Thick-disc model to explain the spectral state transition in NGC 247

Guo, Jiajia; Gu, Wei‐Min; Yi, Tuan

doi:10.1093/mnras/stz581

Cited by 10 publications

(11 citation statements)

References 47 publications

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“…A similar picture was proposed by Guo et al (2019) who argued that the ∼100s transitions can be explained by the viscous timescale with the X-ray flux variability driven by accretion rate fluctuations (atṁ 10). However, local fluctuations in theṀ might also cause variations in the winds, which could alter the source appearance (Feng et al 2016).…”

Section: Accretion Disc and Wind Physicssupporting

confidence: 73%

“…The source is observed at a viewing angle that is high enough that the inner disc is already partly obscured by the wind (soft ULXs, left panel). An increase of the accretion rate pushes up the scale-height of the disc and the optically-thick base of the wind, causing an near-total obscuration of the inner regions and the source appears as an ultraluminous supersoft source (ULS, right panel, see also Pinto et al 2017, 2020b, Guo et al 2019.…”

Section: Discussionmentioning

confidence: 99%

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XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows

Pinto

Soria

Walton

et al. 2021

Monthly Notices of the Royal Astronomical Society

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Most ULXs are believed to be powered by super-Eddington accreting neutron stars and, perhaps, black holes. Above the Eddington rate the disc is expected to thicken and to launch powerful winds through radiation pressure. Winds have been recently discovered in several ULXs. However, it is yet unclear whether the thickening of the disc or the wind variability causes the switch between the classical soft and supersoft states observed in some ULXs. In order to understand such phenomenology and the overall super-Eddington mechanism, we undertook a large (800 ks) observing campaign with XMM-Newton to study NGC 247 ULX-1, which shifts between a supersoft and classical soft ULX state. The new observations show unambiguous evidence of a wind in the form of emission and absorption lines from highly-ionised ionic species, with the latter indicating a mildly-relativistic outflow (−0.17c) in line with the detections in other ULXs. Strong dipping activity is observed in the lightcurve and primarily during the brightest observations, which is typical among soft ULXs, and indicates a close relationship between the accretion rate and the appearance of the dips. The latter is likely due to a thickening of the disc scale-height and the wind as shown by a progressively increasing blueshift in the spectral lines.

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Section: Accretion Disc and Wind Physicssupporting

confidence: 73%

Section: Discussionmentioning

confidence: 99%

XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows

Pinto

Soria

Walton

et al. 2021

Monthly Notices of the Royal Astronomical Society

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“…It is likely due to strong reprocessing in an optically thick environment formed at R sph , where disc inflow is mainly inflated by internal radiation pressure. This picture is consistent with the observed low temperature, large radius, and super-Eddington luminosity (see Shen et al 2015;Soria & Kong 2016;Urquhart & Soria 2016;Guo et al 2019). The hotter component, which we fitted using a DISKBB, dominates the emission above 2 keV.…”

Section: Discussionsupporting

confidence: 88%

“…a change in the funnelling angle, where the region responsible for the harder emission become out of sight in SSUL state, which might also be linked to its compactness in the HID). Guo et al (2019) interpreted the SSUL/SUL transition according to the thick-disc model of Gu et al (2016), where the key physical parameter is the changing geometrical configuration of the system. This model also predicts the highest accretion rate in the SSUL regime where the inner regions are more obscured.…”

Section: The Normal Branch: From Ssul To Sulmentioning

confidence: 99%

The Chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1

D’Aí

Pinto

Santo

et al. 2021

Monthly Notices of the Royal Astronomical Society

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Soft Ultra-Luminous X-ray (ULXs) sources are a sub-class of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems, dips have been observed. The mechanism behind this state transition and the dips nature are still debated. To investigate these issues, we obtained a long XMM-Newton monitoring campaign of a member of this class, NGC 247 ULX-1. We computed the hardness-intensity diagram for the whole data-set and identified two different branches: the normal branch and the dipping branch, which we study with four and three hardness-intensity resolved spectra, respectively. All seven spectra are well described by two thermal components: a colder (kTbb ∼ 0.1-0.2 keV) black-body, interpreted as emission from the photo-sphere of a radiatively-driven wind, and a hotter (kTdisk ∼ 0.6 keV) multi-colour disk black-body, likely due to reprocessing of radiation emitted from the innermost regions. In addition, a complex pattern of emission and absorption lines has been taken into account based on previous high-resolution spectroscopic results. We studied the evolution of spectral parameters and flux of the two thermal components along the two branches and discuss two scenarios possibly connecting the state transition and the dipping phenomenon. One is based on geometrical occultation of the emitting regions, the other invokes the onset of a propeller effect.

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“…Stobbart et al 2006;Urquhart & Soria 2016;Alston et al 2021) are not observed in the light curves of Holmberg II X-1 and NGC 5204 X-1, supporting this interpretation. It is thus likely that the dipping activity in ULSs is associated with direct obscuration of the inflated accretion disc as a result of the increased mass-accretion rate (Guo et al 2019). Hence, the increase in mass-accretion rate confers Holmberg II X-1 and NGC 5204 X-1 with a somewhat harder ULS aspect, because the SSUL aspect of these standard ULXs is not due to a high inclination angle, but rather due to the extreme narrowing of the funnel.…”

Section: Discussionmentioning

confidence: 99%

Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X–1 and NGC 5204 X–1

Gúrpide

Godet

Vasilopoulos

et al. 2021

A&A

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Context. Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by stellar-mass compact objects accreting at super-Eddington rates. While the discovery of evolutionary cycles have marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic black hole binaries, their evidence in the super-Eddington regime has so far remained elusive. However, recent circumstantial evidence hinted at the presence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II X–1 and NGC 5204 X–1. Aims. We aim to build on our previous work and exploit the long-term high-cadence monitoring of Swift-XRT in order to provide robust evidence of the evolutionary cycle in these two sources and investigate the main physical parameters inducing their spectral transitions. Methods. We studied the long-term evolution of both sources using hardness-intensity diagrams (HID) and by means of Lomb–Scargle periodograms and Gaussian process modelling to look for periodic variability. We also applied a physically motivated model to the combined Chandra, XMM-Newton, NuSTAR, and Swift-XRT data of each of the source spectral states. Results. We robustly show that both sources follow a clear and recurrent evolutionary pattern in the HID that can be characterised by the hard ultra-luminous (HUL) and soft ultra-luminous (SUL) spectral regimes, and a third state with characteristics similar to the super-soft ultra-luminous (SSUL) state. The transitions between the soft states seem consistent with aperiodic variability, as revealed by a timing analysis of the light curve of Holmberg II X–1; albeit, further investigation is warranted. The light curve of NGC 5204 X–1 shows a stable periodicity on a longer baseline of ∼200 days, possibly associated with the duration of the evolutionary cycle. Conclusions. The similarities between both sources provide strong evidence of both systems hosting the same type of accretor and/or accretion flow geometry. We support a scenario in which the spectral changes from HUL to SUL are due to a periodic increase of the mass-transfer rate and subsequent narrowing of the opening angle of the super-critical funnel. The narrower funnel, combined with stochastic variability imprinted by the wind, might explain the rapid and aperiodic variability responsible for the SUL–SSUL spectral changes. The nature of the longer periodicity of NGC 5204 X–1 remains unclear, and robust determination of the orbital period of these sources could shed light on the nature of the periodic modulation found. Based on the similarities between the two sources, a long periodicity should be detectable in Holmberg II X–1 with future monitoring.

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Thick-disc model to explain the spectral state transition in NGC 247

Cited by 10 publications

References 47 publications

XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows

XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows

The Chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1

Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X–1 and NGC 5204 X–1

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