it is still raining?!
so we have an east coaster telling us about actual data
on x-ray binaries
in clusters, globular clusters
in other galaxies...
there is an open-to-the-program-members blog over on the cluster09 wikispace.
It has some good summary of yesterdays in depth discussion on runaway mergers in the afternoon session.
so our pivot point is the issue of low mass x-ray binaries (LMXBs) - mass transfer binaries, with compact primary, generally a neutron star, and a companion with a mass lower than some vague mass that is either about a solar mass, or safely less than the mass of the primary, depending on where you are coming from
so - do all low mass x-ray binaries form in clusters, with the ones seen in the field - ie within the galaxy but not in a cluster - actually ejected from some cluster?
Do some of the field LMXBs come from clusters?
In the Milky Way, there are ~ 100 LMXBs in the field and ~ 10 in clusters.
per star, this is a factor of 100 overabundance of cluster LMXBs. Since about 0.1% of stars are currently in clusters.
We think we know how to make x-ray binaries from binary stellar evolution, it is hard, but possible.
We don't know very well how to make LMXBs in clusters, it must involve exchanges and/or binary encounters including stellar collisions, but apart from some "clean exchange" scenarios, the details are not well worked out.
Also in Milky Way the filed LMXBs trace the disk and are kinematically distinct from the clusters; but in elliptical galaxies one might worry that with little star formation going on the last few gigayears that it'd be hard to make LMXBs and some, most?, of the field LMXBs are actually from clusters.
There are LMXBs in extragalactic globular clusters, in ellipticals.
Diagnostics for origin include spatial distribution (does it trace the galaxy stellar population or the cluster population - these have different ρ(r) - but, that could be due to disrupted globulars, which made LMXBs before being disrupted...
Does the number of LMXBs scale with galaxy luminosity or total number of globulars?
ie does it depend on SN, the specific frequency of globular clusters for the galaxies.
Can we get any diagnostic from the x-ray luminosity function? Is it different for the clusters and the field? Is the LMXB lifetime systematically different, if so, why?
How complete is the sample, and are such statistics just selection bias.
What about x-ray colours/hardness?
Argh.
ok, so here is the interesting bit...
the LMXB globular cluster distribution traces the galaxy light in ellipticals, as of course do the field LMXBs.
But the cluster light does not trace the galactic light, it is much more extended.
But, cluster dynamical properties ought to correlate with radial position in the galaxy (data are not good enough to resolve this) - whether from evolution or survivor bias (only dense and massive clusters can survive on orbits in the inner regions).
Hm.
LMXB luminosity functions are also different.
Cluster LMXB LF is flatter at low L.
there are different theoretical models for a break in the XLF depending on whether the donor is a giant or (ultra)compact and depending on whether the accretion disk is irradiated or not.
the models all get the location of the break wrong.
This is interesting - on the one hand we have top data, from the very greatest observatories, analysed with the best tools,
on the other hand we have top models, from the very greatest theorists, with completely ad hoc bolometric corrections...
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The radial distribution of the metal-rich clusters matches the galaxy light pretty well---except for the central core. And since LMXBs are preferentially found in the metal-rich GCs (OK maybe everyone doesn't believe this) then you'd expect the distribution of the cluster LMXBs to follow the field light.
aha, I asked a slightly incorrect version of that question - whether the luminosity weighted GC count tracked galaxy light, and was told, no.
The red weighted light sorta makes sense.
Or would if were not for M15.
central core deficit is survivor bias and field contamination from disrupted GCs, of course ;-)
I'd believe the survivor bias if you showed me that the mass function was different in the inner few kpc.
theoretically it must be there... unless GC initial MF correlates with GC orbit!
you're not going to make me look, are you?
Actually what is observed is that the spatial profile of field LMXBs is more centrally concentrated than the profile of LMXBs in GCs, so the exact color distribution of GCs is irrelevant. Whether or not they exactly follow the galaxy light is a harder question to answer because most LMXB rich galaxies also have hot gas ... completeness issues etc.
>But, cluster dynamical properties ought to correlate with
>radial position in the galaxy (data are not good enough to
>resolve this)
We could always do with better data, but the problem is at the other end. The luminosity functions of MW and M87 GCs basically show no radial gradients, while the dynamical models as you say predict that they ought to. No obvious radial effects seen in LMXBs in GCs either.
And some of the suggested tweaking of orbits doesn't help the GCLF either because there are observational constraints from the present day anisotropy of the orbits.
I should have mentioned that the spatial profiles (and hence the conclusions drawn from them) differ a bit between various groups :-). But it's fair to say that there is general agreement that there are true-blue in situ field LMXBs.
>population - these have different Ï(r) - but, that could be
>due to disrupted globulars, which made LMXBs before being
>disrupted...
This is often mentioned, but there are caveats which make it unlikely IMHO. The luminous LMXBs that we are talking about here have steady accretion lifetimes of 100s of Myr, while destruction happens over a Hubble time. So for this to work GC XRBs would have to have to be transients with "low" duty cycles. With a large reservoir of qLMXBs in GCs... And many (all?) LMXBs in GCs are supposed to be ultracompacts too.