Ultra-compact twin stars with hybrid equations of state from bosonic dark matter
Abstract
The properties of compact stars with a strong first-order phase transition to quark matter and with an additional fluid of self-interacting bosonic dark matter (DM) are studied. We find that the inclusion of DM changes the stability of mass-radius configurations relative to the naive one-fluid criterion. For compact star configurations with similar masses and different radii, so-called twin stars, the presence of DM removes the unstable segment between the hadronic and the hybrid branch, so that the stable mass-radius sequence becomes continuous after the onset of the phase transition to quark matter. We furthermore find stable ultra-compact objects (UCOs), defined by a total compactness C = Mtot/Rgrav ≥ 1/3. We observe two distinct classes of UCOs: a DM-halo class with fDM ≳ 0.9, and a DM-core class at fDM ≲ 0.02. The two classes can be separated by the surface redshift of the normal matter, which reaches z = 0.734 − 0.808 for the DM-core and extreme values of z = 2 − 3.468 for the DM-halo class, allowing us to use their redshift as a way to disentangle the two classes. Furthermore, the high redshift of the DM-halo UCOs could serve as a signal of DM since their redshift exceeds the Buchdahl limit of z = 2. Finally, we find hybrid star solutions of ``ultimate twins" with similar masses and visible radii, but different dark matter content, leading to different tidal deformabilities and surface redshifts. Future X-ray and gravitational measurements of ultra-compact neutron stars with radii and masses outside the allowed neutron star range can thereby probe the presence and the properties of DM in addition to a first-order phase transition to quark matter.
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BibTeX
@misc{rather2026ultracompact,
title = {{Ultra-compact twin stars with hybrid equations of state from bosonic dark matter}},
author = {Ishfaq Ahmad Rather and Sarah Louisa Pitz and Jürgen Schaffner-Bielich},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.03964},
eprint = {2609.03964},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.03964},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Ultra-compact twin stars with hybrid equations of state from bosonic dark matter
AU - Ishfaq Ahmad Rather
AU - Sarah Louisa Pitz
AU - Jürgen Schaffner-Bielich
PY - 2026
DA - 2026/10/26
PB - arXiv
DO - 10.48550/arxiv.2609.03964
UR - https://arxiv.org/abs/2609.03964
AB - The properties of compact stars with a strong first-order phase transition to quark matter and with an additional fluid of self-interacting bosonic dark matter (DM) are studied. We find that the inclusion of DM changes the stability of mass-radius configurations relative to the naive one-fluid criterion. For compact star configurations with similar masses and different radii, so-called twin stars, the presence of DM removes the unstable segment between the hadronic and the hybrid branch, so that the stable mass-radius sequence becomes continuous after the onset of the phase transition to quark matter. We furthermore find stable ultra-compact objects (UCOs), defined by a total compactness C = M_tot/R_grav ≥ 1/3. We observe two distinct classes of UCOs: a DM-halo class with f_DM ≳ 0.9, and a DM-core class at f_DM ≲ 0.02. The two classes can be separated by the surface redshift of the normal matter, which reaches z = 0.734 − 0.808 for the DM-core and extreme values of z = 2 − 3.468 for the DM-halo class, allowing us to use their redshift as a way to disentangle the two classes. Furthermore, the high redshift of the DM-halo UCOs could serve as a signal of DM since their redshift exceeds the Buchdahl limit of z = 2. Finally, we find hybrid star solutions of ``ultimate twins" with similar masses and visible radii, but different dark matter content, leading to different tidal deformabilities and surface redshifts. Future X-ray and gravitational measurements of ultra-compact neutron stars with radii and masses outside the allowed neutron star range can thereby probe the presence and the properties of DM in addition to a first-order phase transition to quark matter.
ER -