IL ~NUOVO CIMENTO VoL. 4 C, N. 3 Maggio-Giugno 1981

Background of Gravitational-Wave Antennas

of Possible Terrestrial Origin - I (*).

:E. AMALDI~ ~E. COCCIA~ S. ~FRASCA~ I. MODE]~A~ P. I{APAG~A1NI a n d F. l:{ICCI

I s t i tu to di F i s i ea dell' U~dversitle - t~oma

Is t i tu to Naz iona le di 2' isiea Nueleare - Sez ione di R o m a

G . V . ~)ALLOTTII~O ~]2d G. PIZZELLA

I s t i tu to di F i s i ca dell' Univers i t~ - t~oma

Is t i t~ to 2Vazionale di ~ i s i c a Nuc lea te - Sez ione di R o m a

I s t i tu to P l a s m a hello Spaz io del C . N . R . - t~rascati

~). ]30.NIFAZI, C. COS~r U. GIOVA~IARDI (**)

V. IAFOLLA~ S. UGAZIO ~nd G. VA~NARO~-I

l s t i tu to P l a s m a nello Spaz io del CNI~ - lXrascati

(ricevuto il 31 Marzo 1981)

Summary. - - The data collected in May 1980 with two gravitational- wave antennas operated simultaneously, one ( M = 20 kg) in Rome, the other (M = 389 kg) in Frascati, show a few high-energy events recorded by the two stations at times which differ by a few seconds. The probabili ty for these <( coincidences >) to be accidental is of the order of 10 -4. Various considerations tend to exclude that these events are due to gravitational waves of extraterrestrial origin. In addition, a few types of trivial local disturbances have been excluded. A study of the occurrence times of the events recorded at Frascati shows the presence of two periods ( ~ = 53.1 and 54.7 rain) equal to those of the free oscillations of the Earth: 0 S+1 and 0S~ 1.

(*) The main points of this paper have been presented by E. A~ALI)I at the Workshop on Gravitational Radiation Detectors of the Tenth Texas Symposium on Relativistic Astrophysics, held in Baltimore (Maryland) on December 15-19, 1980. (**) U. GIOVANAI~I)I died in Geneva on September 2, 1980. All members of the group wish to express their deepest synlpaty for the premature death of such a gifted young physicist, agreeable collaborator and dear friend.

295

296 ~. A:MALDI, :E. COCCIA, S. FI~ASCA, I. MOD:ENA, P: ~ArAGNANI~ i~. RICCI, :ETC.

1 . - I n t r o d u c t i o n .

We have t r ied for the first t ime, f rom May 6 to May 8, 1980, to operate in coincidence our two cryogenic resonant detectors : the <~ tes t ~> an tenna in Rome, M = 20.3 kg (~), and the in te rmedia te ~ntenna in Frascat i , M = 389 kg (2).

The electronic chain, shown schematical ly in fig. 1, is in principle the same for bo th antennas , a l though the values of the pa ramete r s are different in the

h~ ] V(t)=a~(t)

4 - - . . . . . - ~

Fig. 1. - Block diagram of the gravitationM-wave antennas.

y(t)

two cases. A possible incoming gravi ta t ional- radia t ion burs t of ampl i tude ho and durat ion v~ (s) excites the f requency rR of the bar , whose ends v ibra te with an ampl i tude

- - V~(hor~)exp - -2 -~ t s i n ( o ~ t + ~ ) , ( 1 ) ~ ( t ) - - ze

where v is the veloci ty of sound in the ba r mater ia l (A1, v ~ -5 .39 .10 ~ m/s a t T < 4 K) and Q is the mer i t factor of the an tenna (bar + transducer) . This v ibra t ion is t ransfornlcd by the t ransducer (in our ease a piezoelectric ceramic)

into a voltage

(2) V(t) ------ ~ ( t ) ,

(1) ]~. AMALDI, C. COSMELLI, P. BONIFAZI, F. BORDONI, V. FEI~RARI, U. GIOVANAI~DI, G. VANNARONI, G. V, PALLOTTINO, G. PIZZ]]LLA and I. MOD:ENA: ~UOVO Cimento C, l , 341 (1978). (2) E. AMALDI, C. COSMELLI, S. FRASCA, I. MOD:ENA, G. V. PALLOTTINO, G. PIZZELLA, F. RICCI, P. BO~IIFAZI, F. BORDONI, V. FERRARI, U. GIOVANARDI, V. IAFOLLA, B. PAVAN, S. UGAZlO and G. VA~NARONI: 2r Cimento C, 1, 497 (1978). (3) :E. AMALDI and G. PIZZ:ELLA: The search ]or gravitational waves, in Relativity, Quanta and Cosmology in the Development of the Scienti]ic Thought o] Albert Einstein GNew York, N .Y. , 1979; Firenze, 1979).

BACKGROUND 01~ GRAVITATIONAL-WAVE ANTENNAS ETC. - I 2 9 ~

where ~ is the so-called coupling constant . The signal is amplified b y a low- noise F E T wide-band amplifier wi th wide-band noise b i la tera l vol tage spec- t r u m So(nV~/Hz), followed b y a selective amplifier A s and two-phase sensitive detectors (PSD). The PSDs are dr iven in quadra tu re by a synthe t izer (S) a t the resonance f requency of the antenna. I n the absence of signals their out- pu t s x(t) and y(t) are stochast ic variables. These quanti t ies are sampled wi th a sampling t ime

At -=-- to,

where to is the in tegrat ion t ime of the PSDs. Their values x(n.At) , y ( n . A t ) for n = 0, 1, 2 ... are recorded on a magnet ic t ape together wi th the Universal Time (UT).

Each an tenna has its own da ta acquisi t ion sys tem and clock. For the t es t an tenna in R o m e we have recorded only the da ta of its f undamen ta l longi- tudinal v ibra t iona l mode, for the in te rmedia te an tenna in Frasca t i we have recorded the da ta of its fundamen ta l mode as well as of its fifth harmonic , which has a f requency relat ively close to t h a t of the R o m e tes t antenna.

The selective amplifier As, the PSDs and the synthet izer S are different for the two frequencies vl and v5 of the M = 389 kg antenna.

Table I summarizes the ma in features of the two antennas. The t ime ~v = ~-- 2Q/eoR is the damping t ime of the vol tage signal of the bur (see eq. (1) and (2)). The t empera tu res Tn, To and Te~ , are, respectively, the noise t e m p e r a t u r e of the electronics, the equivalent t empe ra tu r e of the an tenna (equal to its the rmo- dynamic t e m p e r a t u r e T plus the increase due to the backreae t ion of the noise current of the F E T amplifier) and the effective t e m p e r a t u r e obta ined b y t rea t ing the ou tpu t da ta wi th the difference filter (4).

While the measured values of To given in table I are in good agreement wi th the corresponding computed values for Vl in R o m e and r~ in Frascat i ,

TABLE I . -- M a i n ]eatures of the two antennas.

v~ c~ Q zv Tn So Te Tel~ At W(vR) (Hz) (V/m) (s) (K) (nV2/Hz) (K) (K) (s) (db)

Rome 8672.482 2.81. l0 s 3.14.104 1.15 0.4 1.36 7.8 1.7 0.1 215 (1st mode)

Fraseati 1795.9349 5.78.107 6.77.105 120 0.20 1.22 9.1 1.2 1.0 240 (1st mode)

F r a s c a t i 7819 .4306 1.65. l 0 s 9 .83 .105 40 0 .20 0 .55 28.8 2 .5 1.0 ~ 230 (Sth mode)

(4) P . BONIFAZI, V. FERRARI, S. FRASCA, G. V. PALLOTTINO a n d G. PIZZELLA: N~OVO Cimento C, 1, 465 (1978).

9 .98 E . A ~ A L D I , E . COCCIA, S. F R A S C A , I . ~/IODENA, P. R A P A G N A N I , F. R I C C I , ETC.

t h e r e is a n a p p r e c i a b l e d i s a g r e e m e n t for v5 in F r a s c a t i due to a n in fe r ior me-

c h a n i c a l f i l t e r ing a t t h i s f r equency .

I n t h e l a s t c o l u m n of t a b l e I we g ive t h e m e c h a n i c a l f i l t e r ' s a t t e n u a t i o n

de f ined as t h e t r a n s f e r f u n c t i o n a t t h e r e s o n a n c e f r e q u e n c y b e t w e e n t h e ac-

c e l e r a t i on of t h e c r y o s t a t ba se a n d t h e a c c e l e r a t i o n of t h e p e r t i n e n t m o d e of

t h e ba r . These v a l u e s h a v e been o b t a i n e d b y c o m b i n i n g t h e r e su l t s of meas -

u r e m e n t s on t h e v a r i o u s p a r t s of t h e f i l ter cha in .

10 s

10 ~-

10 3

10 2 c

(80)

O o O �9

OO O0 0 �9 O0

�9 O0 �9 �9

10 ~ 0 0.5 1.0' 1.5 ~02(V 2)

Fig. 2. - Frequency dis tr ibut ion of the stochastic var iable @2 for the Rome antenna ( l s t harmonic of the longitudinal mode). The open circle with an arrow indicates the number of samples wi th @2> 1.25 V 2.

B A C K G I ~ O L V N D O F G R A V I T A T I O N A L - W A V E A ~ T ] ~ N N A S ]~TC~ - I 299

2 . - T h e i n d i v i d u a l p e r f o r m a n c e o f t h e a n t e n n a s .

The o b s e r v e d d i s t r i b u t i o n s ~) for t h e d a t a co l l ec t ed d u r i n g t h e n i g h t s

M a y 6-7 a n d 7-8, 1980 (for a t o t a l m e a s u r i n g t i m e c o m m o n to b o t h a n t e n n a s

10 s

N(e z)

10 4

10 3

10 2

101

~ ~ �9

�9 o O O o ~ o

o o �9 o o ~

�9 �9 ~ o o ~

(646)

0 ~ I I 10 0.5 1:0 1.5

~(v 2)

Fig. 3. - The same as fig. 2 for the 1st harmonic of the Frasca t i anCenna longitudinal mode.

t m = 94 080 s _~ 26.1 h) a r e s h o w n in fig. 2-4. The q u a n t i t y ~ p l o t t e d in ab-

scissa is de f ined b y

(3)

~2 2 2 ---- xd ~- Yd,

x d = x ( t ~ - At) - - x ( t ) ,

y~ = y ( t + At) - - y ( t ) .

3 0 0 lB. A~IALDI, ~. COCCIA, S. FRASCA, I. lYIODENA, P. ]~APAGNANI~ F. RICCI, ]BTC.

N(eZ) l

1 0 ~ _

103 C(1076) -

"% 1 0 2 ' %,

eo

�9 e �9

oo o

ee e �9 �9

~ e o �9 oe

10 ~ I ~ ~

10 0 I I 0 0.5 1.0 1.5

e 2 (v 2)

Fig. 4. - The same as fig. 3 for the 5th harmonic of the Fraseati antenna longitudinal mode.

This quant i ty represents a good estimate of the square of the force acting on

the bar in a narrow band near o~R. I n complete absence of disturbances the semi-logarithmic plot of the dis-

t r ibut ion of the values of e~ should be a straight line, the slope of which pro-

vides the measured value of Te~ ~. Figure 2 refers to the M = 20.3 kg antenna in Rome. The presence of disturbances clearly appears from the deviation

of the observed points f rom the straight line tha t begins above Q~ ~ 0.5 V s. The tota l number of points above ~2 z 1.25 V ~ is indicated by the circle with an arrow, l~igures 3 and 4 are similar to fig. 2 but refer to the first and fifth

harmonics of the Fraseati antenna~ which show, however, a n appreciably larger number of disturbances.

A detailed examination of these disturbances has shown tha t in a few cases

the Fraseati antenna is excited near and, sometime, up to the saturation level of the PSDs for intervals of t ime of the order of 1 or 2 rain. During all this

BACKGROUND OF GRAVITATIONAL-WAVE ANTENNAS :ETC. - I 301

t ime the variables % and y~ show the behaviour expected for a (almost) mono- chromatic signal, in par t icular t hey keep a constant phase relat ion (sic!). A single event of this t ype contributes to the open circles with one unit per second of its duration.

We have carried on a detailed s tudy of all these large and long events and found tha t their main features could be determined by the ins t rumenta t ion: when the selective amplifier A s is saturated, even by a low-frequency signal (say, for example, f rom a few tens of her tz to a few hundreds of hertz), the PSDs give a signal at the resonance f requency of the antenna.

We noticed also tha t , at least some times, these large pulses occurred more or less simultaneously with large pulses of the Rome antenna which, however, lasted always a short t ime (about i second). Therefore, we felt worthwhile investigating in more detail this point.

3. - Analysis o f the coincidences between large signals in the two antennas.

We repor t here on the coincidences between Frascat i pulses of ampli tude of the 5th harmonic with

(4) ~(~5) ~>10 000 K , sa tura ted

and Rome pulses with

(5) q~>100 K .

Conditions (4) and (5) are arbi t rary, bu t unbiased. They have been chosen in order to limit, for the moment , our considerations only to ve ry large signals.

TABLE I I . - L i s t o] the ~ r a s e a t i events .

k B e g i n n i n g o f e v e n t (a) q*(~l) q~(~5)

t~ ( U T ) t , (m in ) (K) (K)

1 6 d 19 h 20 r a i n 54 .9 s 0 S A T S A T

2 24 2 .4 3 .10 5000 18 000

3 21 16 9.9 115.25 600 S A T

4 23 11 50 .4 230 .93 1200 S A T

5 7 2 39 36 .4 438 .68 2400 S A T

6 3 34 17.4 493 .35 S A T S A T

7 4 25 34 .4 544 .63 640 S A T

8 20 20 19.3 1498.86 5000 10 000

9 23 44 26.3 1703.51 150 S A T

10 0 43 35.3 1762 .66 57 S A T :

(a) Beginning of the sampl ing interval At ~ 1 s in which the event appears.

20 - I1 Nuovo Gimenfo C.

302 E. AMALDI~ ]~. COCCIA~ S. FRASCA~ I. MODENA, P. :RAI?AGNANI~ F. RICCI~ ETC

Table I I shows a list of 10 Frasca t i events which fulfil the above selectio~x cri teria (4) wi th the value of U T corresponding to the beginning of the sampling in terval At ~ i s, in which the event appears . I n the last column we give for each of the ten (, 5 th-mode events ~) the largest value reached b y the corresponding ~(v~) (expressed in K). I n the forelast column we indicate also the largest va lue reached simultaneously b y the ~2(vl) of the 1st mode. These only in some eases correspond to p rominen t (( 1s t -mode event ~).

I n addi t ion to t,, we give also for each event the t ime

(6) tk - ~ tF (k ) - - tF(1)

expressed in minutes . These da ta will be discussed in sect. 5.

TABLE I I I . - L i s t o f the R o m e e v e n t s .

No. B e g i n n i n g o f e v e n t ( t ~ ) ( a ) q~ No. B e g i n n i n g o f e v e n t ( t ~ ) ( ~ ) q2

(UT) (K) (UT) (K)

1 6 d 1 9 h 1 9 m i n 1 .99s 6300 25 7 d 2 h 3 9 m i n 35 .09s 105

2 3.49 1250 26 3 18 3.59 285

3 21 15.69 1340 27 19 36.79 10700

4 23 50.29 258 28 42 10.29 1040

5 39 16.99 235 29 45 59.19 221

6 58 48.29 261 30 56 29.49 210

7 59 32.89 8120 31 4 17 40.59 117

8 59 37.59 1 500 32 45 24.29 196

9 20 9 31.19 184 33 6 13 32.89 105

10 23 38.29 2 150 34 16 58.29 115

11 59 4.39 14210 35 16 20 20.49 338

12 21 0 29.29 665 36 46 30.09 400

13 0 42.99 138 37 51 25.29 365

14 11 23.09 395 38 20 14 45.09 7740

15 16 12.89 5750 39 8 0 22 48.89 13000

16 23 11 52.39 1330 40 39 18.89 670

17 20 26.89 105 41 1 11 33.59 390

18 30 20.79 900 42 2 41 34.59 540

19 32 19.49 16 O00 43 50 46.09 670

20 7 0 5 17.79 120 44 4 37 18.79 230

21 35 56.19 110 45 51 45.79 128

22 38 51.79 540 46 5 0 10.39 260

23 48 14.39 142 47 6 15 29.79 145

24 2 33 6.29 284 48 30 14.39 600

(a) Beginning of the sampling interval At = 0.1 s in which the event appears.

B A C K G R O U N D O F G R A V I T A T I O N A L - W A V E A N T E N N A S E T C . - I 303

T a b l e I I I shows a l i s t of t h e R o m e e v e n t s w h i c h fulfi l t h e se lec t ion cri-

t e r i a (5).

T a b l e I V shows t h a t in 5 cases t h e va lue s of t h e t i m e i n t e r v a l

(7) ~t = t v - t~

is r e l a t i v e l y smal l .

W e m a y n o w c o m p u t e t h e p r o b a b i l i t y for t h e s e f ive e v e n t s to be a c c i d e n t a l

as a f u n c t i o n of t h e r e so lv ing t i m e t~e ' a d o p t e d for t h e i r c o m p u t a t i o n . To b e

TABLE IV. - Coincidences.

No. in Frasca t i No. in Rome dt (s) (4- 0.5 s)

1 3 - -20 .8

2 4 + 12.1

3 15 - - 3.0

4 16 - - 2.0

5 25 + 1.3

m o r e prec i se , we cons ide r two e v e n t s (one in R o m e a n d one in F r a s c a t i ) to b e

(~ in co inc idence ~) if t h e y occur a t t i m e s such t h a t

(s) t v - tro~<t~<t~ + to .

W e s t a r t b y c o m p u t i n g t h e n u m b e r of a c c i d e n t a l co inc idences e x p e c t e d w i t h i n

t h e m e a s u r i n g t i m e t :

N.N~ (9) nRv - - 2tro.,

tm

w h e r e t m = 94080 s. A c c o r d i n g to t a b l e s I I a n d I I I , Nv = 10 a n d -~VR = 48

for ~2~100 , IVR----14 for 0~>1000 . I f we a s s u m e a Po i s son d i s t r i b u t i o n fo r

t h e r e c o r d e d even t s , t h e p r o b a b i l i t y of o b s e r v i n g a n u m b e r of co inc idences

> ~ g is g i v e n b y t h e exp re s s ion

Y--1 n~v exp [ - - nnv] (10) / ) (~>N) = 1 - - ~ k! '

k=O

which , b e c a u s e of (9), is a f u n c t i o n of t h e a s s u m e d r e so lv ing t i m e tro 8. T h e

r e su l t s of such a c o m p u t a t i o n a r e g i v e n in t a b l e V for t w o w i d e l y s e p a r a t e d

v a l u e s of tres a n d t w o va lues of t h e R o m e a n t e n n a t h r e s h o l d .

The p r o b a b i l i t y (10) t u r n s o u t to be smal l in a l l cases.

304 E. AMALDI~ E. COCCIA~ S. FRASCA~ I . /~,IOD:ENA, P . R A P A G N A N I , F . RICCI , ETC.

TABL]~ V. - Probability /or the nnF observed coincidences to be accidental.

t~e~ ~2 > 100 K (a) ~2 ~> 1000 K (~) (N n = 48) (_Am = 14)

nn2, P nnr P

4- 3 s 3 5-10 6 2 4-10 5

4- 30 s 5 2- 10 -5 3 1 . 1 0 -4

(a) Q~ is t he R o m e th re sho ld .

F i n a l l y , as a p p e a r s f r o m t a b l e I I , also t h e e v e n t s of t h e 1s t m o d e of t h e

:F rasca t i d e t e c t o r a r e in co inc idence w i t h t h o s e f r o m t h e R o m e de t ec to r , a t

l e a s t for a v a l u e of ~2(vl)>600 K .

4 . - P o s s i b l e s o u r c e s o f d i s t u r b a n c e s .

B e c a u s e of t h e s m a l l v a l u e of t h e p r o b a b i l i t y for t h e oc c u r r e nc e of t he ob-

s e r v e d co inc idences to b e a c c i d e n t a l , we h a v e e x a m i n e d al l sources of dis-

t u r b a n c e s we cou ld t h i n k of as t h e i r poss ib le or ig in ( t ab le VI ) .

T A B L E V I .

Possible sources of disturbances Agency (*)

power line ENEL (a)

atmospheric conditions

airplane traffic

Ciampino airport

seismic phenomena , ING (b)

cosmic-ray bursts SVIRC0 (r

geomagnetic storms ING (b)

(a) E N E L = E n t e Naz iona l e pe r l ' E n e r g i a E le t t r i ca . (b) I N G = I s t i t u t o N a z i o n a l e di Geoflsica. (0 S V I R C 0 = S taz ione Var i az ione I n t e n s i t k R a g g i Cosmici .

N o d i s c o n t i n u i t y , even v e r y smal l , ha s b e e n f o u n d in t h e r ecords of t h e

v a r i o u s q u a n t i t i e s l i s t e d in t a b l e V I , t a k i n g p l a c e n e a r t h e 5 co inc idences g i v e n

in t a b l e IV. W e h a v e also ve r i f i ed t h a t w e a t h e r cond i t i ons were v e r y good

(no t h u n d e r s t o r m s or l i gh t in ings ) , a n d no p l a n e was f ly ing n e a r R o m e a n d / o r

F r a s e a t i a t t h o s e t imes .

(*) We express our thanks to the agencies l is ted in the table for their kind collab- orat ion.

BACKGROUND OF GRAVITATIONAL-WAVE ANTENNAS ETC. - I 305

5. - Possible correlation of the signals with the free oscillations of the Earth.

I n a n I n t e r n a l R e p o r t of our I n s t i t u t e we h a v e a l r e a d y p r e s e n t e d l a s t

J u l y (5) t h e r e su l t s a n d c o n s i d e r a t i o n s g i v e n in t h e p r e v i o u s sec t ions of t h i s

p a p e r .

S ince t h e n we h a v e n o t i c e d t h a t t h e i n i t i a l U T of t h e t e n <~ F r a s c a t i e v e n t s )~

tk (k = 1, 2, . . . , 10) seems to fo l low a r e g u l a r p a t t e r n .

0.5

{;:at I

&

P = O . 4 = 0 . 0 7 =0 .08 = 1 0 - ~ =0 .08 = 1 0 - * =0.06 =0.04

|

- - 0 . 5 �9 I I I I I I t 58 57 56 55 54 53 52 51

J - ( m i n )

Fig. 5. - Plot of the variable (11) for a few values of the free parameter 3- (period): the spread of the exper imental points is small for J - = 53 and 55 min. The prob- abi l i ty P is defined by relat ion (13).

Such a r e g u l a r i t y is shown, for e x a m p l e , in fig. 5, whe re for 8 va lue s of t h e

p a r a m e t e r J - ( exp res sed in m i n u t e s ) we h a v e p l o t t e d t h e n u m e r i c a l v a r i a b l e

~k (11) r~ = ] - - n k .

The n u m b e r n~ is t h e i n t ege r t h a t , for a f ixed v a l u e of ~- , m i n i m i z e s t h e s p r e a d

(5) E . AMALDI, P. BONIFAZI, F. BORDONI, E. COCCIA, S. ]~RASCA, F. FULIGNI, U. GIO- VANARDI, V. IAFOLLA, I. MODENA, G. V. PALLOTTINO, G. PIZZELLA, ]{. I~APAGNANI, F. RICCI, S. UGAZIO a n d G. VANNARONI: N o r a In te rna de l l ' I s t i tu to di Fisiea G. Marconi dell 'Universit~ di Roma; I N F N , Sezione di Roma, 25 Luglio 1980.

3 0 ~ E. AMALDI, E. COCCIA, S. FRASCA~ I . I~IODENA~ P. RAPAGNAI~I~ :F. RICCI , ETC

of the ~ values:

(12)

Figure 5 shows tha t , for 3----- 53 an4 55 mia , the spread A(3-) is smaller t h a n for the other values and tha t , for example, A(58) covers 90~o of the whole possible interval .

The probabi l i ty t h a t 10 pulses fall b y chance within a f ract ion AI(3-) of the in terval is given b y

(13) P ~ = (A(3-))9.

The values of P x are also given in fig. 5 for each value of 3-. The small values of P for 3-:-- 53 and 55 min suggest the presence of these two periods, which are very close to those of the graves t free oscillations of the E a r t h (3 -=53 .1 min and ~-~-54.7 min) (6). I f we dare to do a step forward and assume t h a t our Frasca t i events are due to the E a r t h oscillations, the coincidences wi th the R o m e detector are natura l ly explained. I t remains not clear whether the observed delays ~t (table IV) are due to the p ropaga t ion of a vibrat ion th rough the E a r t h (Romc-Frasca t i distance ~ 20 kin) or to the lack of infor- ma t ion on the ins tan t of m a x i m u m signal in the Frasca t i detector due to the sa tura t ion of the amplifier output .

I n conclusion, we consider tha t the analysis of the events examined above suggests t h a t they are correlated to the oS2 spheroidal free oscillation of the :Earth. Such a correlation requires, however, some not ye t identified mech- anism which provides the link between the slow oscillation oS2 of the E a r t h and the e x c i t a t i o n - - a t some well-defined p h a s e - - o f our exper imenta l devices. The origin and na ture of such a mechanism is still unclear. I t could belong to anyone of the three following classes:

a) A pure ly ins t rumenta l effect such as a v ibra t ion produced inside the c ryos ta t by the very slow mot ion of the Ea r th , which sa tura tes (or a lmost saturates) the ou tpu t of the amplifier. Such a mechan i sm seems unlikely.

b) A mechanical v ibra t ion propaga t ing th rough the Ear th , which passes th rough the mechanical filters placed below and inside the c ryos ta t (7), reaches the bar and excites the longitudinal modes 1 and 5 of the Frasca t i bar and the longitudinal mode 1 of the Rome bar. This type of signal should bring the

amplifier A s to saturat ion.

(6) See, for example, H. BE~IOFF, F. PRESS and S. SMITtI: J. Geophys. Res., 66, 605 (1961). (7) The saturation of the amplifier can be produced by two subclasses of vibrations: low frequency (i.e. in the region of a few tens of hertz) or acoustical frequency.

B A C K G R O U N D O F G R A V I T A T I O N A L - W A V E A N T E N N A S ~ T C . - I 307

c) F i n a l l y we c a n n o t exc lude t h e fo l lowing u n l i k e l y m e c h a n i s m : a g r a v -

i t a t i ona l - f i e l d v a r i a t i o n due to a t e r r e s t r i a l i n n e r source t r i g g e r e d b y t h e 0S2

v i b r a t i o n of t h e E a r t h w h e n e v e r i t r eaches a r a t h e r we l l -de f ined phase .

A m o r e d e t a i l e d a n a l y s i s of t h e s a m e d a t a as wel l as of d a t a co l l ec t ed in

o t h e r p e r i o d s of t i m e is in p rogress . W e h a v e r e c e n t l y s t a r t e d t o col lec t n e w

e x p e r i m e n t a l i n f o r m a t i o n w i t h t h e hope of c l a r i f y i n g some of t h e a s p e c t s of

t h e o b s e r v e d even t s .

T h e i n t e r e s t of such a n i n v e s t i g a t i o n in v i ew of i t s poss ib l e f u t u r e con t r i -

b u t i o n to a d e e p e r u n d e r s t a n d i n g of t h e E a r t h ' s m o v e m e n t s a n d i t s i m p o r t a n c e

~s a b a s i c i n f o r m a t i o n for a n y sea rch of e x t r a t e r r e s t r i a l sources of g r a v i t a t i o n a l

r a d i a t i o n a p p e a r s to b e b e y o n d a n y d o u b t .

This w o r k has b e e n s u p p o r t e d f i nanc i a l l y b y t h e C N R for t h e F r a s c a t i

a n t e n n a , a n d b y t h e C N R a n d t h e I N F N for t h e R o m e de t ec to r .

W e express o u r t h a n k s t o ou r co l leagues F . BORDONI a n d F . Fu~m~r fo r

t h e i r v a l u a b l e c o n t r i b u t i o n s to t h e c o n s t r u c t i o n of t h e d e t e c t o r s a n d for c r i t i c a l

d i scuss ions a n d to P ro f . S. LESCHIUTTA of t h e I s t i t u t o E t e c t r o t e c n i e o N a -

z iona le Gal i leo F e r r a r i s (Tur in) for p r o v i d i n g t h e U T clock.

W e express also our t h a n k s for t h e i r t e c h n i c a l he lp to Mssr . G. MARTINELLI

a n d P . ~APOLEONI Of t h e C N R , a n d to Mr. E . SERRA~I of t h e U n i v e r s i t y

of R o m e .

�9 R I A S S U N T O

I dat i raecolti durante una serie di misure eseguite in Maggio 1980 con due antenne gravitazionali , funzionanti contemporaneamente, una ( M = 20kg) a Roma, l ' a l t ra (M ~ 389 kg) a Frascat i , mostrano alcuni eventi di e levata energia regis trat i nelle due stazioni a tempi ehe differiseono di qualche secondo. Le probabil i t~ che queste (( eoincidenze ,) siano casuali 6 dell 'ordine di 10 -4. Varie considerazioni tendono ad eseludere che questi eventi siano dovut i ad onde gravi tazional i di origine extraterrestre. Inol t re var i t ip i di dis turbi locali presi da noi ill considerazione sono s tat i esclusi. Uno studio dei tempi di occorrenza degli eventi r i levat i a Frasca t i mostra la presenza di due periodi ( ~ = 53.1 e 54.7 rain) eguali a quelli delle oseillazioni libere della Terra: S+ 1 o 2 e o S ~ 1.

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