Recently-Discovered Double Pulsar

[lpetrich]

I'm not sure if this was mentioned in any other threads here; I could not find any reference to it.

A few weeks ago, a double pulsar was discovered by Andrew Lyne et al.; a binary pulsar where both members are radio-visible pulsars:

J0737-3039A - 23 ms
J0737-3039B - 2.77 s

with an orbit period of around 2.4 hours. The orbit's semimajor axis is around 800,000 km, and its eccentricity around 0.0878.

B was observed to eclipse A, and A apparently modulates B's emissions, changing B's observed luminosity and pulse shape.

I propose names for A and B, these Sesame-Street muppets:
A: Ernie (has a short, broad head)
B: Bert (has a tall, narrow head)
However, those names are likely to run into copyright problems...

Binary pulsars in general are good tests of relativistic-gravity theories, and this double pulsar should offer some especially good tests, since both pulsars being visible increases the supply of observable parameters. Thus making them something like eclipsing double-line binary stars, while previous binary pulsars have been something like single-line binary stars.

In particular, the Newtonian-limit time delays across the orbits fixes the mass ratio very precisely, and the eclipses suggest that the orbit's inclination is close to 90 deg (edge-on).

Relativistic effects have also been observed for A, though not yet for B, like:
Redshift due to motion and the other object's gravitational field
Shapiro extra time delay near a massive object
Precession of the orbit's periapsis

Assuming the correctness of general relativity and that other effects are negligible, some very precise mass estimates can be obtained:
A: 1.34 solar masses
B: 1.25 solar masses

These are a little less than the Chandrasekhar limit; could the difference be partially due to gravitational binding energy? Or gravitational-collapse details?

However, alternative theories like the Generalized Brans-Dicke theory predict a metric that looks much like GR's "post-Newtonian" metric, but with fudge factors, usually called gamma and beta. GR predicts that both are 1; GBD in general predicts non-unity values, but values that can be made arbitrarily close to 1 with appropriate parameter selection, making the theory almost indistinguishable from GR. And GBD-like theories may possibly be a low-energy limit of superstrings instead of pure GR.

The aforementioned redshift can be derived from the Equivalence Principle, so they should be the same in any metric theory of gravity (GR, GBD, etc.).

The Shapiro time delay (and deflection of light) is (1+gamma)/2 * the GR value.
It has two parameters, "r" (the delay value is proportional to it) and "s" (a shape value, equal to sin(inclination))

The precession is (2+2*gamma-beta)/3 * the GR value

The appropriate terms of the post-Newtonian metric are:

g_00 = - (1 + 2*V + 2*beta*V^2)
g_0i = 0
g_ij = delta_ij * (1 - 2*gamma*V)

where c = 1 units are used, velocity-dependent effects and some others are ignored for simplicity, and V is the Newtonian gravitational potential, - sum(GM/r). This metric is, however, correct for a single stationary source.

From Solar-System observations and experiments, however, both gamma and beta agree with GR to within a few times 0.1%.

Finally, it will be interesting to find the orbit-period rate of change, which is most likely due to gravitational radiation. Most alternative theories of gravity predict that the gravitational-radiation-source mass will be different from the inertial mass, on account of different contributions from their gravitational-potential energy, while GR predicts that they will be equal. Alternative theories thus usually predict gravitational-dipole radiation in addition to GR's gravitational-quadrupole radiation.

A gravitational dipole would be zero for two identical objects. But in this case, the two pulsars differ in mass by 10%, perhaps enough to make a gravitational-dipole effect noticeable -- if it exists. In any case, the double pulsar's orbit parameters are determined well enough to make precise predictions possible of how much gravitational radiation one can expect -- without assuming the GR values of the post-Newtonian parameters beta and gamma.


Refs:
PhysicsWeb article (http://physicsweb.org/article/news/8/1/4)
CSIRO article (http://www.csiro.au/index.asp?type=mediaRelease&id=PrPulsars)
Science magazine report (http://www.sciencemag.org/cgi/content/abstract/1094645v1)
Arxiv preprint of that report (PDF) (http://arxiv.org/pdf/astro-ph/0401086)
Nature magazine report (http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v426/n6966/abs/nature02124_fs.html&dynoptions=doi1076253262)
Clifford Will's article on Tests of General Relativity (http://relativity.livingreviews.org/Articles/lrr-2001-4/index.html)
Testing General Relativity with Pulsar Timing, by Ingrid Stairs (http://relativity.livingreviews.org/Articles/lrr-2003-5/index.html)
Binary and Millisecond Pulsars at the New Millennium, by Duncan Lorimer (http://relativity.livingreviews.org/Articles/lrr-2001-5/index.html)

[eburacum45]

So how long before they collide?
Will the result be a type of Gamma ray burster?

[AGN Fuel]

Wow, thanks for the heads-up, LPetrich. This looks like a seriously significant find - a laboratory in space! 8)

It will be fascinating to see some of the research that comes out of this.

[lpetrich]

So how long before they collide?
Will the result be a type of Gamma ray burster?

1: The estimated time, assuming GR, is 85 million years. GBD and similar theories, however, predict a gravitational-dipole contribution, which will speed it up, though how much depends on the theories' fudge factors (GBD can be made arbitrarily close to GR).

2: Very likely; that possibility is being actively investigated. In some simulations, the coalescing neutron stars' temperatures go up to several MeV, which means gamma-ray luminosity.

[lpetrich]

Wow, thanks for the heads-up, LPetrich. This looks like a seriously significant find - a laboratory in space! 8)
This double pulsar is the first-discovered binary-pulsar with two visible pulsars. Several other ones have previously been discovered, but they have only one visible pulsar, and a few have white-dwarf companions. Which means that they provide fewer parameters for testing relativistic-gravity hypotheses.

It was discovered only recently, thus the imprecision of some of the observed parameters. It will take some months to get improved parameters, like improved redshift and precession values and the orbit-inspiral rate, but the results will come.

In particular, it can be shown that for a constant-orientation orbit, the pulsar orbit-induced redshift delays cannot be distinguished from plain orbit-crossing time delay. It takes periapsis precession to help untangle these effects, and, of course, the more the better.

[harlequin]

I propose names for A and B, these Sesame-Street muppets:
A: Ernie (has a short, broad head)
B: Bert (has a tall, narrow head)
However, those names are likely to run into copyright problems...


That would not be copyright but a trademark. I not convinced that it would be a problem since it is very doubtful that the Children's Television Workshop has astronomical objects as part of the scope of its trademark and I don't see how the mere naming could be used to anyone's advantage. If one actually uses images of Ernie and Bert that might change.

Of course consulting with someone is actually a lawyer is adviced.

[lpetrich]

I chose the names out of a certain resemblance of shape.

A is faster-spinning than B, thus making A more oblate than B.

Ernie's head is oblate, and Bert's head is prolate.

Though B would be almost spherical and not prolate like Bert's head. Also, I doubt that A would be as oblate as Ernie's head. The flattening (amount of oblateness) is:

f ~ (angular frequency)^2/(G*density) ~ ((frequency)/(breakup frequency))^2

With a breakup rotation frequency of ~ 1 kHz, this makes A's flattening ~ 1e-3 and B's flattening ~ 1e-7

And as to legal issues around using Ernie and Bert as names for these two pulsars, I invite anyone with legal expertise to respond.

[Mokele Mbembe]

I have to legal training, but I personally I doubt that Sesame Street would object to two pulsars being named after their characters... I mean, it's an "educational" sort of thing, scientific...

[Diamond]

I thought that neutron stars emit material (and hence EM) from their poles. Does this mean that both pulsars' axes are almost 90 degrees to their mutual rotation?

[lpetrich]

They emit from their magnetic poles, not their rotational poles, and that emission is in a cone with an interior angle of something like 15-30 degrees (I don't recall any serious estimates; this is a guess from pulsar pulse shapes).

[Diamond]

They emit from their magnetic poles, not their rotational poles, and that emission is in a cone with an interior angle of something like 15-30 degrees (I don't recall any serious estimates; this is a guess from pulsar pulse shapes).

Quite correct.

Q: What happens when the EM from one pulsar hits the other?

[Amadeus]

They emit from their magnetic poles, not their rotational poles, and that emission is in a cone with an interior angle of something like 15-30 degrees (I don't recall any serious estimates; this is a guess from pulsar pulse shapes).

Quite correct.

Q: What happens when the EM from one pulsar hits the other?

I was think something along those lines too. What would happen if em from the poles hit each others poles?

[lpetrich]

Possibly not very much. The pulsars are very far apart compared to their sizes (800,000 km vs. ~12 km). So if their emissions are localized to near their magnetic pole - surface intersection points, as the "polar cap" theoretical models indicate, then they would get very dilute by they time they got to the other pulsar.

More seriously, however, is the speed-of-light corotation distance: c/(angular frequency) --

A ("Ernie") - 1100 km
B ("Bert") - 132,000 km

This may be why A seems to influence B's luminosity; A is close to B's "near zone", where the field approximates a spinning magnetic field and not outward-going electromagnetic waves.

[ToSeek]

Binary Pulsar Spins Up a Storm (http://skyandtelescope.com/news/article_1348_1.asp)

The only known binary pulsar is giving astronomers a new perspective on the powerful winds that stream from rapidly spinning, highly magnetized neutron stars.