Discovery of the Black Hole X-Ray Binary Transient MAXI J1348–630

Abstract

We report the first half-year monitoring of the new Galactic black hole candidate MAXI J1348–630, discovered on 2019 January 26 with the Gas Slit Camera on board the Monitor of All-sky X-ray Image (MAXI). During the monitoring period, the source exhibited two outburst peaks, where the first peak flux (at T  = 14 days from the discovery of T  = 0) was ∼4 Crab (2–20 keV) and the second one (at T  = 132 days) was ∼0.4 Crab (2–20 keV). The source exhibited distinct spectral transitions between the high/soft and low/hard states and an apparent “q”-shape curve on the hardness-intensity diagram, both of which are well-known characteristics of black hole binaries (BHBs). Compared to other bright black hole transients, MAXI J1348–630 is characterized by its low disk temperature (∼0.75 keV at the maximum) and high peak flux in the high/soft state. The low peak temperature leads to a large innermost radius that is identified as the innermost stable circular orbit, determined by the black hole mass and spin. Assuming the empirical relation between the soft-to-hard transition luminosity ( L trans ) and the Eddington luminosity ( L Edd ), L trans / L Edd  ≈ 0.02, and a face-on disk around a non-spinning black hole, the source distance and the black hole mass are estimated to be D  ≈ 4 kpc and , respectively. The black hole is more massive if the disk is inclined and the black hole is spinning. These results suggest that MAXI J1348–630 may host a relatively massive black hole among the known BHBs in our Galaxy.