Pulsars in supernova remnants

P U L S A R S IN S U P E R N O V A R E M N A N T S

P . R. A m n u 6 1 ' , O. Kh. G u s e i n o v , a n d F . K. K a s u m o v

A study has been made of the radio and x - r ay spec t ra of six supernova remnants . The data on the charac te r i s t i c b reak in the spec t ra indicate that there may be a pulsar (neutron star) in the remnant of SN Tycho (besides the known remnants containing pulsars : Crab and Vela X). There is apparently no active source in the remnants of Cas A, Pup A, and Cyg Loop.

The successes of x - r a y as t ronomy in recent yea r s have led to the local izat ion of about 50 galactic x - r a y sources [1, 2]. It is now clear that there are at least severa l c lasses of x - r ay sources [3, 4], includ- ing x - r a y radiat ion f rom supernova remnants (SNR for short). Reliable indentification has so far been made for six SN-R [5]: Crab, Cas A, Tycho, Vela X,Pup A, and Cyg Loop. No x - r ay radiation has been ob- served f rom the other remnants with known ages: Kepler, PKS 1459, 3C 58, and also Lup Loop. Data on the main observational pa ramete r s of the listed SNR are given in Table 1~

At the present t ime, it is widely believed that neutron s ta rs (pulsars) are formed as a resu l t of catas t rophic p rocesses . One can there fore expect pulsars to be present in the young and closest SNR listed below. However, pulsars have so far been found only in the SNR 1054 (Crab) and Vela X [6]. Since the d i rec - tional d iagram of the pulsar radiat ion may be very narrow, the fai lure to observe pulsating radio radiat ion f rom an SNI~ does not yet mean there is none at all. If this is so, observed x - r ay radiat ion f rom an SNR may be due to the presence in them of pulsars .

There is an indirect way of verifying if there is a pulsar in remnants : f rom the nature of the ob- served radio and x - r a y spectra . If there is a pulsar in SNR, the pumping of energet ic part icles into the nebula must continue into the present t ime. According to [7], the t ime during which pumping continues can be est imated if one knows the magnetic field in the nebula and the frequency Ubr at which the spec t rum of the synchro t ron radiat ion f rom the SNR has a break. To determine the charac te r i s t i c frequency u br of the break, we consider the radio and x - r a y spect ra of six SNR, according to [4, 8-10]. We assume a spec- t rum of the form a v - a keY. cm -3- see- l ; the values of a and (~ are given in Table 1. Under the assumption that the magnetic field s trength and the energy distribution of the re la t iv is t ic e lectrons are the same in the whole of the nebula, the pumping t ime is [11]

1.92.104 pump /r~r:~,~ 1;~ y~ars.

"~br (1)

The values of ~br obtained f rom the spec t ra are given in Table 2. Could Vbr be less than the values given in Table 2? At the break the spect ra l index ~ changes by not less than 0.5 [7, 11]~ Then for Vbr smal le r than the values given in Table 2 we obtain in the x - r a y region anomalously low radiat ion fluxes (for Vela X, the value of ~ br is bounded below by the known optical luminosity of the object). For the Cas A, Tycho, and Pup A remnants we have taken as upper l imits 1 and 0.2 keV, respect ively , because the l imits of the observed x - r a y spec t ra are known.

Shemakha Astrophysical Observatory. Transla ted f rom Astrofizika, Vol. 8, No. 4, ppo 561-566, Octo- b e r - D e c e m b e r , 1972. Original ar t ic le submitted June 9, 1972.

�9 }97t Consultants l~ureau, a division of Plenum Publishing Corporation, 227 ff'est 17th Street, :~'ew York, \ Y. i0011. :\Io part of this publication may be reproduced, stored in a retrieved system, or transmitted, in any'form or by any means, electronic, mechanical, photocopying, microfilming, recordinf~ or otherwise, without written permission of the publisher. 1 copy of this article is available from the publisher for $I5.00.

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TABLE 1

SNR Year of ex~ '

Type ;plosion or age ; R !(years) i kpc

1667 l 2,8 {

1604 15.0

1572 5.0

1181 6.9

1054 2.0

1006 3.9

6.6.10 a 0.5

1.7.104 0.8

2.104 1.8

8.15.104 0.6

3.2

9.5

9.4

I4.0

2.1

29.6

28.4

4I .5

28.0

49.0

L 'asA I1

Kepler i

Tycho I

3C 58

Crab I

PKS 145t} I

Veta X l[

~,g L,mp II (?)

Pup A ---

[.up Loop --

pc radio x-ray

2.118~1.738 I 0.787+0.006 i

1.202~1.152 1 0.59-~0.I0

0.436~0.381 I 0 ,67+0.07 [ - - -

( 9 . 4 6 5 + l . 809), 10 a ] 0.263-Is {2

0.359~0.101 ! 0.601+0.0015

8.23.10 a 023

31.69~I 4.21 0.475-20.025

55.45241.39 0,45-4, 0,05

1.57-108 0.3

, 4,6 2 . 3 + 0 . 6 3 ,722 . 0 __

0 4 4 ~ 2 ) . ] I 1 . 3 4 2 0 . 3

9.0 -~_ 1.0 t, 0 +2{}. 1

1 . { ! 8 2.0~-21,2

1,0 2.2-20.3

6,7 6 .:2~4.6

TABLE 2

SNR Vbr, Hz 105Hobs 1{} s f i t 10 a Tpump days days years

Cas A

Tyeho

Crab Vein X

Cyg Loop

Pup A

(0.9 5.3).10 l~

(0 .07-2 .4 ) ,10 ~;

(0.2 --9.4).10 ta

(0.03~ 1.1).10 ~

(1.6 6.0) ,I0 I~ (0.2 -- 2.3).10 L7

4.6 33 ,0

1 . 8 - - 4 . 3

25- 37

2.0. 2.3

1.2. 1,3

1.9 2.4

2.(1 -3 .6

2 1 - - 5 . 8

I6--57

0 .9--3 .0

0.3 -0.4

0 .2--0 .4

0 .0{i4 0.2

0.02 - - 2.3

0.3 - - 3,2

t .6 --12.1

1.6 - - 3,7

{).3 --- 1.6

We now suppose that in each remnant there is a pulsar that provides continuous pumping. Then tim

pumping time is equal to the age, Tpump = T, and using (I) we can estimate the magnetic field H i in th e nebula. The corresponding quantities are given in Table 2. Independent estimates of the magnetic field

Hob s can be obtained from data on the radiation flux from the objects. The intensity of the synchrotron

radiation is [7]

[ - 1 ; l i -

/,, --- 1 . 3 5 . 1 0 2-'{6.26. !0~8)T-u (-;) s D , / ~ - (2}

w h e r e D i s t h e d i a m e t e r o f t h e S N R ; y = 2 ~ + 1; u ( T ) i s a f u n c t i o n o f T a n d i t s v a l u e s c a n b e f o u n d i n [7] ;

K is the coefficient in the energy spectrum of the electrons, N(E) = KE-T. Assuming [7] that the energy

density Wp of the relativistic particles in the nebula is equal to the magnetic energy density, we obtain

( - - 1 ( " T - - 1 - =7- 1 - ! ,

4"25 10I:/7"+R2"+ ( " T '& 2)

i 2 . . . . . . i u (+i') D ' { 1 . 7 3 ) ~

(3)

where T ;~ 2; F v is the observed flux at the frequency v in W �9 m -2 ~ Hz-i; R is the distance to the SNR in centimeters; and v 2 and v I are the upper and lower limits of the frequencies of the observed flux. Note

that for Wp < H2/8rr, the value of Hob s is increased insignificantly. Thus, when Wp = n(H2/8rc), the value

of Hob s changes by a factor n2/7.

The value of Hob s obtained from (3) is also not very sensitive to a change of R (an indeterminacy in R of a factor n leads to an indeterminacy in Hob s of a factor n-2/7), and therefore the indeterminacy in the

data on the distance to the SNR [8-10] do not introduce large errors into the calculations~

Comparison of Hob s with H i shows that in the Crab, Vela X, and Tycho remnants the values of ~bs are such that one can assume the existence of the characteristic break in the spectrum due to continuous

pumping of relativistic particles during the whole life of the SNR. At the same time, in the three other remnants with known x-ray spectra, Cas A, Pup A, and Cyg Loop, the magnetic fields Hob s are greater

than those needed to explain the characteristic break in the spectra. It is well known [6] thaL pulsars have

3 3 0

been discovered in the Crab and Vela X remnants. With regard to Tycho, the fact that pulsating radiation has not observed in this case may be due to the narrow directional diagram of the pulsar.

We must make special mention of the radio radiation from the Lup Loop, PKS 1459, 3C 58, and Kepler remnants. In the first two cases, the discovery of x-ray radiation has been announced, although the region of localization of the x-ray forces is at 5-7 ~ from the SNR, which makes the identification very unreliable. For Kepler and PKS 1459 the absence of x-rays can be explained by the steepness of the spectrum (see Table I). With regard to the remnant 3C 58, doubts are expressed in [5, 9] whether this is really a SNR.

Thus, although there are appreciable doubts about the SNR data (especially relating to the x-ray spectra) our scheme enables us to distinguish Tycho in addition to Crab and Vela X among the I0 young and near remnants. The presence of a characteristic break in the synchrotron spectrum of Tycho indicates that it may contain a pulsar as a source of relativistic electrons in this SNR.

The remaining remnants (Cas A, Pup A, and Cyg Loop) apparently contain either black holes or neu- tron stars that are not manifested as pulsars for some reason or other. However, a resolution of the prob- lem will only be possible after the SNR spectra have been determined much more accurately.

We may also mention that a genetic relationship between the pulsar P 0833 and the SNR Vela X has not been fully proven, and our conclusion is a further argument that such a connection exists.

LITERATURE CITED

1. R. Giacconi, H. Gursky, E. Kellogg, C. Leong~ S. Murray,E. Schreier, and H. Tananbaum, Preprint (1971).

2. R. Hill, G. Burginyon, R. Grader, T. Palmieri, F. Seward, and I. Stoering, Ap. J., 171, 519 (1972). 3. E. Kellogg, Preprint, ASE-2833 (1971). 4o P.R. Amnu~l' and O. Kh. Guseinov, Astrono Zho, 48,280 (1971). 5. S. llovaisky and Ch. Ryter, Astron. Astrophys, 15,224 (1971). 6. Oo Kh. Guseinov, F. K. Kasumov, V. I. Lazarev, and A. V. Osipchuk, Astron, Zh., 50, 4 (1973). 7. I.S. Shklovski, Supernovae, Interscience, New York (1968). 8. D. Milne, Austr. J. Phys., 2_~3, 425 (1970). 9. D. Dawns, A. J., 766, 305 (1971).

10. L Baars and A. Hartsuijker, Astron. Astrophys., 17, 172 (1972). II. S.B. Pikel'ner, Astron. Zh., 3_~3, 785 (1956).

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