Pulsars and Supernova Remnants

1604–2004: SUPERNOVAE AS COSMOLOGICAL LIGHTHOUSES ASP Conference Series, Vol. 342, 2005 M. Turatto, S. Benetti, L. Zampieri, and W. Shea Pulsars and Supernova Remnants Roger A. Chevalier Dept. of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903, USA Abstract. Massive star supernovae can be divided into four categories depending on the amount of mass loss from the progenitor star and the star’s radius. Various aspects of the immediate aftermath of the supernova are expected to develop in different ways depending on the supernova category: mixing in the supernova, fallback on the central compact object, expansion of any pulsar wind nebula, interaction with circumstellar matter, and photoionization by shock breakout radiation. Models for observed young pulsar wind nebulae expanding into supernova ejecta indicate initial pulsar periods of 10 − 100 ms and approximate equipartition between particle and magnetic energies. Considering both pulsar nebulae and circumstellar interaction, the observed properties of young supernova remnants allow many of them to be placed in one of the supernova categories; the major categories are represented. The pulsar properties do not appear to be related to the supernova category. 1. Introduction The association of SN 1054 with the Crab Nebula and its central pulsar can be understood in the context of the formation of the neutron star in the core collapse and the production of a bubble of relativistic particles and magnetic fields at the center of an expanding supernova. Although the finding of more young pulsars and their wind nebulae initially proceeded slowly, there has recently been a rapid set of discoveries of more such pulsars and nebulae (Camilo 2004). In a number of cases there is interaction with a circumstellar medium, which gives further diagnostics on the system. There is considerable variety in the properties of these systems, and the variety is enhanced by the finding of non-normal pulsar objects in the centers of supernova remnants, such as the central compact object in Cas A (Pavlov et al. 2000). The variety of collapsed objects and their surroundings brings up the question of what determines the outcome of a particular core collapse. An initial approach to this question is to investigate whether there is a relation of the type of core collapse supernova to the nature of the compact star aftermath. The classification of core collapse supernovae is currently confused because different classes depend on different observed aspects of the event. I will use 4 basic categories of events: IIP, IIL/b, Ib/c, and SN 1987A-like. The first three categories refer to the amount of H envelope that is lost before the supernova. In the Type IIP supernovae (SNe IIP), the ones with a plateau light curve, most of the H envelope is retained at the time of the explosion. In the SNe IIL/b, ones which show a linear light curve or a late transition to Type Ib, some of the H envelope is present but most of it has been lost as a result of mass loss. In 422 Pulsars and Supernova Remnants 423 the SNe Ib/c, which have no H lines in their spectra, the H envelope has been entirely lost. The progenitors of SN 1987A-like events are stars that retain most of their H envelopes, like the IIP, but they explode as blue supergiants, unlike the red supergiant explosions of SNe IIP. As described in section 2, these different types of events lead to different young supernova remnant properties and to different environments for a central compact object. In section 3, I describe how observed young SNRs might be placed into these categories. The details of these arguments and full references can be found in Chevalier (2005). 2. Properties of the Supernova Categories The density and composition structure of a supernova is set up soon after the shock front has passed through the star and the gas approaches a state of free expansion. In this process, the presence of strong density gradients at composition interfaces (H/He, He/O, and O/Si) plays an important role. At the edge of a density drop, the flow tends toward free expansion, but is decelerated by the next layer out. The deceleration gives an enhanced pressure which drives a reverse shock wave toward the center of the star and gives rise to Rayleigh-Taylor instabilities at the interface. The instabilities only have a chance to develop fully if the denser layer is completely decelerated, i.e. at the H/He interface the H envelope must be mostly present. At the interface, heavy material is mixed out in plumes, and the lighter inner material is mixed back toward the center in bubbles. Although there have been a number of computer simulation of the instabilities (Kifonidis et al. 2003), the detailed theoretical aspects of the flows remain uncertain and inferences from observations of supernovae in the free expansion phase are necessary. In IIP and 1987A-like supernovae, the H is observed to be present down to low velocities (few 100 km s−1). This is not the case for IIL/b supernovae; for example, in the Type IIb SN 1993J, the H extends over the velocity range 8500 − 10, 000 km s−1 (Houck & Fransson 1996). In both SNe IIL/b and Ib/c, there is evidence for mixing between heavy element layers. In addition to the composition structure, mixing to the center may be important for setting up the surroundings of the central neutron star. Fallback onto the neutron star occurs because some material will be marginally bound to the neutron star. Neutrino losses are high for material that is accreted to the neutron star, so that it can become part of the star. Possible modes of fallback are either material that does not directly escape the gravitational potential and material that is brought back to the center by the reverse shock(s). In the first mode, there is a critical energy below which, or mass above which, there is a great deal of fallback and a black hole is the likely outcome. In the other limit, there may be little fallback. In the case of the reverse shock, the fallback depends on the conditions in the center of the explosion which depend on details of the explosion hydrodynamics. For SN 1987A, Chevalier (1989) estimated a fallback mass of ∼ 0.1 M⊙, but the more recent calculations of Kifonidis et al. (2003) indicate central conditions which imply a smaller mass. An interesting aspect of material brought back by the reverse shock is that it may have greater angular momentum than the mass that is initially close to the compact object, possibly spinning up the neutron star by fallback. 424 Chevalier Another difference between the types of supernovae is the energy in ionizing radiation at the time of shock breakout. The energy primarily depends on the radius of the progenitor star. For an extended red supergiant progenitor (SNe IIP or IIL/b), the radiation can ionize all the mass lost from the star. For a compact progenitor (SNe Ib/c), only a fraction of a solar mass can be ionized. 3. Pulsar Nebulae in Young Supernova Remnants Pulsars are born inside of supernovae, where their nebulae of relativistic particles and magnetic fields can sweep up the surrounding supernova gas. The expansion of the nebula can be described as the expansion of a relativistic bubble of gas. The pulsar is expected to spin down at some point, which reduces the power being produced by the pulsar. Fig. 1 shows the energies associated with the nebula in various forms. Here, it is assumed that the nebula remains within the −1.06 −3 inner part of the supernova density profile, with a form ρsn ∝ (r/t) t based on Matzner & McKee (1999). The constant τ is the initial spindown timescale of the supernova and n is the braking index, assumed to be constant. It can be seen that the ratio Eint/E˙ t provides an estimate of the evolutionary status of the pulsar and nebula, where Eint is the internal energy, E˙ is the current power output of the pulsar, and t is the age. Information on Eint can be obtained from the minimum energy derived from synchrotron emission. Figure 1. The evolution of pulsar nebula internal energy, Eint, kinetic energy of the swept-up shell, Ekin, and the current E˙ t divided by the initial rotational energy, Erot. The 2 models are characterized by the power law index of the supernova density profile, m, and the pulsar braking index, n. The reference time τ is the initial spindown time of the pulsar (from Chevalier 2004). Pulsars and Supernova Remnants 425 The results of applying this model of pulsar nebula expansion are shown in Table 1 (Chevalier 2004). The ages and initial periods, P0, are generally found by taking the current radius of the object and finding a model that fits the radius, assuming expansion into the inner part of a typical supernova. After a model is calculated, it is possible to check that the pulsar nebula is within the inner, moderately flat part of the density profile. This is true for the nebulae discussed here. There are some cases, such as the Crab and 0540–69, for which there is independent information on the age. In general, the preferred models have an internal energy within a factor of a few of the minimum energy required for the synchrotron emission. An age much less than the characteristic pulsar age, P/2P˙ , indicates that the nebula is in an early evolutionary stage.

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