Abstract
Relativistic jets are observed from accreting and cataclysmic transients throughout the Universe, and have a profound impact on their surroundings1,2. Despite their importance, their launch mechanism is not known. For accreting neutron stars, the speed of their compact jets can reveal whether the jets are powered by magnetic fields anchored in the accretion flow3 or in the star itself4,5, but so far no such measurements exist. These objects can show bright explosions on their surface due to unstable thermonuclear burning of recently accreted material, called type-I X-ray bursts6, during which the mass-accretion rate increases7,8,9. Here, we report on bright flares in the jet emission for a few minutes after each X-ray burst, attributed to the increased accretion rate. With these flares, we measure the speed of a neutron star compact jet to be \(v={0.38}_{-0.08}^{+0.11}c\), much slower than those from black holes at similar luminosities. This discovery provides a powerful new tool in which we can determine the role that individual system properties have on the jet speed, revealing the dominant jet launching mechanism.
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Data availability
Data from INTEGRAL are publicly available online (https://www.isdc.unige.ch/integral/archive). Raw ATCA data are provided at the Australia Telescope Online Archive (https://atoa.atnf.csiro.au/query.jsp), under project code C3433. Calibrated radio and X-ray light curve data (as shown in Figs. 1 and 2 and Extended Data Figs. 1 and 2) are available online at https://github.com/russell1/jet-burst-data.git. Radio timing scripts are provided online at https://github.com/tetarenk/AstroCompute_Scripts. Calibrated measurement sets as well as time-lag analysis scripts (Extended Data Figs. 3–5) can be requested from the corresponding author.
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Acknowledgements
We thank participants of the International Space Science Institute (ISSI) Beijing meeting, Charleston and Vasto meetings for very useful discussions. The research was partly based on observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain) and with the participation of Russia and the United States. We thank the INTEGRAL staff for coordinating the X-ray observations possible. We also thank J. Stevens and ATCA staff for scheduling the ATCA observations. ATCA is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) that is funded by the Australian Government for operation as a National Facility managed by the Commonwealth Scientific and Industrial Research Organisation. We acknowledge the Gomeroi people as the Traditional Owners of the ATCA observatory site. T.D.R. acknowledges support as an INAF IAF research fellow. A.J.T. is a former NASA Einstein Fellow and acknowledges partial support for this work from NASA through the NASA Hubble Fellowship grant no. HST–HF2–51494.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract no. NAS5–26555. This work was supported with an XS grant no. OCENW.XS3.096 from the Netherlands Organisation for Scientific Research (NWO), awarded to N.D.
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T.D.R., N.D., J.v.d.E., J.C.A.M.-J. and T.M. coordinated the radio and X-ray observations. C.S.-F. analysed the X-ray data, with help from N.D. T.D.R. carried out and analysed the radio observations, with help from J.C.A.M.-J. A.J.T. performed the time lag analysis. T.D.R., N.D., J.v.d.E., T.M., A.J.T. and C.S.-F. interpreted the results. All authors made significant contributions to the writing of the manuscript. N.D. and J.v.d.E. wrote the NWO XS grant to secure ATCA observing time. T.M. and N.D. worked on the original idea of looking for a jet response to thermonuclear bursts.
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Extended data figures and tables
Extended Data Fig. 1 Light curves.
X-ray and multi-band radio light curves of 4U1728 on 10-minute timescales for our three radio epochs. For the X-rays we show the 1-second 3–25 keV light curves (panels a, b, s). For the radio, we show the flux densities of the target at 5.5 GHz (panels d, e, f; red circles) and 9 GHz (panels g, h, i; blue circles), as well as a nearby check source (grey squares), where error bars are 1-sigma. We detected 10 X-ray bursts coincident with our radio monitoring. For all X-ray bursts, we find clearly defined radio counterparts, with the only exceptions being two that occurred right at the end of our observation when the source elevation was low and the observation ended during the 10-minute time bin, however, radio brightening is detected on shorter timescales for these two bursts (Fig. 2). No radio brightening was detected in the nearby check source. Burst times are shown by the vertical black lines in the lower two rows, while the vertical shaded purple regions mark the time chunks used for our cross-correlation analysis (see Section 1.2.4) of each burst, where the bursts are labelled as in Extended Data Table 3 (we exclude the two bursts close to the end of epoch 1 and 2, labelled as B3 and B11, from our cross-correlation analysis of single bursts (but not for the stacked analysis), as such there is no vertical shaded region for these two bursts).
Extended Data Fig. 2 Combined radio response.
Stacked radio light curves and time-resolved radio spectral indices of 4U1728. To create these light curves (panel a), we stacked all of the radio bursts from 4U1728 separately in the 5.5 GHz and 9.0 GHz bands, with respect to the time of the partner X-ray burst (indicated by the vertical black dotted line). Panel b displays the stacked spectral index showing the flare starting as optically thick. There is a clear time-lag between the X-ray and radio burst signals, where the radio signal lags the X-ray signal by several minutes and this lag is dependent on electromagnetic frequency (smaller electromagnetic frequencies yield longer lags). All errors are 1-sigma.
Extended Data Fig. 3 Cross-correlation functions.
Created with the ZDCF algorithm from stacked versions of the X-ray and radio signals; X-ray to 9 GHz radio (panel a), X-ray to 5.5 GHz radio (panel b), and 9 GHz to 5.5 GHz radio (panel c), with 1-sigma errors. The resulting lag measurements and their uncertainties are shown with the dotted black lines and shaded grey regions, respectively. The lag measurements from these CCFs are also tabulated in Extended Data Table 4.
Extended Data Fig. 4 Time-lags.
Measured time-lags between the X-ray and radio bands in 4U1728. Here we show the lags between the X-ray and stacked signal, as well as those for individual bursts (panel a), as well as the lag between the 9 and 5.5 GHz signal (panel b). In both panels, the data points represent lags measured from cross-correlation functions comparing signals from individual bursts, while the shaded regions represent lags measured from cross-correlation functions comparing signals from the stacked bursts. We show the results of both 2 min and 10 min time-binned radio light curves for the 9 GHz to 5.5 GHz radio lags (which are consistent within errors with each other), while we show only the results from the 10 min time-binned radio light curves for the X-ray to radio lags for clarity. In both cases, the individual burst lags are consistent with the stacked measurement within (the 1-sigma) uncertainties.
Extended Data Fig. 5 MCMC modelling.
Results of modelling the stacked burst time-lags from 4U1728. Panel a: The best-fit model (solid orange line) overlaid on the time-lag measurements, with 1-sigma errors. Here the shaded orange region represents the model predictions from the final position of the MCMC walkers in our parameter space. The three lag measurements correspond to lags between the X-ray and the 9 GHz and 5 GHz radio data, as well as the lag between the 9 GHz and 5.5 GHz radio data (as shown in Extended Data Fig. 3 and Extended Data Table 4). Panel b: The residuals of the fit (data–model/uncertainties). Panel c: Two-parameter correlations in our modelling for the free parameters and those with informative (non-uniform) priors. Our model fits the measured time-lags reasonably well, suggesting the radio counterparts to the X-ray bursts in 4U1728 originate in an out-flowing compact jet.
Supplementary information
Supplementary Table 1
Time resolved spectroscopy of the 4U1636 X-ray bursts.
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Russell, T.D., Degenaar, N., van den Eijnden, J. et al. Thermonuclear explosions on neutron stars reveal the speed of their jets. Nature 627, 763–766 (2024). https://doi.org/10.1038/s41586-024-07133-5
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DOI: https://doi.org/10.1038/s41586-024-07133-5
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