Nine-hour X-ray quasi-periodic eruptions from a low-mass black hole galactic nucleus

Abstract

In the past two decades, high-amplitude electromagnetic outbursts have been detected from dormant galaxies and often attributed to the tidal disruption of a star by the central black hole1,2. X-ray emission from the Seyfert 2 galaxy GSN 069 (2MASX J01190869-3411305) at a redshift of z = 0.018 was first detected in July 2010 and implies an X-ray brightening by a factor of more than 240 over ROSAT observations performed 16 years earlier3,4. The emission has smoothly decayed over time since 2010, possibly indicating a long-lived tidal disruption event5. The X-ray spectrum is ultra-soft and can be described by accretion disk emission with luminosity proportional to the fourth power of the disk temperature during long-term evolution. Here we report observations of quasi-periodic X-ray eruptions from the nucleus of GSN 069 over the course of 54 days, from December 2018 onwards. During these eruptions, the X-ray count rate increases by up to two orders of magnitude with an event duration of just over an hour and a recurrence time of about nine hours. These eruptions are associated with fast spectral transitions between a cold and a warm phase in the accretion flow around a low-mass black hole (of approximately 4 × 105 solar masses) with peak X-ray luminosity of about 5 × 1042 erg per second. The warm phase has kT (where T is the temperature and k is the Boltzmann constant) of about 120 electronvolts, reminiscent of the typical soft-X-ray excess, an almost universal thermal-like feature in the X-ray spectra of luminous active nuclei6,7,8. If the observed properties are not unique to GSN 069, and assuming standard scaling of timescales with black hole mass and accretion properties, typical active galactic nuclei with higher-mass black holes can be expected to exhibit high-amplitude optical to X-ray variability on timescales as short as months or years9.

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Fig. 1: X-ray QPEs in XMM-Newton and Chandra light curves from December 2018 onwards.
Fig. 2: The QPE energy dependence from the longest XMM4 observation.
Fig. 3: Phase-resolved X-ray spectra from the XMM4 observation.

Data and code availability

Most data used in this work are public and available from the corresponding data archives. Some remaining proprietary data will be available immediately after the initial proprietary period expires. Data may be available from the corresponding author on reasonable request. All figures were made in Veusz, a Python-based scientific plotting package developed by J. Sanders and freely available at https://veusz.github.io.

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Acknowledgements

The scientific results reported here are based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA, the Chandra X-ray Observatory, the NASA/ESA Hubble Space Telescope, ATCA, the Karl G. Jansky VLA and the South African MeerKAT radio telescope. ATCA is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO. The NRAO operating the VLA is a facility of the US National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The MeerKAT telescope is operated by SARAO, which is a facility of the National Research Foundation, an agency of the South African Department of Science and Innovation. We thank N. Schartel, B. Wilkes, J. Stevens, M. Claussen and F. Camilo for approving XMM-Newton, Chandra, ATCA, VLA and MeerKAT DDT observations, as well as the operation and scheduling teams of all involved observatories and facilities. We also thank the Neil Gehrels Swift Observatory for supporting a long-term monitoring campaign of GSN 069 over the years. G.M. thanks A. Laor for critically reading the article and suggesting the acronym QPE to describe the observed X-ray variability. G.M. and M.G. thank A. Janiuk, M. Gręzdzielski and A. Różańska for discussions on the physics of disk instabilities and soft excess. G.M. and M.G. acknowledge Spanish public funding through grants ESP2017-86582-C4-1-R and ESP2015-65597-C4-1-R, respectively. This research has been partially funded by the Spanish State Research Agency (AEI) project number ESP2017-87676-C5-1-R and MDM-2017-0737 Unidad de Excelencia “María de Maeztu”-Centro de Astrobiología (INTA-CSIC). K.D.A. is a NASA Einstein Fellow and acknowledges support provided by NASA through NASA Hubble Fellowship grant HST-HF2-51403.001 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. I.H. acknowledges support of the Oxford Hintze Centre for Astrophysical Surveys which is funded through support from the Hintze Family Charitable Foundation. P.G. acknowledges support from STFC and a UGC-UKIERI Phase 3 Thematic Partnership. B.A.G. acknowledges support provided by the Fonds de la Recherche Scientifique—FNRS, Belgium, under grant number 4.4501.19.

Author information

G.M. was PI of all XMM-Newton, HST and Chandra proposals, led the organization of the February 2019 X-ray/radio campaign, performed most of the X-ray data analysis and wrote the article. R.D.S. discovered the source during the XMM-Newton slew and analysed the first XMM-Newton slew data together with A.M.R. R.D.S. also performed part of the XMM-Newton EPIC and OM data analysis. M.G. performed part of the XMM-Newton and Chandra data analysis, and actively contributed to all intermediate results before submission. K.D.A. was PI of the VLA DDT request and performed VLA radio data analysis. M.L.P. led the MeerKAT DDT request. I.H., R.P.F. and I.M. performed MeerKAT data analysis. R.P.F. was PI of the ATCA DDT request. A.K.T. observed GSN 069 with ATCA. M.C. performed ATCA data analysis. C.K. and P.G. were responsible for the analysis of the HST/STIS UV data. B.A.-G. was Co-I of the XMM3 discovery observation. All authors contributed to discussing data analysis and results, read the article since the first draft and helped to improve it throughout the process.

Correspondence to G. Miniutti.

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Extended data figures and tables

Extended Data Fig. 1 Position of GN 069 as inferred from optical, X-ray and radio images.

a, A 12′ × 12′ region from the Digitized Sky Survey centred on the 2MASS position of GSN 069 (RA 01 h 19 min 08.663 s, dec. −34° 11ʹ 30.52″). The galaxy to the north of GSN 069 and close to the edge of the field is ESO 352−G41. The two bright stars in the field are CD-34 503 and CD-34 498. The red box size is 1.7′ × 1.7′. b, c, The same 1.7′ × 1.7′ region as imaged by the VLA at 6 GHz and by Chandra in the 0.4–2 keV band, respectively. The blue box size in b and c is 12″ × 12″. d, The Chandra X-ray image of the blue box region, with superimposed VLA contours (white) and the 2MASS position (red cross) as reference. No boresight correction was applied to the Chandra data.

Extended Data Fig. 2 UV spectrum and variability of GSN 069 from HST and OM observations.

a, The 2014/2018 HST/STIS spectra taken quasi-simultaneously with XMM2 (2014) and XMM3 (2018). b, The 2014 STIS spectrum of GSN 069 is compared with that of intermediate-type main-sequence stars (B3 and B6) from the Pickles Atlas, demonstrating that the UV spectrum is strongly contaminated by starlight and possibly dominated by a relatively young nuclear stellar cluster. c, The OM light curve in the UVM2 filter (approximately 231 nm) during the XMM4 observation. The OM light curve shows no variability, with a reduced χ2 of 0.7 when fitted with a constant, despite the simultaneous X-ray QPEs (see Fig. 1b). The STIS spectra as well as the B3 and B6 spectra from the Pickles Atlas are displayed with no uncertainties. Errors in c represent the 1σ confidence intervals.

Extended Data Fig. 3 The X-ray long-term evolution of GSN 069.

a, The X-ray spectra from the XMM1, XMM2, XMM4 and Chandra observations, excluding time-intervals containing QPEs. All spectra have been divided by the corresponding detector effective area to ease comparison. The XMM3 spectrum is not shown, as it is basically superimposed on the XMM4 one. Spectra have been slightly re-binned for visual clarity. b, The best-fitting SEDs according to the best-fitting models presented in Extended Data Table 2. c, The 0.3–2 keV flux evolution of GSN 069 since first X-ray detection, including the XMM-Newton slew data point. The dashed grey line is a power-law decay model with index fixed at −5/3, while the dotted magenta line is an exponential decay law with best-fitting e-folding timescale of about 5 yr. d, The 0.2–2 keV luminosity of the best-fitting diskbb model as a function of disk temperature (see Extended Data Table 2). The dashed line is the best-fitting relation Ldiskbb T4.5 ± 0.5 to the XMM-Newton data only, consistent with constant-area blackbody emission (L T4). The Chandra data point (green) is far off the L T4 relation, its temperature being too hot to be ascribed to disk emission for the given luminosity. Errors in a represent the 1σ confidence intervals, while error bars in c and d represent the 90% confidence intervals as obtained from X-ray spectral fitting (Extended Data Table 2). Some of the error bars are smaller than the symbol size.

Extended Data Fig. 4 Observation-averaged QPE time evolution.

ac, Time-evolution of the QPE amplitude A (a), duration Tdur (b) and recurrence time Trec (c) since first QPE detection in XMM3. All quantities are averaged over each X-ray observation. d, e, Trec (d) and the duty cycle Δ (e) as a function of the QPE amplitude A. Errors bars represent the 1σ confidence intervals. Some error bars are smaller than the symbol size.

Extended Data Fig. 5 QPE energy dependence.

a, The QPE amplitude (A = N/C) as a function of energy. The maximum A = 93 ± 14 is reached in the 0.6–0.8 keV band. b, N = N(E) and C = C(E), after normalizing them to the detector effective area in each energy bin, that is, the QPE peak (N) and quiescent-level (C) photon spectra. c, d, The QPE duration Tdur (c) and QPE peak time delay ΔTpeak (d) as a function of energy, together with the best-fitting linear relations (see Methods section ‘QPE model-independent properties’). The peak time delay is computed with respect to the full 0.2–2 keV light curve, and the resulting lags are shifted so that the 0.8–1 keV band has zero delay. Errors represent the 1σ confidence intervals, some of the error bars being smaller than the symbol size.

Extended Data Fig. 6 QPE phase-resolved spectroscopy.

QPE spectral evolution throughout the cycle. a, b, Temperature (a) and 0.2–2 keV luminosity (b) of the variable blackbody component throughout the QPE cycle (see Extended Data Table 3b). In a, the shaded area represents the constant temperature of the stable (likely outer) accretion disk. c, d, The corresponding SED evolution (where FE is the model flux spectrum): the best-fitting model during the QPE rise from quiescence (Qpre) to QPE peak (P) (c), and during the QPE decay from peak (P) back to quiescence (Qpost) (d). The model predicts no variability below 0.1 keV, in line with the expectations based on Extended Data Fig. 5a, b. In a and b, errors represent the 90% confidence intervals as obtained from X-ray spectral fits to the phase-resolved spectra (some of the error bars being smaller than the symbol size).

Extended Data Fig. 7 The February 2019 simultaneous X-ray/radio campaign.

a, The X-ray 0.4–2 keV Chandra light curve. bd, The MeerKAT1 (b), VLA (c) and ATCA2 (d) light curves from the simultaneous radio campaign. Notice that the MeerKAT1 and ATCA2 exposures include one X-ray QPE each (vertical shaded areas), whereas the VLA observation was performed during X-ray quiescence. No significant radio variability is detected in any of the radio exposures. The ATCA2 data points are all upper limits, and the horizontal shaded area in d represents the measured time-averaged flux density. We ignore the first data point of the ATCA2 light curve as the source was still very low on the horizon, resulting in a highly degraded image. We also point out that the ATCA2 measurements are contaminated by a nearby unresolved radio source detected by the VLA with a flux density of 71 ± 10 μJy at 6 GHz. Error bars, including the average flux from ATCA (horizontal shaded area in d) represent the 1σ confidence intervals. The ATCA data points (d) are instead 3σ confidence level upper limits.

Extended Data Table 1 Summary of the observations used in this work
Extended Data Table 2 Most relevant best-fitting parameters for the long-term evolution of GSN 069 using X-ray spectra from which time-intervals containing QPEs are excluded
Extended Data Table 3 Most relevant best-fitting parameters from QPE phase-resolved X-ray spectral analysis during the XMM4 observation

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