In the binary system 4U 1700-37, the massive supergiant star HD 153919 sheds a powerful stellar wind. This wind, like an invisible stream, rushes toward its compact companion — likely a neutron star. As it falls, the matter heats to millions of degrees and flares in X-rays. Astronomers pointed the orbiting NuSTAR telescope at this pair to examine the details of the glow.

No pulsations, regular bursts that would reveal rapid rotation, were detected. However, the spectrum showed excess emission around 20 and 40–50 kiloelectronvolts. These could be traces of cyclotron lines, peculiar "fingerprints" of the magnetic field that forces electrons to spiral. But confirming these lines proved elusive: the signal is too weak.

Nevertheless, modeling allowed an estimate of the magnetic field: from 1.7 to 4.4 trillion gauss. For comparison, a refrigerator magnet has only about 100 gauss. With such a field, infalling matter is not stopped by a shock wave but gradually, forming a hot "cap" above the poles. The equilibrium spin period of the star would then be about 32 minutes. This is consistent with the absence of pulsations: too slow to be noticeable.

Such measurements help tie together different observations of the same system. Earlier, different telescopes gave conflicting estimates of the cyclotron line energies. The new data suggest that the lines may shift slightly depending on the source brightness. This hints at a complex geometry of the infalling matter, as if the wind flow waxes and wanes, and the magnetic field responds to these gusts.