Radio monitoring of transient Be/X-ray binaries and the inflow-outflow coupling of strongly magnetized accreting neutron stars

Strongly magnetized (B >= 10(12) G) accreting neutron stars (NSs) are prime targets for studying the launching of jets by objects with a solid surface; while classical jet-launching models predict that such NSs cannot launch jets, recent observations and models argue otherwise. Transient Be/X-ray binaries (BeXRBs) are critical laboratories for probing this poorly explored parameter space for jet formation. Here, we present the coordinated monitoring campaigns of three BeXRBs across four outbursts: giant outbursts of SAX 2103.5+4545, IA 0535+262, and GRO J1008-57, as well as a Type-I outburst of the latter. We obtain radio detections of IA 0535+262 during ten out of twenty observations, while the other targets remained undetected at typical limits of 20-50 mu Jy. The radio luminosity of 1A 0535+262 positively correlates with its evolving X-ray luminosity, and inhabits a region of the L-X-L-R plane continuing the correlation observed previously for the BeXRB Swift J0243.6+6124. We measure a BeXRB L-X-L-R coupling index of beta = 0.86 +/- 0.06 (L-R proportional to L-X(beta)), similar to the indices measured in NS and black hole low-mass X-ray binaries. Strikingly, the coupling's L-R normalization is similar to 275 and similar to 6.2 x 10(3) times lower than in those two comparison samples, respectively. We conclude that jet emission likely dominates during the main peak of giant outbursts, but is only detectable for close-by or super-Eddington systems at current radio sensitivities. We discuss these results in the broader context of X-ray binary radio studies, concluding that our results suggest how supergiant X-ray binaries may host a currently unidentified additional radio emission mechanism.