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Pulsars seen through a new lens

Radio waves produced by cosmic lighthouses called pulsars are distorted by surrounding material. Observations show that this material can act as a lens, focusing the waves and boosting the brightness of the pulsar.
Jason Hessels is at ASTRON (the Netherlands Institute for Radio Astronomy) and the Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1098XH Amsterdam, the Netherlands.
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Much of what we know about the Universe comes from using telescopes to collect and study the electromagnetic waves produced by astronomical objects. These waves can be distorted and absorbed as they travel to Earth, providing valuable information about the otherwise invisible material that lies between stars and between galaxies. Rotating stellar remnants called pulsars have long been known to be excellent probes of interstellar matter1. A pulsar generates a beam of radio waves that flashes across the sky as the object rotates, just like the light from a lighthouse. In a paper in Nature, Main et al.2 report that ionized gas close to a pulsar can greatly amplify the object’s observed brightness, and allow astronomers to zoom in on the source of the emission. The discovery could also help to explain enigmatic astrophysical signals known as fast radio bursts.

Main and colleagues targeted a pulsar3 that is officially termed PSR B1957+20, but is known informally as the black widow (Fig. 1). The pulsar and a companion star exist in a binary system, in which the two bodies orbit around a common centre of mass. The binary system itself is minuscule — it would almost fit inside the Sun.

Figure 1 | The black-widow pulsar. A pulsar is a rotating stellar remnant that generates a powerful beam of radio waves. Main et al.2 report observations of a binary system that comprises a pulsar known as the black widow and a small companion star. A wind of particles from the pulsar strips matter from the companion star and produces a surrounding cloud of ionized and magnetized gas called a plasma. The authors demonstrate that the plasma relatively close to the companion star can act as a lens, focusing the radio waves and boosting the observed brightness of the pulsar.

In addition to its radio beam, the pulsar generates a strong wind of particles that strips matter from the companion star and produces a surrounding cloud of ionized and magnetized gas called a plasma. This is where the ominous name of the pulsar comes from — like the sexually cannibalistic spider after which it is named, the black-widow pulsar is destroying its mate. From the point of view of an observer on Earth, the pulsar is eclipsed once per orbit, when it travels behind the plasma cloud.

Main et al. used the 305-metre-diameter William E. Gordon Telescope at the Arecibo Observatory in Puerto Rico to study the black-widow pulsar in unprecedented detail. The authors meticulously mapped how the observed brightness of the pulsar changes on timescales as short as microseconds. What they found is that the pulsar seems to be much brighter at certain points of its orbit — in particular, at the edges of the plasma cloud, just before and after the pulsar is eclipsed. For only a few milliseconds at a time, and at specific radio frequencies, the pulses can appear to be up to 80 times brighter than average (see Fig. 2 of the paper2).

The authors hypothesize that the edges of the plasma cloud act as a lens, boosting the observed brightness of the pulsar for short periods. Think of how a magnifying glass focuses light; in a broader sense, any medium that causes electromagnetic waves to change direction can serve as a lens. In a plasma lens, radio waves are bent, and the waves arriving at an observer from different angles can overlap to produce a bright spot known as a caustic4. A similar effect with light can be seen at the bottom of a swimming pool on a sunny day.

Main and colleagues also demonstrated that the plasma-lensing effect can be used to zoom in on the pulsar, by studying how the effect changes with time and with observed radio frequency. This is astounding because pulsars are no more than about 30 kilometres in diameter5. They produce their radio waves from a magnetized atmosphere known as a magnetosphere, which rotates along with the pulsar. In the case of a fast-spinning pulsar such as the black widow, the magnetosphere extends only about 100 km above the pulsar’s surface3.

Based on the time- and frequency-dependent way in which the observed pulses are amplified, Main et al. inferred that they could detect the physical offset in emissions that originate from different regions of the magnetosphere. In other words, the lensing effect allowed the authors to measure a kilometre-scale offset despite the fact that the black-widow pulsar is about 1017 km away. Such a feat is equivalent to measuring the width of a hair on Mars from Earth.

The black-widow pulsar provides a stunning illustration of plasma lensing in action, but there are other astrophysical examples of this phenomenon. For instance, variations in the observed radio brightness of distant supermassive black holes called quasars have been ascribed to lensing from poorly understood plasma structures in our Galaxy6,7. Furthermore, echoes of pulses from the Crab pulsar could be caused by lensing from plasma filaments in the pulsar’s surrounding nebula8. Again, plasma lensing not only boosts the brightness of astronomical objects, but also reveals what would otherwise be invisible.

Plasma lensing might allow astronomers to peer deeper into the Universe than would otherwise be possible. More specifically, Main and colleagues suggest that their observations provide a clue to solving the mystery of fast radio bursts — millisecond-duration radio flashes that seem to originate in distant galaxies, but which are bright enough to detect on Earth.

Intriguingly, the effects observed in the black-widow pulsar are similar to distortions seen in pulses from at least one source of fast radio bursts9. Plasma lensing might therefore be responsible for boosting the brightness of fast radio bursts, as has previously been hypothesized and modelled10. However, the story is not complete: the environment around a source of fast radio bursts is probably quite different from, and possibly even more extreme than, that of the black-widow pulsar. It perhaps has more in common with the environment around the young Crab pulsar or at the centre of our Galaxy11.

Main et al. have detected plasma lensing in a pulsar that has been studied for more than 30 years, using a telescope that has been operating since the early 1960s. Why the sudden insight? As computing and data-recording power has grown, so has the ability to use venerable radio telescopes to scrutinize pulsars on shorter timescales and over a wider range of radio frequencies. This suggests that the future is bright for using pulsars to illuminate the invisible Universe.

Nature 557, 494-495 (2018)

doi: 10.1038/d41586-018-05185-y

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