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.
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).