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Celebrating 10 Years of Suzaku

Accretion Powered X-ray Sources

While the energy source of stars like our sun is nuclear fusion, the energy source for many of the brightest X-ray sources on the sky is accretion. Accretion occurs in a variety of cosmic sources and results in the release of gravitational potential energy as matter falls in toward an object. When the accreting object is "compact" like a black hole or a neutron star, accretion is a very efficient energy source. Accretion often proceeds via a disk – the matter orbits the central object, but gradually drifts inward through dynamical friction and gets heated up.

Artist's conception of an accretion disk /

Artist's conception of an accreting black hole. (Credit: NASA/GSFC Conceptual Image Lab)

Accreting black holes and neutron stars are found in binary systems. Supermassive black holes at the center of galaxies can also accrete nearby gas and dust, which make them active ("active galactic nuclei", or AGN). Accretion disks shine at a wide range of wavelengths (visible light, ultraviolet, and X-rays), but X-ray observations have the potential to allow us to measure the motion of matter near the inner edge of an accretion disk, very close to a black hole or a neutron star. This is the regime where we must rely on general relativity (Newton's laws of motion are inaccurate) – so we have the potential to verify some rather exotic inferences drawn from general relativity.

A phenomenon called reflection is an important tool for X-ray astronomers. Low energy X-rays are easily absorbed (hence called "soft"), while high energy ("hard") X-rays are not. When hard X-ray photons, with energies above 10 keV, hit an accretion disk, many are scattered back ("reflection"). Some photons that hit an iron atom knock off an electron; such atoms will then emit X-rays of a specific energy in a process called fluorescence. We can use the Doppler shift and several effects specific to general relativity, to trace the motion of the accretion disk matter.

To use reflection, we ideally need to observe both soft and hard X-rays at the same time, which Suzaku can do using the XIS and the HXD. For almost 7 years, the HXD was the most sensitive instrument for X-rays above 10 keV – until the NuSTAR mission was launched.

Here are four important Suzaku results on AGN and X-ray binaries, three of which used reflection.

black hole animation still image

Still image from an animation of accretion around a black hole. (Credit: NASA/GSFC)

The fact that Suzaku was well suited to the study of reflection in AGN has been widely appreciated since the early days of the mission, and several groups applied this technique to well-known, bright AGN within the first year. In particular, the spin of the accreting black hole – the only property other than mass that characterizes an astronomical black hole – can be inferred from these data. This technique has become well established during the course of the Suzaku mission, although some researchers worry that absorption effects might be distorting the apparent shape of the iron line.
artist's impression of the GX339-4 binary system

Artist's impression of GX 339-4, which is among the most dynamic binaries in the sky. (Credit: ESO/L. Calçada)

General relativity dictates how close the inner edge of an accretion disk can be to the black hole it surrounds. However, the physics of the disk itself can move the inner edge away from this ultimate limit. This is widely thought to happen when the amount of matter flowing through the disk decreases. This study of a black hole binary, in which the accretion rate is known to vary by a large factor, is an example of how Suzaku data can be used to quantify this effect.
artist's conception of hot gas around a neutron star

An artist depiction of a disk of hot gas whipping around a neutron star. (Credit: NASA/Dana Berry)

Probing the Exotic Matter inside Neutron Stars
If an accretion disk surrounds a neutron star, then the inner radius of the accretion disk must be equal to or greater than the radius of the neutron star. This truism can be a powerful tool in studying the interior of neutron stars, because the radius of a neutron star is determined by the exotic physics of nuclear-density matter. These studies showed that studying the accretion disk around a neutron star could help improve our understanding of the neutron star interior.
image of IC 2497 and Hanny's Voorwerp

Ground-based optical image of IC 2497 (top), Hanny's Voorwerp (bottom) and a nearby companion galaxy (left). (Credit: WIYN/William Keel/Anna Manning)

Hanny's Voorwerp is the poster child of the Galaxy Zoo project – a citizen science project that which asked thousands of volunteers to classify galaxies, something human vision is very good at doing. A few objects that defied normal classification schemes were discovered, including Hanny's Voorwerp. One possibility was that it was a nebula ionized by a powerful quasar in a neighboring galaxy, IC 2497, which is hidden from our view by gas and dust. But it's almost impossible to hide X-rays with energies above 10 keV – and no such emission was found in a Suzaku observation. The authors conclude that IC 2497 likely was a quasar 70,000 years or so ago, but not any more.

Publication Date: July 2015