Insight-HXMT monitoring of the Type-C QPO in the black hole candidate EXO 1846-031

ATel #13037; Yi-Jung Yang, Guangcheng Xiao (IHEP), Roberto Soria (UCAS/NAOC/Uni. of Sydney), Dave Russell (NYU Abu Dhabi), Jinlu Qu, Shuang-Nan Zhang (IHEP), on behalf of the HXMT team
on 17 Aug 2019; 06:05 UT
Credential Certification: Roberto Soria (rsoria@physics.usyd.edu.au)

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The Galactic black hole candidate EXO 1846-031 has been in outburst for the past month, as reported in previous ATels (#12968, #12969, #12976, #12977, #12991, #12992, #13012, #13036).

We started monitoring the outburst with Insight-HXMT on 2019 August 2. EXO 1846-031 is being observed regularly, and can be detected by all three instruments onboard the satellite (LE: 1-15 keV; ME: 5-30 keV; HE: 20-250 keV). In our previous Atel #13036, we reported the spectral analysis results from the observations taken on August 2 (MJD 58697.3416 at mid exposure), August 3 (MJD 58698.1730) and August 8 (MJD 58703.1950). Here, we report on the most important timing results from the same three epochs.

A type-C quasi periodic oscillation (QPO) was detected in all three observations, with stronger significance at higher spectral energies. The central frequency of the QPO was at (0.71 +/- 0.01) Hz on August 2, with an amplitude of (10.3 +/- 1.0)% fractional rms in HE (25-250 keV) and (7.7 +/-1.3)% fractional rms in ME (6-30 keV). On August 3, the QPO frequency increased to (1.54 +/- 0.02) Hz with an amplitude of (11.0 +/- 1.3)% fractional rms in HE and (11.4 +/- 1.4)% fractional rms in ME. A broader feature at lower frequencies was also detected, with a centroid around (0.12 +/-0.02) Hz and (0.20 +/-0.01) Hz on August 2 and 3, respectively. Finally, on August 8, the QPO was still detectable in the HE band and its central frequency had increased to (4.58 +/- 0.10) Hz, with an amplitude of (14.2 +/- 2.8)% fractional rms. The increase in the QPO frequency during the rising phase of the outburst is also a well-known feature of low-mass black hole X-ray binaries.

In summary, as already noted for the X-ray spectral properties (Atel #13036), also the timing properties are typical of the canonical outburst evolution from the hard (or hard-intermediate) to the soft-intermediate state in low-mass black hole X-ray binaries.

The power density spectra in the three epochs are shown here:
"http://www.hxmt.org/images/atel/201908/Atel_EXO1846-031.png"

Insight-HXMT will continue to monitor the source until the end of October (end of the current observing window) or until it fades under its detection threshold.