The IUE observations of four active nuclei were coordinated with radio, infrared, and X-ray measurements to obtain simultaneous determinations of their continuous spectra. The results for the BL Lac objects 0735+178 and I Zw 187 indicate sufficient UV and X-ray fluxes to ionize any gas. Comparison of the X-ray measurements with the extrapolated optical UV continuum show a definite X-ray excess for I Zw 187 but none for the other BL Lac object

Bregman, J. N.

Glassgold, A. E.

Huggins, P. J.

English

NASA Technical Reports Server

JSIMULTANEOUS OBSERVATIONS OF ACTIVE
GALACTIC NUCLEI WITH IUE
J.N. Bregman, A.E. Glassgold, and P.J. Huggins
New York University
ABSTRACT
IUE observations of four active nuclei have been coordin-
ated with radio, infrared, and X-ray measurements to obtain
simultaneous determinations of their continuous spectra. The
results for the BL Lac objects 0735+178 and I Zw 187 indicate
sufficient UV and X-ray fluxes to ionize any gas. Comparison
of the X-ray measurements with the extrapolated optical-UV
continuum show a definite X-ray excess for I Zw 187 but none for
the other BL Lac object. •
INTRODUCTION
We present continuum data on four active nuclei of quasar-
like extragalactic objects, with frequency coverage from the
radio to the X-ray band. Such measurements are an essential
first step to the understandin_ of the mechanisms by which the
central energy source produces the emitted radiation. In
particular, the ability to measure the UV flux of these objects
with IUE supplies crucial data for defining the characteristics
of their continuous spectra. Some of the most interesting types
of active nuclei, e.g. the BL Lac objects, present particular
difficulties in that they vary on timescales as short as days or
weeks. Our observations were planned so that simultaneous
observations (within hours or days) could be made at all frequen-
cies in order to obtain definitive spectra for these objectives.
OBSERVATIONS
Table 1 presents the four extragalactic objects observed in
October 1979. The IUE observations were low dispersion, short
and long wavelength (except for I Zw 187) spectra, taken with
the large aperture. Near simultaneous observations were made at
radio frequencies by H. AIler of the Univ. of Mich. (8 and 14
GHz), and by W. Dent of the Univ. of Mass. (32 and 90 GHz); at
infrared wavelengths by R. Rudy of the Univ. of California at
San Diego (H,J,K, and L); and at X-ray frequencies with the
751
https://ntrs.nasa.gov/search.jsp?R=19810017441 2020-03-21T12:06:16+00:00Z
Einstein Observatory by the Columbia Astrophysics Laboratory
(0.3 - 4 key). An estimate of the optical flux (V band) was also
obtained at the IU E immediately before each exposure. All the
measurements were made within two days of the IUE exposures,
except for those at 32 and 90 GHz, which were made within seven
days.
Table i. IUE Observations
Object V z D _ Camera Exp UV slope
(Type) (Mpc) (Min)
0735+178 15 ,0.424 >I020 SW 225 1.7
(BL Lac)
LW 130
I Zw 187 16 0.055 160 SW 285 1.65
(BL Lac)
NGC 3516 13# .0093 28 SW i00 2.3
(Seyfert)
LW 75
NGC 2782 13# .0084 25 SW 165 1.4
(Hotspot)
LW 105
* based on H=I00 km/s Mpc.
$ infrared-optical-ultraviolet slope
# extended underlying galaxy
The simultaneous multifrequency spectra are given in Figs
1-4. The ultraviolet data have been dereddened for the extinction
in the Galaxy using the measurements of gas and dust given by
Burstein and Heiles (1979), and the Seaton (1979) fit for the
selective extinction.
RESULTS
The continuum spectra cover 8 decades in frequency from the
radio region (9.9 GHz) to the key X-ray band (up to 10(+18) Hz).
Although the data are closely simultaneous, the two large gaps in
coverage (submillimeter to far infrared, and extreme UV to soft
X-ray) make suspect any attempt to draw a single continuum through
all the data. Nevertheless, we can see that the BL Lac object
0735+178 is consistent with one smooth continuum, and that the
752
X-ray data can be fit reasonably well by an extrapolation of the
infrared-optical-UV data. This extrapolation is a power law
F_-_with _ =1.3. The total integrated flux at the earth is
i.i (-i0) ergs/cm2s, and the total luminosity is greater than
2.0 (+44) ergs/s, for q=i/2, and H=I00 km/s Mpc. The other BL
Lac object, I Zw 187, differs in several respects: (i) the total
integrated flux is 4.2 (-ii) ergs/cmas, and the total luminosity
is i.i (+43) ergs/s, at least 20 times fainter than 0735+178;
(2) the decrease from the radio to the optical is less, i.e. a
factor of i00 instead of 1000; (3) the extrapolated infrared-
optical-ultraviolet power law is steeper,_ 1.7; and (4) there
is evidence for an X-ray excess, i.e. the observed X-ray flux is
about i0 times greater than that extrapolated from lower frequen-
cies. The Seyfert I galaxy NGC 3516 continues this trend: (i)
the ratio of radio to optical flux is only about 10; (2)_ _ 2.3;
and the X-ray excess is about 50. NGC 2782 presents some inter-
esting contrasts which probably arise because it is not a quasar-
like object. There is almost no change in flux between the
radio and infrared region, and the X-ray flux is much less than
what is expected when the UV continuum is extrapolated into the
X-ray band.
DISCUSSION
Simultaneous UV and X-ray data allow the total flux of ionizing
photons to be estimated more accurately than is possible from op-
tical data alone. The BL Lac object 0735+178 is one of the few to
have a large enough red shift (z>0.424) to shift the Lyman limit to
the short wavelength band of the IUE. The fact that the continuum
shows no dip or change of slope below the Lyman limit places an
upper limit of about i0 (+18) cm on the amount of neutral hydrogen
along the line of sight to 0735+178.
It is interesting to compare the optical-UV continua of the
two BL Lacs reported here with the Red QSOs recently studied by
Rieke et al. (1979) and by Smith and Spinrad (1980). Although the
Red QSIs gave very steep spectra _ 3), They possess broad emis-
sion lines. The fact that the optical-UV continua of the two BL
Lac objects are considerably less steep than those of the Red QSOs
suggests that the 'lineless' property of BL Lac objects arises
from a lack of gas rather than a lack of ionizing flux.
The ability to reliably extrapolate the infrared-optical-UV
flux to higher frequencies is also important for an understanding
of the continuum emission mechanism. The clear detection of an X-
ray excess in I Zw 187 seems consistent with the inverse Compton
process, in which radio frequency photons are raised to X-ray en-
ergies by scattering from relativistic electrons. By use of the
model of Jones, O'Dell, and Stein (1974), the simultaneous data may
be used to estimate the size and magnetic field strength of the
emitting region. For I Zw 187 we estimate a magnetic field strength
753
of about 16 G and a size of about 0.0085 pc (10 light days) - ap-
proximately the size deduced from temporal flux variations. The
data for 0735+178 yield a lower limit of 0.66 pc (2.2 light years)
for the size of the continuum emitting region, which is discrep-
ant with that determined from flux variations (i light week).
This disagreement suggests that some new ingredient must be added
to the canonical model of the inverse Compton process for the case
of 0735+178, such as relativistic flow (Blandford and Konigl, 1979).
REFERENCES
Blandford, R. and Konigl, A., 1979, Ap.J. 232, 34.
Burstein, D. and Heiles, C., 1978, Ap.J. 225, 40.
Jones, T.W., O'Dell, S.L., and Stein, W.A., 1974, Ap.J., 192,201.
Rieke, G.H., Lebofsky, M.J., and Kinman, T.D., 1979, Ap.J., 232, LI51.
Seaton,, M.J., 1979, MNRAS, 187, 73p.
Smith, H.E., and Spinrad, H., 1980, Ap.J., 236, 419.
754
oo o -23
- 23 ° 0735 +178 I Zw187
o% oo
•__-26 _ __-26 _,
! +
29I_ 4-
_ -29
1'o ' 1'2 ' 1:, ' i_, ' 1_ lb ' 1_, ' 1:, ' i_, '
log P log
Figure 1 Figure 2
-23 .... 23
t
m _o
_o
- 29 NGC 2782
-29 NGC 3516 .+.
+
log V log
Figure 3 Figure 4
755


Simultaneous observations of active galactic nuclei with IUE

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server