Multi-frequency radio observations of the radio-loud magnetar XTE J1810-197

We report on the multi-frequency multi-epoch radio observations of the magnetar XTE J1810-197, which exhibited a radio outburst from 2018 December after its 10-year quiescent period. We performed quasi-simultaneous observations with VERA (22 GHz), Hitachi (6.9 GHz and 8.4 GHz), Kashima (2.3 GHz), and litate (0.3 GHz) radio telescopes located in Japan to trace the variability of the magnetar radio pulsations during the observing period from 2018 December 13 to 2019 June 12. The pulse width becomes narrower as the observing frequency goes higher, analogous to the general profile narrowing behavior of ordinary pulsars. When assuming a simple power law in the range of 2.3 GHz and 8.7 GHz, the radio spectrum of the magnetar goes steeper with the average spectral index approximate to -0.85 for the first four months. The wide-band radio spectra gathered from our observations and the literature suggest that XTE J1810-197 would have a double-peaked spectrum with a valley point in 22-150 GHz, where the first spectral peak implies a gigahertz-peaked spectrum (GPS) feature with a peak at a few GHz. The GPS and the high-frequency peak have been identified in the spectra of other radio-loud magnetars, thus they may be intrinsic features that can give a new insight to the understanding of various emission mechanisms and the surrounding environments of radio magnetars. Our study emphasizes the importance of simultaneous long-term broad-band observations toward radio-loud magnetars to capture the puzzling spectral features and establish a link to other types of neutron stars.