Title: Optimal Frequency Ranges for Sub-Microsecond Precision Pulsar Timing,
Abstract: Precision pulsar timing requires optimization against measurement errors and
astrophysical variance from the neutron stars themselves and the interstellar
medium. We investigate optimization of arrival time precision as a function of
radio frequency and bandwidth. We find that increases in bandwidth that reduce
the contribution from receiver noise are countered by the strong chromatic
dependence of interstellar effects and intrinsic pulse-profile evolution. The
resulting optimal frequency range is therefore telescope and pulsar dependent.
We demonstrate the results for five pulsars included in current pulsar timing
arrays and determine that they are not optimally observed at current center
frequencies. For those objects, we find that better choices of total bandwidth
as well as center frequency can improve the arrival-time precision. Wideband
receivers centered at somewhat higher frequencies with respect to the currently
adopted receivers can reduce required overall integration times and provide
significant improvements in arrival time uncertainty by a factor of ~sqrt(2) in
most cases, assuming a fixed integration time. We also discuss how timing
programs can be extended to pulsars with larger dispersion measures through the
use of higher-frequency observations.