Abstract
Galactic nuclei showing recurrent phases of activity and quiescence have recently been discovered. Some have recurrence times as short as a few hours to a day and are known as quasi-periodic X-ray eruption (QPE) sources. Others have recurrence times as long as hundreds to a thousand days and are called repeating nuclear transients. Here we present a multiwavelength overview of Swift J023017.0+283603 (hereafter Swift J0230+28), a source from which repeating and quasi-periodic X-ray flares are emitted from the nucleus of a previously unremarkable galaxy at ∼165 Mpc. It has a recurrence time of approximately 22 days, an intermediary timescale between known repeating nuclear transients and QPE sources. The source also shows transient radio emission, likely associated with the X-ray emission. Such recurrent soft X-ray eruptions, with no accompanying ultraviolet or optical emission, are strikingly similar to QPE sources. However, in addition to having a recurrence time that is ∼25 times longer than the longest-known QPE source, Swift J0230+28’s eruptions exhibit somewhat distinct shapes and temperature evolution compared to the known QPE sources. Scenarios involving extreme mass ratio inspirals are favoured over disk instability models. The source reveals an unexplored timescale for repeating extragalactic transients and highlights the need for a wide-field, time-domain X-ray mission to explore the parameter space of recurring X-ray transients.
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Data availability
All the NICER and Swift data presented here are public and can be found in the NASA archives at the following URL: https://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/w3browse.pl. The VLA data are available from the archives of the National Radio Astronomy Observatory at https://data.nrao.edu/portal/#/. X-shooter spectra will be available from the ESO archive after the 12 months’ proprietary period has passed. Keck/ESI data can be shared by a request to the corresponding authors. The general relativistic magnetohydrodynamic simulation data, described in Supplementary Information, ‘Accretion disk—perturber interaction’, are available by a request to the corresponding author. The data underlying the multiwavelength light curves presented in Fig. 1 are available at https://zenodo.org/records/10238766.
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Acknowledgements
During the refereeing process of this manuscript, Evans et al. 2023 (ref. 3) published a paper presenting a focused investigation of Swift J0230+28. Those authors’ dataset does not include our NICER and radio (VLA) data, but their science case and physical interpretation are like ours. M.G. and S.G. are supported in part by NASA (Grant Nos. 80NSS23K0621 and 80NSSC22K0571). D.R.P. was supported by NASA for this work (Grant No. 80NSSC22K0961). D.R.P. and R.R. acknowledge support from NASA (Grant No. 80NSSC19K1287). M.Z. was supported by the Czech Science Foundation through Junior Star Grant No. GM24-10599M. T.W. warmly thanks the Space Telescope Science Institute for its hospitality during part of this work. P.S. has been supported by the Lumina Quaeruntur fellowship of the Czech Academy of Sciences (No. LQ100032102). This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic through e-INFRA CZ (Grant Nos. 90140 and LM2023047 to V.K.). V.W. was supported by Charles University (Research Programme PRIMUS 23/SCI/019). R.A. acknowledges support from NASA through the NASA Einstein Fellowship (Grant No. HF2-51499) as awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA (Contract No. NAS5-26555). This work was supported by the Australian government through the Australian Research Council’s Discovery Projects funding scheme (Grant No. DP200102471). E.R.C. acknowledges support from the NSF (Grant No. AST-2006684) and from the Oakridge Associated Universities through a Ralph E. Powe Junior Faculty Enhancement Award. E.C.F. is supported by NASA (Award No. 80GSFC21M0002). K.D.A. acknowledges support from the NSF (Grant No. AST-2307668). We recognize and acknowledge the cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. NICER is a 0.2–12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA. Observations were made with European Southern Observatory (ESO) telescopes at La Silla Paranal Observatory under programme ID 110.2599. The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc.
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M.G. led the overall project, wrote a large portion of the paper, performed part of the data analyses and was the principal investigator (PI) of the NICER DDT proposals. D.R.P. reduced the NICER data and performed part of the X-ray analyses. M.Z. and E.R.C. led the theoretical and modelling portion of the project and wrote parts of the paper. V.W., P.S. and V.K. contributed to the modelling and theory portion of the paper. T.W. was the PI of the VLT DDT programme and wrote part of the paper. S.v.V., K.D.A. (PI) and J.M.J. are the team leads of the VLA programme. S.v.V. performed the radio data reduction and analyses. R.R., K.G. and E.C.F. performed the NICER observations. S.G., F.T., Y.Y. and R.A. contributed to gathering, analysing and interpreting data as well as writing the paper and discussions.
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Extended data
Extended Data Fig. 1 Constraint on the beginning of the eruptions in Swift J0230+28.
Historical X-ray light curve, 3σ upper limits from non-detection in 1990 by RASS (pink triangle), in 2005 by XMM-Newton-Slew survey (green triangle) and multiple Swift/XRT observations between 1 December 2021 and 8 January 2022 (red triangles). The multiple consecutive non-detections of XRT constrain that the eruptions may have started between 8 January and 22 June 2022 – date of the first detection by Swift/XRT (blue points).
Extended Data Fig. 2 Eruption shape fitting.
Left: Fit of asymmetric Gaussian profile to the six best-sampled eruptions: around epochs E3, E4, E5, E6, E10, and E11. Right: ratio of σ+ and σ− showing the slight asymmetric nature of Swift J0230+28’s eruption. Error-bars represent 1σ uncertainty.
Extended Data Fig. 3 Residuals of the stacked spectral analyses.
The order (left to right panels) represent distinct eruptions while the color and vertical panels represent distinct phases of each eruption: orange (rises), cyan (peaks) and gold (decays). The order and colors are the same as in Fig. 1. Error-bars represent 1σ uncertainty.
Extended Data Fig. 4 Radio (VLA) images.
A transient radio source is detected in the second radio observation (middle panel) on MJD 59842 with a flux of 93 ± 7μJy (13σ detection). No source is detected in the first and third observations (left and right panels), with upper limits of 15 μJy and 25 μJy respectively. The orange cross marks the peak of the X-ray emission, and the orange circle the Kron radius (12″) of the host galaxy.
Extended Data Fig. 5 Swift J0230+28 Position and host identification.
Top left: Swift/XRT stacked images. Yellow 47″ circle represents the 90% region of the XRT point spread function, and was the radius used for extraction. Green circle is the NICER FoV, no other source is present. Top Right: Pan-STARRS i/g/r bands composed image of Swift J0230+28’s host galaxy. Red cross show the location of the peak of the XRT emission and red circle (radius = 3.4″) represents the 2.7σ uncertainty on the position. The X-ray emission is consistent with the nucleus of the galaxy. Bottom: Continuum normalized X-shooter optical spectrum of the nuclear 1″ of the host galaxy, in the Hβ+[O III] (left) and Hα+[N II] (right) regions.
Extended Data Fig. 6 Diagnostic diagrams of the host galaxy.
Top: Swift J0230+28 in the BPT diagnostic diagram, located above the83 theoretical upper limit for star-formation ionization (red continuous line). Black diamonds represent the 4 known QPE hosts in all panels69. Middle: Swift J0230+28 in the WHAN diagnostic diagram, further showing that the nucleus likely hosts a weak AGN. Bottom: the Lick Hδ absorption index as a function of Hα EW diagram. Grey points show SDSS galaxies for reference; blue circles represent TDE host galaxies. The black dash-delimited (solid) box indicates where QBS (E+A) galaxies are located. These galaxies make up 2.3% and 0.2% of the selected SDSS galaxies, respectively. Error-bars are 1σ uncertainties in all panels.
Extended Data Fig. 7 Broad-band spectral energy distribution (SED) of host galaxy.
Red points show the observed archival photometry, black point the maximum a posteriori (MAP) best-fitted mode, and grey line the MAP best-fitted spectrum. Best-fitted parameters (see text for details) for the model are shown in the lower right. Error-bars are 1σ uncertainties.
Extended Data Fig. 8 Comparison of QPEs light curves.
Top: XMM-Newton-pn light curve of eRO-QPE2. Center: NICER light curve of eRO-QPE1. Bottom: Swift/XRT light curves of Swift J0230+28. All three panels show six consecutive eruptions for each source, the distinct time scales are clearly given the x-axis range: 0.6 days for eRO-QPE2, 4.5 days for eRO-QPE1 and 120 days for Swift J0230+28. Error-bars are 1σ uncertainties.
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Guolo, M., Pasham, D.R., Zajaček, M. et al. X-ray eruptions every 22 days from the nucleus of a nearby galaxy. Nat Astron 8, 347–358 (2024). https://doi.org/10.1038/s41550-023-02178-4
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DOI: https://doi.org/10.1038/s41550-023-02178-4
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