Gravitational waves could reveal the birth of a quark star

19 Feb 2019 Alex Petkov

Simulation of two neutron stars colliding (Credit to: C. Breu, L. Rezzolla)

Gravitational waves could be the key to detecting a new phase transition to quark matter when two neutron stars merge. In simulations of these explosive events, performed independently by two international research groups, distinct signatures of the phase transition were uncovered in the resulting gravitational wave spectra. Both research teams published their findings last week in Physical Review Letters.

The merging of two neutron stars was observed for the first time in 2017, an event labelled GW170817 by astronomers. During such a merger the temperatures and pressures far exceed those that can be achieved in any laboratory, and scientists have wondered whether it’s possible for these extreme conditions to facilitate a new type of phase transition – one to quark matter.

Quarks have so far only been found in a group, forming all known subatomic particles such as protons and neutrons. But scientists have speculated that these subatomic particles could break down at ultrahigh pressures and temperatures to create a uniform sea of quarks.

Researchers believe that evidence for this phase transition might be found in the spectra of the gravitational waves, such as those detected in the GW170817 event. Identifying a specific signature of such a phase transition in the gravitational wave spectra would also allow quark matter to be detected from future merger events – and also enable astronomers to identify new stellar objects such as hybrid or even purely quark stars.

Stars collide

The two research teams searched for this distinct signature by performing simulations of two neutron stars merging, focusing on sizes and masses similar to those observed in GW170817. Some of the models allow quark matter to exist, while others do not – which the researchers hoped would uncover prominent differences in the simulated gravitational-wave spectra that could identify the phase transition.

Meanwhile, Andreas Bauswein and colleagues from Europe and North America analysed the results from 22 different models that all assume asymptotically flat space – of which 7 allow for a phase transition to quark matter at the exact moment of the neutron star merger and 15 are based only on hadronic matter. This multi-model approach should reveal a general trend of behaviour when a quark-matter phase transition occurs that cannot be attributed to any other phenomena.Each group differed in their modelling approach. Elias Most and colleagues from the US and Germany used fully relativistic models, in which they observed a gradual phase transition on millisecond timescales after the neutron stars had merged. Their simulations suggest that the proportion of quark matter reached 20% of the total baryonic mass before the resulting object collapsed as a black hole about 17 milliseconds after the merger. The distinctive signature of the transition, they found, is a dephasing of the gravitational-wave signal as the fraction of quark matter increased.

The signs are there

Most of these models showed a strong dependence between the maximum peak value in the gravitational wave frequency spectrum and the tidal deformability, a parameter that depends on an object’s mass and radius. However, the models that allow for a quark-matter transition exhibit significant deviations from this trend – of the order of half a kilohertz. In these simulations the object resulting from the merger remained gravitationally stable.

The results from these simulations suggest that a phase transition to quark matter does indeed happen in a neutron-star merger, but current measurements are not accurate enough to confirm the findings experimentally. To detect these signatures in future neutron star mergers, more precise measurements will be needed of quantities such as mass, gravitational-wave frequency and tidal deformability.

The ongoing improvement of detection methods should reduce the uncertainties on such measurements in the next few years, which could enable future researchers to identify these signatures and even witness the birth of a new astronomical object – the hypothesized quark star.

physicsworld.com 19/2/2019