The Edge of a Black Hole

X-rays from AGNs have a strong spectral line emitted by iron atoms. In a laboratory, the energy of this line is fixed at 6.4 kilo electron-volts (keV for short). There also is a "continuum" of X-rays in the spectrum, containing X-rays of every conceivable energy from below 1 keV to above 100 keV. (Read more about continuum vs. lines in the spectroscopy primer)

The energy of the iron line doesn't always stay at 6.4 keV, though. The motion of gas around a black hole causes Doppler shifts: when the line-emitting gas is coming towards you as it moves around in the accretion disk, the line is shifted to shorter wavelength (or higher energy) - it's blue shifted. It can also lead to red shifts, when the gas is moving away - this is a shift to lower energy.

But there is more. The gravity of the black hole is so strong that it saps energy out of the X-ray photons as they climb out of the depth of the strong gravitational well of the black hole, causing the line to appear shifted to lower energies. This is called "gravitational red shift." The combination of Doppler shift and gravitational red shift causes the iron line to broaden over a range of energies, with a characteristic shape. (Read about and see an animation about this.)

The exact shape of this broad iron line can tell us a lot about the nature of the black hole and the flow of superheated gas around it. Although scientists have known about the broad iron lines, Suzaku allows the most precise measurements of them to date. This is because the combination of the XIS and the HXD covers X-ray energies from below 1 keV to above 50 keV. So scientists can determine the shape of the spectrum's continuum with unprecedented accuracy. Previous satellites which could measure the broad iron lines only covered energies below about 10 keV, so scientists were uncertain what the continuum's exact shape was. If you don't know the shape of the continuum, you can't be confident about the shape of the broad iron line. Suzaku allows you to measure both.

"The broad iron line is our ticket to view matter and energy very close to a black hole," said Dr. James Reeves of NASA's Goddard Space Flight Center and Johns Hopkins University. He is the leader of a team which studied the iron line in an active galaxy called MCG-5-23-16. This team concluded that the accretion disk in this galaxy is tilted at 45 degrees. Such a precision measurement is a first in X-ray astronomy.

"Across the board, we are finding the broad iron K line to be an incredibly robust measure of black hole properties," said Prof. Andrew Fabian of Cambridge University. He is the leader of the team that studied a different galaxy, MCG-6-30-15. This team confirmed that the black hole at the center of this galaxy is spinning rapidly (See video). Moreover, they found evidence that the black hole is acting as a strong lens, bending the X-rays around it. This is both a nice confirmation of Einstein's general relativity and explains some features of MCG-6-30-15 that used to puzzle researchers.

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