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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.
- ItemThe final SDSS-IV/SPIDERS X-ray point source spectroscopic catalogue(Les Ulis : EDP Sciences, 2020) Comparat, J.; Merloni, A.; Dwelly, T.; Salvato, M.; Schwope, A.; Coffey, D.; Wolf, J.; Arcodia, R.; Liu, T.; Buchner, J.; Nandra, K.; Georgakakis, A.; Clerc, N.; Brusa, M.; Brownstein, J.R.; Schneider, D.P.; Pan, K.; Bizyaev, D.Aims. We look to provide a detailed description of the SPectroscopic IDentification of ERosita Sources (SPIDERS) survey, an SDSS-IV programme aimed at obtaining spectroscopic classification and redshift measurements for complete samples of sufficiently bright X-ray sources. Methods. We describe the SPIDERS X-ray Point Source Spectroscopic Catalogue, considering its store of 11 092 observed spectra drawn from a parent sample of 14 759 ROSAT and XMM sources over an area of 5129 deg2 covered in SDSS-IV by the eBOSS survey. Results. This programme represents the largest systematic spectroscopic observation of an X-ray selected sample. A total of 10 970 (98.9%) of the observed objects are classified and 10 849 (97.8%) have secure redshifts. The majority of the spectra (10 070 objects) are active galactic nuclei (AGN), 522 are cluster galaxies, and 294 are stars. Conclusions. The observed AGN redshift distribution is in good agreement with simulations based on empirical models for AGN activation and duty cycle. Forming composite spectra of type 1 AGN as a function of the mass and accretion rate of their black holes reveals systematic differences in the H-beta emission line profiles. This study paves the way for systematic spectroscopic observations of sources that are potentially to be discovered in the upcoming eROSITA survey over a large section of the sky.
- ItemFirst constraints on the AGN X-ray luminosity function at z 6 from an eROSITA-detected quasar(Les Ulis : EDP Sciences, 2021) Wolf, J.; Nandra, K.; Salvato, M.; Liu, T.; Buchner, J.; Brusa, M.; Hoang, D. N.; Moss, V.; Arcodia, R.; Brüggen, M.; Comparat, J.; de Gasperin, F.; Georgakakis, A.; Hotan, A.; Lamer, G.; Merloni, A.; Rau, A.; Rottgering, H. J. A.; Shimwell, T. W.; Urrutia, T.; Whiting, M.; Williams, W. L.Context. High-redshift quasars signpost the early accretion history of the Universe. The penetrating nature of X-rays enables a less absorption-biased census of the population of these luminous and persistent sources compared to optical/near-infrared colour selection. The ongoing SRG/eROSITA X-ray all-sky survey offers a unique opportunity to uncover the bright end of the high-z quasar population and probe new regions of colour parameter space. Aims. We searched for high-z quasars within the X-ray source population detected in the contiguous 140 deg2 field observed by eROSITA during the performance verification phase. With the purpose of demonstrating the unique survey science capabilities of eROSITA, this field was observed at the depth of the final all-sky survey. The blind X-ray selection of high-redshift sources in a large contiguous, near-uniform survey with a well-understood selection function can be directly translated into constraints on the X-ray luminosity function (XLF), which encodes the luminosity-dependent evolution of accretion through cosmic time. Methods. We collected the available spectroscopic information in the eFEDS field, including the sample of all currently known optically selected z > 5.5 quasars and cross-matched secure Legacy DR8 counterparts of eROSITA-detected X-ray point-like sources with this spectroscopic sample. Results. We report the X-ray detection of eFEDSU J083644.0+005459, an eROSITA source securely matched to the well-known quasar SDSS J083643.85+005453.3 (z = 5.81). The soft X-ray flux of the source derived from eROSITA is consistent with previous Chandra observations. The detection of SDSS J083643.85+005453.3 allows us to place the first constraints on the XLF at z > 5.5 based on a secure spectroscopic redshift. Compared to extrapolations from lower-redshift observations, this favours a relatively flat slope for the XLF at z 6 beyond L∗, the knee in the luminosity function. In addition, we report the detection of the quasar with LOFAR at 145 MHz and ASKAP at 888 MHz. The reported flux densities confirm a spectral flattening at lower frequencies in the emission of the radio core, indicating that SDSS J083643.85+005453.3 could be a (sub-) gigahertz peaked spectrum source. The inferred spectral shape and the parsec-scale radio morphology of SDSS J083643.85+005453.3 indicate that it is in an early stage of its evolution into a large-scale radio source or confined in a dense environment. We find no indications for a strong jet contribution to the X-ray emission of the quasar, which is therefore likely to be linked to accretion processes. Conclusions. Our results indicate that the population of X-ray luminous AGNs at high redshift may be larger than previously thought. From our XLF constraints, we make the conservative prediction that eROSITA will detect 90 X-ray luminous AGNs at redshifts 5.7 < z < 6.4 in the full-sky survey (De+RU). While subject to different jet physics, both high-redshift quasars detected by eROSITA so far are radio-loud; a hint at the great potential of combined X-ray and radio surveys for the search of luminous high-redshift quasars.
- ItemX-ray quasi-periodic eruptions from two previously quiescent galaxies(London [u.a.] : Nature Publ. Group, 2021) Arcodia, R.; Merloni, A.; Nandra, K.; Buchner, J.; Salvato, M.; Pasham, D.; Remillard, R.; Comparat, J.; Lamer, G.; Ponti, G.; Malyali, A.; Wolf, J.; Arzoumanian, Z.; Bogensberger, D.; Buckley, D.A.H.; Gendreau, K.; Gromadzki, M.; Kara, E.; Krumpe, M.; Markwardt, C.; Ramos-Ceja, M.E.; Rau, A.; Schramm, M.; Schwope, A.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.