There have been reports in recent years of the possible observation of bursts in air shower data. If such events are truly of an astrophysical nature then, they represent an important new class of phemonenon since no other bursts have been observed above the MeV level. The spectra of conventional gamma ray bursts are unknown at higher energies but their observed spectra at MeV energies appear generally to exhibit a steepening in the higher MeV range and are thus unlikely to extrapolate to measurable fluxes at air shower energies. An attempt has been made to look for deviations from randomness in the arrival times of air showers above approx. 10 to the 14th power eV with a number of systems and results so far are presented here. This work will be continued for a substantial period of ime with a system capable of recording bursts with multiple events down to a spacing of 4 microns. Earlier data have also been searched for the possible association of air shower events with a glitch of the Vela pulsar

Blair, D. G.

Bruce, T. E. G.

Cinquini, P.

Clay, R. W.

Dawson, B. R.

Protheroe, R. J.

English

NASA Technical Reports Server

OG 5.3-10
266
Search for Bursts in Air Shower Data
Bruce, T.E.G., Clay, R.W., Dawson, B.R., Protheroe, R.J.,
* Blair, D.G. and * Cinquini, P.
Physics Department, University of Adelaide, South Australia, 5001.
* Physics Department, University of Western Australia
Western Australia, 6009.
I. Introduction There have been reports in recent years of the
possible observation of bursts in air shower data (e.g. Smith et al
1983, Fegan et al 1983). If such events are truly of an astrophysical
nature then they represent an important new class of phenomenon since no
other bursts have been observed above the MeV level. The spectra of
convential gamma ray bursts are unknown at higher energies but their
observed spectra at MeV energies appear generally to exhibit a
steepening in the higher MeV range and are thus unlikely to extrapolate
to measurable fluxes at air shower energies (see e.g. Clay et al,
1982). On the other hand, we now know that astrophysical objects are
indeed capable of producing ultra high energy gamma rays and we should
treat seriously the possibility of a burst acceleration mechanism.
We have looked for deviations from randomness in the arrival times
of air showers above ~ I0IW eV with a number of systems and results so
far are presented here. This work will be continued for a substantial
period of time with a system capable of recording bursts with multiple
events down to a spacing of 4 Us - Following the suggestion of Fegan et
al (1983) that their event may be related to a glitch of the Crab
pulsar, we have, also searched our earlier data for the possible
association of air shower events with a glitch of the Vela pulsar.
2. Detecting Systems Four detecting systems were used in this work.
Data from the earlier Buckland Park array was used to search for bursts
from the direction of the Vela pulsar. This system has been described
in detail elsewhere (Crouch et al [1981]). Data from the new Buckland
Park array with a substantially lower energy threshold has been searched
for evidence of any non-random component. This array is described in
this conference (Clay et al 1985a). We have also used two simple air
shower triggers employing, in each case, two scintillators in
coincidence. One detector pair was operated at Adelaide with a 5.6m
spacing (detector area 0.16 m2) and a median detected shower size of
Ne = 4 x 10W particles. The second system, operated at Perth, consisted
of two scintillators (area .07 m2) with a spacing of 6m giving a median
shower size of 3 x I0W particles.
3. Searches for any Non-Random Effect We have previously searched for
non-random effects in our air shower time spacing distribution by
testing our spacing distributions to see whether or not the exponential
form extended to small time spacings. A non-random effect associated
with bursts would be likely to result in an excess of small spacings.
Since we have no 'a priori' reason for expecting a particular burst time
scale, we have fitted exponentials (see Fig I) for each data set above a
OG 5.3-10 
6 
earch f r ursts i  ir ho er ata 
ruce, . .G., lay, . ., awson, .R., rotheroe, .J., 
 lair, . . a   inquini, . 
hysics epart ent, niversity f delaide, outh ustralia, 001. 
 hysics epart ent, niversity f estern ustralia 
estern ustralia, 6009. 
1. I t uction here ave ee  r orts i  t ears f, t  
o sible bservati n f ursts  ir s er at  ( . . ith t l 
3, e a  t l 83).   events r  l  f an st hysical 
at re  t  r sent  i portant  l  f e enon   
t er ursts e een bser ed e  e  l el.  ectra f 
nvential a  ursts r  n t i her nergies ut t eir 
bserved s ectra t e  ergies ear' enera l   hibit  
ning i  t  i her e  e  r  t  nli el  t  xtrapolate 
 easurable f s t i  er nergies (  . . lay t I, 
982). n t  t er a d, e   t at st hysical bjects r  
 apable f r ducing ltr  i  er  a r s d e ul  
t t ri sly  o sibility f  urst ccelerati n echanis . 
e have looked for deviations fro  rando ness i  the arri al ti es 
f ir s ers ove"" 1 14  it   u ber f s d r sults s  
r r  resented ere. his ork ill be nti ed f r  bstantial 
eri d f t e it   s st  capable f r r i g ursts it  ultiple 
ents    aci g f  ~s. oll i g t  s ggestion of egan t 
l ) t at t eir ent ay be r l t  t   li  f t  rab 
ulsar, e ave, l   r arli r ata r  po sible 
ssociati n f ir s er ents ith  lit  f t  ela ulsar. 
. etecti g yst s our etecti g s ere sed i  t is ork. 
ata f  t  arlier uckland ar  r  as sed t  r h r ursts 
f  t  ir cti n f  ela ulsar. his  as een escri ed 
i  etail l here r c  t l 81]). ata f  t   uckland 
ar  rr  ith  bstantia ly l er er  l  has een r ed 
f r i ence f y, non-random ponent. his r  i  escri ed  
t i  nfer ce ( l  t l 985a). e ave l  used t  si ple ir 
er t ers ploying, i   ase, t  i till t rs i  
oi cidence. ne etector air as perated t delaide ith  .  
ci g ( etector r a .  2 ) a d  edian et ct  er i  f 
e    4 articles. he  s st , perated t ert , onsisted 
f t  intill t rs   2) ith  aci g f  i i g  edian 
s er i  f   1 4 articles. 
. earches r  on- an  fect e a e r vi usly s r ed r 
on-r  ff cts i  r ir er t e ci  ist ti n  
t t  ur aci  istr ti ns t   hether r ot t  ~nential 
 t ed t  s a l t  aci gs.  on-r  ff ct ssociated 
ith ursts ould  l l   sult   cess f s a l aci gs. 
i ce e a e n  '  riori' r n r xpecti g  arti ular urst t e 
ale, e a e  xponentials  i  1  r ach ata t ve  
• 
https://ntrs.nasa.gov/search.jsp?R=19850025763 2020-03-20T18:01:26+00:00Z
OG 5.3-10
267
i000
I00
Fig I. Pulse spacing distribution
measured at Adelaide.
200 400
Time (s)
lower time spacing limit chosen subjectively by taking into account the
recording pulse rate and then compared the number of events expected
below this limit with the number actually observed. The results are
shown in table I.
Table 1
Experiment Spacing range Mean event Number exp. Number
(Recording- for fitted spacing below range obs. below
Time) exponential range
Buckland 150-1000s 312.6s 4193±63 4192
Park
(964 hrs)
Adelaide 50-I000s 103s 28732±106 28576
(2139 hrs)
Perth 400-4000s 977s 3139±43 3183
(2488 hrs)
One might also ask whether or nor there is any evidence for bursts
" in the data sets in terms of any series of small time intervals rather
than an excess in the total number of small time intervals. We have
examined our data and calculated for each system the number of times
expected for observing a series of 3, 4 or 5 successive intervals, each
below a certain minimum spacing. The results are shown in table 2.
Table 2
Experiment Number of times consecutive time intervals are <t.
(time-lnterval t) 3 intervals 4 intervals 5 intervals
expected observed exp. obs. exp. obs.
b
Adelaide (20s) 230 221 35.9 44 6.1 8
Buckland Park (2s) 1301 1270 196 245 30 41
Perth (200s) 62.7 71 11.8 I0 2.2 2
.. 
7 
i  1. lse s aci g istri ution 
easured  delaide. 
 
i  
lo er ti e spacing li it chosen subjectively by taking 
recording pulse rate and then co pared the nu ber of 
belo  this li it ith the nu ber actually observed. 
n  l  1  
able  
G 5. -10 
 
(5) 
i t  account the 
events expected 
he results are 
Experiment 
( Recording-
Time) 
Spacing range 
for fitted 
exponential 
ean event 
spacing 
u ber expo 
below range 
u ber 
obs. belo  
range 
uckland 
ark 
 rs) 
delaide 
9 rs) 
erth 
88 rs) 
-1 0s 
-10 0s 
0-4 0s 
2.6s 
s 
7s 
:1:63 2 
: :106 76 
:1:43 3 
ne ight als  ask hether r nor t ere is any evidence f r ursts 
in the data sets ~n ter s of any series of s all ti e intervals rather 
t an an e cess i  t e t tal nu ber f s all ti e i t rvals. e have 
examined our data and calculated for each syste  the nu ber of ti es 
expected for observing a series of 3, 4 or 5 successive intervals, each 
belo  a certain ini um spacing. The results are sho n in table 2. 
able  
xeeriment umber f ti es consecutive ti e i tervals are to 
(t e-interval t)  i tervals  tervals  i tervals 
expected observed expo obs. expo obs. 
delaide ( 0s) 30 21 5.9 4 .  8 
uckland Park ( s) 301 270 6 45 0 1 
erth ( 00s) 2.7  ll.  1  .2 2 
OG 5.3-10
268
It is apparent that our data showed no evidence of any non-random
effects with a total exposure of 5591 array-hours.
4. Discussion Previous experiments which have reported the observation
of UHE bursts have operated for periods of the order of a year. That
is, long monitoring periods are required. In order to design an
experiment to search for such bursts with the greatest efficiency in
terms of data processing effort and use of the available data, it is
instructive to examine tables 1 and 2. Bursts of the type detected by
Fegan et al. and Smith et al. would have been detected readily by the
technique employed in Table 2 but probably not at all by the method used
in Table I. A useful way of searching for bursts would then be to
minotor any short time intervals between air shower events and to search
for any periods which exhibit a series of such short time intervals. We
are now using a time interval measurement device which responds to pairs
of events with time spacings below 0.Ss (compared to a mean rate of one
per 9s with the new Buckland Park array) and records the occurrence of
such an event together with the spacing (in units of 4Bs). Bursts can
then be identified by the observation of a succession of such short
intervals. This is statistically powerful since the probability of
having many successive small intervals by chance falls rapidly with the
number of intervals.
5. The Association of Bursts with a Pulsar Glitch Pegan et al 1983
suggested that their observed burst may have been associated with a
pulsar glitch. The Vela pulsar is at a declination which is easily
observed from Adelaide and we have searched our 1979-1981 data
set (1.3 x 105 events) for all events within our angular uncertainty
arriving from the direction of the Vela pulsar (see Protheroe et al.,
1984). The result of this search is shown in Fig. 2. These data show
no evidence for any clumping. There is one day which contained three
events but such _an occurence has a probability of ~66% in a data set of
this kind. The Vela pulsar exhibited a glitch in this time period as
indicated but no closely correlated events stand out.
1979.0 " ,.
I
I
>- J "" _ ..
<I
r_
Fig. 2 The day of v ..
arrival of events from "
the direction of the
Vela pulsar, Each 0 "
column represents 75 " " I_O_-i_2.0days of data. The time
of a glitch is shown as " • • ..
g. . J.. •
L• ! ,.• .
8 
I OG .3-10 
It is apparent t at our data sho ed no evidence of any non-random 
ffects ith  t tal posure f  rray-hours. 
4. iscussion revious experiments hich have re orted t e bservation 
of UHE bursts have operated for periods of the order of a year. hat 
is, long onitoring periods are required. In order t  design an 
experiment to search f r such bursts ith the reatest efficiency i  
ter s f ata rocessing ffort and se f t e ailable ata, it is 
i structive t  a ine t les  d . ursts f t  t e etected  
Fegan et a1. and S i th et a1. ould have been detected readily by the 
technique e ployed in Table.2 but probably not at all by the ethod used 
i  able 1.  useful ay of searching for bursts ould then be to 
inotor any s ort ti e i tervals bet een ir sho er events and t  search 
f r any periods hich exhibit a series of such s ort ti e i tervals. e 
are no  using a ti e interval easurement device hich responds to pairs 
of events ith ti e spacings belo  0.5s (co pared t  a ean rate of one 
per 9s ith the new Buckland Park array) and records the occurrence of 
such an event together ith the spacing (in units of 4~s). ursts can 
t en be i entified by t e bservation of a s ccession f s ch s ort 
i t rvals. his is st tistically powerful si ce t e robability f 
having any successive s all intervals by chance falls rapidly ith the 
ber f tervals. 
. he ssociation of ursts ith a ulsar litch Fegan t l 1983 
suggested that their observed burst ay have been associat~d ith a 
ulsar lit h. he ela ulsar is t a eclination hich i  sily 
bserved  delaide  e ave r hed r 9-1981 ata 
t .   5 ents) r ll ents ithin r gular certainty 
rri ing fro  t e ir ction f t e ela lsar (see rotheroe t l., 
84). he sult f t i  rch i  n i  i . . hese ata  
no e i ence f r any clu ping. here is one day hich c ntained t r e 
ents ut s h 'a  curence s a r bability f 6% i  a ata t f 
t is i d. he ela lsar e hibited a lit h i  t is ti e eri d as 
indicated but no closely correlated events stand out. 
Fig. 2 The day of 
rrival f ents  
 ir ction f  
Vela pulsar. Each 
column represents 75 
days of data. The ti e 
f a lit h is sho n as 
g. 
.  
9 
198  
Uti b.,.i-1U
269
6. Acknowledgements Dr. G.W. Royle is thanked for providing information
on glitches of the Vela pulsar. This work is suported by the Australian
Research Grants Scheme.
References
I. Clay, R.W., Gerhardy, P.R., and Gregory, A.G. (1982) Ap and Sp.
Sci. _ 279
2. Clay et al (1985a) This conference. OG 9.4-4
3. Crouch, P.C., Gerhardy, P.R., Patterson, J.R., Clay, R.W. and
Gregory, A.G. (1981) Nucl. Inst and Meth., 179, 467
4. Fegan, D.J., McBreen, B., and O'Sullivan, C. (1983) Phys Rev Lett,
51, 2341-2344
5. Smith, G.R. Omgen, M., Bullet, E., and Standil, S. (1983) Phys Rev
D 28 1601-1603
6. Protheroe, R.J., Clay, R.W. and Gerhardy, P.R. (1984) Ap. J. (Lett)
280, L47
• 
OG 5.3-10 
269 
6. Acknowledgements Dr. G. . Royle is t anked for providing infor ation 
on glit es of t e ela pulsar. This work is suported by t e Australian 
Research Grants Scheme. 
References 
1. Clay, R. ., Gerhardy, P.R., and Gregory, A.G. (1982) Ap and Sp. 
Sci. 83, 279 
2. Clay et al (1985a) This conference. OG 9.4-4 
3. Crouch, P.C., Gerhardy, P.R., Pa terson, J. ., Clay, R. . and 
Gregory, A. . (1981) NUcl. lnst and Meth., 179, 7 
4. Fegan, .J., McBreen, B., and 'Sullivan, C:--( 983) Phys Rev Lett, 
51, 2341-23 4 
5. Smith, G.R. Omgen, ., Buller, E., and Standil, S. ( 983) Phys Rev 
D 28 1601-1603 . 
6. Protheroe, R.J., lay, R. . and Gerhardy, P.R. (1984) Ap. J. ( e t) 
280, L47 


Search for bursts in air shower data

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