_NATIJRE_v_oL._304_4A_uo_usr_I98_3 -----NEWS AND VIEWS------391
The second millisecond pulsar On the principle that no celestial object is unique, the discovery of another millisecond pulsar is overdue. That reported on page 417 has an unexpected interest. WHEN the first millisecond pulsating star pected properties- small deceleration and from which X-ray emission was sought (PSR1937 + 214) was discovered more than a relatively small magnetic field (estimated during the lifespan of the Einstein X-ray a year ago, there were two contrasting at about 10 10 gauss). satellite, with the result that it has been views of what might happen next: either the The fact that the new object is a partner possible to use data to set an upper limit of 1.5-ms rotating neutron star was such an in a binary system is the starting point for the X-ray emission from the new system. exceptional object, so near the edge of in the accompanying accounts of how it may The upshot of the argument is that the most stability perhaps, that it would prove to be have arisen. Indeed, the new pulsar is only likely progenitor of the pulsar was not a an isolated phenomenon, or the late the fourth to have been found within a neutron star as such but a white dwarf discovery of this quickly spinning object binary system, much less than the propor which, by accreting matter from its once was a sign that radioastronomers had pre tion (more than a half) of ordinary stars in giant companion, exceeded the critical viously neglected objects in this range, so cluded within binary systems. No doubt the mass of 1.3 times the solar mass, pushing it that the first discovery would quickly be explanation is that the occurrence of a over the threshold separating dwarfs from followed by several others. The truth is supernova explosion within such a system neutron stars. somewhere in between. Far from a flood of will often send the residual neutron star Everybody seems to agree that whatever new discoveries of pulsars with periods in scurrying away. The consequence, the original condition of the system, it must the millisecond range, there has been just however, is that it has been possible to recently have gone through a phase in one - that reported by Boriakoff et at. on glean something of the mass of the system which it would have been recognizable as a page 417. Millisecond pulsars are plainly from half a period's observation of the low-mass X-ray binary of the kind describ uncommon though not exceptional ob Doppler shift of the radio pulsation. The ed by Virginia Trimble in her recent field jects. The new pulsar has the convenience conclusion that the orbit is virtually cir guide to binary stars (Nature 303, 137; of being the principal member of a binary cular is only tentative. The conclusion that 1983). Ruderman and Shaham carry the system, which explains why the report of its the pulsar is more massive than its compa argument a step further in their account of discovery comes complete with no fewer nion is not compelled by the data but is like the origin of the apparently isolated 1.5-ms than half a dozen similar explanations of ly to be true. The accompanying explan pulsar. They conclude that there is a high how it may have been formed. ations agree that the companion is now a chance (determined by the precise details of Even so, it is worth recalling the enormi white dwarf with a mass about 0.3 that of the orbit and the relative masses of the two ty of what needs to be explained. All the Sun. stars) that in a binary system consisting of a pulsars are neutron stars, and nobody has The explanations of the origin of the 6.1- white dwarf primary and a secondary that seriously quarrelled with Chandrasekhar's ms pulsar follow similar lines. The com has become a giant, tidal forces may so estimate that it needs a mass 1.3 times that mon notion is that the now unseen compo disrupt the secondary star when the mass of the Sun to make a neutron star rather nent of the binary system has within the transfer has gone some way that it will than a degenerate dwarf in which atomic comparatively recent past (ten million either be totally fragmented or reduced in nuclei retain their identity. Most pulsars years?) gone through the giant phase in the size to an undetectable planet the size of, have periods measured in seconds, which is normal evolution of a star. To begin with, say, Saturn. explicable only because of the compactness the orbit was much smaller than at present. All this is good clean fun of the kind that and tensile strength of neutron matter When most of the internal hydrogen had keeps astrophysics entertained - and (derived from the strong nuclear forces been converted into helium, the star entertaining. The discovery of the 6.1-ms holding them together), which enable these became a giant, burning hydrogen only in pulsar does, however, reinforce the opi stars to survive the stresses of rapid rota its envelope which by then, in any case, nion that has been growing for some time tion. As it happens, the rotational speed of would have reached beyond the gravita that there are two kinds of pulsars- those a neutron star is limited not so much by its tional equipotential point between the two formed in supernova explosions (usually tensile strength as by the rate at which a stars. Material would then have been with modest rotational speeds) and those rapidly spinning object would lose energy transferred from the companion star to the formed by the accretion of matter onto by means of gravitational waves caused by primary together with angular momentum. white dwarfs or other degenerate objects. readjustments of its shape. So, the argument goes, the end result Pulsars of the former class have relatively Speed apart, the first millisecond pulsar would be that now observed - a rapidly small rotational speeds (with periods had two other exceptional properties. The pulsating neutron star whose mass is com measured in seconds), large velocities rate of deceleration of the spin, measurable parable with that of the Sun and a compa perpendicular to the galactic disk and enough, is small compared with the nion in a nearly circular orbit that has lost strong magnetic fields. The second class, decelerations of more conventional all but the helium core with which it em consisting of pulsars formed by accretion pulsars. But since the deceleration of the barked on its giant phase. It seems to be onto a degenerate star, are represented by rotation of a pulsar is a consequence of the agreed that if the dwarf continues to burn the two millisecond pulsars so far found interaction of matter with its magnetic hydrogen in its envelope, it should be two and possibly by some of those with smaller field, the strength of the magnetic field of orders of magnitude more luminous than rotational speeds. Telling which are which this quickly spinning pulsar must also be the Sun and thus just on the limit of detect will always be difficult at the borderline. small compared with that of more conven ability even at a distance of 3,500 parsecs. Looking for X-ray binaries that may be tional pulsating stars. So far as can be told By sheer good luck, Helfand et at. (see nascent systems like PSR1953 + 29 should, from the data now published, the new page 423) have important information on however, be rewarding- Trimble explains pulsar PSR1953 + 29, with a rotational this point. The new pulsar happens to be that there is a host of candidates. period of 6.1 ms, shares the same unex- within one minute of arc from another John Maddox
0028-0836/83/310391·01:$01.00 © 1983 Macmillan Journals Ltd