letters to nature properties from the emission in the outer or `conal' regions. the very low beaming fraction together imply that we can observe Polarization observations of this pulsar14 at 659 MHz show that only a very small proportion of the total population of such objects the pulse is weakly polarized with a very rapid swing of position in the Galaxy. While extrapolation from the detection of a single angle through the pulse. This, and the narrow and nearly gaussian object is always uncertain (some would say foolhardy), there is no pulse pro®le, suggests that the entire observed pulse is core-type reason to suppose that PSR J2144−3933 is unique. With this caveat, emission with the beam direction close to that of the magnetic axis. this detection implies a Galactic population of similar pulsars of the We note that the period of PSR J2144−3933 is similar to the order of 105, comparable to previous estimates of the size of the total periods of anomalous X-ray pulsars15 (AXPs) and pulsars identi®ed pulsar population22,23. M g 16 with soft -ray repeaters (SGRs). These slowly spinning neutron Received 23 March; accepted 13 July 1999. stars have been interpreted as `magnetars', whose emission is 1. Ruderman, M. A. & Sutherland, P. G. Theory of pulsars: Polar gaps, sparks, and coherent microwave powered not by the rotational kinetic energy of the neutron star, radiation. Astrophys. J. 196, 51±72 (1975). but by the decay of its super-strong magnetic ®eld17. No AXPs or 2. Machabeli, G. Z. & Usov, V. V. Cyclotron instability in the magnetosphere of the Crab Nebula pulsar, and the origin of its radiation. Sov. Astron. Lett. 5, 238±241 (1979). SGRs have yet been detected as radio pulsars, and it has been argued 3. Cheng, A. F. & Ruderman, M. A. Particle acceleration and radio emission above pulsar polar caps. that such pulsars may not emit in the radio band because photon Astrophys. J. 235, 576±586 (1980). splitting inhibits pair-production18. However, we note that the lack 4. Beskin, V. S., Gurevich, A. V. & Istomin, Y. N. Theory of the radio emission of pulsars. Astrophys. Space Sci. 146, 205±281 (1988). of detection of radio emission from the few known AXPs and SGRs 5. Manchester, R. N. & Taylor, J. H. Pulsars (Freeman, San Francisco, 1977). can be explained if their radio beamwidths are similar to that of PSR 6. Manchester, R. N. et al. The Parkes Southern Pulsar SurveyÐI. Observing and data analysis systems − and initial results. Mon. Not. R. Astron. Soc. 279, 1235±1250 (1996). J2144 3933; if this were so, only a very small fraction of the total 7. Lyne, A. G. et al. The Parkes Southern Pulsar SurveyÐII. Final results and population analysis. Mon. population of such objects would be visible from Earth as radio Not. R. Astron. Soc. 295, 743±755 (1998). pulsars. The surface magnetic ®eld of PSR J2144−3933 itself is much 8. D'Amico, N. et al. The Parkes Southern Pulsar SurveyÐIII. Timing of long-period pulsars. Mon. Not. R. Astron. Soc. 297, 28±40 (1998). less than that of magnetars, and no magnetar-like X-ray emission 9. Johnston, S., Nicastro, L. & Koribalski, B. Scintillation parameters for 49 pulsars. Mon. Not. R. Astron. would be expected from this object. Soc. 297, 108±116 (1998). 10. Camilo, F. & Nice, D. J. Timing parameters of 29 pulsars. Astrophys. J. 445, 756±761 (1995). For a given magnetic ®eld con®guration, the locus of P±Bs values 11. Lyne, A. G. & Manchester, R. N. The shape of pulsar radio beams. Mon. Not. R. Astron. Soc. 234, 477± at which radio emission ceases de®nes a `death line' in the P±Bs 508 (1988). diagram (Fig. 2), to the right of which no pulsar should emit radio 12. Rankin, J. M. Toward an empirical theory of pulsar emission. IV. Geometry of the core emission region. Astrophys. J. 352, 247±257 (1990). waves. By invoking a variety of assumptions about ®eld-line 13. Rankin, J. M. Toward an empirical theory of pulsar emission. I. Morphological taxonomy. Astrophys. J. curvature in the emission region above the neutron star, Chen 274, 333±358 (1983). 19 14. Manchester, R. N., Han, J. L. & Qiao, G. J. Polarization observations of 66 southern pulsars. Mon. Not. and Ruderman derived a set of death lines. For a given surface R. Astron. Soc. 295, 280±298 (1998). magnetic dipole ®eld strength, pulsars with strong multipolar ®eld 15. van Paradijs, J., Taam, R. E. & van den Heuvel, E. P. J. On the nature of the `anomalous' 6-s X-ray components will have a highly curved ®eld near the stellar surface pulsars. Astron. Astrophys. 299, L41±L44 (1995). 16. Kouveliotou, C. et al. An X-ray pulsar with a superstrong magnetic ®eld in the soft g-ray repeater permitting the emission to persist to longer periods. In Fig. 2, death SGR1806-20. Nature 393, 235±237 (1998). line B corresponds to the greatest reasonable magnetic ®eld curva- 17. Duncan, R. C. & Thompson, C. Formation of very strongly magnetized neutron stars: Implications for gamma-ray bursts. Astrophys. J. 392, L9±L13 (1992). ture, and death line C is an extreme and unlikely case. No pulsar 18. Baring, M. G. & Harding, A. K. Radio-quiet pulsars with ultra-strong magnetic ®elds. Astrophys. J. should exist with a surface magnetic dipole ®eld strength and period 507, L55±L58 (1998). which place it to the right of these death lines. PSR J2144−3933 lies 19. Chen, K. & Ruderman, M. Pulsar death lines and death valley. Astrophys. J. 402, 264±270 (1993). 20. Weatherall, J. C. & Eilek, J. A. Are there two pulsar emission mechanisms? Astrophys. J. 474, 407±413 to the right of both lines. It is (to our knowledge) the ®rst pulsar (1997). known to do so, and calls into question the assumptions made in 21. Taylor, J. H. & Cordes, J. M. Pulsar distances and the Galactic distribution of free electrons. Astrophys. J. 411, 674±684 (1993). deriving the death lines. 22. Lyne, A. G., Manchester, R. N. & Taylor, J. H. The Galactic population of pulsars. Mon. Not. R. Astron. One possibility is that the neutron-star properties might differ Soc. 213, 613±639 (1985). from those commonly assumed. For example, the derived value of 23. Lorimer, D. R., Bailes, M., Dewey, R. J. & Harrison, P. A. Pulsar statistics: The birthrate and initial spin 6 1/2 periods of radio pulsars. Mon. Not. R. Astron. Soc. 263, 403±415 (1993). Bs is proportional to (I/R ) , where I and R are the neutron-star 24. Tauris, T. M. & Manchester, R. N. On the evolution of pulsar beams. Mon. Not. R. Astron. Soc. 298, moments of inertia and radius, respectively5. For different equations 625±636 (1998). 6 of state, I/R may vary by up to two orders of magnitude. If PSR Acknowledgements. M.D.Y. thanks R. Burman for comments on this manuscript, and R. Burman and J2144−3933 has a larger-than-average value of I/R6, the magnetic B. Kenny for advice. M.D.Y. also thanks the University of Western Australia for partial ®nancial support. The Parkes radio telescope is part of the Australia Telescope, which is funded by the Commonwealth of dipole ®eld strength may be large enough to permit pair production Australia for operation as a National Facility managed by CSIRO. and hence radio emission. Correspondence and requests for materials should be addressed to M.D.Y. (e-mail: matthew@physics. Alternatively, it may be that the radio emission process does not uwa.edu.au). depend on pair-production. Weatherall and Eilek20 have suggested that pulsars lying below line A (Fig. 2) all have conal properties, whereas most of those above it are dominated by core emission; they also suggested that conal emission may be generated by a mechan- ism not dependent on pair production. PSR J2144−3933 has clear Measurement of signatures of core emission, and the presence of this pulsar beyond death line A is inconsistent with the hypothesis of Weatherall and gravitational acceleration Eilek. We note that PSR J1951+1123 also lies well below this death line and has a very narrow pulse, consistent with expectations for by dropping atoms core emission10,12. We suggest that the fact that few core-dominated pulsars are seen beyond death line A is just a selection effect due to Achim Peters, Keng Yeow Chung & Steven Chu the narrow beamwidth of the core emission. Physics Department, Stanford University, Stanford, California 94305-4060, USA PSR J2144−3933 has the lowest spindown luminosity (,3 3 ......................................................................................................................... 1028 erg s 2 1) of any known pulsar. It is also relatively nearbyÐ Laser-cooling of atoms and atom-trapping are ®nding increasing the distance, d, estimated from the dispersion measure8,21 is only application in many areas of science1. One important use of laser- , 2 180 pcÐand so its radio luminosity, L400, is low; L400 ¼ cooled atoms is in atom interferometers . In these devices, an 2 < 2 S400d 0:13 mJy kpc , where S400 is its mean ¯ux density at atom is placed into a superposition of two or more spatially 400 MHz. If the beam is assumed to be circular, the beaming separated atomic states; these states are each described by a fraction (that is, the fraction of the celestial sphere swept across quantum-mechanical phase term, which will interfere with one by the beam) is also extremely small, ,0.01. Its low luminosity and another if they are brought back together at a later time. Atom © 1999 Macmillan Magazines Ltd NATURE | VOL 400 | 26 AUGUST 1999 | www.nature.com 849 letters to nature interferometers have been shown to be very precise inertial by j2; p þ ~ki.