Credit:
NASA/CXC/Univ. of Oklahoma/X. Dai et al.
Like whirlpools in the ocean, spinning black holes
in space create a swirling torrent around them. However, black holes do
not create eddies of wind or water. Rather, they generate disks of gas
and dust heated to hundreds of millions of degrees that glow in X-ray light.
Using data from NASA's Chandra X-ray Observatory and chance alignments across billions of light years, astronomers have deployed a new technique to measure the spin of five supermassive black holes. The matter in one of these cosmic vortices is swirling around its black hole at greater than about 70% of the speed of light.
The astronomers took advantage of a natural phenomenon called a gravitational lens.
With just the right alignment, the bending of space-time by a massive
object, such as a large galaxy, can magnify and produce multiple images
of a distant object, as predicted by Einstein.
In this latest research, astronomers used Chandra and gravitational lensing to study five quasars,
each consisting of a supermassive black hole rapidly consuming matter
from a surrounding accretion disk. Gravitational lensing of the light
from each of these quasars by an intervening galaxy has created multiple
images of each quasar, as shown by these Chandra images of four of the
targets. The sharp imaging ability of Chandra is needed to separate the
multiple, lensed images of each quasar.
The key advance made by researchers in this study was that they took
advantage of "microlensing," where individual stars in the intervening,
lensing galaxy provided additional magnification of the light from the
quasar. A higher magnification means a smaller region is producing the
X-ray emission.
The researchers then used the property that a spinning black hole is
dragging space around with it and allows matter to orbit closer to the
black hole than is possible for a non-spinning black hole. Therefore, a
smaller emitting region corresponding to a tight orbit generally implies
a more rapidly spinning black hole. The authors concluded from their
microlensing analysis that the X-rays come from such a small region that
the black holes must be spinning rapidly.
The results showed that one of the black holes, in the lensed quasar
called the "Einstein Cross," (labeled Q2237 in the image above) is
spinning at, or almost at, the maximum rate possible. This corresponds
to the event horizon, the black hole's point of no return, spinning at
the speed of light, which is about 670 million miles per hour. Four
other black holes in the sample are spinning, on average, at about half
this maximum rate.
For the Einstein Cross the X-ray emission is from a part of the disk
that is less than about 2.5 times the size of the event horizon, and for
the other 4 quasars the X-rays come from a region four to five times
the size of the event horizon.
How can these black holes spin so quickly? The researchers think that
these supermassive black holes likely grew by accumulating most of
their material over billions of years from an accretion disk
spinning with a similar orientation and direction of spin, rather than
from random directions. Like a merry-go-round that keeps getting pushed
in the same direction, the black holes kept picking up speed.
The X-rays detected by Chandra are produced when the accretion disk
surrounding the black hole creates a multimillion-degree cloud, or
corona above the disk near the black hole. X-rays from this corona
reflect off the inner edge of the accretion disk, and the strong
gravitational forces near the black hole distort the reflected X-ray
spectrum, that is, the amount of X-rays seen at different energies. The
large distortions seen in the X-ray spectra of the quasars studied here
imply that the inner edge of the disk must be close to the black holes,
giving further evidence that they must be spinning rapidly.
The quasars are located at distances ranging from 9.8 billion to 10.9
billion light years from Earth, and the black holes have masses between
160 and 500 million times that of the sun. These observations were the
longest ever made with Chandra of gravitationally lensed quasars, with
total exposure times ranging between 1.7 and 5.4 days.
A paper describing these results is published in the July 2nd issue of The Astrophysical Journal, and is available online.
The authors are Xinyu Dai, Shaun Steele and Eduardo Guerras from the
University of Oklahoma in Norman, Oklahoma, Christopher Morgan from the
United States Naval Academy in Annapolis, Maryland, and Bin Chen from
Florida State University in Tallahassee, Florida.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Source: NASA’s Chandra X-ray Observatory
Fast Facts for Q2237+0305:
Scale: Image is 18 arcsec across. (About 500,000 light years)
Category: Quasars & Active Galaxies
Coordinates (J2000): RA 22h 40m 30.34s | Dec 03° 21´ 28.8"
Constellation: Pegasus
Observation Dates: 20 pointings from Dec 31, 2009 to Jun 6, 2014
Observation Time: 130 hours
Obs. IDs: 11534-11539, 13191, 13195, 12831-12832, 13960-13961, 14513-14518, 16316-16317
Instrument: ACIS
References: Dai, X. et al. 2019, AJ, 879, 35 arXiv:1901.06007
Color Code: X-ray intensity: purple
Distance Estimate: About 9.8 billion light years (z=1.69)
