Tuesday, May 10, 2005

GRBs, colliding neutron stars and fast physics

So, GRB 050509b made the press; starting with
Good news article on scienceNow, and associated press put out a very short teaser.

The official NASA press conference is cancelled, and "scientists are cleared to discuss the results".
Since I am unaffiliated with either Swift or the ground based followups, I was free to comment, but was promptly co-opted as a "outside commentator" and therefore under a "no-comment" embargo until release.
NASA has a reason for this, they like co-ordinated, well spaced and timed blitzed on specific news, and preferably stuff that has been published in a refereed paper (and there were high energy astrophysics press events today [on COUP] and another on thursday on GRB041209a (not a NASA event I believe).

But, what do you do when the data and citable telegrams on followups are published immediately on the web?
Journalists aren't stupid, and many good science journalists know where to look for breaking news; they follow the GCN, IAUC and MPEC circulars, and religiously read arXiv every evening. And good for them.

Now, how they're getting my cell number I don't know. Our secretary is not handing it out...

Still, no press is bad press. And it made slashdot - total geek streed cred, d00d.

Oh, science. Its still there; though whether the faint, possibly variable optical counterpart is real or background is very uncertain. The x-ray counterpart is quite well localised and is right next to a whopping big elliptical galaxy at z=0.22.
Those are relatively rare beasts, and this is highly suggestive, but not conclusive, evidence for physical association.
(I guesstimate less than 1:100 chance of a short GRB being that close to a galaxy like this.. post hoc ergo...)

But, deeper Keck imaging reveals "shitloads" of faint blue crap all over the XRT error circle. Which could be high redshift star forming galaxies, which could be the real hosts of the GRBs, which would kinda blow the theory of a delayed offset neutron star merger right out of the water, and incidentally scupper the beautiful model of Bloom, Sigurdsson & Pols (1999).

So what could it be? It fits the NS merger model well, almost scarily well; like coincidence well.
If it is higher z, it is faster (to compensate for (1+z) time dilations, so true time scale of less than 10 millisec (?!) and higher energy. That points to neutron star-black hole merger, with dynamically unstable disruption of the neutron star and a ballistic accretion flow (as opposed to an angular momentum limited disruption with viscous evolution).

That's interesting too. (and thanks to Prof Cole Miller for illuminating discussion on this!) But I still lean to the neutron star merger theory.
Ultimate proof will come when several of these are seen. If they are seen in statistical association with L* galaxies at low redshift, and not ones undergoing a lot of recent star formation; then the NS-NS merger model is likely right.

Interesting possible implications - gravitational radiation sources for ALIGO; tests of extreme nuclear physics, sites for rare nucleosynthetic processes, possible extreme magnetic field evolution, tracer of past star formation, and relativistic physics.
Need lots more, hope Swift picks them up. They're x-ray faint, which is consistent with a relativistic blast wave in low densityh intergalactic medium, but that makes them harder to localise.
Knowing what the beaming fraction is would be very valuable.


Anonymous Anonymous said...

Yes, and knowing the beaming fraction of radio pulsars would be "very valuable" too. Think you'll know the short GRB number first? -A.D.

11:56 PM  
Blogger Steinn said...

I think we are starting to have a pretty good idea of the beaming fraction for radio pulsars, and to some extent how it depends on spin rate and magnetic field morphology.
In particular 0737 suggests the beaming fraction is not 100%, but that the canonical 15% is not a bad estimate. And I think with monitoring and some more precessing binaries we can at least get a good sample of beaming fractions.

For the short GRBs, knowing anything within one or two orders of magnitude would be good. I couldn't tell you whether they are unbeamed, or beamed into solid angles of less thant 0.001 sterads;
If they are NS-NS mergers, and if you believe the NS-NS merger models are accurate to within an order of magnitude (and this is testable by advanced LIGO and with more pulsar observations) then we know something.
If we get a bright short GRB opticall localised we can start playing games with the fireball properties, maybe get radio, even "see" the expansion and deceleration.
So, yes, on 5-10 year time scale I think we will know the short GRB beaming fraction within an order of magnitude; and we will know the radio pulsar beaming about as well by then or sooner.

12:23 AM  
Blogger Sean Carroll said...

I mentioned this on my blog, and in the comments someone pointed to a bulletin claiming that the counterpart isn't actually varying. Is this as bad news for the finding as it appears to be?

2:24 PM  
Blogger Steinn said...

No, the x-ray transient is still well localised, and peculiarly near the low redshift elliptical.
It would be good to have an optical counterpart to really nail it down - but as the images go deeper, more background fluff appears. Could be there is no optical counterpart, if the gas density is low, the optical light curve is too slow and faint to be seen; or could be it is one of those little blurs; will take a few days to nail it down.

3:06 PM  
Anonymous Anonymous said...

Steinn -- how confident are you that they've really got the transient for this? It was about 12 x-ray photons, right? (As I recall, 0.03/s times 400 s.) No background rate listed in the GCN, so it is hard to know the significance from the available data.

And why are you optimistic that the location of the transient will be pinned down? Aren't those early x-ray photons all we've got? Or are there late x-ray observations I've missed somewhere? --A.D.

2:28 AM  
Blogger Steinn said...

Chandra just did a medium depth observation;
GCN note is out
They have a source tentatively identified as the Swift XRT - second observation will be needed to see if it is fading. That will narrow things down further and show if any of the faint blue source are or are associated with the XRT.
I understand additional VLT and Keck observations are underway.

3:50 AM  

Post a Comment

<< Home