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to app ear in Astronomy Letters, vol. 31, No.4, 2005, pp.243-255, with minor corrections Translated from Pis'ma v Astronomicheskii Zhurnal, vol.31, No.4, 2005, pp. 269-283

March 24, 2005

NEW STUDIES OF THE PULSAR WIND NEBULA IN THE SUPERNOVA REMNANT CTB 80
T. A. Lozinskaya1 , V. N. Komarova2 , A. V. Moiseev2 , S. I. Blinnikov
1 2

2

3

Sternberg Astronomical Institute, Universitetskii pr. 13, Moscow, 119992 Russia Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz, Karachai-Cherkessian Republic, 357147 Russia Institute for Theoretical and Experimental Physics, ul. Bol'shaya Cheremushkinskaya 25, Moscow, 117259 Russia

Received -- November 15, 2004 Abstract. We investigated the kinematics of the pulsar wind nebula (PWN) associated with PSR B1951+32 in the old supernova remnant CTB 80 using the Fabry­Perot interferometer of the 6 m Special Astrophysical Observatory telescope. In addition to the previously known expansion of the system of bright filaments with a velocity of 100­200 km s-1 , we detected weak high-velocity features in the H line at least up to velocities of 400­450 km s-1 . We analyzed the morphology of the PWN in the H, [SII], and [OIII] lines using HST archival data and discuss its nature. The shape of the central filamentary shell, which is determined by the emission in the [OIII] line and in the radio continuum, is shown to be consistent with the bow-shock model for a significant (about 60 ) inclination of the pulsar's velocity vector to the plane of the sky. In this case, the space velocity of the pulsar is twice higher than its tangential velocity, i.e., it reaches 500 km s -1 , and PSR B1951+32 is the first pulsar whose line-of-sight velocity (of about 400 km/s) has been estimated from the PWN observations. The shell-like H-structures outside the bow shock front in the east and the west may be associated with both the pulsar's jets and the pulsar-wind breakthrough due to the layered structure of the extended CTB 80 shell. Key words. Supernovae and supernova remnants, pulsar wind nebulae, models.

INTRODUCTION
CTB 80 is a classical example of a sup ernova remnant (SNR) with a fast-moving pulsar at a late stage of the pulsar's interaction with a very old shell. The radio image of CTB 80 is represented by three extended (ab out 30 ) ridges that converge in the region of a bright compact core (Velusami and Kundu 1974; Velusami et al. 1976; Angerhofer et al. 1981; Strom et al. 1984; Mantovani et al. 1985; Strom 1987; Castelleti et al. 2003, and references therein). This unusual (for SNRs) morphology ceased to app ear puzzling in 1988, when, on the one hand, the 39.5-ms pulsar PSR B1951+32 was discovered in the core (Kulkarni et al. 1988; Fruchter et al. 1988) and, on the other hand, Fesen et al. (1988) identified an extended infrared shell that extends the radio ridges in the northeast and assumed that precisely this infrared shell represents a very old SNR. The radio ridges with a steep sp ectrum ( -0.7) corresp ond to the part of the shell into the compressed magnetic field of which the approached pulsar injected fresh relativistic particles, thereby reanimating its synchrotron radio emission. Subsequently, an H I


E-mail: lozinsk@sai.msu.ru

shell that coincides with the infrared shell and expands with a velo city of 72 km s-1 , which yields a kinematic SNR age of 7.7â104 yr, was also identified (Ko o et al. 1990, 1993). The characteristic age of the pulsar PSR B1951+32 is t 105 yr (Kulkarni et al. 1988; Fruchter et al. 1988). The pulsar wind nebula (PWN) is represented by a bright compact (45 ) core with a flat sp ectrum ( 0.0). The core lies at the western b oundary of a 10 â 6 plateau with a steep er sp ectrum ( -0.3) elongated in the east­ west direction. Recently, Migliazzo et al. (2002) detected the motion of the pulsar with a velo city of 240 km s-1 in a direction that confirms the p ossibility of its birth inside the infrared and H I shells. The stage of pulsar wind interaction with a very old shell observed in CTB 80 seems most complex in the evolutionary chain of PWNs, from young SNRs where the pulsar interacts with the SN ejection to pulsars that have already escap ed from the old SNR. The difficulty lies in the fact that the matter density in the co oled old shell b ehind the front of the decelerated sho ck (and, hence, the density of the compressed interstellar magnetic field) is several hundred times higher than the ambient density and the shell structure is unpredictable in advance. In addition,


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LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA

the rotational energy loss by the pulsar PSR B1951+32 is large (E = 3.7 â 1036 erg s-1 , as estimated by Kulkarni et al. 1988), which is comparable to the energy input from the young Vela X pulsar. Various distance estimates for CTB 80 and the pulsar PSR B1951+32 lie within the range 1.5­2.5 kp c (see Ko o et al. 1993; Strom and Stapp ers 2000; and references therein); we use the universally accepted distance of 2 kp c. In this pap er, we present our observations of the PWN in CTB 80 with the 6 m Sp ecial Astrophysical Observatory (SAO) telescop e and analyze narrow-band observational data from the HST archive.

OBSERVATIONS AND DATA REDUCTION Interferometric Observations on the 6 m Telescop e
The core of CTB 80 was observed with the 6 m telescop e as part of a program entitled "The Kinematics of Matter in Pulsar Wind Nebulae" (the main applicant is Yu.A. Shibanov). Our interferometric observations of CTB 80 were p erformed on Octob er 9­10, 2001, at the prime fo cus of the 6 m telescop e using the SCORPIO fo cal reducer; the equivalent fo cal ratio of the system was F /2.9. A description of SCORPIO is given in the pap er by Afanasiev and Moiseev (2005) and on the Internet (http://www.sao.ru/hq/moisav/scorpio/scorpio.html); the SCOPRIO capabilities in interferometric observations were describ ed by Moiseev (2002). The seeing during the observations varied within the range 1. 2­2. 2. We used a scanning Fabry­Perot interferometer (FPI) in the 235th order at the wavelength of the H line; the separation b etween the neighb oring orders of interference, = 28 ° A, corresp onded to a region free from order overlapping of 1270 km s-1 on the radial velo city scale. The width of the instrumental FPI profile was F W H M 2.5 ° A or 110­120 km s-1 . Premono chromatization was p erformed using an interference filter with a half-width of A = 14 ° centered on the H line. The detector was a TK1024 1024 â 1024-pixel CCD array. The observations were carried out with 2 â 2-pixel binning to reduce the readout time. In each sp ectral channel, we obtained 512 â 512-pixel images; at a 0.55 /pixel scale, the total field of view was 4. 7. We obtained a total of 32 interferograms at various FPI plate spacings, so the width of the sp ectral channel was = 0.87 ° or 40 km s-1 near H. A The exp osure time was 240 s p er channel. We reduced the observations using software running in the IDL environment (Moiseev 2002). After the primary reduction (debiasing and flat fielding), the observational data were represented as 512 â 512 â 32-pixel data cub es; here, a 32-channel sp ectrum corresp onds to each pixel. Ma jor difficulties in the data reduction pro cess arose when night-sky emission lines were subtracted from interferograms. All bright image features were masked, and an azimuthally averaged radial sky emission line profile was constructed from the remaining areas; this profile was subtracted from the corresp onding frame in the data

cub e. This technique allows the sky line intensity variations during two-hour observations to b e effectively corrected. However, almost the entire field of view near the core of CTB 80 proved to b e filled with weak emission features, which was also confirmed by our deep H images of the PWN. Therefore, the problem of cho osing a sufficient numb er of "clean" sky areas in a region of almost 5 in size arose. As a result, the weakest features in the field of view could b e "oversubtracted", and the H emission line profile is severely distorted at these lo cations. Our estimates indicate that the line distortion due to the background subtraction may b e disregarded for regions in which the intensity of the H line exceeds 150 photo electrons p er CCD pixel, which corresp onds to a surface brightness of 8.6 â 10-17 erg s-1 / (uncorrected for the interstellar reddening). The large numb er of background stars in the field allowed us to measure and correct the atmospheric transparency and seeing variations in each image. The resulting seeing was 2. 2. Subsequently, we reduced the sp ectra in the data cub e to the same wavelength scale. The formal accuracy of measuring the relative line-of-sight velo cities was ab out 2­4 km s-1 . However, since the emission lines in the ob ject often have asymmetric profiles, the actual measurement accuracy dep ends on the chosen line profile fitting metho d. The sp ectra of the ob ject were smo othed with a F W H M = 1.7 ° (two channels) Gaussian for optimal A filtering of the data cub e. To reliably identify weak emission features, we also smo othed the images in the cub e with a bivariate Gaussian. The resultant angular resolution was ab out 2. 6. The smo othing was p erformed using the ADHOC software package1 . The continuum level in each sp ectrum was determined as a median mean of the eight weakest levels. When constructing the velo city field and the mono chromatic image in the H line, we fitted the sp ectral line profile by a Gaussian using only the p oints offset by ±3 channels from the predetermined line p eak. We calibrated the mono chromatic image in energy units (erg s-1 cm-2 ) by comparison with our calibrated narrow-band image (see b elow). All of the radial velo cities in this pap er are helio centric; the passage to the Lo cal Standard of Rest corresp onds to VHel = VLSR + 17.6 km s-1 .

Medium- and Narrow-Band Images
Observations with the 6 m telescop e. Our mediumand narrow-band observations of the CTB 80 core were p erformed on Octob er 11 and 12, 2001, with the 6 m telescop e using the SCORPIO fo cal reducer (see ab ove) at seeing 1. 4­1. 6 at zenith distance z = 12 ­21 . Filter parameters and exp osure times are given in the Table 1.
The ADHOC software package was developed by J. Boulestex (Marseilles Observatory) and is freely accessible on the Internet.
1


LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA Table 1. Details of the narrow- and medium-band observations of the CTB 80 core with the 6 m telescope (BTA) and HST

3

Range, telescop e H, BTA Continuum, BTA H, HST Continuum, HST [OI I I], HST [SI I], HST

Filter FN657 SED707 F656N F547M F502N F673N

cen , ° A 6578 7036 6563 5479 5013 6732

F W HM , ° A 75 207 20 200 20 20

Exp osure time, s 4 â 300 4 â 120

2600 + 2700 1300 + 1300 2700 + 2700 2700 + 2700
two the the are bright (in H) filawest, which form a east­west direction. ab out 75 â 38 or

To calibrate the images, we observed the sp ectrophotometric standards G138­31 and Feige 110. The HST archive. The PWN in CTB 80 was observed with HST (the main applicant is J. Trauger) in Octob er 1997 using the WFPC2 instrument. Parameters of the filters used and the total exp osure time are given in the Table 1. The primary data reduction is automatically p erformed when the data are queried from the HST archive. A filtering co de prop osed by N.A. Tikhonov (private communication) was used to remove numerous cosmic-ray particle hits. The formal accuracy of the astrometric referencing using WCSTo ols is an order of magnitude lower than the internal accuracy of the USNO-A2.0 star catalog used as a reference one. The resultant value was taken to b e 0. 3.

the outside. We clearly see the mentary shells in the east and spindle-shap ed core structure in The PWN sizes in the H line 0.73 â 0.37 p c.

The PWN Kinematics
The velo cities of PWN filaments. The comparisons of relative line intensities in the core sp ectrum with diagnostic mo dels made by several authors are suggestive of collisional excitation of the gas b ehind the front of a sho ck propagating at a velo city of ab out 120­140 km s-1 in a medium with a density of 25­100 cm-3 (Hester and Kulkarni 1989, and references therein). Gas motions in the PWN with such velo cities have already b een detected. The main metho d used was long-slit sp ectroscopy, which allowed the velo cities of the individual, generally brightest (and, hence, p ossibly slowest) knots and filaments to b e estimated (Angerhofer et al. 1981; Blair et al. 1984). Sp ectroscopic metho ds did not reveal radial velo cities higher than 200 km s-1 with a surface brightness at H 0.5 10-6 erg cm-2 s-1 sr-1 anywhere in the PWN (Blair et al. 1988). Using the echelle sp ectrograph of the 4-m WHT telescop e, Whitehead et al. (1989) found an expansion of the two central shells with velo cities of 200 and 100 km s-1 . Using FPI, we have studied the PWN kinematics for the first time. The advantage of FPI observations is that they give the line profile everywhere in the PWN, and not only in the region cut out by the sp ectrograph slit. Our observations indicate that the line profile, which is single in bright p eripheral filaments, is characterized by a multicomp onent structure or line asymmetry, bright red wings in the central PWN regions. As an illustration, several profiles and their decomp osition into individual Gaussians are given in Fig. 2. Having analyzed all of the observed H profiles, we were able to lo calize the highvelo city gas in the PWN image that emits in the range from 200 to 400 km s-1 (shown at the center in Fig. 2). Figure 3 shows several so-called p osition­velo city diagrams (the change in gas velo city along the chosen direction) constructed from our FPI observations and the

RESULTS OF OBSERVATIONS The PWN Morphology
Previous studies of the CTB 80 core showed that its optical emission is typical of SNRs: an intense (relative to H) [NI I], [SI I], [OI], [OI I I] line emission and a filamentary structure (see Angerhofer et al. 1980; Blair et al. 1984; Whitehead et al. 1989; Hester and Kulkarni 1988, 1989; and references therein). Significant differences in the PWN morphology in different lines were rep orted: only a symmetric central shell is observed in the [OI I I] line, a shell to the east of it app ears in the [SI I] line, and two bright shells to the east and the west of the central structure that form an elongated core structure are observed in the H line. The deep HST images of the PWN demonstrate a staggering filamentary, irregularly shap ed multishell structure and confirm the results of ground-based observations (see Fig. 1, which shows the [OI I I], [SI I], and H images, from top to b ottom panels, resp ectively, with sup erimp osed VLA 1.5-GHz radio isophotes). Only the central horsesho e-shap ed part of the core closest to the pulsar is seen in the [OI I I] line. The PWN morphology in the [OI I I] line clearly shows a structure exp ected for the b ow sho ck pro duced by the pulsar's motion (see the discussion). The H emission is also observed in this central filamentary horsesho e-shap ed shell; the H filaments are adjacent to the [OI I I] filaments from


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LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA

Fig. 1. HST images of the PWN with superimposed VLA 1.5GHz radio isophotes: in the [OIII] line (top), in the [SII] line (midd le), and in the H line (bottom panel). The arrow indicates the motion of the pulsar over a period of 1000 yr, as measured by Migliazzo et al. (2002). 1

lo calization of the corresp onding directions in the PWN image. As we see from this figure, in the bright core structures, the p osition­velo city diagrams show velo cities that agree with the previously measured expansion velo city of 100­ 200 km s-1 ; i.e., they completely confirm the results of sp ectroscopic observations. In addition to the gas velo cities in bright filaments within the range from -200 to +200 km s-1 , our observations clearly reveal weaker high-velo city H emission features in the PWN. Weak emission at high velo cities is clearly seen in the p osition­velo city diagrams up to 400­ 450 km s-1 . This is a lower limit, since the velo city range is limited by the FPI velo city range free from order overlapping and the imp ossibility to prop erly take into account the contribution from the [NI I] 6548 ° line emission. A Figure 3 (scan 3) clearly shows the typical (for an expanding shell) structure of the so-called velo city ellipsoid (its half corresp onding to p ositive velo cities) that is determined by high-velo city motions. The corresp onding p ossible expansion velo city reaches the lower limit mentioned ab ove. The surface brightness of the detected high-velo city comp onent of the H line in the central part of the core lies within the range (1.4­20) â 10-16 erg s-1 cm-2 arcsec-2 . In our estimation, we to ok the color excess E (B - V ) = 0.8 from Blair et al. (1984). Figure 4 shows the line-of-sight velo city field corresp onding to the p eak of the main line comp onent constructed from our observations and sup erimp osed on the H image of the PWN. We emphasize that the line-ofsight velo city field refers only to the core of the line, which often has a complex multicomp onent profile. The velo cities determined from the line p eak lie within the range -100 to +50 km s-1 , in agreement with the sp ectroscopic observations. In Fig. 4, we clearly see the symmetry axis in the velo city distribution of the p eak of the main line comp onent in a direction P 230 , which is close to but do es not coincide with the direction of the pulsar's motion (P = 252 ± 7 ; Migliazzo et al. 2002). We estimated the total flux from the PWN in the main line comp onent to b e 8.4 â 10-12 erg s-1 cm-2 , in close agreement with the estimate of Whitehead et al. (1989). The total flux in the high-velo city line comp onent is 6.4 â 10-13 erg s-1 cm-2 ; the luminosity of the high-velo city gas accounts for ab out 7% of the total luminosity of the core in the H line. The high-velo city emission detected in the core for the first time has confirmed the changes in the PWN fine structure p ointed out by Strom and Blair (1985) -- p ossibly the prop er motions of the filaments of the central shell corresp onding to a velo city of 250d km s-1 (up to 400d km s-1 ), where d is the distance to CTB 80 in kp c, i.e., ab out 500 km s-1 (p ossibly up to 800 km s-1 ). The velo cities of filaments in the CTB 80 shell outside the core. The optical radiation from the extended SNR CTB 80 outside the PWN is represented by


LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA

5

Fig. 2. H profiles at several points of the PWN and their Gaussian decomposition. The features at a velocity of +605 km s -1 are attributable to the [NII] 6548 ° line emission that falls in the wings of the interference filter and that is separated from A the H emission by about 0.5 order of interference. The localization of the high-velocity gas emitting in the range from 200 to 400 km s-1 is shown in the central H image of the PWN (isophotes).

the system of faint filaments clearly seen in the [SI I] and H lines over the entire 16 â 16 field whose image is given in the pap er by Hester and Kulkarni (1989). The filamentary morphology and the intense [SI I] line emission provide strong evidence for the gas emission b ehind the sho ck front. It is interesting to measure the velo cities of these filaments in the extended remnant outside the core, since, in general, the thin filaments can b e lo calized on the front or rear side of the old shell; i.e., these may b e pro jected rather than b e physically asso ciated with the PWN. Besides, the pulsar's velo city with resp ect to the ambient gas is imp ortant in estimating the

exp ected shap e of the b ow sho ck pro duced by the wind from a fast-moving pulsar. Figure 4 (b ottom panel) shows only the brightest outer filaments in a 4 â 4 field for which the background subtraction effect is insignificant (see ab ove). The H brightness of these filaments exceeds 8.6 â 10-17 erg s-1 / . As follows from Fig. 4 (b ottom), the velo cities of the bright filaments in the outer shell of CTB 80 do not differ significantly from the velo cities of the p eripheral filaments of the PWN undistorted by the expansion of the latter. Note, in particular, the filaments that are immediately adjacent to the PWN in the north and the east and that


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LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA

Fig. 3. Position­velocity diagrams constructed from our FPI observations and the localization of the corresponding scans in the PWN image. The emission features at a velocity of about 600 km s-1 are attributable to the [NII] 6548 ° line. A

form a kind of a wake at the b oundary of the shell produced by the wind from a moving pulsar. (These filaments are most distinct in the [SI I] lines in Fig. 2b from Hester and Kulkarni 1989). As our measurements indicate, the velo cities of these filaments do not differ significantly from those of the bright PWN filaments either.

DISCUSSION
The difficulty of explaining the nature of the PWN in CTB 80 lies in the fact the b ow-sho ck structure (in the [OI I I] line, the radio continuum, and the soft X-ray band) and the elongated filamentary shell-like structure in the


LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA

7

action b etween two pro cesses: the ejections of relativistic pulsar plasma in two jets and the motion of the pulsar. The elongated PWN structure is not distorted by the pulsar's motion in any way; the b ow sho ck, in turn, is not distorted by the action of the two plasma jets in any way either. The rough estimates given b elow indicate that individually b oth structures can b e explained adequately.

Estimates for the Sho ck Waves Pro duced by the Pulsar's Jets
If the pulsar's p ower L is constant over time t and is released in a solid angle , blowing a bubble of radius R = R(t) in a homogeneous interstellar medium with a density 0 , then the mass M 0 R3 /3 is swept up from the bubble. Assuming that all of this mass is gathered in a thin layer at radius R and remains in the same solid angle (in fact, it will partially flow around the jet) and equating the pulsar-wind momentum flux L/c to the rate of increase in the momentum of the wind-driven shell, L d = (M v ) , c dt where v = dR/dt, we obtain an acceptable size of the bubble swept up by the wind for a pulsar at rest under reasonable assumptions ab out the ambient density and the wind asphericity. Assuming that R t , we have d L = o c 3 dt hence, = 1/2 at L = const, i.e., R= 6L c
1/4

R4 t

;

t
o

1/2

3 pc â

L

36

4 n0

1/4

t

1/2 6

, (1)

Fig. 4. CTB80 core. Top panel: the velocity field of the H peak superimposed on the PWN image. The arrow indicates the direction of the pulsar's motion, as measured by Migliazzo et al. (2002); bottom panel: the same for the entire FOV of about 5 â 5 in size in the image obtained with the 6 m telescope (only the brightest outer filaments are shown).

H line, which is definitely also of an sho ck origin that, in the opinion of Hester (2003), is asso ciated with the action of the pulsar's jets, are simultaneously clearly seen around the pulsar. We do not see any manifestations of the inter-

where = L/L0 is the fraction of the p ower L0 went into the solid angle and sp ent on blowing the bubble, n0 is the particle numb er density (for hydrogen comp osition), and t6 is the time in Myr. The p ower of the flux in relativistic particles (including all typ es of photons) from the pulsar PSR B1951+32 is L0 = Erot = 3.7 â 1036 erg s-1 (Kulkarni et al. 1988). At n0 = 1 cm-3 , L36 = 3.7, and the pulsar's total age t6 = 0.1, we obtain a bubble size of ab out 1 p c at = 1 even for isotropic radiation ( = 4 ), in agreement with the observed PWN size in the H line. In fact, only a small fraction of the pulsar's p ower go es into blowing the bubble, but is also much less than 4 (by several hundred times for a linear jet op ening angle of several degrees). We emphasize that this is the minimum upp er limit: here, the sho ck waves are assumed to b e radiative. The bubble size will increase if the hot-gas pressure inside the bubble is taken into account (Castelleti et al. 2003). Two cases are p ossible: the radius for radiative sho ck waves is larger than our estimate, but smaller than that for adiabatic sho ck waves (McKee and Ostriker 1977; Blinnikov et al. 1982).


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LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA

If we to ok into account the hot-gas pressure inside the bubble and for adiabatic sho ck waves, we would obtain the maximum estimate at = 4 (Avedisova 1971; Weaver et al. 1977; Ostriker and McKee 1988): R = 28 p cân
-1/5 0

To estimate the sho ck shap e, we can use the solutions by Lipunov and Prokhorov (1984) and Wilkin (1996). The latter author obtained an analytical solution for a normal (nonrelativistic) stellar wind with radiative sho ck waves: R( ) = d0 csc [3(1 - cot )]
1/2

L

1/5 3/5 36 t6

.

(2)

,

(3)

Hence, R could b e even an order of magnitude larger than that from (1). Let us make several remarks suggesting that these rough estimates are uncertain. On the one hand, the Balmer line emission in PWNs is evidence of nonradiative sho ck waves in a partially neutral medium (Chevalier and Raymond 1980); it is pro duced by electron impact excitation or ion charge exchange. In this case, neutral hydrogen can p enetrate into the hot gas and impart o dd shap es to the sho ck waves (Bucciantini and Bandiera 2001; Bucciantini 2002; D'Amico et al. 2003) On the other hand, formula (2) was derived for a hot wind with a normal (Pascal) pressure. Since the flow of magnetized particles or photons in a pulsar wind is directed only along the radius, a simple formula of form (1) the derivation of which assumes no pressure isotropy holds go o d, while formula (2) overestimates the result. Many problems asso ciated with the comp osition of this wind have not yet b een solved (the so-called sigma paradox -- the transition from the fraction of the electromagnetic pressure expressed in terms of the Poynting vector to the kinetic pressure of the particle flux cannot b e reliably calculated (see the review by D'Amico et al. 2003)). A time shorter than the pulsar's age should b e taken as t6 , since it has flown into the dense layers of the shell of the old SNR relatively recently. Castelleti et al. (2003) to ok t = 18 200 yr as the PWN age at an ambient density of n0 = 0.5 cm-3 . According to Mavromatakis et al. (2001), the relative line intensities in the sp ectrum of the filaments of the extended CTB 80 shell are typical of the gas radiation b ehind the front of a sho ck propagating at a velo city of 85­120 km s-1 in a medium with an initial density of ab out 2­5 cm-3 . This density is equal to the mean density in the HI shell estimated by Ko o et al. (1990). However, the dep endence of R on all parameters, including the density and the age, is weak; i.e., clearly, a small fraction of the total p ower that go es into blowing the bubble will suffice. (We also see the pulsar's emission; i.e., clearly, the fraction of the captured p ower, , is appreciably smaller than unity.) Thus, the sizes of the bubbles swept up by the pulsarwind jets can easily b e made close to the observed values even if the density is much higher and the fraction of the pulsar's p ower that go es into the jet is small.

where d0 is the distance from the pulsar to the head p oint at contact discontinuity. This solution describ es well the classical numerical results by Baranov et al. (1971) and van der Swaluw et al. (2003), see Fig.8 in the latter. An approximate solution for adiabatic sho ck waves was given by Chen et al. (1996). The criticism of the applicability of such solutions to known PWNs (see Bucciantini and Bandiera 2001) is based on the fact that these do not include the effects of charge exchange and neutral hydrogen p enetration b ehind the sho ck mentioned ab ove. However, the PWN in CTB 80 is unique in that the b ow sho ck is observed not only in Balmer lines, but also, most clearly, in [OI I I] lines and in the X-ray and radio bands, i.e., in a hot, ionized plasma. Therefore, simple analytical solutions may well b e valid in this case. We obtain the following estimate from the balance b e2 tween the momentum fluxes 1 L/c and 4 d2 o v0 : 0 d0 = L 4 co v
1 1/2 2 0

.

(4)

An Estimate for the Lo cation of the Bow Sho ck
The pulsar's motion in the plane of the sky with a velo city of v0 = 240 km s-1 (Migliazzo et al. 2002) for an isotropic wind must give rise to a b ow sho ck, which is observed in several pulsars (see the review by D'Amico et al. 2003).

Hence, we find for v0 = 240 km s-1 , o = 2 â 10-24 g cm-3 (n0 = 1 cm-3 ), and 1 = 1 that d0 0.044 p c. This value is comparable to the observed distance from the pulsar to the [OI I I] filaments that determine the p osition of the head p oint at the sho ck front, dobs 6 0.057 p c. The theoretical value of d0 will b e close to dobs if we take into account the fact that v0 = 170 km s-1 should b e substituted for v0 = 240 km s-1 , since the pulsar moves in the matter of a remnant expanding with a velo city of 72 km s-1 . In this case, d0 0.043 p c. In Fig. 5 (top panel), the solution by Wilkin (1996) is sup erimp osed on the PWN image in the [OI I I] line and in the radio continuum. The distance from the head p oint at the front to the pulsar is assumed to b e d0 0.043 p c, and the axis of the surface with a p osition angle of 235 do es not coincide with the pulsar's velo city direction in the plane of the sky. An azimuthal asymmetry in the projection of the theoretical surface relative to the observed shap e of the central shell remains noticeable at a p osition angle of P = 252 according to the measurements by Migliazzo et al. (2002). The fact that the shap e of the b ow sho ck observed in the [OI I I] line is appreciably "broader" than the gasdynamical solutions by Wilkin (1996) may suggest that the pulsar moves at a significant angle to the plane of the sky. Allowing for the p ossible inclination of the pulsar's velo city vector to the plane of the sky yields b etter agreement of the theory with the observed morphology of the central shell. By varying the pulsar's velo city vector, we found the b est agreement with the observations for a b ow sho ck whose axis is inclined at an angle of 60 to the plane


LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA

9

1

Fig. 5. Top panel: PWN in the [OIII] line and in the radio continuum (isophotes): the bow shock corresponding to solution (3) for the pulsar's motion in the plane of the sky with a position angle of 235 (dashed line) and the bow shock inclined at an angle of 60 to the plane of the sky ( isolines). Bottom 1 panel: The same bow shock inclined at an angle of 60 chosen by the best agreement with the [OIII] line image superimposed onto the H and X-ray image (isophotes) of the PWN. The arrow indicates the pulsar's motion over a period of 1000 yr, as measured by Migliazzo et al. (2002).

of the sky. Figure 5 (top panel) shows this solution for the b ow sho ck chosen by the b est agreement with the shap e of the central shell observed in the [OI I I] line and in the radio continuum. As we see, the shap e of the pro jection of the theoretical surface defined by relation (3) can b e completely reconciled with the observed PWN morphology. In this case, the p osition angle of the pulsar's velo city vector in the plane of the sky is P = 235 ; i.e., it is close to the direction of the symmetry axis in the velo city field of the main H line comp onent found ab ove. P = 235 matches the value obtained by Strom (1987).

We emphasize that, if the pulsar's velo city vector is inclined at an angle of 60 , its space velo city is twice as high as its velo city measured in the plane of the sky; i.e., it reaches 500 km s-1 (this value agrees with the mean value in the velo city distribution of pulsars; see Arzoumanian et al. 2002.) Since the distance d0 at a higher velo city is a factor of 2 smaller, for close agreement with the observations, we must take a slightly lower gas density in front of the b ow sho ck. Our kinematic studies are also consistent with the suggested mo del. Since the thickness of the p ostsho ck emitting gas, which is determined by the apparent thickness of the bright p eripheral PWN filaments, is an order of magnitude smaller than the distance the pulsar traverses in 1000 yr, (see Fig. 1), we conclude that the radiative co oling time of the p ostsho ck gas is short, ab out 100 yr. In this time, an element of gas is not only compressed, but also acquires a velo city that corresp onds to the pulsar's motion. Without detailed calculations, it is hard to tell at which velo city the gas emission is at a maximum; one may only exp ect the observed velo city to b e a significant fraction of the pulsar's velo city. Gas motions with such velo cities in the PWN have b een detected in our work for the first time (see ab ove). These velo cities refer to the weak emission features in the H line; they are observed b oth in the central region around the pulsar, and near the bright filaments at the PWN b oundary (see Fig. 2). The line-of-sight velo cities of the bright filaments are within the ±200 km s-1 range. This is an ordinary (for SNRs) situation related to the fact that the bright filaments usually represent the front surfaces seen edge-on. Unfortunately, since the velo city measurement range in our FPI observations is limited, at present, we cannot unambiguously determine whether the pulsar is moving toward or away from us. The existence of high p ositive velo cities argues for the motion away from us, but as yet we have no information ab out the p ossible negative velo cities without further FPI observations. The derived velo city distribution of the line p eak could clarify the picture, but here we are restricted by the fact that the density distribution in the closest neighb orho o d of the shell corresp onding to the b ow sho ck is unknown. In particular, the asymmetric expansion of the central shell (according to Whitehead et al. (1989), the motion of the approaching side at a velo city of ab out -200 km s-1 is most clearly observed in it) could b e attributable to the higher brightness of this side due to a nonuniform ambient gas density. In Fig. 5 (b ottom panel), the same theoretical surface of the b ow sho ck that is inclined at an angle of 60 to the plane of the sky and that agrees b est with the PWN emission in the [OI I I] line is sup erimp osed on the H image with X-ray isophotes. The two shell-like H structures in the west and the east are far outside the b ow sho ck. These structures, which form the elongated PWN shap e, were explained by the action of the pulsar's jets (Hester 2000). It should b e noted that, if the pulsar's jets are directed at an angle to its space velo city (e.g., lie in the plane of


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LOZINSKAYA et al.: NEW STUDIES OF THE PULSAR WIND NEBULA

the sky), then the contradiction with the absence of an apparent influence of the jets on the sho ck front in its brightest parts mentioned at the b eginning of the section is removed. Of course, the representation of the sho ck as an infinitely thin layer with shap e (3) is oversimplified. More detailed solutions should b e used, and the structure of two sho cks with a contact discontinuity b etween them should b e taken into account (Baranov et al. 1976), since there is a hint at such a structure in the observations. In particular, Mo on et al. (2004) asso ciate the structure observed in X rays with a backward sho ck wave in the pulsar wind and in the H line with a b ow sho ck in the ambient medium. Note also that, in fact, 1 in formula (4) may b e the fraction of the pulsar's emission absorb ed by an ionized plasma, while in formula (1) for the jet may b e the fraction of the emission absorb ed by neutral hydrogen. For example, hard ultraviolet emission must b e absorb ed completely by neutral hydrogen and only partially by ionized plasma. Another p ossible pro cess, the proton-b eam charge exchange in a neutral gas, cannot take place in an ionized medium. Thus, different comp onents of the pulsar's flux can manifest themselves differently in different comp onents of the ambient medium. A different interpretation of the elongated PWN shap e in the H line is also p ossible. The system of thin filaments mentioned in the Section "Results of Observations", which characterizes the optical emission from the extended remnant outside the PWN, is most likely the layered structure of the old CTB 80 shell (Hester and Kulkarni 1989). Our velo city measurements for the brightest filaments suggest that these are not pro jected, but are physically asso ciated with the PWN. The thickness of the layers (typically, 10­20 or 0.1­ 0.2 p c) and the separation b etween them (ab out 2 ­2.5 or 1­1.5 p c) at the pulsar's velo city of 240 km s-1 in the plane of the sky yield a time of ab out 5000 yr b etween the passages of neighb oring layers and a passage time of a dense layer of 400­800 yr. The pulsar's motion through such a layered medium can pro duce the observed elongated multishell structure of the core as a result of the pulsar-wind breakthrough from a dense layer into a tenuous medium b etween the layers. The breakthrough of the nonrelativistic wind of Wolf­Rayet stars from a dense cloud into a tenuous intercloud gas leads precisely to this effect (see, e.g., Dopita and Lozinskaya 1990).

images taken in the H+[NI I], [SI I], [OI I], [OI I I] lines by Mavromatakis et al. (2001) revealed large-scale filamentary and diffuse structures in the nearby 2 â 2 region. These optical filaments in the north and the northwest closely correlate with the radio images of the remnant (Castelleti et al. 2003) and with the b oundary of the infrared shell, but go far b eyond the infrared and H I shells in the south and the east. (Here, we disregard the extended filaments in the east asso ciated not with CTB 80, but with the remnant of a different sup ernova (Mavromatakis and Strom 2002).) The relative line intensities in the sp ectrum of these large-scale filaments are typical of the radiative co oling of the gas b ehind the front of a sho ck propagating at a velo city of 85­120 km s-1 in a medium with an initial density of ab out 2­5 cm-3 (Mavromatakis et al. 2001). Such a density agrees with the mean density in the H I shell estimated by Ko o et al. (1990) from 21-cm line observations. This argues for the asso ciation of the large-scale filaments with the SNR CTB 80. Therefore, a more complex spatial structure comp osed of two hemispheres with different sizes should probably b e considered as the SNR CTB 80. One hemisphere, which is determined by the northeastern radio ridge and the infrared and H I shells, is the result of the interaction of the sho ck triggered by a sup ernova explosion with a dense medium. The second part of the shell has an aspherical shap e and is determined by the southwestern ridge and the [OI I I] filaments denoted by I I I and IV as well as by the system of bright filaments to the east of IV (see Fig. 2 from Mavromatakis et al. 2001). This part of the shell was most likely formed by the sho ck in a medium with a much lower density. Figure 6 shows approximate b oundaries of the two parts of the shell in pro jection onto the plane of the sky in this mo del. Note that ROSAT observations revealed a conical 1 region of thermal X-ray emission southeast of the pulsar far b eyond the infrared shell (Safi-Harb et al. 1995). The central region of the second part of the shell mentioned ab ove could, in principle, b e resp onsible for this emission. However, the SNR CTB 80 is observed along the Cygnus spiral arm and is immediately adjacent to the giant Sup erbubble pro duced by intense stellar wind from the Cyg OB2 cluster (Lozinskaya et al. 2002, and references therein). Therefore, in general, the thermal X-ray emission in the extended conical region could b e the background emission, i.e., it could b elong not to CTB 80, but to the Sup erbubble. In the prop osed scheme, the morphology of the SNR in the plane of the sky suggests that the ma jor axis of this structure comp osed of two hemispheres with different sizes is oriented at a large angle to the plane of the sky. Therefore, at a p ossible space velo city of 500 km s-1 , the pulsar has not yet gone outside the shell of the extended remnant CTB 80.

Lo calization of the Pulsar in the Extended Remnant CTB 80
If the space velo city of the pulsar actually reaches 500 km s-1 , then, in general, it traverses a distance of 50 p c in a time of 105 yr and could go b eyond the symmetric infrared and H I shells ab out 40 p c in size, which, according to the previous interpretation, determined the total volume of the old remnant CTB 80. However, the deep


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11

Fig. 6. General scheme of CTB 80 from Mavromatakis et al. (2001): the [SII] image of the region, radio ridges (solid line), and the infrared and HI shells (the dashed and dash­dotted lines, respectively). The large and small circles indicate the suggested model for the shell of the old SNR composed of two hemispheres in pro jection onto the plane of the sky.

CONCLUSIONS
Our kinematic study of the pulsar wind nebula in the old sup ernova remnant CTB 80 using the FPI of the 6 m SAO telescop e has revealed weak high-velo city H features in the PWN at least up to a velo city of 400­450 km s-1 . We confirmed the previously measured expansion of the system of bright filaments with a velo city of 100­200 km s-1 . We analyzed the PWN morphology in the H, [SI I], and [OI I I] lines using the HST archival data. The shap e of the b ow sho ck, which is determined by the central horsesho eshap ed shell bright in the [OI I I] line and in the radio continuum, was shown to b e in b est agreement with the theory for a significant (ab out 60 ) inclination of the pulsar's velo city vector to the plane of the sky. The space velo city of the pulsar is twice as high as its tangential velo city measured by Migliazzo et al. (2002); i.e., it reaches 500 km s-1 . This pattern of motion is also confirmed by the high radial velo cities of the gas in the PWN that we found here. Thus,

PSR B1951+32 is the first pulsar whose p ossible light-ofsight velo city (of ab out 400 km s- 1) has b een estimated from the PWN morphology and kinematics. The filamentary shell-like structures observed in the H line in the east and the west outside the b ow sho ck can b e explained not only by the action of the pulsar's jets, but also by the pulsar-wind breakthrough into an inhomogeneous ambient medium. We considered the general scheme of CTB 80 that includes the most recent optical and radio observational data in which the pulsar's high space velo city is consistent with its lo cation in the dense shell of the old SNR. Of course, the prop osed scheme of the PWN in CTB 80 must b e confirmed additionally. The existence of high velo cities in the PWN, the pattern of elongated H structures (p ossibly under the influence of the pulsar's jets or when the wind breaks through from a thin dense layer), and other questions require further observations and detailed hydro dynamic simulations of this interesting ob ject.


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1. ACKNOWLEDGMENTS
This work was supp orted by the Russian Foundation for Basic Research (pro ject nos. 02-02-16500, 03-02-17423, 04-02-16042) and the Federal Science and Technology Program (contract no. 40.022.1.1.1103). A.V. Moiseev thanks the Russian Sciences Supp ort Foundation for partial supp ort of this work. We are grateful to Yu.A. Shibanov for a kind p ermission to use the data obtained as part of his observational program on the 6 m BTA telescop e, B.M. Gaensler for providing the VLA radio data and I.V. Karamyan, V.Yu. Avdeev, and O.V. Egorov for help. This work is based on the observational data obtained with the 6 m SAO telescop e financed by the Ministry of Science of Russia (registration no. 01-43) and on the NASA/ESA Hubble Space Telescop e data taken from the archive of the Space Telescop e Science Institute op erated by the Asso ciation of Universities for research in astronomy under a NASA contract (NAS 5-26555) and data of the Chandra X-ray Observatory Center, which is op erated by the Smithsonian Astrophysical Observatory for and on b ehalf of the National Aeronautics Space Administration under contract NAS8-03060. References
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Translated by V. Astakhov