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

Abstract

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 ${\sigma }_{\Delta \mathrm{B}}\sim 10^{-27}\mathrm{c}{\mathrm{m}}^2$, our constraint as a function of monopole mass $m_M$ is given by $F_{\mathrm{M}}(m_M)\lesssim 6\times 10^{-19}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}\mathrm{s}{\mathrm{r}}^{-1}\times \max (4\times 10^{-6},\min (2\times 10^{11}(GeV/{\mathrm{c}}^2)/m_{\mathrm{M}},1))$. These limits improve previous neutron-star bounds, provide the strongest constraints to date on $F_M$ for $m_M$ between $10^{11}-10^{13}GeV/{\mathrm{c}}^2$, 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.

Comments: 8 pages, 2 figures, 1 table. Supplemental Material (2 pages, 1 table)