Simultaneous 2.25/8.60 GHz observations of the newly discovered magnetar -- Swift J1818.0-1607

TL;DR

This study presents simultaneous radio observations of the fast-rotating magnetar Swift J1818.0-1607, revealing its timing behavior, flux variability, spectral changes, and mode switching over a span of 156 days.

Contribution

It provides detailed timing analysis, flux evolution, and mode switching observations of Swift J1818.0-1607, enhancing understanding of its emission properties and age.

Findings

Long-term spin-down trend confirmed with $ u$ decreasing.

Flux density increased and spectrum became flatter over time.

Detected mode switching between bright and quiet emission states.

Abstract

Swift J1818.0-1607 discovered in early 2020 is not only the fifth magnetar known with periodic radio pulsations but also the fastest rotating one. Simultaneous 2.25 and 8.60 GHz observations of Swift J1818.0-1607 were carried out with Shanghai Tian Ma Radio Telescope (TMRT) from MJD 58936 to 59092. The spin-frequency $\nu$ and first-order derivative $\dot\nu$ of this magnetar were obtained with piecewise fitting method because of its instable timing properties. We found that the amplitude of short-term $\dot\nu$ fluctuations decreased with time, and the long-term declining trend of $\nu$ discovered previously continued in our observations. The best fitted long-term $\dot\nu$ were about $-2.25 \times 10^{-11}~s^{-2}$ using our observation data spanning 156 days. The derived characteristic age was about 522 yr, supporting the recent viewpoint that this magnetar may be older than initially thought shortly after its discovery. The flux density of this magnetar was increased at both 2.25 and 8.60 GHz during our observations, and its radio spectrum became flatter at the same time. We also detected bright-quiet type emission mode switching in Swift J1818.0-1607.