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A Model of the Vela Supernova Remnant

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A MODEL OF THE VELA SUPERNOVA REMNANT VA S I L I I G VA RA M A D Z E ? Abastumani Astrophysical Observatory, Georgian Academy of Sciences, A.Kazbegi ave. 2-a, Tbilisi, 380060, Georgia; Sternberg State Astronomical Institute, Universitetskij Prospect 13, Moscow, 119899, Russia; Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, P.O. Box 586, 34100 Trieste, Italy Abstract. A model of the Vela supernova remnant (SNR) based on a cavity explosion of a supernova (SN) star is proposed. It is suggested that the general structure of the remnant is determined by the interaction of the SN blast wave with a massive shell created by the SN progenitor (15−20 M )star. A possible origin of the nebula of hard X-ray emission detected around the Vela pulsar is discussed. 1. Introduction The Vela SNR is one of the best studied SNRs, primarily because of its large angular extent and high brightness, and to no small degree due to its association with the well-known Vela pulsar. High-resolution observations made at the last ten years revealed many new details of the Vela SNR and revived interest to this remnant. However in despite of the considerable progress in observations there is still no a model proposed to explain the whole complex of the observational data, though many attempts were made to explain one or another peculiarity of the Vela SNR. In this paper‡ we try to make up this deficiency. In Section 2 we review the main observational data. Section 3 contains our model of the Vela SNR. 2. Observational Data The angular size of the main body of the Vela SNR is about 7◦ in radio (Duncan et al., 1996, hereafter D96), optical (Seward 1990, hereafter S90) and X-ray (S90; Aschenbach et al., 1995, hereafter A95) ranges. In all these spectral ranges the Vela SNR shows a distinct asymmetry along the line perpendicular to the Galactic plane. The northeast half of the remnant (faced towards the Galactic plane) has a well ? Address for correspondence: Krasin str. 19, ap. 81, Moscow, 123056, Russia; E-mail: [email protected] ‡This paper summarizes the results published in Gvaramadze 1998a,b, 1999. Astrophysics and Space Science 274: 195–203, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 196 VASILII GVARAMADZE determined circular boundary of radius of ' 3.5◦, whereas the opposite half is very disordered, with some (X-ray) structures (A95) stretched up to ' 6.5◦ from the center (associated with the Vela pulsar position). There also exist several features protruding far outside the northeast rim of the remnant (A95; D96; Gvaramadze, 1998a). At a distance to the Vela SNR of 500 pc (in what follows, we adopt just this value; the contradictory distance estimates for this remnant will be discussed elsewhere) 1◦ corresponds to about 9 pc. The only reliable age estimate of the Vela SNR comes from the estimate of the spin-down age of the Vela pulsar, which is ' (2 − 3) · 104 years (Lyne et al., 1996). 2.1. THE RADIO DATA Early radio observations (e.g. Milne, 1968) showed that the radio emission of the Vela SNR comes from three radio components, which are known as Vela X, Y, and Z. The maximum of emission of the brightest radio component, Vela X, is displaced for ' 450 to the south of the pulsar. High-resolution observations of the Vela SNR showed that it is composed of many filamentary and looplike structures (e.g. D96; Bock et al., 1998a, hereafter B98), and that the northeast half of the remnant is enclosed by an arc of polarized emission which closely matchs its boundary (D96). The Vela X also consists of many strongly polarized filaments with a mean width of few arcminutes (Milne, 1995, hereafter M95; Frail et al., 1997, hereafter F97; B98). Some authors (e.g. Weiler and Panagia, 1980, hereafter W80; Dwarakanath, 1991; B98) believe that the spectrum of Vela X is much flatter than that of other parts of the Vela SNR, while M95 (see also Milne and Manchester, 1986) showed that the spectra of all radio components are nearly the same with a value of the spectral index between –0.4 and –0.8. Based on the high percentage of polarization and on the possible flat spectrum of the Vela X, W80 suggested that this radio source is a plerion, i.e. a pulsar-powered nebula. However Milne and Manchester (1986) pointed out that the large displacement of the Vela pulsar from the center of Vela X could not be explained by the proper motion of the pulsar, since the latter is too small and not in the proper direction (e.g. Nasuti et al., 1997), and concluded that the Vela X is a part of the Vela SNR’s shell. 2.2. THE OPTICAL DATA The [OIII] λ5010 filter photographs of the Vela SNR by Parker et al. (1979, here- after P79) combined in a mosaic and presented in Figure 1 show a very intricate design composed of a multitude of arclike and looplike filaments. It is gener- ally accepted that the origin of optical filaments is connected with the interaction between adiabatic (Sedov-Taylor) SN shocks and interstellar clouds (e.g. Bychkov and Pikel’ner, 1975), or with projection effects in radiative shocks, whose fronts are rippled due to the refraction and reflection by density inhomogeneities in the ambient medium (e.g. Pikel’ner, 1954). The mean expansion velocity of the Vela SNR, inferred from ultraviolet absorption observations, is about 100 km s−1 (e.g. VELA SUPERNOVA REMNANT 197 Jenkins et al., 1984, hereafter J84), whereas some portions of the remnant’s shell expand with much higher speeds (e.g. J84; Danks and Sembach, 1995). The same observations revealed that the line of sight component of the shell velocity does not gradually decrease towards the edges of the remnant (as it should be according to the standard Sedov-Taylor model), but rather shows a chaotic behaviour. J84 suggested that the ‘unusual’ velocity field inherent to the Vela remnant’s shell indicates that some process induces transverse motions in the shell. 2.3. THE X-RAY DATA The soft X-ray emission of the Vela SNR is dominated in the northwest and south- east quadrants of the remnant, and shows some limb-brightening on the northeast edge (Kahn et al., 1985; hereafter K85). K85 found also that the X-ray emission is distributed in a patchy way, with significant spectral and intensity variations on an- gular scales ranging from several arcminutes to a few degrees. Another interesting finding of K85 is that the soft X-ray structures generally correspond with optical filaments. The characteristic X-ray temperature was estimated as ' 0.2keV.Itis generally believed that the soft X-ray emission of the Vela SNR is produced by the hot plasma behind the adiabatic (Sedov-Taylor) SN shock wave (e.g. K85). In this case, the X-ray temperature of 0.2 keV corresponds to the shock velocity of ' 400 km s−1. Subsequent observations of the Vela SNR reveal (S90; A95) that the X-ray extension of this remnant is much larger than that which was found by K85. Particularly, S90 found a long, faint arc in the western part of the SNR, which was interpreted as a rim of the SNR’s shell. A95 discovered a number of radial X-ray structures protruding far beyond the main body of the Vela SNR and interpreted them as bow shocks produced by fragments of the exploded SN star. Willmore et al. (1992; hereafter W92) found an elongated structure of hard X-ray emission (in the 2.5–25 keV band) in the center of the remnant. This structure stretches for about 1◦ on either side of the pulsar in the northeast-southwest direction. W92 suggested that this structure is a synchrotron nebula. Then Markwardt and Ögelman (1995, hereafter M95) discovered a filamentary structure of irregular shape extended from the Vela pulsar position to the center of the Vela X. This feature appears at energies above 0.7 keV (M95) and is visible to energies of at least 7 keV (Markwardt and Ögelman, 1997, hereafter M97). M95 (see also M97; F97) interpreted the X-ray filament as a one-sided jet emanating from the Vela pulsar and supplying the energy from the pulsar into the Vela X. Therethrough they supported the proposal of W80 that the VelaX is a plerion. An important result of X-ray studies of the Vela SNR is the discovery of a net of arcs and loops of intense soft X-rays which covers the whole remnant (Aschenbach, 1997). And finally, an analysis of X-ray images of the Vela SNR presented by Aschenbach (1998) shows that some features visible at radio, optical and/or soft X-ray wavelengths (e.g the X-ray protrusion labelled by A95 as D/D’ and the U-type optical filament on the south edge of the remnant (see Figure 1)) have obvious hard (≥ 1.3 keV) X-ray counterparts. 198 VASILII GVARAMADZE 3. Model of the Vela SNR The main point of our model of the Vela SNR is the statement that the general shape of the Vela SNR might be explained as a result of the interaction between the SN ejecta/shock and the pre-existing wind-driven shell created by the SN progenitor star in an interstellar medium with a density gradient perpendicular to the Galactic plane; the existense of a large-scale region of enhanced density to the northeast of the Vela SNR (i.e.
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