Spin frequency evolution and pulse profile variations of the recently re-activated radio magnetar XTE J1810–197

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.

High-cadence observations and variable spin behaviour of magnetar Swift J1818.0−1607 after its outburst

“…The position angle is typically difficult to interpret, especially since it is shown to change with time. This variability of polarised emission was confirmed when XTE J1810−197 recently switched on as a radio source again ten years after the cease of its radio emission in 2008 (Levin et al 2019;Dai et al 2019).…”

“…Radio-loud magnetars are known to show drastic change in their radio pulse profile within a few weeks from the onset of the X-ray outburst (e.g. Camilo et al 2007a;Levin et al 2019), and this is no different for Swift J1818.0−1607. The initial observations with the Effelsberg and Lovell radio tele- Table 3.…”

“…Firstly, magnetar radio emission is transient. It sometimes appears to be connected to high-energy outbursts (Halpern et al 2005), though the behavior is not consistent (Levin et al 2019). In the unusual case of PSR J1622−4950, the magnetar was first discovered via its radio emission while its X-ray emission was in quiescence (Levin et al 2010).…”

“…The individual pulses of magnetars are also commonly very narrow ('spiky'), spreading over an often wide pulse window (Camilo et al 2007b;Kramer et al 2007;Torne et al 2015), and longitude-resolved modulation indices reveal a high degree of intensity fluctuations on day-to-day timescales and dramatic changes across pulse phase (Serylak et al 2009). This variability in the individual pulses is also reflected in the varying shapes of average pulse profiles (Camilo et al 2007b;Levin et al 2019). The 'spikiness' combined with the large variability, and the high degree of polarisation, resembles some of the properties of repeating Fast Radio Bursts (FRBs) (Spitler et al 2016), and so it is not surprising that magnetars are among the models to explain FRBs (e.g.…”

The Thousand-Pulsar-Array programme on MeerKAT – I. Science objectives and first results

“…This observation highlights the importance of obtaining a TPA legacy dataset with long enough sequences of single pulses for the brightest pulsars to quantify the pulse-to-pulse variability. XTE J1810−197: Following an X-ray outburst (Gotthelf et al 2019), this magnetar recently switched back on in the radio and was observed with the Parkes telescope by Dai et al (2019) and with the Lovell and Effelsberg telescopes by Levin et al (2019). Figure 4 shows a single pulse from the magnetar in data taken on 2019 March 15 showing the spiky nature of its emission.…”