Probing Solar-Wind Structure During Crab Pulsar Conjunctions with Dispersion and Faraday Rotation

Observations of the Crab pulsar during solar conjunction probe plasma structure in the inner solar wind through radio-propagation effects. The new analysis still awaits peer review, but it already lays out the central claim clearly.

That matters because astrophysics becomes persuasive only when an observed signal can be tied to a physically defensible explanation. Compact objects such as neutron stars and black holes are natural laboratories for extreme physics, but the distance and complexity of these systems make interpretation difficult without multi-wavelength coverage and careful modeling. A detection without a mechanism is only half a result. the other half comes from showing that the signal fits quantitatively inside a coherent physical picture rather than merely being consistent with a broad family of models. Observations of the Crab pulsar (PSR B0531+21) during solar conjunction probe plasma structure in the inner solar wind through radio-propagation effects. In our scattering-corrected DM measurements, this feature, $Δ$DM $= (3.95 \pm 0.20) \times 10^{-3}$ pc cm$^{-3}$, is about 20 times the day-to-day DM scatter outside the.

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Have an idea for a project that will add value for arXiv's community. We analyze dispersion measure (DM) and rotation measure (RM) measurements from the annual conjunctions of 2022-2026, obtained with the Canadian Hydrogen Intensity Mapping.

The 2025 DM time series is distinguished from those of the other years by a secondary enhancement on 21 June, about one week after the main conjunction peak. In our scattering-corrected DM measurements, this feature, $\Delta$DM $= (3.95 \pm 0.20) \times 10^{-3}$ pc cm$^{-3}$, is about 20 times the day-to-day DM scatter outside the.

The broader interest lies in turning an observational clue into something that can be weighed against competing models of the underlying physics. Astrophysics does not have the luxury of controlled experiments; everything is inferred from radiation that traveled across cosmic distances under conditions that cannot be reproduced in a terrestrial laboratory. This makes the interpretation chain longer and more uncertain than in bench science, but it also means that a well-constrained measurement of an extreme object carries theoretical information that no earthbound experiment can provide.

The RM time series, measured by combining the single pulses of each observation and corrected for the ionospheric contribution, show conjunction excursions of up to $\sim 60$ rad. From the slope of the RM excess against the DM excess we estimate the electron-density-weighted line-of-sight magnetic field.

Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether independent datasets and physical modeling converge on the same interpretation. Multi-wavelength follow-up, combining X-ray, radio and optical data where possible, is typically what separates a compelling detection from a robust physical characterization. In high-energy astrophysics, results that initially looked definitive have been revised when data from a second messenger arrived; the current result should be read with that history in mind. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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