Spectro-polarimetric study to constrain accretion-ejection properties of MCG-5-23-16 using IXPE and NuSTAR observations

Abstract

Abstract We conducted a study on the X-ray polarisation properties of MCG-5-23-16 by analysing long-term monitoring data from NuSTAR jointly with IXPE observations made in May and November 2022. The re-analysis of IXPE data gives model-dependent polarisation degree, PD (%) = $1.08\pm0.66$ in the energy band 2–8 keV, which agrees with previous studies within error bars. The model-independent analysis of PD poses an upper limit of $\leq3.8$ ( $1\sigma$ level) for the same energy band. The observed upper limit of PD, along with broadband spectral analysis (2–79 keV) using an accretion-ejection based model, allowed us to derive the corona geometry (i.e. radius and height) and the accretion disc inclination ( $\sim33^\circ$ ). Additional NuSTAR observations were also analysed to gain insights into the accretion flow properties of the source and to estimate the expected polarisation during those epochs with PD $\sim 4.3\%$ . The radius and height of the corona varies between $28.2\pm3.1 - 39.8\pm4.6$ r $_s$ and $14.3\pm1.7-21.4\pm1.9$ r $_s$ respectively, with a mass outflow rate from the corona measuring $0.14\pm0.03-0.2\pm0.03$ Eddington rate ( $\dot m_{\mathrm{Edd}}$ ). The estimated PD values were nearly constant up to a certain radial distance and height of the corona and then decreased for increasing corona geometry. The spectral analysis further provided an estimate for the mass of the central black hole $\sim2\times 10^7$ M $_\odot$ and the velocity of the outflowing gas $\sim0.16-0.19c$ . A comparative broadband spectral study using reflection-based models estimates the disc inclination between $\sim 31^\circ\pm8^\circ-45^\circ\pm7^\circ$ , and yields an expected PD of 3.4–6.0%. We also found a weak reflection fraction and a less ionised distant reflecting medium. The expected PD measured using accretion-ejection and reflection models is less compared to the expected PD measured for a given disc inclination of $45^\circ$ . Our modelling of the disc-corona-outflows and polarisation connection can be extended and validated with data from the recently launched XPoSat, India’s first X-ray Polarimeter Satellite, offering potential applications to other sources.