Abstract
Accretion disks around compact objects are expected to enter an unstable phase at high luminosity1. One instability may occur when the radiation pressure generated by accretion modifies the disk viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales2. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole3,4,5. Although there are hints of these cycles in a few isolated cases6,7,8,9,10, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a continuing puzzle11. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum—from radio to X-ray—of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such a new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects.
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Data availability
All raw data from the Swift J1858 August campaign and GRS 1915+105 Chandra/VLA observations are public and can be downloaded from their archives using the reported codes. All the reduced data from this campaign (including spectroscopic observations, which are not presented here) will be made object of a publication and made accessible (Castro Segura et al. in preparation). Further analysis of the WASP and CHIMERA data is in progress, thus they are available on request to the authors.
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Acknowledgements
F.M.V. thanks R. Arcodia, P. Casella, G. Marcel, G. Mastroserio, N. Scepi and L. Stella for insightful discussions. The interpretation of the results benefited from discussions held during the meeting ‘Looking at the disc-jet coupling from different angles’ at the International Space Science Institute in Bern, Switzerland. F.M.V. was supported by the NASA awards 80NSSC19K1456, 80NSSC21K0526 and from grant FJC2020-043334-I financed by MCIN/AEI/10.13039/501100011033 and NextGenerationEU/PRTR. J.N. acknowledges support by the SAO award GO1-22036X. A.J.T. is a NASA Einstein Fellow and acknowledges support for this work provided by 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 NAS5-26555. D.A. and N.C.S. acknowledge support from the Science and Technology Facilities Council (STFC) grant ST/V001000/1. F.M.V., M.A.P. and V.A.C. acknowledge support from the Spanish Ministry of Science and Innovation research project PID2020-120323GB-I00. M.A.P. acknowledges support from the Consejería de Economía, Conocimiento y Empleo del Gobierno de Canarias and the European Regional Development Fund (ERDF) under grant with reference ProID2021010132 ACCISI/FEDER, UE. T.B. acknowledges financial contribution from the agreement ASI-INAF n.2017-14-H.0 and from PRIN-INAF 2019 N.15. T.M.D. acknowledges support from the Spanish Ministry of Science and Innovation project PID2021-124879NB-I00 and the Europa Excelencia grant (EUR2021-122010). T.R. acknowledges the financial contribution from the agreement ASI-INAF n.2017-14-H.0.
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F.M.V. and J.N. drew the new physical scenario for multiwavelength instabilities. F.M.V. carried out the multi-λ timing analysis of the Swift J1858 dataset. A.J.T. modelled the radio variability with the help of S.d.P. and J.v.d.E. Y.C. modelled the phase dependence of the X-ray/IR lag with the help of N.C.S. For Swift J1858, N.C.S. provided the UV and X-ray data; F.J.-I. provided the optical data from LT; G.V., C.D.B., J.M. and M.O.A. provided optical data from Chimera and WASP; J.v.d.E. and T.R. provided the radio data. J.N. is the principal investigator of the Chandra and VLA proposal on GRS 1915; A.J.T. reduced and analysed the radio data. F.M.V., Y.C., G.V., D.A., T.B., N.D., T.M.D. and J.v.d.E. contributed substantially to the development of the luminosity–magnetic field diagram (Extended Data Fig. 1) to compare the different neutron stars. All authors contributed actively to the discussion and to the final version of the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Luminosity versus magnetic field diagram for accreting neutron stars.
The long dashed line represents the limit in the parameter space for which the inner disk is (blue) or is not (red) radiation pressure dominated, that is, where the transition between zones A and B (RAB) of a Shakura and Sunyaev73 disk is greater than the magnetospheric radius (Rm) or the star radius (R*), whichever is larger. Because the disk rotates with Keplerian velocity, for large Rm, the inner disk rotates slower than the neutron star magnetosphere and accretion can be halted owing to the propeller effect74. The dashed grey lines correspond to the propeller threshold luminosities as a function of the magnetic field for different spins74,75. For all these lines, we set ξ = 1, α = 0.1, MNS = 1.4 M⊙ and R = 10 km. We then marked the sources that showed ‘1915-like’ variability patterns (green ticks) in their light curves: the Rapid Burster6, the Bursting Pulsar61,64, the ULX NGC 3621 (ref. 31) and our target, Swift J1858. Along with these, we also showed the different phenomenological classes of accreting neutron stars X-ray binaries, depending on their magnetic field and luminosities. At low magnetic field (≤109 G), we find classical LMXBs, which, depending on their accretion rate, can manifest as atolls, bright atolls or Z sources76,77. X-ray timing studies have also shown that accreting millisecond pulsars (AMXPs), with a magnetic field in the 108 G to 109 G range, are also compatible with atolls in their hard state78. For higher magnetic fields of 109 G, the observed accreting neutron stars usually show pulsations, but—owing to the lower propeller threshold—also have slower spin periods with respect to AMXPs. Above 1011 G, the diagram is mainly populated by high-mass X-ray binaries79,80 and pulsating ULXs81. For all these classes/objects, we also marked which of them show typical phenomenology linked to accretion instabilities, that is, radio ejecta (yellow star) and outflows (cyan waves). We note that these phenomena tend to be present above the radiation pressure disk threshold (see Methods).
Extended Data Fig. 2 Infrared lag analysis.
a, Lag distribution from the five simultaneous NuSTAR–HAWK-I windows after 104 flux randomizations. An evolution of the lag centroid is visible. b, CCF computed between HST and HAWK-I. Excluding the asymmetry at longer lags, owing to the asymmetry of the flares, the CCF peaks at 0.
Extended Data Fig. 3 Averaged flare profile for events.
Owing to the presence of two nearby flares, these are also present not at the centre. However, the overall connection between long and short timescales is still clear.
Extended Data Fig. 4 X-ray versus infrared lag modelling.
a, Fit to the lags obtained including the flares in the light curves. The only parameter allowed to change is the inclination of the binary, i. Long and short dashed curves represent 68% and 99% confidence levels, respectively. b, Histograms of the posterior distributions of the inclinations. In all plots, dotted lines indicate the 0.5th and 99.5th percentiles, dashed lines indicate the 16th and 84th percentiles and solid lines indicate the median. c, Fit to the lags measured excluding the flares from the light curves. d, Histogram of the posterior distribution excluding the flares.
Extended Data Fig. 5 Unconstrained radio modelling of Swift J1858.
Same as Fig. 3a–c but modelling the Swift J1858 radio light curve using no constraints on the ejection times. We found similar results with larger errors.
Extended Data Fig. 6 Comparison of the beats at different wavelengths.
a, Mean-normalized light curves used for quantifying the association between Swift J1858 and GRS 1915: Chandra data from GRS 1915+105 (blue, +1 shift applied), HAWK-I data from Swift J1858 (red), WASP data from Swift J1858 (green, −1 shift applied) and CHIMERA data from Swift J1858 (purple, −2 shift applied). b, Cumulative distribution function (CDF) of the flux distributions of the light curves.
Extended Data Fig. 7 Flux–flux correlation diagram.
a, Flux–flux diagram with respect to the HST measurements for NuSTAR (open circles), LT (open squares) and HAWK-I (filled circles). Although the O/IR is well correlated, the X-rays show a non-linear trend. All bands have been normalized to their average. b, The plot shows the ratio of HAWK-I over LT (filled circles) and HST over LT (opened circles) as a function of the X-ray count rate normalized to 6.5 counts s−1.
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Vincentelli, F.M., Neilsen, J., Tetarenko, A.J. et al. A shared accretion instability for black holes and neutron stars. Nature 615, 45–49 (2023). https://doi.org/10.1038/s41586-022-05648-3
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DOI: https://doi.org/10.1038/s41586-022-05648-3
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