High-energy astrophysics Explore the PUL SAR menagerie Astronomers are discovering many strange properties of compact stellar objects called pulsars. Here’s how they fit together. by Victoria M. Kaspi f you browse through an astronomy book published 25 years ago, you’d likely assume that astronomers understood extremely dense objects called neutron stars fairly well. The spectacular Crab Nebula’s central body has been a “poster child” for these objects for years. This specific neutron star is a pulsar that I rotates roughly 30 times per second, emitting regular appar- ent pulsations in Earth’s direction through a sort of “light- house” effect as the star rotates. While these textbook descriptions aren’t incorrect, research over roughly the past decade has shown that the picture they portray is fundamentally incomplete. Astrono- mers know that the simple scenario where neutron stars are all born “Crab-like” is not true. Experts in the field could not have imagined the variety of neutron stars they’ve recently observed. We’ve found that bizarre objects repre- sent a significant fraction of the neutron star population. With names like magnetars, anomalous X-ray pulsars, soft gamma repeaters, rotating radio transients, and compact Long the pulsar poster child, central objects, these bodies bear properties radically differ- the Crab Nebula’s central object is a fast-spinning neutron star ent from those of the Crab pulsar. Just how large a fraction that emits jets of radiation at its they represent is still hotly debated, but it’s at least 10 per- magnetic axis. Astronomers cent and maybe even the majority. That the bulk of neutron detect the radio signature from these jets as pulses. It turns out stars may be peculiar objects that were unimaginable just a that scientists are finding other decade ago is one of the most startling recent discoveries in classes of pulsars that don’t appear to share traits with astrophysics. It’s time to rewrite the textbooks. the Crab’s central object. Don Dixon for Astronomy © 2014 Kalmbach Publishing Co. This material may not be reproduced in any • 44 Astronomy Octoberform without2010 permission from the publisher. www.Astronomy.com www.Astronomy.com 45 Magnetic field lines were unprecedented in astronomy. Today, example, the 150-millisecond pulsar PSR we understand that these signals are from B1509−58 powers the X-ray nebula rotation-powered pulsars (RPPs): rapidly dubbed the “Hand of God.” This huge Spin axis rotating, highly magnetized neutron stars nebula has fingers of energized material that produce narrow beams of radiation emanating from the central region, which emanating from their magnetic poles, holds the pulsar. with the magnetic axis misaligned from Astronomers recently discovered a Magnetic axis the spin axis. This beaming property likely subclass of radio pulsars called makes radio pulsars the “lighthouses of rotating radio transients (RRATs). Unlike the cosmos.” The beamed radiation spans classical radio pulsars, these objects seem Neutron star the electromagnetic spectrum, although to emit radio waves intermittently and in it generally is most easily observed with short bursts, such that we don’t detect a radio telescopes. Astronomers know of regular pulsation. Why these objects are nearly 2,000 radio pulsars in the Milky seemingly incapable of producing consis- et al. Seward NASA/CXC/SAO/F. Way, most of which inhabit the disk. tent regular signals is a mystery. Careful The Crab Nebula’s pulsar was one of the first Young pulsar PSR B1509–58 emits enough Neutron stars, and thus pulsars, are study of the RRAT bursts has revealed that discovered. This radio pulsar’s fast rotation rate energy to light up this massive nebula dubbed born in supernova explosions. As the they in fact occur at time intervals that are (about 30 spins per second), combined with a the “Hand of God.” The nebula spans some 150 strong magnetic field, generates an intense light-years. The neutron star is in the “hand’s” slowly rotating progenitor star collapses, specific multiples of an underlying period- Radiation wind that energizes the surrounding material. center, with finger-like structures energizing beam physics insists on the conservation of icity, supporting the idea that such objects knots of material in a neighboring gas cloud. Beam rotates angular momentum — a constraint are an extreme form of radio pulsar. Blue represents the most energetic X-rays and around spin axis shared with figure skaters during spec- While researchers know of only about look for isolated single bursts — the red the least. Gas clouds emitting radio waves tacular spinning stunts. Thus some neu- one dozen RRATs, they easily could have RRAT signature. show up as purple. NASA/CXC/SAO/P. Slane et al. tron stars start life spinning rapidly (on missed a huge population of these objects average rotating roughly 50 to 100 times because of the way sky surveys have In isolation that the vast majority of radio pulsars Radiation beams emanate from a radio pulsar’s magnetic axis, which is misaligned from the star’s per second). looked for radio pulsars. Some astrono- Astronomers had long thought radio are also isolated but not classified as spin axis. As the object rotates, the beams flash the observer and appear as pulses. Astronomy: Roen Kelly A classic radio pulsar’s impressively mers think RRATs could outnumber clas- emission was the hallmark of neutron INSs. Researchers have confirmed that strong magnetic field acts to “brake” the sical radio pulsars. Modern radio pulsar stars. But several different subclasses of there are seven INSs, in addition to one Surveying neutron stars lion Gs, if tidal forces hadn’t already star — it emits radiation as any rotating investigations, such as the Pulsar-ALFA radio-quiet neutron stars have emerged candidate object. To properly describe the neutron star shredded you or radiation hadn’t zapped magnet must. Such magnetic fields are survey currently underway at the 305- as attention grabbers in the past decade. These bodies’ defining properties are population as we understand it is to visit you to oblivion.) An open question is just enormous, on the order of 1012 gauss — meter radio telescope in Arecibo, Puerto The so-called isolated neutron stars that they are relatively close to us, have a wild and wonderful “zoo” of objects. how fast one of these objects can spin more than a trillion times as strong as Rico, routinely use special software to (INSs) are rather poorly named given X-ray emission with relatively low X-ray Each seems intent on proving itself the without flying apart. Earth’s magnetic field and too high for luminosity, and have only a visible-light most exotic of the lot — not that they scientists to reproduce in a terrestrial lab- counterpart. Those INSs known are need to try that hard. What’s in the zoo? oratory. Astronomers infer such fields within roughly 1,600 light-years of Earth; All neutron stars share some com- Naïvely, one might think that by being a from the fact that these stars’ rotation beyond that, they’re likely too faint for mon, bizarre properties. Having masses close cousin of the black hole, a neutron rates are slowly decreasing — in other current detectors. They also have rela- upward of half a million Earths crammed star should not have much “hair” — that words, they are “spinning down.” tively long periods as measured with into a sphere some 12 miles (20 kilome- X-ray telescopes, ranging from 3 to 11 ters) wide, these objects are the second seconds. They emit copious X-rays most compact known in the universe — At a neutron star’s center, the density is at least because they are hot due to the decay of a after black holes. At a neutron star’s cen- previously intense magnetic field. ter, the density is at least 10 times that 10 times that within the atomic nucleus; we don’t understand Surprisingly, the typical INS has a within the atomic nucleus; we don’t magnetic field that is somewhat stronger understand the laws of nature within the laws of nature within densities this high. than that of the typical radio pulsar. (The densities this high. reason for this is unclear, although one Even more impressive are neutron theory posits that INSs are regular radio stars’ rapid rotation rates — astronomers is, details. Figuratively speaking, however, The Crab Nebula’s radio pulsar pow- pulsars viewed off-beam so that they know of at least one that completes more researchers now realize that neutron stars ers the surrounding nebula with its appear radio quiet.) than 700 spins per second. Despite the are awfully complicated. The variety of intensely magnetized “wind” of highly tremendous gravitational pull at their sur- properties among the different classes is energetic particles. The pulsar’s ambient Extreme magnets faces, neutron stars rotating at these rates genuinely astonishing. magnetic field accelerates electrons and Then there are magnetars, the “bad boys” are almost certainly slightly pancake- We’ve known about the classic young positrons, which causes the ghostly white of the neutron star population (a descrip- shaped due to the rotation. (On a typical “radio pulsars” since University of Cam- glow within the thermal filamentary tion first suggested by my good friend neutron star, you’d be “pulling” a few tril- bridge graduate student Jocelyn Bell dis- remains of the exploded progenitor star. and colleague Maxim Lyutikov of Purdue covered the first such object in 1967. Astronomers detect “pulsar wind nebu- University). Magnetars are the most Victoria M. Kaspi, a professor of physics at Originally, her research group thought lae” only in the most powerful pulsars. The Arecibo Observatory in Puerto Rico is an important tool for finding pulsars.