The quasar 3C 273 was detected by the Compton Observatory Satellite (COS-B) in the 1970's. Energetic Gamma Ray Experiment Telescope (EGRET) observations of this sky region in Jun. and Oct. 1991 revealed a flux from 3C 273 lower than that measured by COS-B. The flux observed by EGRET in the June period is approximately 0.0000003/sq cm(exp -2) sec(exp -1) for energies greater than 100 MeV. During the Oct. observation it appears to be even lower. For the first observation a preliminary spectrum was derived which was a photon index of 2.4

Bertsch, D. L.

Fichtel, C. E.

Hartman, R. C.

Hunter, S. D.

Kanbach, G.

Kniffen, D. A.

Kwok, P. W.

Lin, Y. C.

Mattox, J. R.

Vonmontigny, C.

English

NASA Technical Reports Server

/k
ASTRONOMY & ASTROPHYSICS
SUPPLEMENT SERIES
Astron. Astrophys. Suppl. Set. I, 1-99 (2001)
JANVIER 2001, PAGE 1
EGRET Observations of 3C 273
C. yon Montigny*, D. L. Bertsch 2, C. E. Fichtel 2, R. C. Hattman 2, S. D. Hunter 2, G. Kanbach 1, D. A.
Kniffen 4, P.W. Kwok 2's, Y. C. Lin 3, :1. R. Mattox 2,s, H. A. Mayer-Hasselwander 1, p. F. Michelson 3, P. L.
Nolan 3, K. Pinkau 1, H. Rothermel I, E. Schneid s, M. Sommer 1, P. Sreekumat 2,7, and D. J. Thompson 2
] Max Planck Institut i_r extraterrestrische Physik, D-8046 Garching,
2 Code 662, NASA/Goddard Space Flight Center, Greenbelt, MD 20771
3 Stanford University, Stanford, CA 94305
4 Hampden-Sydney College, Hampden-Sydney, VA 23943
Grumman Aerospace Corporation, Bethpage, NY, 11714
6 GRO SSC, Computer Sciences Corporation, Code 668.1, NASA/GSFC;
7 Universities Space Research Association, NASA/GSFC
s NAS/NRC Research Associate
Received; accepted
Abstract. -- The quasar 3C 273 was detected by COS-B in the 1970%. EGRET observations of this sky region
in June and October 1991 revealed a flux from 3C 273 lower than that measured by COS-B. The flux observed by
EGRET in the June period is approximately 3 x 10 -T cm-2sec -1 for energies greater than 100 MeV. During the
October observation it appears to be even lower. For the tlrst observation a preliminary spectrum has been derived
which has a photon index of 2.4.
Key words: gamma rays - Quasars: 3C 273 - Instrument: EGRET
1. Introduction
The optically very bright quasar 3C 273 (z=0.158) is a well
known source in all energy regimes ranging from the radio,
infrared, optical and UV up to the X-rays. Intense studies,
including simultaneous multiwaveband observations, are
made in all these bands to reveal the nature of this object
(see e.g. Courvoisier 1987).
The currently favored model discussed for AGN's is
a supermassive black hole surrounded by an accretion
disk. This scenario is believed to be able to provide the
enormous luminosities of the order of 104_--4e erg/sec and
the observed time variabilities of the order of weeks (or
even hours) in the X-ray range. The major problem, how-
ever, is to find mechanisms which are able to explain
in detail the generation of the emitted radiation. Sev-
eral possible processes have been proposed, among them
Bremsstrahlung, blackbody radiation, synchrotron radia-
tion, inverse Compton scattering and Penrose processes.
3C 273 has been detected in 7-rays by COS-B in two 4-
week observations in July 1976 and June 1978. The flux of
3C 273 was about 0.6x10 -6 cm-lsec -I for E _> 100 MeV.
Send offprint requests to: C. v. Montigny
No variationof the flux was observed within the 500£
uncertainty(Swanenburg etal.1978,Bignami etal.1981).
Here, preliminary resultsfrom two observations of
3C 273 with the EGRET instrument aboard the Compton
Observatory ate reported.
2. Observations
EGRET is a spark chamber experiment in which photons
with energies from 30 MeV to 20 GeV ate detected via
their conversion into electron-positron pairs. For a more
detailed description of the instrument see e.g. Hughes
e_ al. 1980 or Kanbach etal. 1988. The effective area of
EGRET reaches .--1500 cm 2 at 500 MeV, about a factor of
20 larger than that of COS-B. Since EGRET is designed to
have practically no internal background, only the celestial
background will limit the detection of weak sources.
The instrument response functions have been deter-
mined at the Stanford Linear Accelerator (SLAC) and
the results of the calibration are given by Thompson et
al. , 1992.
https://ntrs.nasa.gov/search.jsp?R=19940014991 2020-06-16T17:06:38+00:00Z
2TheComptonobservatoryhas observed the sky region
which contains the quasars 3C 279 and 3C 273 from June,
15th - 28th and from October, 3rd - 17th, 1991.
3. Results
The incident photon directionswere binned into maps
with a pixel size of 0.5 deg x 0.5 deg for four dif-
ferent energy intervalsand have been analysed with a
cross-correlationmethod. This method searchesforpoint
sourcesby lookingfor photon arrivaldistributionsconsis-
tent with the expected response of EGRET to point-like
sources.
First, the very strong source 3C 279 (Hartman et el.
1992 / about 110 away from 3C 273 has to be subtracted in
order to analyse 3C 273. Otherwise it is totally suppressed
by the strong source. The result of this subtraction is
shown in figure 1 for the energy range 200 - 500 MeV
and the June period. The cross-correlation algorithm is
applied to the four energy intervals with a photon index
of 2.3 for the source. For this index the number of cross-
correlated counts reached their maximum in each energy
interval.
0.)
(.)
I,I
C3
v_O " ........ t ......... i ......... i ......... i ......... i .........
........ I ......... I ......... I ......... I ......... I ........
210 200 190 180 170 160
RA (deg)
--10
--2O
--30
220
FIGURE 1: Contour map of cross-correlated counts for
3C 273 (3C 279 subtracted) in the energy range 200 - 500
MeV (June 1991/.
The position for 3C 273 is then determined from the
average of the positions derived from the energy intervals.
It is found to be RA - (187.5 + .6) deg and DEC = (1.98
-4- 0.16) deg, about 0.2 degrees (or 12 arcmin) from the
known position of 3C 273: RA = 187.3 deg, DEC = 2.05
deg (epoch 2000). This is consistent with the uncertainty
in the EGRET position determination for a low intensity
source.
The energy iiitervals, the number of correlated counts
and the corresponding positions for 3C 273 are given in
table 1.
TABLE 1. Energy intervals, cross-correlated counts and
positions .for 3C _73.
E (MeV) corr. counts*" RA(deg) DEC(deg)
70 - 200 156 186.8 1.94
200- 500 56 187.5 2.16
500-2000 9 188.1 1.85
2000-5000 1 (186.7) (2.25)
"" assuming E -2"3spectrum
>
_D
L)
('q
E
£)
m
C
0
0
c-
13_
10-8
10-9
10-1o
10-11
10-12
10-13 n t tJ[ .. _ a ' _ _llln z l n I J t,
1O0 1000 10000
Energy (MeV)
FIGURE 2: Preliminary spectrum of 3C 273 during the
observation period in June, 1991.
In order to reproduce the measured counts a powerlaw
spectrum has been convolved with the energy resolution of
the instrument and the appropriate exposure for the June
observation. The number of counts obtained from this
model is then compared and fitted to the cross-correlated
counts.
The spectrum (figure 2) can be represented by a power
law of the form:
dI (3.0 -1- 1.7) 10 -4 E (-2.394-0.13) ph
d--'-E= cmZsecMeV (i)
This implies a flux of about (3 -t- .5 ) 10 -_ cm -z sec -1
for E > 100 MeV.
>(D
¢u
03
c-q
E
¢.J
v
(13
Z
O
F--
O
I
-4
10
10--6
-8
10
-10t0
10-12
COMPTEL
4- EGRET
--¢-
-I<-,
1 10 100 1000 10000
Energy (MeV)
FIGURE 3: Comparison of the COMPTEL and EGRET
spectra for 3C 273 in June, 1991. The dotted line rep-
resents the powerlaw fit to the EGRET data given by
equation (1).
The comparison with the COMPTEL spectrum (figure
3) shows that the EGRET spectrum fits well (see also:
Hermsen et al. 1992), although the extrapolation of the
EGRET powerlaw comes up too high in the COMPTEL
regime. However, since the energy region between 30 and
100 MeV is crucial, the analysis of the EGRET data in
this energy domain has to be done very carefully and will
be the subject of further study.
During the October observation 3C 273 appeared to
be tess intense than in June. But due to the very low
intensity and the decrease in the spark chamber efficiency
because of gas age a more detailed study is neccessary to
confirm this suggested decrease in intensity.
4. Acknowledgements
The German contribution to the EGRET project was
funded by the Bundesministerium ffir Forschung und
Technologic under grant 50 QV 90955.
References
Bignami G.F. et al. , 1981, A&A 93, 71
Courvoisier T.J.-L., 1987, A&A 176, 197
Hartman, R.C el al. , 1992, ApJ, 358, L1
Hermsen W. el al. , 1992, these proceedings
Hughes E.B. et al. , 1980, IEEE Trans. Nucl. Sci., NS-27,
364
Kanbach G. et al. , 1988, Space Science Reviews, 49, 69
Swanenburg B.N. el al. , 1978, Nature 275, 298
Thompson D.J. et al. , 1992, submitted to Ap.J. Suppl.


EGRET observations of 3 C 273

EGRET observations of 3 C 273

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