Enter the ‘Anti-Transit’

by Paul Gilster on April 22, 2014

Gravitational lensing is a technique rich enough to help us study not only distant galaxies but exoplanets around stars in our own Milky Way. As gravity warps space and time, light passing near a massive object takes the shortest route, from our perspective seeming to be bent by the gravitational field. Inside the Milky Way, such effects are referred to as ‘microlensing,’ capable of magnifying the light of a more distant object and sometimes revealing the presence of an unseen planet around the intervening star. Now we have a Kepler find with implications for binary stars.

Working with Eric Agol at the University of Washington, graduate student Ethan Kruse has discovered a ‘self-lensing’ white dwarf eclipsing binary system. He made the find while looking for transits in the Kepler data, the signatures of planets crossing in front of their stars as seen from Earth. KOI-3278 turned out to have an unusual signal, says Kruse:

“I found what essentially looked like an upside-down planet. What you normally expect is this dip in brightness, but what you see in this system is basically the exact opposite — it looks like an anti-transit.”

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In other words, a transiting planet causes a dip in the overall light of the star that shows up in the well known lightcurves that have flagged the presence of so many Kepler planets. Kruse was seeing not a dip but a surge in brightness, the apparent result of movement within this binary star system. 2600 light years away in the constellation Lyra, KOI-3278 is now known to be made up of two stars with an orbital period of 88.18 days, one of them a white dwarf, separated by about 70 million kilometers. The brightness surge is the white dwarf’s lensing effect upon the star it passes in front of as we view the system. The lensing effect allows the mass of the white dwarf to be measured as roughly 63 percent the mass of our Sun.

Image: An image of the Sun used to simulate what the sun-like star in a self-lensing binary star system might look like. Credit: NASA.

It was about a year ago that Philip Muirhead (Caltech) and colleagues published their own findings, likewise based on Kepler data, of a white dwarf being orbited by an M-class dwarf that was, although larger, less massive than the white dwarf it circled. KOI-256 looked at first glance to show the signature of a gas giant planet eclipsing the red dwarf, but radial velocity follow-up studies using the Hale instrument at Palomar Observatory demonstrated that the intervening object was a white dwarf. The KOI-256 data showed the same brightening effects that Kruse found with KOI-3278, although the microlensing in the former was not nearly as powerful.

In both cases, refined mass measurements of the white dwarf have followed, as well as more accurate analysis of the mass and temperature of both stars. Kruse and Agol think the effect can be used in follow-up observations to reveal the white dwarf’s size, and have applied for time on the Hubble Space Telescope to study the system in greater detail. We may or may not find more systems like this in the Kepler data, but the KOI-3278 discovery gives us yet another way to use the extremely subtle effects of microlensing. In this case, the lensing is a repeatable phenomenon as the two stars orbit each other, not the case with most microlensing events.

The paper is Kruse and Agol, “KOI-3278: A Self-Lensing Binary Star System,” Science Vol. 344, No. 6181 (18 April 2014), pp. 275-277 (abstract). The KOI-256 paper is Muirhead et al., “Characterizing the Cool KOIs. V. KOI-256: A Mutually Eclipsing Post-Common Envelope Binary,” The Astrophysical Journal Vol. 767, No. 2 (2013), 111 (abstract).

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CharlesJQuarra April 22, 2014 at 12:30

I guess you would see an anti-dip also as the planet albedo reflected more starlight in the orbit’s opposition, if the orbital plane is just at an angle less than 90º. You wouldn’t expect to see a dip when the planet is between the star and us, because there orbital plane’s angle wouldn’t allow it. I wonder if they were able to dismiss that possibility by some other reason that I’ve missed

Andrew Palfreyman April 22, 2014 at 17:45

I am fascinated by the gravitational lensing opportunities available as popularised by Claudio Maccone in respect of using our own Sun as a lens (although I just wish he’d make his books more affordable!). The big fly in the ointment is the 550+ AU trek out beyond the focus – at least fifteen times more distant than Pluto. So I got to thinking whether we couldn’t do something along the lines of synthetic aperture optics right here in the inner solar system, which would entail flying circular pseudo-orbits of radius roughly equal to that of the Sun. Have any papers been published on this technique?

Dave Moore April 22, 2014 at 23:43

I was thinking about gravitational lens imaging, and given that your first mission using the sun is going to be a multi-year affair, you really want to know you are going to get crisp images at the end of it.

So, I was thinking possible gravitational lens imaging missions closer to home to perfect the technology. This led me to the Proxima occultation post

http://www.centauri-dreams.org/?p=27914

something to look forward to latter this year. This, of course, wont be imaging the distant star using Proxima, but looking at the distant star’s light curve to see if Proxima has any planets.

But I was thinking, if we could find a nearby star in front of an emission nebula, we could use a small space telescope and an external occultation disk to produce images of the nebula. The actual images might not be very interesting, but it would test out the technique. Another possible target would be the Andromeda galaxy.

Lionel April 23, 2014 at 2:02

I have a question about how this white dwarf happens to be so close to it’s companion star KOI-3278. My understanding of system evolution is roughly zero. This white dwarf has a separation of about 0.46 AU from KOI-3278.

This white dwarf was previously a red giant presumably with a radius of several AU that would completely engulf KOI-3278 if it were in the same position today.

Now that we’re on this subject, why wasn’t this white dwarf spotted from doppler shift or physical movement of KOI-3278? The white dwarf’s gravity must drag KOI-3278 around all over the place. Was it never investigated?

Thanks for your replies

Lionel April 23, 2014 at 10:23

Regarding the FOCAL probe, perhaps a more advanced civilisation (optimistically ourselves in a few centuries?) would achieve a spherical swarm of millions of FOCAL probes, one probe for watching each star its planetary system. One of my favourite thoughts is the diffusions of self-replicating mining robots that spread from NEOs to main-belt asteroids, Jupiter trojans, Centaurs, Neptune trojans, KBOs and then detached and Oort cloud objects, eventually reaching beyond 550AU to give us our FOCAL torus or sphere. Interesting to note that Sedna spends most of its time beyond 550 AU, though it’ll be a couple of thousand years before it’s back at that distance.

@Dave Moore

Thanks for the link to the Proxima occultation post, I hadn’t read it before and it’s very exciting as it looks like the occultations this year and in 2014 will be several months long, giving lots of data for the experts to play planet hunting detective with.

Michael April 24, 2014 at 8:35

@Lionel April 23, 2014 at 2:02

‘I have a question about how this white dwarf happens to be so close to it’s companion star KOI-3278…This white dwarf was previously a red giant presumably with a radius of several AU that would completely engulf KOI-3278 if it were in the same position today.’

The most likely process occured when the more massive star turned into a white dwarf and shed it layers it was further out and migrated closer due to tidal drag interactions with the other star (the other star can still orbit within its atmosphere as it is quite thin). The other star can also steal material during that period gaining mass and angular momentum. There are examples of white dwarfs orbiting very close together, the fastest I think at only 39 minutes with a separation distence of a couple of 100 000 miles!

Lionel April 27, 2014 at 12:30

@Michael

Thanks, and wow

ljk June 24, 2014 at 13:20

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