Long-term X-ray spectral evolution of Ultraluminous X-ray sources: implications on the accretion flow geometry and the nature of the accretor

Abstract

The discovery of pulsations in several Ultraluminous X-ray sources (ULXs) demonstrated that a fraction of ULXs are powered by super-Eddington accretion onto neutron stars (NSs). This opened the debate as to what is the NS to black hole (BH) ratio within the ULX population and what physical mechanism allows ULXs to reach luminosities well in excess of their Eddington luminosity: strong magnetic fields or rather strong outflows that collimate the emission towards the observer. To distinguish between supercritically accreting BHs, weakly or strongly magnetised NSs, we study the long-term X-ray spectral evolution of a sample of 17 ULXs, 6 of which are known to host NSs. We combine archival data from \chandra, \xmm\ and \nustar\ observatories to sample a wide range of spectral states for each source and track each source's evolution in a hardness-luminosity diagram (HLD). We find NS-ULXs to be among the hardest sources in our sample with highly variable high-energy emission. On this basis, we identify M81 X-6 as a strong NS-ULX candidate, whose variability is shown to be akin to that seen in NGC 1313 X-2. Most softer sources with unknown accretor show the presence of three markedly different spectral states that we interpret invoking changes in the mass-accretion rate and obscuration by the supercritical wind/funnel structure. Finally, we report on a lack of variability at high-energies ($\gtrsim$ 10 keV) in NGC 1313 X-1 and Holmberg IX X-1, which we argue may offer means to differentiate BH from NS-ULXs. We argue that the hardest sources in our sample might harbour strongly magnetised NSs, while softer sources may be explained by weakly magnetised NSs or BHs, in which the presence of outflows naturally explains their softer spectra through Compton down-scattering, their spectral transitions and the dilution of the pulsed-emission, should some of these sources contain NSs.

arXiv comment44 pages, 13 figures, 7 tables. Accepted for publication to A&A

References in corpus (125)

• On the Absorption of X-rays in the Interstellar Medium
• The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections
• The European Photon Imaging Camera on XMM-Newton: The MOS Cameras
• The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission
• Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and Ions
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• Astrophysics Source Code Library
• The Atlas3D project -- I. A volume-limited sample of 260 nearby early-type galaxies: science goals and selection criteria
• Binary Black Hole Population Properties Inferred from the First and Second Observing Runs of Advanced LIGO and Advanced Virgo
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Ultraluminous X-Ray Sources in External Galaxies
• The Nature of Accreting Black Holes in Nearby Galaxy Nuclei
• X-ray emission from star-forming galaxies - I. High-mass X-ray binaries
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• Ultraluminous X-Ray Sources
• Cosmicflows-3
• An Intermediate-mass Black Hole of Over 500 Solar Masses in the Galaxy ESO 243-49
• An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907
• The Ultraluminous State
• Black Hole Winds
• Our Peculiar Motion Away from the Local Void
• Discovery of a 0.42-s pulsar in the ultraluminous X-ray source NGC 7793 P13
• Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13
• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• Discovery of pulsations from NGC 300 ULX1 and its fast period evolution
• The small scatter of the baryonic Tully-Fisher relation
• Observational evidence for intermediate-mass black holes
• A Simple Comptonization Model
• Stingray: A Modern Python Library For Spectral Timing
• Groups of Galaxies in the Two Micron All-Sky Redshift Survey
• The ultraluminous state revisited: fractional variability and spectral shape as diagnostics of super-Eddington accretion
• Resolved atomic lines reveal outflows in two ultraluminous X-ray sources
• A 400 solar mass black hole in the Ultraluminous X-ray source M82 X-1 accreting close to its Eddington limit
• On the maximum accretion luminosity of magnetized neutron stars: connecting X-ray pulsars and ultraluminous X-ray sources
• Masses, Beaming and Eddington Ratios in Ultraluminous X-ray Sources
• Black hole accretion discs: reality confronts theory
• Global simulations of axisymmetric radiative black hole accretion disks in general relativity with a sub-grid magnetic dynamo
• The Three Spectral Regimes Found in the Stellar Black Hole XTE J1550--564 in its High/Soft State
• A spectral-timing model for ULXs in the super-critical regime
• A Complete Sample of ULX Host Galaxies
• The ultraluminous X-ray sources NGC 1313 X-1 and X-2: a broadband study with NuSTAR and XMM-Newton
• The discovery of weak coherent pulsations in the ultraluminous X-ray source NGC 1313 X-2
• Propeller effect in action in the ultraluminous accreting magnetar M82 X-2
• Discovery of a 2.8 s pulsar in a 2 d orbit High-Mass X-ray Binary powering the Ultraluminous X-ray source ULX-7 in M51
• Observing the effects of the event horizon in black holes
• 2XMM Ultraluminous X-Ray Source Candidates in Nearby Galaxies
• Evidence for Pulsar-like Emission Components in the Broadband ULX Sample
• Clumpy Outflow from Supercritical Accretion Flows
• X-ray Variability and Hardness of ESO 243-49 HLX-1: Clear Evidence for Spectral State Transitions
• Spectral variability of ultraluminous X-ray sources
• NICER and Fermi GBM Observations of the First Galactic Ultraluminous X-ray Pulsar Swift J0243.6+6124
• Bright radio emission from an ultraluminous stellar-mass microquasar in M31
• From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link
• Chandra X-ray Observations of the Spiral Galaxy M81
• Magnetic field strength of a neutron-star-powered ultraluminous X-ray source
• Broad absorption features in wind-dominated ULXs?
• Diagnosing the accretion flow in ultraluminous X-ray sources using soft X-ray atomic features
• Pulsing ULXs: tip of the iceberg?
• Broadband X-ray Spectra of the Ultraluminous X-ray Source Holmberg IX X-1 observed with NuSTAR, XMM-Newton and Suzaku
• Pulsator-like spectra from Ultraluminous X-ray sources and the search for more ultraluminous pulsars
• ULXs: Neutron Stars vs Black Holes
• Optically thick outflows in ultraluminous supersoft sources
• Optically thick envelopes around ULXs powered by accreating neutron stars
• ULX spectra revisited: Accreting, highly magnetized neutron stars as the engines of ultraluminous X-ray sources
• Investigating slim disk solutions for HLX-1 in ESO 243-49
• Spectra of Black Hole Accretion Models of Ultra-Luminous X-ray Sources
• Formation of Overheated Regions and Truncated Disks around Black Holes; Three-dimensional General Relativistic Radiation-magnetohydrodynamics Simulations
• Spectral States and Evolution of Ultraluminous X-ray Sources
• An Iron K Component to the Ultrafast Outflow in NGC 1313 X-1
• X-ray Binaries
• Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase
• Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in NGC 1313
• Super-Eddington Accretion onto the Neutron Star NGC 7793 P13: Broadband X-ray Spectroscopy and Ultraluminous X-ray Sources
• First evidence for spectral state transitions in the ESO243-49 hyper luminous X-ray source HLX-1
• On the weakness of disc models in bright ULXs
• A 78 Day X-ray Period Detected from NGC 5907 ULX1 by Swift
• A Potential Cyclotron Resonant Scattering Feature in the ULX Pulsar NGC 300 ULX1 seen by NuSTAR and XMM-Newton
• Finite-temperature extension for cold neutron star equations of state
• No magnetars in ULXs
• Identification of the X-Ray Thermal Dominant State in an Ultraluminous X-Ray Source in M82
• Why Is Supercritical Disk Accretion Feasible?
• Spectral and temporal properties of the ultra-luminous X-ray pulsar in M82 from 15 years of Chandra observations and analysis of the pulsed emission using NuSTAR
• Super-Eddington accretion discs with advection and outflows around magnetized neutron stars
• X-ray spectral evolution in M33 X-8
• Pulsing and Non-Pulsing ULXs: the Iceberg Emerges
• A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13
• X-ray spectral states and metallicity in the ultraluminous X-ray sources NGC 1313 X-1 and X-2
• Super-Eddington accretion onto a magnetized neutron star
• NuSTAR, XMM-Newton and Suzaku Observations of the Ultraluminous X-ray Source Holmberg II X-1
• M51 ULX-7: super-orbital periodicity and constraints on the neutron star magnetic field
• Multiwavelength Follow-up of the Hyperluminous Intermediate-mass Black Hole Candidate 3XMM J215022.4-055108
• The Unusual Broadband X-ray Spectral Variability of NGC 1313 X-1 seen with XMM$-$Newton, Chandra and NuSTAR
• Ultra Luminous X-ray Sources: a deeper insight into their spectral evolution
• A Hard X-Ray Study of Ultraluminous X-ray Source NGC 5204 X-1 with NuSTAR and XMM-Newton
• The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1
• Revisiting the Dynamical Case for a Massive Black Hole in IC10 X-1
• XMM-Newton Campaign On Ultraluminous X-ray Source NGC 1313 X-1: Wind vs. State Variability
• On Neutral Absorption and Spectral Evolution in X-ray Binaries
• Pulsating ULXs: large pulsed fraction excludes strong beaming
• Revisiting Putative Cool Accretion Disks in Ultraluminous X-ray Sources
• Nature of the soft ULX in NGC 247: super-Eddington outflow and transition between the supersoft and soft ultraluminous regimes
• A radiation-hydrodynamic model of accretion columns for ultra-luminous X-ray pulsars
• NGC 300 ULX1: A test case for accretion torque theory
• The local Tully-Fisher relation for dwarf galaxies
• A long-term X-ray monitoring of the ultraluminous X-ray source NGC 5408 X-1 with Swift reveals the presence of dips but no orbital period
• All at once: transient pulsations, spin down and a glitch from the Pulsating Ultraluminous X-ray Source M82 X-2
• Revisiting the ultraluminous supersoft source in M101: an optically thick outflow model
• Chandra monitoring observations of the ultraluminous X-ray source NGC 5204 X-1
• Thermal stability of winds driven by radiation pressure in super-Eddington accretion disks
• General relativistic radiation magnetohydrodynamics simulations of supercritical accretion onto magnetized neutron star; -modeling of ultra luminous x-ray pulsars
• X-ray spectral state is not correlated with luminosity in Holmberg II X-1
• On the magnetic field in M51 ULX-8
• Evidence for precession due to supercritical accretion in ultraluminous X-ray sources
• Swift monitoring of M51: A 38-day super-orbital period for the pulsar ULX7 and a new transient ULX
• Supercritical Accretion onto a Non-Magnetized Neutron Star: Why is it Feasible?
• Modeling the light curves of ultraluminous X-ray sources as precession
• The Broadband Spectral Variability of Holmberg IX X-1
• Investigating ULX accretion flows and cyclotron resonance in NGC 300 ULX1
• Theoretical modeling of Comptonized X-ray spectra of super-Eddington accretion flow: origin of hard excess in Ultraluminous X-Ray Sources
• Optically Thick Outflows of Supercritical Accretion Discs: Radiative Diffusion Approach
• A global solution to a slim accretion disk with radiation-driven outflows
• Chandra probes the X-ray variability of M51 ULX-7: evidence of propeller transition and X-ray dips on orbital periods
• The slim-disk state of the ultraluminous X-ray source in M83
• Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1
• Pulsed fraction of super-critical column accretion flows onto neutron stars: modeling of ultraluminous X-ray pulsars

Cited by in corpus (36)

• Ultraluminous X-ray sources
• XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows
• Can nebular HeII emission be explained by ultra-luminous X-ray sources?
• Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X-1 and NGC 5204 X-1
• MUSE spectroscopy of the ULX NGC 1313 X-1: a shock-ionised bubble, an X-ray photoionised nebula, and two supernova remnants
• Statistical study of a large and cleaned sample of ultraluminous and hyperluminous X-ray sources
• The rare X-ray flaring activity of the Ultraluminous X-ray source NGC 4559 X7
• Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2
• Evolution of the spin, spectrum and super-orbital period of the ultraluminous X-ray pulsar M51 ULX7
• Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1
• Bright X-ray pulsars as sources of MeV neutrinos in the sky
• Propeller states in locally super-critical ULXs
• Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925
• On the nature of the ultraluminous X-ray source Holmberg II X-1
• Simultaneous NICER and NuSTAR observations of the Ultraluminous source NGC 4190 ULX-1
• The ultraluminous X-ray source M81 X-6: a weakly magnetised neutron star with a precessing accretion disc?
• Hard X-ray flares and spectral variability in NGC 4395 ULX1
• The High Energy X-ray Probe (HEX-P): Studying Extreme Accretion with Ultraluminous X-ray Sources
• A transient ultraluminous X-ray source in NGC 55
• Multi-wavelength spectroscopic analysis of the ULX Holmberg II X-1 and its nebula suggests the presence of a heavy black hole accreting from a B-type donor
• jaxspec : a fast and robust Python library for X-ray spectral fitting
• Digging a little deeper: characterising three new extreme ULX candidates
• The hunt for new pulsating ultraluminous X-ray sources: a clustering approach
• The restless population of bright X-ray sources of NCG 3621
• Broadband X-ray spectral analysis of the ULX NGC 1313\,X-1 using JeTCAF: Origin of the ULX bubble
• XMM-Newton multi-year campaign on NGC 55 ULX-1: Resolving the wind and its variability with RGS
• The population of hyperluminous X-ray sources as seen by XMM-Newton
• Spectral variability in NGC 1042 ULX1
• Identification of three new ultraluminous X-ray sources in NGC 4631 and NGC 1097: Evidence for stellar-mass black holes
• Exploring the nature of an ultraluminous X-ray source in NGC 628
• Probing spectral variability in NGC 4490 ULX-8 over 24 years of XMM-Newton, Chandra and Swift-XRT observations
• A comparative study of occurrence rates and nature of Ultraluminous X-ray sources in spiral and elliptical galaxies
• Ultraluminous X-ray sources in the first eROSITA survey I. Candidate catalogs
• Unveiling the biconical geometry of the outflow in the ultraluminous X-ray source NGC 5204 X-1
• Spectral and Temporal Properties of Ultra-luminous X-ray Source NGC 55 ULX1
• Ultraluminous X-ray sources in the group-centric elliptical galaxy NGC 5813