Radio Detection of PSR J1813–1749 in HESS J1813–178: The Most Scattered Pulsar Known

The 44.7 ms X-ray pulsar in the supernova remnant G12.82−0.02/HESS J1813−178 has the second highest spin-down luminosity of known pulsars in the Galaxy, with E ̇ = 5.6 × 10 37 erg s −1 . Using the Green Bank Telescope, we have detected radio pulsations from PSR J1813−1749 at 4.4–10.2 GHz. The pulse is highly scattered, with an exponential decay timescale τ longer than that of any other pulsar at these frequencies. A point source detected at this position by Dzib et al. in several observations with the Jansky Very Large Array can be attributed to the pulsed emission. The steep dependence of τ on observing frequency explains why all previous pulsation searches at lower frequencies failed ( τ ≈ 0.25 s at 2 GHz). The large dispersion measure, DM = 1087 pc cm −3 , indicates a distance of either 6.2 or 12 kpc according to two widely used models of the electron density distribution in the Galaxy. These disfavor a previously suggested association with a young stellar cluster at the closer distance of 4.8 kpc. The high X-ray measured column density of ≈10 23 cm −2 also supports a large distance. If d ≈ 12 kpc, HESS J1813−178 would be one of the most luminous TeV sources in the Galaxy.