A Multiwavelength Study of the Hard and Soft States of MAXI J1820+070 During Its 2018 Outburst
Srimanta Banerjee, G. C. Dewangan, C. Knigge, Maria Georganti, P. Gandhi, N. P. S. Mithun, Payaswini Saikia, D. Bhattacharya, D. M. Russell, Fraser Lewis, A. A. Zdziarski
Abstract
Abstract We present a comprehensive multiwavelength spectral analysis of the black hole (BH) X-ray binary MAXI J1820+070 during its 2018 outburst, utilizing AstroSat far-UV, soft X-ray, and hard X-ray data, along with (quasi-)simultaneous optical and X-ray data from the Las Cumbres Observatory and NICER, respectively. In the soft state, we detect soft X-ray and UV/optical excess components over and above the intrinsic accretion disk emission ( kT in ∼ 0.58 keV) and a steep X-ray power-law component. The soft X-ray excess is consistent with a high-temperature blackbody ( kT ∼ 0.79 keV), while the UV/optical excess is described by UV emission lines and two low-temperature blackbody components ( kT ∼ 3.87 and ∼0.75 eV). Employing continuum spectral fitting, we determine the BH spin parameter ( a = 0.77 ± 0.21), using the jet inclination angle of 64° ± 5° and a mass spanning 5–10 M ☉ . In the hard state (HS), we observe a significantly enhanced optical/UV excess component, indicating a stronger reprocessed emission in the outer disk. Broadband X-ray spectroscopy in the HS reveals a two-component corona, each associated with its reflection component, in addition to the disk emission ( kT in ∼ 0.19 keV). The softer coronal component dominates the bolometric X-ray luminosity and produces broader relativistic reflection features, while the harder component gets reflected far from the inner disk, yielding narrow reflection features. Furthermore, our analysis in the HS suggests a substantial truncation of the inner disk (≳51 gravitational radii) and a high disk density (∼10 20 cm −3 ).