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

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 ± 0.66 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 ≤ 3.8 (1σ 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 (∼ 33◦). 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 ∼ 4.3%. The radius and height of the corona varies between 28.2 ± 3.1 − 39.8 ± 4.6 rs and 14.3 ± 1.7 − 21.4 ± 1.9 rs respectively, with a mass outflow rate from the corona measuring 0.14 ± 0.03 − 0.2 ± 0.03 Eddington rate (m˙ 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 ∼ 2 × 107 M and the velocity of the outflowing gas ∼ 0.16 − 0.19c. A comparative broadband spectral study using reflectionbased models estimates the disc inclination between ∼ 31◦ ± 8◦ − 45◦ ± 7◦, 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◦. 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.