We report results from X-ray and optical observations of the Galactic black hole candidate MAXI J1828 - 249 , performed with Suzaku and the Kanata telescope around the X-ray flux peak in the 2013 outburst .
The time-averaged X-ray spectrum covering 0.6–168 keV was approximately characterized by a strong multi-color disk blackbody component with an inner disk temperature of \sim 0.6 keV , and a power-law tail with a photon index of \sim 2.0 .
We detected an additional structure at 5–10 keV , which can be modelled neither with X-ray reflection on the disk , nor relativistic broadening of the disk emission .
Instead , it was successfully reproduced with a Comptonization of disk photons by thermal electrons with a relatively low temperature ( \lesssim 10 keV ) .
We infer that the source was in the intermediate state , considering its long-term trend in the hardness intensity diagram , the strength of the spectral power-law tail , and its variability properties .
The low-temperature Comptonization component could be produced in a boundary region between the truncated standard disk and the hot inner flow , or a Comptonizing region that somehow developed above the disk surface .
The multi-wavelength spectral energy distribution suggests that the optical and UV fluxes were dominated by irradiated outer disk emission .