Two years ago I called shens about whether Quark Soup exists. Last year the Lead Radius Experiment (Pb-R-Ex, pronounced "prex" I guess) cast further doubt. Zaven Arzoumanian's team at NICER, and others, looked at pulsars J0030 then J0740; Arzoumanian promised some results on a third pulsar at that time. [h/t Nyrath's twitter.]
Neutron-star radii can actually be measured, even from this distance. Their magnetic poles - like ours - don't always align with their rotational poles. Such are "pulsars" which send X-Rays our way. Then we know their rotation-period... to the nanosecond. Apparently when the magnetic pole runs on the other side of the pulsar, the gravity of the ex-star bends the radiation just so. Now we got radius, and gravity. Even if the ex-star doesn't have planets or other companions (Kepler, yo!), which some do.
As for which pulsar they want next, formal publication is slow. I had to dig in the arxiv. Turns out: this one's famous. They're looking into J0437−4715, the closest (at about 391.79 light years away
) and brightest. (Told you pulsars are measurable... and that publication is slow.) As to why NICER didn't measure it yet, maybe that's because it's in an orbit with a white dwarf perhaps muddying the readings.
LOW MASS 10/24 This might be a quark star.
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