X-ray and Optical Observations of the Black Hole Candidate MAXI J1828-249

TL;DR

This study presents X-ray and optical observations of the black hole candidate MAXI J1828-249 during its 2013 outburst, revealing spectral features and states that inform models of accretion and emission mechanisms.

Contribution

First detailed X-ray and optical spectral analysis of MAXI J1828-249 during outburst, identifying a unique 5-10 keV feature explained by low-temperature Comptonization.

Findings

Detected a 5-10 keV spectral feature not explained by reflection or relativistic effects.

Inferred the source was in an intermediate accretion state.

Optical/UV flux dominated by irradiated outer disk emission.

Abstract

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 ~0.6 keV, and a power-law tail with a photon index of ~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 (<~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.