|
Items in cart [0] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 251
Quasi-Per~odic Oscillations in Low-Mass X-ray Binaries
W.H.G. LEWIN*, J. VAN PARADI3St, AND M. VAN DER KLIST
Variability on short time scales in the X-ray flux (Lewin et al. 1968) Is a
very general property of binary X-ray sources. Until recently, most efforts
in studying such variability were spent on two types of intensity variations,
periodic X-ray pulsations and X-ray bursts. Due to the lack of a direct
and transparent interpretation, relatively little attention was paid to noise
in X-ray intensity vanations. An outstanding exception has been the study
of the very fast variability of the black-hole candidate Cyg X-1 and sources
long thought to be of a similar nature such as Cir X-1 and GX 3394 (for
a comprehensive review of the studies of noise in X-ray intensity variations
of binary X-ray sources up to 1980, we refer to Bradt et at 1982~.
Consequently the bright persistent X-ray sources in the central regions
of the Galaxy (the galactic bulge sources) which showed neither pulsations
nor bursts were somewhat neglected. Not until after the discovery (Van der
Klis e' al. 1985) of intensiW-dependent quasi-penodic oscillations (QPO)
and associated red noise from these luminous low-mass X-ray binaries (see
Figure 1) were systematic studies of the shape of these power-spectral
components made. In this note we give a brief account of the main
developments since this discovery which have led to a new picture of the
properties of LMXB. Since this is not intended to be a review paper, we
will not give extensive references here but instead refer the reader to We
*Center for Space Research and Department of Physics, Massachusetts Institute of Technology
37~27, Cambridge, MA 02139, USA
t Astronomical Institute "Anton Pannekoek", University of Amsterdam, Roetersstraat 15, 1018
WB Amsterdam, The Netherlands and Center for High-Energy Astrophysics, NIKHEF-H, Am-
sterdam, The Netherlands
251
OCR for page 252
252
AMERICAN AND SOVIET pERspEcTrvEs
comprehensive reviews of Lewin et al. (1988), Lamb (1988, 1990), Van der
Klis (1989), and a recent article by Hasinger and Van der Klis (1989).
Slow QPO (frequencies In the range one to tens of milk-Hertz) had
been reported prior to 1985 from several sources. It is likely that these
QPO have a different origin from those which are the subject of this note.
Short trains of fast oscillations during a number of type ~ X-ray bursts
(i.e., runaway thermonuclear events) had been reported (for a review see
Lewin and Joss 1983~. The association of these oscillations with X-ray bursts
makes it difficult to compare them with the QPO seen in the persistent flux
of galactic bulge X-ray sources.
Sara et al. (1982) discovered quasi-periodic (A 2 Hz) oscillations in 2
out of 64 long type II X-ray bursts from the Rapid Burster. The frequency of
these oscillations differed between the bursts and drifted within each burst,
so Sara e! al. concluded that they could not be a direct manifestation of
the rotation of the neutron star. There were at least two reasons why this
discovery of QPO in the Rapid Burster received much less attention than
the QPO discovery in GX 5-1 a few years later. (i) The Rapid Burster was
(and still is) a very peculiar source (Lewin and Joss 1983), and the 2-Hz
QPO were seen as "just another" strange phenomenon. (ii) The frequency
of the QPO in GX 5-1 depended strongly on the source intensity; there
were no obvious connections between the QPO in the Rapid Burster and
other characteristics of this source.
Alpar and Shaham (19$5) proposed that the intensity-dependent QPO
discovered in GX 5-1 (Van der Klis et al. 1985) are caused by some
interaction between the magnetic field of the rapidly rotating neutron star
and the inner accretion disk (bounded by the magnetosphere) in which
matter orbits the neutron star in appro~nateh,r Keplerian orbits. The QPO
frequency is then the difference ("beat frequency'') between the Keplerian
frequency at the inner disk edge and the neutron star spin frequency.
Modulation of the accretion rate through magnetic gating was proposed
as a feasible mechanism for causing the X-ray intensity variations, and it
was shown that the red-noise component was a natural consequence of
accretion-modulation mechanism (Lamb e! al. 1985~. From the observed
intensity dependence of the QPO frequency, Alpar and Shaham derived a
neutron star rotation period of 10 ms and a magnetic dipole field strength of
a few 109 G. These parameters agree very well with the spin-up scenario for
the binary msec radio pulsars. In this scenario the msec radio pulsars are
the descendants of low-mass X-ray binaries (see, e.g., On den Heuvel 1986
for a renew of the evolution of X-ray binaries). For this reason the "beat
frequenter" model of Alpar and Shaham was greeted with enthusiasm.
However, if 10 me were the correct rotation period, it was somewhat
purling why this period did not show up as coherent pulsations in the
OCR for page 253
HIGH-ENERGY ASTROPHYSICS
3.8
3.4
3.0 l
2.6
2.2
1.8
3.0
2.6
2.2
1.8
3.0
Con 2.6
3
o
2.2
1 .8
3.0
2.6
2.2
1.8
3.0
it
t1
.:
.1
~';~1
t, ",'
1-
~ 'I
J in
2277-2486 c/s
~ ~l it: U 1
2486-2695 C/S
2695-2904 c/s
2904-3113 c/s
1.8
3.0
2.6
2.2
r.
3113-3322 c/s
3322-3531 C/S
1.8 ~ ~ ~'111 u up v II I AL ~',-3 ~ ~ ~ - ~ I L an 11
0 10 20 30 40 50 60 70 80 90 100
FREQUENCY ( Hz )
253
FIGURE 1 Display of average power spectra from GX 5-1 in six different source intensity
integrals Indicated in the panels3. The lines drawn through the data indicate fits described
lay a function which is the sum of a constant (Poisson noise), a line with a Lorentzian
profile (the QPO peak), and an exponentially rising red noise component. This figure is
Mom Van der Klis et al. 1985.
OCR for page 254
2~4
AMERICAN AND SOVIET PERSPECTIVES
power spectrum since the existence of a magnetosphere implies that in the
vicinity of the neuron star the magnetic field influences the accretion flow.
QPO discovered subsequently in Sco X-1 (Middleditch and Priedhorsly
1986; Van der Klis et al. 1986) had quite different properties. Their
frequency was sometimes low (near 6 Hz) and appro~natel!,r constant or
even slightly anti-correlated with X-ray intensity, while at other times, it
was high (10-20 Hz) and either positively correlated or varying erratically
as a function of source intensity depending on the intensity of the source.
Red noise was weak compared to the QPO. Both results seemed to pose a
problem for the beat-frequency model.
The "beat-frequenc~r" model was clearly unable to account for the
complicated picture presented by the observations; alternative models were
proposed and ways were worked out to broaden the range of phenomena
that could be explained within the beat-frequency model.
An underlying organization in the phenomena revealed itself when
correlations were found between the spectral properties of the sources and
their QPO characteristics. Three of the newly discovered QPO sources
were known to exhibit two different X-ray spectral states, distinguishable as
"branches" in an X-ray hardness versus intensity diagram. In OX 5-1 the
2040 Hz QPO were only and always observed when the source was found in
the so-called "horizontal-branch" of this diagram (Van der Klis et al. 1987,
see Figure 23. Its red noise was shown to consist of two components; one of
these is the low-frequenc~r noise associated with the QPO and only present
in the honzontal-branch state; the other ("very low frequency noise") is a
power-law component that Is seen in all specual states, and dominates the
power spectrum below ~ 0.1 HE A similar pattern was observed in Cyg X-2
(Hasinger 1987a). In Sco X-1 a strong correlation was also found between
QPO and the spectral state; two-branched spectral behavior occurred with
strongly intensity-dependent high-frequency QPO in one spectral branch
and weakly intensity-dependent ~ 6 Hz QPO in the other. However, bow
the morphology of the branches and the way in which the QPO frequency
varied among them were quite different from that In OX 5-1 and Cyg X-2.
A key contribution came from Hasinger (19~) who observed that the
normal branch of Cyg X-2, known to be connected at its upper end to the
horizontal branch, showed a sharp bend near its lower end suggesting a
transition to yet another branch. Hasinger, therefore, proposed that there
are three branches: the horizontal branch, the normal branch, and the
flaring branch which form a Zshaped pattern in the X-ray hardness vs.
intensity diagram and each of which has its characteristic power spectral
behavior. In OX 5-1 and Cyg X-2 it was the upper part of the Z that
had so far been observed; in Sco X-1 it was the lower part. Subsequent
observations have beautifully confirmed Hasinger's proposal, and the class
of "Z sources" is now well established (see Figure 3~.
OCR for page 255
HIGH-ENERGY ASTROPHYSICS
255
r I
0.70
o
060
z
C)
CY 0.50
~:
I
0.40
TYPI CAL
STAT I STI CA L
ERROR BAR
· .
.
NB ° °
o o
o
o
o o ° o
o
o
o
8 o ° o
° ° 8 o °
o
o
o o
o o
. HB
: ..~::~:.,.: ..·
o
o o o o o
° o
o
o o
o
o
o
o
80 100 120
140 160 180 200 220
I NTE NSITY (c/s)
FIGURE 2 Spectral hardness (ratio of the 6-10 keV and 3~ keV count rate) vemus
intensi~ (3-10 keV) diag~m for some EXOSAT data on GX 5-1. The solid dots in
the "honzontal branch" (HB) represent data when strong 2040 Hz QPO were observed;
the power spectra represented by open circles on the "normal branch" (NB) showed no
significant high-frequengy QPO. Ihis figure is ~om Van der Klis et al. (1987~.
W~th this clanfication, attempts to explain all obse~ved QPO behavior
within the framework of the beat frequenc~y model were abandoned, and
the model was proposed to be valid onl~r for the honzontal-branch high-
frequengy QPO (and associated LFN). Models for the 6 Hz normal-branch
QPO have been proposed in terms of radiation-dominated accretion Dows
near the Eddington limit ~amb 198S, 1990, Hasinger 1987a).
Millisecond time lags were discovered in the intensity-dependent hon
OCR for page 256
256
G.75
.
:
a_
V
~ O.
AMERICAN AND SOVIET PERSPECTIVES
Cyg x-2
0 . 7O
0.65
0.60.
s5~
, .,, ., , . . .
11B -
..,,
· ~
.\~ .. e
~' ~ ~ S. . I:I]
.
_
O.50 ;
0 75 C .80 0.85 0. SO
1 o2
-
U2
Am
~10
1 Al
10 o
10
10
-
1 O
,., -. - --- --''''' ''
~ ~ Cyg X-Z
~ ~ ·1
~ ~!
L_
1~' '. . . ~
10-3 10-2 1o-l 10 ° 1ol 1o2
GX 17+2
0.70~
~lII3`
0.60
a. 50
SOFT COLOR
1
1 ol
10 o
1 O
1 o-2
1 o-3
1 ~4
Or:
,~ FIN
+
0. 35 C.~O 0.45 0.50
. .
NLFX GX 17+2
l LEN
\~ _1
W~0>
At
1 :1
1~5 . 1'
10-3 10-2 io-i 10 ° 101 ~o2
FREQUENCY (Hz)
FIGURE 3 Display of the X-ray color color diagrams and power spectra for the Z sources
C;yg X-2 and GX 17 + 2 illustrating the Correlation between the QPO behavior and the
location of the source on the Z-shaped track This figure has been adapted from Van der
Klis (1989~.
zontal-branch QPO between X-ray spectral bands (Hasinger 1987b). This
suggested that X-rays originating from the near vicinity of the neutron star
were Compton scattered in a surrounding hot plasma before they could
escape. This Comptonization model had been proposed before to account
for the absence of coherent millisecond X-ray pulsations through smearing
and was consistent with some interpretations of the shape of the X-ray
spectrum. The problem of accounting for the effect of scattering on QPO
and beamed pulsar radiation stimulated a substantial theoretical effort. The
idea that the magnetosphere in these systems is very small (a few neutron
star radii, rather than hundreds as ~ X-ray pulsars) stimulated a reassess-
ment of magnetospheric theories. It was pointed out that magnetosphenc
OCR for page 257
HIGH-ENERGY ASTROPHYSICS
2S7
formation might be qualitatively different in these systems from that in
massive X-ray binaries since the boundaries of such small magnetospheres
are located in the radiation-pressure dominated part of the disk (White
and Stella 19884.
Further observations showed that not all low-mass X-ray binaries con-
formed to the above Z scheme. Some of them should have shown QPO,
but they did not while others excited QPO with properties that did not
fit into the Z scheme. Hasinger and Van der Klis (1989) distinguished, in
addition to the Z sources, a second class of low-mass X-ray sources that
shows a different pattern of correlated X-ray spectral and power-spectral
behavior, many of these sources are X-ray bursters. In their X-ray color-
color diagrams one can distinguish a "banana" shaped spectral branch and
isolated "islands". Both spectral branches have a characteristic associated
type of power spectrum (see Figure 4~. These so-called "atoll sources"
are, on average, less luminous than the Z sources, and they do not exhibit
the QPO found in the latter. Most of the persistently accreting low-mass
binary X-ray sources can be classified as either a Z source or an atoll
source; however, some (mostly transient) sources can not (e.g. the Rapid
Burster, and Cir X-1~.
Besides the fast-variability characteristics, other properties of low-mass
X-ray binaries are correlated to their X-ray spectral states as well. It was
recently discovered that the radio intensities of the Z sources OX 17 + 2,
C:yg X-2, and Sco X-1 (Penninx et al. 1988; Hjellming et al. 1989, 1990) are
strongly correlated with the location of these sources on the Z shaped track
in the X-ray color-color diagram (see Figure 5~. When these sources are in
the haring-branch state their radio brightness is relatively low, it increases
when the source moves up the normal branch, becoming relatively high
on the horizontal branch Since the radio emission is probably caused
by relativistic electrons (synchrotron radiation), this correlation suggests
that perhaps the required non-thermal processes and particle acceleration
are connected with the boundary layer between the inner disk and the
magnetosphere of the neutron star.
A second, very recent, development is the discovery that for atoll
sources there is a strong correlation between the spectral state of the
source (i.e. "island" vs. "bananas state) and the properties of X-ray bursts
(Van der Klis et al. 1990~. In particular, during the island state the duration
of the bursts is much longer than during the banana state. This may be
indicative of a difference in the hydrogen content of the layers in which the
thermonuclear hash occurs that gives rise to the burst. Although it is likely
that the accretion rate is the governing parameter for the properties of
both the X-ray bursts and the persistent X-ray spectrum, the nature of the
connection between the flashing layer and the (superficial) regions where
the persistent (accretion) luminosity is emitted is presents unclear.
OCR for page 258
o 1.00
o o.so
V
0.80
¢
~ c.60
258 AMERICAN AND SOVIET PERSPECTIVES
GX 13+1 4U 1705-44
....... , , , ,
i'''' ' ' 1120, art.
I O.90 ~
0.70 .~p! - _ 0.80 .
0.70
1 . i 0 1 . 8 0 1 . 9 0 2 . ~ 0 i . i 0 1 . 2 0 1 . 3 0 I; 4 0 i; 5 0 i . 6 0
SOFT COLOR
1
lo
lo c
lull
~3
1~4
., ... . - _ - . .
GX 13+1
'~\
1~5 .. ..
1~3 1~2 1~1 10 °
1
lol
10 o
loll
~2
1~3
1~4
1~5
1~3 1~2 1~1 10 ° 1 ~1
4U 1705-44
~- ~
l
FREQUENCY (Hz)
FIGURE 4 Display of the X-ray color color diagrams and power spectra of the atoll
sources GX 13 ~ 1 and 4U 170544 illustrating the oo:Telation between power spectrum
and the source spectral state ("island" versus "banana" state). This figure has been adapted
Tom Van der Klis (1989~.
In general, the rms variations of the QPO is only a few percent,
and therefore most studies discussed so far have been based not on X-
ray mtensibr curves but on power spectra of the intensity variations. An
exception is the Rapid Burster. QPO rms variation up to ~ 30% have been
observed from this source which made it possible with Toga to observe
trains of individual oscillations (Dotani et al. 1990~. These observations
showed that the finite width of the QPO peak in the power spectrum is
caused by frequency modulation of the signal. Since the Rapid Burster is a
very peculiar source, it is unclear whether the undertring mechanism holds
for over sources as well.
Finally, studies of the power spectra of some high-mass X-ray binaries
OCR for page 259
HIGH-ENER~ ~TROP~SICS
in_
i: 0.06
1
2`
~ ~ 0.05
0 ~
. ~
ID
a: 0 04
1
Go
-
-
0.03
0.02
259
I I I I I ~I I I I I I I I T S ~I
1 1 1 1 1
- Horizontal - I\
Branch ~I/,
Normal I, ~ s,
Bra a_
0.8 1 1.2 0.8 1 1.2
soft colour
(4.7-9.3 keV/1-4.7 key)
FIGURE 5 Display of the correlation between the radio intensity and X-ray spectral
properties of the Z source GX 17 + 2. In the left-hand panel the three branches are
indicated in the X-ray color color diagram. In the nght-hand panel the size of the octagon
is a measure of the radio intensity; its lo~tion indicates the spectral state of the source.
(most of them pulsars) have shown that these power spectra also contain
QPO and broad noise components, somewhat similar to those found in
low-mass X-ray binaries (Ebisawa et al. 1989; Belloni and Hasinger 1989~.
In the case of Me pulsating high-mass transient source EXO 2030 + 375
(Angelini et al. 1989) the variation of the QPO frequency with X-ray
luminosity combined with the known spin frequenter of the neutron star
was consistent ninth the beat frequency relation. However, recent results
obtained by Norns et al. (1989) and by Mitsuda e! al. (19891 mav Dose
problems for He beat frequency model
~, ~
In closing, the recent division of the low-mass X-ray binaries in Z
sources and atoll sources has clarified matters a great deal. It seems that
the beat frequency model is at present a promising (though not generally
accepted) model for the honzontal-branch QPO in Z sources. The normal-
branch and the flaring-branch QPO in Z sources probably have a common
origin which is different from the origin of the horizontal-branch QPO.
OCR for page 260
260
AMERICAN AND SOVIET PERSPECTIVES
REFERENCES
Alpar, M A., and J. Shaham. 1985. Nature 316: 239.
Angelini, L, L Stella, and ~N. Parmar. 1989. Astroph. J. In press.
Belloni, 1:, and G. Hasinger. 1989. Astron. Astroph. In press
Bradt, H.V., RL" Kelley, and LD. Petrol 1982. In: Sanford, P.W., et al. (eds). Galactice
X-ray Sour~es. W~ley. 89.
Dotani, T., K. Mitsuda, H. Inoue, Y. Tanaka, N. Kawai, Y. Tawara, K Makishima, J. Van
Paradi~s, W. Penniox, M. Van der Klis, J. Tan, and W.H.G. Lewin. 1990. Astroph. J.
350: 395.
Ebisawa, K, K Mitsuda, and H. Inoue. 1989. Publ. Astron. So~ Japan 41: 519.
Hasinger, G. 1987a. Astron. Astroph. 186: 153.
Hasinger, G. 1987b. In: White, N., and 3.-H. Huang (eds.~. The Origin and Evolution of
Neutron Stam. IAU Symp. 125: 333.
Hasinger, G. 1988. Adv. Space Res. Vol. 8, No. 2: 377.
Hasinger, G., and M. Van der Klis. 1989. Astron. Astroph. In press.
Hjellming, R.M., X.H. Han, F.A Cordova, and G. Hasinger. 1989. Astron. Astroph. In
press.
Hjellming, RM., R.l: Stewart, G. L White, R Strom, W.H.G. Lewin, P. Her~, K Wood,
K Mitsuda, W. Penninx, and J. Van Paradijs. 1990. Astroph~ J. In press.
Lamb, F.K 1988. Adv. Space Res. Vol. 8, No. 2: 421.
Lamb, F.K 1990. Astroph. J. In press.
Lamb, F.K., N. Shibazaki, M.A. Alpar, and J. Shaham. 1985. Nature 317: 681.
Lewin, WH.G., G.W. Clark, and W.B. Smith. 1968. Astroph. J. 152: L55.
Lewin, W.H.G., and P.C Joss. 1983. In: Lewin, W.H.G., and E.PJ. van den Heuvel (eds.~.
Accretion-dnven Stellar X-ray Sources. Cambndge Unrversity Press. 41.
Lewin, W.H.G., J. Van Paradijs, and M. Van der Klis. 1988. Space Sci. Rev. 46: 273.
Middled itch, J., and W. C P riedhotsly. 1986. Astroph. J. . 306 : 230.
Mitsuda, K, e' al. 1989. Proceedings of the Bologna conference, and paper submitted to
Publ. Astron. Soc Japan.
Noms, J., P. Hertz, K Wood, B. Vaughan, M. Michelson, K Mitsuda, and T. Dotani. 1989.
Prooeedings of the Bologna Conference, and Astrophys J. Submitted.
Penniox, W., W.H.G. Lewin, AN Zijlstra, K Mitsuda, J. Van Paradijs, and M. Van der
Klis 1988. Nature 336: 146.
ldwara, Y., S. Hayakawa, a al. 1982. Nature 299. 38.
Van den Heuvel, E.PJ. 1986. In: ~mper, J., W.H.G. Lewin, W. Brinkman (ed~). Ihe
Evolution of Galactic X-ray Binanes. Reidel Publ. Cry. 107.
Van der Klis, M. 1989. Ann. Rev. Astron. Astroph. Z7: 517.
Van der Klis, M., F. Jansen, J. Van Parad~js, W.H.G. Lewin, E.PJ. Van den Heuvel, J.
l~mper, and M. Sztajno. 1985. Nature 316: 225.
Van der Klis, M., F. Jansen, J. Van Paradijs, W.H.G. Lewin, M. Sztajno, and J. l~mper.
1987. Astroph. J. 319: L13.
Van der K1LS, M., E. Damen, W. Penninx, J. van Paradijs, and W.H.G. Lewin. 1990.
Astroph. J. (Lett) 360: Ll9.
White, N.E., and L" Stella. 1988. Monthly Not. Roy. astron. Soc. 231: 325.
Representative terms from entire chapter:
der klis
