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Koji Mukai

posted Mar 27, 2009, 9:45 AM by Mike Simonsen   [ updated Mar 27, 2009, 9:54 AM ]
Koji Mukai was born in Osaka, Japan, and graduated from University of Tokyo
with a Rigakushi (B.Sc.) degree.

For his graduate study, he chose University of Oxford. There, he worked with
Phil Charles, Robin Corbet and Alan Smale, among others, on interacting
binaries, completing his Ph.D. thesis on AM Her type systems in less than 3
years and 1 month. He then spent 3 years at Mullard Space Science
Laboratory, working with Keith Mason, Alan Smale, Simon Rosen and Coel

During his 6 years in England, he was mostly an optical astronomer, and had
6 observing trips to La Palma, 3 to South Africa, 2 to Australia, and 1 to
Hawaii, using world-class telescopes with state-of-the-art instruments
located at the best sites in the world.

After moving to the United States, Koji spent 2 years at UC Berkeley's
Center for EUV Astrophysics, working on preparations for the EUVE all-sky
survey. In January 1992 Koji joined NASA Goddard Space Flight Center as an
USRA research scientist. Initially, his work was on the Japanese-US ASCA
mission. More recently he has worked on the ill-fated ASTRO-E mission, and
now the Astro-E2 mission, renamed Suzaku after launch.

In recent years, Koji has focused his research efforts into the X-ray observations of magnetic cataclysmic variables and other accreting binaries.
It is these magnetic CVs, specifically intermediate polars (IPs) that will
be the topic of out interview.

CVnet: Hello, Koji. Thank you for agreeing to participate in this CVnet
interview series.

IPs are the CV class with the most ferocious debates about membership or
not. On your web site you refer to yourself as the 'Curmudgeon', referring
to your strict requisites for inclusion as a member of the IP class, so let'
s begin with the definition of an intermediate polar.
Intermediate polars are systems between non-magnetic CVs and the AM Her
systems. The accretion process in IPs is through a disc with a disrupted
inner radius, or an accretion stream (as in the polars), or both. By virtue
of a lower magnetic field (as compared to polars) the accreted area on the
white dwarf is larger, and typically extends over a hemisphere. The white
dwarf spin is not synchronous with the orbital period, as in the AM Her
systems. The spin period of the white dwarf is shorter than the orbital
What other characteristics would you include to define a "genuine IP"?

Mukai: Nothing at all. The definition is fine, I think pretty much
everybody agrees. Any differences of opinion are in how to apply
this definition to real-life data. What I'm trying to do, as the
Curmudgeon, is to curb the enthusiasm of some observers. I mean,
if you see a peak in the periodogram of a three-hour stretch
of photometry of a CV, you shouldn't jump to the conclusion that
you've discovered an IP. After all, all CVs vary on a variety of
timescales, and that will produce a peak in the periodogram somewhere.
You need more than just half a night's photometry to be sure you're
looking at a persistent, periodic, characteristic of the system.

CVnet: The intermediate polars with the shortest spin periods and weakest
magnetic fields are called DQ Herculis stars, although there has been some
discussion concerning whether they really deserve to be called a separate
class of object. Are all DQ Her systems IPs, are all IPs DQ Hers, or is
there a distinction?

Mukai: I personally don't think there are two distinct classes - there
are quantitative differences, yes, but not qualitative one. So, to me,
they are all IPs. You can call them all DQ Her systems, like Joe Patterson
prefers to do.

CVnet: How does the relative stability of the spin period provide evidence
for white dwarf, as opposed to neutron star, nature of the compact object.

Mukai: Speaking of Joe, I think he was the first person to point this out,
way back in 1981 in his Nature paper. Basically, if you compare the radius
of a white dwarf versus that of a neutron star, a white dwarf is about a
thousand times bigger. And that makes them more stable - it's much harder
to change the spin of a white dwarf than the spin of a neutron star. So,
observationally, if you see the spin of an X-ray pulsar changing quickly,
then you suspect it's a neutron star. If the spin doesn't change much,
either you were unlucky or the system has a white dwarf.

CVnet: Will you explain what a spin up or down is, and how it relates to the
study of these systems?

Mukai: If you measure the spin period of an IP accurately one year,
and come back the following observing season, you might find that
the spin period is now slightly shorter - that's spin up - or slightly
longer - that's spin down. Since white dwarf spin is stable, it takes
years of collecting a lot of photometry to really see if an IP is spinning
up or spinning down. In some cases, one system might do both!

IPs probably last hundreds of millions of years as IPs, gradually
changing its orbital period and such. If IPs never deviate from such long
term average, it should be exceedingly difficult to detect any spin ups
or downs. The fact that we do see them mean that IPs do deviate from
their long-term average on timescales shorter than, say, millions of
years. In simple terms, if the accretion rate goes up, the torque
exerted by the accreting gas wins and the white dwarf spins up.
If the accretion rate goes down, the braking effects of the rotating
magnetic field wins and the white dwarf spins down. But the physics
is rather complicated - any time you have an interaction of plasma and
magnetic field, it's hard to figure out exactly what will happen (just
ask the people trying to build nuclear fusion reactors!). So, getting
numbers out of spin ups and spin downs is not easy - or rather, you can
extract numbers but you never know how much you can trust them.

CVnet: Some IPs, [GK Per, DO(YY) Dra, HT Cam, and EX Hya] , exhibit
outbursts. These are probably the systems most familiar to CVnet observers
and participants. What is the current thinking on the cause of these
outbursts, and why are these systems different than the IPs that don't
exhibit outbursts?

Mukai: You would think that the mechanism is the same for these IPs
as in ordinary (non-magnetic) dwarf novae. By that logic, the IP
outbursts must also be due to instability in the accretion disk.

CVnet: Some IPs have shown occasional low states in archival plate
photometry, but probably not as frequently as in AM Her type systems. What
are the possible causes for this behavior?

Mukai: I'm mostly going to punt on this question, because nobody really
knows why some CVs go into low states. Yes, some clever people have ideas,
but there are far more questions than answers on this topic, in my opinion.
There are some ideas why AM Her type systems go into low state more often
- perhaps because they don't have a reservoir of mass in the form of an
accretion disk, perhaps because the magnetic field would force the plasma
to climb up the gravitational potential (that's hard to do!) under certain

But mostly, I'd like to appeal to the readers of CVnet to watch out for
IPs going into low states. Because, as far as I know, the only known
examples of IPs going into low states are from archival plates, discovered
many years after the fact, and nobody has caught one in action. Low states,
if you can catch one, are a great opportunity to learn about the underlying
stars uncontaminated by accretion. That would be great! In contrast, AM Her
type systems go into low states so often that many of us now consider low
states to be an annoyance as far as AM Her systems are concerned.

CVnet: Let's talk about the evolutionary track of these systems a bit. The
commonly proposed evolutionary history of non-magnetic and magnetic CVs has
been assumed to be similar. Recent results, infrared spectroscopy of two
dozen CVs in Harrison et al (2004, 2005) and UV spectroscopy by Gaensicke et
al 2003, find evidence for peculiar abundance ratios in the secondaries of
non-magnetic CVs; specifically deficits of carbon and enhancements of
nitrogen. If magnetic CVs follow the same evolutionary path, one would
expect to find similar abundances in the secondary stars of magnetic
systems. In Harrison et al, Astrophys.J. 632 (2005), several
polars were examined and the spectra of these secondaries are consistent
with normal late type dwarfs, suggesting that the evolution of secondaries
in magnetic systems is different than that of non-magnetic systems.
Have there been similar studies of the secondaries in IPs, and if so, are
the secondaries of these systems normal late type dwarfs or do they exhibit
the same peculiar abundance ratios as non-magnetic systems?

Mukai: By and large, the secondary of IPs have never been convincingly
detected - with the exception of GK Per and YY Dra, I think. Accretion
rates are so high in IPs, it's hard to see the secondary (and believe me,
I've tried). That's another reason to want to see them go into low states.

CVnet: One of the goals of CVnet is to bring professional and amateur
astronomers together to the mutual benefit of both. What can interested
amateur observers with modest telescopes do to help in the understanding of
these systems?

Mukai: CVnet observers can watch out for outbursts and low states of
IPs - at the moment, I don't have anything specifically set up to observe
outbursts, but it's always useful to know. As I said, if an IP is found
in a low state, that can be a gold mine of information. The other main
thing is to follow the spin ups and spin downs - the best way may be to
join Joe Patterson's CBA network, which always have campaigns set up to
make sure there are enough data on IPs to be able to track their spin

CVnet: Thank you very much, Koji. I hope you enjoyed this and will agree to
talk with us again in the future.

Mukai: Sure, it's been a pleasure.

The Curmudgeon's IP Home Page:

Recent Publications
Kuntz, K.D., Gruendl, R.A., Chu, Y.-H., Chen, H.-C. R., Still, M., Mukai, K., Mushotzky, R.F. 2005, "The optical counterpart of M101 ULX-1," ApJLett 620, L31-L34
Mukai, K., Orio, M. 2005, "X-ray Observations of the Brigt Old Nova V603 Aquilae," ApJ 622, 602-612
Belle, K.E., Howell, S.B., Mukai, K., Szkody, P., Nishikida, K., Ciardi, D.R., Fried, R.E., Oliver, J.P. 2005, "Simultaneous X-ray and Optical Observations of EX Hydrae," AJ 129, 1985-1992
de Martino, D., Matt, G., Mukai, K., Bonnet-Bidaud, J.-M., Gaensicke, B.T., Gonzalez Perez, J.M., Haberl, F., Mouchet, M., Solheim, J.-E. 2005, "X-ray Confirmation of the Intermediate Polar HT Cam," A&A 437, 935-945
Parker, T.L., Norton, A.J., Mukai, K. 2005, "X-ray Orbital Modulations in Intermediate Polars," A&A, 439, 213-226
Mukai, K., Still, M., Corbet, R.H.D, Kuntz, K.D., Barnard, R. 2005, "The X-ray Properties of M101 ULX-1 = CXOKM101 J140332.74+542102," ApJ 634, 1085-1092