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Center of the galaxy

An X-ray lamp

Gas disk around a black hole lights up every seven days

068-069_BuracoNegro_190In 1991, the astrophysicist Thaisa Storchi Bergmann, from the State of Rio Grande do Sul, discovered a disk of matter, a flattened cloud of ionized gas, revolving around a black hole in the center of NGC 1097, a beautiful spiral galaxy in the Fornax constellation, 45 million light-years from Earth. Once a year, for ten years, the researcher from the Federal University of Rio Grande do Sul (UFRGS) observed the galaxy and found that the gas disk was not uniform. The cloud had a spiral arm that every five and a half years moved completely around the black hole once. The astrophysicist also noticed that sometimes the disk became brighter than usual. These peaks of luminosity were interpreted as arising from the fact that the black hole had at these times swallowed up more matter coming from the cloud, because perhaps in that particular place there was a greater density or quantity of gas to be sucked up.

New observations made with the Gemini Sul telescope on Cerro Pachon, in Chile, between the end of last year and early 2011, corrected the periodicity at which the cycle of the complete circuit of the spiral arm occurs to an interval of a year and a half. They also identified a second variation in the luminosity of the disk around the black hole in the galaxy, occurring often, about once a week. X-ray emissions from the innermost and hottest part of the gaseous cloud that directly surrounds the black hole fluctuate in a matter of days, as if they were a lamp, the beam of light shining from the center to the edges of the disk. As it takes approximately a week for the light to travel from the center to the edge of the cloud, the size of the ray of the disk of matter must be seven light-days. “We only perceived this variation because we carried out weekly observations of the galaxy for three solid months,” said Thaisa. Working alongside people from Brazil and abroad, among who was the master’s degree student Jáderson Schimoia, she has submitted an article to a scientific journal that describes the phenomenon.

An analogy with day-to-day objects helps one to visualize the phenomenon. The system, comprising the black hole plus gas disk, which is held around it by gravity, can be compared to the parts of an old song album recorded on vinyl, the long playing record (LP). The black hole would be the equivalent of the hole in the middle of the record. The innermost part of the gas disk would be the paper label in the center of the LP. The outermost part would be similar to the large black region of the vinyl, where the songs are recorded; they start right up against the label and go right to the edge of the disk.

The comparison is useful but not perfect. An LP is a disk in which all parts have the same thickness. The disk of matter of NGC 1097 has irregularities. Its central region (the label on the vinyl) is thicker and fatter than the more distant sectors of the black hole. Technically, it has the shape of a toroid, a figure that is reminiscent of a tire or a cookie with a hole in the middle. “It’s as if this donut was a high energy lamp fixed on a post that’s a little higher than the rest of the gas disk,” compares Thaisa. “It lights up or intensifies because of the amount of gas that falls into the black hole.”

In the study, the researchers analyzed data obtained by Gemini relative to the so-called H-alpha spectral line, the emission of more intense and visible energy of hydrogen atoms, coming from the peripheral area of the disk. They concluded that the emission variation in this region was due to the reverberation of the luminosity, originating in the “donut.” It is not precisely known why the lamp “blinks” at even-day intervals, but this is probably connected with variations in the amount of matter sucked up by the black hole. “It was used to a gas capture diet and suddenly it saw itself obliged to swallow more matter,” compares Brazilian astrophysicist, Rodrigo Nemmen, another author of the work, who did post-doctoral studies at NASA’s Goddar Space-Flight Center.

As is known, it is impossible to observe a black hole directly, a region in space that is so dense and compact, endowed with an enormous gravitational field, from which nothing escapes, not even light. However, an object with these characteristics supplies indirect clues of its presence. When a mysterious source of radiation, especially X-rays, is discovered at some point in the Universe, like the center of an active galaxy, one of the possible explanations for the phenomenon is the existence of a black hole. A little before being dragged in by the gravitational field of the black hole, the matter of the gas disk becomes so hot that it releases energy in the form of radiation. Therefore, when there is a peak of matter absorption, it is expected that the innermost region of the disk, the lamp, will increase in luminosity and reverberate this extra energy to its edges.

Knowing the time the light takes to travel from the most central part to the edge of a gas cloud allows an estimate to be made of the dimension of the disk of matter, regardless of other theoretical models. “Having the dimension of the disk and the speed of the gas around it, which we infer from the optical emissions and which can reach 10,000 km a second, we can obtain the mass of the black hole,” explains Thaisa. Using this alternative approach, the Brazilian astrophysicists recalculated this parameter of the black hole in the center of NGC 1097, obtaining a result of the order of 100 million solar masses, a figure that coincides with estimates made using other techniques.