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Aneta Siemiginowska, Jill Bechtold,
Kim-Vy Tran, Adam Dobrzycki
Center for Astrophysics, Cambridge, MA 02138 USA Steward Observatory, Tucson, AZ 85721 USA
We present results of the modeling of the spectrum of the high redshift
(z=2.72), high luminosity (10
ergs s
)
quasar, HS1700+6416 (Reimers et al. 1989), observed with HST FOS (Reimers
et al. 1992) and GHRS (Reimers 1995). We analyzed the HST spectra together
with optical spectrophotometry and identified absorption systems towards
the quasar. We corrected the spectra for the intervening Lyman limit
absorbers and defined the intrinsic optical-UV continuum. The continuum
extends down to 310Å. We included the X-ray spectrum from the ROSAT PSPC
observation (Reimers et al. 1995) from 0.37 keV to
10 keV in the rest
frame. The X-ray energy index for the best fit power law model is equal to
1.24. It is steep, but not unusual for radio-quiet quasars at this redshift
(Bechtold et al. 1994). The observed
=1.69 is consistent with
the prediction given by the relation between
and optical
luminosity for radio-quiet quasars (Bechtold et al. 1994).
We fit the HS1700+6416 spectral energy distribution with several theoretical models including accretion disk models and free-free optically thin emission. None of the applied models can fit the UV continuum of HS1700+6416.
Keywords: Quasar - spectra, models; HS1700+6416
IRAS
IRAS observations were taken between January and November 1983. The satellite
scanned HS1700+6416 several times during this period. Addscan analysis revealed
a detection at 60 m (Tanner et al. 1995).
HST FOS
We have extracted data for HS1700+6416 from the HST Archive. We re-calibrated
the FOS data using the CALFOS in STSDAS and the latest calibration files
provided by the STScI. We corrected the spectra for interstellar reddening
assuming E(B-V)=0.051, which corresponds to a Galactic column density of
N=2.46
10
cm
.
There are seven optically thin Lyman limit systems present in the FOS spectra. We have corrected the spectrum for those systems using the optical depth and redshifts given by Vogel & Reimers (1995). The continuum was found using FINDSL, the iterative fitting procedure described in Aldcroft (1993).
Optical
Figure: Spectral energy distribution of HS 1700+641, corrected for
the absorption due to the Lyman limit systems.
Solid line represents a continuum fit.
The data are also corrected for the interstellar reddening.
The optical spectrum of HS1700+6416 was taken on July 4, 1995, using the FLWO Tillinghast 1.5-meter telescope with the FAST spectrograph (exposure time of 900 seconds). The spectrum has been corrected for the interstellar reddening with E(B-V)=0.051.
X-ray Data
ROSAT PSPC pointed towards HS1700+6416 twice in 1992 and 1993 (Reimers
et al. 1995). We have extracted both data sets from the ROSAT archive
and fitted them with a single power-law model, assuming only
Galactic absorption (N=2.46
10
cm
). We found an energy
index of the best fit power law model (
=32 for 42 dof)
equal to
=1.24
.
Accretion disks
We considered the standard -disk models in both Schwarzschild and Kerr
geometries (as in Laor & Netzer 1990, Sun & Malkan 1989). We include the
modification due to electron scattering and Comptonization of soft photons in
the disk atmosphere again for both Schwarzschild (Czerny & Elvis 1987,
Maraschi & Molendi 1990) and Kerr geometries (Siemiginowska et al. 1995). The
detailed description of the applied model can be found in Siemiginowska et
al. (1995).
An underlying power law with =0.58, normalized at the IR data
point, has been added to the accretion disk emission.
Thermal bremsstrahlung
We also considered (free-free) from a single temperature optically thin cloud (as in Barvainis 1993).
Figure: SED from IR to X-rays in the rest frame (=50 km s
Mpc
, q
=0)
for HS1700+6416. IRAS 3
upper limits are indicated with the arrows.
ROSAT PSPC best fit power law is plotted with the solid line and 1
error on the power law fit is indicated with the dashed lines. We plot the
modified blackbody emission from an accretion disk (Schwarzschild geometry)
plus the underlying power law (
=0.58) from IR to X-ray band.
Figure: Spectral energy distribution in the UV for HS1700+6416.
Models indicated in the plot: F-F - optically thin free-free emission; MBB
- modified blackbody emission from an accretion disk in the Schwarzschild
geometry: BB
- local blackbody emission from an accretion disk in the Kerr
geometry.
The UV continuum of HS1700+6416 flattens (in
)
towards high frequencies which may indicate the presence of a luminosity
peak. The continuum should turn-over at higher frequencies, since the X-ray
luminosity is more than 2 orders of magnitude lower than the extrapolated UV
continuum would predict.
is equal to 1.69.
This is consistent with the prediction given by the relation between
and optical luminosity for radio-quiet quasars (Bechtold
et al. 1994).
Modified blackbody emission from an accretion disk in
the
Schwarzschild geometry does the best at fitting the observed far UV continuum.
However, it does not fit the long wavelength part of the
spectrum. It also requires a high accretion rate, slightly above the Eddington
value.
Local blackbody emission from an accretion disk in the Kerr
geometry can describe the optical data, but its continuum drops
sharply below the HST observed spectra in the
far-UV region.
Optically thin free-free emission does not have the correct
shape in the optical-UV band.
The entire broad-band SED of quasars should be used to model their
continua.
The far-UV spectral region observed with HST gives powerful
constraints on the models.
None of the applied models can fit the far UV
continuum of HS1700+6416.
Support for this work was provided by NASA through grants number AR-5294.01-93A and AR-05785.01-94A from the Space Telescope Science Institute, which is operated by AURA, Inc. under NASA contract NAS5-26555.
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