A&A 369, 174–177 (2001) Astronomy DOI: 10.1051/0004-6361:20010104 & c ESO 2001 Astrophysics On the velocity of the Vela pulsar V. Gvaramadze1,2,3,? 1 Abastumani Astrophysical Observatory, Georgian Academy of Sciences, A. Kazbegi ave. 2-a, Tbilisi 380060, Georgia 2 Sternberg State Astronomical Institute, Universitetskij Prospect 13, Moscow 119899, Russia 3 Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, PO Box 586, 34100 Trieste, Italy Received 10 May 2000 / Accepted 14 December 2000 Abstract. It is shown that if the shell of the Vela supernova remnant is responsible for nearly all the scattering of the Vela pulsar, then the scintillation and proper motion velocities of the pulsar can only be reconciled with each other in the case of nonzero transverse velocity of the scattering material. A possible origin of large-scale transverse motions in the shell of the Vela supernova remnant is discussed. Key words. pulsars: individual: Vela – ISM: individual objects: Vela supernova remnant – scattering 1. Introduction Aschenbach et al. 1995; Bocchino et al. 1999; Cha et al. 1999; Cha & Sembach 2000)1. One of the arguments (pro- The Vela pulsar is one of the best studied radio pulsars posed by Ogelman¨ et al. 1989 and repeated by Oberlack and was the first one found to be associated with the su- et al. 1994; Cha et al. 1999) was based on the compari- pernova remnant (SNR) (Large et al. 1968). In spite of its son of the Vela pulsar transverse velocity (inferred from relative proximity to the Earth, there is still no consensus the new estimate of the pulsar proper motion by Ogelman¨ on the value of its (transverse) velocity, which is connected et al. 1989) with the scintillation velocity (reported by with the yet unsolved problem of the distance to the Vela Cordes 1986). It is known that proper motion velocities of pulsar/Vela SNR. The first attempt to estimate the pulsar pulsars show significant correlation with pulsar velocities velocity was made by Bignami & Caraveo (1988), whose inferred from interstellar scintillation measurements (e.g. optical measurments gave an upper limit on the pulsar Lyne & Smith 1982; Gupta 1995). Therefore, assuming proper motion (<60 mas yr−1). However, even the max- that the scintillation velocity V isa“true”valueofthe imum admissible value of the pulsar proper motion was iss pulsar transverse velocity, Ogelman¨ et al. (1989) suggested found to be too low to explain the pulsar offset from the −1 that Viss =53 5kms (Cordes 1986) could be recon- apparent geometrical centre of the Vela SNR, which ques- − ciled with the proper motion µ ' 38 8masyr 1 if the tions the pulsar/SNR association. Later, it was recognized distance to the Vela pulsar (and the Vela SNR) is about (e.g. Seward 1990; Aschenbach et al. 1995) that the real 290 80 pc. The distance reduction might be even more extent of the Vela SNR is much larger than was accepted dramatic if one takes the recent high-precision estimate of in early studies, so now there can be no doubt that the − the Vela pulsar proper motion (52 3masyr 1) obtained Vela pulsar and the Vela SNR are the remnants of the by De Luca et al. (2000; see also Nasuti et al. 1997). same supernova explosion. However, this association has The situation with the distance to the Vela SNR was caused some problems in estimating the pulsar velocity. “improved” after Gupta et al. (1994) showed that the Wallerstein & Silk (1971) questioned for the first scintillation velocity calculation formula used by Cordes time the “canonical” value of the distance to the (1986) underestimates V by factor of 3. The revised Vela SNR of 500 pc given by Milne (1968; see also iss value of the scintillation velocity of the Vela pulsar of Taylor et al. 1993) and suggested that this distance − 152 km s 1 better corresponds to the proper motion veloc- should be reduced to some smaller value (' 250 pc). − ity of 123 km s 1 (for the distance to the pulsar of 500 pc Since that time many additional arguments in support of this suggestion have been put forward (Ogelman¨ et al. 1 1989; Oberlack et al. 1994; Jenkins & Wallerstein 1995; We critically analysed these arguments (Gvaramadze 2000a,b) and came to the conclusion that ? Address for correspondence: Krasin str. 19, ap. 81, Moscow there are no weighty reasons to revise the “canonical” distance 123056, Russia; e-mail: [email protected] of 500 pc. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010104 V. Gvaramadze,: On the velocity of the Vela pulsar 175 and µ =52masyr−1). However, the real situation is a bit more complicated. It should be noted that the calculations of Gupta (1995) were based on the assumptions that the scatter- ing material is concentrated in a thin screen and that the screen is placed midway between the observer and the pulsar. Although the first assumption is realistic, the second one is not suitable in the case of the Vela pulsar. Indeed, it is believed that the scattering irreg- ularities responsible for the enhanced scattering of the Vela pulsar (Backer 1974) are localized in a thin screen rather than uniformly distributed along our line of sight to the pulsar (Backer 1974; Lee & Jokipii 1976, see also Williamson 1974), and that the scattering screen resides close to the pulsar and could be associated with the shell of the Vela SNR (Desai et al. 1992; Taylor & Cordes 1993; Gwinn et al. 1993, see however Gwinn et al. 1997, 2000 and cf. Cordes & Rickett 1998). The asymmetrical loca- tion of the screen implies (see Sect. 2) that the actual value of the scintillation velocity should be considerably larger Fig. 1. The 843 MHz image of the central part of the Vela SNR than that given by Gupta (1995) and that the scintillation (adopted from Bock et al. 1998). Position of the Vela pulsar velocity is not equal to the proper motion velocity. is indicated by a cross. The arrow shows the direction of the In this paper, we show that if the scattering of the pulsar proper motion velocity (Bailes et al. 1989). North is up, Vela pulsar indeed occurs in the shell of the Vela SNR east at left. The horizontal bar is 1◦ long then the scintillation velocity could be reconciled with the pulsar proper motion velocity only if the scatterer has a observer’s reference frame, the scintillation velocity is con- nonzero transverse velocity. A possible origin of large-scale nected with the pulsar proper motion velocity transverse motions in the Vela SNR’s shell is discussed. −1 Vpm =4.74µDkpc km s , (2) −1 2. Scintillation and proper motion velocities where µ is measured in mas yr , by the following rela- tionship (cf. Gupta et al. 1994; Cordes & Rickett 1998): The scintillation velocity for an asymmetrically placed h 2 2 − thin scattering screen is (Gupta et al. 1994; Gupta 1995): Viss = x Vpm 2x(1 + x)VpmVscr,k i1/2 1/2 2 2 2 2 4 (νd,MHzDkpcx) −1 +(1 + x) Vscr,k +(1+x) Vscr,⊥ , (3) Viss =3.85 10 km s , (1) fGHztd where Vscr,k and Vscr,⊥ are the components of the trans- where νd,MHz and td are the scintillation bandwidth and verse velocity of the screen, correspondingly, parallel and the time-scale measured respectively in MHz and sec- perpendicular to the vector of the pulsar proper motion onds, Dkpc is the distance from observer to pulsar in kpc, velocity. In (3) we neglected small contributions from x = Do/Dp, Do and Dp are the distances from observer to the differential Galactic rotation and the Earth’s orbital screen and from screen to pulsar, fGHz is the frequency of motion around the Sun. If Vscr = 0, one has Vpm = −1 observation in units of GHz. For νd,MHz =0.001, td =5.6, Viss/x ' 38 km s (Dkpc =0.5), i.e. about 3 times smaller fGHz = 1 (Cordes 1986), Dkpc =0.5, and assuming that than that from Eq. (2). These velocity estimates could −1 x = 1, one finds for the Vela pulsar that Viss = 152 km s be reconciled only if the distance to the Vela pulsar is −1 (Gupta 1995). As we mentioned in Sect. 1, Desai et al. Dkpc =0.05(x/15.7) , which is too small to be likely. (1992) showed that the scattering screen is close to the The pulsar, however, could be placed at its “canonical” pulsar. Assuming that D = 500 pc, they found that distance if Vscr =0.6 Do/D ' 0.81, and that this value could be increased up to Figure 1 shows the 843 MHz image (Bock et al. 1998) 0.96 if 5% of the scattering of the Vela pulsar is due to the of the central part of the Vela SNR, known as the ra- effect of the Gum Nebula. The latter value of Do/D is ex- dio source Vela X (Milne 1968). A considerable fraction pected if the scattering material is mainly concentrated in of the radio emission from Vela X originates in filamen- the shell of the Vela SNR of angular diameter of 5◦ (the fig- tary structures, one of which crosses the Vela pulsar po- ure accepted in early studies of the Vela SNR). Assuming sition. This filament (or rather its part to the south of that the Vela SNR’s shell is indeed the main scatterer of the pulsar) is known as a radio counterpart of the Vela the Vela pulsar and using the currently adopted angular X-ray “jet” discovered by Markwardt & Ogelman¨ (1995).