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OCR for page 77
The Cyclotron Absorption Line arid Eclipse Transition
Phenomena of 4U 1538-52
GEORGE W. CLARK
Massachusetts Institute of Technology
ABSTRACT
Observations of the eclipsing binary X-ray pulsar 4U 153~52 by the
Japanese satellite Ginga have revealed a cyclotron absorption feature at 20
keV in the X-ray spectrum. The pulse-phase dependence of the intensity
and spectrum can be mimicked by a model of X-ray emission from thin
accretion-heated slabs at the magnetic poles of a rotating neutron star
with its magnetic dipole axis inclined at 45 from the rotation axis. The
observations also yielded data on the eclipse transitions which demonstrate
that the radial density function at the base of the supersonic wind of the
O-Wpe supergiant pnma~y has the forth of an exponential like that which
characterizes the density run in the similar region of the O-bpe superg~ant
primary of Cen X-3. As in the Cen X-3 system, the scale height of the
exponential implies a temperature ~ the base region much greater than
that of the supersonic wind.
Recent observations of the eclipsing binary X-ray pulsar 4U 1538-52
with the Ginga satellite have yielded results bearing on two quite different
topics. The first is the X-ray spectra and beaming pattern of the pulsar; the
second is the density distribution in the winds of the early-trpe supergiant
companions of this and other X-ray pulsars.
Ginga was developed and launched by the Institute of Space and
Astronautical Science of Japam It cames several detectors including a
4,000 cm2 Urge Area Counter ~AC) developed by the Leicester University
group and specially suited to the measurement of the spectra and variability
of compact X-ray sources like binary pulsars. The LAC is sensitive from
77
OCR for page 78
78
AMERICAN AND SOVIET PERSPECTIVES
1 to 38 keV and records data in 48 pulse-height channels with an energy
resolution of 20% at 5.7 keV and a maximum time resolution of 1 msec.
The Japanese group has made tune available to U.S. observers in
a guest observer program supported on the U.S. side by NASA In this
program an observation of 4U 153~52 was earned out over a complete
3.7~ay orbital cycle in March 1988 in a collaboration between myself and
J. Woo of MIT and F. Nagase, K Makishima and T. Sakao at ISAS.
The X-ray spectra of strong magnetized plasmas on neutron stars
has been the subject of much theoretical work since the pioneering in-
vestigations of Sunyaev and coworkers following the discovery of beady
X-ray pulsars by Giacconi and colleagues with Uhuru in 1972. In 1975
Basko and Sunyaev suggested that the effects of cyclotron resonance in
the Compton scattering cross section might be observed in the form of
X-ray emission lines. The subsequent discovery in a balloon experiment
by lumper et al. (1977) of the cyclotron feature near 50 keV in the Her
X-1 spectrum focused attention on the problem of radiative diffusion in
plasma with fields > 10~2 G. One other clear cyclotron line was found in
4U 0115+63 by Wheaton et al., and indications of a line in 4U 1626~7 by
Pravdo (1979) and Koyama (1989~. Mazets e! al. (1981) found evidence of
cyclotron features in gamma ray burst spectra, though other interpretations
have been put forward. And just recently clear evidence of first and second
harmonic absorption lines at 20 and 40 keV have been found with Ginga
in two gamma-ray bursts.
Our Ginga observation of 4U 153~52 adds a third definite example
of cyclotron absorption line formation in an X-ray binary pulsar under
what appear to be specially favorable circumstances for analysis (Clark
al. 1989~. This object has a 530-second pulse period and an eclipse with
a half-angle of about 30 . Considering first the data unaffected by the
eclipse, we divided it into ten spectral intervals and plotted the counting
rate as a function of pulse phase, as shown in Figure 1. We also divided
the data into eight equal intervals of pulse phase and plotted the spectra of
each portion, as shouts in Figure 2. Four salient properties of the spectra
and vanabili~ are clearly evident:
1) The pulse profile has symmetrical primary and a secondary peaks
of unequal amplitude and separated by 180 in phase.
2) The primary peak has a dimple at low energies.
3) There is an absorption line at 20 keV with a maximum equivalent
width in the middle of the secondary peat Indeed, the line is
so strong as to essentially blot out the secondary peak in the
spectral internal centered on 20 keV.
4) The pulse fraction increases with energy.
Properties 1), 2), and 4) have been seen in previous observations of
OCR for page 79
20
0
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40
20
o
US 60
ED
as 40
o
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20
o
80
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40
20
o
40
20
HIGH-ENER~ ASTROP~ICS
1 1 ~ 1 1 1 ' '
1 n ~-12 6 keV
1 ~: ~ J
1 1 ~ 1 1 1 1
1 1
8.0-10.3 keV
N-,~
1 1 i 1 1 1 1
5.7 - 8.0 keV
1 1 1 1 1 1 1
1 1
3.4-5.7 keV
_~ I_
_ _
1 1 1 1 1 1 1
_ 1.1-3.4 keV
~-
.5
.5
o
2
o
4
2
o
5
0
o
30
20
0
o
o
~ ,~k _
1 1 1 1 1 1 1
20.9 - 25.7 keV
r ~
, 1 , 1 , 1 ,
~ r ' I ' I
16.1 -20.9 keV
_~ I_
1 1 1 1 1 1 1
12.6 - 16.1 keV
_~ ;~4' ~ _
, 1 , 1 ,
.5 ~ 1.5 2
PULSE PHASE PULSE PHASE
FIGURE 1 X-ray counting rates of 4U 1538 52 plotted against pulse phase (pulse period
= 530 s) for ten energy channels.
4U 1538-52. The absorption line at 20 keV is new and opens interestung
possibilities for detailed comparisons between the observed phenomena and
the results of recent theoretical treatments of the emissions of magnetized
slabs and columns of accretion-heated plasmas at the poles of magnetized
neutron stars.
OCR for page 80
80
1oo
o ~
l o 2
10.3
10'
10°
10':
~ c'
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0
Y
z In
O z
lo,
o o
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AMERICAN AND SOVIET PERSPECTIVES
-
i\
3 ~
10°
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101
10°
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ENERGY (keV)
1 0 20 30 40 50
ENERGY (keV)
FIGURE 2 Pulse height spectra and inferred inadent energy spectra of 4U 153~52 for
eight intervals of pulse phase.
OCR for page 81
HIGH-ENERGY ASTROPHYSICS
81
OCR for page 82
82
AMERICAN AND SOVIET PERSPE~IIVES
go
11
- 5 keV
- If- ~- 20 keV
- 25 keV
:~""~
p = zoo
' 1 ' 1 1 1 ,
25 keV I
~-
L:i - W
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1 1 1 1 1 -
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5 keV ~
TV
, 1 , 1 , 1 ,
0 .5 1
PULSE PHASE
-
v,
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180 ~
5 7 10 20 30 50
ENERGY (keV)
FIGURE 3 Calculations of the pulse light curve (middle panel) and pulse-phase resolved
energy spectra (bottom panel) for a model X-ray pulsar with the radiation pattern illustrated
in the top panel and the orientations of magnetic dipole axis and spin axis specified in the
text.
OCR for page 83
HIGH-ENERGY ASTROPHYSICS
83
gradual, though one abrupt transition was apparently observed by Davison
et al. (1977) with Ariel.
A gradual eclipse immersion or emersion of an eclipsing X-ray binary,
of which six are known, is obviously caused by absorption in an extended
atmosphere of the primary, which opens a new direct way to a determination
of the radial density distinction. The situation can of course be complicated
by streams or blobs of matter flying around in the system as in Vela X-1
and sometimes in Cen X-3. But often in the case of Cen X-3, SMC X-1,
EMC X4, Her X-1 and 4U 1538-52 the transitions are fairly clean and
uniform, indicating that the measured column densities are fair measures
of the radial density functions. The situation can be ideal because the
size of the X-ray source, i.e. the neutron star, is negligible compared to
the scale size of the atmosphere, and X-ray absorption from 1 to 10 keV
is a relatively simple measure of column density along the line of sight.
Compare this with the indirect arguments that must be used to interpret
spectroscopic, radio and IR data in terms of atmospheric structure.
The first remark about the significance of a gradual eclipse transition
was made by Schreier et al. (1972) In the paper announcing the discovery of
the binary nature of Cen X-3. They characterized the eclipse transition by
ascribing a "scale height" of 5 x 10~° cm to the atmosphere of the primary
implying thereby an exponential form of the density function
Following the discovery of intense, cool (30,000-50,000 K), supersonic
winds in O and B stars by Morton (1967) in rocket UV spectroscopy,
theories of the wind acceleration process developed around the idea of
radiation pressure arising from scattering of light from the Doppler-shifting
UV lines of the metals in the wind. This works well in the supersonic regime,
and since the radiation intensity varies as 1/r2, the resulting acceleration
leads to velocity cones with a characteristic rapid initial rise and then a slow
approach to an asymptotic terminal velocity with values in the range of 1000
to 3000 Ems, as observed in the P Cygni profiles of UV absorption lines.
There has always been a problem, though, in how the winds get started
because the passage from sub to supersonic is blocked by the sound barrier
as expressed in the singularity in the hydrodynamic equation governing the
how. Only by carefully tailoring the outward force by judicious combinations
of thermal and radiation pressure can one achieve a steady flow from sub
to supersonic with the requisite mass flux and cool temperature. Hearn
suggested in 1975 that there may be a coronal layer at the base of the wind
in which thermal pressure can cause the early acceleration, with sudden
cooling and radiation pressure taking over when the velocity is high enough
for Doppler shifting to prevent line saturation. Then Castor, Abbott and
Klein (1975) showed He way to a theory of a radiation pressure driven
cool wind from start to finish, and came up with velocity curies with the
same rapid initial change, and then a slow approach to asymptotic terminal
OCR for page 84
84
AMERICAN AND SOVIET PERSPECTIVES
velocity, which turns out to be, in general, about three times the escape
velocity from the stellar surface (Abbott 1978~.
Near the end of the SAS 3 mission we undertook a detailed study
of Cen X-3 eclipses using data from a continuous two-week observation
when Cen X-3 was in a high luminosity state (Clark et al. 1988~. The
transitions are well accounted for by an exponential density function with
a scale height of 6 x 10~° cm, and not by a density function implied by
the conventional 1/r2 radiation force-driven wind theories. Here, as in
the case of 4U 153~52, the opticaVUV luminosity of the companion is
much greater than the X-ray luminosity of Cen X-3, so the X-ray heating
effects are presumably negligible. Day et al. (19883, using data from several
continuous observations of eclipse transitions by EXOSAT, drew the same
conclusion, lie. that the density run is exponential Such a scale height, if
interpreted as the characteristic of an isothermal, hydrostatic atmosphere
of a 19 solar mass star, corresponds to a temperature of 106 K Following
Hearn's idea that there may be a coronal layer at the base of the wind,
heated by some mysterious process, in which the initial acceleration takes
place, we constructed an ad hoc model with a 106 K base corona governed
by the usual hydrodynamic flow equation with a gravity reduced by half by
radiation pressure, and terminating at 1.4 R* at the high temperature sonic
point where we assumed the heating mechanism turns off, the temperature
drops to 50,000 K, and radiation driving takes over in the cool but supersonic
regime. This hybrid model yields a density curve that can be fit to the data
for reasonable mass loss rates. A major problem of the model is that its
coronal layer has an enormous soft X-ray luminosity (about 10% of the
optical luminosity of KRZ) which must be absorbed by the outer wind.
Other ways to get more mass loss out of a cool subsonic flow Is to fine tune
the effective gravity by allowing for centrifugal force (which may amount to
1/6 to 1/3 of the gravity) and radiation pressure without the benefits of the
large Doppler shifts that keep it effective in the supersonic regime. But I
am not aware of rigorous treatment of this subsonic regime that yields a
density curve that explains the Cen X-3 eclipse transition data.
Now, with the 4,000 cm2 sensitive area of Ginga, we have good data on
4U 1538-52, another massive binary pulsar, providing similar results. The
variation with orbital phase of column density deduced by spectral analysis is
displayed in Figure 4 along with the results of least squares fittings of three
different functions-an exponential, an isothermal hydrostatic function, and
a 1/r2 force-driven wind. The latter fails again because of its too rapid initial
fall in density. The isothermal hydrostatic function fits well, allowing, to
be sure, for the fluctuation that cannot be fitted by any reasonably smooth
function. Of course, the situation is not hydrostatic, so a transition to a
radiation~riven regime must occur, and one can undoubtedly construct
another ad hoc hybrid model, combining an initial subsonic acceleration
OCR for page 85
HIGH-ENERGY ASTROPHYSICS
Cot
2
A
x
-
3
z
LU
As
~ 2
o
~ I ' 1 ' 16 '
ISOTHERMAL HYDROSTATIC ~
1
~1
-.3 ORBITAL PHASE +-3
~ · T i T ~ I · l
EXPONENTIAL
~.1 ~
::1 1_
Fit -.
. . . . . . . . . . .
ORBITAL PHASE +.3
4 0
-4.2
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.
ORBITAL PHASE +.3
85
FIGURE 4 Measured column densities (H-atoms/cm2) along the lines of sight to 4U
1538 52 plotted against orbital phase during one eclipse ingress and egress. The solid curves
are least-squares fits of column density curves derived from three trial atmospheric density
functions of the primary BO star plus constant terms before and after eclipse. Deviations
of the data from the fitted column densities are displayed below each plot. (In each case
the line integration is terminated at two orbital radii).
OCR for page 86
86
AMERICAN AND SOVIET PERSPECTIVES
with an exponential density run with a radiation-dr~ven supersonic regime.
But what is needed now is theoretical attention to the facts of X-ray eclipse
transitions and the direct information they provide about He mysterious
subsonic acceleration phase of early star wind generation.
REH:RENCES
Abbott, D.C 1978. Ap. J. 225: 893.
Basko, M.M., and R^. Sunyaev. 1975. Astr. Ap. 42: 311.
Castor, J.I., D.C. Abbott, and R.I. Klein. 1975. Ap. J. 195: 157.
Clark, G.W., J.W. Woo, F. Nagase, K Makishima, and 1: Sakao. 1990. Ap. J. 353-274.
Clark, G.W., J.R. Minato, and G. Mi. 1988. Ap. J. 324: 974.
Hampton, D., J.B. Hutchings, and A P. Cowley. 1978. Ap. J. (Letters3 Z5: L63.
Day, ~ 1988. Thesis, Cambridge University.
Davison, PJ.N., M.G. Watson, and J.P. Pye. 1977. M.N.R.\S. 181: 73P.
Giacconi, R., H. Gursly, E. Kellogg, E. Schreier, 1: Matilsky, D. Koch, and H. Idnanbaum.
1971. Ap. J. Suppl. Z37(27): 37.
Hearn, A.G. 1975. Astr. Ap. 40: Z77.
Koyama, K et al. 1989. Pub. Astr. Soc Japan, in press.
Mazets, E.P., S.V. Golonetskii, AL Aptekar', Y^. Gur~yan, V.N. Al'inskii. 1981. Nature
290: 378.
Mesz~ros, P., and W. Nagel. 1985. Ap. J. 298: 147.
Morton, D.C 1967. Ap. J. 147: 1017.
Nagel, ~ 1981. Ap. J. 251: Z78.
Pravda, S.H., N.E. White, EN Boldt, S.S. Halt, PA. Serlemitsos, J.H. Swank, and AK.
Szymkowia~ 1979. Ap. J. =1: 912.
Schreier, E., R. Levinson, H. Gursly, E. Kellogg, H. Tananbaum, and R Giacconi. 1972.
Ap. J. (Letters) 172: L79.
Hamper, J., W. Pietsch, ~ Reppin, W. Voges, R Staubert, and E. Kendziom. 1978. Ap.
J. (Letters) 219: L105.
Wheaton, Wm. ~ et al. 1979. Nature 282: 240.
Representative terms from entire chapter:
radiation pressure
