Jumps in a Pulsar's Period the Evaporating Sun?

_29-'--2-------------------NEWSANDVIEWS------------NA_ru_RE_vo_L_.m_24_MAR_c_H_t_983 the pulsar during a glitch is necessary. Radioastronomy Clearly the imperfections in periodicity, as observed in the Vela pulsar and many others, are all derived from the steady Jumps in a pulsar's period 2 decrease in the rotation rate • It is not from Andrew Lyne surprising, therefore, that those pulsars which have a small period derivative show a IN this issue of Nature, McCulloch, occasions in the Vela pulsar. The changes remarkable uniformity of period without Hamilton, Royle and Manchester 1 report in coupling between the crust and the fluid glitches or other 'restless' behaviour. In observations of a large jump or 'glitch' in interior can be explained by the rather un particular, the recently discovered milli the period of the Vela pulsar PSR0833-45, familiar processes caused by vorticity in a second pulsar PHS 1937 + 21 in 4C21.53 followed by a series of smaller changes in superfluid neutron sea and its interactions with a period of only 1.56 ms has a period period. Over short time periods pulsars with the crystal lattice of the crust. derivative of less than about w- 19s s-l so usually show remarkably uniform rates of The new element introduced by that its period will take more than about 9 3 4 rotation - as indeed would be expected of McCulloch et a/. is in a series of 10 years to double • • This compares with a spinning body with a large moment of in observations which missed a glitch by only about 10 15s s- 1 for an average pulsar. The ertia isolated in space. Over longer time a few hours. As well as the recovery over resulting stability in rotational period, periods the rotation rate usually 'spins several hundred days, they show that some along with a remarkably well defined and down' slowly as the pulsar loses angular of the recovery occurs on a much shorter narrow pulse which allows timing accurate momentum through the emission of elec time scale of only 1.6 days. They interpret to about l J.IS, may provide us with a clock tromagnetic dipole radiation or particles. this as being due to the effect of a third to surpass the best terrestrial clocks in 4 Irregularities in pulsar rotation, including component, presumably liquid, in the precision • Unfortunately, unless we find the jumps observed by McCulloch and his neutron star. The short time scale indicates another similiar object we will of course colleagues, are themselves related to the that it must be closely coupled to the never know how good it really is. D steady spin-down and can tell us a great neutron star crust. This may give a new of 1. McCulloch, P.M .. Hamilton, P.A., Hoyle, G.W.H & deal about the pulsar's internal structure. insight into the structure and properties Manchester, H.N. Na!Ure 302, 319 ( 19831. The most obvious theoretical connection the exotic material of which neutron stars 2. Cordes, J.M. & Helfand, D.J. Asrrophys. J. 239, 640 (1980). between the changes in period and the slow are composed. Before the nature of the 3. Ashworth, M .. Lyne, A.G. & Smilh, F.G. Nature 301, 313 glitches can be undestood in detail, (19831. reduction in rotation rate of a pulsar is 4. Backer, D.C., Kulkarni, S.H. & TayiPr, J.H. Nature301, through the changes in ellipticity that must however, an observation of the spin-up of 314 (1983). occur as centrifugal forces fall. The solid crust of the pulsar must adjust to the Solar flares changing ellipticity in a series of steps in which the moment of inertia changes and the rotation rate is correspondingly modi The evaporating Sun? fied abruptly to conserve angular momen tum. Such steps might be large enough to from John C. Brown explain the glitches observed in pulsars as the Crab pulsar PSR0531 + 21. IN a recent article in the Astrophysical tion of the hot tenuous corona from the For the Vela pulsar, however, the glit Journal (263, 409; 1982), Acton and his cooler denser chromosphere by the thin ches which have been observed so far are so colleagues claim direct evidence of the transition zone which carries the stabilizing large that they require a change in ellipticity 'evaporation' of chromospheric matter in conductive flux. If such a plasma is tran of the order of 1 per cent in a single step. As to the corona of the Sun during a solar siently heated, extra mass can become un the ellipticity of the pulsar must be very flare. They carried out a detailed study of stable and heat and expand greatly towards small to produce its observed rotation data from the Solar Maximum Mission new temperature and pressure equilibria. period of 0.089 seconds and glitches occur (SMM) and other well coordinated obser Readjustment of this kind can be impor every 2 or 3 years, a different explanation is vations of a solar flare on 7 May 1980. The tant in the explosive behaviour of tran clearly required. This is provided by a two work constitutes the best evidence yet for sients such as supernovae and solar flares. component model of the neutron star evaporation in flares although com In the case of the solar flare, the addi rather than the single-rigid-body model en plications may arise from the sort of tran tional heating should depress the transition visaged above. The outer component is a sient features in flare energy release zone, causing chromospheric matter to solid crust while the interior is composed of reported by Takakura and his colleagues in heat and expand into the hot coronal a liquid which rotates independently, but is this issue of Nature (see p.317) and the regime - a mechanism known widely by loosely coupled to the solid crust. The quantitative work cannot yet be considered the misnomer 'chromospheric evapora steady slowdown affects the crust but watertight. tion'. It is well established observationally leaves the interior rotating faster by a fixed Evaporation is ultimately thought to that the transition zone is depressed during amount which depends on the frictional take place as a consequence of an impor solar flares but not until the new work by forces coupling the two components. The tant property of tenuous astrophysical Acton and his colleagues has there been discontinuity in rotation rate is seen when a plasmas - their radiative cooling rates rise direct evidence that the chromospheric sudden increase in coupling between the steeply up to temperatures of around 105K mass actually 'evaporates' into the corona. two speeds up the crust and slows down the and then fall at higher temperatures. This The main data employed by Acton et at. liquid interior. Decoupling than follows occurs largely because of spectrum line comprise hydrogen alpha (Ha) line profiles and differential rotation is slowly re emission from trace elements, similar to from Sacramento Peak Observatory, as in established. The picture fits very well the that causing radiation losses in fusion dicators of chromospheric structure, and pattern of an increase in rotation rate at the machines when trace atoms ablate from the SMM X-ray polychromator (XRP) data on glitch and a subsequent exponential decay walls. The important consequence is that a flare coronal structure. SMM hard X-ray to steady slowdown over the following few plasma heated above l0 5K becomes burst spectra (HXRBS) and imaging (HXIS) hundred days which has been seen on five radiatively unstable, its temperature run data were used to distinguish possible heating ning away to much higher values until a mechanisms driving the evaporation. Andrew Lyne is in the University ofManchester stabilizing mechanism, such as thermal By matching the observed Ha profiles Nuffield Radio Astronomy Laboratories, conduction, sets in to balance the heating. with profiles computed for a grid of em Jodre/1 Bank, Cheshire SKJJ 9DL. In the Sun, the effect is seen in the separa- pirical model chromospheres in 0028-0836183/ 120292-02$01.00 C! 1983 Macmillan Journals Lid .

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