Low-frequency optical QPO in MAXI J1820+070 detected with IFI+IQUEYE@Galileo

ATel #11723; 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 16 Jun 2018; 18:09 UT
Credential Certification: Luca Zampieri (luca.zampieri@oapd.inaf.it)

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We observed the black hole transient MAXI J1820+070 (ASASSN-18ey, see ATels #11399, #11400, #11403, #11404, #11406, #11418, #11420, #11421, #11423, #11424, #11425, #11426, #11432, #11437, #11439, #11440, #11445, #11451, #11478, #11481, #11488, #11490, #11510, #11533, #11539, #11540, #11574, #11576, #11578, #11591, #11596, #11609) with the fiber-fed fast optical photon counter IFI+IQUEYE, mounted at the 1.2 m Galileo telescope in Asiago. The source was observed in white light on April 18-19 and May 21-22. On April 18 and 19, seven 900s observations were carried out between MJD 58226.991338 (Apr 18 23:47:31.6 UT) and MJD 58227.144271 (Apr 19 03:27:45.0 UT). On May 21 and 22, eight observations of different duration (between 300s and 900s each) were performed between MJD 58259.974418 (May 21 23:23:09.7 UT) and MJD 58260.173968 (May 22 01:46:30.8 UT). We retained only the observations with better sky conditions, for a total useful on-source time of 3600s (4 observations) in April 18-19 and 1200s (3 observations) in May 21-22. The fractional root-mean-square (rms) variability of the 1s binned light curve was 8.1% on April 18-19, when the background-subtracted average source rate was ~8700 counts/s (background rate ~2200 c/s). The sky background was regularly monitored with acquisitions performed between on-target observations. Also a nearby field star with comparable count rate was pointed in between on-target observations and showed a significantly lower rms (3.7%). On May 21-22, the source net rate and fractional rms were ~900 c/s and 5.1%. Power spectra of the non-background subtracted light curves with a time bin of 1 ms were computed for each night and were then averaged over intervals of 130s duration. In the observations taken on April 18-19 we detect a significant quasi-periodic oscillation (QPO) on the top of three broad-band noise components. The QPO has a centroid frequency of 128 +/- 2 mHz, a full-width-half-maximum of 24 +/- 5 mHz, and a fractional root-mean-square variability of 3.1 +/- 0.3 %. Another QPO-like feature at lower frequency and with lower significance is present in the power spectrum with centroid frequency 71 +/- 4 mHz, full-width-half-maximum 36 +/- 16 mHz, and fractional root-mean-square variability 3.2 +/- 0.6 %. The 128 mHz QPO has a width consistent with that of the optical QPO reported in ATel #11591. The frequency difference between the two measurements, that are approximately 3.7 days apart, is 29 mHz. The fractional increment of the centroid frequency of the optical QPO is consistent with that calculated for a similar QPO detected in the X-rays with NuSTAR and reported in ATel #11578. The properties (frequency and width) of the 128 mHz optical QPO are also consistent with those of the QPO at ~0.12 Hz detected in the power spectrum of the quasi-simultaneous Swift observations of MAXI J1820+070 taken between Apr 16 and 20. In the observations taken on May 21-22 no significant QPO was detected. Despite the weather conditions were poor and the useful observing time short, we can constrain the fractional rms of a QPO with the centroid and width of the 128 mHz QPO to be smaller than ~2% (95% confidence).