Insight-HXMT detection of the hard-to-soft state transition in the black hole binary GX339-4

ATel #14504; Hexin Liu (IHEP); Liang Zhang (Uni. of Southampton); Lian Tao; Yupeng Chen; Jinlu Qu; Shu Zhang; Shuang-nan Zhang on behalf of the HXMT team
on 2 Apr 2021; 10:03 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Liang Zhang (liang.zhang@mail.bnu.edu.cn)

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The black hole X-ray binary GX 339-4 entered a new outburst in January 2021 (ATels #14336, #14351, #14352, #14354, #14367, #14384, #14400, #14419, #14440). Insight-HXMT has monitored the outburst since 2021 February 18 (MJD 59263.16) with a nearly daily cadence.

The results of the broadband spectral analysis (1-150 keV) show that the power-low photon index remains almost constant around ~1.5 during the initial two months of the outburst, suggesting that the source was in the hard state during this period. Starting from MJD 59298.2 (25^th Mar) to MJD 59301.2 (28^th Mar), the 1-150 keV band flux rose rapidly from 1.65E-08 erg/cm^2/s to ~1.15E-06 erg/cm^2/s. The photon index increased from ~1.6 to ~2.3, suggesting that the source was undergoing a rapid hard-to-soft transition.

Type-C quasi periodic oscillations (QPOs) were detected during the bright hard state with frequencies below 1 Hz (~0.14 Hz on 19^th Feb). The central frequency of the QPO increased rapidly from 1.98 +/-0.06 Hz (25^th Mar) to 3.18 +/-0.22 Hz (28^th Mar), with the fractional rms decreasing from 5.3 +/- 0.2% to 4.6 +/- 0.4% in the LE (1-10 keV) band. The fast change of the QPO properties also indicates that the source was under a state transition.

Insight-HXMT is China's first X-ray astronomical satellite. It consists of three collimated telescopes: the High Energy X-ray Telescope (HE, 18 cylindrical NaI(Tl)/CsI(Na) phoswich detectors), the Medium Energy X-ray Telescope (ME, 1728 Si-PIN detectors), and the Low Energy X-ray Telescope (LE, Swept Charge Device (SCD)), with collecting area/energy-range of ~5100 cm^2/20-250 keV, ~952 cm^2/5-30 keV and ~384 cm^2/1-10 keV. Insight-HXMT will continue to monitor GX 339-4 until it fades under its detection threshold.