How did they make a video of the center of the galaxy, and what is it exactly that's flashing there? The ESA video ESOcast 173: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole includes a clip of images of stars at the center of our galaxy orbiting around SgrA*, a presumed supermassive black hole. This isn't visible light because it's obscured by dust, so it may be radio or long wave infrared, but I don't know.

In the middle, I can see something flashing at whatever wavelength this image has been produced from.

Question: 


How are these images obtained, and
what process is it that is believed to be causing that flashing?


GIF made from video at around 02:50:





Six annotated frames from GIF highlighting the flashing that I'm seeing.
Question: How are these images obtained?

Later in the video the narrator says they took the images using ESO's VLT.


  03:40 [Narrator] 14.​ Making these measurements pushed the power of ESO’s Very Large Telescope to the limits.


(Source: ESO transscript)

Over the whole observation period multiple telescopes and imaging instruments were used. Early observations were accomplished using the NTT. Since around the year 2002 the VLT observed Sagittarius A* with the NACO and GRAVITY instruments too.

This graph puts the rotation period of Sagittarius A* in relation to the time and the observing instruments:



(Source)

Question: What process is it that is believed to be causing that flashing?

ESO provides older footage over a minutes-timescale showing a flare of Sagittarius A* in May 2003: Flashes of light from disappearing matter


  ESO Press Video eso0330 shows the detection of a powerful flare from the centre of the Milky Way galaxy. These and other adaptive optics (AO) images (with resolution 0.040 arcsec in the near-infrared H-band at wavelength 1.65 µm) of the central region of the Milky Way were obtained with the NACO imager on the 8.2-m VLT YEPUN telescope at the ESO Paranal Observatory on May 9, 2003. [...] The position of the 15-year orbiting star S2 (cf. ESO Press Release eso0226) is marked by a cross and the astrometric location of the black hole is indicated by a circle.


The cause of the flickering is probably the same for the video sequence you showed.

A October 2018 ESO publication states:


  ESO’s GRAVITY instrument on the Very Large Telescope (VLT) Interferometer has been used by scientists from a consortium of European institutions, including ESO [1], to observe flares of infrared radiation coming from the accretion disc around Sagittarius A*, [...]. The observed flares provide long-awaited confirmation that the object in the centre of our galaxy is, as has long been assumed, a supermassive black hole. The flares originate from material orbiting very close to the black hole’s event horizon — making these the most detailed observations yet of material orbiting this close to a black hole.


(Source)

ESO also has another video showing the therory with more detail: skip to about 1:00.

Flares associated with Sgr A* are also described in this answer.

Artist's illustration of heated gas orbiting the black hole:



(Screengrab from 1:25 of that video)



See also 


Phys.org (2019): Black hole at the center of our galaxy appears to be getting hungrier
Do et al. 2019 Unprecedented Near-infrared Brightness and Variability of Sgr A*
Chen et al. 2019 Consistency of the Infrared Variability of SGR A* over 22 yr




Figure 1 from Do et al. 2019


  Figure 1. Top row: a series of K' images taken on 2019 May 13 centered on Sgr A* showing the large variations in brightness throughout the night. The first image from the left is the brightest measurement ever made of Sgr A* in the near-infrared. Also labeled are nearby stars S0-2 (K' = 14 mag) and S0-17 (K' = 16 mag) for comparison. Bottom panel: K' (black) and H-band light curves of Sgr A* from 2019 May 13. On this night, we alternated between H and K' observations. The H-band magnitudes are offset using H − K' = 2.45 mag. There appear to be no significant color changes during the large change in brightness. Red circles show the location of the four images in the panels above.