The rare X-ray flaring activity of the Ultraluminous X-ray source NGC 4559 X7

TL;DR

This study presents the first long-term analysis of the ultraluminous X-ray source NGC 4559 X7, revealing unusual flaring activity, spectral variability, and outflow signatures, supporting a super-Eddington accretion model involving a stellar mass compact object.

Contribution

It provides the first comprehensive long-term light curve and spectral analysis of NGC 4559 X7, highlighting its flaring activity and outflow features, advancing understanding of ULX accretion mechanisms.

Findings

Detected flux variations up to an order of magnitude.

Observed spectral hardening during flares and softening away from flares.

Identified narrow absorption and emission lines indicating outflows.

Abstract

Ultraluminous X-ray sources are considered amongst the most extremely accreting objects in the local Universe. The recent discoveries of pulsating neutron stars in ULXs strengthened the scenario of highly super-Eddington accretion mechanisms on stellar mass compact objects. In this work, we present the first long-term light curve of the source NGC 4559 X7 using all the available Swift, XMM-Newton, Chandra and NuSTAR data. Thanks to the high quality 2019 XMM-Newton and NuSTAR observations, we investigated in an unprecedented way the spectral and temporal properties of NGC 4559 X7. The source displayed flux variations of up to an order of magnitude and an unusual flaring activity. We modelled the spectra from NGC 4559 X7 with a combination of two thermal components, testing also the addition of a further high energy cut-off powerlaw. We observed a spectral hardening associated with a luminosity increase during the flares, and a spectral softening in the epochs far from the flares. Narrow absorption and emission lines were also found in the RGS spectra, suggesting the presence of an outflow. Furthermore, we measured hard and (weak) soft lags with magnitudes of a few hundreds of seconds whose origin is possibly be due to the accretion flow. We interpret the source properties in terms of a super-Eddington accretion scenario assuming the compact object is either a light stellar mass black hole or a neutron star.