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

Abstract

We study the multi-wavelength spectral properties of the black hole X-ray binary MAXI J1348$-$630 using quasi-simultaneous $\textit{ALMA}$, $\textit{NICER}$, and $\textit{Swift}$ observations during the decay phase of the January 2019 outburst. In millimeter wavelengths, radio continuum emissions in the frequency range of 89.56$-$351.44~GHz are measured. We found that the flux densities at millimeter wavelength varied between 12.18 mJy and 18.47 mJy with spectral index ($\alpha $) of $0.28\pm 0.02$. 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 $\textit{NICER}$ spectrum is fitted by a combined model of disk blackbody component $(\textit{diskbb})$ along with a comptonization component $(\textit{simpl})$ which explains the power-law continuum caused by the thermal Comptonisation of soft disk photons in a hot gas of electrons. The $\textit{NICER}$ 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 $\textit{ALMA}$ and $\textit{NICER}$ data to understand the source nature by locating the source in the $L_{X}$-$L_{R}$ 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.