Supersoft, luminous X-ray sources in galactic nuclei

Abstract

Tidal disruption events (TDEs) are usually discovered at X-ray or optical wavelengths through their transient nature. A characteristic spectral feature of X-ray detected TDEs is a "supersoft" X-ray emission, not observed in any other extragalactic source, with the exception of a few, rapidly variable hyper-luminous X-ray sources (HLXs) or supersoft active galactic nuclei (AGN) that are however distinguishable by their optical emission. The goal of our work is to find extragalactic supersoft sources associated with galactic centres. We expect this category to include overlooked TDEs, supersoft AGN and nuclear HLXs. Finding such sources would allow for the study of extreme regime accretion on different black hole mass scales. We searched for supersoft X-ray sources (SSS) by cross-correlating optical and X-ray catalogues to select extragalactic near-nuclear sources and we then filtered for very steep spectra (photon index $\Gamma >3$) and high X-ray luminosities ($L_X>10^{41}$~erg~s${}^{-1}$). With our blind search, we retrieved about 60 sources including 15 previously known supersoft AGN or TDEs, so demonstrating the efficiency of our selection. Of the remaining sample, 36 sources, although showing steeper-than-usual spectra, are optically classified as AGN. The remaining nine, previously unknown sources show spectral properties consistent with the emission by extremely soft-excess dominated AGN (five sources) or TDE (four sources). An {\it XMM-Newton} follow-up observation of one of these sources confirmed its likely TDE nature. Our work is the first attempt to discover TDEs by their spectral features rather than their variability and has been successful in retrieving known TDEs as well as discovering new extreme ultrasoft sources, including four new TDE candidates, one of which is confirmed via follow-up observations.

arXiv comment15 pages, 13 figures, 2 tables. Accepted for publication in A&A

References in corpus (83)

• The 2dF Galaxy Redshift Survey: Spectra and redshifts
• The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey
• The Sixteenth Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra
• The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III
• Galaxy and Mass Assembly (GAMA): survey diagnostics and core data release
• The 6dF Galaxy Survey: Final Redshift Release (DR3) and Southern Large-Scale Structures
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Intrinsic disc emission and the Soft X-ray Excess in AGN
• Galaxy And Mass Assembly (GAMA): end of survey report and data release 2
• Ultraluminous X-Ray Sources
• The Las Campanas Redshift Survey
• An Intermediate-mass Black Hole of Over 500 Solar Masses in the Galaxy ESO 243-49
• The XMM-Newton view of PG quasars I. X-ray continuum and absorption
• The Sloan Digital Sky Survey Quasar Catalog: Fourteenth Data Release
• Tidal Disruption Events
• The XMM-Newton serendipitous survey IX. The fourth XMM-Newton serendipitous source catalogue
• The Ultra-Luminous X-ray Source Population from the Chandra Archive of Galaxies
• Six Months of Multi-Wavelength Follow-up of the Tidal Disruption Candidate ASASSN-14li and Implied TDE Rates from ASAS-SN
• Nine-hour X-ray quasi-periodic eruptions from a low-mass black hole galactic nucleus
• The XXL Survey: I. Scientific motivations - XMM-Newton observing plan - Follow-up observations and simulation programme
• Large-Amplitude X-ray Outbursts from Galactic Nuclei: A Systematic Survey Using ROSAT Archival Data
• The Sloan Digital Sky Survey quasar catalog: tenth data release
• The 2M++ galaxy redshift catalogue
• Luminous Thermal Flares from Quiescent Supermassive Black Holes
• The soft X-ray/NLR connection: a single photoionized medium?
• X-ray Quasi-Periodic Eruptions from two previously quiescent galaxies
• Testing warm Comptonization models for the origin of the soft X-ray excess in AGN
• X-ray quasi-periodic eruptions from the galactic nucleus of RX J1301.9+2747
• A spectral-timing model for ULXs in the super-critical regime
• On the origin of soft X-rays in obscured AGN: answers from high-resolution spectroscopy with XMM-Newton
• Evolution of tidal disruption candidates discovered by XMM-Newton
• The destruction and recreation of the X-ray corona in a changing-look Active Galactic Nucleus
• Ultra-luminous X-ray Sources in nearby galaxies from ROSAT HRI observations I. data analysis
• A Catalog of Narrow Line Seyfert 1 Galaxies from the Sloan Digital Sky Survey Data Release 12
• A Population of Bona Fide Intermediate Mass Black Holes Identified as Low Luminosity Active Galactic Nuclei
• Compton Thick AGN: the dark side of the X-ray background
• ROSAT monitoring of persistent giant and rapid variability in the narrow-line Seyfert 1 galaxy IRAS 13224-3809
• A tidal disruption-like X-ray flare from the quiescent galaxy SDSS J120136.02+300305.5
• 2XMM Ultraluminous X-Ray Source Candidates in Nearby Galaxies
• XMM-Newton discovery of a sharp spectral feature at ~7 keV in the Narrow-Line Seyfert 1 galaxy 1H 0707-495
• Possible X-ray Quasi-Periodic Eruptions in a Tidal Disruption Event Candidate
• The XMM-Newton Serendipitous Survey I. The role of XMM-Newton Survey Science Centre
• Candidate tidal disruption events from the XMM-Newton Slew Survey
• From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link
• A Spectroscopic Survey of X-ray Selected AGN in the Northern XMM-XXL Field
• Optically thick outflows in ultraluminous supersoft sources
• A likely decade-long sustained tidal disruption event
• The Spitzer mid-infrared AGN survey. I - optical and near-infrared spectroscopy of candidate obscured and normal AGN selected in the mid-infrared
• X-ray spectral properties of the AGN sample in the northern XMM-XXL field
• The XMM-Newton view of AGN with intermediate mass black holes
• A population of highly energetic transient events in the centres of active galaxies
• Luminosity and Redshift Dependence of the Covering Factor of AGNs viewed with WISE and SDSS
• Observations of Galactic and Extragalactic Novae
• Comparison of the X-ray emission from Tidal Disruption Events with those of Active Galactic Nuclei
• A ~3.8 hour Periodicity from an Ultrasoft Active Galactic Nucleus Candidate
• A high Eddington-ratio, true Seyfert 2 galaxy candidate: implications for broad-line-region models
• The missing link: a low mass X-ray binary in M31 seen as an ultraluminous X-ray source
• A long decay of X-ray flux and spectral evolution in the supersoft active galactic nucleus GSN 069
• Discovery of an Ultrasoft X-ray Transient Source in the 2XMM Catalog: a Tidal Disruption Event Candidate
• Aperture-free star formation rate of SDSS star-forming galaxies
• Soft X-ray spectroscopy of Compton-thick Seyfert 2 galaxies with BeppoSAX
• RX J1301.9+2747: A Highly Variable Seyfert Galaxy with Extremely Soft X-ray Emission
• A Spectro-photometric Search for Galaxy Clusters in SDSS
• Evidence of a tidal disruption event in GSN 069 from the abnormal carbon and nitrogen abundance ratio
• An Ultrasoft X-ray Flare from 3XMM J152130.7+074916: a Tidal Disruption Event Candidate
• Fainter harder brighter softer: a correlation between ${\alpha }_{\mathrm{ox}}$ , X-ray spectral state and Eddington ratio in tidal disruption events
• New Compton-thick AGN in the circumnuclear water maser hosts UGC3 789 and NGC 6264
• A Candidate Active Galactic Nucleus with a Pure Soft Thermal X-ray Spectrum
• Nature of the soft ULX in NGC 247: super-Eddington outflow and transition between the supersoft and soft ultraluminous regimes
• A Catalog of Hyper-luminous X-ray Sources and Intermediate-Mass Black Hole Candidates out to High Redshifts
• SRG/eROSITA prospects for the detection of stellar tidal disruption flares
• The Low-mass, Highly Accreting Black Hole Associated with the Active Galactic Nucleus 2XMM J123103.2+110648
• Large Decay of X-ray Flux in 2XMM J123103.2+110648: Evidence for a Tidal Disruption Event
• Central Engine and Host Galaxy of RXJ 1301.9+2747: A Multiwavelength View of a Low-mass Black Hole Active Galactic Nuclei with Ultra-soft X-ray Emission
• Active Galactic Nuclei from HeII: a more complete census of AGN in SDSS galaxies yields a new population of low-luminosity AGN in highly star-forming galaxies
• Disentangling AGN and Star Formation in Soft X-rays
• PHL 1092 as a transient extreme X-ray weak quasar
• Chandra View of the Ultra-Steep Spectrum Radio Source in Abell 2443: Merger Shock-Induced Compression of Fossil Radio Plasma?
• Classification and environmental properties of X-ray selected point-like sources in the XMM-LSS field
• The XXL survey XIV. AAOmega redshifts for the southern XXL field
• Discovery of the Candidate Off-nuclear Ultrasoft Hyper-luminous X-ray Source 3XMM J141711.1+522541
• The XMM-Newton Wide Angle Survey (XWAS)
• Large-amplitude rapid X-ray variability in the narrow-line Seyfert 1 galaxy PG 1404$+$226

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