X-ray quasi-periodic eruptions from two previously quiescent galaxies

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.

Milli-Hertz Gravitational Wave Background Produced by Quasi-Periodic Eruptions

“…The mass of XMMSL1 J0249-041244 is inferred from the correlation between the mass of a MBH and the velocity dispersion of the bulge of the host galaxy (from Wevers et al 2019). For eRO-QPE1 and eRO-QPE2, since the masses of their host galaxies have been derived in previous works (Arcodia et al 2021), we use them to estimate the masses of the MBHs according to the empirical scaling relation log(M BH /M ) = 7.45 + 1.05 log(M stellar /10 11 M ) (1) (Reines & Volonteri 2015).…”

“…Interestingly, the EM counterpart to the above WD EMRI may have been found in a new type of transient called "quasi-period eruption" (QPE, Miniutti et al 2019;Giustini et al 2020;Arcodia et al 2021;Chakraborty et al 2021). Five QPEs have been discovered so far and they share a distinctive feature: within an hour the X-ray count rate surges by one to two orders of magnitude and such an eruption recurs every few hours.…”

“…In this case, little GW radiation is expected. These models, however, have difficulties explaining two of the QPEs which are found in quiescent galaxies showing no sign of accretion disks (Arcodia et al 2021).…”

Searching for Quasi-periodic Oscillations in Active Galactic Nuclei of the Chandra Deep Field South

“…A natural time bin is imposed by the ACIS frame time of 3.2 s. However, due to the intrinsic faintness of most CDF-S sources, we instead adopt 𝑇 bin = 100 s, which helps suppress the Poisson noise without losing sensitivity to AGN QPOs that empirically exhibit a typical period of hours. We further restrict the period searching range between 200 s and 20 ks, since most currently known AGN QPOs exhibit a period shorter than 20 ks, with the exception of XMMU J134736+173403 (85.7 ks; Carpano & Jin 2018) as well as two QPEs (32.4 ks in GSN 069, Miniutti et al 2019;66.6 ks in eRO-QPE1, Arcodia et al 2021). The size of the frequency grid is taken to be Δ𝜈 = 1/(𝑛 0 𝑇), where 𝑛 0 is known as the oversampling factor and chosen to be 5 in our analysis.…”

“…The recent advent of the eROSITA all-sky survey led to the discovery of new QPEs in two otherwise quiescent galaxies, named eRO-QPE1 and eRO-QPE2, with a period of 18.5 hours and 2.4 hours, respectively (Arcodia et al 2021). The origin of QPEs and their relation with normal QPOs are not clear.…”