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 subclass 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 dataset 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 ($k{T}_{\mathrm{bb}}$ $\sim$ 0.1-0.2 keV) black-body, interpreted as emission from the photo-sphere of a radiatively-driven wind, and a hotter ($k{T}_{\mathrm{disk}}$ $\sim$ 0.6 keV) multicolour 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 the 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.

13 pages, accepted by MNRAS on 2021/08/16

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• Evolution of a Relativistic Outflow and X-ray Corona in the Extreme Changing-Look AGN 1ES 1927+654
• Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1
• A new pulsating neutron star in the Ultraluminous X-ray source NGC 4559 X7?
• Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925
• Propeller states in locally super-critical ULXs
• Identification of a Helium Donor Star in NGC 247 ULX-1
• On the nature of the ultraluminous X-ray source Holmberg II X-1
• Ultraluminous X-ray sources with He star companions
• XMM-Newton multi-year campaign on NGC 55 ULX-1: Resolving the wind and its variability with RGS
• Spectral variability in NGC 1042 ULX1
• Probing extreme black-hole outflows on short timescales via high spectral-resolution X-ray imagers
• Spectral and Temporal Properties of Ultra-luminous X-ray Source NGC 55 ULX1