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
Summary
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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