Hybrid neutron stars among mass gap objects are excluded by twin stars at 1.4,M

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera, and with me are Jocelyn and Subrahmanyan, guest researcher.

Jocelyn: Today's paper: "Hybrid neutron stars among mass gap objects are excluded by twin stars at 1.4,M".

Vera: The gist: Mass-gap hybrid stars are excluded as candidates for observed mass-gap compact objects by twin star constraints at 1.4 M⊙,

Jocelyn: First, who's behind it and why it matters.

Title and authors: Vera: So we're looking at the paper titled "Hybrid neutron stars among mass gap objects are excluded by twin stars at one point four,M," and I think the title itself tells you a lot right away <ref:2606.02593#pg1,stars among mass gap objects are excluded by twin stars at 1>. It suggests they’re finding that some of these potential compact objects in that mass gap just can't fit the model when you look at twin star constraints.

Jocelyn: Exactly, Vera, because we know there are these interesting mass gaps between two point five and five M⊙ objects detected by gravitational waves, and this paper is putting a hard limit on what those objects could actually be if they're hybrid stars <ref:2606.02593#pg1>.

Subrahmanyan: From a theoretical standpoint, the core idea here seems to be testing whether these hybrid stars can exist in that mass gap region, which we know is currently a bit of an open question because standard hadronic neutron stars stop around two point one M⊙ because of how the equation of state behaves at very high densities

Yamamoto et al <ref:2606.02593#pg1>. two thousand sixteen: <ref:2606.02593#pg1>.

Vera: Right, so they're using some kind of hybrid equation of state to see if these stars can reach those higher masses in the gap, and the authors are using twin star data at one point four M⊙ to put a real constraint on that possibility <ref:2606.02593#pg1>.

Jocelyn: And what they're implying is that based on their analysis, certain hybrid star configurations simply aren't allowed when you consider those mass-twin relationships we see in some of the data from pulsars like PSR J0437-four thousand seven hundred fifteen.

Subrahmanyan: It’s a direct test of whether a specific type of exotic matter—the hybrid EoS they are using—can actually support the required properties for those objects in that mass gap.

The paper's summary: Vera: So, looking at what the authors summarized in "Hybrid neutron stars among mass gap objects are excluded by twin stars at one point four,M," they’re basically taking a generic hybrid equation of state and mapping out all the possible configurations in a Seidov diagram to see where the mass-gap objects fall <ref:2606.02593#pg1,stars among mass gap objects are excluded by twin stars at 1>.

Jocelyn: They’re using that diagram to confront it with the mass-radius constraints we get from multi-messenger astronomy, specifically focusing on those twin star properties, which shows two different stars having the exact same gravitational mass but different radii.

Subrahmanyan: The crucial part of their summary is that they find that for typical color superconducting quark matter models, like one where the sound speed squared is zero point five, they can find mass-gap objects below two point five n0, which means they can technically be hybrid stars in those regions Contrera et al. two thousand twenty-two <ref:2606.02593#pg1>.

Vera: But the paper points out that to find third-family members among the high-mass hybrid stars, you need the density jump at the deconfinement transition to exceed a specific value set by that modified Seidov criterion, which is shown by the black line on their diagram.

Jocelyn: So what they're saying is that while mass gap objects *could* be hybrid stars based on some parameter settings, finding them in the third family requires pushing those EoS parameters into a very specific corner of that diagram.

Subrahmanyan: And then they apply a Bayesian analysis scheme, using the quark matter parameters nonset and delta n instead of the Lagrangian model parameters, to see what solutions have high posterior probability based on those mass-radius constraints.

The paper's improvements: Vera: The authors suggest that by applying this specific Bayesian analysis scheme with the quark matter EoS parameters, they can rule out certain parameter sets entirely. They find that the solutions with high posterior probability for twin star solutions around one point four M⊙ just reach maximum masses that are below two M⊙ <ref:2606.02593#pg1>.

Jocelyn: That’s a significant restriction because those mass-twin sequences around one point four M⊙ could be what we expect from pulsars like PSR J0614-three thousand three hundred twenty-nine and PSR J0437-four thousand seven hundred fifteen and this analysis suggests that the maximum mass for hybrid stars in those sequences cannot reach the mass gap region.

Subrahmanyan: This finding is important because it supports the idea that if we confirm those pulsars are indeed mass twin stars at one point four M⊙, then any hybrid EoS supporting them would be limited to a maximum mass less than two point two M⊙

Ayriyan et al <ref:2606.02593#pg1,twin stars at 1.4 M>. two thousand twenty-five: <ref:2606.02593#pg1>.

Vera: So what this means for the bigger picture is that if those twin star constraints hold true, then the objects we see in the mass gap of two point five to five M⊙ cannot be hybrid neutron stars; they would have to be black holes instead <ref:2606.02593#pg1>.

Jocelyn: That’s a strong implication because it directly addresses whether these observed mass gap objects are fundamentally different from standard black holes or if they could be exotic compact stars.

Conclusion: Vera: So, to wrap up the paper "Hybrid neutron stars among mass gap objects are excluded by twin stars at one point four,M," the main finding is that for a generic twoparametric hybrid neutron star equation of state, there are parameter sets where the maximum mass is in the mass gap region, meaning it's greater than two point five M⊙

Ayriyan et al <ref:2606.02593#pg1,stars among mass gap objects are excluded by twin stars at 1>. 2026b: .

Jocelyn: However, if we take the conjecture that PSR J0614-three thousand three hundred twenty-nine and PSR J0437-four thousand seven hundred fifteen are indeed mass twin stars at one point four M⊙, then the maximum mass for those hybrid EoS would be limited to less than two point two M⊙ Ayriyan et al. 2026b.

Subrahmanyan: And this suggests that if those twin star constraints are confirmed, then the observed mass gap objects in that range must be black holes rather than hybrid stars Ayriyan et al. 2026b.

Vera: So, it’s a tight constraint: the existence of these observed mass gap objects as hybrid stars seems unlikely if those pulsar twin star constraints hold true.

Jocelyn: It really brings the whole picture together, linking the high-mass compact object mystery to specific properties of binary pulsar systems.