Not a Quark Star After All?

Don't try this at home.
The sizes of a neutron star and hypothetical quark star compared to the Grand Canyon, which is 29 kilometers from rim to rim.
Illustration by D. Berry / Chandra X-ray Center.
Last April NASA organized a press conference to herald some remarkable news: the possible discovery of two quark stars, objects even smaller than neutron stars. These long-theorized objects would be so dense that their neutrons would be squashed apart to form a sea of free quarks — a totally new state of matter. Now two Stanford University astronomers have bolstered critics who said the announcement was premature.

RX J1858–3754
The superdense star RX J1858–3754, located about 380 light-years away in Corona Australis, shines fiercely in X-rays. Inset: The star is practically invisible at optical wavelengths, as shown in this deep image from one of the European Southern Observatory's 8.2-meter reflectors.
X-ray image: NASA / SAO / CXC /J. Drake et al. Optical: ESO VLT.

Timothy M. Braje and Roger W. Romani examined the spectrum of
one of the stars, RX J1856–3754, in X-rays, ultraviolet, and visible light. They find evidence that the spectrum comes from an ordinary, normal-sized neutron star about 27 kilometers in diameter with a smaller, hotter, brighter polar cap that creates the illusion of a smaller body. The quark-star interpretation is "strongly excluded" by their work, Braje and Romani write in a paper submitted to the Astrophysical Journal. They conclude that the star is probably a normal young pulsar whose radio beams are directed away from our line of sight.

But Jeremy Drake (Harvard-Smithsonian Center for Astrophysics), who proposed the quark-star model, isn't convinced. A small quark star might yet produce the observed spectrum, he says. "Model atmospheres [of such dense stars], with very strong magnetic fields for example, haven't been investigated in detail. There may be model atmospheres that fit the spectrum with only one temperature."