High-cadence observations and variable spin behaviour of magnetar Swift J1818.0-1607 after its outburst (2009.03568v1)

Abstract: We report on multi-frequency radio observations of the new magnetar Swift J1818.0-1607, following it for more than one month with high cadence. The observations commenced less than 35 hours after its registered first outburst. We obtained timing, polarisation and spectral information. Swift J1818.0-1607 has an unusually steep spectrum for a radio emitting magnetar and also has a relatively narrow and simple pulse profile. The position angle swing of the polarisation is flat over the pulse profile, possibly suggesting that our line-of-sight grazes the edge of the emission beam. This may also explain the steep spectrum. The spin evolution shows large variation in the spin-down rate, associated with four distinct timing events over the course of our observations. Those events may be related to the appearance and disappearance of a second pulse component. The first timing event coincides with our actual observations, while we did not detect significant changes in the emission properties which could reveal further magnetospheric changes. Characteristic ages inferred from the timing measurements over the course of months vary by nearly an order of magnitude. A longer-term spin-down measurement over approximately 100 days suggests an characteristic age of about 500 years, larger than previously reported. Though Swift J1818.0-1607 could still be one of the youngest neutron stars (and magnetars) detected so far, we caution using the characteristic age as a true-age indicator given the caveats behind its calculation.

• The paper presents detailed timing analysis on Swift J1818.0−1607’s spin behavior, identifying four distinct events with significant variability.
• It employs high-cadence radio observations to quantify the steep spectral index and polarisation properties linked to the magnetospheric emission geometry.
• The findings challenge the magnetar's age estimation, underscoring the need for continuous monitoring to accurately characterize neutron star evolution.

Observations and Spin Behavior of Magnetar Swift J1818.0−1607

This paper presents an in-depth analysis of the radio observations of Swift J1818.0−1607, a recently-activated magnetar, captured over a period exceeding one month following its outburst. Magnetars, a distinct class of neutron stars, exhibit high energic phenomena due to their intense magnetic fields. Swift J1818.0−1607, the subject of this paper, was observed with distinctly high-frequency and high-cadence radio telescopes to determine its timing and polarization properties post-outburst. The findings underscore the variability in its spin-down rate and emphasize the challenges in assessing its characteristic age.

Summary of Key Findings

Swift J1818.0−1607 is characterized by an unusually steep radio spectrum and a relatively narrow emission profile, likely attributable to our line-of-sight grazing the edge of its emission beam. This phenomenon may also rationalize the steep spectral characteristics observed. Crucially, the paper reveals significant variability in the magnetar's spin-down rate over four distinct timing events within the observation period.

The variability observed in Swift J1818.0−1607’s profile points to complex underlying magnetospheric processes. The timing analysis indicates an average spin frequency derivative that suggests a characteristic age notably larger than initially reported figures. Notably, the calculated characteristic age, derived from long-term spin-down rate measurements (~100 days) is approximately 500 years, which is significantly higher than earlier determinations. However, the authors caution against using the characteristic age as a definitive marker of the magnetar's true age, given potential fluctuations in the braking index and the birth period.

Radio Emission and Profile Modulation

Magnetars like Swift J1818.0−1607 often display intense radio emission that is polarised and changes rapidly in profile shape, which was evident from the data collected. The observations established that magnetar emissions are not consistently flat in spectrum and showed a range of polarisation properties, highlighting unique magnetospheric activities. This variability in emission, both in single pulses and averaged profiles, resembles characteristics observed in Fast Radio Bursts (FRBs).

Timing Variability and Spin-down Measurements

Time-resolved measurements underscored substantial variability in spin parameters, with periods of increased spin-down rates and discrete timing anomalies. This sporadic behavior complicates the direct inference of age and necessitates a cautious approach to extrapolating long-term evolution from short-term data. The observed timing solutions advocate for a comprehensive methodology when analyzing magnetar spin-down behavior, to delineate between deterministic and stochastic timing noise.

Implications and Future Research

The data and conclusions drawn emphasize the need to further explore the interplay between magnetars' spin characteristics and magnetospheric dynamics. Observations of such rapid timing events furnish insights into neutron star crustal and magnetospheric properties, posing implications for studies investigating the correlation between magnetars and FRBs. With Swift J1818.0−1607 being one of the fastest-spinning known magnetars, its characteristics prompt further comparisons within the known population to refine models of magnetar formation and evolution.

Future studies should consider continuous monitoring of newly activated magnetars, using both high-cadence radio and X-ray observations to achieve a detailed picture of their spin and emission evolutions. These efforts could pave the way to elucidating nuanced magnetospheric changes and constrain theoretical models related to the emission mechanisms and the underlying physical processes driving such energetic neutron star phenomena.