Pulsars serve as cosmic beacons, beaming radiation into space. But not all of these beacons look the same. Now, astronomers have used NASA's Chandra X-ray Observatory to provide a clearer picture of the nebulae that form around pulsars to reconstruct their geometry and explain the differences observed from pulsar to pulsar.
While some pulsars emit radio signals, others emit more energetic gamma rays; some emit both, while some emit only one or the other. With so many variations on a theme possible, astronomers have struggled to create a comprehensive model of pulsars that explains all the available observations. Currently, it’s thought that the differences are a function of geometry — the pulsar’s orientation in space with respect to the Earth can cause us to see or miss certain signals as they sweep out over the cosmos.
In addition to these focused beams of radiation, pulsars also emit a “wind” of charged particles, which form a glowing nebula around the pulsar. These pulsar wind nebulae (PWN) emit radiation of their own, often in X-rays. As the pulsar and its nebula move through the interstellar medium, the PWN can become warped as the interstellar medium pushes against and deforms it.
New Chandra X-ray observations of the Geminga pulsar, one of the closest to the Earth at 800 light-years away, show a PWN with three distinct tails. Two of these tails, which appear to emanate from the pulsar’s poles, stretch out in long arcs away from the star for over half a light-year, like an archer’s bow. The third tail is much smaller, and appears to emanate from the radial region of the pulsar itself.
A Different View
Geminga is a gamma-ray pulsar, emitting beams of highly-energetic radiation. But it is radio quiet, meaning it doesn’t emit detectable radio pulses. By contrast, pulsar B0355+54 is a bright radio pulsar, but isn’t detectable in gamma rays. Not only does B0355+54 emit at different wavelengths, its PWN also looks markedly different from that of the Geminga pulsar. Chandra’s picture of B0355+54 looks more like a jellyfish than a bow — it shows a thick “cap” with long, extended tails that stretch out more than five light-years.
Roger W. Romani, a professor of physics at Stanford University and the principal investigator of the Chandra PWN project, explains in a recent press release that, “By making the 3-D structure of these winds visible, we have shown how one can trace back to the plasma injected by the pulsar at the center.” This information in turn allows astronomers to create more detailed and accurate models of the pulsar and its surroundings, ultimately teasing out the reason for the differences in observed pulses.
Bettina Posselt, a senior research associate in astronomy and astrophysics at Penn State, says that based on these observations, Geminga’s magnetic poles appear to be oriented at the top and bottom of the neutron star from our point of view, which also align with its spin poles. Because these areas are where a pulsar’s radio emission should originate, it makes sense that no radio waves are detected. The pulsar’s gamma rays, however, are created over a larger area at higher altitudes, causing them to sweep out over a larger area of the sky and making them detectable from Earth. The observed tails in the Chandra image may be due to a process called Fermi acceleration, which occurs at the interface between the PWN and the interstellar medium. These results were published in the most recent issue of The Astrophysical Journal.
Oleg Kargaltsev, an assistant professor of physics at George Washington University who studied the B0355+54 pulsar, says that the very different picture of this pulsar is also explained with the new Chandra observations. “For B0355+54, a jet points nearly at us so we detect the bright radio pulses while most of the gamma-ray emission is directed in the plane of the sky and misses the Earth.”
George Washington University graduate student Noel Klingler explains that the angles between a pulsar’s spin axis, velocity, and our line-of-sight to the object can have a significant effect on the way we see it. “In particular, it may be tricky to detect a PWN from a pulsar moving close to the line-of-sight and having a small angle between the spin axis and our line-of-sight, “she says. Klingler is the lead author on a recent paper also published in The Astrophysical Journal outlining these new observations of B0355+54.
Although these explanations are still just one interpretation of the data, the Chandra observations used to reach them remain invaluable. These X-ray images are allowing astronomers to use pulsars much like particle physicists use accelerators on Earth. Through pulsars, they can observe the behaviors of particles under extreme conditions that accelerators cannot recreate, turning these objects into laboratories for further particle physics studies. While the current observations can explain either of two possible scenarios for Geminga’s behavior, Posselt concludes that, “In both scenarios, Geminga provides exciting new constraints on the acceleration physics in pulsar wind nebulae and their interaction with the surrounding interstellar matter."