Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://hea-www.harvard.edu/QEDT/Papers/hst2_paris.ps Äàòà èçìåíåíèÿ: Tue Jan 16 00:28:53 1996 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:34:14 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 8

Science with the Hubble Space Telescope -- II
Space Telescope Science Institute, 1996
P. Benvenuti, F. D. Macchetto, & E. J. Schreier, eds.
Modeling of the Blue Bump Emission in High Redshift Quasars.
Aneta Siemiginowska 1 , Jill Bechtold 2 , Kim­Vy Tran 2 , Adam Dobrzycki 1
Abstract. High luminosity and high redshift combine in quasars to give conditions
that allow studies of the intergalactic medium, as well as the emission mechanism in
quasars. High luminosity makes quasars bright enough to study in detail at z = 3.
High redshift also brings the EUV part of the emitted spectrum into observable
wavelengths, and it is in the EUV where the power output of quasars peaks, and
where the photoionizing continuum is produced.
We present results of the modeling of the spectrum of the high redshift (z=2.72),
high luminosity (L ¸ 5\Theta10 48 ergs s \Gamma1 ) quasar, HS1700+6416 (Reimers et al. 1989),
observed with HST FOS (Reimers at al 1992) and GHRS (Reimers 1995). We an­
alyzed the HST spectra together with optical spectrophotometry and identified ab­
sorption systems towards the quasar. We corrected the spectra for the intervening
Lyman limit absorbers and defined the intrinsic optical­UV continuum. The con­
tinuum extends down to 310 š A. We included the X­ray spectrum from the ROSAT
PSPC observation (Reimers et al. 1995) from 0.37 keV to ¸10 keV in the rest frame.
The X­ray energy index for the best fit power law model is equal to 1.24. It is steep,
but not unusual for radio­quiet quasars at this redshift (Bechtold et al 1994). The
observed ff ox =1.69 is consistent with the prediction given by the relation between
ff ox and optical luminosity for radio­quiet quasars (Bechtold et al 1994).
We fit the HS1700+6416 spectral energy distribution with several theoretical
models including accretion disk models and free­free optically thin emission. None
of the applied models can fit the UV continuum of HS1700+6416.
1. Spectral Energy Distribution of 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 detec­
tion at 60 ¯m (Tanner et al. 1995).
HST FOS We have extracted data for HS1700+6416 from the HST Archive. We recali­
brated 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(B­V)=0.051,
which corresponds to a Galactic column density of NH =2.46\Theta10 20 cm \Gamma2 .
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).
1 Center for Astrophysics, Cambridge, MA, USA
2 Steward Observatory, Tuscon, AZ, USA
1

2
Optical
The optical spectrum of HS1700+6416 was taken on July 4, 1995, using the FLWO
Tillinghast 1.5­meter telescope with the FAST spectrograph (exposure time of 900 seconds).
The spectrum has been corrected for the interstellar reddening with E(B­V)=0.051.
X­ray 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 power­law model, assuming only Galactic absorption (NH =2.46\Theta10 20 cm \Gamma1 ).
We found an energy index of the best fit power law model (ü 2 =32 for 42 dof) equal to
ff E =1.24 +
\Gamma 0:06.
Table 1. Lyman Limit Systems a
Redshift Ü
2.433 0.31575
2.315 0.4460
2.1678 0.4460
1.8465 0.3542
1.725 0.70687
1.1572 0.4460
0.8642 0.1410
a Vogel & Reimers, 1995
Table 2. DATA
Instrument Observed Band Rest Frame Date Ref
IRAS 100--12 ¯m 26.8--3.2 ¯m Jan­Nov 1983 Neugebauer et al. 1984
FLWO 7540­3650 š A 2027­981 š A July 1995 This Work
HST FOS 3240­1140 š A 871­306 š A G270H: Dec 13/14 1991 Reimers et al. 1992
G190H Dec 13/14 1991
G130H Feb 13/14 1992
ROSAT PSPC 0.1­2.5 keV 0.37­9.3 keV Nov 13/14 1992 Reimers et al. 1995
July 21/22 1993

3
2. Modelling the Spectral Energy Distribution of HS1700+6416
ffl Accretion disks.
We considered the standard ff­disk 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 ff=0.58, normalized at the IR data point, has been added to the
accretion disk emission.
ffl Thermal bremsstrahlung
We also considered (free­free) from a single temperature optically thin cloud (as in Barvainis
1993).
Table 3. MODELS
Model M 8
—
M cos` L/LEdd T [K]
[M fi yr \Gamma 1]
Blackbody (S) 2.8 120 1.0 15.6
Modified Blackbody (S) 20 100 1.0 1.8
Blackbody (K) 100 21.2 0.5 0.3
Modified Blackbody (K) 100 56.5 0.5; 0.75 0.8
Opt. Thin Free­Free 6\Theta10 5
S -- Schwarzschild, K -- Kerr geometry.
3. Results
ffl The UV continuum of HS1700+6416 flattens (in log šL š ) towards high frequencies which may
indicate the presence of a luminosity peak. The continuum should turn­over at higher frequencies,
since the X­ray luminosity is more than 2 orders of magnitude lower than the extrapolated UV
continuum would predict.
ffl ff ox = log(F 2keV =F
3000
š A
)=2:685 is equal to 1.69. This is consistent with the prediction given by
the relation between ff ox and optical luminosity for radio­quiet quasars (Bechtold et al. 1994).
ffl 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.
ffl 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 far­UV region.
ffl Optically thin free­free emission does not have the correct shape in the optical­UV band.
4. Conclusions
ffl The entire broad­band SED of quasars should be used to model their continua.
ffl The far­UV spectral region observed with HST gives powerful constraints on the models.
ffl None of the applied models can fit the far UV continuum of HS1700+6416.

4
Figure 1. Spectral energy distribution of HS 1700+641, corrected for the ab­
sorption due to the Lyman limit systems. Solid line represents a continuum fit.
The data are also corrected for the interstellar reddening.
Acknowledgments. Support for this work was provided by NASA through grants number AR­
5294.01­93A and AR­05785.01­94A from the Space Telescope Science Institute, which is operated by
the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS5­26555.
References
Aldcroft,T.L., 1993, Ph.D. Thesis, Appendix A, Stanford University
Barvainis, R. 1993, ApJ, 412, 513
Bechtold J., et al 1994, AJ, 108, 374
Czerny, B. & Elvis, M. 1987, ApJ, 321, 305
Laor,A. & Netzer H., 1990, MNRAS,238, 897
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5
Figure 2. SED from IR to X­rays in the rest frame (H 0 =50 km s \Gamma1 Mpc \Gamma1 , q 0 =0)
for HS1700+6416. IRAS 3oe upper limits are indicated with the arrows. ROSAT
PSPC best fit power law is plotted with the solid line and 1oe 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
(ff=0.58) from IR to X­ray band.
Figure 3. Spectral energy distribution in the UV for HS1700+6416. Models
indicated in the plot: F­F ­ optically thin free­free emission; MBB S ­ modified
blackbody emission from an accretion disk in the Schwarzschild geometry: BBK ­
local blackbody emission from an accretion disk in the Kerr geometry.