A pulsar’s gamma rays brighten when its X-rays fade

In some binary systems, gas from a companion star forms a disk around a rapidly rotating neutron star. Transitional millisecond pulsars are systems that can move between a disk state and a state in which the neutron star’s rotation powers its emission. In the disk state, PSR J1023+0038—the first confirmed transitional millisecond pulsar—switches rapidly between two X-ray modes. Maksat Satybaldiev and Manuel Linares ask whether its gamma-ray output changes with those switches, and what the pattern might reveal.

Matching X-ray modes to gamma-ray data

The researchers analyzed X-ray observations from NICER, XMM-Newton, Swift and NuSTAR. They identified intervals in the high and low X-ray modes from changes in brightness, using statistical methods to distinguish sustained modes from random fluctuations. They also separated out brighter flaring intervals.

Next, they matched those intervals to observations by NASA’s Fermi Large Area Telescope (LAT), which detects gamma rays. Fermi gathers too few photons from this source to follow each rapid switch individually. Instead, the researchers combined gamma-ray data from many intervals classified by their X-ray mode. This is like sorting a faint signal into two large piles before comparing them.

The analysis detected gamma-ray emission in both modes, but at different levels. The source was brighter in gamma rays during the X-ray low mode and fainter during the X-ray high mode. The reported difference between the gamma-ray fluxes was significant at 3.9 sigma, a statistical measure of how strongly the data distinguish the two measurements. It does not establish which physical process produces the emission.

A mismatch with a proposed picture

The finding conflicts with a model in which gamma rays are produced mainly when a wind of particles from the neutron star interacts with the nearby disk, making gamma-ray emission strongest in the X-ray high mode. The observed pattern puts the gamma-ray bright phase in the X-ray low mode instead.

What produces those gamma rays? The authors discuss possible explanations but do not settle the question. One is emission from a compact jet, a narrow outflow of material. The system is also known to brighten in radio during the X-ray low mode. Another possibility is self-synchrotron Compton emission in a “propeller” state: in this proposed state, the rotating neutron star pushes some disk material outward, and energetic particles could produce gamma rays. The study does not present a complete quantitative model confirming either explanation.

Nor does the result directly establish a link between gamma-ray and radio emission. The authors infer a possible connection from the gamma-ray/X-ray anticorrelation they report and a previously observed radio/X-ray anticorrelation. That makes a jet interpretation worth testing, but this analysis did not measure simultaneous radio and gamma-ray changes.

What remains uncertain

The gamma-ray measurements rely on stacking data from many intervals. They show an average difference between the two groups, not how gamma rays respond to each individual switch. Fermi also observed only part of the total X-ray exposure assigned to each mode, and the mode-by-mode gamma-ray spectra have limited statistics. To fit those spectra, the researchers fixed the cutoff energy—the point where the spectrum steepens—to the value measured for the average disk state. Alternative fits could not determine the mode-specific cutoff energies precisely.

The study concerns one system, so it cannot show whether other transitional pulsars behave the same way. And an observed anticorrelation does not, by itself, identify what causes either signal. More observations and detailed modeling will be needed to distinguish a jet, a propeller-related process or another explanation.

The work is available as an arXiv preprint. The source acknowledges an anonymous referee’s comments, but the supplied material does not establish the paper’s publication or review status; its findings and interpretations should therefore be assessed with that limitation in mind. For now, PSR J1023+0038 presents an intriguing puzzle: when its X-rays dim, its gamma rays seem to brighten.