172 research outputs found
The Intrinsically X-ray Weak Quasar PHL 1811. II. Optical and UV Spectra and Analysis
This is the second of two papers reporting observations and analysis of the
unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line
quasar PHL 1811. The first paper reported that PHL 1811 is intrinsically X-ray
weak, and presented a spectral energy distribution (SED). Here we present HST
STIS optical and UV spectra, and ground-based optical spectra. The optical and
UV line emission is very unusual. There is no evidence for forbidden or
semiforbidden lines. The near-UV spectrum is dominated by very strong FeII and
FeIII, and unusual low-ionization lines such as NaID and CaII H&K are observed.
High-ionization lines are very weak; CIV has an equivalent width of 6.6A, a
factor of ~5 smaller than measured from quasar composite spectra. An unusual
feature near 1200A can be deblended in terms of Ly\alpha, NV, SiII, and CIII*
using the blueshifted CIV profile as a template. Photoionization modeling shows
that the unusual line emission can be explained qualitatively by the unusually
soft SED. Principally, a low gas temperature results in inefficient emission of
collisionally excited lines, including the semiforbidden lines generally used
as density diagnostics. The emission resembles that of high-density gas; in
both cases this is a consequence of inefficient cooling. PHL 1811 is very
unusual, but we note that quasar surveys are generally biased against finding
similar objects.Comment: Accepted for publication in ApJS. Full resolution figures available
here: http://www.nhn.ou.edu/~leighly/phl1811_paper1.pd
Acceleration and Substructure Constraints in a Quasar Outflow
We present observations of probable line-of-sight acceleration of a broad
absorption trough of C IV in the quasar SDSS J024221.87+004912.6. We also
discuss how the velocity overlap of two other outflowing systems in the same
object constrains the properties of the outflows. The Si IV doublet in each
system has one unblended transition and one transition which overlaps with
absorption from the other system. The residual flux in the overlapping trough
is well fit by the product of the residual fluxes in the unblended troughs. For
these optically thick systems to yield such a result, at least one of them must
consist of individual subunits rather than being a single structure with
velocity-dependent coverage of the source. If these subunits are identical,
opaque, spherical clouds, we estimate the cloud radius to be r = 3.9 10^15 cm.
If they are identical, opaque, linear filaments, we estimate their width to be
w = 6.5 10^14 cm. These subunits are observed to cover the Mg II broad emission
line region of the quasar, at which distance from the black hole the above
filament width is equal to the predicted scale height of the outer atmosphere
of a thin accretion disk. Insofar as that scale height is a natural size scale
for structures originating in an accretion disk, these observations are
evidence that the accretion disk can be a source of quasar absorption systems.
Based on data from ESO program 075.B-0190(A).Comment: 14 emulateapj pages, 7 figures, ApJ in pres
The Intrinsically X-ray Weak Quasar PHL 1811. I. X-ray Observations and Spectral Energy Distribution
This is the first of two papers reporting observations and analysis of the
unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line
quasar PHL 1811, focusing on the X-ray properties and the spectral energy
distribution. Two Chandra observations reveal a weak X-ray source with a steep
spectrum. Variability by a factor of 4 between the two observations separated
by 12 days suggest that the X-rays are not scattered emission. The XMM-Newton
spectra are modelled in the 0.3--5 keV band by a steep power law with \Gamma =
2.3\pm 0.1, and the upper limit on intrinsic absorption is 8.7 x 10^{20}
cm^{-2}. The spectral slopes are consistent with power law indices commonly
observed in NLS1s, and it appears that we observe the central engine X-rays
directly. Including two recent Swift ToO snapshots, a factor of ~5 variability
was observed among the five X-ray observations reported here. In contrast, the
UV photometry obtained by the XMM-Newton OM and Swift UVOT, and the HST
spectrum reveal no significant UV variability. The \alpha_{ox} inferred from
the Chandra and contemporaneous HST spectrum is -2.3 \pm 0.1, significantly
steeper than observed from other quasars of the same optical luminosity. The
steep, canonical X-ray spectra, lack of absorption, and significant X-ray
variability lead us to conclude that PHL 1811 is intrinsically X-ray weak. We
also discuss an accretion disk model, and the host galaxy of PHL 1811.Comment: 45 pages, 9 figures, accepted for publication in Ap
ASCA observations of two steep soft X-ray quasars
Steep soft X-ray (0.1-2 keV) quasars share several unusual properties: narrow
Balmer lines, strong FeII emission, large and fast X-ray variability, rather
steep 2-10 keV spectrum. These intriguing objects have been suggested to be the
analogs of Galactic black hole candidates in the high, soft state. We present
here results from ASCA observations for two of these quasars: NAB0205+024 and
PG1244+026. Both objects show similar variations (factor of about 2 in 10 ks),
despite a factor of about ten difference in the 0.5-10 keV luminosity (7.3E43
erg/s for PG1244+026 and 6.4E44 erg/s for NAB0205+024, assuming isotropic
emission, H_0 = 50.0 and q_0 = 0.0). The X-ray continuum of the two quasars
flattens by 0.5-1 going from the 0.1-2 keV band toward higher energies,
strengthening recent results on another half dozen steep soft X-ray AGN.
PG1244+026 shows a significant feature in the `1 keV' region, which can be
described by either as a broad emission line centered at 0.95 keV (quasar
frame) or as edge or line absorption at 1.17 (1.22) keV. The line emission
could be due to reflection from an highly ionized accretion disk, in line with
the view that steep soft X-ray quasars are emitting close to the Eddington
luminosity. Photoelectric edge absorption or resonant line absorption could be
produced by gas outflowing at a large velocity (0.3-0.6 c).Comment: Accepted for publication in MNRAS, 10 page
A Population of X-ray Weak Quasars: PHL 1811 Analogs at High Redshift
We report the results from Chandra and XMM-Newton observations of a sample of
10 type 1 quasars selected to have unusual UV emission-line properties (weak
and blueshifted high-ionization lines; strong UV Fe emission) similar to those
of PHL 1811, a confirmed intrinsically X-ray weak quasar. These quasars were
identified by the Sloan Digital Sky Survey at high redshift (z~2.2); eight are
radio quiet while two are radio intermediate. All of the radio-quiet PHL 1811
analogs are notably X-ray weak by a mean factor of ~13. These sources lack
broad absorption lines and have blue UV/optical continua, suggesting they are
intrinsically X-ray weak. However, their average X-ray spectrum appears to be
harder than those of typical quasars, which may indicate the presence of heavy
intrinsic X-ray absorption. Our radio-quiet PHL 1811 analogs support a
connection between an X-ray weak spectral energy distribution and PHL 1811-like
UV emission lines; this connection provides an economical way to identify X-ray
weak type 1 quasars. The fraction of radio-quiet PHL 1811 analogs in the
radio-quiet quasar population is estimated to be < 1.2%. We have investigated
correlations between relative X-ray brightness and UV emission-line properties
for a sample combining radio-quiet PHL 1811 analogs, PHL 1811, and typical type
1 quasars. These correlation analyses suggest that PHL 1811 analogs may have
extreme wind-dominated broad emission-line regions. Observationally,
radio-quiet PHL 1811 analogs appear to be a subset (~30%) of radio-quiet
weak-line quasars. The existence of a subset of quasars in which
high-ionization "shielding gas" covers most of the BELR, but little more than
the BELR, could potentially unify the PHL 1811 analogs and WLQs. The two
radio-intermediate PHL 1811 analogs are X-ray bright. One of them appears to
have jet-dominated X-ray emission, while the nature of the other remains
unclear.Comment: ApJ accepted; 25 pages, 11 figures and 8 table
Formation of Structure in Snowfields: Penitentes, Suncups, and Dirt Cones
Penitentes and suncups are structures formed as snow melts, typically high in
the mountains. When the snow is dirty, dirt cones and other structures can form
instead. Building on previous field observations and experiments, this work
presents a theory of ablation morphologies, and the role of surface dirt in
determining the structures formed. The glaciological literature indicates that
sunlight, heating from air, and dirt all play a role in the formation of
structure on an ablating snow surface. The present work formulates a
mathematical model for the formation of ablation morphologies as a function of
measurable parameters. The dependence of ablation morphologies on weather
conditions and initial dirt thickness are studied, focusing on the initial
growth of perturbations away from a flat surface. We derive a single-parameter
expression for the melting rate as a function of dirt thickness, which agrees
well with a set of measurements by Driedger. An interesting result is the
prediction of a dirt-induced travelling instability for a range of parameters.Comment: 28 pages, 13 figure
A Self-Consistent NLTE-Spectra Synthesis Model of FeLoBAL QSOs
We present detailed radiative transfer spectral synthesis models for the Iron
Low Ionization Broad Absorption Line (FeLoBAL) active galactic nuclei (AGN)
FIRST J121442.3+280329 and ISO J005645.1-273816. Detailed NLTE spectral
synthesis with a spherically symmetric outflow reproduces the observed spectra
very well across a large wavelength range. While exact spherical symmetry is
probably not required, our model fits are of high quality and thus very large
covering fractions are strongly implied by our results. We constrain the
kinetic energy and mass in the ejecta and discuss their implications on the
accretion rate. Our results support the idea that FeLoBALs may be an
evolutionary stage in the development of more ``ordinary'' QSOs.Comment: Accepted for publication in ApJ/removed misleading remarks about
CLOUDY in section
X-ray Observations of the Seyfert galaxy LB 1727 (1H 0419-577)
We discuss the properties of the Seyfert 1.5 galaxy LB 1727, also known as 1H
0419-577, from X-ray observations obtained by ASCA and ROSAT along with optical
observations from earlier epochs. ASCA shows only modest (< 20%) variations in
X-ray flux within or between the observations. In contrast, a daily monitoring
campaign over 1996 Jun - Sept by the ROSAT HRI instrument reveals the soft
X-ray (0.1-2 keV) flux to have increased by a factor ~3.
The 2 - 10 keV continuum can be parameterized as a power-law with a photon
index Gamma ~ 1.45-1.68 across ~0.7 - 11 keV in the rest-frame. We also report
the first detection of iron Kalpha line emission in this source. Simultaneous
ASCA and ROSAT data show the X-ray spectrum to steepen sharply at a rest-energy
\~0.75 keV, the spectrum below this energy can be parameterized as a power-law
of slope Gamma ~3.6. We show that LB 1727 is one of the few Seyferts for which
we can rule out the possibility that the presence of a warm absorber is solely
responsible for the spectral steepening in the soft X-ray regime. Consideration
of the overall spectral-energy-distribution for this source indicates the
presence of a pronounced XUV-bump visible in optical, ultraviolet and soft
X-ray data.Comment: 27 pages, 7 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journa
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