Radio spectra of most pulsars follow a power law, but some show a break: below a certain frequency, the flux drops sharply. Such gigahertz-peaked spectra (GPS) are often attributed to absorption in a dense medium rather than to the emission mechanism itself. In binary systems, especially 'spiders' with close companions, plasma from the donor star and the pulsar wind create a complex environment. Earlier studies noted that spectra of eclipsing millisecond pulsars vary with orbital phase, but systematically tracking the evolution of the break itself across the orbit had not been possible due to source faintness and a lack of wideband observations. This work joins the line of research on pulsar spectral properties and fills a gap: it is the first to continuously track the break frequency over nearly the entire orbit.
The pulsar PSR J2144−5237 was observed with the 64-meter Parkes radio telescope in Australia using the Ultra-Wideband Low receiver, which simultaneously covers the range from 704 to 4032 MHz. Total observing time was about 50 hours, split into 10 epochs over roughly two years. The system's orbital period is about 10 days, so each epoch captured a narrow interval of orbital phase, and together they formed an almost complete picture. Data were calibrated for polarization and cleaned of radio frequency interference, then divided into 20-minute segments. In each segment, the flux was measured separately for the two pulse components (first and second), which correspond to different magnetic poles of the pulsar. The spectrum of each segment was fitted with a broken power-law model, and Bayesian estimation (Dynesty nested sampling) determined the break frequency with uncertainties.
The break frequency of the radio spectrum smoothly varies with orbital phase: from about 0.8 GHz near phase 0.35 to nearly 3.0 GHz around phase 0.64, then decreases again. This pattern is reproduced across different observing epochs: points taken at different times at the same phase fall on a single curve. Both pulse components, the first and the second, show nearly identical evolution of the break frequency. The Pearson correlation coefficient between them reaches 0.98 at some epochs with high significance (p-value on the order of 10⁻⁹). The difference and ratio of the break frequencies of the two components remain almost constant along the orbit, except for a few segments with low signal-to-noise ratio. A simple broken power-law model does not always fit the spectra well: at some phases, additional features are visible, indicating more complex absorption processes. Nevertheless, the main trend is clear: the break frequency continuously 'breathes' in time with the orbital motion.
This work differs from neighboring studies in that it is the first to trace the evolution of the break frequency continuously across the entire orbit, rather than comparing individual phases. Neighboring works either studied general spectral properties of pulsars or noted spectral changes in limited phases. Here, it is shown that the break frequency serves as a sensitive indicator of the plasma state along the line of sight. The correlation between the two pulse components rules out internal causes and points to a common external absorbing screen. This turns the gigahertz break from a static characteristic into a dynamic diagnostic tool.
A natural next step is to move from a phenomenological broken power-law model to physically motivated radiative transfer models that account for specific absorption mechanisms: cyclotron resonance, free-free absorption, and synchrotron self-absorption. Joint analysis of the break frequency evolution with polarization, rotation measure, and dispersion measure will allow reconstructing the magnetic field geometry and density distribution in the system. Expanding the sample to other binary millisecond pulsars, especially with the future Square Kilometre Array, will show how universal this effect is. Three-dimensional magnetohydrodynamic modeling of the pulsar wind interaction with the companion will link the observed evolution to the shock physics.
The method is applicable to any binary pulsar with a gigahertz spectral break. It provides a new way to estimate the companion's magnetic field and the electron density in the pulsar wind.
Build a physical model of cyclotron absorption and fit it to the orbital dependence of the break frequency. Conduct simultaneous spectral and polarization observations for several systems.
For pulsar physics, the structure and dynamics of intra-binary plasma in 'spider' systems remain an open question. This work directly links the observed spectral evolution to the density and magnetic field distribution in that environment, bringing us closer to solving the problem of pulsar wind interaction with the companion's material.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors have, for the first time, continuously traced the evolution of the gigahertz break in the radio spectrum of the double millisecond pulsar PSR J2144−5237 across orbital phase, using broadband Parkes observations. They showed that the break frequency varies smoothly, and that its behavior is identical for both pulse components, pointing to external absorption in the companion's plasma.
Where its strength is
The strength of the work lies in the systematic orbital monitoring of the spectral break and the demonstration of its coordinated variation in the two components, which makes it possible to use the break as a probe of the medium in the binary system.
Where it could go next
Worth trying
Why this may matter
If the orbital dependence of the break turns out to be common among binary pulsars, this will provide a new independent method for probing the plasma in close systems and for refining the parameters of the pulsar wind.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.59, against a usual 0.64. It looks like few have drilled nearby.
The break frequency changes by a factor of four over the orbit: from 0.8 to 3 GHz. It is as if a radio receiver were automatically retuning from the FM band to the Wi-Fi range, following the motion of an invisible star.