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


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!