Abstract
Quasi-periodic eruptions (QPEs) are very-high-amplitude bursts of X-ray radiation recurring every few hours and originating near the central supermassive black holes of galactic nuclei1,2. It is currently unknown what triggers these events, how long they last and how they are connected to the physical properties of the inner accretion flows. Previously, only two such sources were known, found either serendipitously or in archival data1,2, with emission lines in their optical spectra classifying their nuclei as hosting an actively accreting supermassive black hole3,4. Here we report observations of QPEs in two further galaxies, obtained with a blind and systematic search of half of the X-ray sky. The optical spectra of these galaxies show no signature of black hole activity, indicating that a pre-existing accretion flow that is typical of active galactic nuclei is not required to trigger these events. Indeed, the periods, amplitudes and profiles of the QPEs reported here are inconsistent with current models that invoke radiation-pressure-driven instabilities in the accretion disk5–9. Instead, QPEs might be driven by an orbiting compact object. Furthermore, their observed properties require the mass of the secondary object to be much smaller than that of the main body10, and future X-ray observations may constrain possible changes in their period owing to orbital evolution. This model could make QPEs a viable candidate for the electromagnetic counterparts of so-called extreme-mass-ratio inspirals11–13, with considerable implications for multi-messenger astrophysics and cosmology14,15.
Highlights
Two follow-up observations triggered with the XMM-Newton X-ray telescope confirmed the remarkable bursting nature of the source (Fig. 1c, d)
In terms of intrinsic 0.5–2-keV luminosity, after an initial quiescent phase at about 2.3 × 1040 erg s−1 the first burst was characterized by a fast rise and slower decay lasting around 30 ks and peaking at approximately 3.3 × 1042 erg s−1; it was followed by a second fainter burst and by a third, which was the brightest but was only caught during its rise
The second XMM-Newton observation showed an eruption very similar to the first seen in eRO-QPE1-XMM1 in terms of amplitude and luminosity, lasting for >40 ks, that is, for almost as much as the three in eRO-QPE1-XMM1 combined (Fig. 1c)
Summary
The optical counterpart of eRO-QPE1 is classified as a passive galaxy from the absence of emission lines (Extended Data Fig. 1b), and in eRO-QPE2 the strong narrow emission lines that are observed classify it as a star-forming galaxy (Extended Data Fig. 2b and Methods section ‘The host galaxies of the QPEs’) This in turn suggests that the two galaxies reported here have not been active for approximately at least the last 103–104 years, assuming narrow-line region light-travel timescales[19]. Future X-ray observations on longer temporal baselines (months or years) will help to constrain or rule out this scenario and to monitor the possible orbital evolution of the system This picture is reminiscent of a suggested formation channel of extreme-mass-ratio inspirals[24,25] and it could make QPEs their electromagnetic messenger[13,26]. Low-mass supermassive black holes (~105M☉–107M☉) and finding more will help us to understand how black holes are activated in low-mass galaxies, which— so far a poorly explored mass range in their co-evolution history27,28—is crucial for synergies with future Laser Interferometer Space Antenna (LISA) gravitational wave signals[29]
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