SUPERNOVA REMNANTS, PULSARS AND THE INTERSTELLAR MEDIUM - SUMMARY OF A WORKSHOP HELD AT U SYDNEY, MARCH 1999
Vikram Dwarkadas , Lewis Ball , James Caswell , Anne Green , Simon Johnston , Brian Schmidt , Mark Wardle, PASA, 17 (1), 83.
Next Section: Masers associated with SNRs
Title/Abstract Page: SUPERNOVA REMNANTS, PULSARS AND
Previous Section: Exotics I
Contents Page: Volume 17, Number 1

Exotics II

Dick Manchester (ATNF) and Don Melrose (SRCfTA) talked on the observations and theory of Anomalous X-ray Pulsars (AXPs). AXPs have periods of 6-12 seconds (cf. `normal' pulsar periods range from 0.025 s to several seconds), soft X-ray spectra, and relatively low X-ray luminosities of 10²⁸ - 10²⁹ W -- significantly below the Eddington limit 10³¹ W. Their X-ray emission is relatively steady on time scales longer than the pulse period - much more so than for accretion-powered binary sources - and they exhibit no evidence that they are binary star systems.

The pulse periods of AXPs increase with time (Mereghetti, Israel & Stella 1998). If the associated loss of rotational energy is attributed solely to magnetic dipole radiation, the inferred surface field is $B \sim 3.2\times 10^{15} (P \dot{P})^{1/2}$ Tesla, whence $B\sim3\times 10^{10}$ T for typical AXP parameters: $P\sim 10$ s and $\dot{P}\sim 10^{-11}$ (corresponding to 3 ms/year). This is much stronger than the inferred fields for `normal' ($B\sim 10^8$ T) and millisecond pulsars ($B\sim 10^5$ T). The idea that their strong magnetic field may be the defining characteristic of AXPs has led to them being referred to as `magnetars' (Thompson & Duncan 1993).

More specifically, a magnetar is a neutron star whose surface field exceeds the critical field strength $B_{\rm c}=4.4\times 10^9$ T at which the energy corresponding to the cyclotron frequency $\Omega_{\rm e}$ equals the electron rest energy $\hbar\Omega_{\rm e} = m_{\rm e} c^2$. Electric fields of energy densities exceeding that of the critical field decay spontaneously via electron-positron pair creation. Magnetic fields which exceed $B_{\rm c}$ cannot decay in this way because of kinematic restrictions -- the process of pair creation would violate momentum conservation.

The strong inferred fields of magnetars may arise in one of two ways. Usov (1992) has shown that the if the strong magnetic fields associated with some white dwarf stars are frozen in when they collapse as Type 1a supernovae, then neutron star fields of 10⁷ T may result. Duncan & Thompson (1992) have shown that dynamo action could generate the inferred fields.

The energy loss rates, $4\pi^2 I/(P\dot{P})$ where I is the moment of inertia, for normal and millisecond pulsars are much higher than the observed radiation luminosities, and these objects are thought to be rotation powered. In contrast, the spin-down luminosity of a neutron star with $P\sim 10$ s and $\dot{P}\sim 10^{-11}$ is 4 x 10²⁵ W, much less than the observed X-ray luminosities of AXPs. It is therefore thought that AXP emission is not powered by rotation, but rather by the decay of their strong magnetic fields.

Some the eight known AXPs are associated with supernova remnants and some with Soft Gamma-ray Repeaters (SGRs). There is some evidence that the AXPs associated with SGRs have the strongest inferred magnetic fields. The idea that a strong neutron star magnetic field suppresses radio emission has recently been placed on a more firm theoretical foundation by Baring and Harding (1998), invoking suppression of electron-positron pair formation due to increased photon splitting.

The best known SGR was the source of the 5 March 1979 event which attained a luminosity of 10³⁷ J and had a clear 8.1 s periodicity. It is believed to be associated with a supernova remnant, N49, in the Large Magellanic Cloud. A specific model for this object involves the release of magnetic energy through fractures of the neutron star crust (Thompson & Duncan 1995).

In a supercritical magnetic field the cross section for the scattering of radiation with frequencies well below the gyrofrequency is highly anisotropic. In particular, scattering of the extraordinary mode is strongly suppressed with respect to that of radiation in the ordinary mode. The consequences of this effect are subtle: it allows extraordinary mode emission to escape even from close to the neutron star, and it clearly affects the interpretation of the Eddington `limit' for accretion powered sources.

The Parkes Multibeam Pulsar Survey (Lyne et al., 1999), which has a flux sensitivity of 150 $\mu$Jy and is seven times more sensitive than any previous survey, may double the number of radio pulsars from the 750 known before it began. It has already discovered 362 new pulsars, including PSR J1814-17 which has a period of around 4 s and a high $\dot{P}$ which places it in the part of $P-\dot{P}$-space occupied by AXPs. The AXP 1904+09, which has P=5.16 s, $\dot{P}=1.23\times10^{-10}$, and which is associated with SGR1900+14, has recently been claimed as a radio pulsar (Shitov 1999).

AXP/SGR/SNR associations, and the relationship between magnetic field strength and radio emission, may ultimately shed light on the apparent deficiency of radio pulsars that are associated with supernova remnants.

The collapse of a star and the resulting supernova explosion that produces a neutron star depends on neutrinos to revive the shock and eject the outer layers of the star (Bethe & Wilson 1985). Four neutrino flavours are necessary to explain all known neutrino anomalies, but only three ordinary neutrinos are allowed. Yvonne Wong (University of Melbourne) is investigating the possibility that the fourth flavour may arise through oscillations into `sterile' neutrinos - which do not participate in weak interactions as ordinary neutrinos do. The physics of such oscillations, in matter rather than in vacuo, has important implications for the understanding of supernova shocks (Nunokawa et al. 1997).

Roberto Soria (ANU/SRCfTA) and Amy Mioduszewski (SRCfTA) discussed observations of the sources GRS J1655-40 and CI Cam which have answered some questions and raised others. Optical spectra of GRS J1655-40 display both broad lines in absorption and emission ( $>1000\;{\rm km\,s^{-1}}$), and emission lines which are narrower than the minimum allowed if they originate in an accretion disk. This can be explained if the system is a black hole binary, and the narrow lines originate in an extended envelope surrounding the disk. The nature of the source CI Cam remains a mystery. It has been classified as a symbiotic star and as a Herbig B object. It is a bright emission-line star which exhibited a single uncomplicated X-ray brightening on 1 April 1998, detected by RXTE and CGRO/BATSE, brightening from $\sim 0$ to $\sim 2$ Crabs in less than 1 day and then slowly decaying. An associated optical brightening by 2 magnitudes was recorded (Fontera et al. 1998). A radio flare was detected with the VLBA on day 1 and then at intervals of a few days. The images, with a resolution of just a few AU show a slowly-expanding synchrotron shell, with a speed of just $200\;{\rm km\,s^{-1}}$, and no evidence of the jet-like collimated outflows seen in all other soft X-ray transient-related radio transients observed with sufficient resolution.

Next Section: Masers associated with SNRs
Title/Abstract Page: SUPERNOVA REMNANTS, PULSARS AND
Previous Section: Exotics I
Contents Page: Volume 17, Number 1