J/A+A/649/A104ivo://CDS.VizieR/J/A+A/649/A104doi:10.26093/cds/vizier.36490104CDSGurpide A.Godet O.Koliopanos F.Webb N.Olive J.-F.2021-09-17T07:29:33Z2021-05-21T13:04:54ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frx-ray-binary-starsThe discovery of pulsations in several ultraluminous X-ray sources (ULXs) has demonstrated that a fraction of them are powered by super-Eddington accretion onto neutron stars (NSs). This has raised questions regarding the NS to black hole (BH) ratio within the ULX population and the physical mechanism that allows ULXs to reach luminosities well in excess of their Eddington luminosity. Is this latter the presence of strong magnetic fields or rather the presence of strong outflows that collimate the emission towards the observer? In order to distinguish between these scenarios, namely, supercritically accreting BHs, weakly magnetised NSs, or strongly magnetised NSs, we study the long-term X-ray spectral evolution of a sample of 17 ULXs with good long-term coverage, 6 of which are known to host NSs. At the same time, this study serves as a baseline to identify potential new NS-ULX candidates. We combine archival data from Chandra, XMM-Newton, and NuSTAR observatories in order to sample a wide range of spectral states for each source. We track the evolution of each source in a hardness-luminosity diagram (HLD) in order to identify spectral changes, and show that these can be used to constrain the accretion flow geometry, and in some cases the nature of the accretor. 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 of NGC 1313 X-2. For most softer sources with an unknown accretor, we identify the presence of three markedly different spectral states, which we interpret by 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 (>~10keV) in NGC 1313 X-1 and Holmberg IX X-1, which we argue may offer a means to differentiate BH-ULXs from NS-ULXs. We support a scenario in which the hardest sources in our sample might be powered by strongly magnetised NSs, meaning that the high-energy emission is dominated by the hard direct emission from the accretion column. Instead, 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.2021A&A...649A.104Ghttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/649/A104CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/649/A104https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/649/A104http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/649/A104https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/649/A104https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/649/A104http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/649/A104GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/649/A104/table1 (sources selected for this study with their respective distance)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/649/A104/table1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/649/A104/table1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/649/A104/table1?GETtext/xml+votable180.050000true124.870791770.70538890.0055555555555555566/3506 7606 7799 11127 11375 16441 33193 35536 41586 43420https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/649/A104?format=asciidefaultJ/A+A/649/A104/table1sources selected for this study with their respective distanceSimbadNameSimbad column added by the CDSmeta.idcharNameNamemeta.id;meta.maincharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintn_Name[P NS] Note on Name (1)meta.notecharDistDistanceMpcpos.distancefloatr_DistDistance reference (2)meta.ref;pos.framecharNHGalactic absorption (3)cm**-2phys.absorption.galdoubleEpochEpoch number (G1)meta.idcharTelTelescope of the observationinstr.tel;obscharInstInstruments considered, indicates that a certain instrument was not used, Inst1/Inst2/Inst3 (4)instr.setupcharDateDate of the observationtime.epochnullableObsIDObservation idmeta.id;obsExp1? Good exposure time for Inst1kstime.duration;obs.exposurefloatnullableExp2? Good exposure time for Inst2kstime.duration;obs.exposurefloatnullableExp3? Good exposure time for Inst3kstime.duration;obs.exposurefloatnullable_RARight Ascension (J2000) from SIMBAD (not part of the original data)degpos.eq.ra;meta.mainnullable_DEDeclination (J2000) from SIMBAD (not part of the original data)degpos.eq.dec;meta.mainnullableJ/A+A/649/A104/table2Best-fit spectral fit parameters )NameNamemeta.id;meta.maincharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintEpochEpoch number (G1) (1)meta.idcharnHExtragalactic neutral hydrogen absorption column1e+20cm**-2phys.absorptionfloatE_nH? Error on nH (upper value) (2)1e+20cm**-2stat.error;stat.maxfloatnullablee_nH? Error on nH (lower value) (2)1e+20cm**-2stat.errorfloatnullablen_nH[b] Note on nH (3)meta.notecharkTsoftTemperature of the soft diskbbkeVphys.temperaturefloatE_kTsoft? Temperature of the soft diskbb (2)keVstat.error;stat.maxfloatnullablen_E_kTsoft[a] Note on kTsoft (3)meta.notechare_kTsoftTemperature of the soft diskbb (2)keVstat.errorfloatNormsoftNormalization of the soft diskbbstat.fit.paramfloatE_NormsoftNormalization of the soft diskbb (2)stat.error;stat.maxfloate_NormsoftNormalization of the soft diskbb (2)stat.errorfloatkThardTemperature of the hard diskbbkeVphys.temperaturefloatE_kThard? Temperature of the hard diskbb (2)keVstat.error;stat.maxfloatnullablen_E_kThard[a] Note on kThard (3)meta.notechare_kThardTemperature of the hard diskbb (2)keVstat.errorfloatNormhardNormalization of the hard diskbbstat.fit.paramfloatE_NormhardNormalization of the hard diskbb (2)stat.error;stat.maxdoublee_NormhardNormalization of the hard diskbb (2)stat.errorfloatGamma? Power law index of the simpl modelmeta.code;spect.index;meta.modelledfloatnullableE_Gamma? Power law index of the simpl model (2)stat.error;stat.maxfloatnullablee_Gamma? Power law index of the simpl model (2)stat.errorfloatnullablen_Gamma[b] Note on Gamma (3)meta.notecharfscatt? Fraction of scattered photons of the simpl model%stat.paramdoublenullableE_fscatt? Fraction of scattered photons of the simpl model (2)%stat.error;stat.maxintnullablen_E_fscatt[a] Note on fE_scatt (3)meta.notechare_fscatt? Fraction of scattered photons of the simpl model (2)%stat.errorintnullablen_e_fscatt[a] Note on e_fscatt (3)meta.notecharchirReduced chi squarestat.fit.chi2floatdofDegrees of freedomstat.fit.dofintJ/A+A/649/A104/table3*Estimated unabsorbed luminosities for each epoch using the distances from Table 1 and models from Table 2recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameNamemeta.id;meta.maincharEpochEpoch number (G1)meta.idcharLsoftUnabsorbed luminosity in the 0.3-1.5keV band1e+32Wphys.luminosityfloatE_LsoftError on Lsoft (upper value)1e+32Wstat.error;stat.maxfloate_LsoftError on Lsoft (lower value)1e+32Wstat.errorfloatLhardUnabsorbed luminosity in the 1.5-10keV band1e+32Wphys.luminosityfloatE_LhardError on Lhard (upper value)1e+32Wstat.error;stat.maxfloate_LhardError on Lhard (lower value)1e+32Wstat.errorfloatLtotUnabsorbed luminosity in the 0.3-10keV band1e+32Wphys.luminosityfloatE_LtotError on Ltot (upper value)1e+32Wstat.error;stat.maxfloate_LtotError on Ltot (lower value)1e+32Wstat.errorfloatLsoftdbbLuminosity of the soft diskbb component1e+32Wphys.luminosity;em.X-rayfloatE_LsoftdbbError on Lsoftdbb (upper value)1e+32Wstat.error;stat.maxfloate_LsoftdbbError on Lsoftdbb (lower value)1e+32Wstat.errorfloatLharddbbLuminosity of the hard diskbb component1e+32Wphys.luminosity;em.X-rayfloatE_LharddbbError on Lharddbb (upper value)1e+32Wstat.error;stat.maxfloate_LharddbbError on Lharddbb (upper value)1e+32Wstat.errorfloatLsimpl? Unabsorbed luminosity of the simpl*diskbb_hard model component (Flux luminosity for entire component)1e+32Wphys.luminosity;em.X-rayfloatnullableE_Lsimpl? Error on Lsimpl (upper value)1e+32Wstat.error;stat.maxfloatnullablee_Lsimpl? Error on Lsimpl (lower value)1e+32Wstat.errorfloatnullableJ/A+A/649/A104/table7Results of the F-test between (diskbb+ diskbb) (model 1) and (diskbb+ simpl*diskbb) (model 2) modelsNameNamemeta.id;meta.maincharEpochEpoch number (G1)meta.idcharchir2Reduced chi square of model 2 (diskbb+simpl*diskbb)stat.fit.chi2floatrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintchir1Reduced chi square of model 1 (diskbb+diskbb)stat.fit.chi2floatdof1Degrees of freedom of model 1 (diskbb+diskbb)stat.fit.dofintdof2Degrees of freedom of model 2 (diskbb+simpl*diskbb)stat.fit.dofintDeltachi? Delta chi improvement between model 1 and model 2stat.fit.chi2floatnullablePrej? Rejection probability of model 1 as given by the F-test%statfloatnullablesimpl?[yes/no ] indicating if the simpl model was usedmeta.codecharn_simpl?[cd] Note on simpl? (1)meta.notechar