The light of a distant lighthouse does not go out all at once: first it flares brightly, then slowly fades into darkness. That is how the X-ray pulsar 4U 0115+63 behaves — a neutron star that periodically brightens when gas from a companion ordinary star falls onto it. A neutron star is the remnant of a massive star that exploded, compressed to the size of a city, with a magnetic field billions of times stronger than Earth's. Such a pulsar's outbursts last weeks or months, and then it quiets down.
Using the NICER space telescope, a team of astronomers has for the first time traced the entire fading of 4U 0115+63—from the bright phase to complete quiescence. Previously, it was thought that the decay was explained by the "propeller effect": the star's magnetic field spins up the gas and flings it away, like fan blades. But the new data show something different. The gas disk around the star simply cools and stops flowing inward, like cooled tea stops emitting steam. The star, meanwhile, continues to shine for a long time, slowly releasing accumulated heat.
This picture is well described by the thermal instability model of the disk — the same one that explains outbursts in ordinary, non-magnetic binary systems. The scientists applied this model to a whole set of X-ray pulsars with different parameters and found agreement with observations. This means that the propeller effect is not needed as the main mechanism for the fading. Instead, the very fact of fading allows estimating the strength of the star's magnetic field — by how quickly it fades.
Thus, a distant lighthouse, extinguished after an outburst, revealed its hidden properties. Its light, fading into darkness, left behind not only an image but also a key to the magnetic force that otherwise cannot be measured.
as written by the authors arXiv:2608.10733v1
The final stages of outbursts in transient X-ray pulsars (XRPs), which are characterised by a significant decline in the mass accretion rate, provide valuable insight into the physics of the accretion disc and its interaction with the strong magnetic field of the neutron star (NS). In particular, the `propeller effect', or centrifugal inhibition of accretion, has been proposed as a key mechanism governing both the onset luminosity and the timescale of the rapid transition to the quiescent state. In addition, it offers an independent method for estimating the magnetic field strength of the NS. On the other hand, the decrease in the mass accretion rate itself is driven by processes occurring in the accretion flow at larger distances from the NS. Recovering the information encoded in the light curve therefore requires sensitive high-cadence X-ray monitoring capable of capturing the rapid and often unpredictable transition from the accreting regime to the quiescent regime. In this study, we present the results of the first comprehensive monitoring campaign that tracks the entire transition to quiescence in the transient XRP 4U 0115+63 utilising observations by the NICER X-ray telescope. We show that the observed behaviour can be explained by the thermal-viscous disc instability model (DIM), with the emission observed immediately after an outburst possibly arising from the ongoing accretion from the recombined (`cold') disc and the subsequent quiescent emission being produced by the cooling NS. We further applied this model to a larger sample of XRPs encompassing a broad range of physical parameters. Ultimately, our findings indicate that the temporal behaviour of XRPs, including the quiescent state, can be consistently explained within the DIM framework without requiring the propeller effect as the primary mechanism governing the observed transition.
The neutron star 4U 0115+63 rotates on its axis in 3.6 seconds — making it one of the slowest known X-ray pulsars.
In the series "Star Trek," neutron stars often serve as navigational beacons for starships, just as in reality, their periodic signals help determine position in space.