# Broad-band spectral study of X-ray transient MAXI J1820+070 using Swift/XRT and NuSTAR

## TL;DR

This study presents a detailed broad-band spectral analysis of the X-ray transient MAXI J1820+070 using simultaneous Swift/XRT and NuSTAR observations, revealing key accretion disk properties and state evolution.

## Contribution

It provides the first simultaneous broad-band spectral modeling of MAXI J1820+070, constraining disk parameters and relativistic reflection features during its hard state.

## Key findings

- Detected relativistically broadened Iron-Kα line and Compton hump.
- Constrained inner disk radius to approximately 4.1 R_ISCO.
- Indicated the source was in a hard, evolving accretion state.

## Abstract

We report on a \textit{NuSTAR} and \textit{Swift}/XRT observation of the newly discovered X-ray transient MAXI J1820+070. \textit{Swift}/XRT and \textit{NuSTAR} have concurrently observed the newly detected source on 14 March 2018. We have simultaneously fitted the broad-band spectra obtained from \textit{Swift}/XRT and \textit{NuSTAR}. The observed joint spectra in the energy range 0.6--78.0 keV are well modeled with a weak disk black-body emission, dominant thermal Comptonization and relativistic reflection fraction. We have detected a fluorescent Iron-K$\alpha$ line relativistically broadened, and a Compton hump at $\sim$ 30 keV. We constrain the inner disk radius as well as the disk inclination angle and their values are found to be 4.1$^{+0.8}_{-0.6}$ R$_{ISCO}$ (where R$_{ISCO}\equiv$ radius of the innermost stable circular orbit) or 5.1$^{+1.0}_{-0.7}$ r$_{g}$ (where r$_{g}\equiv$ gravitational radius) and 29.8$^{+3.0}_{-2.7}$ $^\circ$ respectively. The best fit broad-band spectra suggest that the source was in the hard state and evolving. The source emission is best described by weak thermal emission along with strong thermal Comptonization from a relatively cold, optically thick, geometrically thin and ionized accretion disk. X-ray spectral modeling helps us to understand the accretion and ejection properties in the vicinity of the compact object.