A Missing Link

We've been pretty sure that low-mass X-ray binaries turn into millisecond pulsars for a while now. But no one had ever seen the one turn into the other. Thanks to the drift-scan survey, now we have.

The system, J1023, is currently behaving just like a normal millisecond pulsar: it spins very regularly, and we see radio pulses every cycle. But in 2001, before anyone had any idea there might be a pulsar there, optical observations showed that it was flickering, blue, brighter than usual, and had double-peaked emission lines. All of these are signs of hot gas flowing in the system, and the double-peaked lines in particular are a pretty clear indication that the system had an accretion disk. Unfortunately nobody knew to point an X-ray telescope at J1023 while it was doing this, but it seems pretty clear in retrospect that it was an X-ray binary at the time.

In 2002, observations showed the emission lines were gone, the brightness and color were back to 1999 levels, and the flickering was tailing off to nothing. So the optical story stands today: no sign of an accretion disk. But there is clearly a radio pulsar there, and we're able to make some extremely good measurements of the system because of it. We know that the pulsar is 7.2 times as massive as its companion, for example, and if the pulsar has a typical neutron star mass of 1.4 solar masses, the system is 1.3 kiloparsecs away (about 4200 light-years) and we're seeing it at an angle of 46 degrees.

The image above is a pair of artists' renditions of it (well, sort of, I did most of the work using the software binsim and I'm hardly an artist, though Joeri van Leeuwen improved them significantly). They assume the pulsar's mass is 1.4 solar masses, and show the system as we think it was in 2000 and now. The hot disk of matter, present in 2000 but absent now, produced the optical emission that was observed, but (we think) it was blown away when a drop in the accretion rate allowed the radio pulsar to turn on, producing not just the beam of radio waves pictured in the image but also a powerful wind.

Since we know the system went through one roughly two-year active phase, it seems entirely possible that it will do so again within the next few years. If that happens, we'll be able to watch the formation of an accretion disk in a system where we have very good measurements of the orbit and system geometry from pulsar timing. That's never been seen before, and will be very exciting.

While I was the one who found this source, everyone involved in the drift-scan survey did essential work in making it possible for me to find it, and I worked with many other people to carry out the follow-up observations, so let me thank all my collaborators: Ingrid H. Stairs, Scott M. Ransom, Victoria M. Kaspi, Vladislav I. Kondratiev, Duncan R. Lorimer, Maura A. McLaughlin, Jason Boyles, Jason W. T. Hessels, Ryan Lynch, Joeri van Leeuwen, Mallory S. E. Roberts, Frederick Jenet, David J. Champion, Rachel Rosen, Brad N. Barlow, Bart H. Dunlap, and Ronald A. Remillard.
The paper describing the discovery has just been published in Science, and can also be read on arxiv.org.

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