Fast optical flaring in the suspected black-hole binary MAXI J1820+070 (ASASSN-18ey)

ATel #11421; Colin Littlefield (University of Notre Dame)
on 15 Mar 2018; 12:42 UT
Credential Certification: Colin Littlefield (clittlef@alumni.nd.edu)

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The transient MAXI J1820+070 (= ASASSN-18ey; ATel #11399, #11400) is an outbursting low-mass X-ray binary in which the accreting object is suspected to be a black hole (ATel #11418). Between approximately 8-11 UTC on 2018 March 15, I obtained unfiltered, time-series photometry of this object with the 80-cm Sarah L. Krizmanich Telescope at the University of Notre Dame in South Bend, Indiana (USA). Individual exposures were 2 seconds long, with an overall cadence of 5 seconds per image.

As shown in the light curve at the link below, the system displayed chaotic variability during these observations. The top panel of the linked figure shows the full light curve, while the bottom panel zooms in on a representative 30-minute segment so that individual points may be seen more clearly. The source flickered almost constantly, undergoing many flares that lasted for only a few seconds. A few exceptional flares were a half-magnitude in amplitude, but the typical flare amplitude was 0.2-0.3 mag. The system's average magnitude was approximately 13.0 (bandpass: unfiltered with a Johnson V zeropoint).

To confirm that the fast variability was not an artifact caused by scintillation noise or seeing variations, I inspected light curves of several check stars in the field, and even though they were considerably fainter than MAXI J1820+070, they showed comparatively little scatter (RMS = 0.02 mag).

Given that much of the observed variability occurred on timescales of less than 5 seconds, it is important to utilize as rapid a cadence as possible when obtaining time-series photometry of this object.

Light Curve