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- ItemX-ray emission from a rapidly accreting narrow-line Seyfert 1 galaxy at z = 6.56(Les Ulis : EDP Sciences, 2023) Wolf, J.; Nandra, K.; Salvato, M.; Buchner, J.; Onoue, M.; Liu, T.; Arcodia, R.; Merloni, A.; Ciroi, S.; Di Mille, F.; Burwitz, V.; Brusa, M.; Ishimoto, R.; Kashikawa, N.; Matsuoka, Y.; Urrutia, T.; Waddell, S.G.H.The space density of X-ray-luminous, blindly selected active galactic nuclei (AGN) traces the population of rapidly accreting super-massive black holes through cosmic time. It is encoded in the X-ray luminosity function, whose bright end remains poorly constrained in the first billion years after the Big Bang as X-ray surveys have thus far lacked the required cosmological volume. With the eROSITA Final Equatorial-Depth Survey (eFEDS), the largest contiguous and homogeneous X-ray survey to date, X-ray AGN population studies can now be extended to new regions of the luminosity-redshift space (L2-10 keV > 1045 erg s-1 and z > 6). Aims. The current study aims at identifying luminous quasars at z > 5:7 among X-ray-selected sources in the eFEDS field in order to place a lower limit on black hole accretion well into the epoch of re-ionisation. A secondary goal is the characterisation of the physical properties of these extreme coronal emitters at high redshifts. Methods. Cross-matching eFEDS catalogue sources to optical counterparts from the DESI Legacy Imaging Surveys, we confirm the low significance detection with eROSITA of a previously known, optically faint z = 6:56 quasar from the Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs) survey. We obtained a pointed follow-up observation of the source with the Chandra X-ray telescope in order to confirm the low-significance eROSITA detection. Using new near-infrared spectroscopy, we derived the physical properties of the super-massive black hole. Finally, we used this detection to infer a lower limit on the black hole accretion density rate at z > 6. Results. The Chandra observation confirms the eFEDS source as the most distant blind X-ray detection to date. The derived X-ray luminosity is high with respect to the rest-frame optical emission of the quasar.With a narrow Mgii line, low derived black hole mass, and high Eddington ratio, as well as its steep photon index, the source shows properties that are similar to local narrow-line Seyfert 1 galaxies, which are thought to be powered by young super-massive black holes. In combination with a previous high-redshift quasar detection in the field, we show that quasars with L2-10 keV > 1045 erg s-1 dominate accretion onto super-massive black holes at z _ 6.
- ItemDeciphering the extreme X-ray variability of the nuclear transient eRASSt J045650.3-203750: A likely repeating partial tidal disruption event(Les Ulis : EDP Sciences, 2023) Liu, Z.; Malyali, A.; Krumpe, M.; Homan, D.; Goodwin, A.J.; Grotova, I.; Kawka, A.; Rau, A.; Merloni, A.; Anderson, G.E.; Miller-Jones, J.C.A.; Markowitz, A.G.; Ciroi, S.; Di Mille, F.; Schramm, M.; Tang, S.; Buckley, D.A.H.; Gromadzki, M.; Jin, C.; Buchner, J.Context. During its all-sky survey, the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) observatory has uncovered a growing number of X-ray transients associated with the nuclei of quiescent galaxies. Benefitting from its large field of view and excellent sensitivity, the eROSITA window into time-domain X-ray astrophysics yields a valuable sample of X-ray selected nuclear transients. Multi-wavelength follow-up enables us to gain new insights into understanding the nature and emission mechanism of these phenomena. Aims. We present the results of a detailed multi-wavelength analysis of an exceptional repeating X-ray nuclear transient, eRASSt J045650.3-203750 (hereafter J0456-20), uncovered by SRG/eROSITA in a quiescent galaxy at a redshift of z ∼ 0:077. We aim to understand the radiation mechanism at different luminosity states of J0456-20, and provide further evidence that similar accretion processes are at work for black hole accretion systems at different black hole mass scales. Methods. We describe our temporal analysis, which addressed both the long- and short-term variability of J0456-20. A detailed X-ray spectral analysis was performed to investigate the X-ray emission mechanism. Results. Our main findings are that (1) J0456-20 cycles through four distinctive phases defined based on its X-ray variability: an X-ray rising phase leading to an X-ray plateau phase that lasts for abouttwo months. This is terminated by a rapid X-ray flux drop phase during which the X-ray flux can drop drastically by more than a factor of 100 within one week, followed by an X-ray faint state for about two months before the X-ray rising phase starts again. (2) The X-ray spectra are generally soft in the rising phase, with a photon index & 3:0, and they become harder as the X-ray flux increases. There is evidence of a multi-colour disk with a temperature of Tin ∼ 70 eV in the inner region at the beginning of the X-ray rising phase. The high-quality XMM-Newton data suggest that a warm and hot corona might cause the X-ray emission through inverse Comptonisation of soft disk seed photons during the plateau phase and at the bright end of the rising phase. (3) J0456-20 shows only moderate UV variability and no significant optical variability above the host galaxy level. Optical spectra taken at different X-ray phases are constant in time and consistent with a typical quiescent galaxy with no indication of emission lines. (4) Radio emission is (as yet) only detected in the X-ray plateau phase and rapidly declines on a timescale of two weeks. Conclusions. J0456-20 is likely a repeating nuclear transient with a tentative recurrence time of ∼223 days. It is a new member of this rare class. We discuss several possibilities to explain the observational properties of J0456-20. We currently favour a repeating partial tidal disruption event as the most likely scenario. The long-term X-ray evolution is explained as a transition between a thermal disk-dominated soft state and a steep power-law state. This implies that the corona can be formed within a few months and is destroyed within a few weeks.