During the first talk this morning, S. Bhattacharya claimed that the spin frequency of NS must be measured simultaneously with the radius and the mass in order to constrain the EoS. Does anyone has a comment about that ? Is it really crucial to measure the spin frequency ?
Another comment that I would like to make concerns a question following the talk by R. Wijnands. He expressed doubts about the qLMXBs nature of those objects detected in GC since they have been observed in outbursts. I find it a quite interesting opinion, however, I don't think that we should exclude those objects from being quiescent accreting neutron stars. Only a few qLMXBs are known in globular clusters and for a several years only. The quiescent period can last more than 10 years in some cases of known field qLMXBs therefore I believe that time will tell whether or not they are transient objects and if they will go through an outburst stage.
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Any equation of state needs to be able to predict neutron stars that spin at the rates observed without the star flying apart. So maximum spin rate can, on its own, provide a constraint. However in order for a measured spin to rule out any of the current equations of state it would need to be very high (well above 1000 Hz). The current record holder, at 716 Hz, isn't fast enough to do this.
Spin is also useful, however, in more indirect constraints on the equation of state. Efforts have been made, for example, to use spectral lines seen during X-ray bursts to infer redshift, and hence mass and radius. Rapid spin would broaden any spectral lines and affects their modeling, but if you know the spin you can try to deal with this in your calculations.
The many soft low-luminosity X-ray sources in globular clusters are, in my opinion, very likely neutron star transients in their quiescent states. However, the question by Didier Barret was triggered by the text 'but are they qNS?' on one of my slides (due to time constraints I did not have time to elaborate on this). The reason for this is that we have several tens of systems now found in GC and it is expected that 100 to 150 of those systems should be present in all Galactic GCs (see papers by Pooley or Webb, also see Nathalie Webb's presentation during E15).
If such a number of qNS's indeed exist in the GCs *and* they have outburst properties similar (recurrence times of years to a few decades; luminosities >1e36 erg/s) to the known neutron star transients, we should see very often systems go in outburst.
We do not see that. Possible reasons could be:
1: the recurrence time is very very long (centuries or even thousands of years). If true, this would mean some serious problems for the disk instability models which are used to explain the outbursts.
2: the peak outburst luminosity is
very weak, 1e34 to 1e35 erg/s. Might well be, especially that we know about such very-faint transients in our Galaxy (see, e.g., the talk by Nathalie Degenaar in E15). This can be tested by frequent monitoring observations of many gobular clusters with sensitive instruments like Chandra or XMM-Newton. ASMs typically cannot detect such faint outbursts, although MAXI might see a bunch because it is more sensitive.
3: the systems are not qNSs. But
what are they then?
Solution 3 made me raise the question during my talk because unless we have detected outbursts from several of these low-luminosity soft X-ray sources in GCs, option 3 is still valid.
I will agree with Anna watts on the spin contribution to the NS EOS.
As remembered, 1000 Hz spin may have an influence on NS mass at
scale of 20%. Even though the spin 1122 Hz is a valid detection, we still have problem to distinguish the EOS of neutron star from that of quark star.
(Chengmin Zhang)
The statistical significance of the 1122 Hz burst oscillation claim is sufficiently low (if you take into account the number of other sources whose bursts have been searched for oscillations) that this should at present be treated as a tentative detection. This is why I quoted the 716 Hz result as the upper limit on observed spin. If the 1122 Hz turns out to be reproducible it would of course be extremely exciting - but it needs to be seen in another burst from the same source to be confirmed.
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