A radio receiver tuned to one station suddenly starts picking up another because someone invisible is turning the tuning knob. That is roughly how the radio spectrum of the millisecond pulsar PSR J2144−5237 behaves: the frequency at which its emission sharply weakens smoothly shifts from 0.8 to 3 gigahertz as the star completes an orbit around its companion. This spectral "break," resembling a bend in a graph, has now been traced by astronomers at the Indian center NCRA-TIFR not sporadically but continuously over nearly the entire ten-day orbit.

About fifty hours of observations with the 64-meter Parkes radio telescope in Australia, equipped with a wideband receiver that simultaneously captures waves from 704 to 4032 megahertz, allowed them to assemble a mosaic of ten epochs. Each epoch captured a narrow slice of the orbit, and together they formed a complete picture. The pulsar's pulse consists of two components corresponding to different magnetic poles of the neutron star. Remarkably, both components "breathe" in unison. The break frequency of the first and second components changes synchronously, with a correlation coefficient of 0.98. Such agreement rules out internal causes related to the star itself and points to a common external screen.

This screen is most likely plasma: a cloud of charged particles enveloping the companion star. As the pulsar moves along its orbit, the line of sight passes through different parts of this cloud. In dense and magnetized regions, radio waves are absorbed more strongly, and the spectral break shifts to higher frequencies. Modeling showed that the main absorption mechanism is cyclotron resonance: plasma particles spiraling around magnetic field lines extract energy from waves at a specific frequency. Simple absorption in ionized gas, often considered the primary cause, does not work here.

Previously, astronomers noticed that the spectra of some binary pulsars change with orbital phase, but tracing the break itself continuously was not possible: the sources are too faint, and wideband observations were lacking. Now the break frequency transforms from a static characteristic into a sensitive diagnostic tool. Its variations reveal the electron density and magnetic field strength in the plasma surrounding the companion.

This method applies to any binary pulsar with a spectral break. It offers a new way to estimate the companion's magnetic field and the electron density in the pulsar wind: the stream of charged particles the star ejects into space. And then the invisible hand turning the tuning knob becomes visible: we begin to discern the shape and motion of the plasma cloud through which the pulsar looks at us.

In the future, astronomers plan to move from simple descriptions to physical models of radiative transfer that account for specific absorption mechanisms. Combined analysis of the break frequency evolution with polarization and other characteristics will allow reconstructing the magnetic field geometry in the system. Expanding the sample to other binary pulsars, especially with the future Square Kilometre Array radio telescope, will show how universal this effect is. And then pulsar radio spectra will become not just graphs but true maps of invisible plasma clouds.