“Variable and polarized radio emission from the T6 brown dwarf WISEP J112254.73+255021.5”

2017 July 18

I should post something about this paper — it only came out on Arxiv eleven months ago. What can I say, it’s been a busy year! Here’s the official journal version.

This paper was inspired by a neat result from Matt Route & Alex Wolszczan that came out last year (free Arxiv version here). They detected yet another radio-emitting brown dwarf with Arecibo, adding to the very small number of T-type brown dwarfs detected in the radio. These objects are fascinating because they’re at the cutting edge of what we can detect with current facilities — they are the physical size of planets like Jupiter, and not that much more massive. Radio observations tell us about the magnetic fields of these objects, and I’ve argued recently that the takeaway is that these brown dwarfs have magnetic fields that are basically like amped-up versions of the one we find around Jupiter and the Earth. Which is cool because Jupiter’s magnetic field turns out to be bananas as well as very important scientifically — it drives immensely complex space plasma physics and tells us about Jupiter’s internal structure. Studies of T-type brown dwarfs give us the first glimpse of what the magnetic fields of planets around other stars might look like.

Not only did Route & Wolszczan discover an interesting new object — full name WISEP J112254.73+255021.5, “WISE 1122+25” for short — they made a bold claim: that it might be rotating as rapidly as one full revolution every 18 minutes. Keep in mind that this is a ball of gas the size of Jupiter. You can do the calculations and it’s not physically impossible for WISE 1122+25 to rotate that quickly, but only just barely. This thing would be dramatically egg-shaped, and who knows what kind of neat physical effects that would cause.

Route & Wolszczan made their conjecture based on the timing of when they detected bursts of radio waves from WISE 1122+25. If the bursts are sweeping past us like the beam of light from a lighthouse, their periodicity tells us about how long it takes the object to make a complete spin. Unfortunately, the data weren’t definitive — there were only five bursts detected over a span of months. If every burst arrives at a perfectly rigid cadence, you can still measure a rotation period, but with that few bursts spread over that long of a time, you worry about any periodic signals being spurious.

It’s definitely a neat possibility, though! So, as has happened before, I teamed up with some of my regular collaborators, John Gizis and Edo Berger, to observe WISE 1122+25 with the NRAO Very Large Array (the legendary “VLA”) and see if we could verify the claims. Arecibo is better at detecting very rapid bursts than the VLA, but the VLA can sit on one object for longer and detect fainter events — perfect for checking whether we could, say, see any bursts happening every 18 minutes like clockwork. We got these observations through the VLA Director’s Discretionary Time channel — thanks to the Director!

And what did we find? The image below shows how the radio brightness varies over the course of several hours, with the radio emission being broken down in several different ways.

I won’t describe the data in detail, but the first big takeaway is that we do not see evidence for periodic signals every 18 minutes or so. A more formal periodogram analysis supports this conclusion, and we don’t see any evidence for variation in optical data either. Overall, these data make us think that the 18-minute periodicity found by Route & Wolszczan was spurious.

But we do see a lot of complicated variability in the radio emission! What I found to be the most exciting was the measurement shown in the lowest panel of the figure above. It shows the level of circular polarization of the radio waves we get from WISE 1122+25. Almost all astronomical objects emit light that is unpolarized in the circular sense — that is, 0% circular polarization. Radio-emitting brown dwarfs are among the rare objects that can produce high levels of circular polarization. The way we quantify things, circular polarization levels can range between +100%, meaning fully right- handed circular polarization, to –100%, for fully left-handed circular polarization.

What’s neat about the VLA data is that the polarization fraction seems to swing back and forth between the two states. We’ve seen brown dwarfs emit radio bursts with both kinds of handedness, but I’m not aware of any data showing these kind of long-lasting, abrupt transitions. And if you get a bit ambitious in the interpretation, you can imagine that maybe this handedness flips back periodically with the rotation of the brown dwarf. Looking at the different pieces of data, we found some evidence for periodicity at 116 minutes. But the whole observation only spanned 162 minutes, so that’s a very tentative idea — you’d want to see multiple flips back and forth all in sequence to be more confident.

But … if you want to get even more ambitious … You might be able to explain that kind of behavior if this object has a magnetic field that has a major axis that’s very misaligned with the rotation axis. With the right viewing geometry and magnetic polar caps that emitted strongly polarized radiation, you can get curves that look similar to the data.

You don’t have to take my word for it! Here’s an interactive simulator I put together that shows the scenario. You can drag the sliders around to adjust the key physical parameters of the (very simplistic) model. “LOC” is the “latitude of center”, which is how inclined the object’s equator is relative to the viewer. “CML” is “central meridian longitude”, which tracks where we are in a rotation of the object. The other two sliders set the size of the magnetic caps and their separation from the rotation axis.

On the right-hand side, the solid line is the total brightness and the dashed line is the fractional polarization. If you set LOC = 25 and θ = 50, you get curves that resemble the VLA data … if you squint, and get enthusiastic about the possibility that the emission might be repetitive every two hours, at least. It’s hard to see how the magnetic field would end up so misaligned with the object’s rotation, so it’d be really interesting if this was the case!

As always, what we need is more data. I’ve obtained more, longer observations, and am in the process of analyzing them now. Stay tuned!