About 3.9 billion years ago, a star that passed close to a monster black hole was shredded by the hole’s intense tidal pull. On March 28, 2011 the X-rays generated during this event first reached Earth, and NASA's Swift satellite was able to detect them and alerted astronomers located in various parts of the world.
In this artist's rendering, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close. Stellar debris has fallen toward the black hole and collected into a thick chaotic disk of hot gas. Flashes of X-ray light near the center of the disk result in light echoes that allow astronomers to map the structure of the funnel-like flow, revealing for the first time strong gravity effects around a normally quiescent black hole. (Credits: NASA/Swift/Aurore Simonnet, Sonoma State University)
Scientists decided that the outburst, christened Swift J1644+57, signified both the sudden flare-up of a previously inactive black hole and the tidal disruption of a star.
Currently, astronomers utilizing archival observations from Swift, the Japan-led Suzaku satellite and the European Space Agency's (ESA) XMM-Newton observatory have spotted the reflections of X-ray flares erupting during the event.
The team led by Erin Kara, a postdoctoral researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland, College Park (UMCP), have utilized these light reverberations or echoes, to map the gas flow near a recently revived black hole for the first time.
While we don't yet understand what causes X-ray flares near the black hole, we know that when one occurs we can detect its echo a couple of minutes later, once the light has reached and illuminated parts of the flow. This technique, called X-ray reverberation mapping, has been previously used to explore stable disks around black holes, but this is the first time we've applied it to a newly formed disk produced by a tidal disruption.
Erin Kara, Postdoctoral Researcher, NASA's Goddard Space Flight Center
Debris of stars falls toward a black hole and accumulates into a rotating structure referred to as an accretion disk. The gas is compressed here and heated to over a million degrees and finally spilling over the event horizon of the black hole, which is the point where nothing can get away and astronomers cannot monitor.
The Swift J1644+57 accretion disk was thicker, more chaotic and highly tumultuous than stable disks, which have had time to stabilize into a systematic routine.
The research findings are presented in a paper published online in the Nature journal, June 22 issue.
NASA Goddard astronomer Erin Kara discusses the discovery of X-ray echoes from Swift J1644+57, a black hole that shattered a passing star. X-rays produced by flares near this million-solar-mass black hole bounced off the nascent accretion disk and revealed its structure. (Credits: NASA's Goddard Space Flight Center)
There was one surprise from the study - high-energy X-rays occurring from the inner region of the disk. Astronomers had believed that a major portion of this emission started off from a thin jet of particles racing up to near the speed of light. In blazars, which is the most luminous galaxy class powered by supermassive black holes, jets generate a majority of the highest-energy discharge.
We do see a jet from Swift J1644, but the X-rays are coming from a compact region near the black hole at the base of a steep funnel of inflowing gas we're looking down into. The gas producing the echoes is itself flowing outward along the surface of the funnel at speeds up to half the speed of light.
Lixin Dai, Postdoctoral Researcher, UMCP
X-rays emanating near the black hole stimulate iron ions in the rotating gas, causing them to fluoresce with a unique high-energy glow referred to as iron K-line emission. Just like an X-ray flare fades and brightens, the gas trails in turn after a brief delay based on its distance from the source.
"Direct light from the flare has different properties than its echo, and we can detect reverberations by monitoring how the brightness changes across different X-ray energies," said co-author Jon Miller, a professor of astronomy at the University of Michigan in Ann Arbor.
The Swift J1644+57 is a part of the only three tidal disruptions that have generated high-energy X-rays, and to date it remains the only event captured at the heights of this emission.
Astronomers would not know about black holes if these star shredding incidents had not temporarily stimulated black holes. Astronomers now believe that for every black hole vigorously accreting gas and producing light, nine others are dark and dormant. Even when the universe was younger, these quiescent black holes were active, and they had a significant role in how galaxies emerged. Therefore, tidal disruptions provide a quick look at the large number of silent supersized black holes.
If we only look at active black holes, we might be getting a strongly biased sample. It could be that these black holes all fit within some narrow range of spins and masses. So it’s important to study the entire population to make sure we’re not biased.
Chris Reynolds, Professor of Astronomy, UMCP
The mass of the Swift J1644+57 black hole has been estimated by the researchers at approximately a million times that of the sun. They however did not measure its spin. With future enhancements in modeling and understanding accretion flows, the team believes it could possibly accomplish that as well.
In December 1999, ESA's XMM-Newton satellite was launched from Kourou, French Guiana. NASA funded some phases of the XMM-Newton instrument package and offered the NASA Guest Observer Facility at Goddard, which is open for use by U.S. astronomers.
Suzaku operated from July 2005 to August 2015 and was created at the Japanese Institute of Space and Astronautical Science, which is part of the Japan Aerospace Exploration Agency, in partnership with NASA and other U.S. and Japanese institutions.
In November 2004, NASA's Swift satellite was launched and is managed by Goddard. It is operated in partnership with Penn State University in University Park, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corp. in Dulles, Virginia, with international partners in the U.K., Germany, Italy, and Japan.