Journal
ASTROPHYSICAL JOURNAL
Volume 896, Issue 1, Pages -Publisher
IOP Publishing Ltd
DOI: 10.3847/1538-4357/ab8db4
Keywords
Black holes; Compact objects; Low-mass x-ray binary stars
Categories
Funding
- China National Space Administration (CNSA)
- Chinese Academy of Sciences (CAS)
- National Program on Key Research and Development Project from the Minister of Science and Technology of China (MOST) [2016YFA0400800]
- Strategic Priority Research Program of the Chinese Academy of Sciences [XDB23040400]
- National Natural Science Foundation of China [11673023, 11733009, 11603037, 11973052, U1838201, U1838115, U1938103, U1838202]
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We study the evolution of the temporal properties of MAXI J1820+070 during the 2018 outburst in its hard state from MJD 58,190 to 58,289 with Insight-HXMT in a broad energy band 1-150 keV. We find different behaviors of the hardness ratio, the fractional rms and time lag before and after MJD 58,257, suggesting a transition occurred around this point. The observed time lags between the soft photons in the 1-5 keV band and the hard photons in higher energy bands, up to 150 keV, are frequency-dependent: the time lags in the low-frequency range, 2-10 mHz, are both soft and hard lags with a timescale of dozens of seconds but without a clear trend along the outburst; the time lags in the high-frequency range, 1-10 Hz, are only hard lags with a timescale of tens of milliseconds; they first increase until around MJD 58,257 and decrease after this date. The high-frequency time lags are significantly correlated to the photon index derived from the fit to the quasi-simultaneous NICER spectrum in the 1-10 keV band. This result is qualitatively consistent with a model in which the high-frequency time lags are produced by Comptonization in a jet.
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