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4th of April 2024
Plots showing radio flares detected with the ATCA following X-ray bursts detected with the INTEGRAL satellite.
Thermonuclear explosions on neutron stars reveal the speed of their jets
by Russell et al.
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.

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