Detection of optical and X-ray QPOs at similar frequencies in MAXI J1820+070

ATel #11510; Wenfei Yu (SHAO), Jujia Zhang (YNAO), Zhen Yan (SHAO), Xiaofeng Wang (THU) and Jinming Bai (YNAO)
on 6 Apr 2018; 22:18 UT
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Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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We report the V-band photometric observation of MAXI J1820+070 (also named ASASSN-18ey, see ATELs #11399, #11400, #11403, #11404, #11406, #11418, #11420, #11421, #11423, #11424, #11425, #11426, #11432, #11437, #11439, #11440, #11445, #11451, #11478, #11481) with the Lijiang 2.4m telescope (+YFOSC) at Lijiang Gaomeigu Station of Yunnan observatories, in comparison with the Swift/XRT observation of MAXI J1820+070 on the same day.

Optical photometry observations of the source were performed on March 30th, April 1st, and April 4th. The optical observation from UT 19:53:35 to 20:19:45 on April 1st 2018 is the only observation among three with enough exposure and short frame time to investigate variability on time scales of seconds or longer. The exposure time of each image was set to 3 seconds, and the readout time of each image was about 3.5 second. A total of 240 images were obtained during the observation of about 1570 seconds. The average time resolution is then around 6.5-6.6 seconds. The variation of the V-band optical flux of this transient was measured via 12 local reference stars. The optical light curve shows variability as compared with the reference stars. An average Fourier power spectrum was generated in segments of 64 images each. We found a quasi-periodic oscillation at around 49.53 +-0.5 mHz with a FWHM of 4.6 +- 1.9 mHz (the power spectrum was modeled as a Lorentzian or a zero-centered Lorentzian plus a Lorentzian - we obtained similar results). The average fractional rms is about 3.2% with a significance of around 4.7 sigma. An investigation of the individual power spectra suggested that the signal persisted in most of the segments. The mHz optical QPO might be different from those reported in ATEL #11426, due to its lower fractional amplitude and shorter period, but consistent with the evolution of the X-ray QPO as seen with Swift/XRT observations in the past few weeks.

It is interesting to compare optical variability to its X-ray variability, as suggested by ATEL #11432. There was a Swift/XRT observation taken at 09:35:04 on April 1st, 2018 (ObsID 00010627021), which is close the our optical observation on the same day. The XRT was used in the Windowed-Timing mode with exposure time 932.34 seconds. We generated an average Fourier power spectrum from the event mode data. The average X-ray power spectrum was modeled with two zero-centered Lorentzians plus a QPO at 58.04 +- 3.97 mHz with a FWHM of 35.32 +- 15.66 mHz. The fractional rms amplitude was around 13%. The X-ray QPO is likely the low frequency C-type QPO seen in the rising phase of black hole transient outburst, although we still need more data to confirm its origin with more monitoring towards possible spectral transition. The optical QPO we found is at a frequency remarkably close to the X-ray QPOs seen in the Swift/XRT observation within a day, and we suspect that they are related, as inferred from the previous report of correlated optical/X-ray variability. Black hole low frequency QPO has been proposed as originated from the Lense-Thirring precession of the innermost accretion flow (see Ingram, Done & Fragile 2009, 397, L101). However, the smaller FWHM of the optical QPO as compared with that of the X-ray QPO raises questions about whether and how the two are related. The higher coherence of the optical QPO hints itself is not a reprocessed signal as compared with the X-ray QPO. Simultaneous multi-wavelength timing observations are therefore highly encouraged.

We would like to thank the Swift team and other colleagues for providing continuous Swift monitoring observations of MAXI J1820+070 to the public.