2022ApJ...930L..27K

Making precise measurements of pulsar dispersion measures (DMs) and applying suitable corrections for them is among the major challenges in high-precision timing programs such as pulsar timing arrays (PTAs). While the advent of wideband pulsar instrumentation can enable more precise DM measurements and thence improved timing precision, it also necessitates doing careful assessments of frequency-dependent (chromatic) DMs that were theorized by Cordes et al (2016). Here we report the detection of such an effect in broadband observations of the millisecond pulsar PSR J2241-5236, a high-priority target for current and future PTAs. The observations were made contemporaneously using the wideband receivers and capabilities now available at the Murchison Widefield Array, the upgraded Giant Metrewave Radio Telescope, and the Parkes telescopes, thus providing an unprecedentedly large frequency coverage from 80 MHz to 4 GHz. Our analysis shows the measurable changes in DM that scale with the observing frequency (ν) as δDM ∝ ν^2.5±0.1. We discuss the potential implications of such a frequency dependence in the measured DMs and the likely impact on the timing noise budget and comment on the usefulness of low-frequency observations in advancing PTA efforts.