How Pulsars Get Their Spins a Net Kick at Birth Has Been Mounting for Years

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Neutron stars would not cause the same problem. Evidence that neutron stars experience How pulsars get their spins a net kick at birth has been mounting for years. Pulsars are the fastest population in Adam Burrows the Galaxy3, with an average velocity of ~450 km s–1. Such speeds are far larger supernova explosion announces the ‘kick’ from an asymmetrical supernova could than any orbital motion left over from spectacular death of a star heavier exert a torque on a nascent neutron star, they birth in a binary. In the pulsar binaries,

Athan about eight solar masses. Its posit a unified theory of pulsar spins, kicks PSR J0045–7319 and PSR1913+16, the spin brightness can rival that of its host galaxy, and magnetic fields. First, they suggest that axes and the orbital axes are misaligne8d, and its total explosion energy is a prodigious magnetic fields in massive stars couple core suggesting that the explosions that created 1051 ergs. The usual residue of such an and mantle strongly enough to leave the pre- the pulsars were not spherical (ref. 4, and explosion, a neutron star, is no less extreme supernova core in solid-body rotation. Such I. Wasserman, J. Cordes and D. Chernoff, an animal. With a density that of the atomic initial magnetic fields indeed yield neutron personal communication). In fact, for the nucleus, these stars the size of a city are stars with fields of 1012 gauss, as seen in radio former the orbital motion seems retrograde observed to be rotating with periods from pulsars. But the core would rotate far too relative to the spin5 — the explosion may milliseconds to seconds, and travelling at slowly (with a period of ~100 seconds) to have kicked the pulsar backwards! The up to 1,500 km s–1 (Fig. 1, overleaf). Radio leave young neutron stars spinning as fast as young supernova remnant Cassiopeia A, pulsars are neutron stars, and as such have they do. So Spruit and Phinney propose that formed in about AD 1680, has calcium, been studied for 30 years. Approximately 103 the mechanism that gives the neutron star its sulphur and oxygen element distributions of probably 105 radio pulsars in the Galaxy linear kick may also generate its spin. that are clumped and have gross back–front have been discovered (among perhaps 108 Second, the authors show that a series of asymmetry6 — no simple shells are seen. neutron stars). Their masses, magnetic random impulses naturally yields a quanti- Many other supernova remnants, such as fields, spin periods, spin-down rates, and tative connection between spin and kick N132D and SN0540–69.3, have velocities7 proper motions across the sky are cata- magnitudes, and at the same time partially relative to the local interstellar medium of up logued, and theories for their emission are decorrelates spin and transverse proper to 900 km s–1. The supernova SN1987A is a devised. But the origin of these basic pulsar motion vectors and magnitudes. A single case study in asphericity: its X-ray, gamma- parameters has all but been ignored as kick would produce a spin perpendicular to ray and optical fluxes and light curves untestable. On page 139 of this issue1, Spruit the velocity change, and so give a strong cor- require that shards of the radioactive isotope and Phinney attempt to bridge the gap relation between spin and apparent motion 56Ni were flung far from the core in which between the supernova and pulsar commu- on the sky — which radio pulsar data do not they were created; the infrared line profiles of nities with a provocative thesis. show. Many randomly placed kicks, perhaps its oxygen, iron, cobalt, nickel and hydrogen Taking a lead from the suggestion2 that a from a convecting supernova explosion, are ragged and show a pronounced red–blue Entomology asymmetry; its light is polarized (something a sphere cannot accomplish); and Hubble Beauty and the beetle Space Telescope pictures of its inner debris8 reveal large clumps and hint at a preferred .

AL Coloured beetles are of two basic types. expansion direction.

ET Some have a layer of pigment in their Finally, SN1997X is one of the most exoskeleton (pigmentary colours), while intriguing recent finds. This is a so-called ARKER others rely on the interaction of light type-Ic supernova, thought to be the ex-

A. R. P with features in their integument plosion of the bare carbon/oxygen core of (structural colours). These structurally a massive star that has been stripped of reflecting colours can be very different its envelope. SN1997X has the greatest opti- — Calloodes grayanus (pictured right), cal polarization of any known supernova for example, appears a weak green in (L. Wang, personal communication), which colour, whereas Anoplognathus implies that supernova cores, and ex- parvulus is a strong metallic gold. How plosions, are asymmetrical. are these different colours produced? layer that scatters light. The resulting All in all, it seems clear that neutron stars Writing in the Journal of reflection is diffuse, appearing the same can be given a hefty extra kick at birth, and Experimental Biology (201, 1307–1313; when viewed from any direction; this that asymmetries in the supernova explosion 1998), Andrew R. Parker and colleagues green colour seems to provide excellent are implicated. How might these asymme- provide the answer. In both cases, the camouflage against a leafy background. tries arise? No doubt, instabilities in the structural colour arises from so-called The layers that make up the A. outer envelopes of supernova progenitors ‘multilayer reflectors’ — layers of a parvulus reflector, however, decrease in make debris clouds clumpy and mixed. The transparent material separated by layers spacing (and consequently in optical observation of hydrogen deep in SN1987A’s with a different refractive index. thickness) with depth in the structure. ejecta strongly suggests the work of such Reflected beams from all of the layers The resulting metallic gold colour can be mantle instabilities. But overall, the data — interfere, and when this interference is seen only from certain directions, but it particularly for the heavier elements pro- constructive for a particular wavelength makes A. parvulus very conspicuous duced in the inner core — point to asymme- of light, that colour is observed. nonetheless, perhaps allowing it to be tries in the central engine of the explosion. But the similarity ends there. Parker recognized by members of the same Supernova theorists have not been et al. have found that the C. grayanus species. Unfortunately, its striking napping, and over the past decade have reflector is composed of regularly spaced vesture could also mark it as a tasty determined that supernova cores are indeed layers of the same optical thickness, morsel for predators. grossly unstable to convective instabili- 2,9,10 overlaid by an irregular transparent Alison Mitchell ties . Between the ‘bounce’ (when some material falls in and effectively bounces off

NATURE | VOL 393 | 14 MAY 1998 121 Nature © Macmillan Publishers Ltd 1998 news and views

Daedalus

. WINKLER Deceptive appearances

ETRE/F Few women have ever been satisfied with their figures. Hence the foundation- garment industry. Perhaps its peak was the late nineteenth century, when the

fashionable Victorian young lady seems to WDEN/C. BECKER/R. P have carried nearly as much rigging as a 8 sailing ship. S. SNO Since then, elasticated undergarments have grown simpler and less fashionable. Modern women are urged to perfect their figures by diet and exercise. This state of affairs is not an improvement — elastic at least worked. So Daedalus is studying the basics of the technology. Tensioned clothing cannot truly shape; it can only squeeze. The natural resulting cross-sections are circles or arcs of circles. The human figure, however, tends to have elliptical or ovoid cross-sections. To impose such shapes, a garment needs not tightness, but directional rigidity. So DREADCO corsetières are devising fabrics woven, not of fibres which can only carry tension, but of micro-girders which can resist bending. These tiny I-beams have polyimide compression faces, carbon fibre tension faces, and thermoplastic Figure 1 The 3,700-year-old supernova remnant Puppis A, imaged in X-rays by the satellite Rosat. The central webs. As with some steel girders, bright spot has been interpreted as its neutron star, even though no pulsations are seen, because it has they are initially made in T-section, with a a large ratio of X-ray to optical emission. Its inferred transverse speed is ~1,000 km s–1. Interestingly, sawtooth vertical web. When two such the spot is on the opposite side to the fast, oxygen-rich knots, as one might expect in some models of sections are welded together by their tooth neutron-star recoil during a supernova explosion. points, the resulting I-beam has diamond- shaped holes along its web. The warp and the surface of the forming neutron star) and far larger than the average pulsar surface weft half-girders of the new fabric are laid the explosion, there is a delay while the out- field of 1012 gauss. Perhaps the pre-explosion on a shaped former, the corresponding top ward-moving shock wave travels through convective motions themselves can generate half-girders are brought down on them, the star. During this 100 to 1,000 millisec- the required fields by dynamo action. and all the welds are made at once by a fast- onds, the core is strongly convective, boiling It is now reasonable to imagine a theory set adhesive. Warp and weft micro-girders and churning at sonic speeds (~3 ǂ104 km that unifies the spins and velocities of thus pass through each other’s web holes. s–1). Any slight asymmetry in collapse can neutron stars, the anisotropies observed in They can move angularly against each amplify this jostling, and result in vigorous supernova ejecta, and stellar collapse and other, giving the new fabric the soft ‘drape’ kicks and torques to the residue that can be explosion. To date, dialogue between the of cloth. But it has a definite shape derived either systematic or random11. pulsar and the supernova communities from its former; and will impose that Whatever the details, it would seem odd if has been rare — but the paper by Spruit shape, subtly but firmly, on the wearer. the nascent neutron star were not left with a and Phinney is a good conversational Until the complex micro-engineering of net recoil and spin, although it is not known gambit. production has been optimized, whether pulsar speeds as high as 1,500 km s–1 Adam Burrows is in the Department of Astronomy, DREADCO’s ‘Shapers’ will be vastly can be reached through this mechanism. But University of Arizona, Tucson, Arizona 85721, USA. expensive. In the fashion trade, fortunately, asymmetries in the matter distribution may e-mail: [email protected] this is not necessarily a disadvantage, also cause asymmetries in the emission of 1. Spruit, H. & Phinney, E. S. Nature 393, 139–141 (1998). especially for such a revolutionary product. neutrinos, which carry away most of the 2. Burrows, A., Hayes, J. & Fryxell, B. A. Astrophys. J. 450, Instead of merely squeezing the wearer at 830–850 (1995). binding energy of the neutron star (around 3. Lyne, A. & Lorimer, D. R. Nature 369, 127–129 (1994). one point and displacing the tissue 53 3ǂ10 ergs). Amazingly, a net angular 4. Kaspi, V. M., Bailes, M., Manchester, R. N., Stappers, B. W. & elsewhere, they will truly shape her figure. asymmetry in the neutrino radiation of Bell, J. F. Nature 381, 584–586 (1996). Thin and seemingly insubstantial, they will 5. Lai, D., Bildsten, L. & Kaspi, V. M. Astrophys. J. 452, 819–824 only 1% would give the residue a recoil of (1995). flatten an excessive stomach without –1 300 km s . Not surprisingly, many theorists 6. Lawrence, S. S. et al. Astron. J. 109, 2635–2651 (1995). cramping the rest of the waist, and will have concentrated on producing such a 7. Kirshner, R. P., Morse, J. A., Winkler, P. F. & Blair, J. P. form thighs, buttocks and breasts into pert, neutrino asymmetry, invoking anisotropic Astrophys. J. 342, 260 (1989). becoming contours without even feeling 8. Pun, C. S. J., Kirshner, R. P., Garnavich, P. M. & Challis, P. accretion, exotic neutrino-flavour physics, or Bull. Am. Astron. Soc. 191, 9901 (1998). tight. Even if quite overweight, the happy the influence of strong magnetic fields on 9. Herant, M., Benz, W., Hix, J., Fryer, C. & Colgate, S. A. wearer will still display a perfect figure, but neutrino cross-sections and transport. The Astrophys. J. 435, 339 (1994). slightly scaled up. 10.Janka, H.-T. & Müller, E. Astron. Astrophys. 290, 496–502 (1994). last is particularly interesting, but generally 11.Burrows, A. & Hayes, J. Phys. Rev. Lett. 76, 352–355 (1996). David Jones 12 14 16 requires magnetic fields of 10 to 10 gauss, 12.Lai, D. & Qian, Y.-Z. Astrophys. J. (in the press).