Partial Coverage Analysis (Misawa et al. 2005)

1
Dramatically Variable C IV mini-Broad Absorption
Line System in the Quasar HS 16033820 T.
Misawa, M. Eracleous, J. C. Charlton, G. Chartas
(Penn State), N. Kashikawa (NAOJ)
SUMMARY We observed the quasar HS 16033820 (zem
2.542, first discovered by Dobrzycki et al.
1996) six times over an interval of 4.2 yrs (1.2
yrs in the quasar rest frame) using the High
Dispersion Spectrograph on Subaru telescope. The
purpose was to study the mini-broad absorption
line (mini-BAL FWHM 1,000km/s) that was
blue-shifted from the quasar by 9,500 km/s. We
found in the mini-BAL both time-variability and
partial coverage, which supported the physical
association of the mini-BAL gas and the quasar
(not arising in intervening galaxies or IGM). We
propose two possible scenarios for this
variability (i) rapid continuum fluctuations,
coupled with coverage fraction fluctuations, and
(ii) scattering of continuum photons around the
absorber. We also used archival Chandra X-ray
data to study the X-ray properties of this
quasar, and found that intrinsic X-ray absorption
in this quasar (log (NH/cm-2) lt 22) is much
weaker than those in BAL quasars, while it has
intermediate optical-to-X-ray flux ratio (aox)
compared to BAL and normal quasars. These results
would constrain geometrical structures of the
quasar central region. This work was
supported by NASA grant NAG5-10817.


Partial Coverage Analysis (Misawa et al. 2005)
(What is the typical size of absorber?)
Time Variability Analysis (Misawa et al.
2007) (How stable of absorbers physical
condition?)
We have monitored the C IV mini-BAL (for
which we have already found partial coverage) for
more than 4 years with SubaruHDS, and found a
dramatic variability (Figure A). We fitted
models to two of the C IV components at
?5284-5318Å (enclosed in blue rectangle) in the
system, with four free parameters redshift (z),
column density (log N), Doppler parameter (b),
and Covering factor (Cf). Temporal variations of
fit parameters (i.e., total observed-frame
equivalent width (Wobs), log N, b, Cf, and
ejection velocity (Vej) are plotted in Figure B
(from top to bottom).
The optical depth ratio of C IV or other UV
doublet lines sometimes deviates from the values
expected from atomic physics, 21. This
discrepancy can be explained if the absorber
covers the continuum source only partially, and
an unabsorbed continuum changes the relative
depths of the lines (e.g., Wampler et al. 1995).
absorber
(A)
(B)
QSO
A C IV mini-BAL with an ejection velocity of
Vej 9,000 km/s from the quasar (left) has
clearly partial coverage, while a C IV NAL
(right) has full coverage (Cf 1). Mini-BALs
have the advantages of both BALs (i.e., high
probability of being intrinsic to the quasars)
and NALs (i.e., line profiles can be resolved and
fitted with Voigt models.)
Possible Origins of Time Variability (Misawa
et al. 2007a)
(1) Scattering of continuum photons around the
absorbers
(2) Change of ionization state by a variable
screening material
If there exists scattering materials that
redirect continuum photons toward the observer,
Covering factor can change independent of the
other fit parameters. Validity of this idea can
be checked by future spectropolarimetric
observations, because scattered light are highly
polarized.
If there exists a screen of variable optical
depth between the continuum source and the
absorber, it can change the absorbers ionization
condition much faster than any expected
variations in the UV continuum of such a luminous
quasar. We can test this scenario with X-ray
observations because X-ray spectra can probe
warm (partly-ionized) X-ray absorbers.
Lamy Hutsemékers (2004)
Gallagher Everett (2006)
X-Ray Properties of quasars with mini-BALs
(Misawa et al. 2007b) (Where do the mini-BALs
arise?)
(B) Optical-to-X-ray Spectral Index (a OX)
The index of a power law, connecting the
rest-frame SED at 2500Å and 2keV (aOX), is
another indicator of X-ray absorption (Tanambaum
et al. 1979). After correcting the value based on
the UV luminosity (Steffen et al. 2006), we do
not see any significant X-ray absorption in the
quasar.
(A) Model Fits to the Observed Spectrum

• We test two competing scenarios for the location
  of the NAL gas in an accretion-disk wind
• NAL/mini-BAL gas locate at very low latitudes
  above the accretion disk, showing very large
  column densities comparable to BAL quasars (i.e.,
  logNH 22-23 Elvis 2000),
• NAL/mini-BAL gas are at high latitudes having
  lower column densities (Ganguly et al. 2001.)

We fitted the archival Chandra X-ray spectrum of
HS16033820 using the XSPE, with 3 free
parameters photon index (G), photon flux at 1keV
(N0), and intrinsic column density (NH) . We
found NH much smaller than in BAL quasars.
CONCLUSION Mini-BALs arise in the shearing zone
between the outflow gas and highly-ionized
low-density medium as suggested by Ganguly et al.
2001.
References
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305
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