2021MNRAS.505..713J

We present broad-band X-ray spectral-timing analysis of the new Galactic X-ray transient MAXI J1348-630 using five simultaneous AstroSat and NICER observations. Spectral analysis using AstroSat data identify the source to be in the soft state for the first three observations and in a faint and bright hard state for the next two. Quasi-periodic oscillations at ∼0.9 and ∼6.9 Hz, belonging to the type-C and type-A class are detected. In the soft state, the power density spectra are substantially lower (by a factor >5) for the NICER (0.5-12 keV) band compared to the AstroSat/LAXPC (3-80 keV) one, confirming that the disc is significantly less variable than the Comptonization component. For the first time, energy-dependent fractional rms and time lag in the 0.5-80 keV energy band was measured at different Fourier frequencies, using the bright hard state observation. Hard time lag is detected for the bright hard state, while the faint one shows evidence for soft lag. A single-zone propagation model fits the LAXPC results in the energy band 3-80 keV with parameters similar to those obtained for Cygnus X-1 and MAXI J1820+070. Extending the model to lower energies, reveals qualitative similarities but having quantitative differences with the NICER results. These discrepancies could be because the NICER and AstroSat data are not strictly simultaneous and because the simple propagation model does not take into account disc emission. The results highlight the need for more joint coordinated observations of such systems by NICER and AstroSat.