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

Abstract

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.

Spectral and Temporal Properties of Ultra-luminous X-ray Source NGC 55 ULX1

“…The X-ray flux dipping episodes have been observed in many ULXs, for example NGC 55 ULX1 (Stobbart et al 2004;Pintore et al 2015), NGC 628 (Liu et al 2005), M94 (Lin et al 2013, NGC 5408 (Pasham & Strohmayer 2013;Grisé et al 2013), NGC 247 (Feng et al 2016;D'Aì et al 2021;Alston et al 2021;Pinto et al 2021), M 51 ULX-7 (Vasilopoulos et al 2021;Hu et al 2021), M51 and M81 (Urquhart & Soria 2016). Significant X-ray dipping behaviour has been reported in NGC 55 ULX1 using the XMM-Newton observations conducted in 2001 (Stobbart et al 2004).…”

“…Although the depth of the dips is not strong as seen in 2001 XMM-Newton observations, we observed the probable dips in different intensity levels, which suggests that the dips are not associated with any particular flux levels. Multiple origins have been proposed for dips in ULXs, which include occultation of obscuring material, propeller scenario and outflows driven by super-Eddington accretion (Stobbart et al 2004;Feng et al 2016;D'Aì et al 2021;Alston et al 2021). However, its origin is still debated.…”

“…However, we cannot completely rule out these interpretations for ULSs. Another interesting feature detected in ULXs is the X-ray flux dipping (Stobbart et al 2004;Liu et al 2005;Lin et al 2013;Pasham & Strohmayer 2013;Feng et al 2016;Urquhart & Soria 2016;D'Aì et al 2021;Alston et al 2021;Hu et al 2021). X-ray dips are not generally periodic and lasted for a few hundreds of seconds to several ks.…”

“…X-ray dips are not generally periodic and lasted for a few hundreds of seconds to several ks. In addition, the dips are often associated with the change in spectral hardness (Lin et al 2013;Feng et al 2016;D'Aì et al 2021) however, such a change is not seen in some ULXs (Pasham & Strohmayer 2013;Hu et al 2021). These dips can be explained by different mechanisms such as the absorption effect by the structures (accretion stream-disc interaction region or the companion star's atmosphere) of the binary system, the geometrical occultation of the emit-ting regions, propeller effect and outflows driven by supercritical accretion (Stobbart et al 2004;Feng et al 2016;D'Aì et al 2021;Alston et al 2021;Hu et al 2021).…”

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

“…Motta et al 2020;Pintore et al 2021) and flux-dips in soft ULXs (e.g. Stobbart et al 2004;Feng et al 2016;Alston et al 2021;Pinto et al 2021;D'Aì et al 2021). Pointed, deep observations with high-throughput telescopes are limited and cannot enable a detailed study of the long-term behaviour.…”