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

Abstract

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.

Can nebular He II emission be explained by ultra-luminous X-ray sources?

“…The SEDs of ULXs are relatively poorly known, and they show a large diversity and also variability (e.g. Gladstone et al 2009;Sutton et al 2013a,b;Walton et al 2015;Pintore & Mereghetti 2016;Gúrpide et al 2021a). In general, the X-ray spectra of ULXs is modelled by the sum of two components related to the emission from the accretion flow produced close to the compact object: a multi-colour black body for the accretion disc and a power law representing the coronal emission (Kaaret et al 2017).…”

Simultaneous X-ray/UV observations of ultraluminous X-ray source Holmberg II X-1 with Indian space mission AstroSat

“…2), one can see flux changes with a factor of 2-3 from the mean level at time scale of ∼ 10 ks or even shorter in five data sets (#1, 6, 7, 9, 10). Such a behavior is known for Ho II X-1 (Gúrpide et al 2021, Kajava et al 2012. Kajava et al (2012) note that fast variations of the X-ray flux do not lead to spectral changes.…”

“…). Later it became much harder (2013, Γ ∼ 1.9, Gúrpide et al 2021, Kobayashi et al 2019) and remained so by the time of our AstroSat observations. Moreover, the blue points in Fig.…”

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

“…Swift instead allows us to perform X-ray spectroscopic studies over a long (∼ yearly) baseline with weekly visits, as proved by previous studies (e.g. Kaaret & Feng 2009;Grisé et al 2010;La Parola et al 2015;Brightman et al 2020;Gúrpide et al 2021b).…”

“…The long-term behaviour is reported only for a small fraction of the whole ULX population (e.g. Brightman et al 2019Brightman et al , 2020Gúrpide et al 2021b), containing 1843 ULX candidates according to the recent catalogue of Walton et al 2021. Thus, to enlarge this sample, it is crucial to perform studies of long-term variability, possibly correlated with spectral changes, and to look for periodicities.…”