When a star passes close to a black hole (BH), the enormous gravitational force of the BH pulls the stellar material. This phenomenon is called “stellar tidal disruption”. During this process an  immense amount  of energy is released and the surroundings of the BH are brightened in an event  called a flare. These flares contain high-energy radiation, including ultraviolet and X-ray light, that destroys any dust up to a radius around the black hole.The exact details of the mechanism that triggers this phenomenon are not well understood.

Recently, astronomers detected four such flares. The detection was made using NASA’s Wide-field Infrared Survey Explorer (WISE) telescope. Using a technique called “light echoes” they measured the delay between the optical light flare and the subsequent infrared light, that is produced when the flare reaches the areas of the black hole that the dust has not been destroyed. This time delay allows scientists to measure the distance between the black hole and the dust. Astronomers found that as the flare heats the dust, an infrared signal is generated that is detectable up to a year after the flare is at its most luminous! This confirms previous findings that the black hole is surrounded by a patchy web of dust located a few trillion miles away from the BH!!

The WISE satellite, after scanning the entire sky twice, has now been assigned a new name and mission. It is now called NEOWISE and identifies potentially hazardous near-Earth objects!

Publications: Van Velzen et al. 2017, Jiang et al. 2017

Source: NASA