MAPPING THE GALACTIC CENTER WITH GRAVITATIONAL WAVE MEASUREMENTS USING PULSAR TIMING

We examine the nHz gravitational wave (GW) foreground of stars and black holes (BHs) orbiting SgrA* in the Galactic center. A cusp of stars and BHs generates a continuous GW spectrum below 40 nHz; individual BHs within 1 mpc to SgrA* stick out in the spectrum at higher GW frequencies. The GWs and gravitational near-field effects can be resolved by timing pulsars within a few pc of this region. Observations with the Square Kilometer Array may be especially sensitive to intermediate-mass BHs in this region, if present. A 100 ns-10 mu s timing accuracy is sufficient to detect BHs of mass 1000 M-circle dot with pulsars at distance 0.1-1 pc in a 3 yr observation baseline. Unlike electromagnetic imaging techniques, the prospects for resolving individual objects through GW measurements improve closer to SgrA*, even if the number density of objects steeply increases inward. Scattering by the interstellar medium will pose the biggest challenge for such observations.