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The 511 keV annihilation line from binaries
Chin. Ash-on. Astrophys. A translation
(1993)
1712,
@
183-186
Acta Astrophys.
Pergamon
Press Ltd
Printed in Great Britain
of Sin. (1993) 13/l,
FAST RESEARCH
0275-lOfJ2/93%24.00+.00
98-100
REPORT
The 511 keV annihilation line from binaries t WU Mei
ZHANG Chun-sheng
LI Ti-bei
Abstract The electron-positron annihilation process in accreting binary systems, typically Cygnus X-3, was investigated. Our Monte Carlo calculation shows that, under certain conditions such systems can emit a rather narrow annihilation line, whose flux and width depend on the intensity of the emitted y photons, the state, size and temperature distribution of the X-ray halo, and the density and distribution of the electrons in the disk. We also discussed the possibility of observing the line by current experiments including the OSSE detector on board the CR0 satellite. Key words: accreting binaries-X-rays-T-ray
spectrum
1. INTRODUCTION Positron creation is an important physical process in high energy astronomical bodies while the signature of existence of positrons, the 511 keV electron-positron annihilation line, carries a great deal of information on the temperature, electron density, gravitational redshift etc. at the site of their generation. Therefore, the search for the 511 keV line has been an important objective in astrophysical observation. Since 1968 when Rice universityI’ detected this line from the direction of the galactic centre using a balloon-borne y-ray telescope, a large volume of observation has been done 12--75. So far, this line has only been found in solar outbursts, y-ray outbursts and in the direction of the galactic centre. These observations seem to have two features in common. 1) They seem to indicate that the formation of the line includes contributions from both electron-positron pairs (decaying into either three or two photons) and the direct annihilation of electrons and positrons. Fig. 1 is a typical spectrum from the largest experimental sample I6-8l. It has a large extension towards the low energy end from the vicinity of 511 keV, showing the three-photon decay at work. 2) The flux of the 511 keV line shows variation in time, probably corresponding to violent changes in star activity at the site of generation (similar to the X-rays emitted by binaries having a high and a low state). t
Supported by National Natural Science Foundation Received 1991 October 8; revised version 1992 September
14
184
WU Mei et al.
Fig. 1 A typical energy spectrum around 511 keV from the direction of the Galactic centre
Lingenfelter et al’s analysisi line requires
stringent
physical
showed that the generation of the low-extending 511 keV conditions,
for example,
the temperature
must
be below
5x lo* K, the matter density must be greater than lo5 H/cm3,
the ambient photon density must be below lo3 erg/cm3, etc.. Also, it should be pointed that, the magnetic field at the site of annihilation must be sufficiently small, otherwise, because the ratio of the probabilities of two-photon and three-photon decays is proportional to B2, the shape of the spectrum would differ greatly from that shown in Fig. 1 if B - 10’ G. So far, there has been little discussion on the 511 keV line emitted by binary accreting systems. We thought that, if the neutron star in the system is a source of high-energy yrays and if, further, an X-ray halo exists around it, then the process 7 + X + e* may take place15*gl. A secondary product, the positron, could then be the source of the annihilation line. The newly created positron has a very large kinetic energy, so, even if annihilation
takes place, the 511 keV line will not be observed because of the doppler shift. The kinetic annihilation cross section is small, and the great majority of the positrons will not undergo annihilation before they leave the system. But the strong magnetic field around the neutron star will confine almost all of them to the binary system. For example, at a distance of 10gcm from the neutron star, the Larmor radius of a l-GeV electron or positron is about lo3 cm, while the 7 + X --$ e* process occurs mainly within 1O’cm from the starlg*‘Ol. At the same time, as these positrons move in the magnetic field, they continually lose energy through elastic scattering by the electrons of the binary system. The e* scattering cross section diverges at zero scattering angle; for a minimum scattering angle of 0.1 radian (corresponding to a minimum energy loss of 0.2% for the positron), the e* total scattering cross section is shown by the solid line in Fig. 2, to be compared with the annihilation cross section shown by the dashed line. The former is two orders of magnitude higher than the latter: scattering dominates over annihilation here. It is only when the positron has
185
The 511 keV Line
io,, .
. . t ,
.
1
P
c
3
‘2 10.
s
Y 6 1
;_,;,_
;
i-x.
0
. _#------J
O*f
450
.
Fig. 2
Total cross sections of elastic
sections
--L-__-----.
! . . _ . 550! . . .
500
600
5 04
-%+%
scattering (minimum scattering angle 0.1 rad, solid line) and of pair-annihilation (dashed) in units of classical electron cross
’
Fig. 3 Calculated r-ray spectrum around
511 keV for Cygnus X-3. tograms
are for coronal
The two his-
electron
column
densities of 10’” and 102’cmS2 , respectively.
lost much of its energy to, say, a state of near rest that annihilation with electron takes place. Clearly, annihilation of this kind will not produce the shape of Fig. 1, for the site of generation of the positrons has a rather high temperature (generally of the keV order), and electron-pairs cannot be stably formed; moreover, as the magnetic field is strong, even if pairs are formed, it may almost be impossible for thre~photon decay to occur. Therefore, we can expect that, under certain conditions, binary accreting systems will probably emit rather pure 511 keV line. Taking Cyg X-3 as example, we have made a detailed Monte Carlo simulation of the physical process of generating the 511 keV line, in which the cascading absorption of high-energy y-rays in X-photon field and the spectrum and extent of the latter were taken from another paperl’“l, the e* scattering and annihilation cross sections were taken from above
Fig. 2 and the Compton scattering between the emitted y-photons and the electron field was taken into account. The main calculated results are shown in Fig. 3. Here, the X-photon field is in the high state. The solid and dashed lines correspond to ambient electron column densities of lOa and 10” cme2, respectively. It clearly shows that, under certain conditions, the binary system can emit a rather pure 511 keV line, with a shape that is quite different from that of the line seen in the direction of the galactic centre. The line here has a small extension at the high energy end, indicating a small contribution from kinetic annihilation. The contrast and width of the line is directly related to the physical parameters including the electron column,density around the neutron star, the temperature distribution of the X-photon field, its being in the high or low state and its size. For example, if the X-photon field is in the low state, the 7 + X -+ e * process will produce few positrons and the 511 keV will be almost undetectable; or, if the electron field has a large column density, Compton absorption of the 7 photons around 511 keV will be severe and the intensity of the line will be greatly reduced, etc.. Therefore, to search and observe the 511 keV ~~hilation line emitted by binaries will help the study of the physical properties and the process of accretion in such systems. If we take the flux intensity of the high-energy r-rays from the
186
WU Mei et al.
binary to be ls(> 100 MeV) N 10m6 photons/cm2.&‘l, then for an e- matter field with a column density of 102* cmm2, the calculated value of the intensity of the annihilation line will be about 6 x lo- 5 photons/cm2*s. This value is a little above the detection threshold of the OSSE detector on board the GRO satellite (3~10~~ photons/cm2.s). We expect that the recent y-ray and X-ray telescopes of GRO will observe and analyze such 511 keV electron-positron annihilation line from binary systems, particularly Cyg X-3.
References Johnson, W. N. & Haymes, R. C., ApJ, ill Leventhal, M. et al., ApJ, 1980, 240,338 [21 131 [41 [51 [61 171
Albernhe, F. et al., A&A,
1981,94,
1973,184,
103
214
Riegler, C. R. et al., ApJ, 1981, 248, L13 Positron-Electron Pairs in Astrophysics (section IV) AIP Conference Proceeding, No. 101, 1983 Paul, J. et al., In: Gamma Ray Line Astrophysics, eds. N. Prantzos and P. Duroudroux, New York, AIP, 1999 Sunyaev, R. et al., In: Gamma Ray Line Astrophysics, eds. N. Prantzos and P. Durouchoux, New York, AIP, to be pubiished
f81
Bertram Schwarzschild, Physics Today, 1991,3,
191 WI 1111
LI Ti-bei, WU Mei and TANG X&-ping,
17
CAA, 1999,14,473.
WU Mei, ZHANG Chun-sheng and LI Ti-bei, 1993,3 Li,T.P.
& Wu,M.,
ApJ, 1992, 346,391.
= AApS, 1990,113, 291
(1993)
1712,
@
183-186
Acta Astrophys.
Pergamon
Press Ltd
Printed in Great Britain
of Sin. (1993) 13/l,
FAST RESEARCH
0275-lOfJ2/93%24.00+.00
98-100
REPORT
The 511 keV annihilation line from binaries t WU Mei
ZHANG Chun-sheng
LI Ti-bei
Abstract The electron-positron annihilation process in accreting binary systems, typically Cygnus X-3, was investigated. Our Monte Carlo calculation shows that, under certain conditions such systems can emit a rather narrow annihilation line, whose flux and width depend on the intensity of the emitted y photons, the state, size and temperature distribution of the X-ray halo, and the density and distribution of the electrons in the disk. We also discussed the possibility of observing the line by current experiments including the OSSE detector on board the CR0 satellite. Key words: accreting binaries-X-rays-T-ray
spectrum
1. INTRODUCTION Positron creation is an important physical process in high energy astronomical bodies while the signature of existence of positrons, the 511 keV electron-positron annihilation line, carries a great deal of information on the temperature, electron density, gravitational redshift etc. at the site of their generation. Therefore, the search for the 511 keV line has been an important objective in astrophysical observation. Since 1968 when Rice universityI’ detected this line from the direction of the galactic centre using a balloon-borne y-ray telescope, a large volume of observation has been done 12--75. So far, this line has only been found in solar outbursts, y-ray outbursts and in the direction of the galactic centre. These observations seem to have two features in common. 1) They seem to indicate that the formation of the line includes contributions from both electron-positron pairs (decaying into either three or two photons) and the direct annihilation of electrons and positrons. Fig. 1 is a typical spectrum from the largest experimental sample I6-8l. It has a large extension towards the low energy end from the vicinity of 511 keV, showing the three-photon decay at work. 2) The flux of the 511 keV line shows variation in time, probably corresponding to violent changes in star activity at the site of generation (similar to the X-rays emitted by binaries having a high and a low state). t
Supported by National Natural Science Foundation Received 1991 October 8; revised version 1992 September
14
184
WU Mei et al.
Fig. 1 A typical energy spectrum around 511 keV from the direction of the Galactic centre
Lingenfelter et al’s analysisi line requires
stringent
physical
showed that the generation of the low-extending 511 keV conditions,
for example,
the temperature
must
be below
5x lo* K, the matter density must be greater than lo5 H/cm3,
the ambient photon density must be below lo3 erg/cm3, etc.. Also, it should be pointed that, the magnetic field at the site of annihilation must be sufficiently small, otherwise, because the ratio of the probabilities of two-photon and three-photon decays is proportional to B2, the shape of the spectrum would differ greatly from that shown in Fig. 1 if B - 10’ G. So far, there has been little discussion on the 511 keV line emitted by binary accreting systems. We thought that, if the neutron star in the system is a source of high-energy yrays and if, further, an X-ray halo exists around it, then the process 7 + X + e* may take place15*gl. A secondary product, the positron, could then be the source of the annihilation line. The newly created positron has a very large kinetic energy, so, even if annihilation
takes place, the 511 keV line will not be observed because of the doppler shift. The kinetic annihilation cross section is small, and the great majority of the positrons will not undergo annihilation before they leave the system. But the strong magnetic field around the neutron star will confine almost all of them to the binary system. For example, at a distance of 10gcm from the neutron star, the Larmor radius of a l-GeV electron or positron is about lo3 cm, while the 7 + X --$ e* process occurs mainly within 1O’cm from the starlg*‘Ol. At the same time, as these positrons move in the magnetic field, they continually lose energy through elastic scattering by the electrons of the binary system. The e* scattering cross section diverges at zero scattering angle; for a minimum scattering angle of 0.1 radian (corresponding to a minimum energy loss of 0.2% for the positron), the e* total scattering cross section is shown by the solid line in Fig. 2, to be compared with the annihilation cross section shown by the dashed line. The former is two orders of magnitude higher than the latter: scattering dominates over annihilation here. It is only when the positron has
185
The 511 keV Line
io,, .
. . t ,
.
1
P
c
3
‘2 10.
s
Y 6 1
;_,;,_
;
i-x.
0
. _#------J
O*f
450
.
Fig. 2
Total cross sections of elastic
sections
--L-__-----.
! . . _ . 550! . . .
500
600
5 04
-%+%
scattering (minimum scattering angle 0.1 rad, solid line) and of pair-annihilation (dashed) in units of classical electron cross
’
Fig. 3 Calculated r-ray spectrum around
511 keV for Cygnus X-3. tograms
are for coronal
The two his-
electron
column
densities of 10’” and 102’cmS2 , respectively.
lost much of its energy to, say, a state of near rest that annihilation with electron takes place. Clearly, annihilation of this kind will not produce the shape of Fig. 1, for the site of generation of the positrons has a rather high temperature (generally of the keV order), and electron-pairs cannot be stably formed; moreover, as the magnetic field is strong, even if pairs are formed, it may almost be impossible for thre~photon decay to occur. Therefore, we can expect that, under certain conditions, binary accreting systems will probably emit rather pure 511 keV line. Taking Cyg X-3 as example, we have made a detailed Monte Carlo simulation of the physical process of generating the 511 keV line, in which the cascading absorption of high-energy y-rays in X-photon field and the spectrum and extent of the latter were taken from another paperl’“l, the e* scattering and annihilation cross sections were taken from above
Fig. 2 and the Compton scattering between the emitted y-photons and the electron field was taken into account. The main calculated results are shown in Fig. 3. Here, the X-photon field is in the high state. The solid and dashed lines correspond to ambient electron column densities of lOa and 10” cme2, respectively. It clearly shows that, under certain conditions, the binary system can emit a rather pure 511 keV line, with a shape that is quite different from that of the line seen in the direction of the galactic centre. The line here has a small extension at the high energy end, indicating a small contribution from kinetic annihilation. The contrast and width of the line is directly related to the physical parameters including the electron column,density around the neutron star, the temperature distribution of the X-photon field, its being in the high or low state and its size. For example, if the X-photon field is in the low state, the 7 + X -+ e * process will produce few positrons and the 511 keV will be almost undetectable; or, if the electron field has a large column density, Compton absorption of the 7 photons around 511 keV will be severe and the intensity of the line will be greatly reduced, etc.. Therefore, to search and observe the 511 keV ~~hilation line emitted by binaries will help the study of the physical properties and the process of accretion in such systems. If we take the flux intensity of the high-energy r-rays from the
186
WU Mei et al.
binary to be ls(> 100 MeV) N 10m6 photons/cm2.&‘l, then for an e- matter field with a column density of 102* cmm2, the calculated value of the intensity of the annihilation line will be about 6 x lo- 5 photons/cm2*s. This value is a little above the detection threshold of the OSSE detector on board the GRO satellite (3~10~~ photons/cm2.s). We expect that the recent y-ray and X-ray telescopes of GRO will observe and analyze such 511 keV electron-positron annihilation line from binary systems, particularly Cyg X-3.
References Johnson, W. N. & Haymes, R. C., ApJ, ill Leventhal, M. et al., ApJ, 1980, 240,338 [21 131 [41 [51 [61 171
Albernhe, F. et al., A&A,
1981,94,
1973,184,
103
214
Riegler, C. R. et al., ApJ, 1981, 248, L13 Positron-Electron Pairs in Astrophysics (section IV) AIP Conference Proceeding, No. 101, 1983 Paul, J. et al., In: Gamma Ray Line Astrophysics, eds. N. Prantzos and P. Duroudroux, New York, AIP, 1999 Sunyaev, R. et al., In: Gamma Ray Line Astrophysics, eds. N. Prantzos and P. Durouchoux, New York, AIP, to be pubiished
f81
Bertram Schwarzschild, Physics Today, 1991,3,
191 WI 1111
LI Ti-bei, WU Mei and TANG X&-ping,
17
CAA, 1999,14,473.
WU Mei, ZHANG Chun-sheng and LI Ti-bei, 1993,3 Li,T.P.
& Wu,M.,
ApJ, 1992, 346,391.
= AApS, 1990,113, 291