through an array of pulsars?

Ray: Well, if you were timing an array of pulsars scattered throughout the sky what you would see is as a gravitational wave passes through the earth it stretches and squeezes space-time in opposite directions and so what you will see is the pulses from some of the pulsars arrive a little bit early and pulses from some of the other pulsars arrive a little bit late. That is, early or late compared to when they were expected because they work very much like nice regular clocks. In fact, because a gravitational wave is polarized, a quadrapolar polarization like the dipolar polarization of electro-magnetic waves, that early and late arrival times would have a very characteristic pattern on the sky that is unique to a gravitational wave. So it is a signature that we would know quite convincingly that what we are seeing are gravitational waves.

(8:11)

John: How far off would the gravitational wave shift the pulses?

Ray: Much less than a microsecond actually. The gravitational waves make a very small impact on the pulses and so in order to make this measurement we need pulsars where the arrival times can be measured to small fractions of a microsecond.

(8:26)

John: Why are millisecond pulsars more useful than pulsars that spin more slowly in the search for gravitational waves?

Ray: Millisecond pulsars have two crucial characteristics. First their very fast pulses and very narrow pulse profiles can be measured very precisely and that can give us the sub-microsecond arrival time accuracy that I mentioned that we needed. Secondly, because they are very old and are losing their rotational energy very slowly they are extremely stable and predictable clocks and that is crucial also because we are looking for, as I said arriving early or late relative to some prediction, and so the fact that millisecond pulsars have extremely predictable pulses makes them perfect for this experiment. Younger pulsars, the ones that are recently born, are slowing down much more rapidly and do a lot of this unpredictable wobbling–the clocks are very unstable and we call the process “timing noise”. So that makes them much less suitable for this kind of precise timing experiment.

(9:27)

John: What other methods have been employed to try and detect gravitational waves?

Ray: Several ground based methods have been tried starting from bars a couple of decades ago ,big bar detectors, to the current best bet for the detection of gravitational waves which is laser interferometers. A big project is going on in the US called LIGO, the Laser Interferometer Gravitational Wave Observatory, which is a pair of interferometers, laser interferometers, with two perpendicular arms that are four kilometers in length. Basically, they are using this interferometer to try and measure a very tiny change in the length in one arm relative to the other as one of these gravitational waves passes though. Now LIGO has already been operating at its initial sensitivity level without any detections–but they didn’t really expect any detections–so far proving out the technology. But they are about to take the instrument apart and rebuild it with a number of sensitivity improvements over the next few years. And if those improvements meet their sensitivity goals they fully expect to be able to detect gravitational waves directly in a few years from now.

(10:34)

John: What would the detection of gravitational waves mean?

Ray: Well, of course it would be a tremendous triumph for the theory of General Relativity proving that Einstein was right once again, but it would also open a new way to probe the universe beyond just studying the electro-magnetic spectrum like we normally do in astronomy. Gravitational waves give us a look into some of the most violent collisions in the universe–that is neutron stars spiraling into other neutron stars, small black holes falling into big black holes at the centers of galaxies–things that we could never see with a normal electro-magnetic observation–like light or radio waves or gamma rays as we do in most of astronomy.

(11:14)

John: How do you see this approach to detecting gravitational waves going forward over the next few years?

Ray: Well, as we continue to find new pulsars like these millisecond pulsars that are useful for a pulsar timing array and we keep building improved instruments at the world’s largest radio telescopes to improve our timing capabilities. I hope that we can convince these observatories to devote enough time to the timing programs that we could have a shot at detecting these gravitational waves with the pulsar timing array within about a decade from now. Although it might seem like there is a competition to see who gets the first detection between this kind of technique and LIGO, that is certainly somewhat true, but in fact they are extremely complementary because the frequencies that they are sensitive to are completely different and that means that the sources of gravitational waves that each one would be sensitive to are completely different. So, they really start to give us a capability of doing gravitational wave astronomy that we have never had before and if you add to that something like the proposed space interferometer mission called LISA, then that probes yet another frequency range that is intermediate between LIGO high frequency gravitational waves and the pulsar timing array at extremely low frequency gravitational waves. It would tell us even more about binary star systems and a host of other things. So, if we put those together it is almost like having a suite of observatories almost like what we have now with radio observatories on earth, optical observatories, and x-ray and gamma-ray observatories in space. So I hope to see that we have a lot of practicing gravitational wave astronomers some day.

End of podcast:

365 Days of Astronomy
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