Magnetar XTE J1810-197: Spectro-temporal evolution of average radio emission

TL;DR

This study tracks the long-term changes in radio emission from magnetar XTE J1810-197, revealing spectral steepening, profile narrowing, and spectral turnover shifts, providing insights into magnetar radio emission mechanisms.

Contribution

First systematic long-term spectro-temporal monitoring of XTE J1810-197 across 300-1450 MHz, showing spectral evolution and turnover shifts over time.

Findings

Radio spectrum steepened over time.

Spectral turnover shifted to lower frequencies.

Profile width decreased gradually.

Abstract

We present the long-term spectro-temporal evolution of the average radio emission properties of the magnetar XTE J1810-197 (PSR J1809-1943) following its most recent outburst in late 2018. We report the results from two and a half years of monitoring campaign with the upgraded Giant Metrewave Radio Telescope carried out over the frequency range of 300 - 1450 MHz. Our observations show intriguing time variability in the average profile width, flux density, spectral index and the broadband spectral shape. While the average profile width appears to gradually decrease at later epochs, the flux density shows multiple episodes of radio re-brightening over the course of our monitoring. Our systematic monitoring observations reveal that the radio spectrum has steepened over time, resulting in evolution from a magnetar-like to a more pulsar-like spectrum. A more detailed analysis reveals that the radio spectrum has a turnover, and this turnover shifts towards lower frequencies with time. We present the details of our analysis leading to these results, and discuss our findings in the context of magnetar radio emission mechanisms as well as potential manifestations of the intervening medium. We also briefly discuss whether an evolving spectral turnover could be an ubiquitous property of radio magnetars.