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On the Hunt for Magnetic Field Winds with Jon Miller


June 21, 2006 ::
Animation of a Black Hole Pulling Matter from Companion Star
(Credit: NASA/CXC/A.Hobart)
Scientists have used Chandra X-ray Observatory to make a breakthrough in understanding how huge amounts of light are generated by supermassive black holes. Here, the leader of this study, Jon Miller, gives some personal and scientific insight into his team's discovery.

CXC: What's the level of significance and potential impact of this discovery?

JM: This result is an important step toward understanding accretion onto black holes, which is ultimately central to understanding how black holes and galaxies co-evolve. So, potentially, the impact could be quite broad.

Illustration of a Stellar-Mass Black Hole with Spectrum
Illustration of a Stellar-Mass Black Hole with Spectrum
(Illustration: NASA/CXC/M.Weiss; X-ray Spectrum: NASA/CXC/U.Michigan/J.Miller et al.)
JM: It is important to recognize, though, that it is but one step, and that the evidence -- while strong -- is indirect. We need more observations, and more sensitive observations. Chandra has allowed us an important glimpse; a future mission like Constellation-X has the potential to reveal the infall of matter onto black holes in even more exquisite detail.

CXC: How do you expect the scientific community will react to your discovery?

JM: I hope it will be a positive reaction, of course. However, I think it is the mark of a good scientist to be skeptical, even when a given result is one that you might want to see for various reasons. I think it would be apporpriate to view this result as an important step toward understanding the inner workings of accretion onto black holes observationally. It is certainly not the endpoint -- there is much work left to do.

CXC: What was your own reaction?

Edwin Hubble at the 100-inch Hooker Telescope. (Mt. Wilson Obs.)
Illustration of Magnetic Fields in GRO J1655-40
(Credit: NASA/CXC/M.Weiss)
JM: When it became clear that the wind we observed had to be driven magnetically, and that the field has to arise in the disk, it was clear what it might mean and I was very excited. My initial reaction was that this was a much bigger step forward than I could have hoped for. That said, within the research team, we did not simply accept the most exciting interpretation. We had a very detailed discussion in which we questioned our results, interpretation, and assumptions skeptically. It was only after this process that we decided that the an exciting interpretation was indeed demanded by the data.

CXC: Why is the discovery important?

JM: Accretion onto central black holes can have a profoud impact on how galaxies evolve. It is also through accretion, that black holes in the center of galaxies grow to be monstrous -- millions to hundreds of millions of solar masses. More broadly, by some estimates, 20% of the light in the universe is due to accretion onto black holes. The mechanisms which cause accretion onto black holes to be a strong source of radiation (and sometimes also matter if a wind is driven) are therefore important for us to understand.

Animation of a Black Hole Pulling Matter from Companion Star
Animation of a Black Hole Pulling Matter from Companion Star
(Credit: NASA/CXC/A.Hobart)
CXC: What was the scope of the research? What resources were needed to complete it? What led to your involvement?

JM: Chandra was the central tool in this investigation. The unprecedented spectral resolution that Chandra provides in the X-ray band was the key to measuring the detailed parameters of the accretion disk wind, learning that the wind had to be driven magnetically, and inferring that magnetic fields are very likely the mechanism making accretion onto black holes not only possible, but a very luminous process. The Rossi X-ray Timing Explorer was also vital to discovering that the black hole we studied -- GRO J1655-40 -- had entered a bright phase. RXTE is unique in its ability to monitor the X-ray sky. We also made use of atomic spectroscopy databases, which are painstakingly calculated and provided to the astronomical community by dedicated scientists.

Illustration of Rossi X-ray Timing Explorer Illustration of Chandra Spacecraft
Illustrations of the Rossi X-ray Timing Explorer and the Chandra Spacecraft
(Illustration: CXC/NGST)
JM: My involvement in this work came about because accretion onto black holes has been a central theme in my research for several years, and Chandra has been a principal tool in that research.

CXC: How will this discovery change or trigger future research? What's the next step?

JM: We need to observe more black holes in this same level of detail. Chandra can help us do that. We need to better understand the nature of the magnetic fields working in accretion disks -- for instance, is the field configuration always the same, and how does that affect how much matter flows onto the black hole? Are there phases during which the magnetic field is coiled around the black hole like a snake, and are there later phases where it sticks out of the disk like the quills on a porcupine?

JM: In the near future, this result may cause astronomers working on accretion onto other compact objects -- white dwarfs, neutron stars, and supermassive black holes -- to consider whether or not there is also evidence for winds which are driven (at least in part) by a magnetic accretion disk.

In the long run, I think this result shows that planned future missions like Constellation-X will fulfill their promise of revealing the nature of accretion onto black holes, both in terms of the relativistic effects close to black holes, and the physics which determines how gas flows onto black holes.

Constellation-X Concept
Constellation-X Concept
(Credit: NASA/GSFC)
CXC: How did the notion of magnetic field winds first emerge?

JM: We first examined whether or not thermal pressure, and then radiation pressure, could plausibly drive the X-ray wind we observed. When it became clear that these mechanisms were very unlikely, we were forced to consider magnetic fields. It is fortunate that computing power is now sufficient to allow theorists to develop simulations examining the effects of magnetic fields in the context of accretion onto black holes. These simulations provided us with insight into how magnetic fields in the disk might facilitate winds, and how they affect the disk itself.

CXC: How powerful are these winds?

JM: Quite powerful, relative to everyday experience. Our observations suggest that half of the material which might otherwise flow onto the black hole in a disk, can actually be driven outward in this wind. Much of the wind seems to be running away from the black hole at 500 km/s (approximately 300 miles/s). While that is very fast, it is important to remember that it is a very small fraction of the speed of light.

CXC: Were there any unexpected surprises encountered during your research? JM: I think the quality of the data was a big surprise. We are used to excellent data from Chandra, but this data was truly special. When I first reduced the data, and found such a large number of strong spectral lines, I thought I might have made a mistake, so I repeated the reduction twice more to be sure the spectral lines were real. Soon therafter, I met with the second author -- John Raymond -- and we looked at a large printed version of the spectrum in a hallway. We spent the afternoon identifying features and soon realized that we had something very special on our hands.

Evidence for Wind in the GRO J1655-40 Spectrum
Evidence for Wind in the GRO J1655-40 Spectrum
(Credit: NASA/CXC/U.Michigan/J.Miller et al.)
CXC: Is there anything else that you would like to add?

JM: I am very grateful to the Chandra X-ray Observatory staff. At every turn, Chandra proves to be a wonderful mission. I am also grateful to NASA for its support of space science; without the funding NASA provides to support this research, this result would not have been possible. I think the future of X-ray astronomy is particularly bright; like Hubble, Chandra will endure for many years to come, but I look forward to a day when we can use Chandra's successor -- Constellation-X -- to learn even more.

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