VLA monitoring of LS V +44 17 reveals scatter in the X-ray--radio correlation of Be/X-ray binaries

TL;DR

This study reports the first radio detection of LS V +44 17 during an X-ray outburst, revealing a correlation between X-ray and radio luminosity and evidence of scatter in this relationship among Be/X-ray binaries.

Contribution

It provides the first radio counterpart detection of LS V +44 17 and analyzes its X-ray--radio correlation, highlighting scatter similar to low-mass X-ray binaries.

Findings

Radio emission detected during the X-ray brightest peak

X-ray and radio luminosity are correlated with a power law slope of ~1.25

Evidence of scatter in the X-ray--radio correlation among Be/X-ray binaries

Abstract

LS V +44 17 is a persistent Be/X-ray binary (BeXRB) that displayed a bright, double-peaked period of X-ray activity in late 2022/early 2023. We present a radio monitoring campaign of this outburst using the Very Large Array. Radio emission was detected, but only during the second, X-ray brightest, peak, where the radio emission followed the rise and decay of the X-ray outburst. LS V +44 17 is therefore the third neutron star BeXRB with a radio counterpart. Similar to the other two systems (Swift J0243.6+6124 and 1A 0535+262), its X-ray and radio luminosity are correlated: we measure a power law slope $\beta = 1.25^{+0.64}_{-0.30}$ and a radio luminosity of $L_R = (1.6\pm0.2)\times10^{26}$ erg/s at a $0.5-10$ keV X-ray luminosity of $2\times10^{36}$ erg/s (i.e. $\sim 1\%$ $L_{\rm Edd}$). This correlation index is slightly steeper than measured for the other two sources, while its radio luminosity is higher. We discuss the origin of the radio emission, specifically in the context of jet launching. The enhanced radio brightness compared to the other two BeXRBs is the first evidence of scatter in the giant BeXRB outburst X-ray--radio correlation, similar to the scatter observed in sub-classes of low-mass X-ray binaries. While a universal explanation for such scatter is not known, we explore several options: we conclude that the three sources do not follow proposed scalings between jet power and neutron star spin or magnetic field, and instead briefly explore the effects that ambient stellar wind density may have on BeXRB jet luminosity.