The one dimensional distribution of Fe 14 emission has been determined along a radius of the Cygnus Loop through the use of a tilting filter photometer. The observed emission extends at least 5 arc minutes outside the optical filaments. A simple Sedov solution model of the temperature and density distribution behind the shock agrees with the observations if the shock front is near the extent of the Fe 14 emission, the shock velocity is from 300 to 250/kms and the density external to the remnant is about 0.7-1.4 cm to three minus 3 power. These parameters are in reasonable agreement with X-ray maps and optical radial velocities

Balon, R. J.

Kirshner, R. P.

Woodgate, B. E.

English

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RADIAL DISTRIBUTION OF [Fe XIV] EMISSION 
IN THE CYGNUS LOOP 
BRUCE E. WOODGATE 
ROBERT P. KIRSHNER 
RONALD J. BALON 
(NASA - TM - X-71 3 3) RArI Al DISTRIBUTION OF N77-J 3001 
f Fe XIV 1 EM I.s ~ IC N IN 'IHE C1GNUS l CC f ( ASA ) 
13 p 6C AO IMi A01 CSCL 03A 
AUGUST 1977 
Uncla.s 
G3/89 485 7 
-- GODDARD SPACE FLIGHT CENTER ---
GREENBELT, MARYLAND 
Radial Distribution of [Fe XIV] Emission
in the
Cygnus Loop
Bruce E. Woodgate*
NASA - Goddard Space Flight Center
Robert P. Kirshner
and
Ronald J. Balon
Department of Astronomy
University of Michigan
*Visiting Astronomer at Kitt Peak National Observatory, which is operated by
the Association of Universities for Research in Astronomy, Inc., under contract
with the National Science Foundation.
Abstract
O
The one dimensional distribution of [Fe XIV] X5103A emission has been
determined along a radius of the Cygnus Loop through the use of a tilting
filter photometer. The observed emission extends at least 5 arc minutes out-
side the optical filaments. A simple Sedov solution model of the temperature
and density distribution behind the shock agrees with the observations if the
shock front is near the outer extent of the [Fe XIV] emission, the shock ve-
locity is from 300 to 250 kms -I and the density external to the remnant is
about 0.7-1.4 cm-3 . These parameters are in reasonable agreement with X-ray
maps and optical radial velocities.
I. Introduction
This work concerns the relation between the shock in the interstellar
gas and the observed X-ray and optical emission of the Cygnus Loop. Following
the discovery of X-rays from the Cygnus Loop by Grader, Hill, and Stoering (1970)
and subsequent determinations that the X-rays were likely to have a thermal
origin (Gorenstein et al 1971, Tucker 1971), attempts were made to observe the
[Fe XIV] X5303 emission that should accompany thermal X-rays. Kurtz, Vanden Bout
and Angel (1972) searched for the emission and it was detected by Woodgate et.al .
(1974). Those observations were consistent with a thermal origin for both the
X-rays and the [Fe XIV] with a temperature of 2.8+0.3x106K, and a normal abundance
of iron relative to C, 0, Si and S, the elements whose lines are responsible for
most of the X-ray emission (Woodgate, Angel, and Kirshner (1975)).
The high temperature was puzzling because the radial velocity measurements
of Minkowski (1958) made from optical emission lines in bright filaments gave an
expansion of about 116 kms -1 , which gives a post-shock temperature of order
3.5x105K. This numerical difficulty evaporated with the discovery by Kirshner and Taylor
(1976) of	 Ha emission at velocities up to 300 kms -1 . However, the connection
between the shock front, the hot gas and the optical filaments remained unclear.
In particular, the alternative models for production of the hot gis - that of Tucker
(1971), in which the hot gas is immediately behind the shock and that of Cox (1972)
in which the hot interior is responsible for the X-rays and [Fe XIV] cannot be
distinguished without moderately good spatial resolution.
II. Observations.
In order to determine whether the [Fe XIV] emission and the hot gas that
produces it is located inside, outside or coincident with the optical filaments,
measurements of the [FeXIV] flux were made in seven locations on a radial line passing
through the bright optical filaments in the northeast segment of the loop (NGC 6992) as
shown in Figure 1. The same observational techniques employed b y Woodgate et.al . (1974)
were used, except with the #1 0.9m telescope at KPNO rather than a 6-inch refractor.
by tilting the filter. Measures were made on and off the source, and the
excess counts on the source and in the band computed in the same way as before.
The entrance aperture to the photometer measured 5 1 34" by 2 1 43" for a total
solid angle of 1.28 x 10-6 sterad. Slight adjustments in the positions of the 	 rr
apertures were made to avoid bright stars.
Data were obtained on 6 photometric nights in May and August 1976. As
illustrated schematically in Figure 2, the [Fe XIV] emission extends 5 arc
min outside the "front" of the optical filaments. If the Loop is at the
conventional (though perhaps unreliable, see Kirshner 1976) distance of 770
parsecs, this separation corresponds to about 1 parsec.
III. Interpretation
If the Cygnus hoop can be considered an adiabatic blast wave in a
homogenious medium, then the similarity solution given by Sedov (1959) pro-
vides a description of the temperature and den:.ity as a function of radius
in terms of the post-shock temperature, density and the shock radius. We
have integrated along the line of sight through such spherical models to
derive the expected surface brightness of [Fe XIv] emission as a function
of radius. The parameters of the Sedov solution were provided in a convenient
form by Roger Chevalier of KPNd and the atonic parameters for the X5303 line
were taken from Kurtz et al. (1972).
Curves of surface brightness versus radius are compared to the data in
Figure 3. For velocities higher than 300 kms -1 , [Fe XIV] emission is produced
3.
most efficiently just behind the shock front, for example if vsn3S0 kms-I
the surface brightness reaches a maximum at R/Rs= 0.96. For lower shock ve-
locities, the maximum in [Fe XIV] surface brightness is farther toward the center
because the temperature at which [Fe XIV] is best produced is higher than the
temperature just behind the shock, and the temperature rises toward the inside
in a Sedov solution. This effect is compounded by the increased path length
through the sphere for small values of R/Rt. We found a reasonable match to
the data of Figure 2 for shock velocities between 250 and 300 kms -1 . For lower
velocities, the peak surface brightness was too broad and too far toward the
inside to match the data, while for higher velocities, the peak was too narrow
and too close to the outside. Both of the fits shown in Figure 3 fail to
match the innermost point. The calculations always show a substantial [Fe XIV]
surface brightness, even at R/R s = 0 because the line of sight always traverses
(twice!) the shell that represents the temperature at which [Fe XIV] is best
produced (about 2.lx10 6K). The failure to reproduce this behavior must mean that
the assumption of a homogenious sphere is defective. That is no surprise, since
the distribution of X-rays over the surface of the Loop is far from uniform.
The peak surface brightness observed depends on the density of the ambient
gas. For 300 kms - I , the observed peak is matched for a pre-shock density of 0.7 cm-3,
and for 250 kms - I , it is matched for a density of 1.4 cm- 3 . For lower shock
velocities, the required density becomes improbably large. With these parameters,
the initial energy and the age can be derived. For 250 kms -I , the energy is
8 x 1050 erg, and the age is 29,000 years. For 300 kms - I , the numbers are 6 x 1050
erg and 24,000 years.
A scan was also made over the same fields of view by the same technique in
the [Ar IV]X4740 line. A positive detection was only made in patch (iy) (see
Figure 1), where a surface brightness of (18 ± 4) x 10 -8 erg cm-2 s-I sterad-I
was measured. All the other patches were weaker than 10 x 10 -8 erg cm-2 s-1
sterad-1 (3a). The emission appears in the outermost patch of the optical filaments
1
4.
Since the line is produced at about 10 5 OK, this observation is consistent with
the hypothesis that the filaments are formed by instabilities caused by
radiative cooling, (see McCray, Stein and Kafatos, 1975.)
IV. Discussion
Of course, it is possible that the location of the hot gas relative to
the optical filaments and the shape of the [Fe XIV) distribution are purely
accidental. However we do not believe this to be the case because of the good
agreement between the parameters derived here and independent evidence from
X-rays and other optical data. The X-ray map of Rappaport et.al . (1974) showed
a slightly larger diameter for the X-rays than the optical filaments: 2°75
compared to 2°65. This agrees with our result that the hot gas seen in [Fe XIV)
extends -5' farther out than the optical edge. They also find that the overall
distribution of the X-rays requires a shock velocity 400 ± 100 kms -1 , and a mean
density of 0.25 ± 0.10cm-3 . Our results are more sensitive to the shock velocity
since we fit the shape of the distribution, not just its radius. Our velocity
of 300-250 kms -I
 requires somewhat higher density to match the observed surface
brightness than the high velocity models of Rappaport et.al . Although this could
represent a local density fluctuation it is more likely to reflect the uncertainty
in the parameters. We also note that the shock velocity required by the present
data agrees well with the velocity of the Ha emission at 300 kms -I found by
Kirshner and Taylor (1976).
It will be extremely interesting to compare the present data with X-ray
observations of high spatial resolution from HEAO and other new X-ray experiments.
Even without detailed X-ray observations, it is clear that the direct interaction
of the supernova shockwave with interstellar gas is more clearly tied to the high
i
pAG
QUWO
eF
S.
temperature phenomena of [Fe XIV] and X-ray emission than to the optical filaments.
We would like to thank R. Chevalier for providing us with the Sedov solution in a
useable form, and Burt Johnson of KPNO for assistance with Forth.
References
Cox, D. 1972 A . J. 13. 169.
Gorenstein, P., Harris, B., Gursky, H., Giacconi, R., Novick, R., and
Vanden Bout, P. 19 1 1. Science, 17^?, 329.
Grader, R.J., Hill, R.W. and Stoering, J.P. (1970) Ap.J. (Letters), W, L4S.
Kirshner, R.P. 1976 P . A.S.P. a585.
Kirshner, R.P. and Taylor, K. 1976 A . J. (Letters) ?, L83.
Kurtz, D.W., Vanden Bout, P . A. and Angel, J.R.P. 1972, Ap . J., 1 8, 701.
McCray, R., Stein, R.F. and Kafatos, M. 1975, Ap. J . 1g, 565.
Minkowski, R. 1958 Rev. Mod. Phys., LO,, 1048.
Rappaport, S., Doxey, R. and Solinger, A. 1974 Ap.J. 1U, 329.
Sedov, L. 1959, Similarity and Dimensional Methods in Mechanics (New York:
Academic Press).
Tucker, W. 1971, Science, 1172, 372.
Woodgate, B.E., Angel, J . R.P. and Kirshner, R.P. 1975, ^p.J. 3A, 715.
Wnnigate, B.E., Stockman, H.S., Angel, J.R.P., and Kirshner, R.P. 1974,
Ap.J, (Letters), 18, L79.
I_I	 t	 l
Figure Captions
Figure 1. Locations of the entrance apertures for the [Fe XIV] measures are
shown. North is up, and east to the left. The apertures measure 5 1 34" by
2'4311 . The approximate position of the center aperture is (1950) 20hS5m
+31°30'.
Figure 2. Surface brightness in [Fe XIV]. The maximum surface brightness,
S max, is 7.0 x 10 -8 erg cm-2
 s -I sterad, about 3% of the night sky surface
o
brightness in the 4 A bandpass. The abscissa is the fractional radius RJR`,
where Rs
 is the shock radius, taken to be about 82'. The outside of the 3
Loop is to the left, and the inside to the right, as in Figure 1.
Figure 3. Similarity solution models for [Fe XIV) surface brightness. The
observations shown in Figure 2 have been shifted horizontally to best match
the models. In (a) the shock velocity is 300 kms -1 , while in (b) it is
250 kms -1 . Higher velocities have the maximum [Fe XIV] surface brightness
too close to the shock front while lower velocities require very large
densities to match the observed surface brightness.

..L 
Sma. 
t.O 
(a) 
-t V, -300 km I 
0.85 0.80 
4-R/R.-


Radial distribution of Fe XIV emission in the Cygnus Loop

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