Finding Radio Pulsars in and beyond the Galactic Center

Radio wave scattering is enhanced dramatically for Galactic center sources in a region with radius ≳15. Using scattering from Sgr A* and other sources, we show that pulse broadening for pulsars in the Galactic center is at least 6.3ν^-4 s (ν = radio frequency in GHz) and is most likely 50-200 times larger because the relevant scattering screen appears to be within the Galactic center region itself. Pulsars beyond—but viewed through—the Galactic center suffer even greater pulse broadening and are angularly broadened by up to ~2'. Periodicity searches at radio frequencies are likely to find only long-period pulsars, and then only if optimized by using frequencies ≳7 GHz and by testing for small numbers of harmonics in the power spectrum. The optimal frequency is ν ~ 7.3 GHz[Δ_0.1 P(α^1/2)]^-1/4, where Δ_0.1 is the distance of the scattering region from Sgr A* in units of 0.1 kpc, P is the period (in seconds), and α is the spectral index. A search for compact sources using aperture synthesis may be far more successful than searches for periodicities because the angular broadening is not so large as to desensitize the survey. We estimate that the number of detectable pulsars in the Galactic center may range from ≤1 to 100, with the larger values resulting from recent, vigorous starbursts. Such pulsars provide unique opportunities for probing the ionized gas, gravitational potential, and stellar population near Sgr A*.