Abstract

Investigation of spectral evolution of four black hole candidates was carried out by using color-color diagram as well as spectral fitting on Swift/XRT data. Newly found candidates, which are classified as low-mass X-ray binary system based on their transient nature, are the focus of our work. We compare their spectral evolutions to that of XTE J1752-223, a transient system and a more convincing black hole candidate whose mass has been determined from spectral-timing correlation scaling. In addition, comparison to Cygnus X-1, a well-known stellar-mass black hole, was done despite its persistent nature. The spectral fitting, by using a combination of thermal disk and non-thermal component model, results in the innermost temperature values in the range of the typical innermost temperature of black hole binary which is 0.7 – 1.5 keV. The spectral evolutions of the candidates bear a resemblance to both Cygnus X-1 and XTE J1752-223. We note that during Swift/XRT observations, the spectra of Cygnus X-1 and IGR J17451-3022 are mostly dominated by the non- thermal component. We conclude that the compact object of MAXI J1535- 571 and MAXI J1828-249 is highly likely to be a black hole. However, the lack of data rendered conclusive result impossible for IGR J17454-2919.

Highlights

  • Direct evidence of astrophysical black hole remains difficult to obtain even after the first image of black hole’s shadow, which happened to be a supermassive black hole in the center of M87 [1], has been obtained by Event Horizon Telescope (EHT) team

  • We found that the majority of data of each candidate reveal that the innermost temperatures are in the range of black hole binary, which is between 0.5 to 1.5 keV

  • MAXI J1828-249, IGR J17454-2919, and IGR J17451-3022 do not show full spectral evolution tracks, we cannot rule out the possibility that MAXI J1828-249 and both IGR objects follow the spectral evolution track of Cyg X-1 and XTE J1752-223

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Summary

Introduction

Direct evidence of astrophysical black hole remains difficult to obtain even after the first image of black hole’s shadow , which happened to be a supermassive black hole in the center of M87 [1], has been obtained by Event Horizon Telescope (EHT) team. Their extremely compact dimension give rise to much better opportunities to study their variability. Many well-known stellar-mass black hole candidates are discovered in X-ray binaries. Done & Gierlinski (2003) have shown that spectral evolution of black hole can be distinguished from that of neutron star in X-ray binary systems.

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