Eureka?


The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny ...'
-Isaac Asimov

I recently found a very exciting new millisecond pulsar. But my first thought was not "Wow! A new millisecond pulsar!" but "Isn't that a suggestive bit of interference?"

To explain myself a bit further, I was looking at candidates from the drift-scan survey. These are all the periodic signals we picked up with the GBT, and they naturally include every cell phone call, car ignition, laptop computer, and worn-out electric blanket in the vicinity. Most are easy to distinguish from real pulsars, but some aren't. One characteristic pulsars generally have is they're noisy: after all, they're faint astronomical sources, so it would be very strange if they were so strong we couldn't see any noise in the observation. But in the plot showing 1023's signal, seen above, you can see, there's no noise. Turns out 1023 is just plain bright. I was not the first to misclassify it, though.

J1023 normally looks like a fairly ordinary 17th magnitude star. It's in star catalogs and photographic plates (enter RA 10 23 47.67 and Dec 00 38 41.2) going back as far as 1952, but it never got any attention in particular until 2002.

In 1998, the FIRST sky survey was carried out with the VLA. This is a rather different beast from a pulsar sky survey; it gives average brightnesses over several-minute integration times, so it's not going to have any luck detecting pulsations. But because the VLA is an interferometer, it's able to generate quite high-resolution images. This kind of survey, called a "continuum" sky survey (as opposed to a survey for spectral lines or a pulsar survey) is good for finding nebulae, radio stars, and galaxies. J1023, or to give it its full name, FIRST J102347.67+003841.2, caught the attention of people analyzing the FIRST data because it was variable: in the three observations just days apart, its radio brightness varied by a factor of at least three. Galaxies generally don't change so quickly, so they thought the source was worth following up.

Since the source had an optical counterpart, they chose to follow it up by taking optical spectra. These optical spectra, and those by another group, showed that it was blue and had double-peaked emission lines. A blue spectrum indicates very hot gas, emission lines indicate hot diffuse gas, and the fact that they were double-peaked is normally a sign that they were coming from an accretion disk: gas on one side of the disk is moving rapidly towards us, so its emission line is shifted towards the blue end of the spectrum, while gas on the other side is moving rapidly away, so its emission line is shifted towards the red end. Since the hottest gas is the nearest the center and the fastest moving, it produces the strongest emission, and you get a double-peaked spectrum. Other observers also looked at the source with high-speed photometry (just looking at how bright the object was) and found it was flickering, which is normal for accreting systems, as turbulent knots in the disk come and go. So people looked at all this data and classified J1023 as an unusual "cataclysmic variable", that is, an accreting white dwarf.

In 2002, observations showed that the emission lines were gone, the spectrum had gone back to the Sun-like colors observed in 1999 and earlier, and the flickering had tailed off. All that was left was a Sun-like star that varied in brightness by 0.4 magnitudes in a predictable fashion as it travelled around its orbit. This return to quiescence is normal for cataclysmic variables: they go through active phases and passive phases. But John Thorstensen and Eve Armstrong decided to try to come up with a model that explained the light curve (the brightnesses and colors).

When you have a binary system like this in which the stars are close together, the companion is heated by the white dwarf. When the hot side is facing us, the star is brighter and bluer than when the cool side is facing us. So it shouldn't be too hard to look at the light curve and figure out how bright the white dwarf is.

Well, as always, it's not as easy as it sounds, but by dint of great effort, Thorstensen and Armstrong managed to come up with a model that fit. But there was a problem: it needed a massive bright white dwarf. So massive, in fact, that it couldn't be a white dwarf, and so bright that we should have seen it in the spectrum, as a blue bump. There are many possible explanations for this sort of thing; software bugs, calibration problems, and so on. But they did a careful analysis, didn't find any of those, and decided to go out on a limb and suggest that the prevailing opinion was wrong: J1023 didn't contain a white dwarf at all, but a neutron star.

There the question stood for a few years. Homer et al. took X-ray observations of J1023, finding it was bright in X-rays, which supported Thorstensen and Armstrong but wasn't definitive. Thorstensen kept an eye on the system, taking a spectrum now and then to see if it was doing anything new. It wasn't. Then we came along.

When I found the pulsar, we weren't very sure of its position. We were using the low frequency of 350 MHz so that our beam would be big enough to see any piece of sky for two minutes as the Earth turned. But that big beam means that when we find a pulsar, all we know is a very rough position. Nevertheless, when we found a bright fast millisecond pulsar, we knew it was going to be interesting, so we requested a follow-up observation with the GBT.

The night before we were supposed to take the data, Jim Condon of the NRAO emailed us to point out that J1023 was in our beam. Ingrid Stairs, one of my collaborators on the survey, did some reading and found Thorstensen and Armstrong's paper. Suddenly there was the possibility that our millisecond pulsar might actually have been accreting in 2001. This was a big deal - Duncan Lorimer bet us all a drink that it wasn't the same source (covering his bets, I think - he was hoping it was the same source as much as the rest of us), and we were all excited. So we definitely wanted to find out whether it was the same source as soon as possible.

Unfortunately, the observation we had planned was another 350 MHz observation, just to see whether the source was real and to start to build up a timing solution for it. So as Scott Ransom and I prepared to run the observation, we argued: I wanted to look at 350 MHz first, to make sure there was something there at all, and Scott wanted to take a 2 GHz observation pointed at the position of J1023, so that the much smaller beam would tell us whether the source was really J1023 or not. In the end we compromised: we started at 350 MHz, and the pulsar came booming in right away. So we retracted the prime focus arm and switched receivers and pointed the GBT at J1023, and sure enough, there it was, loud and clear right at the position of J1023. We took the rest of the observation at 2 GHz, and immediately began requesting follow-up observations.

We initially planned to follow up with the GBT, since it was what we were used to, but Paulo Freire emailed us and asked us to please propose for time with Arecibo: there was not much else for Arecibo to look at at the time of day 1023 is visible there, and Arecibo's funding is threatened and it could really use a splashy discovery. (We were keeping the discovery quiet at this point, but Paulo was sharing an office with one of my collaborators, so there was no keeping it from him.) With Arecibo, this already-bright pulsar comes in beautifully, and we get nice clean timing.

Timing the pulsar, we quickly came up with a model of the orbit of the pulsar, and sure enough it agreed with Thorstensen and Armstrong's orbit. In fact, not only did the orbital period come out the same, if we extended our solution back to 2005, we got the same orbital phase as they did: over those three years, we were able to account for every single turn of the companion around the pulsar. Needless to say, this removed all doubt that our system was actually J1023.

The follow-up observations also revealed some peculiar phenomena, like plasma floating around the system and orbital period variations (very small, needless to say), but the essence of it is there: J1023 is a system with a neutron star and a companion that had an accretion disk for about a year around 2001 but is now a millisecond pulsar. The paper has just been published in Science, and is available on arxiv.org.

1 comment:

Nicole said...

Hey! Got to your blog from Cosmic Variance.

Another blogging female grad student in radio astronomy? *gasp* Congrats on the MSP discovery :-)

There was an error in this gadget