It is now more than 20 years since the first ground based gamma ray experiments involving atmospheric Cerenkov radiation were undertaken. The present highlights in observational ground-based very high energy (VHE) gamma ray astronomy and the optimism about an interesting future for the field follow progress in these areas: (1) the detection at increased levels of confidence of an enlarged number of sources so that at present claims were made for the detection, at the 4 to 5 sd level of significance, of 8 point sources; (2) the replication of the claimed detections with, for the first time, confirmation of the nature and detail of the emission; and (3) the extension of gamma ray astronomy to the ultra high energy (UHE) domain. The pattern, if any, to emerge from the list of sources claimed so far is that X-ray binary sources appear to be copious emitters of gamma rays over at least 4 decades of energy. These X-ray sources which behave as VHE and UHE gamma ray emitters are examined

Turver, K. E.

English

NASA Technical Reports Server

399
GROUND-BASED VERY HIGH ENERGY GAMMA RAY ASTRONOMY
- OBSERVATIONAL HIGHLIGHTS.
K E Turver
Department of Physics,
University of Durham, Durham DHI 3LE, UK.
It is now more than 20 years since the first ground based gamma ray
experiments involving atmospheric Cerenkov radiation were undertaken. The
present highlights in observational ground-based very high energy (VHE)
gamma ray astronomy and the optimism about an interesting future for the
field follow from progress in three areas:-
(i) the detection at increased levels of confidence of an enlarged
number of sources so that at present claims have been made for the
detection_ at the 4-5 sd level of significance, of emission from 8 point
sources (Cen A in 1973 (1_and more recently Crab pulsar_2), Crab
nebula(_), Vela pulsar('_, Cygnus X-3(".6.?,"_, Hercules X-1(9.1°.I*),
4U0115+63c_2), PSR 1953 (COS B source 2CG065 ?)"_),) plus three reports
of diffuse emission from the galactic plane_14.*".16)
(it) the replication of the claimed detections (e.g. Cygnus
X-3(_.6.7.e)_ Her X-1(9,11)) _ith_ for the first time_ confirmation of
the nature and detail of the emission; we are also seeing the beginnings
of the detailed studies of the emission (e.g. Cygnus X-3_*?), Crab
pulsar_,)).
(iii) the extension of gamma ray astronomy to the ultra high energy
(UHE) domain (10"-_eV) with numerous reports of Cygnus X-3(_9.=°_and
single reports of Her X-I(2_)_ and yet to be confirmed at either VHE or
UHE, Vela X-I_22) and LMC X-4(2_.
At an energy around 1000 GeV (VHE) all observations employ the
ground based atmospheric Cerenkov light technique(2"). The higher energy
studies around 100-10000 TeV (UHE) involve air shower detecting arrays
usually involving particle detectors (the FIy's Eye detector operating in
the Cerenkov mode is an exception).
The pattern, if any, to emerge from the list of sources claimed so
far is that X-ray binary sources (with or without a pulsar as the
collapsed object) appear to be copious emitters of gamma rays over at
least 4 decades of energy. Looking in more detail at these X-ray sources
which behave as VHE and UHE gamma ray emitters:-
(a) CYGNUS X-3. This is a very topical object which will be
mentioned frequently during this Conference. It is also the source which
has been most involved in the development of the present subject in
recent years. Following the radio outburst in 1972, Stepanian at hie
Crimean Astrophysical Observatory reported the first of a series of
detections of 1000 GeV gamma rays from the object. Initially (1973) these
were detections of a DC signal(_m), with the first indications at VHE
gamma ray energie of the now well known X-ray periodicity at 4.8 hr
following in 1975_) Since then similar detections at energies of 1000
GeV or thereabouts have been made by 5 groups ( Tien Shan_=?),
FLWO/Dublin_), FLWO/Dublin/Durham/Hawaii/Iowa/HongKon9(?),
JPL/Iowa/UCR(_), and Durham at Dugway,Utah_e_).Typical measurements made
since 1980 are shown in Figure I.
An Invited Highlight Paper Presented at
19th International Conference on Cosmic Rays
San Diego, 11-24 August 1985.
https://ntrs.nasa.gov/search.jsp?R=19860022030 2020-03-20T13:31:23+00:00Z
400
.: . 1
'_' R_Y MAX of these observations made: FIG i. X- A similar sto y emerges from al
:.3 "_ through the early 19BOs -•'_ intermittent emission (the chance
... (a) lj of observing during the ON state
1: seems to be about I per I0-20 4.8_i hr cycles ?) with a peak flux of: about lO-_°cm-2s-1 for energy >o.... -_.. I000 GeV always about the phase
i_-_'T_ J 0.62 in the 4.B hr period,_ corresponding to X-ray maximuml
FIFL.WO/DUBLIN and lasting for a short time,
; - i ; , , , about I0 min or so _e) In• ' ' .e
addition there have been
1.12 (b) suggestions of time variability
in the emission which is either
!_ sporadic or perhaps regular with
long ter periodicity nvolving
_ I._ _p 06,_@! the complete modulation ofthebeabout 19 days -
I_ VHE intensity (the long term
l_ -. .... period may
which is interpreted as possibly
arising from apsidal motion
OiL _CR according to the X-ray
observations- has been mentioned
0 L,/IOWA/ : in the context of radio, X- and
02 O'a VHE ga ma rays).
= . At the highest energies the
(c) discovery by the Kiel group of a
" similar 4.8 hr modulated emission
! = : at an energy around 101_eV
(_9_has been followed by other
confirmatory analyses of the
i arrays ((2o_, but with possible
_._. variability in the strength and a
definite change in the 4.B hr
-, DURHAM phase of the emission being
,iiii*i
............ suggested by later observations -
,...= ....... see Fi ure 2.
, &C,,,,I,=,, I
t5 ( a ) CYGt4U$ _'3 _ IVan der Kh_ 160nnet 6,daodl
FIB 2.
HAVERAH PARK
0 I I I l I I
t_ 01 C2 03 O'& 05 0'_ 0'7 0'_ 0"9 1"0
PM_5[
401
, . The VHE and UHE gamma ray
-,o flux measurements combine to give
an energy spectrum which is
i iO _ •
I • e exceedingly flat (integral
e$ spectrum _ 1.1) - see Figure 3.
_== q A question which has
E =o remained unanswered at all
u
wavelengths is whether or not
x Cygnus X-3 contains a pulsar as
_ the powerhouse of the system.J
-z 4 O This has been suggested by
i oj _ Vladimirsky (CAO) as a
consequence of a recentsupernova, leading to a rotation
W I driven process. Alternatively,
-I . it has been suggested by Brechmr
10
m (2e)that the mechanism is that of
I= 14 16 a unipolar inductor involving the10 10 10
accretion disc of a low field
PHOTON ENERGY {IV) pulsar rotating at equilibrium
when accretion is the origin of
FIG 3. I the energy. To date no
, suggestions have been made for
> -_ ' ' ' the pulsar period. Recently our
j _ge_ SEF _: (a) group in Durham <_)have analysed
" new data taken late in the life
-_ of the Dugway facility when our
o telescopes were operating with
maximum sensitivity. At a level
= -_ of significance which exceeds
- __that of many of the reported VHE
wj l_L___,__]jgamma ray detections (PSR 0532,
-= PBR 1953, Her X-I, 4U0115 +63
_etc), we find prima face evidence
_for ms periodicity (conservativeO
j o chance prob <3xi0-7). This occurs
__=._so _=._o _='_t those times around X-ray
' ' ' - maximum in the 4.B hr cycle when
a _ge_ oc_ = (b) emission manifested as an
increase in the count rate of the
telescopes has been detected. The0
= observed period is 12.5908
0.0003 ms and some of the
evidence for the claim is shown
in Figure 4.
> Confirmation of this
= oeriodicity (which may well have
:=.meo _:.mgo :0 be made at VHE gamma Fay
o _o_ , _ ) energies if dispersion and
a scattering in the coccoon or
FIG 4. stellar wind is occurring) would
represent a real highlight !!
402
:l.......1 b HeRcuue x-lJ A.short (3min)ofh_ U t b U r S _3,5 (a) ' (I,24 s) pulsed gamn,a rays wasdetected by the Durham VHE gammaL ray experiment in Utah in AprilO 1903 <9_- see Figure 5(a). ]'he.G 2 effect was at phase 0.77 in the
1.7 d orbit and , as far ms we
P 2 can tell, 35 d before an observed
R switch on of X-ray activity to
0 1.5 the high state. The temptation to
B associate a burst of VHE gamma
i rays with X-ray switch on should
perhaps be resisted - later
6.5 observations only partially
confirm the pattern. The VHE
emission is characterized by a
I. 18 I. 22 I •26 I • 3 br'oad (X-ray like) light curve
PERIOD (S) end a peak flux of 10-9 cm-_
s-_at E > 1000 GeV.
| .... , ' i_i' I .... I ' '" _ Replication of this observation,
Io-6 _ (b) i_; H[R X-I _ complete in many details, has
O-MAY-B4 been reported by the FLWO:0-2 i : collaboration 'I°_- see Figure
-1°-4 i ! At energies I00-I000 times
_°-3 i higher, the University of Utah
• , . Fly's Eye group has recently
_°-2 li I. reported a Ouly 1983 burst of (40
:o-) _' _ min duration also showing the
.IV _ % 1.24 s periodicity of the X-ray
1.23 ].23_ ].24 _.245 ].2_ puls_r. The light was i0% duty
PERIOD (SEC) cycle end the observed effect was
very strong - an increase of 40%
above the cosmic ray background
from 50 sq deg of sky - see
Figure 5(c). At the same time the
6o_ (c) + colo_ated I000 Bey Durhami " .] Cerenkov light experiment saw no
" _ such marked effect but did
X_, 40- observe a rather longer interval
_ of activity (2-3 d) which
included the time of the Fly's
20- Eye outburst. This is the first
example of simultaneous
I observations with two systems and
0 i].23 ].24 ].2s certainly with two detectors with
PERIOD(,) such dissimilar energy
........ ! thresholds; it should be the
FIG 5. i first of many. If true, these two
results suggesting very different
origins for the gamma rays of different energies mus_ constraln the
choice of models for the Her X-1VHE and UHE emitting system.
403
(c) 4UO115+&3
Following the discovery of
Her X-I as a 1000 GeV gamma ray
emitter, the University of Durham
project targetted 4U0115+63 as
the binary system which is most
-7 _ J similar to Her X-I (period and
X-ray luminosity) and which
L -6 shares a claim to cyclotron line
0 emission. A successful search was
G -5 made in September 1984 (I=) and
p -4 the resulting periodogram showing
R the 3.6 sec periodicity for the
0 -3 VHE gamma ray data is shown in
B Figure 6. This is the strongest
-2 time averaqed source of 1000 GeV
-1 gamma rays detected in the Durham
project (7xlO-11 cm-_ s-2).
There is no evidence for other
3.6144 3.6146 than a steady output over B or 9
PERIOD (S} days - it is certainly not the
case that bursts of a few mins of
FIG 6. intense activity provide the
signal as so often seems to have
been the case (e.g. Crab pulsar
and Hercules X-I).
(d) VELA X-I and LMC X-4.
At PeV energies the EAS group at Adelaide has reported evidence for
an excess of Sho_ers from the directions of Vela X-I and L_C X-4 showing,
in each case, the characteristic orbital periodicity but with no
indication of the (long) pulsar period in the case of Vela X-I.
ORBII FLUX CHANCE F'ROB LUMINOSITY
-II. -2 -I -4 _
VELA X-1 8.9 d 9 _ 3 x I0 m s 10 2x 10 ergsls
-II -2 -I 3e
LMC X-4 1.4 d 5 ± 2 x 10 m s 0.009 10 ergs/s
TO sum up what we know about X-ray binaries as VHE and UHE gamma ray
OU r C e S . PtJLSAR O;RB I T LONGTERM
V_A _-, E_ _3 _._ _ --
LM_ X--4 -- I . 4 d
[Information in square brackets relates to X-re.y data]
404
The VHE light curve of the binaries is interesting - all X-ray
binaries detected so far at TeV energies have broad light curves with
duty cycles 30 % or so of the cycle (rather like.the X-ray light curves)
- see Figure 7.
Her X-I PSR 1953
20 J "i
i .
12 1
4 4
1
IlJlllill
O 03 I.e e. i.e
Pha_arbitrary units) _mn ¢_MBZT_MRY)
Cyg X-3 4U0115+63
v I I I | I
ZN
' I
(d)
lo I_3 _E_T a2
! I !
I I • I / 1o II
FIG 7.
As far as other VHE gamma ray sources are concerned the Vela pulsar
was detected by the TATA group <e)and was found to show the double peaked
light curve observed at radio wavelengths - see Figure 8. The detailed
study of the VHE emission from the Crab pulsar by the Durham group<IB>is
an e>_ampleof a new phase in gamma ray astronomy - the systematic study
of the detail of the VHE emission. The width of the light curve around
the radio main pulse is observed to be < I% (<0.4 ms FWHM) - see Figure
9. This is the shortest duty cycle of emission from the Crab pulsar at
any wavelength other than radio and confirms the trend first reported 12
years ago by Greisen and his collaborators (_°>during gamma ray
observations at energies of a few GeV. The constraints placed on models
for 1000 GeV gamma ray production in pulsars by such observations are
severe,
4O5
lhe 6 ms low-field radio
pulsar (PSkI953) discovered
in a bir,_ry system with a
I17,3 d orbit and locatedP VELA PULSAR (PSR0833-45)
z within the error box o_ the
W 4-'-'_'---- 0'4 2 _
> COS B source 2CG065 (but notW 4'4 •
18°° _"22-1 firmly associated with it)0
m 17oo has been seen to produce
w 1000 6eV gamma rays _I=) -
• "',: C._ r #= i= see Figure I0. Although
L recently the COS B
Z 1500
collaboration has expressedJ
o 0.2 0.4 o-6 o-s ro doubt that some of the first
PHASE quadrant point sources are
genuine (and maybe the
FIG B. result of enhanced amounts
of molecular material
irradiated with cosmic
4488 ,rays), our VHE measurement
would suggest that 2CG065
C 4388 a weak radio puls_rl is also
O a copious VHE gamma ray
U 4288
N source,
_he VHE data 4rom the
488___j original 1,5 ms high-field
468 radio pulsar has recently
448 been analysed with the
428 benefit of a radio ephemeris
488 of unprecedented
_ccuracy _. An indication
,92 8 .88 of VHE emission in phase
PHRSE with the radio main pulse
and significant at the
3x10 -< chance level has been
FIG 9, obtained from the Durham
o. ...........................
•--5 PSR1953 • _e-'_.
L ............ _ , ,
0 -4
6
| .
p -3 ; ....
0 -2
B l,.o °
--1 I "" i 'II6.133162 166 17B ....
o.w
PERIOD (MS) ..... _ ......... _'"
rZG Io. rxG I._.__:,
406
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(2) Dowthwaite JC et al.,(1984),Ap J.,2B6,L35.
(3) Cawley MF, etal.,(1985),This Conference OG 2.3-2.
(4) Bhat PN et al.,(1980),Astron Astrophys.,81,L3.
(5) Danaher Set al.,(1981),Nature,289,5aB.
(6) Lamb RC et al.,(1982),Nature,29a,543.
(7) Cawley Met al.,(1985),Ap J (in the press)
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(10) Baltrusaitis RM et al.,(1985),This Conference OG 2.2-7.
(11) Cawley MF etal.,(1985),This Conference OS 2.2-9.
(12) Chadwick PM et al.,(1985),Astron Astrophys Lett (in the press)
(13) Chadwick PM et al, (1985),Astron Astrophys, (in the press)
(14) Fomin VP etal.,(1973),Proc Denver Conf 1,12.
(15) Weekes TC etal.,(1979),Proc Kyoto Conf 1,133.
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(17) Chadwick PM et al,(1985),This Conference OG 2.1-8.
(18) Chadwick PM et al,(1985),This Conference OG 2.3-9.
(19) Stamm S and Samorski M,(1983),Ap J.,268,L17.
(20) Lloyd-Evans Jet al.,(1983),Nature, 305,784.
(21) Balutaitis RM et al.,(1985),Ap J ,in the press.
(22) Protheroe RJ et al.,(1984),Ap J.,280,L47.
(23) Protheroe RJ & Clay,RW.,(1985),This Conference OG 2.6-10.
(24) Porter NA and Weekes TC,(1978), SAO Spec Report 301.
(25) Vladimirsky BM et al.,(1973),Proc Denver Conf 1,456.
(26) Vladimirsky BM et al.,(1975),Proc Munich Conf,1,118.
(27) Mukanov JB et al.,(1979),Proc Kyoto Conf,I,143.
(28) Chanmugan G & Brecher K,(1985),Nature,313,767.
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(30) Macbreen Bet al, (1973), Ap.J.184,571.
(31) Chadwick PM et al.,(1985),unpublished.


Ground-based very high energy gamma ray astronomy: Observational highlights

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