4. Super strong magnetic field
Let us also ponder how far we are justified in assuming a superstrong G for a very hot new born pulsar or accretion torus. As to the origin of such magnetic fields in young pulsars, Duncan & Thompson (1992) suggest that the convective turbulence associated with strong neutrino transport with erg/s/cm2 causes such magnetic field by the dynamo action. This tends to suggest that the strong magnetic field is an aftermath of strong neutrino cooling and it need not be present in the nascent and hot NS. In a normal astrophysical or laboratory plasma, microscopic turbulence may convert part of the bulk kinetic energy into magnetic field. However, at the same time it should be remembered that turbulence on macroscopically significant scales may have a tendency to destroy the pre-existing strong magnetic field by producing spatially and temporally incoherent current systems and by means of ohmic heating. Thus, there may be an optimal level of turbulence conducive to production of strong magnetic field and beyond which turbulence may be counter productive.
In general, linear extrapolation of magnetic field generation ideas (in relativistically hot plasma), which are originally meant to explain much lower fields, may not be proper because note that there is a natural quantum unit of for B. Here e is the electronic charge and is the electron mass. For , the Larmour radius of the electrons, whose current ultimately gives rise to B, becomes equal to the electron Compton wavelength . Thus it may be possible that one can realize a value of B comparable to only when there is some degree of macroscopic quantum behaviour of the underlying medium. Atleast, in cold neutron stars this is the case in that the source of the magnetic field can be ascribed to the existence of strong internal current systems in the form of quantized flux tubes or fluxoids carrying elementary flux (Bhattacharya & Srinivasan 1995, and ref. therein). The existence of strong internal currents may be inextricably linked to the existence of a superconducting NS -interior (protons and neutrons behave like superconducting medium in which the electron current flows). The fluxoids have the same sense of current and , where is the number of fluxoids threading each cm2. Such type II (or any other) superconductivity is a manifestation of macroscopic quantum behaviour and can be operative only below certain critical temperature, which, for NS interiors, is . Therefore, it might be possible that, a new born NS, which must be very hot does not possess the observed strong field, and, the strong field is set up later at the expense of the turbulent and internal energy as the star cools below and becomes quantum mechanically organized.
On the other hand, the catastrophic NS-NS collision process, which is far from the spherically symmetric near-adiabatic core-collapse scenario and results in a central compact object of MeV (Ruffert et al 1997), might destroy the preexisting order and the magnetic field. The same is even much more true for the resultant disk which is very hot (MeV) and macroscopically turbulent. Recall here that, there is a critical temperature above which the NS core or the disk ceases to be a superconductor. Earlier theoretical estimate was that in the density range of g cm-3, the transition temperature lies between MeV. But more recent and and refined estimates find MeV even at the highest densities (Bhattacharya & Srinivasan 1995). Thus, the hot compact object or the disk resulting from tidal distortions is most unlikely to be the site of a magnetic field whose value exceeds the characteristic quantum value G. Probably, the modest macroscopic quantum behaviour implied by a type II superconductor can not explain a G, the maximum value for radio pulsars, and it may require a greater degree of quanum coherence to have a field stronger than this. If one reqires a value of , it may be more logical to conceive that superconductivity is due to flow of protons rather than of electrons. This will, however, demand, quantum coherence on much larger scale.
It is thus particularly difficult to conceive how the hot, turbulent, and incoherent accretion disk resulting from a NS-NS or NS-BH collision may possess a value of .
At any rate the sustenance of a strong NS magnetic field is certainly not due to any strong macroscopic turbulence, because, in a cold NS core or crust, there is hardly any macroscopic turbulence. It must be mentioned now that recently, in a remarkable observation, the so called Soft Gamma Ray Repeaters (SGRs) have been identified to be associated with a class of NS with strong magnetic field few G (Kouveliotou et al. 1998). These class of NSs with superstrong magnetic fields have been termed as "magnetars". But, from our view point, the important point is that the magnetars are extremely slow rotators with few s. Their spin down luminosity is very low having a value of only erg/s. The stored magnetic energy of such stars is in the range of erg, which far exceeds the RKE erg (at the present epoch). It is interesting to note that the X-ray and particle emissions from the magnetars, which may fuel the SGR activity, are powered not by rotation but by the sporadic release of the stored magnetic energy. The magnetars discovered now may be several thousand years old with temperature, certainly, much below 1 MeV.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998