THE ANGULAR BROADENING OF THE GALACTIC CENTER PULSAR SGR J1745-29: A NEW CONSTRAINT ON THE SCATTERING MEDIUM

The pulsed radio emission from the Galactic Center (GC) pulsar SGR J1745-29 probes the turbulent, magnetized plasma of the GC hyperstrong scattering screen through both angular and temporal broadening. We present measurements of the angular size of SGR J1745-29, obtained with the Very Long Baseline Array and the phased Very Large Array at 8.7 and 15.4 GHz. The source sizes are consistent with the scatter-broadened size of Sagittarius A* at each frequency, demonstrating that SGR J1745-29 is also located behind the same scattering medium. Combining the angular broadening with temporal scattering obtained from pulsar observations provides a complete picture of the scattering properties. A best-fit solution for the distance of a single thin screen is Δ = 5.8 ± 0.3 kpc, consistent with being located in the Scutum spiral arm. The scattering is also consistent with a uniform scattering medium or a series of thin screens distributed between the GC and the Earth. This result is a substantial revision of the previously held model in which the scattering screen is located very close to the GC. As also discussed in Spitler et al., these results suggest that GC searches can detect millisecond pulsars gravitationally bound to Sgr A* with observations at ≳ 10 GHz and ordinary pulsars at even lower frequencies.