Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2

Robba, A.; Pinto, C.; Walton, D. J.; Soria, Roberto; Kosec, P.; Pintore, Fabio; Roberts, T. P.; Alston, W. N.; Middleton, M.; Cusumano, G; Earnshaw, Hannah P.; Fürst, Felix; Sathyaprakash, R.; Kyritsis, E.; Fabian, A. C.

doi:10.1051/0004-6361/202140884

Cited by 15 publications

(16 citation statements)

References 64 publications

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“…In the brighter (𝐿 𝑋 up to 10 40 erg/s) and pulsating source NGC 1313 ULX-2 there is a clear anti-correlation between the bolometric luminosity and the temperature of the cool blackbody-like component (Robba et al 2021). In that source the trend is in agreement with the L ∝ 𝑇 −4 relationship predicted by X-ray emission from a wind photosphere rather than a disc (e.g.…”

Section: Comparison With Other Ulxssupporting

confidence: 78%

“…Therefore, whilst fitting the spectra of these six short exposures we preferred to fix the 𝑁 𝐻 to the average value obtained in the latter observations as previously done in e.g. Robba et al (2021) for NGC 1313 ULX-2. Parameter units: 𝐸 1 0 and 𝐸 2 0 (in keV unit) refer to the centroids of the first and second gaussian, respectively.…”

Section: Spectral Modelling Of Individual Observationsmentioning

confidence: 99%

“…Although the best-fit values of temperatures might not exactly correspond to gas temperatures due to the unknown structure of the emitting regions (because we have modelled a thick disk with a thin disk template), it is still very useful to compare them with the bolometric luminosities of the blackbody components. This is indeed a commonly adopted procedure to understand the behaviour of the thermal components and, in particular, to place some constraints on the disc structure and accretion regime, possibly providing some information on the nature of the compact object, especially for not too high accretion rates or luminosities (𝐿 BOL ∼ 10 39 erg/s, e.g., Urquhart & Soria 2016;Earnshaw & Roberts 2017;Walton et al 2020;Gúrpide et al 2021;Robba et al 2021;D'Aì et al 2021).…”

Section: Comparison With Disc Modelsmentioning

confidence: 99%

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Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1

Barra¹,

Pinto²,

Walton³

et al. 2022

Preprint

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Ultraluminous X-ray sources (ULXs) are the most extreme among X-ray binaries in which the compact object, a neutron star or a black hole, accretes matter from the companion star and exceeds a luminosity of 10 39 erg s −1 in the X-ray energy band alone. Despite two decades of studies, it is still not clear whether ULX spectral transitions are due to stochastic variability in the wind or variations in the accretion rate or in the source geometry. The compact object is also unknown for most ULXs. In order to place constraints onto such scenarios and on the structure of the accretion disc, we studied the temporal evolution of the spectral components of the variable source NGC 55 ULX-1. Using recent and archival data obtained with the XMM-Newton satellite, we modelled the spectra with two blackbody components which we interpret as thermal emission from the inner accretion flow and the regions around or beyond the spherization radius. The luminosity-temperature (L-T) relation of each spectral component agrees with the L ∝ T 4 relationship expected from a thin disc model, which suggests that the accretion rate is close to the Eddington limit. However, there are some small deviations at the highest luminosities, possibly due to an expansion of the disc and a contribution from the wind at higher accretion rates. Assuming that such deviations are due to the crossing of the Eddington or supercritical accretion rate, we estimate a compact object mass of 6-14𝑀 , favouring a stellar-mass black hole as the accretor.

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Section: Comparison With Other Ulxssupporting

confidence: 78%

Section: Spectral Modelling Of Individual Observationsmentioning

confidence: 99%

Section: Comparison With Disc Modelsmentioning

confidence: 99%

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Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1

Barra¹,

Pinto²,

Walton³

et al. 2022

Preprint

Self Cite

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“…The fractional variability has been often used to study the variability properties of ULXs within individual observations, typically considering timescales of minutes -hours (e.g. Sutton et al 2013;Pintore et al 2020;Mondal et al 2021;Robba et al 2021). Here, we used this estimator on longer timescales (using 6 days bins), applying it to sparse sampled lightcurves, using the 'ensemble' approach proposed in Allevato et al (2013), as explained in section 2.7.…”

Section: Temporal Variabilitymentioning

confidence: 99%

Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925

Salvaggio,

Wolter,

Pintore

et al. 2022

Preprint

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Variability is a powerful tool to investigate properties of X-ray binaries (XRB), in particular for Ultraluminous X-ray sources (ULXs) that are mainly detected in the X-ray band. For most ULXs the nature of the accretor is unknown, although a few ULXs have been confirmed to be accreting at super-Eddington rates onto a neutron star (NS). Monitoring these sources is particularly useful both to detect transients and to derive periodicities, linked to orbital and super-orbital modulations. Here we present the results of our monitoring campaign of the galaxy NGC 925, performed with the Neil Gehrels Swift Observatory. We also include archival and literature data obtained with Chandra, XMM-Newton and NuSTAR. We have studied spectra, light-curves and variability properties on days to months time-scales. All the three ULXs detected in this galaxy show flux variability. ULX-1 is one of the most luminous ULXs known, since only 10% of the ULXs exceed a luminosity of ∼5×10 40 erg s −1 , but despite its high flux variability we found only weak spectral variability. We classify it as in a hard ultraluminous regime of super-Eddington accretion. ULX-2 and ULX-3 are less luminous but also variable in flux and possibly also in spectral shape. We classify them as in between the hard and the soft ultraluminous regimes. ULX-3 is a transient source: by applying a Lomb-Scargle algorithm we derive a periodicity of ∼ 126 d, which could be associated with an orbital or super-orbital origin.

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“…We note that to fully understand this, exploration of the luminosity-temperature plane (e.g. Robba et al 2021) via a continual monitoring in X-rays and at other wavelengths is necessary, rather than the two snapshots we have obtained; however we will proceed with this caveat in mind.…”

Section: The Big Picturementioning

confidence: 99%

A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2

Sathyaprakash,

Roberts,

Grisè

et al. 2022

Preprint

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NGC 1313 X-2 is one of the few known pulsating ultraluminous X-ray sources (PULXs), and so is thought to contain a neutron star that accretes at highly super-Eddington rates. However, the physics of this accretion remains to be determined. Here we report the results of two simultaneous XMM-Newton and HST observations of this PULX taken to observe two distinct X-ray behaviours as defined from its Swift light curve. We find that the X-ray spectrum of the PULX is best described by the hard ultraluminous (HUL) regime during the observation taken in the lower flux, lower variability amplitude behaviour; its spectrum changes to a broadened disc during the higher flux, higher variability amplitude epoch. However, we see no accompanying changes in the optical/UV fluxes, with the only difference being a reduction in flux in the near-IR as the X-ray flux increased. We attempt to fit irradiation models to explain the UV/optical/IR fluxes but they fail to provide meaningful constraints. Instead, a physical model for the system leads us to conclude that the optical light is dominated by a companion O/B star, albeit with an IR excess that may be indicative of a jet. We discuss how these results may be consistent with the precession of the inner regions of the accretion disc leading to changes in the observed X-ray properties, but not the optical, and whether we should expect to observe reprocessed emission from ULXs.

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Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2

Cited by 15 publications

References 64 publications

Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1

Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1

Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925

A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2

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