Simultaneous NICER and NuSTAR observations of the ultraluminous source NGC 4190 ULX-1

Combi, Jorge A.; Fogantini, Federico A.; Saavedra, Enzo A.; Romero, Gustavo E.; Abaroa, Leandro; García, Federico; Luque-Escamilla, Pedro; Martí, Josep; Cruz-Sanchez, Nelson
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Astronomy and Astrophysics

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Aims: We present an X-ray analysis of three different XMM-Newton observations together with simultaneous NICER and NuSTAR observations of the ultraluminous X-ray source NGC 4190 ULX-1. Our goal is to constrain the structure of the accretion disk and the geometrical properties of the source.
Methods: We performed temporal and spectral analyses in the 0.4−30 keV energy range in which the source is significantly detected in dedicated XMM-Newton, NICER, and NuSTAR observations.
Results: The temporal analysis shows no flaring activity in the light curves. No pulsation is detected throughout. The source exhibits a typical ULX spectrum, which can be fitted with two thermal blackbody components plus a Comptonization tail at high energies. The luminosity-temperature (L − T) relation of each thermal spectral component is consistent with the L ∝ T2 relation expected from an advection-dominated supercritical disk.
Conclusions: We interpret these results as a super-Eddington accreting black hole seen almost face-on. A dense wind ejected from the disk obscures the central source, and a hot electron plasma is evacuated through the funnel formed above the hole. Geometric beaming is responsible for the ULX soft emission, whereas the hard tail is the result of the Comptonization of soft photons by the electrons ejected through the funnel.