Hakucho satellite operations and the problems that emerged from the neutron star observations are illustrated. X-ray pulsars and bursters are reviewed. Future satellite programs are outlined

Tanaka, Y.

English

NASA Technical Reports Server

X-Ray Astronomy Program i n  Japan 
Yasuo Tanaka 
Ins t i t u te  o f  Space and Astronautical Science, 
Komaba, Meguro, Tokyo 153 
Japan 
1. Introduction 
One o f  the great achievements i n  the short h istory o f  X-ray astronomy i s  t o  
have established the real  existence o f  neutron star rather than j u s t  a theoretical 
conception. 
undoubtedly continue t o  produce important science through t h i s  decade and beyond. 
amusing as well as amazing t o  f i n d  that t h i s  96 kg s a t e l l i t e  w i h i  l imi ted capabil i- 
t i e s  gives us plenty o f  interesting resul ts and signi f icant questions concerning 
neutron stars. These questions i n  fact  constitute the basis fo r  our planning of 
the future progran, I would, therefore, l i k e  to  i l l u s t r a t e  some o f  the problems 
that emerged from the Hakucho observations and are t:, Le understood by future 
investigations . 
Pursuing t h i s  extreme astronomical body and i t s  environment w i l l  
It i s  We have been operating the Hakucho s a t e l l i t e  f o r  over two years. 
2. X-ray pulsars 
o f  Vela X-1. Vela X-1 i s  a slow pulsator with the approximate pulsation period 
o f  283 sec and the binary period o f  8.97 days. Fig. 1 shows the history o f  the 
pulsation period variation. It i s  clear that the neutron star had been spinning 
up u n t i l  1979, consistent with the picture o f  acceleration by the accretion torque 
(e.9. Rappaport and Joss 1981 1. 
features. 
continues (Nagase e t  a l .  1981). The average i )  / P 2 3.5 x 
1979 - 1981. 
the order o f  the orb i ta l  period i s  s igni f icant ly resolved fo r  the f i r s t  time 
(Nagase 1981). 6 / P o f  t h i s  f luctuation ranges over (-3 % 4) x yr-', an 
order o f  magnitude larger than that for the long-term variation. The observed 
long-term slowing down i s  not readi ly explained. There has been no signi f icant 
Among many X-ray pulsars, we have conducted so f a r  most extended observation 
Hakucho observations revealed two remarkable 
i) The spin period started to  slow down since 1979 and t h i s  trend 
yr-' f o r  the period 
ii) An up-down f luctuation of the spin period with a time scale o f  
https://ntrs.nasa.gov/search.jsp?R=19820018188 2020-03-21T07:36:51+00:00Z
i 
220 ORIGINAL PAGE IS 
OF POOR QUALITY 
change i n  the average X-ray 
luminosity through past 
several years. Would t h i s  
be due t o  the change o f  the 
d i rect ion o f  accretion f low 
wi th  respect t o  the spin 
vector o f  the neutron s tar? 
A r  en t i re l y  d i f f e ren t  idea 
suggested by Makishima i s  
tha t  the secular var ia t ion 
o f  the pulsation period 
might due t o  the Doppler 
e f fec t  by the o rb i ta l  motion 
o f  the Vela X-1 binary 
around an unseen t h i r d  body 
with a period o f  the order 
Vela X-1 
I I 1 1 I 1 
t 
Q s3* CP A5 53 c0 08 BA 0 
+oe 
COPefnlCS 
Arid-5 
sns-3 
COS-0 
050-8 
Balloon 
bkUCh0 
t -I 
I 1 I I I 1 I 
75 76 77 78 79 80 81 
282.7L 
Fig.1 Histroy o f  the pulsat ion period o f  Vela X-1 
10 years. 
hence implying a giant black hole. 
o f  the neutron s tar  revolut ion i s  also observed f o r  GX 301-2 (Kelley, Rappaport and 
Petre , 1980). 
The short-term up-down f luc tua t inn  o f  the pulsat ion per iod poses an in t r igu ing  
question. If the f luc tua t ion  i s  o f  an external or ig in ,  the d i rect ion o f  the accre- 
t i o n  flow may change rather quickly. 
due t o  the coupling and decoupling between the crust  and the superf lu id i n t e r i o r  
such as proposed by Lamb, Pines and Shaham (1978). 
outside neutron stars by accurately monitoring the behavior o f  X-ray pulsars. 
I f  i t  were so, the mass of the t h i r d  body would then be roughly 100 Me, 
It i s  worth not ing tha t  a s imi lar  slowing down 
I f  i t  i s  o f  an in ternal  or ig in,  t h i s  may be 
The example o f  Vela X-1 demonstrates tha t  we can learn a l o t  about inside and 
3. X-ray bursters 
As a matter o f  fact, one o f  the main emphasis o f  Hakucho was placed on the obser- 
vations o f  X-ray bursts. D is t r ibu t ion  o f  burst sou-:es including those we found 
i n  the general d i rect ion o f  the galact ic center i s  shown i n  Fig, 2. A d i s t i n c t  
c luster ing towards the galact ic  center i s  evident. This d i s t r i bu t i on  by i t s e l f  
suggests the Pop. I 1  nature o f  burst  sources, which i s  strongly enhanced by the 
fac t  tha t  nearly h a l f  as many burst  sources are associated w i th  globular clusters. 
radius of the neutron star. Owing much t o  the SAS-3 group (Lewin and Joss 19811, 
it i s  by now established tha t  the "black-body" radius i s  conserved during the burst  
decay and o f  the order 10 km. We estimated the "black-body" r a d i i  f o r  several 
X-ray burst  i s  another outstanding phenomenon tha t  neutron s tar  exhibi ts.  
O f  par t i cu la r  importance o f  bursts i s  tha t  one can i n  pr inc ip le  measure the 
I 
221 I 
1743: 
1743-29 
(P BURST SCURCES 
I Y  GLOBULAR CLUSTERS 
0 OTHEFC BURST SOURCES 
. -  . .  .. . .  
Fig.2 D is t r ibu t ion  o f  burst sources around the galact ic center 
burst sources, including three globular c luster  sources, tha t  are most probably 
near the galact ic center. They a l l  come close t o  10 km w i th in  20%, implying tha t  
they are about the same size. I must caution, however, tha t  i t  i s  too ear ly  t o  
take t h i s  number as the actual neutron s tar  radius f o r  t h i s  requires correct  i n te r -  
pretat ion o f  the burst  spectrum which i s  yet  the theme o f  fu r ther  investigations. 
Nevertheless, i t  i s  very important tha t  we have means t o  measure the neutron s ta r  
radius i n  our hands. While, X-ray pulsars enable us t o  measure the mass of neutron 
stars. 
We have sofar learned tha t  every indiv idual  burst  source can produce a var ie ty  
o f  bursts i n  the time p ro f i l e ,  peak luminosity and t o t a l  energy. The thermonuclear 
f lash  model provided a persuasive p ic ture o f  the burst  phenomenon. There are, 
however, several observational facts that  challenge theoret ical  interpretations. 
i )  Successive bursts a t  a very short in te rva l  
We observed three such events; 10 min. f o r  1608-52 (Murakami e t  a l .  19801, 
8 min. f o r  Terzan 5 source (see Fig. 31, and 8 min. f o r  MXB 1636-53. l h e  SAS-3 
group observed three bursts i n  such short succession from a galactic-center source 
(Lewin e t  a t ,  1976). Such an in terva l  i s  de f in i te ly  too short t o  replenish the 
required nuclear fue l  by the mass accretion. The nuclear fue l  ought t o  be hidden 
somewhere on the neutron s tar  surface, but where? and how? are the b i g  questions. 
222 ORIGINAL PAdE 1s 
OF POOR QUALln 
ii) Possible supercri t ical luminosity 
A puzzling resu l t  was 
obtained from examination o f  
the burst peak luminosities 
(Inoue e t  a l .  1981). The 
peak 1 uminos i t y  d is t r ibut ions 
for f i v e  burst sources near 
the galactic center are shown 
i n  Fig. 4. If the distance 
t o  the galactic center i s  
taken t o  be 10 kpc, the 
largest peak luminosity ob- 
served f o r  each source large- 
l y  exceeds the Eddington 
1 i m i t  f o r  a 1.4 Me neutron 
star. This conclusior remains 
va l id  unless the galactic 
center distance i s  much smaller 
- 
m 
1Q) 
e-nluv 
2 6 6 8 w n Is di 
T I M E  ( Y I N )  
Fig.3 X-ray bursts a t  8 min. in terva l  from - 
T e r s n  5. 
than 7 kpc. 
The same can be concluded f o r  a Terzan 2 
ourst (Grindlay e t  a l .  1980) and f o r  the bursts 
from NGC 6624 (Clark e t  a l .  1976), the d is tawe t o  
the l a t t e r  being p re t t y  well estimated. 
can be exceeded by blowing o f f  the neutron s tar  
atmosphere. However, the observational resu l t  
indicates tha t  the "black-body" radius remains 
constant from the burst peak through the decay, 
excluding the expanding photosphere. Strong 
magnetic f i e l d  can reduce the Thomson cross section, 
thereby increase the c r i t i c a l  luminosity. This 
e f fec t  i s  s ign i f i can t  only i f  the f i e l d  i s  o f  the 
order o r  greater than 10'' Gauss. Why then does 
the d i s t i n c t  difference i n  the behavior and the 
spectral hardness between X-ray pulsars and X-ray 
bursters ex is t?  
We assess a great significance upon this 
problem o f  possible supercr i t ica l  1 uminosi t y  . 
There may very well be physics i n t r i n s i c  t o  the 
superdense photons. 
One might think tha t  the c r i t i c a l  luminosity 
Fig.4 
Peak Flux ( e r g c d s d )  
Dist r ibut ions o f  burst 
peak luminosities f o r  
f i v e  sources near the 
galactic center. 
I 
i 
223 
The rapid bursts from MXB 1730-335 are interpreted by Lewin e t  a1 . (1976) as 
due t o  the :hopped accretion f low caused by a cer ta in  i ns tab i l i t y .  Fortunately, 
we met i t s  remarkable benavior i n  August 1979 (Inoue e t  al. 1980). By then, the 
rap id  burster produced a long t r a i n  o f  bursts with trapezoidal shape. The length 
o f  the f l a t  top o f  these trapezoidal bursts ranged from h a l f  a minutes t o  over 
10 minutes. The famous l inear  re la t i on  between the burst  size and the time t o  the 
next burst s t i l l  holds f o r  these bursts, implying a reservoir  o f  the capacity i n  
excess of lo2' g o f  matter. 
a trapezoidal burst, the luminosity as w e l l  as temperature are kept constant. 
Hence, the emit t ing area i s  also constant. This seems t o  indicate that the trape- 
zoidat burst  i t s e l f  represents a stat ionary state. Another stationary state exists,  
which corresponds t o  the low level  ( 6 ~  1/10 the burst  peak) persistent emission tha t  
appears between two bursts. We, therefore, revise the p ic tu re  o f  the rap id  burster 
tha t  i t  undergoes rap id switchings between the high and low states rather  than a 
s ingle shot o f  matter onto the neutron star. However, the mechanisms o f  the 
reservoir, switching and cont ro l l ing  the mass f low are key questions t o  be answered 
by fu turo study. 
I n  two o f  the trapezoidal bursts, c lear  osc i l la t ions  were discovered. As 
shown i n  Fig. 6, the power spectrum f o r  each exhib i ts  a prominent spike a t  about 
0.5 sec. The osc i l l a t i on  does not  appear as due t o  the neutron s tar  revolution, 
since the observed spikes show a structure and are s l i g h t l y  but s ign i f i can t l y  
d i f f e ren t  from each other, The o r i g i n  o f  t h i s  o s c i l l a t i o n  i s  unknown. 
As can be seen i n  Fig. 5, an important resu l t  i s  that, during t h  f l a t  top of 
Fig.5 Example o f  tho trape- 
zoidal burst  from the 
rap id burster. 
FREQUENCY (HER21 
Fig.6 Power spectra f o r  two trapezoidal bursts, 
f o r  which a prominent spike i s  seen. 
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224 ORIGINAL PAOE IS 
OF POOR QUALll'Y 
4. Future sate1 1 i te program 
vations have put forward concerning neutron star and i t s  system. Following Nakucho, 
we have two more approved X-ray astronomy missions, ASTRO-B and ASTRO-C. The task 
o f  these missions i s  t o  bet ter  understand the above-described problems and t o  
explore fur ther  . 
vehicle a t  ISAS has been steadi ly increasing. The present ISAS launcher, M-3S, i s  
capable o f  launching a Q 250 kg s a t e l l i t e  i n to  a 500 km a l t i t ude  c i r cu la r  o rb i t .  
New development e f f o r t  has started t o  upgrade the launch power. l h e  f i r s t  step 
i s  M-3S I 1  which can carry a payload w e r  400 kg f o r  a 500 km orb i t .  The second 
step, M-3S 111, would achieve substai1,ial increase of the launch capabt l i ty  over tha t  
of M-3S 11. 
So far, I have b r i e f l y  covered the c ruc ia l  problems tha t  the Hakucho obser- 
Since the time o f  the Hakucho launch, the capabi l i ty  o f  the s a t e l l i t e  launch 
Fig. 7 i l l u s t r a t e s  the current mission plan a t  ISAS. 
are approved misr.ions t o  date, whereas those wi th  dashed c i r c l e s  are yet  t o  be 
approved. Mzin features o f  ASTRO-B and ?qTRn-C are l i s t e d  i n  Table I ,  i n  comparison 
with those of Hakucho. 
ware i s  presently under fabr icat ion.  The main instrument o f  ASTRO-B i s  gas sc in t i -  
2 l l a t i o n  proportional counters with the t o t a l  area o f  about 9dOO cm . Major increase 
Those wi th  so l i d  c i r c les  
ASTRO-8 weighing 220 kg i s  due for launch i n  ear ly  1983, and the f l i g h t  hard- 
' Fiscal Year (Apr.4 1979 I1980 ' 1981 I 1982 1 1983 I 1984 I 1985 119% 1 1 9 7  
EXOS-Series 
I 
STP-Plasma I '  
I ' 1  
I 
'X-Ray" 
:- I '- Test s a t e l l i t e s  
CVGT 
L -ge S a t e l l i t e s  OPEN-J 
/ \  A ,\\"X-Rayn 
I - 
1 -Lb(- \ I  V \ I  
-. - ._ .- - x- -- ---- -
A 
SOP-2 
i'\ 
Tether 
0 SE PAC f"? I 
In ternat lonal  1 
Col labotat  ion 
SEPAC 
SL- 1 
', ? 
I RTJ 
Other SPACELAB ?xperiments 
i 
Fig.7 Current mission plan a t  ISAS for 10 years. The ser ia l  number o f  
s c i e n t i f i c  s a t e l l i t e ,  miss% name and launch vehicle are ind i -  
cated fo r  each. 
PAGE 1s 
w p o o f t v  
225 
as coatpared to Hakucho i s  made i n  the sens i t i v i t y  as w e l l  as i n  the spectral reso- 
lution. ASTRO-B w i l l  consequently provide us with much bet ter  temporal and spectro- 
scopic infomation about the galactic sources than we are having from Hakucho. 
As a s ign i f icant  feature, the energy resolut ion c f  the gas s c i n t i l l a t i o n  proportional 
counter which i s  twice that  of the conventional proportional counter would enable 
us to study the spectral de ta i l  and in par t i cu la r  w i s s i o n  l i nes  o r  absorption 
features. 
ASfRo-C i s  the Coltow-on X-ray astronomy s a t e l l i t e  scheduled f o r  launch i n  
1986-87. f f  2 spacecraft w i l l  be approximately 400 kg i n  weight and 3-axis stabi- 
lized. A t  present, we are i n  the biginning of the prototype design phase. The 
slain instrument w i l l  be low-background proportional counters w i t h  the to ta l  area 
2 of about 5000 an . Gas s c i n t i l l a t i o n  proportional counters may well be a pa r t  o f  
it. An al l-sky monitor w i l l  also be included. 
The primary mission objective of ASTRO-C i s  the study of the time var iab i l i t y .  
b i n  euqhasis here i s  not only accurate timing analysis o f  the galactic sources 
but the xrutiny o f  the time v a r i a b i l i t y  o f  act ive galactic nuclei. Exoticism 
o f  act ive galactic nuclei i s  inwense but  yet  l i t t l e  i s  explored. We consider that  
study o f  the time va r iab i l i t y  i s  o f  cruc ia l  importance o f  the physics involved i n  
the power house problem. 
We hope that the ASTRO-series missions with the improved M-launcher w i l l  
Table I .  Main features o f  Hakucho, ASTRO-B and ASTRO-C. 
HAI(u;Ho Asm-B ASTRO-C 
~ - ~~~ ~ ~ ~~~ ~ 
UUKH 1979 1983 1906-7 - _I 
WEIW (kg)  % 220 .\r 400 
MII ImTw. PROP. C W E R  G.S.P.C. P.C. rmr1-cyr 1 
EFFECTIVE 6" x 6" 80 cmz 3" x 3" 1000 c192 z 1' x 3" 
z5OOo 02 2 AREA 170 17" m tll 
SC IERTIFIC L. HIDL F.O.V. RElc 0 SPECTRAL All) 0 TINE bARIATION 
OBJECTIVES BURST SURVEY TEMPOPAL VARIATIOIl OF EKTRAGALACTIC 6 - IIDI1ITORING SEVE- OF W T I C  SOURCES U U A C T I C  SOURCES 
RAL SOURCES AT A O W E  = 10 I AT 6 keV 0 0.5 UJy SOURCES 
T Int POWERFUL FOR I N  10 HIN. 
> 300 GMAXIES 1m L I N S  z 0 SOURCE LOCATION 
c#c 0.5" CYCLOTRON L I R S  0 S I Z E  OF ACTIVE 
FMC 0.05" 1-50 keV I N  GALACTIC W L E I  
128 CH / 0.5 S 
32 CH / 0.125 S 
STRUCTURE OF 
MQIIERIITE STARS 
--Awl IL IART 0 XMCIING P.C. 0 1-DIM. HADAWAW) TEL. ' ALL SKY MWIITOR 
> 100 4 y  
- STERADIrn 
-I_-_-- --_ 
t 
1. 
70 c d  x 2 2 I m m m s  35m n 2  
1.7' x 37' zoo x 20" 
Q WR-Cey X - M Y  0 smrffi P.C. 
1co C J  x 2 
~ c o ~ n ~ ~  20- 1 2  
#wc I TOR 
80 CUI2 x 2 P 0 I -DIM.  REFLECTOR 
2 
226 
continue through 1990's. Besides th is  stream, we also envisage t o  carry out a 
larger X-ray astronomy mission than the planned capabil i ty of the !+launcher. This 
mission was or ig inal ly named the Cosmic X-ray and --ray Telescope (CXGT). Over 
past years, thc concept o f  CXGT has evolved. As X-ray astrophysics advances, more 
and more sensi t iv i ty and spectroscopic resolution are inevitably needed. It i s  
therefore logical t o  consider a focusing optics f o r  CXGT. 
Finally, I would l i k e  t o  mention that the government level agreement on the 
U.S.-Japan col laboration programs i n  space science including X-ray astronomy was 
made. We very much hope t o  enhance th t i e  through possible collaborations in  
th is  f ie ld .  
References 
Clark, G.W., Jernigan, 6.. Bradt, H., Canizares, C., Lewin, W.H.G., Li, F.K., 
Mayer, W. and McClir.tock, J.E. 1976, Ap.J. ( letters), 207, - L195. 
Grindlay, J.E., Marshall, H.L., Hertz, P., Soltan, A., Ueisskopf, M.C., Elsner, R.F., 
Ghosh, P., Darbro, W. and Sutherland, P.G. 1980, Ap.3. (Letters), - 240, 1121, 
Inoue, H., Koyama, K., Makishima, K. e t  al. 1980, Nature, - 283, 358. 
!noue, H., Koyama, K., Makishima, K. e t  al.  1981, Ap.3. (Letters), i n  press. 
Kelley, R., Rappaport, S. and Petre, R. 1980, Ap.J., - 238, 699. 
Lamb, F.K., Pines, D. and Shaham, 3. 1978, Ap.J., 224, %9. 
Lewin, W.H.G., Hoffman, J.A. and Doty, 3. e t  a l .  1976, H.N.R.A.S., - 177, 83. 
Lewin, W.H.G. and Joss, P.C. 1981, Space Science Rev., 28, 3. 
Wjrakami, T., Inoue, H., Koyama, K. e t  al.  1980, Publ. Astron. Soc. Japan, - 32, 543. 
Nagase, F., Hayakawa, S., Kunieda, H. e t  a l .  1981, Nature, - 290, 572. 
Nagase, F. 1981, ta l k  presented a t  15th ESLAB Symposium on X-Ray Astronaqy, 
Rappaport, S. and Joss, P.C. 1981, 'X-Ray Astronomy", ed. R. Giacconi, Proceedings 
June 1981, k te rdam.  
o f  the HEAO-AAS Meeting, January 1980, Cambridge. 


X-ray astronomy program in Japan

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