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Low-frequency QPOs in MAXI J1820+070 as seen by INTEGRAL/SPI

ATel #11488; I. A. Mereminskiy, S. A. Grebenev, S. V. Molkov, I. A. Zaznobin, G. A. Khorunzhev, R. A. Burenin (Space Research Institute, Moscow), M. V. Eselevich (Institute of Solar-Terrestrial Physics, Irkutsk)
on 29 Mar 2018; 07:19 UT
Credential Certification: Sergei Grebenev (sergei@hea.iki.rssi.ru)

Subjects: X-ray, Transient

Referred to by ATel #: 11490, 11576, 11578, 11591, 11723, 11820, 11951

We report the detection of low-frequency QPOs in the X-ray power spectra of the new X-ray nova and black hole candidate MAXI J1820+070 (ATEL #11399, #11400, #11406, #11418, #11423, #11427). The data were collected during the dedicated ToO observations of the source with INTEGRAL in revolutions 1931-1935 (March 16-27, 2018, ATEL #11478). Since the source is exceptionally bright (>3.5 Crab in the 15-50 keV band) we, willing to avoid systematic effects usual for the IBIS/ISGRI data at high count rates, used NRT data from the SPI telescope in order to perform our timing analysis. We selected the events with energies in the 20-50 keV range and barycentered their times of arrival. All the power spectra show a strong band-limited noise at frequencies below 0.1 Hz. In revolutions 1932-1934 we saw the QPOs with the centroid frequency growing from 0.030+/-0.001 Hz (in rev. 1932) to 0.035+/-0.001 Hz (rev. 1933) and later 0.042+/-0.001 Hz (rev. 1934). Our analysis of the JEM-X monitor X-ray data obtained in rev. 1934 has confirmed the presence of QPOs at 0.04+/-0.01 Hz in the power spectrum of the source but with smaller confidence.

Note that the source flux was stable or very slowly increased during this time interval, according to the Swift-BAT transient monitor data (3.6 Crab in the middle of rev. 1932 and 3.8 Crab in the middle of rev. 1934). The similar trend of the flux was found in optical during our photometric and spectroscopic monitoring of the source at the 1.6-m telescope of the Sayan observatory. Note also that the observed shape of the power spectrum, with a broadband noise and a prominent QPO peak, is typical of the low/hard or hard/intermediate states of black hole transients.

Follow-up timing observations of the source in X-rays or optical/IR bands are strongly encouraged. We will also continue to monitor the source.

Evolution of the QPO frequency in MAX J1820+070