Compact stars in a large-tension braneworld: mildly negative Weyl coupling consistent with NICER and gravitational-wave data

We use Bayesian inference on multi-messenger observations to constrain the parameter space of compact stars in a phenomenological braneworld model inspired by the effective Sahni--Shtanov scenario. Stellar structure is described by modified Tolman--Oppenheimer--Volkoff equations including local quadratic brane corrections and a phenomenological closure for the nonlocal Weyl sector, parametrized by the brane tension lambda, the Weyl coupling alpha U, and the Weyl equation-of-state parameter w U. Using the SLy equation of state, we perform an affine-invariant ensemble Markov Chain Monte Carlo analysis combining mass--radius posteriors from GW170817 (LIGO/Virgo) and NICER observations of PSR J0740 + 6620 and PSR J1231 - 1411. The posterior yields 10(lambda/km-2)=3.98+1.52-1.44 (68%), indicating a large-brane-tension regime where local high-energy corrections are subdominant. The Weyl coupling is constrained to alpha U =-0.15+0.30-0.08 (68%), while w U remains weakly constrained, with a 95% credible interval of [-1.25,,1.02]. The inferred stellar properties are M=2.30+0.14-0.08,M and R 1.4=13.31+0.54-0.57, km (68%), exceeding the corresponding General Relativity predictions for the SLy equation of state. The 95% posterior interval extends into the GW190814 secondary-mass range, although the median and best-fit values remain below it. These results show that a large-tension braneworld with a mildly negative Weyl coupling is consistent with current NICER and gravitational-wave constraints without requiring large deviations from General Relativity.