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
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!

IW Andromedae is a Z Cam star!

This is not a newsbreak. Taichi Kato, Ryoko Ishioka and Makoto Uemura described this system as a Z Cam dwarf nova back in 2003, in the International Bulletin on Variable Stars (IBVS).

http://www.konkoly.hu/cgi-bin/IBVSpdf?5376

Based on 55 observations made over a period of about four months this team caught IW And in a standstill, which is the defining characteristic of Z Cam dwarf novae.

Light curve of IW And from Kato et al, 2003, IBVS 5376

Since the earliest days of the Z CamPaign http://arxiv.org/abs/1104.0967 I had noticed that the light curve of IW And was unlike that of any other system in the Z Cam candidate sample. It exhibited a quasi-periodic behavior whose light curve looked more like an eclipsing system than a dwarf nova.

This never before seen behavior led me to believe that perhaps we had discovered some new animal in the CV Zoo. "This doesn't look like a Z Cam light curve," I told myself. "We may have stumbled on to something important here!"

My ego quickly overruled the facts, and I have been hoping ever since that I had uncovered some unique, astrophysically interesting class of CVs. I convinced a lot of people to pay special attention to this system, hoping that my “discovery” would pan out.

Fortunately, the stars couldn’t care less about my ego, and IW Andromedae has once again gone into a prolonged standstill, as evidenced by the AAVSO light curve, confirming that it is indeed a member of the Z Cam class of dwarf novae.

 AAVSO light curve of IW And, 

clearly showing the standstill after an active period of outbursts and quiescence.

Welcome to the club, IW And. I still think you are special, and you will remain one of my favorite variable stars forever. 

As if to add to the lesson in humility, V513 Cas, my other "special case" appears to be in the early stages of a Z Cam standstill also!

A Z CamPaign update will be coming soon. In the meantime, thank you to all who have made this campaign a success and keep up the excellent coverage.

Wild Stars Pictorial Review

Steve Howell, head cowboy, and coiner of the famous acronym TOADs (tremendous outburst amplitude dwarf novae) welcomes everyone to the conference and explains where the bathrooms are.

His other main task for this conference seems to be getting everyone who wants one, a receipt for their expense reports. Poor Steve.

His poster on magnetic CVs has an awesome visualization. I'll try to get permission to reproduce it here. It is way cool....err, I mean hot.




ChritsianKnigge opens the paper session by reviewing what we know about the secondaries in CVs and their role in the evolution of these systems.

Check out the visualization of that bloated, star spotted, crazy looking secondary. Wild stars indeed!

More interesting is the fact that CV donor stars are larger and cooler than individual main sequence stars of equal mass. Observing these secondary properties may tell us a lot about the evolutionary track of these systems. Fascinating stuff presented very well. Two Simothumbs up for this one.

What's that? You say you don't understand magnetic braking? Don't worry, I'm in a room full of PhDs who will talk about it all day, but they don't understand it either!


Hands down winner of the animated visualizations for the conference thus far definitely goes to D.V. Bisikalo from the Russian Academy of Sciences, Moscow. His illustrations of accretion and the outflow of material into the envelope around the binary were fascinating to watch and quite detailed. The parameters and science used to achieve these results may be unrealistic, but the animations were glorious! Not only that, but watching the accretion overflow, I had an 'aha moment' for something I've been working on regarding Z Cam outbursts.




For those of you who remember my blog on 'Dusty Toads', here is one of the authors, Don Hoard, talking about surprising dusty environments around cataclysmic variables. They went hunting for information about the red secondary star of WZSge with the Spitzer Space Telescope and found so much dust they couldn't observe the secondary! A surprising result that may lead to, well, who knows?

I'll be interviewing Steve and Don about their dusty toads and where this new result may lead CV research.

On a personal note: it has been a lot of fun meeting the people and associating the names with the faces. I met several Japanese observers and important contributors to CVnet-AkiraArai, Hiroshima University, IzumiHachisu, University of Tokyo, AkiraImada, Kagoshima University, DaisakuNogami, Kwasan Observatory, Kyoto University. I also got to meet and talk with astronomers using AAVSO data for their papers here at this conference or elsewhere: Brad Schaefer, Louisiana State University, Christian Knigge, University of Southampton.

AAVSO was well represented with Arne Henden giving a poster presentation with hundreds of AAVSO light curves and Paula Szkody talking about pulsating white dwarfs in SDSSCVs.

Boris Gaensicke, who I met for the first time in Cambridge, UK last spring, seems to have his fingers in so many pies here it is quite remarkable. He is listed as a co-author or principle investigator on at least 40% (UNSCIENTIFICSIMO-ESTIMATE) of the papers being presented.

And on a personal basis, I had the pleasure of meeting Kurtis Williams, of Professor Astronomy's Astronomy Blog .
He has been kind enough to support the AAVSO Writers Bureau with his blogs and is an all around nice guy who it is my pleasure to have met finally.

It's been a good time so far. More later.

SimoCowboy Ready to Roll!

Next week is a conference I have been excited about attending for a long time. It's an entire week devoted to my specialty, cataclysmic variables. The list of attendees is a literal who's who of CV research.

The conference is called Wild Stars in the Old West II. This special get together doesn't come around that often. I wouldn't miss it for the world.


From the website:

"It has been ten years since the last North American Workshop on Cataclysmic Variables and nearly five years since the last international meeting on cataclysmic variables and their kin. Of particular interest since these last meetings are new results based on observational platforms such as GALEX, Spitzer, Chandra, XMM INTEGRAL and Swift/BAT, large surveys such as SDSS and planned Pan-STARRS and LSST, smaller but equally important surveys such as All Sky Automated Survey (ASAS), Catalina Sky Survey, ‘Pi of the Sky’, ROTSE, results from large aperture ground-based telescopes, theoretical advances, and evolutionary relationships of CVs to other binary stars."

I plan to blog about the proceedings and talks, and I'm taking a digital audio recorder to do some one on one interviews with some of the leaders in CV research. These will be turned into podcasts for Slacker Astronomy and Restless Universe.

Sunday is a travel day, and the welcome get together in Tucson. Monday, the real stuff begins. Check back for updates next week.

Yeehaw!

Still On The Rise

As a quick update to my blog about V630 Cas, the anticipated outburst is still under way. This is pretty unusual for the types of dwarf novae I normally follow. Three weeks ago we suspected it was going to go into outburst, and here it is still slowly rising.


Most of the time, a dwarf novae would have risen to maximum in a day or two, remained there for a few days and then began to trail off in brightness until it reached minimum after a week or so. Obviously, V630 Cas is a horse of a different color.

Refining the Distance Scale

I scan the new astrophysics papers regularly. I almost always find something I end up downloading and reading, either right then and there, or later when I have time to concentrate. Rarely do I stumble across a paper that I can't take my eyes away from, like a great novel. One of those times when, forsaking all else, you must get to the last page.

Yesterday I found a great paper. What I was most pleased with was the fact it was written in plain English, with good grammar and organization. I understood every bit of it from start to end! That almost never happens. I couldn't stop reading it.

I don't have a PhD in astronomy or physics. I do this because I love it, period. So I often find myself part way into a paper on some astrophysical phenomena that the author is trying to explain, but no lights are going on in my brain. Either the subject is too technical for me to grasp, or the author is writing about things at a level only the top five experts in the world would ever understand. Add the poor English skills of foreign scientists writing in a second language and things can get ugly fast.

At some point I have to decide to either suck it up and plow through, hoping that a light will come on somewhere in the process, or skim through the rest to see if anything interesting develops with the plot.

Rarely do I find myself whooping it up and commenting out loud about the paper in my hands.

Okay, enough teasing. The paper is Absolute Magnitudes of Dwarf Novae: Murmurs of Period Bounce by Joe Patterson. Obviously, the subject appeals to me because dwarf novae are my special area of interest. But let me quote you some examples of why I was so impressed with this paper.

The first paragraph:
"Distance is the sine qua non of astrophysics. A distance estimate is required to convert flux to luminosity, and stellar physics is all about luminosity, not flux. Unfortunately, distances to cataclysmic variables are particularly difficult to estimate, because the dominant light source is not a star, but an accretion disk- preventing straightforward application of physical methods developed for single stars."

That will never be stated more clearly, ever. Yet it has a conversational tone to it that invites you in to take a look around. Remember, this is a scientific paper!

There are some other gems near the beginning that particularly caught my attention.

"In the 1980's available data on dwarf nova eruptions consisted of a blend of photographic and visual magnitudes. But now we have access to searchable variable star records, especially that of the AAVSO. The human eye is the ideal detector for this purpose, since it is immune to changes in technology, and used by thousands of observers. Furthermore, the central wavelengths of the eye and the commonly used Johnson V filter are similar; and both detectors are broad enough to render line emission insignificant."

He gives praise to the observations of amateur observers, the AAVSO and explains why visual observations are scientifically valuable all in one breath! I have a new hero.

I won't spoil it by giving away the end, and if you want to find out what period bouncers are you're going to have to read the paper.

I'm going to print it out on fine paper, have it bound in a nice little cover, and get Joe to autograph it for me when I see him in Tucson later this month at the CV conference, 'Wild Stars in the Old West.'

Precursor To An Outburst?

Dwarf novae are compact binaries where one star is a sun-like star and the other member is a white dwarf, orbiting so close that it is literally stripping the outer atmosphere off its partner. The material streams over to the white dwarf but can’t slam down to the surface. Instead, it goes into orbit around the white dwarf, forming what is known as an accretion disk.

Image used with permission. Copyright Mark A. Garlick. Do not use this image without permission.

Eventually, the accretion disk builds up enough material to become unstable. The disk material falls down onto the surface of the white dwarf, causing a thermonuclear explosion that releases all kinds of energy across the electromagnetic spectrum. Optically, we see these as sudden brightenings of several magnitudes in a matter of hours. These outbursts can last from days to weeks. The system eventually simmers down into quiescence and the whole process starts over again.

The majority of these binaries have periods measured in hours. Think about that for a second. Imagine a white dwarf racing around our sun in a few hours, so close it is stripping material from the surface. Everything about these systems is extreme.

Some dwarf novae, like SS Cygni, go into outburst every couple weeks. Some may take years or decades to build up enough steam to explode into outburst. No one knows when the next outburst will occur, making these a favorite target for amateur astronomers to monitor on a nightly basis.

Occasionally, one of these cataclysmic variables will tip its hand to an upcoming outburst, by becoming active a week or more before the big event. Sometimes they will actually have a minor precursor outburst, fade to quiescence and then go into a major outburst.

Over the last couple nights, several UK observers have reported an increase in the quiescent level of V630 Cas. It has been measured peeking its head into the 15th magnitude range, slightly brighter than its normal quiescent magnitude around 16.5V.
V630 Cas is rare example of a dwarf nova with a long orbital period. Its period is measured in days, not hours.

The last recorded outburst of V630 Cas was in 1992, and lasted about three months from beginning to end. That is a long time for an outburst to last! The only other recorded outburst was in 1950. Obviously, outbursts of this system are very rare, so astronomers will be excited to catch every last detail from beginning to end of the next outburst.

This current 'activity' could be the precursor to an upcoming outburst. Observers will be paying close attention to V630 Cas in the coming weeks to make sure that a rare, and possibly long, outburst isn't missed.

I’ll let you know what happens.

Professional and Amateur Collaboration

Amateur astronomers and the American Association of Variable Star Observers (AAVSO) have done it again. This time AAVSO observers were able to provide timely observations to alert UK radio astronomers of the early stages of an outburst of SS Cygni. The results, the first ever detection of a radio jet from a dwarf nova, have been published in the journal Science. The pre-print of the paper can be downloaded from arXiv.org.

What I found most intriguing about this paper is that the jets they were looking for were transient phenomena that only occur at a certain stage of the outburst (onset). The astronomers predicted that the jets would exist, and at which stage of the outburst they would be detectable _before_ observing them. This brought to mind a comment Joe Patterson made in an interview for CVnet we did a couple years ago. "The dwarf-nova outburst itself has become generally well understood -- in the sense that there's a theory which successfully reproduces the observed phenomena. But it's noteworthy that all of that theory was crafted to fit previously known data -- it has never actually predicted something not known in advance."

Congratulations to Dr. Elmar Koerding and his team for their discovery, and kudos to the amateur variable star observers all over the world who made this observation possible. There is a very nice optical and radio light curve of the outburst on page 6 of the paper. Multi-million dollar radio telescope observations coupled with visual observations from backyard telescopes, the very definition of pro-am collaboration.