Supernova Springs New Surprise

595 .:..N..:..A:..:T..:U..:..R:..:E:..._Y_O:_L:_·:..:3..:..37_16_F_E_B_R:..:U_A_R_Y___.:_l9_;_89_______ NEWS AND VI EWS------------------_ Supernova springs new surprise What seems to be the pulsating star left by the supernova whose explosion was observed two years ago has surprised everybody and could, if confirmed, stand theory on its head. AsTRONOMERS have been growing a little 1968.629-Hz periodicity of the new pulsar leads to a simple relation between bright anxious in the past few months, wonder changes that picture. Its rotational energy ness, magnetic field and rotation rate. The ing when the pulsar left by the explosion of is about the same as its gravitational bind dipole field strength of the new pulsar can supernova 1987A would emerge from the ing energy, which means that it is on the be no more than about 10' gauss, whereas cooling remnant. The longer they waited, point of being physically flung apart. (For 10 12 gauss is what astrophysicists would the more they suspected that the pulsar some theoretical nuclear equations of state, have guessed. would be feeble and undistinguished. it ought to fly apart, which is another If the rapid rotation and low field are Instead, if observations announced last problem.) Moreover, the energy of the odd, the apparent 8-hour orbital period is week stand up to scrutiny, astronomers explosion was only a small fraction of that bizarre to the point of incredibility. The are faced with three surprises-and a raft now tied up in rotation, which means that companion apparently has the mass of of theoretical puzzles. refinements of the distribution of angular Jupiter and an orbital radius putting it The bare facts, given in IAU Circular momentum in the progenitor yield order inside what would have been the outer No. 4735 by John Middleditch of Los of-magnitude effects in the explosion hydrogen layer of the progenitor. Alamos National Laboratory, Carl dynamics. While it is on the fringes of possibility Pennypacker of Lawrence Berkeley ·For Stan Woosley of Lick Observatory, that a condensed object might have lived Laboratory and many others, are that who has exploded many stars by computer, for a little while in the cool atmosphere of optical variations between magnitudes 18 the problem is that no one knows the dist a giant star, it is quite impossible that any and 19 were detected during a 7-hour ribution of angular momentum in a giant such object could have survived the explo observing period on 18 January with the 4- star, or how to calculate angular momen sion. Some unusual hypotheses come to m telescope at Cerro Tololo Observatory tum transport through a convective turbu mind: if the pulsar was spun up by recent in Chile. From these observations comes a lent medium evolving on a millisecond accretion, then the companion could be a pulsar with a period of half a millisecond, timescale. blob accreting material; if the core was making it the fastest known. And there is The angular momentum of a giant star, rotating very fast as it collapsed, then it also an 8-hour variation suggesting that conserved through collapse to a neutron might have passed through a "dumb-bell" the pulsar is a member of a binary system. star, is enough to create a millisecond phase and fractured into two pieces. The three surprises are these. How can pulsar. But hoping to follow the collapse The recent case of PSR1957+20, in a new-born pulsar have a sub-millisecond in detail, and to make useful predictions which radiation from a pulsar is presumed period, when it has been thought for years about the outcome, is close to impossible. to be eroding a nearby companion, that pulsars are born rotating perhaps ten Woosley is understandably waiting to suggests to Edmund van den Heuvel of times a second? Why is the pulsar's mag see whether the observations are con Amsterdam University a similar but more netic field so small, which it must be firmed before he attempts to explain the extreme history for the pulsar in SN1987A. because luminosity goes as the square of new pulsar, and is not averse to models in Because of its very rapid rotation , the the field strength and the fourth power of which the pulsar was born slow two years newly formed pulsar may have the power the rotation rate, and because the pulsar is ago, but has been spun up by accretion to boil away a companion of one solar mass certainly no brighter than the much more since then. But others, such as Andrew in a matter only of years, rather than the slowly rotating Crab pulsar? And how has Fabian, of the Cambridge Institute for millions of years taken by PSR1957+20. it acquired a companion which, had it Astronomy, are sceptical. Although To do this, a much stronger magnetic field been present at the time of explosion, dumping about half a solar mass of than the simple-minded limit allows is would have been sitting in the outer layers material onto a fledgling neutron star needed but because the system is evid of the giant progenitor star? could spin it up to a millisecond period, ently evolving on a timescale of years, Most pulsars have periods of some the energy generated would be far above even months, the usual arguments may be modest fraction of a second; the few with the Eddington limit (meaning that radia quite wrong. millisecond periods belong (with a couple tion pressure would be enough to repel the The reality of the 8-hour periodicity is of exceptions) to binary systems. So, the in falling material). certain to be questioned. But Penny story goes, all pulsars are born rotating The Eddington limit can be evaded, for packer and his colleagues are the best in slowly, and spin down as they age. But a example by supposing gas to accrete at the this business. and they spent several pulsar in a close binary can accrete mass equator and radiation to emerge from the weeks analysing their data because they from its companion, and be spun up to a poles, but this requires special pleading. too were taken aback by what they found. millisecond period as mass spirals in, con And the extra gas must come from some New observations are scheduled two serving angular momentum. The handful where: if the supernova explosion has to weeks from now. The new pulsar may be a of isolated millisecond pulsars are sup be such that the core collapses, the atmos freak but van den Heuvel at least seems posed somehow to have lost their compa phere is blown off yet a little gas is left prepared for a wholesale revision of ideas: nions, perhaps through encounters with stranded for subsequent accretion, the the notion that pulsars are born slow may other stars or, as in the celebrated case of pleading becomes even more special. turn out to be an unfounded guess, an PSR1957+20, by blasting them away by Like its rapid rotation, the low magnetic unwarranted extrapolation to the moment radiation. field of the new pulsar will probably just of birth from 1000-year old objects such as This tidy picture absolves astrophysi have to be accepted for what it is. The the Crab pulsar. The infancy of the new cists who calculate the dynamics of super elementary assumption that a pulsar pulsar will be followed minutely, and will novae explosions from the immense obtains its luminosity from electrons put adulthood and old age in a new light. trouble of dealing with rotation, but the accelerated by the rotating magnetic field David Lindley .

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