NICER / ISS Science Nugget
for December 5, 2019

Simultaneous X-ray and Optical observations of MAXI J1820+070

Between March and November 2018, NICER intensively monitored the outburst of the relatively nearby (10,000 light-years, or 3 kiloparsecs, away) black-hole binary MAXI J1820+070, which shone for a time as the second-brightest X-ray emitter in the sky. A significant number of NICER's observations were performed in coordination with other observatories, both in space and on the ground across the electromagnetic spectrum, offering complementary views of the black hole, its companion star, and the luminous accretion disk of material spiraling in to its ultimate fate.

One set of coordinated, simultaneous observations was made using a novel, ultra-fast camera called HiPERCAM on the Gran Telescopio Canarias (GTC) at La Palma in the Canary Islands. At rates of over 100 frames per second, in five visible-light filter bands simultaneously, HiPERCAM data were compared with NICER's X-ray intensity and spectrum measurements to delve deep into the variable emissions of MAXI J1820, in search of correlations between the two wavelength bands. The results were recently published by John Paice et al. (Univ. of Southampton, UK) as a peer-reviewed Letter in the journal Monthly Notices of the Royal Astronomical Society.

What the researchers found was violent flickering of both the visible and X-ray light. A video demonstrating this behavior illustrates what one might see by venturing (too!) close to the system. The movie was made using real data, but slowed down to 1/10th of actual speed to allow the most rapid flares to be discerned by the human eye.

Here, we see how the material around the black hole is so bright that it outshines the star that it is consuming, and the fastest flickers last only a few milliseconds. Researchers also found that dips in X-ray levels are accompanied by a rise in visible light (and vice-versa), and the fastest flashes in visible light were found to emerge a fraction of a second after the corresponding X-rays. This is important information when it comes to black hole systems - by analyzing this flickering, we can reconstruct the geometrical configurations of structures surrounding the system that are otherwise far too small to see. The hottest regions deep in the center of the system emit primarily in X-rays, while larger and more distant regions emit at progressively lower energies (longer wavelengths of light). The researchers interpret the fraction-of-a-second time delay as corresponding to the physical separation between the center of the system and the regions that emit in visible light, which they associate with violent 'jets' of outrushing plasma.

Optical and X-ray light curves for MAXI J1820+070 show
anti-correlated flickering

Figure: A portion of optical (top, GTC) and X-ray (bottom, NICER) lightcurves. The left panel shows a long-term trend, binned with a moving average function over 150 points (0.5 seconds), while the right panel shows a zoom-in around the 4-sec mark (dotted vertical lines) with no binning. The optical bands are ugriz (blue/green/red/dark-red/black traces, bottom-to-top). Representative error bars on individual time bins are shown. Note the rapid red flaring of the source, down to approximately 10 ms timescales (e.g., at 3.4 s and at 3.96 s).

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