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We present a spectral study of the black hole candidate MAXI J1348-630 duringits 2019 outburst, based on monitoring observations with Insight-HXMT andSwift. Throughout the outburst, the spectra are well fitted with power-law plusdisk-blackbody components. In the soft-intermediate and soft states, weobserved the canonical relation L ~ T_in^4 between disk luminosity L and peakcolour temperature T_in, with a constant inner radius R_in (traditionallyidentified with the innermost stable circular orbit). At other stages of theoutburst cycle, the behaviour is more unusual, inconsistent with the canonicaloutburst evolution of black hole transients. In particular, during the hardrise, the apparent inner radius is smaller than in the soft state (andincreasing), and the peak colour temperature is higher (and decreasing). Thisanomalous behaviour is found even when we model the spectra withself-consistent Comptonization models, which take into account theup-scattering of photons from the disk component into the power-law component.To explain both those anomalous trends at the same time, we suggest that thehardening factor for the inner disk emission was larger than the canonicalvalue of ~1.7 at the beginning of the outburst. A more physical trend of radiiand temperature evolution requires a hardening factor evolving from ~3.5 at thebeginning of the hard state to ~1.7 in the hard intermediate state. This couldbe evidence that the inner disk was in the process of condensing from the hot,optically thin medium and had not yet reached a sufficiently high optical depthfor its emission spectrum to be described by the standard optically-thick disksolution.

Peculiar disk behaviors of the black hole candidate MAXI J1348-630 in the hard state observed by Insight-HXMT and Swift