Russell et al. monitored the accreting neutron star 4U 1728-34 with a simultaneous radio and X-ray observing campaign. The radio observations, which probe the jet emission, were taken over a 3-day period with the Australia Telescope Compact Array (ATCA). Radio data were recorded simultaneously at two frequency bands, 5.5 and 9 GHz. X-ray monitoring, tracing the accretion flow and detecting the thermonuclear explosions, consisted of a single long observation with the International Gamma-Ray Astrophysics Laboratory (INTEGRAL). Fourteen X-ray bursts occurred during the INTEGRAL monitoring, ten of which occurred when the source was visible to ATCA. The X-ray bursts all had a similar duration of roughly 10 secondss, but differed in peak brightness by a factor of approximately two. Following every X-ray burst, a clear radio flare was detected minutes later. The timing of the radio flare peak was frequency dependent; on average, the 5.5 GHz radio emission peaked ~3.5 min after the onset of the X-ray flare, before fading back to preburst levels 20–25 min after the X-ray burst. By contrast, at 9 GHz, the emission peaked ~2.5 mins after the start of the burst, fading back to preburst levels within ~12 min. The combination of the flare arriving first at higher frequencies and strong variations between individual flares points to a jet origin of the radio enhancement.

The figures above show the 3–25 keV count rate in two-second bins for (a) 2021 April 3, (b) April 4 and (c) April 5. The corresponding radio plots show the flux densities of the target during each epoch, measured at 5.5 GHz (red circles) and 9 GHz (blue squares) for 10 min time bins. The timing of the X-ray bursts in the X-ray light curves are shown by the grey vertical lines in the lower panels. For all X-ray bursts clearly defined radio counterparts are found, although for the final bursts of epoch 1 and 2 the data are not as clear owing to the low source elevation and the radio observation ending close to the burst.