Radio observations of the 2022 outburst of the transitional Z-Atoll source XTE J1701-462

TL;DR

This paper reports on extensive radio observations of the 2022 outburst of the neutron star X-ray binary XTE J1701-462, revealing multiple flare events, long-lasting radio emission, and polarization constraints, with comparisons to its 2006/7 outburst.

Contribution

It provides the first detailed radio analysis of the 2022 outburst, including flare characterization, polarization limits, and comparison with previous outbursts, enhancing understanding of jet behavior in Z-source neutron stars.

Findings

Detected radio emission for over three months during the outburst.

Identified at least three flare-like events with high minimum energy.

Placed the source in the radio--X-ray plane consistent with other Z-sources.

Abstract

XTE J1701-462 is a neutron star low mass X-ray binary (NS LMXB) discovered in 2006 as the first system to demonstrate unambiguously that the `Atoll' and `Z' classes of accreting neutron stars are separated by accretion rate. Radio observations during the 2006/7 outburst provided evidence for the formation of a relativistic jet, as now expected for all accreting neutron star and black hole X-ray binaries at high accretion rates. The source entered a new outburst in 2022, and we report 29 observations made with the MeerKAT radio telescope. The first radio detection was on the 16th September 2022, we continued detecting the source until mid-December 2022. Thereafter, establishing radio upper limits till 25 March 2023. We present the radio analysis alongside analysis of contemporaneous X-ray observations from MAXI. The radio light curve shows evidence for at least three flare-like events over the first hundred days, the most luminous of which has an associated minimum energy of $1\times10^{38}$ erg. We provide a detailed comparison with the 2006/7 outburst, and demonstrate that we detected radio emission from the source for considerably longer in the more recent outburst, although this is probably a function of sampling. We further constrain the radio emission from the source to have a polarisation of less than 9% at the time of 2022 IXPE detection of X-ray polarisation. Finally, we place the source in the radio -- X-ray plane, demonstrating that when detected in radio it sits in a comparable region of parameter space to the other Z-sources.