BL Lac objects are of considerable current interest both because they were recently realized to be a new class of objects and because they were found to be similar in many significant respects to QSOs and Seyfert Galaxies. Since at least one model for the X-ray emission predicts a flattening of the X-ray spectrum above a few keV, several of these sources were studied using the high energy scintillation telescopes on OSO-8 and Ariel-5. With the possible exception of Markaryan 421, only upper limits were obtained for the X-ray emission in the energy ranges 25-60 and 60-130 keV, thereby imposing constraints on any spectral breaks

Coe, M. J.

Crannell, C. J.

Dennis, B. R.

Dolan, J. F.

Engel, A. R.

Frost, K. J.

Orwig, L. E.

English

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Technical M	 andum 80239
.High Energy X-Ray Observations
of BL Lacertae Objects
(NASA-TR-80239) HIGH ENERGY I-RAY	 179-20943
OBSERVATIONS OF BL LACEBTAE OBJECTS (NASA)
10 p HC A02/!!F A01	 CSCI. 03B
Onclas
G3/89 19632
M. J. Coe, B. R. Dennis, J. F. Dolan,
C. J. Crannell, K. J. Frost, L. E. Orwig
and A. R. Engel
FEBRUARY 1979
National Aeronautics and
Space Administration
Goddard Space Flight C~
Greenbelt, Maryland 20771 INroe,,
a
M.J. Coe
B. R. Dennis
J. F. Dolan
C. J. Crannell
K. J. Frost
L. E. Orwig
Laboratory for Astronomy and Solar Physics
Goddard Space Flight Center, Greenbelt, MD 20771, USA
A. R. Engel
Blackett Laboratory, Imperial College, London SW7, UK
Febniary 1979
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland
--	 A. R. Engel
Blackett Laboratory, Imperial College, London SW7, UK
ABSTRACT
BL Lac objects are of considerable current interest both because they were
recently realised to be a new class of objects and because they have been found to
be similar in many significant respects to QSOs and Seyfert Galaxies. Since at least
one model for the X-ray emission predicts a flattening of the X-ray spectrum above
a few keV, several of these sources have been studied using the high energy scintil-
lation telescopes on OSO-8 and Ariel-5. With the possible exception of Markaryan
421, only upper limits were obtained for the X-ray emission in the energy ranges
25-60 and 60-130 keV, thereby imposing constraints on any spectral breaks.
iii
_	
-
HIGH ENERGY X-RAY
 -OBSERVATIONS OF BL LACRRTAE OBJECM
BL Lac objects are of interest not only because they were recently identified as a new
class of objects, - but also because they have been found to be similar in many significant
respects to QSOs and Seyfert Galaxies. Since at least one model s for the X-ray  emission
predicts a flattening of the X-ray spectrum above a few keV, several of these sources have
been studied using the high energy scintillation telescopes on OSO-8 and Ariel-5. With the
possible exception of Markaryan 421, only upper limits were obtained for the X-ray emir
sion in the energy ranges 25-60 and 60-130 keV, thereby imposing constraints on spectral
hardening.
The prototype of this class of object, the rapid variable BL Lacertae, was established
as an unusual object from observations obtained during 1968-69. A review of its discovery
and the subsequent realisation that it represented a new class of objects can be found in
Stein et al. 2 and references therein. X-ray emission in the 2-10 keV band has been estab-
lished definitely for five of these objects 3 , including Mkn 501 (ref. 4) and Mkn 421 (ref. 5),
for which the detailed spectra have been measured over the range 2-40 keV 6 . An upper
limit of 2.4 X 10' t t erg CM-2 S-1  in the 2-10 keV range has been set by Mushotzky et A6
for the rc--naining members of the list of Stein et a1. 2 , and for the recently discovered new
member, 1418+54 (Mushotzky, private communication).
The results presented here were obtained with two high energy scintillation telescopes,
one on the OSO-8 satellite and the other on the Ariel-5 satellite. The OSO-8 instrument
consists of an actively collimated CsI(Na) detector with an effective area of 28 cm  and a
field of view (FWHM) of 5 0 . It has been described in detail by Dennis et a1. 8 . The Ariel-5
instrument also consists of an actively collimated CsI(Na) crystal, but with an effective area
of 8 cm 2 and a field of view (FWHM) of 8 0 . It has been described by Engel and Coe 9 . Data
- _I
_ —
	 from both theme	 entt flare beery ana^y^t t^ study a-tof tof—note BL i.ac objects,
including Mkn 501 and Mkn 421.
Apart from Markaryan 421, which is discussed in the following paragraph. no evidence
was foun&in the OSO.8 and Ariel-5 data for any X-ray emission in the 25-250 keV photon
energy range. Two standard deviation upper limits on the flux have been calculated for the
,
	
photodxnergy ranges 25
-60 and 60-130 keV. These limits are presented in Table 1. The
different values obtained arise from the different instrument backgrounds, the length of the
observations, and the positions of the sources within the field of view. The upper limits on
Mkn 501do not require any break in the extrapolation of the spectrum observed with
HEAO-1 (ref. 6), and serve only to eliminate the possible existence of an extreme hardening
above 40 keV. It must be noted, however, that even though there is evidence for little or nc
variability in Mka 501 (ref. 4), our measurements and the HEAO - I spectrum are separated
by two years and, as such, may not represent a true composite spectrum at any one time.
Markaryan 421, on the other hand, shows considerable variability above 1 keV.
Mushotzky et a1. 6 observed a factor of 6 change in 6 months in the energy range 2-10 keV,
and Ricketts et al. 10 observed a 20 fold increase in 10 days in the energy range 2-18 keV.
Consequently, it is important to obtain concurrent observations if the overall spectrum is
to be considered. This has been done by combining the results from the proportional
counter on OSO-8 (Mushotzky et a1. 6 ) with simultaneous high energy observations from the
scintillation telescope on the same satellite. The resulting composite spectrum is shown in
Figure ', . The low energy data tip to 5 keV are well fitted by a power-law photon-number
spectrum with a spectral index of —2.5. At higher energies there is an excess flux above an
extrapolation of this power law. The tipper limits imposed by the high energy detector re-
quire that any power-law spectrum in the 10-100 keV range cannot be flatter than E-1.0
While our data lend support to a spectral feature at — 25 keV suggested by the proportional
2 J
counter data, the statistical significance of our data is not sufficient to warrant a more
detailed analysis.
A total of nine BL Lac objects have been observed with the high energy X-ray spec-
trometers on Ariel-5 and OSO-8 including two out o: 't.e five known low energy X-ray
emitters3. Of these nine sources Mkn 501 and Mkn 421 are optically the brightest 2 , and
are therefore possibly the closest objects in this class. Consequently, the lack of detection
of all but Mkn 421 in this search is perhaps not surprising. Nevertheless, the results pre-
sented here suggest that these sources are not bright in hard X-rays and hence support the
contention by Boldt 11 that they do not contribute significantly to the diffuse cosmic back-
ground at these energies. The possibility of a feature in the spectrum of Mkn 421 at — 25 keV,
while intriguing at the moment, is in need of further confirmation before it can be considered
a constraint on models for these sources.
The work of the OSO-8 Project Scientist, Dr. Roger Thomas is greatly appreciated, as
is the assistance in the OSO-8 data analysis by the personnel of Comtech Incorporated. The
Ariel-5 satellite project is fund--d by the Science Research Council of Great Britain and
operated by the Appleton Laboratory. MJC acknowledges receipt of an NAS/NRC Resident
Research Associateship.
M.J. Coe
B. R. Dennis
J. F. Dolan
C. J. Crannell
K. J. Frost
L. E. Orwig
Code 684, Laboratory for Astronomy and Solar Physics, Goddard Space Flight Center,
Greenbelt, MD 20771, USA.
A- R. Engel
Space Physics Group, Blackett Laboratory, imperial College, London SW7, UK.
3
--REFERENCES
I. Jones, T. W., O'Dell, S. L. and Stein, W. A. (1974) Ap.J. 188, 353.
s
2. Stein, W. A., O'Dell, S. L. and Strittmatter, P. A. (1976) Ann. Rev. Astron. and Astro.
14,173.
3. Schwartz, D. A., Doxsey, R. E., Griffiths, R. E., Johnston, M. D. and Schwarz, J.
(1978) Submitted to Ap.J.	 1
t
1
4. Schwartz, D. A., Bradt, H., Doxsey, R., Griffiths, R., Gursky, H., Johnston, M. and 	 1
Schwarz, J. (1978) Ap.J. 224. ;.103.
5. Marshall, F. J. and Jernigan, J. G. (1978) I.A.U. Circular 3224.
6. Mushotzky, R. F.. Boldt, E. A., Holt, S. S., Pravdo, S. H., Serlemitsos, P. J., Swank,
J. H. and Rothschild, R. H. (1978) Ap.J. 226, L65.
7. Miller, H. R. (1978) Ap.J. 223, L67.
8. Dennis, B. R., Frost, K. J., Lencho, R. J. and Orwig, L. E. (1977) Space Sci. Inst. 3,
325.
9. Engel, A. R. and Coe, M. J. (1978) Space Sci. Inst. 3, 407.
10. Ricketts, M.J.. Cooke, B. A. and Pounds. K. A. (1976) Nature 259. 546.
11. Boldt, E. A.0978) NASA TM number 78106.
4
T
Object SatelliteUsed
Date of
Observation
2 Sigma Upper Limits
(Photons cm-2 s-i keV-1)
25-60 keV 60-i 30 keV
1101+384
(Mkn 421)
OSO-8 1977
May
6.2 X 10-4 1.2 X 10-4
1215+303 Ariel-5 1974
Nov
4.4 X 10-3 4.6 X 10-5
1219+285 Ariel-5 1974
Nov
4.2 X 10-3 4.4 X 10-5
1225+206 Ariel-5 1974
Nov
5.6 X 10-3 5.8 X 10-4
1418+540 OSO-8 1977
June
3.8 X 10-4 9.2 X 10-5
1514+197 Ariel-5 1977
June
3.2 X 10-3 6.6 X 10-4
1652+398
(Mkn 501)
OSO-8 1975
Aug
4.8 X 10-4 8.9 X 10-5
1727+502 Ariel-5 1977
Feb
2.0 X 10-3 6.4 X 10-5
2200+420 Ariel-5 1974 3.4 X 10-3 6.7 X 10-4
(BL Lac) Oct
MKN 421
0 PROPORTIONALCOUNTER
SCINTILLATION
TELESCOPE
10-2
q-
mY
N
10-3
H
z
OR
O
S
CL
10-4
10.5
1
PHOTON ENERGY Nee
Figure 1. The spectrum of Markaryan 421 observed concurrently with the proportional
counter on OSO-8 (ref. 6) and the scintillation spectrometer also on OSO-8 (this work)
for the period 1977 May 18-24. The fitted power-law is dN/dE = 0.06E -2•5 photons
cm-2 s-1 keV-1 . Upper limits are indicated at the two standard deviation level.
6
4. -


High energy X-ray observations of BL lacertae objects

High energy X-ray observations of BL lacertae objects

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