Optical timing observations of MAXI J1820+070 with IFI+IQUEYE and AQUEYE+ soon after state transition

ATel #11936; Luca Zampieri (INAF-Astronomical Observatory of Padova), Michele Fiori (University of Padova), Aleksandr Burtovoi (INAF-Astronomical Observatory of Padova), Giampiero Naletto, Cesare Barbieri, Paolo Ochner, Gabriele Umbriaco (University of Padova), Mauro Barbieri (University of Atacama)
on 11 Aug 2018; 09:15 UT
Credential Certification: Luca Zampieri (luca.zampieri@oapd.inaf.it)

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We are continuing the optical timing monitoring of the black hole transient MAXI J1820+070 (ATel #11723, #11824), in parallel with dedicated photometric and spectroscopic observations (ATel #11899). Here we report further observations with our fast optical photon counters IFI+IQUEYE (mounted at the 1.2 m Galileo telescope) and AQUEYE+ (mounted at the 1.8 m Copernico telescope) in Asiago, taken soon after the reported hard to soft accretion state transition (ATel #11820, #11823, #11827, #11831, #11833, #11855, #11887). Observations were carried out on four nights between: MJD 58317.860313 (Jul 18 20:38:51.0 UT) and 58318.091458 (Jul 19 02:11:42.0 UT), MJD 58318.906753 (Jul 19 21:45:43.5 UT) and 58319.053264 (Jul 20 01:16:42.0 UT), MJD 58322.893250 (Jul 23 21:26:16.8 UT) and 58322.998957 (Jul 23 23:58:29.9 UT), MJD 58327.871594 (Jul 28 20:55:05.7 UT) and 58327.939637 (Jul 28 22:33:04.6 UT). We retained only the observations with better sky conditions. The average background-subtracted rate of MAXI J1820+070 showed night-to-night variations and was in the range 2300-5700 c/s for IFI+IQUEYE and 35700-39000 c/s for AQUEYE+. The sky background was regularly monitored between on-target observations. The fractional root-mean-square (rms) variability of the 1s binned (non background-subtracted) IFI+IQUEYE light curve was between 2.7% and 4.2%, and that of the AQUEYE+ light curve between 2.7% and 3.8%. The two values are slightly larger than those of a nearby reference star pointed in between on-target observations (1.9-3.8% for IFI+IQUEYE and 2.5-3.0% for AQUEYE+). Power spectra of the non-background subtracted light curves with a time bin of 10 ms were computed for each night and were then averaged over intervals of 130s duration. The power spectra are characterized by low frequency broad-band noise components mostly induced by the sky variability. No significant low frequency quasi periodic oscillations (QPOs) were detected in any of the observations. Using the AQUEYE+ acquisitions of July 18-19, that have the highest background-subtracted source rate, we can constrain the fractional rms of a QPO with properties similar to those detected in April and June (centroid ~100 mHz, width ~20 mHz) to be smaller than ~0.5% (95% confidence).