PSR J1840-1419: A very cool neutron star

We present upper limits on the X-ray emission for three neutron stars. For PSR J1840$-$1419, with a characteristic age of 16.5 Myr, we calculate a blackbody temperature upper limit (at 99% confidence) of $k{T}_{\mathrm{b}\mathrm{b}}^{\mathrm{\infty }}<24_{-10}^{+17}$ eV, making this one of the coolest neutron stars known. PSRs J1814$-$1744 and J1847$-$0130 are both high magnetic field pulsars, with inferred surface dipole magnetic field strengths of $5.5\times {10}^{13}$ and $9.4\times {10}^{13}$ G, respectively. Our temperature upper limits for these stars are $k{T}_{\mathrm{b}\mathrm{b}}^{\mathrm{\infty }}<123_{-33}^{+20}$ eV and $k{T}_{\mathrm{b}\mathrm{b}}^{\mathrm{\infty }}<115_{-33}^{+16}$ eV, showing that these high magnetic field pulsars are not significantly hotter than those with lower magnetic fields. Finally, we put these limits into context by summarizing all temperature measurements and limits for rotation-driven neutron stars.