The complex time and energy evolution of quasi-periodic eruptions in eRO-QPE1

Abstract

Quasi-periodic eruptions (QPEs) are recurrent X-ray bursts found so far in the nuclei of low-mass galaxies. Their trigger mechanism is still unknown, but recent models involving one or two stellar-mass companions around the central massive (≈ 10 5 − 10 6 M ) black hole have gathered significant attention. While these have been compared only qualitatively with observations, the phenomenology of QPEs is developing at a fast pace, with the potential to reveal new insights. Here we report two new observational results found in eRO-QPE1, the brightest QPE source discovered so far: i) the eruptions in eRO-QPE1 occur sometimes as single isolated bursts, and at others as chaotic mixtures of multiple overlapping bursts with very different amplitudes; ii) we confirm that QPEs peak at later times and are broader at lower energies, with respect to higher energies while, for the first time, we find that QPEs also start earlier at lower energies. Furthermore, eruptions appear to undergo an anti-clockwise hysteresis cycle in a plane of hardness ratio versus total count rate. Behavior i) was not found before in any other QPE source and implies that if a common trigger mechanism is in place for all QPEs, it must be able to produce both types of timing properties, regular and complex. Result ii) implies that the X-ray emitting component does not have an achromatic evolution even during the start of QPEs, and that the rise is harder than the decay at a given total count rate. This specific energy dependence could be qualitatively compatible with inward radial propagation during the rise within a compact accretion flow, the presence of which is suggested by the stable quiescence spectrum observed in general for QPE sources.right panel of Fig. 2). It is loosely based on that proposed in Norris et al. (2005) for gamma-ray bursts, and defined as:which is evaluated at zero for times smaller than the asymptote at t peak − t as , where t as = √ τ 1 τ 2 . Whereas A is the amplitude at the peak and λ = e t λ a normalization to join rise and decay, Article number, page 2 of 11 R. Arcodia et al.: Evolution of quasi-periodic eruptions in eRO-QPE1

Variability in a low-mass active galactic nucleus: oscillation or eruption?

“…• While the case against gravitational lensing, as described by Ingram et al (2021), for the mechanism in QPE sources is strong in light of the asymmetry seen in the eruptions of eRO-QPE1 (Arcodia et al 2022), we can make no similar claim for the variability in 2XJ1231. It does, however, still appear to be an unlikely mechanism, as with GSN 069 and RX J1301.9+2747, as the black hole mass being as low as 10 5 𝑀 would produce eruptions which are either not of a large enough amplitude or not long enough in duration without the inclusion of a complex geometry for the emitting regions.…”

Quasiperiodic Erupters: A Stellar Mass-transfer Model for the Radiation

“…If the heating is strong enough to unbind the gas, the speed of the large-scale eddies is comparable to the escape speed. If, as is commonly the case in turbulent dynamos (Kulsrud & Anderson 1992;Schekochihin et al 2004;Bott et al 2021;Seta & Federrath 2021;Arcodia et al 2022), the magnetic field energy density is amplified to rough equipartition with the turbulent kinetic energy density, the Alfvén speed would then be comparable to the star's escape speed. On the main sequence, this is almost always ;0.002c, or ∼2 × 10 −2 v orb when a ∼ 100r g .…”

“…A potential consequence of strong, irregular pressure fluctuations is turbulence. A large body of work on turbulent dynamos (Kulsrud & Anderson 1992;Schekochihin et al 2004;Bott et al 2021;Seta & Federrath 2021;Arcodia et al 2022) has shown that large-amplitude (transonic) turbulence can amplify the magnetic field enough to make the Alfvén speed v A ∼ c s , for c s the local sound speed (in conditions of high opacity like a star, the sound speed reflects the combination of gas and radiation pressure). As the gas being transferred accelerates away from the star (pulled by the black hole's tidal gravity and pushed by its internal pressure gradient), one might expect v A to increase above c s as the gas density drops while the component of the magnetic field parallel to the flow is unchanged.…”

“…In galaxies hosting quasiperiodic erupters (QPEs), bright X-ray flares close to the galaxies' centers repeat every several hours, sharply raising the X-ray flux (Miniutti et al 2019;Giustini et al 2020;Arcodia et al 2021Arcodia et al , 2022. In one case, GSM 069, the ratio of peak flare flux to quiescent flux is at least 2 orders of magnitude (Miniutti et al 2019).…”

Quasi-periodic eruptions from mildly eccentric unstable mass transfer in galactic nuclei

“…12). The large variation in flare luminosities, as well as occasional overlapping flares (Arcodia et al 2022), could arise either from variation in the mass stripped from the star in a given orbit and/or variations in the circularization efficiency. Given the complex disk-star interactions ( §4) and stellar debris circularization ( §5), it is natural to expect such variations.…”