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Free keywords:
TIDAL-DISRUPTION EVENT; UNSTABLE MASS-TRANSFER; PHOTON IMAGING CAMERA;
STAR-DISK COLLISIONS; DEPTH SURVEY EFEDS; BLACK-HOLE; XMM-NEWTON; GSN
069; STELLAR POPULATION; NUCLEAR TRANSIENTAstronomy & Astrophysics; accretion, accretion disks; surveys; galaxies: active; galaxies: nuclei;
X-rays: bursts; X-rays: galaxies;
Abstract:
X-ray quasi-periodic eruptions (QPEs) are a novel addition to the group of extragalactic transients. In this work, we report the discovery of two further galaxies showing QPEs, eRO-QPE3 and eRO-QPE4, with the eROSITA X-ray telescope on board the Spectrum Roentgen Gamma observatory. Among the properties in common with those of known QPEs are: the thermal-like spectral shape in eruption (up to kT similar to 110-120 eV) and quiescence (kT similar to 50-90 eV) and its evolution during the eruptions (with a harder rise than decay); the lack of strong canonical signatures of active nuclei (from current optical, UV, infrared and radio data); and the low-mass nature of the host galaxies (logM(& lowast;)approximate to 9-10) and their massive central black holes (logM(BH)approximate to 5-7). These discoveries also bring several new insights into the QPE population: i) eRO-QPE3 shows eruptions on top of a decaying quiescence flux, providing further evidence for a connection between QPEs and a preceding tidal disruption event; ii) eRO-QPE3 exhibits the longest recurrence times and faintest peak luminosity of QPEs, compared to the known QPE population, excluding a correlation between the two; iii) we find evidence, for the first time, of a transient component that is harder, albeit much fainter, than the thermal QPE spectrum in eRO-QPE4; and iv) eRO-QPE4 displays the appearance (or significant brightening) of the quiescence disk component after the detection of QPEs, supporting its short-lived nature against a preexisting active galactic nucleus. Overall, the newly discovered properties (e.g., recent origin and/or transient nature of the quiescent accretion disk; lack of correlation between eruption recurrence timescales and luminosity) are qualitatively consistent with recent models that identify QPEs as extreme mass-ratio inspirals.
