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

We report on a NuSTAR and Swift/XRT observation of the newly discovered X-ray transient MAXI J1820+070. Swift/XRT and NuSTAR have concurrently observed the newly detected source on 2018 March 14. We have simultaneously fitted the broad-band spectra obtained from Swift/XRT and NuSTAR. The observed joint spectra in the energy range 0.6-78.0 keV are well modeled with a weak disc black-body emission, dominant thermal Comptonization, and relativistic reflection fraction. We have detected a fluorescent Iron-Kα line relativistically broadened and a Compton hump at ∼30 keV. We constrain the inner disc radius as well as the disc inclination angle, and their values are found to be 4.1^{+0.8}_{-0.6}R_ISCO (where R_ISCO ≡ radius of the innermost stable circular orbit) or 5.1^{+1.0}_{-0.7} r_g (where r_g ≡ gravitational radius) and 29.8^{+3.0}_{-2.7}^°, respectively. The best-fitting 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 disc. X-ray spectral modeling helps us to understand the accretion and ejection properties in the vicinity of the compact object.