Constraints on magnetic monopoles from X-ray observations of neutron stars

Magnetic monopoles are captured efficiently by neutron stars, and if they catalyze nucleon decay, the decay products would thermalize and generate observable X-ray surface emission. We use archival Chandra, XMM-Newton, and Swift-XRT data for old isolated millisecond pulsars to place conservative limits on the Galactic monopole flux (F M). For a benchmark cross-section of sigma B about 10-27 cm squared, our constraint as a function of monopole mass m M is given by F M(m M) 6 times 10-19 cm-2s-1sr-1 times (4 times 10-6, (2 times 10 11 (GeV/c 2)/m M, 1)). These limits improve previous neutron-star bounds, provide the strongest constraints to date on F M for m M between 10 11 - 10 13 GeV/c squared, and are competitive to existing constraints for this scenario. We also derive complementary constraints from the measured thermal emission of the Magnificent Seven. Our results demonstrate that neutron star X-ray observations provide a powerful probe of magnetic monopoles and motivate dedicated X-ray searches for old neutron stars as a means to test monopole-induced heating.