Multi-wavelength observation of MAXI J1348–630 during the outburst in 2019

We study the multi-wavelength spectral properties of the black hole X-ray binary MAXI J1348–630 using quasi-simultaneous ALMA, NICER, and Swift observations during the decay phase of the January 2019 outburst. In millimeter wavelengths, radio continuum emissions in the frequency range of 89.56–351.44 GHz are measured. We found that the flux densities at millimeter wavelength varied between 12.18 mJy and 18.47 mJy with spectral index (α) of 0.28±0.02. The broadband spectrum suggests that the source was accompanied by weak synchrotron emission from the compact jets. Broadband spectral study indicates that MAXI J1348–630 falls in the regime of "radio-quiet" during the decay phase of the outburst. The NICER spectrum is fitted by a combined model of disk blackbody component (diskbb) along with a comptonization component (simpl) which explains the power-law continuum caused by the thermal Comptonisation of soft disk photons in a hot gas of electrons. The NICER spectrum is dominated by the comptonised components during the decay phase of the outburst close to the hard state of the source. We have investigated the correlation between X-ray and radio luminosity using quasi-simultaneous ALMA and NICER data to understand the source nature by locating the source in the LX-LR diagram. The correlation study of radio/X-ray luminosity suggests that MAXI J1348–630 did not follow the well-known track for black holes and it is a new member of a restricted group of sources.