Exponential increase in X-ray QPO frequency with time in MAXI J1820+070

ATel #11578; Douglas Buisson (Cambridge), Andy Fabian (Cambridge), Will Alston (Cambridge), Dom Walton (Cambridge), Erin Kara (Maryland), Javier Garcia (Caltech), Jeroen Homan (MIT) and John Tomsick (Berkeley)
on 26 Apr 2018; 21:10 UT
Credential Certification: Douglas Buisson (djkb2@cam.ac.uk)

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The X-ray transient MAXI J1820+070, discovered through its ongoing outburst beginning on 11th March (ATEL #11399), is likely a black hole binary in the hard state (e.g. ATEL #11418, #11423, #11420, #11426).

Several observations have been made with NuSTAR. A low frequency QPO, often with 2:1 and occasionally 3:1 harmonics, is detected in all observations since the outburst peak.

The characteristic frequency of the QPO increases exponentially with time, as log₁₀(f/Hz) = 0.01869*t - 2.9158 (for time in days from 2018-01-01, MJD 58119), i.e. an e-folding time of 23 days. A plot of the QPO frequencies can be found at the link below.

Low frequency QPOs have also been detected with INTEGRAL (ATEL #11488), Swift (ATEL #11510) and NICER (ATEL #11576) at frequencies matching this relation. The QPO seen by NuSTAR is present at all energies from 3-78 keV, implying that the same phenomenon is responsible for all these detections.

Despite the large change in QPO frequency, the X-ray flux (as measured by Swift/BAT) is decreasing slowly and any spectral evolution (across 0.3-78 keV, measured by NuSTAR, Swift and NICER) is minimal.

The NuSTAR spectrum comprises a hard powerlaw and reflection spectrum (broad iron line and Compton hump), as expected for a corona and accretion disc around a rapidly spinning black hole.

NuSTAR results will be presented in full in a future publication (Buisson et al. in prep.).

Figure showing evolution of QPO frequency with time