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Pulsar Wind Nebulae in Supernova Remnants A&A 380, 309–317 (2001) Astronomy DOI: 10.1051/0004-6361:20011437 & c ESO 2001 Astrophysics Pulsar wind nebulae in supernova remnants Spherically symmetric hydrodynamical simulations E. van der Swaluw1,2,A.Achterberg1,Y.A.Gallant1,3,4,andG.T´oth5 1 Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands 2 Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Republic of Ireland 3 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy 4 Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France 5 Department of Atomic Physics, P´azm´any P´eter s´et´any 1, 1117 Budapest, Hungary Received 19 December 2000 / Accepted 27 September 2001 Abstract. A spherically symmetric model is presented for the interaction of a pulsar wind with the associated supernova remnant. This results in a pulsar wind nebula whose evolution is coupled to the evolution of the surrounding supernova remnant. This evolution can be divided in three stages. The first stage is characterised by a supersonic expansion of the pulsar wind nebula into the freely expanding ejecta of the progenitor star. In the next stage the pulsar wind nebula is not steady; the pulsar wind nebula oscillates between contraction and expansion due to interaction with the reverse shock of the supernova remnant: reverberations which propagate forward and backward in the remnant. After the reverberations of the reverse shock have almost completely vanished and the supernova remnant has relaxed to a Sedov solution, the expansion of the pulsar wind nebula proceeds subsonically. In this paper we present results from hydrodynamical simulations of a pulsar wind nebula through all these stages in its evolution. The simulations were carried out with the Versatile Advection Code. Key words. ISM: supernova remnants – stars: pulsars: general – hydrodynamics – shock waves 1. Introduction expansion of the SNR. Therefore, the dynamical influence of the pulsar wind on the global evolution of the supernova The explosion of a massive star at the end of its life as remnant itself will be small. From an observational point a supernova releases an amount of energy roughly equal 53 of view, however, the presence of a pulsar wind can lead to to 10 erg. Roughly 99% of this energy is radiated away a plerionic supernova remnant, where the emission at ra- in the form of neutrinos as a result of the deleptonization dio wavelengths shows an extended, flat-spectrum central of the ∼1 M stellar core as it collapses into a neutron 51 source associated with a Pulsar Wind Nebula (PWN). The star. The remaining (mechanical) energy of about 10 erg best-known example of such a system is the Crab Nebula, is contained in the strong shock propagating through the and about a half-dozen other PWNs are known unam- stellar mantle, and ultimately drives the expansion of the biguously around pulsars from radio surveys (e.g. Frail & supernova remnant (SNR). Scharringhausen 1997). These surveys suggest that only In those cases where a rapidly rotating neutron star young pulsars with a high spindown luminosity produce (pulsar) remains as a “fossil” of the exploded star, a pul- observable PWNs at radio wavelengths. At other than ra- sar wind, driven by the spindown luminosity of the pulsar, dio wavelengths, in particular X-rays, there are about ten can be formed. The precise magnetospheric physics lead- detections of PWN around pulsars both in our own galaxy ing to such a pulsar wind is not fully understood, but it is and in the large Magellanic cloud (LMC) (e.g. Helfand believed that a major fraction of the spin-down luminosity 1998; Table 1 of Chevalier 2000). of the pulsar is converted into the mechanical luminosity of such a wind. The expansion of an isolated SNR into the general The total rotational energy released by a Crab-like pul- interstellar medium (ISM) can be divided in four differ- sar over its lifetime is of order 1049−1050 erg. This is much ent stages (Woltjer 1972; see also Cioffi 1990 for a re- less than the mechanical energy of ∼1051 erg driving the view): the free expansion stage, the Sedov-Taylor stage, the pressure-driven snowplow stage and the momentum- Send offprint requests to:E.vanderSwaluw, conserving stage. In the models presented here we will only e-mail: [email protected] focus on the evolution of a pulsar wind nebula (PWN) Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20011437 310 E. van der Swaluw et al.: Pulsar wind nebulae in supernova remnants during the first two stages of the SNR: the free expansion and a linear velocity profile as a function of radius, stage and the Sedov-Taylor stage. We will assume that r r the pulsar is stationary at the center of the remnant, ex- Vej(r)= = V0 , (2) cluding such cases as CTB80 (e.g. Strom 1987; Hester & t Rej Kulkarni 1988), PSR1643-43 and PSR1706-44 (Frail et al. with Rej = V0t the radius of the front of the ejecta. The 1994), where the pulsar position is significantly excentric value of V0 is determined by the requirement that the with respect to the SNR, presumably due to a large kick kinetic energy of the ejecta equal the total mechanical velocity of the pulsar incurred at its birth, assuming of energy E0 of the SNR: course that SNR and pulsar are associated and we are Z V0t 2 not dealing with a chance alignment of unrelated sources. 1 r 2 3 2 The case of a pulsar moving through the remnant with a E0 = ρej(t) 4πr dr = MejV0 . (3) 2 0 t 10 significant velocity will be treated in a later paper. In this paper we compare (approximate) analytical ex- This yields: s pressions for the expansion of a PWN in a supernova rem- 10 E0 nant with hydrodynamical simulations carried out with V0 = · (4) the Versatile Advection Code1 (VAC). We confirm earlier 3 Mej analytical results (Reynolds & Chevalier 1984; Chevalier We assume that the stellar ejecta swept up by the strong & Fransson 1992) which state that the PWN is expand- shock which bounds the PWN collect in a thin shell, and ing supersonically when it is moving through the freely that this material moves with the post-shock velocity. expanding ejecta of the SNR. Due to deceleration of the Neglecting the contribution of the thermal energy we can expanding SNR ejecta by the interstellar medium (ISM), write the total (kinetic) energy of this shell, E ,as: a reverse shock propagates back to the center of the SNR shell (e.g. McKee 1974; Cioffi et al. 1988). Due to the presence 2 1 3 ˙ 1 Rpwn of reverberations of the reverse shock in the SNR, the ex- Eshell(t)= Msw(t) Rpwn(t)+ , (5) 2 4 4 t pansion of the PWN goes through an unsteady phase when this reverse shock hits the edge of the PWN. After these where reverberations have decayed, the expansion of the PWN 3 through the ejecta of the SNR progenitor star continues Rpwn Msw(t) ≡ Mej (6) subsonically with the PWN almost in pressure equilibrium V0t with the interior of the SNR. is the ejecta mass swept up by the pulsar wind nebula. This paper is organised as follows. In Sects. 2 and 3 we In deriving the post-shock velocity, we assumed that the discuss the aforementioned two stages of the PWN/SNR ejecta behave as an ideal non-relativistic gas with adia- system. In Sect. 4 the hydrodynamical simulations will be batic heat ratio γej =5/3 and used the Rankine-Hugoniot presented and compared with the analytical expressions jump conditions for a strong shock. from Sects. 2 and 3. The interior of the PWN is dominated by thermal en- ergy. The sound speed in a realistic PWN is close to the speed of light c, while the expansion velocity is much less 2. Pulsar Wind Nebula in a freely expanding than c. Perturbations in the pressure will be smoothed ∼ Supernova Remnant out rapidly, on a sound crossing time ts Rpwn/c,much less than the expansion time scale texp ∼ Rpwn/R˙ pwn. In the early stage of the evolution of a PWN, the SNR Therefore, we can assume a nearly uniform pressure Ppwn consists mostly of the stellar ejecta expanding freely into in the PWN. The internal energy of the PWN then equals the interstellar medium. The PWN expands into these ejecta. The sound velocity in the interior of the SNR is 4π 3 Epwn = Rpwn Ppwn. (7) much smaller than the expansion velocity of the PWN. 3(γpwn − 1) The supersonic expansion of the PWN results in a shock Here we take γ =4/3 because the pulsar wind nebula propagating into the ejecta (see Fig. 1). pwn material is relativistically hot. The pressure of the interior An analytical equation for the radius of this shock can of the PWN is a few times (∼4whenγej =5/3) the pres- be derived for a constant spindown luminosity. Using this sure in the shocked ejecta just downstream of the outer solution, the assumption of supersonic expansion will be shock of the pulsar wind nebula (Chevalier 1984, Fig. 4). checked a posteriori. For simplicity we assume that the In the Appendix, we give a derivation of this result using ejecta have a uniform density, the thin shell approximation for the motion of the shocked α ejecta, assuming a power-law expansion: Rpwn ∝ t .There 3Mej we show that the ratio of the shock radius Rs and the ra- ρej(t)= 3 , (1) 4πRej dius Rc of the contact discontinuity separating the pulsar wind material from the shocked ejecta satisfies (Eq.
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