A signal (chance probability = .0002) with the 1.24 s period of Hercules X-1 has been observed using the Utah Fly's Eye. The signal's relatively long period and high shower energy conflict with some popular models of particle acceleration by pulsars. Optical and X-ray data suggest a picture in which energetic particles produce multi-TeV gamma rays by collisions with Hercules X-1's accretion disk

Baltrusaitis, R. M.

Cassiday, G. L.

Copper, R.

Elbert, J. W.

Gerhardy, P. R.

Loh, E. C.

Mizumoto, Y.

Sokolsky, P.

Sommers, P.

Steck, D.

English

NASA Technical Reports Server

111 OG 2.2-7
500 TeV GAMMARAYS FROMHERCULESX-1
Baltrusaitis, R.M., Cassiday, G.L., Cooper, R., Elbert, J.W., Gerhardy,
P.R., Loh, E.C., Mizumoto, Y., Sokolsky, P., Sommers, P. and Steck, D.
Physics Department, University of Utah, Salt Lake City, UT 84112 USA
ABSTRACT
A signal (chance probability : 2 x 10-4) with the 1.24 s
period of Hercules X-1 has been observed using the Utah
ny's Eye. The signal's relatively long period and high
shower energy conflict with some popular models of
particle acceleration by pulsars. Optical and X-ray
data suggest a picture in which energetic particles
produce multi-TeV y-rays by collisions with Hercules
X-l's accretion disk.
1. Introduction. A detection of TeV y-ray emission by Hercules X-1
has been reported by Dowthwaite et al. (1984). We have studied the
same object at much higher energies by detecting Cerenkov flashes from
atmospheric air showers. The use of the _y's Eye to search for ultra-
high energy y-rays has been described elsewhere (Boone et al. 1984).
The 67 mirror units and 880 photomultipliertubes of _y's Eye I recor-
ded Cerenkov flashes which triggered 6 or more tubes. This selected
showers with energies above about 200 TeV, with mean energies near 500
TeV. The angular resolution radius is about 3.5°, therefore a 7°
square target region was used centered on the direction of Hercules X-I.
2. Observations and Data Analysis. The only nights for which
Hercules X-1 was visible and the detector was recording Cerenkov data
were July 10-14, 1983 (UT). Expected rates within the target region
(if y-ray emission were absent) were found by observing rates in
regions outside the target region in the same declination strip. The
total number of showers recorded in the target region was 301, with an
expected number of 271.9. This amounts to a 1.8_ excess. A more
significant result is obtained by a test for periodicity in the data.
Because Dowthwaite et al. (1984) observed very sporadic emission from
Hercules X-l, the data frownthe 5 nights were analyzed separately.
The shower arrival times were corrected for the motion of the X-ray
source in its binary system and adjusted to the solar system barycenter
using results from Deeter, Boynton, and Pravdo (1981). The pulse
period was obtained from 1983 May X-ray satellite results by extra-
polation, using the period and period derivative given by Nagase et al.
(1984). The period used to fold the data was 1.2377872 s. Although
the X-ray data obtained a period, an absolute phase determination was
not possible. Our choice of phase is arbitrary.
A x2 test was applied to the distribution of phases within the
~1.24 s period, or light curve. Using 10 phase bins the data were
compared to a constant background prediction. To remove effects of
arbitrary bin boundaries, four X2 values were obtained for each data
,. 
1  7 
  MMA S OM CULES -I 
altr saitis, . ., assi , . ., ooper, ., l ert, J. ., er r , 
. ., oh, . ., i zu oto , ., okolsky, ., So ers, P. and teck, D. 
Physics epart ent, niversity of tah, Salt Lake ity, UT 84112 USA 
T 
 signal (chance probability = 2 x 10- 4 ) ith the 1.24 s 
erio  f erc les -I as een served sin  the tah 
~yls Eye. The signal IS relatively long period and high 
sho er energy conflict ith some popular odels of 
particle acceleration by pulsars. ptical and -ray 
ata s est a ict re i  hich e er etic articles 
produce ulti-TeV y-rays by collisions ith ercules 
-li  ion i  
. I trod tion.  t tion f eV -ray ission  er les -I 
s  r rted  o t aite t l. ( ). e  studied th  
sa e object at uch higher energies by detecting erenkov flashes from 
at s eric ai r s ers. he se f t e ~  I S ye t  search f r ul tra-
high energy y-rays has been described elsewhere ( oone et al. 1984). 
he 67 irror units and 880 photo ultiplier tubes of ~ ls Eye I recor-
ded erenkov flashes hich triggered 6 or ore tubes. his selected 
s ers it  e er ies above a t 200 e , it  ean e er ies ear 500 
Te . The angular resolution radius is about 3.5°, therefore a 7° 
square target region was used centered on the direction of ercules -I. 
. bser ations and ata nal sis. he l  i ts f r hich 
er l  x I as i i le  th  t tor s r rding renkov ta 
ere July 10-14, 1983 ( ). xpected rates ithin the target region 
(if y-ray e ission ere absent) ere found by observing rates in 
regions outside the target region in the same declination strip. The 
t t l er f s o ers rec rded in the t r t region as , ith an 
expected nu ber of 271.9. This a ounts to a 1.8a excess.  ore 
significant result is obtained by a test for periodicity in the data. 
ecause o th aite t l. (1984) served ery s ra ic e ission from 
er l s -I, th  ta from the  i ts er  lyzed s r tely. 
  ival im s er  ted  h  ot on  he - ay 
s rce in its i ary s stem and a j sted to the s lar s stem ar ce ter 
using results from eeter, oynton, and ravda (1981). The pulse 
period as obtained from 1983 ay -ray satellite res lts e tra-
lation, sing the eriod a d eriod erivative iven by a ase t l. 
( ). e riod ed to f ld t  ta as .237 872 . lthough 
t  -r y t  t ined  rio , a  s l te s  t rmi tion as 
not possible. ur choice of phase is arbitrary. 
 x2 test as applied to the distribution of phases ithin the 
~1.24 s period, or light curve. sing 10 phase bins the data ere 
r d to  st t round r i tion. o rem ve ffects f 
arbitrary bin boundaries, four x2 values ere obtained for each data 
https://ntrs.nasa.gov/search.jsp?R=19850025620 2020-03-20T17:54:59+00:00Z
I I 2 OG2.2-7
set by uniform shifts of the phase bin boundaries. Then the
maximum x2 was selected. This procedure prevented a narrow signal
from being split between adjacent bins and thereby diminishing its
apparent significance. Of the 5 nights, only 1983 July ii had a
statistically significant ×2. Next, the data from that night were
divided into two equal parts and it was observed that the signal was
present only in the data taken in the earlier part of the night. The
light curve for this case is shown in Figure 1. An excess is present
in only one
bin. The
uncertainty
zs- in the back-
ground is very
o small and the
1o Poisson proba-
bility for0
excess counts
to be due to
s
_ background
fluctuations
z is found to
0 e 7 x 10-7.
0.0 0.2 0.4 06 0.8 1.0 The number
PHASE (l.24s period) of tries used
in getting
this result is
Fig. 1 Phase dependence of the shower arrival times, the product of
The dashed line is the expected number, the number of
bins (10), the
number of phase
increments (4),
and the number
of data sets
i _ tried (5 nights
6o and 2 hal f
nights). This
- gives 280
tries, yielding
X2 4o- _, , _ " a chance "
_ __ probability of
2x10-4.
20-
3. Results.
A fixed value
0 l of the period
1.23 1.24 1.25 was used while
PERIOD (s) performing the
%2 tests
Fig. 2 The X2 dependence on the period used to fold described
the data. The arrow marks the period above. Figure
obtained from Nagase et al. 1984. 2, however,
11  G -7 
t  if r  ifts f t   i  daries. n t  
axi um 2 as selected. is r ce ure re ented a arr  si al 
fro  ei  s lit et ee  a jacent i s and t ere y i inishing its 
apparent significance. f the 5 nights, only 1983 July 11 had a 
t tistically sig ificant x2 • e t, the ta from th t ig t r  
divided into two equal parts and it as observed that the signal as 
present only in the data taken in the earlier part of the night. The 
light curve for this case is shown in Figure 1. An excess is present 
II') 
~ 15 
w 
~ 
 
:x: 
II') 0 
u. 
o 
5 
- --
0.2 0.4 0.6 O.B .0 
PHASE(1.24s period) 
Fig. 1 Phase dependence of the sho er arrival times. 
e s ed line i  th  ted ber. 
0 
2 40 
 
 
O~--------------,-------------~ 
1.23 1.24 
PERI D(s) 
25 
i .  he x2 e e e ce on t e erio  used t  f ld 
the data. The arrow arks the period 
obtained from agase et al. 1984. 
in only one 
  
uncertainty 
  
ground is very 
l d he 
oisson proba-
bil ity for 
ess ts 
   o 
background 
luctuations 
s ound o 
   7 • 
  
of tries used 
in getting 
   
the product of 
t e er f 
bins (10), the 
number of phase 
increments (4), 
   
 t  ts 
tried (5 nights 
  al  
nights). This 
gives 280 
tries, yielding 
a chance 
probabil ity of 
x - 4 • 
. esult  
 xed  
of the period 
as s  hil  
performing the 
x2 tests 
described 
above. Fi gure 
2, ho ever, 
113 OG 2.2-7
shows the X2 as a function of the period. The x2 is quite specific in
preferring a period near that of Nagase et al. (1984). Since the
signal was received during a relatively short 40 minute interval, the
period measurement is crude compared with other experiments. The
barycentric time at the center of this time interval was JD 2445526,719.
This corresponds to orbital phase 0.66 (Deeter, Boynton, and Pravdo
1981) and 0.63 in the 35 day period (Delgado, Schmidt, and Thomas
1983). The orbital phase is such that the companion star, HZ Herculis,
was not near the line of sight to the pulsar. It was therefore not
positioned so that the edge of its atmosphere could serve as a target
or converter to produce high energy y-rays from energetic protons.
The approximate y-ray flux was estimated using the signal shown
in Figure 1. The resulting flux is 3.3±1.1 x 10"12cm-2s'1. This is
the (apparently sporadic) flux observed in the first part of the 1983
July 11 data. It is the average flux within the 1.24 s period. The
uncertainties given above are statistical, only. Using a distance of
5 kpc for Hercules X-I and assuming the y-rays are emitted isotropi-
cally, the peak observed luminosity above 5 x 1014 eV is about
103? erg s-l. This value is close to the total luminosity estimated
for the system (Bradt et al. 1979).
4. Discussion. The charged particles which produced these y-rays
must have energies above 10I_ eV. Given the relatively long rotational
period (1.24 s) of Hercules X-I, this energy exceeds the maximum
expected from Hercules X-1 according to certain accelerationmodels.
The magnetic field in the vicinity of the pulsar surface is
3-5 x 1012 Gauss (Trumper et al. 1978). According to the models of
Goldreich and and Julian (1969), and Cheng and Ruderman (1977), the
maximum energy of produced particles would be about 2-3 x 10IB eV.
If we assume the model of Gunn and Ostriker (1969) and allow particles
to be accelerated from the speed of light cylinder radius out to the
companion star, the maximum energy is near 1013 eV. Some models,
however, do predict sufficiently high energies from this system (Kundt
1983, Chanmugam and Brecher 1984).
Optical (Delgado, Schmidt, and Thomas 1983) and X-ray (Parmar et
al. 1985) data from Hercules X-1 were taken during the time interval
of our observations. Hercules X-I displays a 35 day cycle of X-ray
intensity variations in addition to the 1.24 s pulsar period and the
1.7 day orbital period. High emission normally occurs during about 10
days of the cycle. During 1983 June to August, however, Hercules X-I
remained at levels _ 5% of the normal peak intensities. This might
suggest that X-ray production did not occur during this time. This
conclusion is not supported by optical observationsmade in 1983 June
and August. These show the normal (~1.5 mag) variation of the
optical emission in the 1.7 day orbital cycle. This variation is
attributed to extra emission due to X-ray heating of the side of the
companion star which faces the X-ray source. The optical variability
implies that X-rays were being produced during this interval. The
conclusion of Parmar et al. and Delgado, Schmidt, and Thomas was that
the accretion disk may have thickened and blocked the line of sight to
113 OG 2.2-7 
s s t  x2 as a f tion of t  peri . he  is uit  s ecifi  i  
r f ing  peri  e  t at f aga  t l. ( . i  t  
si l as r iv  duri  a r l tiv ly s rt  inut  i t r l. t  
er  easur ent i   c par  it  t  er ent  Th  
barycentric ti e at the center of t is time interval as J  2445526.719. 
hi  c rr p  t  orbit l phas  0.  ( eet r. oynt .  r  
 a  .  i  t   d  peri  ( el , c idt.  h a  
983). The orbit l s  i  s  t t t  pa i n st r. Z erculis. 
as not ear t e li e f si t t  t e pulsar. It as t eref re t 
osi ione  s  t at t   of i  at os  l  s  a  a t t 
r nvert  t  r  i  e  y-  from er eti  pr t . 
The r i t  -  fl  as estim t  usi  t  si l s  
i  i  . h  r lting fl  i  3. .   -I - - I . hi  i  
t  ( r tly r i  fl  s  i  t  fi t art f t   
J l  1  at . It i  t  a er  fl  it i  t  .   eri . he 
uncertai t es gi   r  st t . l  si  a i an  f 
  f r ercul  -I  i  t  -r  r  e itt  i tr i-
call . t e peak ser e  l i osity a e   101  e  is a t 37 er  s-1. hi  al  i  cl  t  t  t t l l i sity esti t  
f r t e s stem ( ra t t al. 9). 
. isc ssi . The r  parti l s hic  r  t s  -r s 
ust ha  er i  a  15 e e i  t  r l ively l  r t ion  
peri  ( .  s) f ercul s -I. t is e er  e c s t  axi u  
e t  from erc l  -I in  t  ert i  el tion odels. 
The agneti  fi l  i  t  vici it  f t  ul  r  i  
-  x 1  aus  ( u  t al. 8). cor ing t  t  odel  f 
oldr i    J li  ( ). a  he   uderman ( ). t  
axi  e  f r  parti  oul  b  a ut 2-  x 13 e e 
I  e a um  t  odel f un  a  stri r ( ) a  allow arti l  
t  be ccel ed f om t  s  of l t l  r  t t  t  
pani  st r. t  axi   i  near 13 e e  odels. 
ho ever, do predict s ffi ie tly i  er i s from t is s stem ( un t 
983. h am a  re  984). 
pti  ( el . c idt.  h a    -r   t 
al. ) at  from ercul  -I er  t  uri  t  tim  i t l 
f our observati  ercul  -I di  a   cycl  f -r  
i t ity vari tio  i  a ditio  t  t  .  s puls r peri  a  t  
.   orbit l peri . i  e issi  nor all  c r  uri  a t  
a  f t  c cl . uri   J  t  ugust. h ever. erc l  -I 
r ai  at l el  ~ 5  f t  n l pe  i t siti . hi  ight 
s gest t t -r  r ction i  t occ r rin  t i  ti . hi  
c ncl o  i  t r ed  pti  bser t ons ade i    
 ugust. hes  s  t  n r l (  ag) ari tion f t  
ptical e ission i  t e .  da  r ital c cle. his ariation is 
at ib e  t  extr  iss   t  -r  eat n  of t   f t  
c pani  st  hi  f  t  -r  r . h  pti l variabilit  
i pli s t at -r s er  ei  pr  ri  t is i t r l. The 
c ncl  of ar a  t l. a  el  c idt.  h a  as t t 
t  r tion i  a  ha  t ick  a  l  t  li  f i t t  
1 1 4 OG 2.2-7
the earth for X-rays originating near the neutron star.
If energetic protons are produced near the neutron star, then
the occulting material mentioned above may have served as target
material for the generation of ultra-high energy _° mesons which
decayed to produce the energetic y-rays. The resulting y-rays are
essentially parallel with the parent protons. The y-rays could be
produced reasonably efficiently by column thicknesses of 5-200 g/cm-2,
which would absorb keV X-rays very effectively. Such a model may be
rejected in the future if ultra-high energy y-rays are detected
simultaneously with X-rays. If the model is correct the y-ray emission
by Hercules X-1 may occur only during unusual conditions.
Although the signal reported by Dowthwaite et al. (1984) was at
much lower energy and was not simultaneouswith our signal, our result
is supportive of their conclusion that TeV y-rays are produced by
Hercules X-I.
5. Acknowledgements. This work was supported by the U. S. National
Science Foundation under grants PHY-8201089 and PHY-8415294,
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500 TeV gamma rays from Hercules X-1

500 TeV gamma rays from Hercules X-1

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