TITLE: GCN CIRCULAR NUMBER: 24280 SUBJECT: LIGO/Virgo S190426c: Insight-HXMT/HE observation DATE: 19/04/27 13:10:14 GMT FROM: QiBin Yi at IHEP, HXMT Q.B.Yi, Q. Luo, C. Cai,S. Xiao , C. K. Li, X. B. Li, G. Li, J. Y. Liao, S. L. Xiong, C. Z. Liu, X. F. Li, Z. W. Li, Z. Chang, A. M. Zhang, Y. F. Zhang, X. F. Lu, C. L. Zou (IHEP), Y. J. Jin, Z. Zhang (THU), T. P. Li (IHEP/THU), F. J. Lu, L. M. Song, M. Wu, Y. P. Xu, S. N. Zhang (IHEP), report on behalf of the Insight-HXMT team: Insight-HXMT was taking data normally around the GW trigger time (T0=2019-04-26 15:21:55 UTC). At T0, about 100% of the LIGO localization region was covered by the Insight-HXMT without occultation by the Earth. Within T0 ± 100 s, no significant excess events (SNR > 3 sigma) are found in a search of the Insight-HXMT/HE raw light curves. Assuming the GW counterpart GRB with three typical GRB Band spectral models, two typical duration timescales(1 s, 10 s) from the peak position of the LIGO-Virgo location probability map, the 5-sigma upper-limits fluence (0.2 - 5 MeV, incident energy) are reported below: Band model 1 (alpha=-1.9, beta=-3.7, Ep=70 keV): 1s: 7.4e-07 erg cm^-2 10s: 3.9e-06 erg cm^-2 Band model 2 (alpha=-1.0, beta=-2.3, Ep=230 keV): 1s: 1.0e-06 erg cm^-2 10s: 5.6e-06 erg cm^-2 Band model 3 (alpha=-0.0, beta=-1.5, Ep=1000 keV): 1s: 9.6e-07 erg cm^-2 10s: 5.0e-06 erg cm^-2 Further analysis will be reported in the following circulars. All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 80-800 keV (record energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. Insight-HXMT is the first Chinese space X-ray telescope, which was fundedjointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information could be found at: http://www.hxmt.org.