EXOSAT results obtained with the imaging instrument (CMA) and the medium energy proportional counters (ME) are discussed. Assuming that the featureless power-law spectrum obtained in the 2 to 10 keV range is synchrotron radiation from relativistic electrons, one derives constraints on magnetic field strength and age of the nebula. The energy spectra of the electrons responsible for the emission in the radio and X-ray ranges are discussed. The great similarity of the physical properties of G 29.7-0.3 and of three synchrotron nebulae containing a compact object observed to pulse in X-rays makes G 29.7 - 0.3 a very promising candidate for further search for pulsed emission. Further observations at infrared wavelengths might reveal the break(s) in the emitted spectrum expected from the radio and X-ray power-law indices and give us more information on the production of the electron populations responsible for the emission of the nebula

Becker, R. H.

Braun, R.

Jansen, F. A.

Koch-Miramond, L.

Rocchia, R.

English

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Jansen, F.A.

Becker, R.H.

Leiden University Scholary Publications

The electron spectra in the synchrotron nebula of the supernova remnant G 29.7-0.3

NASA Technical Reports Server

THE ELECTRON SPECTRA IN THE SYNCHROTRON NEBULA 
OF THE SUPERNOVA REMNANT G 29.7 - 0.3 
L. Koch-Miramond and R.Rocchia, Service dlAstrophysique, Saclay, France 
F.A. Jansen and R.Braun, Huyghens Laboratory, Leiden, 
R.H. Becker, University of California, Davis 
I. Introduction. The current observational situation concerning the presence or 
absence of  synchrotron nebulae, presumably fed by an active pulsar (PSR) i n  the 
central region of  supernova remnants (SNRs) is evolving rapidly, thanks t o  the high 
resolution maps obtained wi th  the Very Large Array (VLA) i n  the radio range and 
wi th  the Einstein and Exosat observatories i n  the X-ray range. The larger spectral 
coverage (0,04 - 20 keV) o f  Exosat and i t s  spectro-photometric capabilit ies are 
decisive i n  this respect. Observations i n  the whole range of frequencies between 
radio and X-rays would help determining the range of  synchrotron frequencies and 
hence of  injected part icle energies, establish the physical size of the nebula and 
advance our understanding of part icle acceleration by PSRs. 
The remnant G 29.7 - 0.3 has been studied intensively. The radio properties 
has been discussed by (I), (2), (3). High resolution maps obtained w i t h  the VLA show 
two spectrally distinct components, a f la t  spectrum core surrounded by a shell. The 
central component is 25% linearly polarized and has a central ly peaked radio 
brightness distribution characteristic of Crab-like remnants. While not visible a t  
optical wavelengths due t o  the high visual absorption on the l ine o f  sight, G 29.7 - 
0.3 is a very bright object i n  X-rays (2) (3), X-ray diffuse emission coincident wi th  
the radio core was observed wi th  the Einstein Observatory. For  a distance of 19 kpc 
the estimated luminosity o f  the nebula was .V 4 1036 erg s-1 i n  the 0.2 - 4.0 keV 
range and i t s  linear diameter N 2  PC. The present paper shows Exosat results 
obtained wi th  the imaging instrument (CMA) and the medium energy proportional 
counters (ME). Assuming that the featureless power-law spectrum obtained i n  the 2 
t o  10 keV range is synchrotron radiation f rom relativist ic electrons, one derives 
constraints on magnetic f ie ld strength and age of the nebula. The energy spectra o f  
the electrons responsible for  the emission i n  the radio and X-ray ranges are 
discussed. 8 
2. Results o f  the Exosat Observation. G 29.7 - 0.3 was observed on 29 August 
1984, for 4 104  s wi th  the Exosat imaging instrument (CMA) and the medium 
energy proportional counters (ME) (4).  he source was not seen i n  the CMA, the 
3 c  upper l im i t  on the counting ra te  being 4 10-4 s-1 . The counting ra te  due to the 
source i n  the ME Argon counters half-array was 2.7 s - l .  A f t e r  background 
subtraction the observed spectral distribution is shown on f igure 1. The best f i t  was 
obtained assuming a featureless power-law spectrum dN/dE = 1.3  hoton on 
cm- ls-1 k e v - l  and an absorbing hydrogen column density o f  - 2.3 1 0 ~ ~ c m - ~ .  A 
3wlower l im i t  on the N value of  - 3 lo2' ~ m - ~  has been deduced f rom the upper 
l im i t  o f  the CMA count!ng rate. Then the consistency between the CMA and ME 
data imposes: a = - 1.0 + .15 and NH = (3.3 - + .3) 1 0 ~ ~ c m - ~ .  The f igure 2 shows the 
incident X-ray spectrum, 
Search for pulsations. Regular pulsations were searched for  in  the ME data 
by performing a fast Fourier transform. No statist ical ly significant period has been 
found between 32ms and 104 s. The resulting upper l i m i t  of pulsed fraction i n  this 
period range is n, 1.5%. 
3. Discussion. The value o f  N we have deduced can be compared w i th  the lower 
l im i t  o f  NHI = 2 . 1 0 ~ 9 c ~ ~ ? n f e r r e d  f rom neutral hydrogen radio absorption 
394 OG 6.2-10 
      
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395 OG6.2-10
measurements by (3). The incident X-ray flux from the source is (3.8.,.1)10 -11
erg.cm -_- s'iin the 2 to 10 KeV range, corresponding to an intrinsic luminosity
of 2 10_$erg.s'lfor a distance of 19 kpc. This large distance was estimated by (5)
from the surface brightness/diameter relationship for SNRs and confirmed by the
VLA results of (3). The total X-ray luminosity in the 0.1 to 10 keV range is about 5
1036 erg.s-_ assuming that the same power-law holds. We assume a synchrotron
nebula on the basis of our X-ray data and of the radio polarization. The emission at
all wavelengths is synchrotron emission from relativistic electrons accelerated by
thecentralstellaremnant.
Assuming that the nebulacontainsa uniform magnetic fieldH, that all
particleshave energysuch thatmaximum synchrotronemissionisat i keV and that
the energy of the nebula is equallydividedbetween relativisticelectronsand
magnetic field(7),one findsH..7 10-_'Caussand Eo = E_, 6 I0_6 ergs,with an
angularradiusof thenebulaof i0 arcsec.from (2).T-hesyh_hrotronlifetimeof the
X-ray emitting electronsis then _NI50 years.Doing that we have ignored
electronsnon radiatingin X-rays;ifnow we considerthe entireelectromagnetic
spectrum we get H=2 i0"_ G, E-= 1.610#_erg and T'forelectronsemittingat i
keY of 35 years.The age of theP'SNRhas been estimatedby (8)from the surface
brightness_. at 408 MHz assumingthat the dynamicalevolutionof the shelli,_
describedby the Sedov equationi.e.unaffectedby thepresenceof a centralpulsar,
then ,_.= 1.25 10"lSt'_givingt = 660 years.Hence continuousinjectionof energy
near the centerisinferredfrom the shortsynchrotronlifetimeof the electronsas
compared to theSNR's age.
4.Energy and spectraof theelectrons. If_2isthe frequencycorrespondingto the
rfiaximurnof the synchrotronspectrum _= 4.6 10-$H E2" (9).Assuming H = H
equipartition=._.10"z_G,one findsthatthe radioemittinqelectronshave energy
E ,_ 10S eV and the X-ray emittingelectronsE._3 101.L_eV. Since the energy
spectrum of the X-ray emittingelectronsisdominated by synchrotronlossesthe
observedX-ray spectralindexof -1.0 + 0.15impliesa power-law'distributionf
the radiatingelectronswith N(E)_ E-_z-._. Because synchrotronlossessteepena
continuouslyinjectedspectrum of electronsby one power the inferredinjection
spectrum isproportionalto E -_-_-'3.Also thereshouldbe a break atlow energies
to a radiation spectrum Sv _ _-O.13_.tP. (figure 3). For the equipartition field H :
2 10-4G and an age of 660 years that should occur at a frequency of about 10f_ Hz.
Considering the spectrum of G 29.7 - 0.3 over 9 decades in frequencies (figure 3)
and taking into account the uncertainties on measured spectral indices, a break (if
unique) should be expected in the frequency range 10t:' to 10 t5 Hz encompassing
the infrared range.
The radio flux of the synchrotron nebula integrated over the range 10 _ to
10H Hz with the same spectral index _. =-0.25, as observed around I GHz by (2), is
4 10 $_ erg s-t. Then the L× /LI_ .ratiols_'l(_ell above the theoritical minimum of
,_17 deduced from (6), who were modeling the evolution of a _ power-law
particle spectrum in the Crab-like SNRs. Hence a different origin for the X-ray and
radio producing electrons is not required. This conclusion still marginally holds if
one considers a flat radio spectrum (0< =0, Lx /Lp.,_500and (Lx /L,, ) . N 600 in (6).
. ,'7. mln .
These considerations require that the difference of spectral lnolcesoetween the
radio and X-ray domains is 0,5. But this difference could be as large as 1, as
suggested by our X-ray observations and by (3). [n that case the electron spectral
distribution could be more complex and its understanding would require the
knowledge of the photon spectrum in the interval between radio and X-rays. Infra-
red inforrnations are much valuable from this point of view.
396 G . -10 
measurements by (3). The incident X-ray flux from the source is (3.8:t.l) 10-11 
erg.cm-1 s·1 in the 2 to 10 KeV range, corresponding to an intrinsic lu inosity 
of 2 3'erg.s-1for a distance of 19 kpc. his large distance as estimated by (5) 
from t e s rface rightness/diameter relationship for s a  c nfirmed  the 
 results f (3). e t tal -ray lu inosity i  t e .  to  eY range is a t  
1036 erg.s-1 assu ing that the sa e po er-law holds. e assu e a synchrotron 
nebula on the basis of our -ray data and of the radio polarization. he e ission at 
all avelengths is synchrotron e ission from relativistic electrons accelerated by 
 ral ellar r . 
ssu ing t at the nebula contains a uniform agnetic field , t at all 
particles have energy such that axi u  synchrotron e ission is at 1 keY and that 
the energy of the nebula is equally divided bet een relativistic electrons and 
magnetic field (7), one finds H", 7 10-S' Gauss and E = E ,..., 6 1046 ergs, with an 
angular radius of the nebula of 10 arc sec. from (2). ?he sYh6hrotron li fetime of the 
-ray e itting electrons is then 1'" rv 150 years. oing that e have ignored 
electrons non radiating in -rays; if no . e consider the entire electromagnetic 
spectrum e get =2 10- ~ , Ep'= 1.6 10'+? erg and T" for electrons e itting at 1 
ke  of 35 years. The age of the SNR has been estimated by (8) fro  the surface 
rightness ~ at  Hz assuming t at the a ical e lution f the s ell is 
described by the Sedov equation i.e. unaffected by the presence of a central pulsar, 
then ~ = 1.25 1O-15t-<X-giving t = 660 years. ence continuous injection of energy 
ear t e t r is inferred from t e s rt s c rotron lifetime f the electrons as 
. compared to the SNR's age. 
4:. Energy, and spectra of the electrons. If Y is the frequency corresponding to the 
maximum of the synchrotron spectrum -y = 4.6 10- 6H E2. (9). Assuming H = H 
equipartition = ~_ 10-4 G, one finds that the radio e itting electrons have energy 
 '"" 109 e  and the -ray e itting electrons EAI 3 101.~ e e Since the, energy 
spectrum of the -ray e itting electrons is do inated by synchrotron losses the 
observed -ray spectral index of - 1.0 + 0.15 i plies a po er-Iaw'distribution of 
the radiating electrons. ith (E) 0< E -3.i". 45. ecause synchrotron losses steepen a 
continuously injected spectrum of electrons by one power the inferred injection 
spectrum is proportional to E -1 ±. 3 • lso there should be a break at low energies 
to a radiation spectrum S)J t>l. ))-0.131.12. (figure 3). For the equipartition field H = 
2 10- 4 G and an age of 660 years that should occur at a frequency of about 1011fo Hz. 
onsidering the spectru  of  29.7 - 0.3 over 9 decades in frequencies (figure 3) 
and taking into account the uncertainties on easured spectral indices, a break (if 
unique) should be expected in the frequency range 10 12. to 1015 z enco passing 
the infrared range. . 
he J'adio flux of the synchrotron nebula integrated over the range 10'" to 
10 11 z ith the sa e spectral index ~ =-0.25, as observed around 1 z by (2), is 
4 10~! erg s-i. Then the Lx /LR, ratiois",,"lo;..,ell above the theoritical minimum of 
'" 17 deduced from (6), who were modeling the evolution of a single power-law 
particle spectrum in the Crab-like SNRs. Hence a different origin for the X-ray' and 
radio producing electrons is not required. This conclusion still arginally holds if 
one considers a flat radio spectrum ( 0<, :::0, Lx ILJ:\.""SOO and (L~ /LR,) . N 600 in (6). 
These considerations require that the difference of spectral indicg§J~etween the 
radio and -ray do ains is 0.5. ut this difference could be as large as 1, as 
suggested by our -ray observations and by 0). In that case the electron spectral 
distribution could be ore co plex and its understanding ould require the 
knowledge of the photon spectrum in the interval between radio and X-rays. Infra-
red informations are uch valuable fro  this point of vie . 
.5. Infra-red emission. The IRAS catalogue of point and small extended sources 
(extension s 1 arc  min) gives a t  w (1950) = 18h43m46.5~; 5 (1950)~-03002'33" a 
source flux of 4.84 Jy a t  1 2 ~  and 8.63 Jy a t  24)r (10). The corresponding points 
shown on figure 3 lie 2 decades above the  extrapolated radio emission but the  infra- 
red source does not coincide with the central synchrotron nebula and lies closer t o  
the  brightest shell emission region A. So these data  cannot be confidently used t o  
constrain the  electron spectrum. More work on the  infra-red diffuse emission of 
this region is required. 
6. Conclusions. The great  similarity of the  physical properties of G 29.7 -0.3 and 
of the three synchrotron nebulae containing a compact object observed t o  pulse in 
X-rays ( l l ) ,  (12) and Table 1, makes G 29.7 - 0.3 a very promising candidate for  
further search for pulsed emission. Further observations a t  infra-red wavelengths 
might reveal the  break(s) in the emitted spectrum expected from t h e  radio and X- 
ray power-law indices and give us more information on the  production of t h e  
electron populations responsible for the emission of the  nebula (13, (14), (15). 
7. Acknowledqements. We thank the  Exosat Observatory team a t  ESOC for their  
support during the  observation and real-time reduction of the  data,  A.Peacock and 
collaborators a t  ESTEC for their help on ME data analysis and J.Jorrand a t  Saclay. 
References 
1. Becker,R.H., Kundu, M.R., 1976, Ap.J. 3 , 4 2 7 .  
2. Becker, R.H., Helfand, D.J., Szymkowiak, A.E., 1983, Ap.J.Lett. 263L93. 
3. Becker, R.H., Helfand, D.J., 1984, Ap.J. 283, 154. 
4. Koch-Miramond, L.,Rocchia, R., e t  al., 1984, 18th ESLAB Symposium on 
X-ray astronomy, in press. 
5. Milne, D.K., 1979, Australian J. Phys. 32, 83. 
6. Reynolds, S.P., Chanan, G.A., 1984, Ap.J. 281, 673. 
7. Pacholczyck, A.G., 1970, Radio Astrophysics, Freeman 
8. Clark, D.H., Caswell, J.L., 1976. M.N.R.A.S. 170, 267. 
9. Ginzburg, V.L., Syrovatskii, S.I., 1964, The Origin of Cosmic Rays, Pergamon. 
10. Perault, M., 1985, Private communication 
11. Seward, F.D., 1983, in  Supernova Remnants and their X-ray emission, Symposium 
IAU 101, ed. J.Danziger and P. Gorenstein, p.405. 
12. Seward, F.D., Harnden Jr, F.R., Helfand, D.J., 1984, Ap.J.Letters, =,L19. 
13. Pacini, F., Salvati, M., 1973, Ap.J. 186, 249. 
14. Reynolds, S.P., Chevalier, R.A., 1984, Ap.J. 278, 630. 
15. Reynolds, S.P., 1985, Ap.J. 291, 155. 
TABLE 1. PROPERTIES OF DIFFUSE SYNCHROTRON NEBULAE 
Names Distance Size Lx H * EP * r * 
~ P C  PC erg r'l ~ a u s s  ergs Years 
G 186.6-5.8 (Crab) 2 .O 0.6 2 103~ 2 10'~ 2 1046 30 
G 29.7-0.3 (KES 75) 19 1.8 5 103~ 7 10'~. 6 1o46 150 
LMC 0540-693 55 1 1103' 2 loe4 llo47 33 
G 320.4-1.2 (MSHl5-52) 4.2 6.4 1.6 1035 9 10'6 2 lo66 4103 
* Based on the  consideration of X-ray emitting electrons only (11). 
398 
OG 6.2-10 
5.   5    
( t i  ~   i ) t ~  6.5s; S = °02'33" 
   .    2tl  .   t ,.. .    
        
       
 l  50     
    i   
 
  t     .3  
   i     
11 , )   ,      i t  r 
  r   t t  
l  (s)   t  t       
 i         
 r   , 
g     t r   t  r  
rt i     t ,   
l r t rs t  r  l    t  l sis   t e
r es 
. er,R.H., , .R., 1 , .J. 20 ,427. 
 r ., l , .J., zymkowiB ,"" .E., 1 , .J.Lett. 8,
r ., l .J., 1 , .J. , 1 -
 ra occhia, , t ., 1 , 18th SLAB y posiu  n 
str in 
  O ., 1 , tr li   
6  y l , P., ., 1 .J. 67
 acholczy .G., 19 , adio cs, r e an 
 l r , ., aswel J ., 1 . . .R A.S. 4 . 
9 i r ., Syrov tskii 5.1., 196 ,  rigin of osmic y , erg .
10. Perault, , 19 , Pri te co unication 
11. e ar , F 0., 19 , in Supernova e nants and t eir e i si , y posium . 
IAU 101, e J.  and . r t i p. . 
12. Seward, F 0., rnden Jr, F ., l J., 198 , . e ters, 287,L19. 
13. Pacini F , Salv ti , 1973, .J. 6, 2 -
1 . eynol , S P., hevali ., 198 , .J. 8, 63 .
15. eynol , S ., 1985, .J. , 15 . -
A LE 1 R ES  FFUSE SYNCHROTRON  
allle. i tance S ze Lx H • Ep • T 11 
kpc pc era ,-1 G us. e ' . 
G 1 4. -5.8 (Crab) 2.0 0.  2 1037 2 10-4 2 1046 30 
G 2 . -0.3 (KES 75) 19 1  5 1036 7 10-5 6 1046 150 
LHC 0540-69.3 55 1 1 1037 2 10-4 1 10 7 33 
G 3 . -1.2 (HS 1 52) 4  6 1 1035 9 10-6 2 104  4103 
• Based on the consideration of X-ra  e it ing electrons only (11). 
f 
EXOSAT HE 619.7 - 0.3 OG 6.2-10 
lo't I I I 6 1  r l l  
G.29.7 -0.3 
Synchrotron nebula 
. R*S 
Z s
EXOSAT ME G29.7 - 0.3 
1Q' I I I r r r n r m  
f iqure 2: The incident photon 
spectrum o f  G 29.7-0.3 as 
deduced f rom the EXOSAT ME 
data. 
.Figure 1: The observed ME 
spectrum of G 29.7-0.3 ( fu l l  
array). The thin bars represent 
the observed data points wi th  
1Q' 
Figure 3: Emission s?ectrum of  
the synchrotron nebula i n  G 
29.7-0.3 over 9 decades i n  
frequency. Radio spectrum 
f rom (1,2,3), IRAS flux might 
be due t o  shell emission, X-ray 
spectrum from the present 
work. A spectral break is 
expected i n  the shaded area. 
3 l+ + 1 6 errors. The thick 3 .  fiistogram is the predicted 
1 E distr ibution for a power-law 
h4 spectrum wi th  energy index 
9=-.77 and an absorbing 
column density o f  NH= 
2.3.10a2cm-2: 
I I I I I l l  
1.0 10 . 
Energy IkeVI 
r - 
- 7 1 .  . . * . . . . . . . . .  1 8 10 12 14 16 18 20 22 
LoglO Frequency (Hz) 
lo" - + +------- --++ - 
+*+* 
"ttt : 
+t+ 
I 1 1 I , , I 1  
1.0 10 
Energy lkeVl 
387 
[XOSA T HE G29.7. OJ 
tOl __ -=~':":"""''':'''''''::':T--'--r"T'""''T'"T'] 
> 
• ..
: 
..... 10' 
i 
<J 
1.0 
Enlt9J (kaV) 
G297 • OJ EXOSAT HE 
10.1 ~--"':"":":'r---,.....-.-r--,,-r"T'"1 
> 
• ..
..; 
• ..
... 
.:: 
.. 
i j 
A. 
10· Z 
to·) I:-
o 
-;. -1 
:: 
1.0 to 
Energy (keVI 
~-::----~;: 
..... 'c:; 
............ 
\ 
(j.29.7 -0.3 
Synchrotron nebull 
+ VlA 
\ \ \ 
\ 1\ \ I' \ ,\ 
\\\ 
\ 1\ 
", \~ \\ II, 
• _AS 
.. EXOSAT 
\ 
Log,o Frequency (Hzl 
OG 6.2-10 
-Figure 1: The observed ME 
spectrum of G 29.7-0.3 (full 
array). The thin bars represent 
the observed data points with 
+ 1 f1" errors. The thick 
histogram is the predicted 
distribution for a power-law 
spectrum with energy index 
~ =-.77 and an absorbing 
column density of NH= t.3.10'a.~cm·1: 
oF igure 2: The incident photon 
spectrum of G 29.7-0.3 as 
deduced from the EXOSA T ME 
data. 
Figure 3: Emission sflectrum of 
the synchrotron nebula in G 
29.7-0.3 over 9 decades in 
frequency. Radio spectrum 
from (1,2,3), IRAS flux ight 
be due to shell emission, X-ray 
spectrum from the present 
ork.  spectral break is 
expected in the shaded area. 


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The electron spectra in the synchrotron nebula of the supernova remnant G 29.7-0.3

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