Koji Mukai on X-rays and Dwarf Novae

This is the second time Koji Mukai has granted me
an interview. The first time we discussed magnetic CVs,
intermediate polars specifically. That interview can be
read here.

Now Koji is back to discuss RU Peg and the X-ray
behavior of dwarf novae with massive white dwarfs.

CVnet: Hi, Koji. Thank you for granting us another interview. Let's start with
where are you working now and what are your primary responsibilities?
Also, what are you current areas of research?

Mukai: I work at NASA's Goddard Space Flight Center, although my employer
is University of Maryland, Baltimore County. I work at the
US Guest Observer Facility for the joint Japan-US Suzaku mission,
and also work on the education and public outreach group of the
astrophysics science division here. My research has always focused
on accreting white dwarfs - it still does, but over the last few
years it has expanded from just CVs to CVs and symbiotic stars.
I'm interested both in accretion and mass ejection during nova

CVnet: Are you still maintaining the Intermediate Polars pages?

Mukai: Yes, although I haven't had the time to make a substantial update
for the last year or so. There are quite a few new confirmed
and candidate IPs to add to the site!

CVnet: AAVSO Alert Notice 459 states you are requesting monitoring of the dwarf nova,
RU Peg, in anticipation of the next outburst. Let's discuss why RU Peg is so interesting,
and what you hope to learn by observing it with Swift.

Mukai: RU Peg is a bright dwarf nova that has been neglected, relatively
speaking, for X-ray observations. For dwarf novae, it is very
important to conduct X-ray monitoring campaigns through an outburst.
Now that RXTE has been decomissioned, Swift is the only observatory
for this type of campaign.

CVnet: Since your observations will be in the X-ray, where do X-rays in dwarf novae originate?

Mukai: In a dwarf nova, half the available gravitational potential energy is
radiated away in the accretion disk - that's a source of infrared,
visible, and ultraviolet light. The other half of the potential
energy has been converted into the kinetic energy of the disk material,
moving at several thousand kilometers per second. Since the white
dwarf is rotating much more slowly than this, that motion must suddenly
cease in a very small region - what we call the boundary layer. That's
where the X-rays originate in dwarf novae.

CVnet: How does the amount of X-rays emitted change between the quiescent and outburst
phases of the dwarf novae?

Mukai: That actually depends on what you mean by "X-rays." But if you mean
X-rays in the traditional band (photon energies of 2-10 keV, or
wavelengths of about 1-5 Angstroms), dwarf novae become fainter during
outburst than in quiescence.

Below are the AAVSO and RXTE light curves of WW Cet from
a recent paper I was involved in. This shows what I now think of
as "typical" behavior. X-ray bright in quiescence, X-ray faint in outburst, 
with sudden a transition and no intermediate states.

From 2011PASP..123.1054F  Fertig, D.; Mukai, K.; Nelson, T.; Cannizzo, J. K. 
The Fall and the Rise of X-Rays from Dwarf Novae in Outburst: RXTE Observations of VW Hydri and WW Ceti

CVnet: What do we think is happening as the outburst begins in the accretion disc
to cause this X-ray suppression?

Mukai: In quiescence, the boundary layer is optically thin - that is, X-ray
photons, once emitted, escape the boundary layer without interacting
with matter. In outburst, much more matter is flowing through the
boundary layer, so the density is much higher. In this case, the
boundary layer becomes optically thick - X-ray photons emitted by
the ions interact with surrounding matter several times before
they are able to escape. In this situation, the temperature of
the boundary layer drops, and only lower energy X-rays ("soft"
X-rays, as in X-rays that cannot penetrate matter that much) are
emitted - with energies below 0.5 keV. The optically thin case
is like the corona of the sun, the optically thick case is like
the photosphere of the sun. In fact, during outburst, the boundary
layer has both the photosphere-like region and the corona-like region.

If the line of sight to the dwarf nova is relatively free of
interstellar matter, then we can observe dwarf novae brighten
dramatically during outburst in soft X-rays and extreme ultraviolet.

CVnet: Isn't this the opposite of what has been observed in prior campaigns on SS Cygni?

Mukai: No, not really. During the peak of the outburst (as determined by
visible light observers), SS Cyg is fainter in hard X-rays and brighter
in soft X-rays. It's in the time of transitions that SS Cyg has
shown a behavior pattern that has not been seen in other dwarf novae.
Other systems have shown "quiescent" (hard X-ray bright) and
"outburst" (hard X-ray dim) states, and nothing else. SS Cyg,
on the other hand, initially brightens in hard X-rays (near the
time of the peak visible light) before switching to hard X-ray
faint/soft X-ray bright state. There is another hard X-ray brightening
near the end of the outburst. So, in hard X-rays, it goes from
bright-brighter-faint-brighter-bright through an outburst.

You can see this in the light curves here.

CVnet: Does this mean SS Cygni is actually the exception to the rule, and not the
prototype as most people have always assumed?

Mukai: You can still consider SS Cyg to be the prototype of the hard X-ray
bright (quiescence) - dim (outburst) behavior. It appears to be
an exception in showing the bright-brighter-faint-brighter-bright

CVnet: How does the mass of the white dwarf come into play in the whole process?

Mukai: The accretion rate at which the boundary layer switches from the
optically thin regime to the optically thick regime is believed to
be a strong function of the white dwarf mass, according to theoretical
studies. The higher the white dwarf mass, the higher the accretion
rate at which the transition occurs. The state change of the disk,
between quiescence and outburst, is governed by the conditions in
the disk, and is far less sensitive to the white dwarf mass. When
the disk goes into outburst, the accretion rate through the boundary
layer rises, making it optically thick for an average mass white
dwarf, while making it brighter but still optically thin for a
high mass white dwarf - at least that''s a physically motivated
explanation of why SS Cyg might behave differently from the average
dwarf novae.

CVnet: Is this the main reason for selecting RU Pegasi as your target for the Swift campaign?

Mukai: Yes, we believe that the white dwarf in the RU Peg system is among the
most massive for a dwarf nova. Also, it is one of the X-ray brightest
dwarf novae for which an X-ray monitoring campaign has never been

CVnet: How do you know the mass of the white dwarf in RU Peg?

Mukai: In the optical spectra of RU Peg, you can see both the mass donor and
the accretion disk, so the radial velocity motion of both stars can
be measured, with the usual caveats.

CVnet: So what if we don't see the same X-ray behavior as SS Cyg when RU Peg goes into outburst?
Will the campaign still prove useful scientifically?

Mukai: That would be a very important result, because it would have disproved
our current hypothesis. We will have to go back to square one in terms
of trying to understand why SS Cyg is different, but that's how science
is supposed to work.

CVnet: Thanks, Koji. Any final comments or advice for our observers?

Mukai: Thank you, and thanks to all the AAVSO observers out there who make
this kind of research possible!

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