The compact object HESS J1731-347, with a mass of about 0.77 solar masses and a radius of about 10.4 km, may be a strange quark star rather than an ordinary neutron star. If so, a deconfinement transition from the two-flavor color-superconducting (2SC) phase to the color-flavor-locked (CFL) phase in its core could eject strangelets — hypothetical stable lumps of strange quark matter — whose annihilation or decay might imprint monochromatic gamma-ray lines on the source's TeV spectrum. The open problem is to determine whether such strangelets are detectable or meaningfully constrainable with the Cherenkov Telescope Array (CTA): develop the microphysical theory of 2SC-to-CFL production that determines whether strangelet formation is viable and, where it is, predicts concrete line energies, branching ratios, cross sections, and strangelet mass distributions; complete CTA Monte Carlo detectability studies over the full multi-TeV range (including above ~2 TeV toward the source's ~7–10 TeV continuum cutoff, where existing simulation limits are unsatisfactory); and convert CTA line-flux limits or a detection into robust constraints on strangelet population properties. The search is posed conditionally on the still-unresolved nature of HESS J1731-347 (light neutron star versus strange star versus hybrid star).

Background

Strange quark matter (SQM) is a hypothetical state of collapsed baryonic matter containing roughly equal numbers of up, down, and strange quarks; since Bodmer and Witten it has been conjectured to be the true ground state of QCD at high density, more strongly bound than ordinary nuclei. Small, metastable or stable clumps of SQM are called strangelets. Decades of searches — accelerator experiments such as NA52 at CERN and cosmic-ray instruments such as PAMELA — have found none and set upper limits on their flux. If SQM is stable, some compact stars could be strange quark stars: self-bound objects made essentially of SQM, possibly surrounded by a nuclear crust or embedded in a hybrid star with a quark core inside a hadronic mantle.

At the supra-nuclear densities of compact-star cores, QCD is expected to enter color-superconducting phases, in which quarks near the Fermi surface pair analogously to electrons in a BCS superconductor. The two-flavor color-superconducting (2SC) phase pairs up and down quarks of two colors while strange quarks stay unpaired; at higher density the color-flavor-locked (CFL) phase pairs all three flavors and all fermionic modes are gapped, lowering the free energy. A transition from 2SC to CFL inside a compact object could nucleate strangelets, with mixed 2SC–CFL regions as candidate nucleation sites — although the finite-size surface and Coulomb energies of small quark droplets may destabilize them, and magnetic fields and chiral-symmetry restoration (which reduces the effective strange-quark mass) modify formation probabilities in a model-dependent way.

HESS J1731-347 is a shell-type supernova remnant hosting a central compact object whose measured mass M=0.77−0.17+0.20 M⊙M=0.77^{+0.20}_{-0.17}\,M_\odot and radius R=10.4−0.78+0.86R=10.4^{+0.86}_{-0.78} km (Doroshenko et al. 2022, from X-ray spectral and Gaia-based distance modeling) make it the lightest and smallest compact object known. Since standard supernova simulations hardly produce neutron stars below ∼1.1 M⊙\sim 1.1\,M_\odot, this measurement triggered a wave of competing interpretations that continues through 2026: a 2SC-phase hybrid star in a chirally symmetric relativistic density functional (Sagun et al. 2023); the lightest neutron star formed through a binary-evolution channel (Zhang, Rueda Hernandez and Negreiros 2025); a CFL strange star heated by viscous dissipation of r-mode oscillations (rotational instabilities of the star), predicting a present spin of 6–9 ms (Yuan and Zhou 2025); a dark-matter-admixed quark star (Mandal, Pal and Chaudhuri 2026); and kaon-condensate cooling models (Nanopoulos et al. 2026). The nature of the object remains genuinely unsettled.

The remnant is also a bright very-high-energy (VHE, 0.1–100 TeV) gamma-ray source whose continuum (roughly dN/dE∝E−2.3dN/dE\propto E^{-2.3}) cuts off at about 7–10 TeV (Doroshenko, Pühlhofer and Santangelo 2023). The Cherenkov Telescope Array (CTA) is a next-generation array of imaging atmospheric Cherenkov telescopes, which detect VHE gamma rays through the Cherenkov light of the particle showers they trigger in the atmosphere; its planned 1600-hour Galactic Plane Survey is expected to reach a narrow-line sensitivity of order 10−1310^{-13} ph/cm²/s at 1 TeV for a 100-hour deep exposure, about ten times better than H.E.S.S. (the High Energy Stereoscopic System, the current-generation array of imaging atmospheric Cherenkov telescopes in Namibia). In the proposal of Das (2025), strangelets ejected from HESS J1731-347 could annihilate (for example via S~S~→γγ\widetilde{S}\widetilde{S}\to\gamma\gamma, producing a monochromatic photon pair) or interact with the interstellar medium, generating line-like spectral features. Existing H.E.S.S. data cap such line fluxes at roughly 10−1210^{-12} ph/cm²/s, which — under assumed interstellar hydrogen density nHn_H and strangelet cross-section σs\sigma_s — translates into limits on the strangelet number density (nS<3×10−22n_S<3\times10^{-22} cm⁻³), total mass (<10−17 M⊙<10^{-17}\,M_\odot), and production rate (<10−12<10^{-12} s⁻¹). What does not yet exist is (i) a microphysical theory predicting definite line energies, branching ratios, cross sections, and strangelet mass/charge distributions from the 2SC-to-CFL transition, (ii) CTA Monte Carlo simulations of strangelet-induced line signals across the full multi-TeV band — the source paper explicitly calls its simulation limits beyond ~2 TeV 'unsatisfactory' because the source continuum cuts off at 7–10 TeV — and (iii) a defensible mapping from CTA line limits to strangelet population constraints under controlled model assumptions (effective nHn_H, σs\sigma_s, mass distribution, production-rate history). A same-author follow-up (Das 2025, arXiv:2511.19799) built a reproducible maximum-likelihood and injection-recovery framework (Gammapy/ctools — two open-source gamma-ray data-analysis packages — on standardized GADF/DL3 gamma-ray data, the community's open formats for event-level data and instrument responses, with CTA instrument response functions) reproducing the ∼10−13\sim10^{-13} ph/cm²/s line sensitivity at 1 TeV, and parameterized production-rate scenarios — but predicted no concrete line energies or branching ratios, left the multi-TeV simulation gap untouched, detected nothing, and reported internal numbers (continuous-injection fluxes near 10−2310^{-23} ph/cm²/s, ten orders of magnitude below CTA sensitivity) that leave the detectability question wide open.

Problem Statement

Determine whether strangelets produced by the compact object HESS J1731-347 — should it be a strange or hybrid star whose core has undergone or is undergoing the transition between the two-flavor color-superconducting (2SC) and color-flavor-locked (CFL) phases — give rise to gamma-ray signatures detectable with the Cherenkov Telescope Array, and what constraints on strange quark matter follow either way. This single objective comprises three sub-goals: (1) a microphysical theory of strangelet production in the 2SC-to-CFL transition that determines whether mixed 2SC-CFL phase nucleation survives surface and Coulomb finite-size destabilization and how magnetic fields and chiral-symmetry restoration (through the effective strange-quark mass) alter formation probabilities, and that, where production is viable, predicts concrete line energies, branching ratios (for channels such as S~S~→γγ\widetilde{S}\widetilde{S}\to\gamma\gamma), interaction cross sections, and strangelet mass-to-charge distributions; (2) completed CTA Monte Carlo simulations of strangelet-induced monochromatic line signatures from HESS J1731-347 over the full 0.1-10 TeV range, explicitly including the regime above ~2 TeV up to the ~7-10 TeV continuum cutoff where the source analysis reports existing Monte Carlo limits as unsatisfactory, yielding a defensible detectability assessment for the multi-TeV regime; and (3) the conversion of projected CTA line-flux limits derived from CTA instrument-response simulations (no real deep CTA Galactic Plane Survey data on this source exist yet) — or of a detection — into robust constraints on strangelet population properties (number density, total strangelet mass, production rate), with the model assumptions (effective interstellar hydrogen density, strangelet cross-section, mass distribution, production-rate history) controlled or themselves constrained rather than freely assumed. Assume HESS J1731-347 may be a strange or hybrid star whose core has undergone the 2SC-to-CFL transition. If mixed 2SC-CFL phase nucleation does not survive surface and Coulomb finite-size destabilization, no strangelet signature arises and the problem is resolved negatively. Where production is viable, a detection attributable to strangelets after conventional origins are excluded would confirm the strange/hybrid-star branch, and an exclusion at CTA sensitivity would bound the strangelet population: an assumption-controlled conversion of projected CTA narrow-line limits into bounds on the strangelet flux, number density, total mass, and production rate that improve upon the existing H.E.S.S.-derived limits for those quantities (number density nS<3×10−22n_S<3\times10^{-22} cm⁻³, total mass <10−17 M⊙<10^{-17}\,M_\odot, production rate <10−12<10^{-12} s⁻¹) under explicitly stated and controlled model assumptions, thereby constraining the strangelet-production scenarios that are consistent with the strange/hybrid-star branch and satisfy those assumptions.

References

  1. C. R. Das, 'Finding strangelets in cosmic rays from HESS J1731-347, a possible strange quark star using the Cherenkov Telescope Array Observatory', 2025, arXiv:2507.12817, DOI 10.48550/arxiv.2507.12817, https://arxiv.org/abs/2507.12817; published as Physics of Atomic Nuclei 88, 1142-1148 (2026), DOI 10.1134/S1063778825601982
  2. C. R. Das, 'Strangelet Searches from Neutron Stars, Binary Mergers, and Gamma-Ray Bursts with Current and Future Observatories', 2025, arXiv:2511.19799, DOI 10.48550/arxiv.2511.19799, https://arxiv.org/abs/2511.19799
  3. V. Doroshenko, V. Suleimanov, G. Pühlhofer, A. Santangelo, 'A strangely light neutron star within a supernova remnant', Nature Astronomy 6, 1444 (2022), DOI 10.1038/s41550-022-01800-1, https://doi.org/10.1038/s41550-022-01800-1
  4. V. Doroshenko, G. Pühlhofer, A. Santangelo, 'Expansion and ongoing cosmic ray acceleration in HESS J1731−347', Astronomy & Astrophysics 679, A152 (2023), DOI 10.1051/0004-6361/202346142, https://doi.org/10.1051/0004-6361/202346142
  5. V. Sagun, E. Giangrandi, T. Dietrich, O. Ivanytskyi, R. Negreiros, C. Providência, 'What Is the Nature of the HESS J1731-347 Compact Object?', The Astrophysical Journal 958, 49 (2023), DOI 10.3847/1538-4357/acfc9e, https://doi.org/10.3847/1538-4357/acfc9e
  6. Shu-Rui Zhang, J. A. Rueda Hernandez, R. Negreiros, 'Can the Central Compact Object in HESS J1731–347 Be Indeed the Lightest Neutron Star Observed?', The Astrophysical Journal 978, 1 (2025), DOI 10.3847/1538-4357/ad96b5, arXiv:2411.19382, https://doi.org/10.3847/1538-4357/ad96b5
  7. Ya-Jing Yuan, Xia Zhou, 'Thermal Evolution of the Central Compact Object in HESS J1731−347 as Evidence for a Color-flavor-locked Strange Star', Research in Astronomy and Astrophysics 25, 055016 (2025), DOI 10.1088/1674-4527/adce4e, https://doi.org/10.1088/1674-4527/adce4e
  8. Haimanti Mandal, Suman Pal, Gargi Chaudhuri, 'Exotic nature of HESS J1731-347: Dark-admixed quark star?', Physical Review D 113, 123071 (2026), DOI 10.1103/z8bx-c8dv, https://doi.org/10.1103/z8bx-c8dv
  9. D. G. Nanopoulos, P. S. Koliogiannis, V. Petousis, M. Veselsky, Ch. C. Moustakidis, 'Impact of Kaon Condensation on the Thermal Evolution of the CCO in HESS J1731--347 Supernova Remnant', 2026, arXiv:2607.28107, https://arxiv.org/abs/2607.28107