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On the Hunt for Magnetic Field Winds with Jon Miller
June 21, 2006
Scientists have used Chandra X-ray Observatory to make a breakthrough in
understanding how huge amounts of light are generated by supermassive
Here, the leader of this study, Jon Miller,
gives some personal and scientific insight into his team's discovery.
What's the level of significance and potential impact of this discovery?
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
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
How do you expect the scientific community will react to your discovery?
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.
What was your own reaction?
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.
Why is the discovery important?
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.
What was the scope of the research? What resources were needed to complete it? What led to your involvement?
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
My involvement in this work came about because accretion onto black
has been a central theme in my research for several years, and
Chandra has been a principal tool in that research.
How will this discovery change or trigger future research? What's the next step?
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?
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
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.
How did the notion of magnetic field winds first emerge?
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
How powerful are these winds?
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
Were there any unexpected surprises encountered during your research?
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.
Is there anything else that you would like to add?
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
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.