The detection of 1000 GeV gamma-rays with the characteristic 6.1 ms periodicity of the radio pulsar PSR 1953 +29 is reported. This result, significant at the 5.4 beta level, provides the first direct evidence for the association of the 6 ms radio pulsar PSR1953+29 with the gamma-ray source 2CG065+0. Extensive observations of the 1.5 ms pulsar PSR 1937 are also reported

Chadwick, P. M.

Dowthwaite, J. C.

Kirkman, I. W.

Mccomb, T. J. L.

Orford, K. J.

Turver, K. E.

English

NASA Technical Reports Server

OG 2.3-12
161
I000 BmV BAMMA RAYS FROM ms PULSARS
P.M.Chadwickt J.C.Dowthweite, I,g.Kirkman,
T.J.L.McComb, K.J,Orford, and k.E.Turver
Department of Physics,University of Durham,
Durham DHt 3LE_ England,
ABSTRACT
We here report the detection of 1000 BeY Y-rays with the
characteristic 6.1 ms periodicity of the radio pulsar PSR
1953 +29 '_, This result,.significant at the 5.4 _ level,
provides the first direct evidence for the association of
the 6ms radio pulsar PSRI?53+29 with the Y-ray source
2CG065+0. Extensive observations of the 1.5 ms pulsar PSR
1937 are also reported.
i. INTROOUCTION,
Observations of PSRI95]+29 and PBRI937.21 were made between 19B3 fipril
and 19B4 September using tilearray of four telescopes of the Dugway VME
Y-ray facility which have been described elsewhere `=) . The telescopes
have an energy threshold dependent on zenith angle of approximately 1
TeV. The duration of the individual observations varied between 1 and
hrs during which the pulsar was observed with all telescopes in the
trackinq mode. The time of arrival of a Cerenkov light flash was recorded
in Coordinated Universal Time (UTC) to an absolute accuracy of 0.5 ms and
with a resolution of I _s, The time of each event has been converted
from UTC to Barycentre Corrected Oulian Ephemeris Time using the NIT
so]mr system ephemeris '_ and the position of the radio pulsar. A
further correction to allow for the orbital motion of the PSR 1953 has
been made by adjuJting the times to the focus of the binary orbit
according to radio measurements c,, which are summarized in Table I.
RA I l?h 53m 26.673s
DEC I 29 ° O' 44.1"
EPOCH t 244542B.66
d t 117.3 + <0.1 d.
• : <O,O01-
p s 6.133166 ms,
asin(i) s 31.29 Is.
TABLE I.
G .3-12 
61 
1  GeY G   OM l ARS 
. . hadwick, J. . owthwlite, I.W.Kirkman, 
.J. . cComb, .J.Orford, and K.E.Turver 
epartment of hysicl,University of urha., 
urha  l , gland. 
TRACT 
e re report t  etection f  Ge  -rays ith t e 
haracteristic b.1 IS eriodicity f t e r dio lsar R 
1953 +29 C1 ). his result, significant at the 5.4 ~ level, 
rovides t e irst irect idence f r t e sociation f 
the 6 s radio pulsar PS 1953+29 ith the -ray source 
065+0. ~ ensive servations f t e .5 s lsar  
1937 are also reported. 
1. D . 
servations f 1953+29 a  S 1937+21 ere ade et een 8  April 
 8  t ber si  tne rr  f f r t lesc s f t  a  H  
-ray acility hich   scribed lsewh re C2 ).  t lescopes 
have an energy t reshold dependent on zenith angle of approximately 1 
.  ration  e ndividual servations aried t en   6 
hrs during hich the pulsar as observed ith all telescopes in the 
trackinq ode. The ti e of arrival of a .renkov light flash al recorded 
in ordinated niversal i e ( ) t  a  solute racy f .5 s  
ith a resolution f 1 ~s. he ti e f eac  e ent as ee  c verted 
fro  UTC to 8arycentre orrected J lian phemeris i l uling t . MIT 
s lar s ste  e e eris (3) a  t e sition f t e radio lsar.  
urther rection  low r e Orbital tion  e   l 
been ade by adj~ltinq the ti es to th. focul of the binary orbit 
according to radio easurements C4) hich are su arized in able I. 
A 
 
CH 
 
e 
p 
sin(i) I 
 1  
19h . .673s 
D ' 4.P 
5428. 6 
.3  0.1  
0.0 1-
. 331 6 i. 
. 29 s. 
https://ntrs.nasa.gov/search.jsp?R=19850025635 2020-03-20T17:54:31+00:00Z
162 OG 2.3-12
2.RESULTS - (A) PBR 1953 - The 6 ms Pulsar,
A homogeneous dataset was compiled comprising.only those observations
of at least 4 hours duration. This enables the results of the periodicity
searches on 8 individual observations to be combined with equal
statistical weights, The uncertainty in the binary orbit causes
progressive phase errors and precludes the use of data=elm of 24 hours or
more, in particular the assembly of a single large dataset spanning the
whole observation period. Events were selected which triggered any two of
the four telescopes (this has been shown '=._) to increase the
sensitivity of the telescope array to Y-rays since twofold telescope
responses define a narrower effective aperture). The PBR 1953 data=el
(comprising 14284 events in all) was obtained during the half of the
orbit when the pulsar was approaching the earth, but spanned a year, or
approximately three orbits. Each individual observation was tested for
periodicity over a restricted range of period (6.133162 - 6.133170 ms}.
This limited searching for periodicity was necessary to allow (with
decreasing effect) for (a) uncertainties in the orbital ephemeris, (b)
the effects of statistical sampling on a periodicity in sparse data, and
(c) residual uncertainties in the precise rate of the stabilised IMHz
system clock, The Rayletgh test ,a.=_ was selected as the appropriate
test for searching for periodicity in the absence of specific knowledge,
or a reasonable guess_ of the light-curve duty cycle. The test is
Uniformly Most Powerful for distinguishing between the alternative
hypotheses of the fundamental power being zero or non-zero.
Each of the eight observations was analysed independently and the
eight probabilities of chance occurrence of each trial period were
combined ,7_. This combined probability is shown as a function of the
trial period in Figure 1, A 3.5% periodic excess is found within the
trial period range, which has a probability of chance occurrence of
1,6x10-". The number of independent trial periods in the range=
P1=6.133162 ms to P==b.133170 ms is (T/P= - T/P=) = 3.4 (T = mean
duration of an observation), increasing the probability of the detection
being due to chance to 5.4x10 -=. Further independent data from the
single-telescope responses is available. Such responses are less
sensitive to Y-rays because of the larger aperture which allow the
registration of more proton-induced showers. However, one of the four
telescopes had been equipped with new mirrors and has a narrower field of
view to improve its signal/noise to approximately the same as
two-telescope responses. This data=st, comprising 17302 events,
independently shows a 3.2% pulsed signal at a period within the sampling °
range of the periodicity shown by the 2 telescope responses. This signal
is at a significance level of 4x10 -= - see Figure 1. With a3.5% signal
strength in narrow aperture detection systems we would expect the other .
three wider aperture telescopes to have a signal/noise ratio of about
1,5% and hence not show significant effects, This is seen to be the case,
The overall probability of the effect being of chance origin is therefore
about 4x10 -= which corresponds to a 5.4_ signal.
82  .  
.   SR    IS llar. 
 e e s taset as c piled c prising,only t se servations 
of at least 4 hours ration. his enables the results of the periodiCity 
searches on B i ividual servations t  be co bined ith equal 
tatiltical lights. . ertainty i  t  inary rbit ses 
rogressive s  rrors a  r cludes t e s  f tasets f  rs r 
lore, in particular the asse bly of a single large dataset spanning the 
le servation eriod. ents ere selected ich tri gered any t o f 
t  ur lescopes this    CD,.)  crease t e 
sensitivity of the telescope array to V-rays since t ofold telescope 
res onses efine a arrower ffective erture). he PSR  taset 
(comprising 6 e ents i  ll) as tained ring t e lf f t e 
rbit hen the pulsar as approaching the earth, but spanned a year, or 
proximately t r e rbits.  i dividual servation  t sted r 
riodicity r i estricted nge  iod / . 33162 . 33170 s). 
is limited s rching f r riodicity WII cessary t  llow ( ith 
decreasing effect) for (a) uncertainties in the orbital ephemeris, (b) 
t  ff t  f t ti ti l s pli    ri icit  in s rs  lt , a  
( ) r i l certainti s in t  r cil  t  f t  t ili  lH  
sYlte  clock. ht ayltigh test C6,81 as selected as the appropriate 
test for searching for periodicity in the absence of specific kno ledge, 
or a reasonable guess, of the light-curve duty cycle. he test is 
niformly ost Po erful for distinguishing bet een tht alternative 
hypotheses of the fundamental po er being zero or non-zero. 
ach of the eight observations as analysed independently and the 
eiQht robabilities of chance occurrence of each trial period ere 
c bined (7'. his c bined robability is sho n as a function f t e 
trial period in Figure 1.  3.~X periodic excess is found ithin the 
trial riod range, hich as  robability f ance currence Df 
1.6xlO-D• he nUlber of independent trial periods in the rangel 
, .1 3162 IS  2 6.1 3170  . IP, / 2)  .4    
duration of an observation), increasing the probability of the detection 
eing e t  c ance t  ~ 4x10 D  urther independent ata fr  t e 
Singl -telescope responses is ailable.  responses re le s 
sensitive to V-rays because of the larger aperture hich .llow the 
registration of ore proton-induced showers. o ever, one of t e four 
telescopes a   i ped ith ne  irrors I  s  arrower field f 
ie  t  i prove itl signal/noise to a proximately t e sa e as 
t o-telescope responses. his ataset, c prising 2 e ents, 
independently s s a , X ulsed signal at a eriod ithin t e sa pling 
range of the eriodicity sho n by the 2 telescope responses. his signal 
is at a Significance level of 4xl0-~ - see igure 1. ith a 3.SX signal 
strength in arrow perture etection s stems e ould eKpect the ther 
three ider aperture telescopes to have a signallnoise ratio of about 
1.5  and hence not sho  significant effects. This is seen to be the case. 
he verall robability f t e effect eing f c ance rigin is therefore 
out xl0- a hich o responds to  .4a ignal. 
163
OG 2.3-12
!
r I i
H _ _M
l IO"
l - -[
"I
I I I Z *o
&,l_li2 & 1331_ 6.133170
I
P_RXOD imp) O l.O
Fiqure I i The probability of chance
occurrence as a function of the trial PHA_ (AR_Z_ARy)
per|od. The horizontal bar shows the
expected position o# the radio period *
Illow|nl for the uncertainties in the Figure 2 I The light curve of VHE _riy
emission on 19B3 August 6.binary orbit. Tha solid line refers to
the two-telescope responses and the
broken line to the independent simple
from the single (sensitive) telescope
response.
The Y-ray signal has been investigated on a night-to-night basis
during the eight longest observations. The average signal (i-rays)/noise
(cosmic ray protons in the field of view) for a 2-telescope response is
3.5% and fluctuates from below the background noise level to 8.5X. No
pattern is apparent in the variations in that the signal averaged over
observations which are at similar orbital phases remains at about 3.5%,
although in some cases the observations are a year apart. Assuming that
the Y-ray samples have a Van Mimes distribution, we can test for
constancy of the signal from night to night using an established test '¢_.
We find that the variation of signal strength from night to night is
consistent with sampling variations from a constant signal strength, but
cannot preclude variations from zero to about three times the average
strength. A similar test of the data on a time scale of about 30 mini
also thaws no evidence of variation. We can however exclude as the source
of the present effect the type of strong (_30X) but brief (_few minutes
duration ) outbursts detected by us from the Crab pulsar '*' and from
Hercules X-I'_,. The time-averaged flux of Y-rays of energy in excess of
2 TeV is 3xlO-_' cm-= i-=. The VHE Y-ray luminosity, assuming a
distance to the pulsar of 3.5 kparsec and a differential power law index
of 3.0 is 3xlO=" lrgi i -=. The light curve for the 2-telescope responses
on the occasion of the apparently strongest signal (1983 August 6) has
been derived for the radio period (6.133166 ms) and is shown in Figure 2,
t. 
tot 
J 
tot 
II 
« 
II 
0 
t: 
a. 
III' 
U 
Z 
« 
I 
U 
63 
 -12 
l- IDO 
" , \ 
I , 
I 1- I 80' 
I 
I 
I 
10-' . I 
J 
60 
6.1~1.2 •• d 1" •• 133170 
PER%CD ( ., 0 
Figur. I I Th. probability of chanci 
rrtnc ...  f nction f t  trial ASE RB%TRARV) 
l iod. l rizontal l   l 
,xplctld pOlition of the radio period • y 
Illowina for the unclrtlinties in thl Figure 2 I Th. light curve of VHE g-r.v 
binary orbit. Thl solid line rlflrl to I~ill on  J u   
thl t o-tiIISCOPI reiponsis and thl 
broken lin. to the Ind.p.ndtnt li~pll 
fro. the linall (sensitive' telescope 
flspOnlt. 
1.0 
The -ray signal has been investigated on a night-to-night basis 
during the eight longest observations, The average signal (Y-rays)/noi~e 
(cosmic rlY protons in thl field of view) for a 2-telescope responle il 
3.5% and fluctuates from below the background noise level to 8.5%. No 
pattern is apparent in the variation, in that the signal averaged over 
observations hich are at si ilar r ital phases re ains at about 3.5%, 
although in so e Cisel the observations are a year apart. Assu ing that 
the -ray sa ples have a Von ises distribution, we can test for 
constancy of the signal from night to night using an established test'SI. 
e find that the variation of signal strength from night to night i. 
consiltlnt ith lampling variations from a constant signal strength, but 
cannot preclude variationl fro  zero to about three ti es .the average 
• strength. A si ilar test of the data on i time scale of about 30 ini 
so IhoWI  dence  ation. e n r lude  he ce 
of the present effect the type of strong (~30y' ) but brief (~few inutes 
duration) outbursts detected by us fro  the rab pulsar C., and fro  
Hercules X-l'91. The time-averaged flux of Y-rays of energy in excesl of 
2 TeV is 3x10- 11 c - 2 S-I. he VHE -ray lu i sity, assuming a 
distance to the pulsar of 3.5 kparsec and a differential power law index 
of 3.0 il 3x10:S D ergs I-I. The light curve for the 2-telescope responses 
on the occasion of the apparently strongest signal (1983 ugust bl has 
been derived for the radio period (6.1331bb s) and is shown in Figure 2. 
164 ,_:
OG 2.3-12 :
RESULTS (B) PSR 1937 - The 1.5 ms Pulsar,
Between 19B3 April 13 and 19B4 September 29 PSR 1937 was observed on
46 separate occasions for a total of 129 hrs durtng which 301597 Cerenkov
light flashes were recorded. The extremely short period of the pulsar
precludes the possibility of maintaining phase and combining data
recorded on separate nights as a separate dataset, The data from each
observation have been analysed for periodic content using the Rayletgh
test. No evidence for a pulsed content characterized by a broad light
curve has been found. A 3e flux limit equal to 1.5% of the cosmic ray
background (a Y-ray flux limit of 5xlO-'lcm-=s-l) has been derived, The
analysis of the data continues with a search for a pulsed emission
contatn|ng a very much shorter duty cycle.
This work was funded by the United Kingdom Science and Engineering
Research Council and carried out at Dugwiy Proving Bround t Utah, We are
indebted to the Commander and Staff of Dugway Proving 6round who
supported our experimental programme and Professor R. Bucchert who
provided the data on the 6 ms radio pulsar,
References
1. Bortakoff. V.t it all 19831 Nature1 3041 417-419.
2. Dowthwaitel et el, 19831Aitro. Astrophyt., 1261 1-6. i!
3. Ash1N.E.t Shapiro1 I.I.t and Smith, W.S.1 19671Aitron. J•l i
hi
7_22,33B-350. _,!
4, Succhert, R.,(1985), private communication. '._:
_. Dowthwatte t OtC, it al I 19841Astrophys. O.i 28&, L35-38. _i_i
6. BlbSOnl A.I.t et allgSll Nature1 29__6t S33-S35.
7, Eadiel W.T i it alt 19711 "Statistical Hethodi in Experimental '_
Physics", North-Holland1 283
S. Nirdti,K.,lg721 "Statistics of Directional Data", Academic Press
(London and New York).
?. Dowthwatte, S.C.1 eL all 19641 Nature, 30__91691-693.
10. ChadwiCk1P.H.t et el, 19651 submitted to Rstro. Astrophys. Letters• "
11. Chadwick, P.H., et al,t?S51iubmitted to Aitro. Astrophys. Letters.
12. Samorskit H. and Stamml W.I 19fl31Astrophye. O.(Letters)126_._SB_ L17-L21
13. Lloyd-Evani, a.et el, 19831 Nature13051 784-767.
14 
 -12 
S    . ~ .1 lslr. 
t e  8  pril  I  8  t ber  R  WIS ed  
 s plrate ccasions for I t tal f  rl ring ic   ere ov 
light fllshes ere r corded. e tremely s ort riod f t e lsar 
precludes the ssibility of ~aintlinlng phase and co bining data 
r corded  l irate ightl il  l arate taset.  lta ro~  
observation have been analYled for periodic content using the ayleigh 
t lt. o . i ence f r I lsed ntent aracterized   rold light 
rve l  und.  a lux imit I l  . Y.  l ic  
ground  V-ray lull hit  1110- 11cl 2 s-1) IS  rived. h  
aly is f t e ti continues ith  s rch f r  lsed I ission 
containing a very luch I orter duty c cle. 
il r  WII f ed  t e nited ingd  cience a  gineering 
esearch ouncil and carrild out at ug ay Proving Ground, tah. e are 
i debted t  t e o ander and taff f ug ay r ving Ground ho 
s orted r e perimental progra e and rofessor . uccheri ho 
rovided t e ta  t e  l radio lsar. 
ferences 
1. oriakoff. ., et aI, 1983, ature, 304, 417-419. 
2. Dowthwaih, It ai, 1983, Astro. Astrophy •• , 126, 1-6. 
. s , H.E., apiro, 1010,  IIIith, .B., 7, Astron.J., 
72, 338-350. 
. Bu cheri, .,(1985), rivate unication. 
5. o thwa1te, J, , .t 11, 1984, Astrophys.J., 28b, L35-39. 
. Giblon, .I., t al1981, ature, 6, B 3-83~ 
. adie, .T., et i, , · t thti l ethods i  xperillltntal' 
ysics-, orth-Holland,  
8. Mardia,K.,1972, Stati tics f irectional ata", cade ic ress 
on    ork). 
9. Dwthwaite, J. ., et aI, 1984, ature, 309, 691-693. 
10. had iCk, P.H., et ai, 1985, sub itted toAstrD. strophys. etters. 
11. hadWick, . ., et al,1985,submitted t  stro. strophys. etters. 
12. Suorski, H. and tau, ., 1983, Astrophys.J. (Letters) ,268 9 Ll7-L21 
13. loyd-Evans, J.et aI, 1983, ature,305, 784-787. 


The 1000 GeV gamma rays from ms pulsars

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server