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

TL;DR

The study uses NICER timing of four magnetars (2017–2024) to map long-term spin evolution and pulse-profile changes, identifying 10 timing events (5 glitches, 2 anti-glitches, 1 state transition) and highlighting source-dependent behaviors. Key results include an anti-glitch in 1E 2259+586 with exponential recovery ($\tau_d = 89(17)$ days) and two anti-glitches in 4U 0142+61, plus a state-transition episode in 1RXS J1708, all pointing to complex internal torques and crust-magnetosphere coupling. Pulse-profile evolution is observed in 1E 2259+586 and 4U 0142+61, supporting the notion that magnetars exhibit gradual profile changes in addition to abrupt timing events. The findings advance magnetar timing theory by constraining internal superfluid dynamics and crustal processes and underscore the value of sustained, high-cadence X-ray timing for understanding FRB associations and magnetar physics, with future capabilities anticipated from eXTP.

Abstract

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.

Figures (10)

• Figure 1: The long-term timing evolution of 1E 2259+586. Three vertical dashed lines represent the times of the glitches within the NICER observation range. Panel(A), Frequency as a function of time with a linear trend in frequency subtracted. The inset graph zooms in on the results of the anti-glitch, revealing the existence of an exponential term. Panel(B), Frequency derivative as a function of time. Panel(C), The timing residuals of all TOAs, black, magenta, and blue data represent from NICER, Swift and IXPE , respectively.
• Figure 2: Comparison results of normalized pulse profiles of 1E 2259+586 . The numbers 1-3 represent the average pulse profiles for the three time segments, divided based on the glitch events shown in Figure \ref{['timing:a']} (with the first glitch excluded). The residuals for each pair of normalized pulse profiles, obtained by subtracting one profile from the other, are plotted below the corresponding panels.
• Figure 3: The timing residuals of the anti-glitch for 1E 2259+586 obtained from three different models. Panel(A), excluding the exponential term, the model utilized only $\nu$, $\dot{\nu}$, $\Delta \nu$, $\Delta \dot{\nu}$, with an RMS residual of 68.988 ms. Panel(B), the second derivative of frequency $\ddot{\nu}$ was added to the analysis from Panel(A), with an RMS residual of 57.448 ms. Panel(C), the exponential term $\Delta\nu_{d}$ was added to the analysis from Panel(A), with an RMS residual of 46.865 ms.
• Figure 4: The long-term timing evolution of 4U 0142+61. Two black vertical dashed lines represent the times of the glitches within the NICER observation range. Panel(A), Frequency as a function of time with a linear trend in frequency subtracted. Panel(B), Frequency derivative as a function of time. Panel(C), The timing residuals of all TOAs.
• Figure 5: Comparison results of normalized pulse profiles of 4U 0142+61 . The numbers 1-3 represent the average pulse profiles for the three time segments, divided based on the glitch events shown in Figure \ref{['timing:b']}. The residuals for each pair of normalized pulse profiles, obtained by subtracting one profile from the other, are plotted below the corresponding panels.