Long-term timing evolution of four Anomalous X-Ray Pulsars

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are believed to be manifestations of magnetars. Typically, AXPs exhibit higher X-ray luminosities, whereas SGRs are generally fainter and display significantly high signal-to-noise ratios only during their outburst phases. In this work, we report the long-term timing evolution of four AXPs: 1E 2259+586, 4U 0142+61, 1RXS J170849.0-400910 and 1E 1841-045, which were regularly monitored with NICER from 2017 to 2024. Over this period, we identify a total of 10 timing events. In addition to one glitch and one anti-glitch in 1E 2259+586 reported in literature, we detect another 8 new timing events: 5 glitches, 2 anti-glitches, and 1 unusual state transition event. Notably, both anti-glitches were observed in 4U 0142+61, making it the most frequent source of such events, and there is a hint of regular evolution in its pulse profile. In the case of 1RXS J170849.0-400910, it continues to exhibit pronounced high-frequency timing anomalies and undergoes a state transition event. Finally, we study the evolution of the pulse profiles and find that the profiles of 1E 2259+586 and 4U 0142+61 both evolve. This is consistent with the earlier finding that pulse profile evolution is a generic feature of magnetars.

💡 Research Summary

This paper presents a comprehensive long‑term timing study of four anomalous X‑ray pulsars (AXPs) – 1E 2259+586, 4U 0142+61, 1RXS J170849.0‑400910, and 1E 1841‑045 – using data from the Neutron Star Interior Composition Explorer (NICER) collected between 2017 and 2024. The authors processed all NICER observations with the latest NICERDAS tools, applying standard filtering, background subtraction, and barycentric corrections, and retained only exposures longer than 50 s for timing analysis. They complemented the NICER data with archival Swift observations for 1E 2259+586 and two IXPE pointings for 1E 2259+586 and 1RXS J170849.0‑400910 to improve coverage of key events.

Timing analysis began with a χ² search to obtain an initial spin frequency for each segment of data, followed by the construction of time‑of‑arrival (TOA) measurements. These TOAs were fitted with polynomial spin‑down models (including ν, ν̇, and ν̈ terms) to produce phase‑connected ephemerides where possible. Deviations from the smooth model – sudden jumps in frequency (Δν) or its derivative – were identified as timing events. The authors modeled each event with a step in frequency and, when appropriate, an exponential recovery term to quantify the glitch amplitude and recovery timescale.

During the eight‑year span, ten distinct timing events were detected. Two of these (a glitch and an anti‑glitch) had been previously reported for 1E 2259+586; the remaining eight are newly identified: five spin‑up glitches, two spin‑down anti‑glitches, and one unusual state‑transition event characterized by a rapid spin‑down accompanied by a change in pulse morphology. Notably, both anti‑glitches occurred in 4U 0142+61, making it the most prolific source of such events among known magnetars. The anti‑glitches were observed during radiatively quiet intervals, yet subtle, systematic changes in the pulse profile were detected, suggesting a link between the internal torque change and magnetospheric configuration.

Pulse‑profile evolution was examined by constructing energy‑resolved profiles for each source and tracking the relative amplitudes of the fundamental and higher harmonics over time. Both 1E 2259+586 and 4U 0142+61 show gradual, monotonic variations in their harmonic content, indicative of slow re‑configuration of the magnetic field geometry. 1RXS J170849.0‑400910 exhibits frequent high‑frequency timing noise and undergoes a state‑transition event, reinforcing its reputation as a highly active timing source. 1E 1841‑045 remained relatively quiet for most of the monitoring period, but entered an active phase in 2024, displaying a new glitch concurrent with enhanced X‑ray activity.

The discussion evaluates two primary mechanisms for anti‑glitches: (1) inward motion of superfluid vortex lines driven by crustal fractures induced by magnetic‑field evolution, and (2) time‑variable external torques arising from magnetospheric re‑configurations (e.g., twist decay or particle wind changes). The authors note that while many glitches are accompanied by radiative outbursts, a substantial fraction (~70 %) occur without detectable flux or spectral changes, implying that internal superfluid dynamics can operate independently of magnetospheric activity. Conversely, the anti‑glitches in 4U 0142+61 are tightly coupled to subtle pulse‑profile changes, supporting an external‑torque contribution.

Overall, the study demonstrates the power of high‑cadence, high‑resolution X‑ray timing to uncover the diverse phenomenology of magnetar spin behavior. The detection of multiple anti‑glitches, state‑transition events, and systematic pulse‑profile evolution across four bright AXPs strengthens the view that pulse‑profile changes are a generic feature of magnetars and that both internal and external processes shape their timing irregularities. The authors advocate for continued NICER monitoring and coordinated multi‑wavelength campaigns with upcoming missions such as eXTP and Athena to further disentangle the interplay between magnetic field evolution, crustal dynamics, and magnetospheric torques in these extreme neutron stars.