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| Paper: |
Tracing disk instability in MAXI J1820+070 with MAXI/GSC, Swift/XRT, and NuSTAR |
| Volume: |
536, The Twelfth Pacific Rim Conference on Stellar Astrophysics |
| Page: |
91 |
| Authors: |
Saraswati, T. A.; Fadhilah, A. G. N.; Vierdayanti, K.; Premadi, P. W. |
| Abstract: |
We analyze MAXI J1820+070 spectral and light curve data from Swift/XRT, NuSTAR, and MAXI/GSC during the 2018 outburst and the subsequent 2019-2020 mini-outbursts to trace the signs of spectral transition that indicate the presence of instability on the disk surrounding the black hole. Swift/XRT and NuSTAR spectral data both complement and agree with each other, showing hard-to-soft transition only in the second peak of the main outburst. The first peak and mini-outbursts were dominated by a non-thermal component. Measurement of light curve parameters and morphology shows a major similarity with outbursts from GRO J1655-40 (2005) and GX 339-4 (2002), suggesting that a similar process could have governed these phenomena. The main outburst peaked at 4.6 Crab intensity and radiated a total energy of 9.3 × 1044 erg, and lasted for 199 days. It was then followed by the first mini-outburst, which occurred 160 days later. The quiescent period between outburst and mini-outburst is a lot shorter than the typical viscous diffusion timescale for black hole binaries (∼1,000 days), suggesting it could still be part of the main outburst cycle. Similar patterns were also found in the 1998-2003 outburst-mini-outburst event of XTE J1550-564 despite the rarity of this event. From the results and comparison to notable outburst events, limit-cycle instability in the accretion disk is evident. We tend to agree with the disk instability model for black hole binary as it predicts the spectral evolution, double-peak light curve, and mini-outburst. However, the parameter values need further constraints for an accurate explanation of the physical processes. |
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