New Paper on Einstein@Home Radio Pulsar Discovery

Hi all,

today you can find the latest Einstein@Home paper here on the arXiv preprint server. If you want to read it, just click on the PDF button on the upper right on the arXiv page.

What is this new paper about?
Our paper presents the discovery of a millisecond pulsar found in Pulsar ALFA survey observations with the Arecibo telescope, processed on Einstein@Home as part of the BRP4 search. The pulsar rotates once every 4.3 milliseconds and is in 22-day orbit with a companion star of ~0.3 solar masses. This makes it look like a so-called “recycled†pulsar that was spun up to this rapid rotation in the past by [url=http://en.wikipedia.org/wiki/Accretion_(astrophysics)]accretion of matter[/url] from its companion. This recycling process usually leads to almost perfectly circular orbits in the binary systems. For PSR J1950+2414 (that's its name) however, this did not happen!

What is special about this radio pulsar?
The odd – and interesting – thing about PSR J1950+2414 is that the orbit is not circular, but elliptic with an orbital eccentricity of e=0.08 (e=0 would be a circle; e=0.08 is just short of the famously elliptical orbit of Mars around the Sun). That is far above that what is known from other recycled pulsars (usually e < 0.001 and much smaller) and it is not compatible with the standard “recycling scenarioâ€. This pulsar system must have formed in some other way! There are several possibilities how this system might have evolved. We currently cannot say for certain through which of these possible channels the system formed.

Interestingly, there are three other very similar systems (in terms of their orbits and spins). So one might speculate that there is a common mechanism that generates this kind of pulsar system; again we do not know for certain which mechanism that could be. Also, it could of course be simply a coincidence that these system look similar.

How does the Universe make pulsars like this one?
In our paper we look at four different mechanisms and how well they can explain PSR J1950+2414's characteristics and those of the other three known similar pulsars:

a) initial evolution of the pulsar in a triple system which became dynamically unstable.
Here you have initially a triple system with a pulsar (yes, this does exist and is really awesome!), which over time ejects one of its members. The four known systems could of course have formed this way, however their similarity would have to be a (unlikely) coincidence. Three-body systems are chaotic and the resulting binaries should be very different from one another.

b) origin in an exchange encounter in an environment with high stellar density, like that of the core of a globular cluster.
In this case the pulsar is born via the standard scenario and in an almost circular orbit. It is inside a region of space with lots of other stars (like a globular cluster). There are lots of millisecond pulsars in globular clusters, so this is something well known. At some point the binary system has a close encounter with a third star. A new binary system with an elliptical orbit forms and it gets ejected from the globular cluster. The problem here is that there is no globular cluster near the pulsar, and that the pulsar is very close to the disk of our Galaxy – unlike your average globular cluster. Again: possible, but unlikely.

c) rotationally delayed accretion-induced collapse of a super-Chandrasekhar white dwarf.
Here, your neutron star/pulsar does not form in a supernova and gets spun up afterwards via accretion from its companion star over hundreds of thousands or billions of years. (This long time and the tidal forces during this period are what circularizes the orbits of “normal†millisecond pulsars.) Here, after the accretion phase the system consists of two white dwarf stars in a circular orbit. One of them is actually too massive to exist as a (non-rotating) white dwarf (that's the “super-Chandrasekhar†in the name, it's above the Chandrasekhar mass limit of ~1.4 solar masses). Luckily, it's spinning rapidly enough to avoid collapsing to a neutron star under its own gravity – at least for some time. When it has spun down (various ways of losing angular momentum are possible here), at some point it has to collapse to neutron star. This releases a lot of gravitational binding energy and ejects some matter into a disk around the system and it also makes the orbit eccentric. Making reasonable assumptions, these leads to the values observed for the four similar pulsars. You can read more about this in this paper.

d) dynamical interaction of the binary with a circumbinary disk.
Here again the rapidly rotating pulsar forms through the usual channel, and the orbital eccentricity arises from the gravitational interaction with a circumbinary disk ejected from the stars at earlier times. This is a possibility that would explain the similar systems. Again, here is the paper describing this mechanism in more detail

So none of the models is out of the race, although a) and b) seem unlikely. Both c) and d) make rather precise predictions for the masses of the pulsar and the companion star. Unfortunately, we cannot determine either precisely enough at the moment. But further observations with the Arecibo telescope are ongoing and should let us make those assessments in the future. At this point we might be able to say whether c) or d) or neither is a good candidate to explain systems like J1950+2414.

Thank you!
Big thanks to all of you for supporting Einstein@Home and making this possible! If you have any questions, just let me know. I know that there are a lot of technical terms in what I wrote up here, and I tried to link to additional info. If you have questions, I'd be happy to answer them here. :-)

Cheers, Benjamin

P.S.: I have also submitted the paper to The Astrophysical Journal for publication. It will now go the usual path of all scientific publications. It has passed first quality control and internal review and will now be sent to two independent scientists for peer-review. Theses referees will send back reports about our paper to the journal. The reports can be anything from “please accept for publication as is†to “should not be published in this journalâ€, but in most cases they will ask for clarifications, re-writes of sections, or offer insights that can help to improve the science in the paper. The paper will be re-submitted by us after we have addressed the comments and it goes to the referees for another round of comments until they are happy with it and recommend it for publication. We'll keep you posted about the progress of that.

P.P.S.: If your computer finds a pulsar, you will not only get our lovely certificates, you will also get mentioned in the paper. If you look at the acknowledgements section of this paper, you will find: “We thank all Einstein@Home volunteers, especially those whose computers found PSR J1950+2414 with the highest statistical significance: David Miller, Cheltenham, Gloucestershire, UK and `georges01'.â€