The barn and the blazar
An undergraduate’s story of pulsars, black holes, and some very high energy gamma rays.
When you walk up to the Science Center at Nevis Labs in Irvington, New York, you’re struck by how much it looks like a barn instead of a physics building. That’s because it was a barn, sitting on the old estate of Alexander Hamilton’s son, James Hamilton, until the land was donated to Columbia University in 1934. Once a home for horses, the now-refurbished barn is temporarily home to a gaggle of astrophysicists, working here for a week while the main research building undergoes roof renovations — and I’m among them.
I’m at Nevis for Columbia’s REU program, one of a set of NSF-funded summer programs around the United States. In February, I applied to several astrophysics REUs and was lucky enough to get one, and so as June turns to July, I find myself sitting in this former barn studying high-energy gamma rays from matter appearing to travel faster than light, coming from a monstrous supermassive black hole almost a billion light-years away.
Ah, yes, I thought that sentence might get your attention.
This blog post is going to be less technical than most I write, and a bit longer. REUs are funded by the National Science Foundation, so your tax dollars might be supporting my research, a few thousandths of a cent per person. Interested in where that money goes? Read on to find out what’s happening in that barn by the Hudson River.
How do we see light that never reaches Earth?
If you look at things superficially, ground-based gamma ray astronomy should be completely impossible.
The problem is, gamma rays can’t reach Earth’s surface (fortunately for us!). Most of them interact with atoms a couple dozen kilometers up in the atmosphere, never coming near the ground. This is why some of the most famous gamma ray observatories — like Fermi — are in space, far above all that pesky matter. No air means plenty of gamma rays.
Nonetheless, there are several major gamma ray observatories on land, including HESS, MAGIC, HAWC, and VERITAS, the telescope I work with. All four of these telescopes use an ingenious method to detect the gamma rays they can’t see: instead of trying to detect the high-energy photons directly, they detect signals from the air showers they create after colliding with a nucleus. These air showers create pairs of electrons and positrons, which emit more radiation through interactions with neighboring atoms, and in turn create more electrons and positrons.
Now, all of these particles are moving faster than the speed of light in air, which is still quite fast, but not quite the same as the universal speed limit — the speed of light in a vacuum. Many of the electrons and positrons travel somewhere between the two speeds, and so they give off something called Cherenkov radiation, which is what VERITAS and the other observatories detect— not the gamma rays themselves — with optical telescopes. It’s a clever idea that’s only seen widespread success in the last two decades.
Where are these high energy gamma rays coming from? Some gamma ray sources lie within the Milky Way: high-mass binary stars, pulsar wind nebulae, and supernovae remnants. Very high energy gamma rays often come from outside the galaxy, primarily from active galactic nuclei, or AGN— supermassive black holes lurking in the center of galaxies, accreting matter and spewing it out as jets of relativistic particles, which in turn produce photons (and, in some cases, neutrinos) which can reach Earth.
For all of these cases, gamma rays are produced through one of two types of processes, called leptonic processes and hadronic processes. Leptonic processes involve electrons and positrons interacting with radiation, while hadronic processes involve high-energy nuclei interacting with matter in the interstellar medium, producing more hadrons. We don’t fully understand which process is more important for some sources. The characteristic double bump in the spectra of some AGN can be explained well by leptonic models, but it’s not out of the question that hadrons are involved.
A cosmic crab and a beastly black hole
There are a zoo of gamma ray sources out there — and you can check out the most energetic of them online — but this summer, I’m only interested in two of them. The first is the Crab pulsar, a rapidly-rotating neutron star, and the pulsar wind nebula that surrounds it. The Crab is the brightest source in the gamma ray sky for VERITAS, and given how constant its flux is, it’s an excellent calibration source. We look at it throughout most of the observing season, from early autumn to late spring. If a telescope’s acting up, running a quick observation of the Crab should give us weird data, so we can use the pulsar as a check to make sure that all systems are working as they should.
The Crab is what I’m doing data analysis on this week, which is why I’ve spent the last few days downloading hundreds of gigabytes of data, organizing it, and sending some of it over to the lab servers to be analyzed. The analysis I’m doing is a modified version of the normal process, involving checking for the sort of anomalous behavior I talked about before, and the Crab is the perfect testbed.
My fingers are firmly crossed that this is going to give us the results we want. If it doesn’t, that’s problematic for the data analysis pipeline. I’ll have to keep crossing my fingers for about a week or two, until the final results are all finished, but when I do, I’ll get to move on to my second source: the active galactic nucleus BL Lacertae. BL Lac is a type of AGN called a blazar, which means one of its two jets is pointed very close to Earth, so we see interesting gamma ray emission . Some blazars even emit neutrinos, like the one IceCube found almost two years ago coming from the blazar TXS 0506+056.
Like all blazars, BL Lac’s spectrum shows two peaks: one from synchrotron radiation from high-energy electrons, and one from something called inverse Compton scattering, involving those same electrons. The lower energy peak, the result of that synchrotron radiation, is at relatively low energies, so we call BL Lac a low-frequency peaked BL Lac object, or an LBL. Blazars with high-frequency peaks are known as HBLs.
Now, for a long time, there no known LBL had ever displayed very high energy emission, emitting photons with energies of a few teraelectronvolts (TeVs). There was plenty very high energy data on HBLs, but none of LBLs — until recently. In 1998, the Crimean Observatory observed TeV emission from BL Lac, and seven years later, so did MAGIC (Albert et al. 2007). This was unexpected, but things got even weirder from there. In June of 2011, the VERITAS team saw BL Lac flare, reaching an apparent brightness 125% that of the Crab, dying off within an hour (Arlen et al. 2013). Interestingly, though, was that the flare wasn’t seen at x-ray or lower energy gamma ray wavelengths, nor in radio waves. What happened?
A knotty situation
Four months after the TeV flare, radio astronomers saw another flare, this time at 15.4 GHz, 37 GHz, and 230 GHz — much lower energies. It turned out that this flare might be related to the gamma ray flare seen during the summer — just delayed. Two models were able to explain the behavior:
- A conical shock in the jet could provide photons, to jumpstart inverse Compton scattering. A high-density region of electrons passing through would produce gamma rays, and later radio waves, taking the form of a knot that would appear, thanks to an optical illusion, to be traveling faster than light.
- A high density region of matter could start upstream of the radio core, producing gamma rays. As it flows downstream, it would gradually become transparent at longer and longer wavelengths, eventually producing radio synchrotron radiation.
Credence was lent to these theories by the discovery by the Very Long Baseline Array of a radio knot emerging from the core around the same time as the radio flare.
The really exciting part of all this? We’ve seen more flares since 2011, including one this past spring. That’s where I come in. Assuming the Crab tests work, I’ll be applying the same methods to BL Lac data, trying to understand this spring’s flare and seeing how it compares to past ones. Is the AGN changing its flaring behavior, maybe getting brighter or dimmer? Are the flares all the same, or are they different? What does this mean for the structure of the knots passing through? Hopefully, I’ll be able to start taking steps towards answering these questions.
Back to the barn
Four days ago, we had the annual Nevis picnic, complete with burgers, volleyball, and drum lessons with the lab sysadmin. With all the scientists together — astrophysicists studying blazars, electricians designing circuit boards for ATLAS, physicists calibrating dark matter detectors, and many more — Nevis does feel like a family.
We dare to try to study some of the most extreme objects in the universe, from tiny subatomic particles zipping through Earth to supermassive black holes millions of light-years away. Like much of science these days, these are collaborative endeavors, made possible only by diverse groups of researchers coming together. At its best, science crosses borders and disciplines.
So far, that collaboration has helped us understand the universe better than ever before. I’m only a small part of that, nestled in a barn by the Hudson River, peering across the cosmos. But from this barn — from this old farm — we’re figuring out how the universe works, one photon at a time.
