A Pulsar's Gamma Rays Flicker in Step With a Puzzle

Some objects in the sky can't decide what they are. PSR J1023+0038 is one of them: a rapidly spinning neutron star that keeps flipping between two lives. In one, it siphons gas from a close companion, glowing as material spirals inward. In the other, the inflow shuts off and it sweeps the space around it as a radio pulsar. That double identity is why it earned the label transitional millisecond pulsar, and why it has become the reference case for a whole class of neutron stars caught between accretion and rotation.

Even within its feeding state, the star refuses to hold still. It rapidly toggles between two X-ray brightness levels, a high mode and a low mode, on timescales of seconds. Now Maksat Satybaldiev and Manuel Linares report, in a preprint posted to arXiv, that its gamma rays keep time with those flips too, and that they do so in a way the standard picture struggles to accommodate.

By stacking years of data from the Fermi Gamma-ray Space Telescope's Large Area Telescope and matching it against simultaneous X-ray records, the pair found the source alternating between distinct gamma-ray modes. The surprise is the direction. The gamma-ray flux runs higher when the X-rays are in their low mode, and lower when the X-rays are high, an inverse relationship rather than a matched one.

That anti-correlation is the crux. The favoured explanation for the GeV-band light from these systems invokes the pulsar wind, a stream of high-energy particles blown off the neutron star, colliding with the surrounding accretion flow and radiating through synchrotron and inverse-Compton processes. On that model, more infalling material should feed more gamma-ray emission, not less. Seeing the gamma rays peak when the X-rays sink points the other way.

Satybaldiev and Linares offer a reinterpretation. They argue the pulsar wind is likely switched off during the gamma-ray-high, X-ray-low mode, a mode that also shows up as brighter in the radio band. In its place, they propose, a jet, a narrow outflow launched near the neutron star, does the work of producing the GeV emission. If that holds, the high-energy glow of these in-between pulsars would trace jet activity rather than the wind, a meaningful shift in how the class is understood.

The findings come with significant caveats. The study is a single preprint that has not yet undergone peer review, and its main result is based on accumulated observations of one source. Before the jet interpretation, which the authors propose to explain a pattern that existing models cannot account for, can be treated as more than a promising lead, it will need independent analysis and, ideally, confirmation in other transient systems. The study's value lies in the puzzle itself: a well-studied object is behaving in a way that textbooks would not predict, with its gamma-ray emission flickering in precise counterpoint to the light it is expected to follow.