Further detection of the optical low frequency QPO in the black hole transient MAXI J1820+070

ATel #11591; Wenfei Yu, Jie Lin, Dongming Mao (SHAO), Jujia Zhang (YNAO), Zhen Yan (SHAO), and Jinming Bai (YNAO)
on 1 May 2018; 02:50 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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We report on the optical photometric observation of MAXI J1820+070 with the 2.4m telescope at Lijiang Gaomeigu Station of Yunnan observatories with our Fast Optical Camera (FOC) on April 22, 2018, following the detection of low frequency QPO in the optical band (ATEL #11510).

Photometry observations of MAXI J1820+070 in white light were performed with FOC on April 22 from UT 20:20 for nearly two hours to investigate short-term variability on sub-second time scales, including the low frequency QPO detected in the optical (ATEL #11510). A total of 200+ thousand images were taken with a frame-time of 22.9 milliseconds in a field of view of 1.5 arcmin x 1.5 arcmin. The variation of the optical flux in the optical counterpart of MAXI J1820+070 was calibrated via four brightest nearby reference stars which were simultaneously detected in the field-of-view. Our preliminary analysis shows the optical low frequency QPO was at around 0.157 +- 0.002 Hz with a FWHM of 0.030 +- 0.006 Hz during our FOC observations, consistent with the primary of the low frequency QPO in frequency seen in the X-ray power spectrum of the Swift XRT observation performed in the same period of time around April 22 20:30 (UT). The average fractional rms of the optical low frequency QPO was about (4.1 +- 0.3)%. Our observation of the low frequency QPO is consistent with the frequency evolution of the X-ray QPO as seen with Swift/XRT observations in the past few weeks (see ATEL #11488, #11578). The optical low frequency QPO shown in the FOC observation on April 22 is the highest in frequency among detections of optical, UV or Infrared low frequency QPOs, which were in the frequency range 0.05-0.13 Hz (Motch et al. 1983; Hynes et al. 2003; Durant et al. 2009; Veledina et al. 2015; Kalamkar et al. 2016; Pahari et al. 2017; Yu et al. 2018 in ATEL #11510).

We appreciate the efforts of the Swift PI Brad Cenko and the HXMT PI Shuangnan Zhang for scheduling coordinated observations. The FOC project was initiated in 2009 and the first light was taken on Jan 15, 2010 installed on the 1.56 m telescope in Shanghai. Results of the test runs was presented in EWASS 2015. The FOC is a custom version of the Cambridge fast photon counting systems equipped with an E2V CCD 97. We would like to thank Craig Mackay of Cambridge Univ. and Henk Spruit of MPA for persistent help and encouragement.