Spin frequency evolution and pulse profile variations of the recently re-activated radio magnetar XTE J1810-197 (1903.02660v2)

Abstract: After spending almost a decade in a radio-quiet state, the Anomalous X-ray Pulsar XTE J1810-197 turned back on in early December 2018. We have observed this radio magnetar at 1.5 GHz with ~daily cadence since the first detection of radio re-activation on 8 December 2018. In this paper, we report on the current timing properties of XTE J1810-197 and find that the magnitude of the spin frequency derivative has increased by a factor of 2.6 over our 48-day data set. We compare our results with the spin-down evolution reported during its previous active phase in the radio band. We also present total intensity pulse profiles at five different observing frequencies between 1.5 and 8.4 GHz, collected with the Lovell and the Effelsberg telescopes. The profile evolution in our data set is less erratic than what was reported during the previous active phase, and can be seen varying smoothly between observations. Profiles observed immediately after the outburst show the presence of at least five cycles of a very stable ~50-ms periodicity in the main pulse component that lasts for at least tens of days. This remarkable structure is seen across the full range of observing frequencies.

• The paper analyzes the spin frequency and pulse profile variations of radio magnetar XTE J1810-197 following its re-activation in December 2018.
• Observations reveal a significant increase in the spin frequency derivative ($|"{\dot{\nu}}"|$) by a factor of 2.6 and dynamic changes in the spin-down mechanism shortly after re-activation.
• The study highlights the necessity of sustained monitoring of magnetars during quiescent phases to capture transient behaviors and advance theoretical emission models.

Analysis of Spin Frequency Evolution and Pulse Profile Variations of the Magnetar XTE J1810–197

The research presented in the paper by Levin et al. explores the intriguing behaviors of the radio magnetar XTE J1810–197 following its re-activation in December 2018, after nearly a decade of radio silence. Upon its resurgence, the magnetar exhibited notable variations in both spin frequency and pulse profiles, adding valuable insights to our understanding of these enigmatic astrophysical objects.

Key Findings and Observations

XTE J1810–197 is distinguished as one of the rare magnetars known for emitting radio waves, alongside its X-ray emissions. The research underscores several pivotal observations:

1. Spin Frequency Evolution: The re-activation phase was marked by a significant increase in the spin frequency derivative, $|\dot{\nu}|$, by a factor of 2.6 over the first 48 days of observation. Notably, the detailed timing solution reveals variations in the spin-down mechanism, indicating dynamic changes shortly after re-activation. The researchers reported $|\dot{\nu}|$ reaching magnitudes similar to those observed during previous emission periods.
2. Pulse Profile Variations: The paper documents variations in total intensity pulse profiles across multiple frequencies, ranging from 1.5 to 8.4 GHz. Unlike the erratic behavior reported in earlier active phases, the current profile evolution appears smoother, with a stable 50-ms periodicity observed immediately after the outburst. This stable periodic structure persisted across all observed frequency bands for several days, a phenomenon unreported in other known radio pulsars.
3. Comparison to Historical Data: By comparing the current behavior of XTE J1810–197 with its past active phases, the paper provides context regarding the magnetar's stability and variability. It further highlights the necessity of sustained monitoring to capture such transient phenomena.

Implications and Future Research Directions

The re-activation of XTE J1810–197 and its subsequent behavior have significant implications both for theoretical models of magnetar emission and for practical observational strategies:

• Theoretical Models: The observed step-change in $\dot{\nu}$ shortly after the outburst, coupled with the persistent periodicity in the pulse profile, suggests complex dynamics in the magnetosphere. Models must account for how such structural changes in the emission zones can occur and stabilize post-outburst.
• Long-Term Monitoring: The findings emphasize the requirement for persistent, high-cadence observations of magnetars, especially during quiescent phases, to better time the transitional behaviors like the ones observed. This could improve the predictive models of magnetar activity.
• Comparison with Quasi-periodic Oscillations: The stable 50-ms profile periodicity presents a new frontier in studying emission geometry and is particularly compelling when considered alongside the quasi-periodic oscillations observed in X-ray bursts of magnetars. This cross-wavelength phenomenon could provide clues into the underlying physical processes at play.

Conclusion

This paper of XTE J1810–197 post-re-activation enriches our understanding of magnetar behavior. The significant changes in spin frequency evolution and profile characteristics not only advance theoretical modeling efforts but also guide future observational practices in capturing dynamic astrophysical phenomena. Continued exploration of such magnetars is likely to yield further insights into the complexities of neutron star magnetospheres and their emissions.