Other low-frequency optical QPO-like features in MAXI J1820+070 detected with IFI+IQUEYE@Galileo

ATel #11824; Michele Fiori (University of Padova), Luca Zampieri (INAF-Astronomical Observatory 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), Piergiorgio Casella (INAF-Astronomical Observatory of Rome)
on 7 Jul 2018; 17:39 UT
Credential Certification: Luca Zampieri (luca.zampieri@oapd.inaf.it)

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On Jun 9 and 10 we carried out the third observing run targeting the black hole transient MAXI J1820+070 (ASASSN-18ey) with the fiber-fed fast optical photon counter IFI+IQUEYE, mounted at the 1.2 m Galileo telescope in Asiago (see ATel #11723 and other ATels referenced therein). Several observations were carried out between MJD 58278.041748 and 58278.079276 (Jun 9 01:00:07.0 UT through 01:54:09.4 UT) and between MJD 58278.881996 and 58279.083934 (Jun 9 21:10:04.5 UT through Jun 10 02:00:51.9 UT). We retained only the observations with better sky conditions, for a total useful on-source time of 4500s (4 observations taken on Jun 10). During these observations the average background-subtracted source rate was 2900 c/s. The sky background was regularly monitored with acquisitions performed between on-target observations and had an average rate of 1700 c/s. The fractional root-mean-square (rms) variability of the 1s binned light curve of the target was 4.2%. A nearby field star pointed in between on-target observations showed a significantly lower rms (2.6%). 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 65s duration. We detected two broad QPO-like features on the top of a broad-band noise component (that were fitted with Lorentzians). The higher frequency QPO-like feature has a centroid frequency of 268 +/- 12 mHz, a full-width-half-maximum of 150 +/- 39 mHz, and a fractional root-mean-square variability of 1.9 +/- 0.2 %. The lower frequency QPO is less significant and has a centroid frequency of 151 +/- 6 mHz, a full-width-half-maximum of 33 +/- 16 mHz, and a fractional root-mean-square variability of 1.2 +/- 0.3 %. We obtained acceptable fits of the power spectrum also with two harmonically-related QPOs, that have width and significance similar to those reported above. Acceptable fits are obtained for 1:2, 2:3, or 3:5 centroid frequency ratios. A fit with harmomically-related QPOs performed on the April IFI+Iqueye observations (ATel #11723) gives similar results for the same harmonic ratios (1:2, 2:3 or 3:5). We note also that, within the errors, the QPO centroid frequencies of the June observations are two times higher than those of the April observations. However, the 268 mHz QPO has a significantly larger width than the 128 mHz QPO observed in April and its frequency is not consistent with the fractional increment of the centroid frequency reported in April.