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Koji Mukai RU Peg Campaign





This is the second time Koji Mukai has granted CVnet 
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
       outbursts.

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
       behavior.

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
       done.

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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