2,634 research outputs found
On variability and spectral distortion of the fluorescent iron lines from black-hole accretion discs
We investigate properties of iron fluorescent line arising as a result of
illumination of a black hole accretion disc by an X-ray source located above
the disc surface. We study in details the light-bending model of variability of
the line, extending previous work on the subject.
We indicate bending of photon trajectories to the equatorial plane, which is
a distinct property of the Kerr metric, as the most feasible effect underlying
reduced variability of the line observed in several objects. A model involving
an X-ray source with a varying radial distance, located within a few central
gravitational radii around a rapidly rotating black hole, close to the disc
surface, may explain both the elongated red wing of the line profile and the
complex variability pattern observed in MCG--6-30-15 by XMM-Newton.
We point out also that illumination by radiation which returns to the disc
(following the previous reflection) contributes significantly to formation of
the line profile in some cases. As a result of this effect, the line profile
always has a pronounced blue peak (which is not observed in the deep minimum
state in MCG--6-30-15), unless the reflecting material is absent within the
innermost 2--3 gravitational radii.Comment: 24 pages, 22 figures. Accepted for publication in MNRA
Nonthermal X-Rays from Supernova Remnant G330.2+1.0 and the Characteristics of its Central Compact Object
We present results from our X-ray data analysis of the SNR G330.2+1.0 and its
CCO, CXOU J160103.1--513353 (J1601). Using our XMM-Newton and Chandra
observations, we find that the X-ray spectrum of J1601 can be described by
neutron star atmosphere models (T ~ 2.5--3.7 MK). Assuming the distance of d ~
5 kpc for J1601 as estimated for SNR G330.2+1.0, a small emission region of R ~
1--2 km is implied. X-ray pulsations previously suggested by Chandra are not
confirmed by the XMM-Newton data, and are likely not real. However, our timing
analysis of the XMM-Newton data is limited by poor photon statistics, and thus
pulsations with a relatively low amplitude (i.e., an intrinsic pulsed-fraction
< 40%) cannot be ruled out. Our results indicate that J1601 is a CCO similar to
that in the Cassiopeia A SNR.X-ray emission from SNR G330.2+1.0 is dominated by
power law continuum (Gamma ~ 2.1--2.5) which primarily originates from thin
filaments along the boundary shell. This X-ray spectrum implies synchrotron
radiation from shock-accelerated electrons with an exponential roll-off
frequency ~ 2--3 x 10^17 Hz. For the measured widths of the X-ray filaments (D
~ 0.3 pc) and the estimated shock velocity (v_s ~ a few x 10^3 km s^-1), a
downstream magnetic field B ~ 10--50 G is derived. The estimated maximum
electron energy E_max ~ 27--38 TeV suggests that G330.2+1.0 is a candidate TeV
gamma-ray source. We detect faint thermal X-ray emission in G330.2+1.0. We
estimate a low preshock density n_0 ~ 0.1 cm^-3, which suggests a dominant
contribution from an inverse Compton mechanism (than the proton-proton
collision) to the prospective gamma-ray emission. Follow-up deep radio, X-ray,
and gamma-ray observations will be essential to reveal the details of the shock
parameters and the nature of particle accelerations in this SNR.Comment: 26 pages, 3 tables, 7 figures (4 color figures), Accepted by Ap
Simultaneous EUVE/ASCA/RXTE Observations of NGC 5548
We present simultaneous observations by EUVE, ASCA, and RXTE of the type~1
Seyfert galaxy NGC 5548. These data indicate that variations in the EUV
emission (at keV) appear to lead similar modulations in higher
energy (\ga 1 keV) X-rays by 10--30 ks. This is contrary to popular
models which attribute the correlated variability of the EUV, UV and optical
emission in type~1 Seyferts to reprocessing of higher energy radiation. This
behavior instead suggests that the variability of the optical through EUV
emission is an important driver for the variability of the harder X-rays which
are likely produced by thermal Comptonization. We also investigate the spectral
characteristics of the fluorescent iron K line and Compton reflection
emission. In contrast to prior measurements of these spectral features, we find
that the iron K line has a relatively small equivalent width
(eV) and that the reflection component is consistent
with a covering factor which is significantly less than unity (--0.5). Notably, although the 2--10 keV X-ray flux varies by % and the derived reflection fraction appears to be constant throughout our
observations, the flux in the Fe~K line is also constant. This behavior
is difficult to reconcile in the context of standard Compton reflection models.Comment: 13 pages, 6 figures, LaTeX, uses emulateapj.sty and apjfonts.sty,
submitted to Ap
Testing Comptonizing coronae on a long BeppoSAX observation of the Seyfert 1 galaxy NGC 5548
We test accurate models of Comptonization spectra over the high quality data
of the BeppoSAX long look at NGC 5548, allowing for different geometries of the
scattering region, different temperatures of the input soft photon field and
different viewing angles. We find that the BeppoSAX data are well represented
by a plane parallel or hemispherical corona viewed at an inclination angle of
30. For both geometries the best fit temperature of the soft photons
is close to 15 eV. The corresponding best fit values of the hot
plasma temperature and optical depth are 250--260 keV and
0.16--0.37 for the slab and hemisphere respectively. These values
are substantially different from those derived fitting the data with a
power-law + cut off approximation to the Comptonization component (kT_{\rm
e}\lta 60 keV, 2.4). This is due to the fact that accurate
Comptonization spectra in anisotropic geometries show "intrinsic" curvature
which reduces the necessity of a high energy cut-off. The Comptonization
parameter derived for the slab model {is} larger than predicted for a two phase
plane parallel corona in energy balance, suggesting that a more
``photon-starved'' geometry is necessary. The spectral softening detected
during a flare which occurred in the central part of the observation
corresponds to a decrease of the Comptonization parameter, probably associated
with an increase of the soft photon luminosity, the {hard} photon luminosity
remaining constant.Comment: 36 pages, 9 figures, accepted by Ap
X-ray absorption and rapid variability of the dwarf Seyfert nucleus of NGC4395
We report the detection of an absorbed central X-ray source and its strong,
rapid variability in NGC4395, the least luminous Seyfert nucleus known. The
X-ray source exhibits a number of flares with factors of 3-4 flux changes
during a half day ASCA observation. Such X-ray variability is in constrast to
the behaviour of other low luminosity active galaxies. It provides further
support for an accreting black hole model rather than an extreme stellar
process in accounting for the nuclear activity of NGC4395. The soft X-ray
emission below 3 keV is strongly attenuated by absorption. The energy spectrum
in this absorption band shows a dramatic change in response to the variation in
continuum luminosity. A variable warm absorber appears to be an explanation for
the spectral change. The absorption-corrected 2-10 keV luminosity is 4e39 erg/s
for a source distance of 2.6 Mpc, and at 1 keV is one order of magnitude above
previous ROSAT estimates. Our X-ray results infer the nuclear source of NGC4395
to be a scaled-down version of higher luminosity Seyfert nuclei, with an
intermediate mass (10^4-10^5 Msun) black hole, unlike the nearby low luminosity
active galaxies in which underfed massive black holes are suspected to reside.Comment: 12 pages, accepted for publication in MNRA
WALLABY Early Science - I. The NGC 7162 Galaxy Group
We present Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY) early
science results from the Australian Square Kilometre Array Pathfinder (ASKAP)
observations of the NGC 7162 galaxy group. We use archival HIPASS and Australia
Telescope Compact Array (ATCA) observations of this group to validate the new
ASKAP data and the data reduction pipeline ASKAPsoft. We detect six galaxies in
the neutral hydrogen (HI) 21-cm line, expanding the NGC 7162 group membership
from four to seven galaxies. Two of the new detections are also the first HI
detections of the dwarf galaxies, AM 2159-434 and GALEXASC J220338.65-431128.7,
for which we have measured velocities of and km s,
respectively. We confirm that there is extended HI emission around NGC 7162
possibly due to past interactions in the group as indicated by the
offset between the kinematic and morphological major axes for NGC 7162A, and
its HI richness. Taking advantage of the increased resolution (factor of
) of the ASKAP data over archival ATCA observations, we fit a tilted
ring model and use envelope tracing to determine the galaxies' rotation curves.
Using these we estimate the dynamical masses and find, as expected, high dark
matter fractions of for all group members. The
ASKAP data are publicly available.Comment: 20 pages, 11 figures, accepted for publication in MNRA
XMM-Newton spectroscopy of high redshift quasars
We present XMM-Newton X-ray spectra and optical photometry of four high
redshift (z=2.96-3.77) quasars, [HB89] 0438-436, [HB89] 2000-330, [SP89]
1107+487 and RX J122135.6+280613; of these four objects the former two are
radio-loud, the latter two radio-quiet. Model fits require only a power law
with Galactic absorption in each case; additional intrinsic absorption is also
needed for [HB89] 0438-436 and RX J122135.6+280613. The spectra are hard (Gamma
\~1.7 for [HB89] 0438-436, [HB89] 2000-330 and ~1.4 for RX J122135.6+280613)
with the exception of [SP89]~1107+487 which is softer (Gamma ~2.0); the
combined Galactic and intrinsic absorption of lower energy X-rays in the latter
source is much less significant than in the other three. The two intrinsically
unabsorbed sources have greater optical fluxes relative to the X-ray
contributions at the observed energies. While there is no need to include
reflection or iron line components in the models, our derived upper limits (99%
confidence) on these parameters are not stringent; the absence of these
features, if confirmed, may be explained in terms of the high power law
contribution and/or a potentially lower albedo due to the low disc temperature.
However, we note that the power-law spectrum can be produced via mechanisms
other than the Comptonization of accretion disc emission by a corona; given
that all four of these quasars are radio sources at some level we should also
consider the possibility that the X-ray emission originates, at least
partially, in a jet.Comment: Accepted for publication in MNRA
A Structure for Quasars
This paper proposes a simple, empirically derived, unifying structure for the
inner regions of quasars. This structure is constructed to explain the broad
absorption line (BAL) regions, the narrow `associated' ultraviolet and X-ray
warm absorbers (NALs); and is also found to explain the broad emission line
regions (BELR), and several scattering features, including a substantial
fraction of the broad X-ray Iron-K emission line, and the bi-conical extended
narrow emission line region (ENLR) structures seen on large kiloparsec scales
in Seyfert images. Small extensions of the model to allow luminosity dependent
changes in the structure may explain the UV and X-ray Baldwin effects and the
greater prevalence of obscuration in low luminosity AGN.Comment: 35 pages, including 8 color figures (figures 4abc are big).
Astrophysical Journal, in press. Expanded version of conference paper
astro-ph/000516
Recommended from our members
Failure of Working Memory Training to Enhance Cognition or Intelligence
Fluid intelligence is important for successful functioning in the modern world, but much evidence suggests that fluid intelligence is largely immutable after childhood. Recently, however, researchers have reported gains in fluid intelligence after multiple sessions of adaptive working memory training in adults. The current study attempted to replicate and expand those results by administering a broad assessment of cognitive abilities and personality traits to young adults who underwent 20 sessions of an adaptive dual n-back working memory training program and comparing their post-training performance on those tests to a matched set of young adults who underwent 20 sessions of an adaptive attentional tracking program. Pre- and post-training measurements of fluid intelligence, standardized intelligence tests, speed of processing, reading skills, and other tests of working memory were assessed. Both training groups exhibited substantial and specific improvements on the trained tasks that persisted for at least 6 months post-training, but no transfer of improvement was observed to any of the non-trained measurements when compared to a third untrained group serving as a passive control. These findings fail to support the idea that adaptive working memory training in healthy young adults enhances working memory capacity in non-trained tasks, fluid intelligence, or other measures of cognitive abilities.</p
Reddening, Emission-Line, and Intrinsic Absorption Properties in the Narrow-Line Seyfert 1 Galaxy Akn 564
We use Hubble Space Telescope UV and optical spectra of the narrow-line
Seyfert 1 (NLS1) galaxy Akn 564 to investigate its internal reddening and
properties of its emission-line and intrinsic UV absorption gas. We find that
the extinction curve of Akn 564, derived from a comparison of its UV/optical
continuum to that of an unreddened NLS1, lacks a 2200 A bump and turns up
towards the UV at a longer wavelength (4000 A) than the standard Galactic, LMC,
and SMC curves. However, it does not show the extremely steep rise to 1200 A
that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The
emission-lines and continuum experience the same amount of reddening,
indicating the presence of a dust screen that is external to the narrow-line
region (NLR). Echelle spectra from the Space Telescope Imaging Spectrograph
show intrinsic UV absorption lines due to Ly-alpha, N V, C IV, Si IV, and Si
III, centered at a radial velocity of -190 km/s (relative to the host galaxy).
Photoionization models of the UV absorber indicate that it has a sufficient
columnand is at a sufficient distance from the nucleus (D > 95 pc) to be the
source of the dust screen. Thus, Akn 564 contains a dusty ``lukewarm absorber''
similar to that seen in NGC 3227.Comment: 25 pages, 4 figures. LaTeX with encapsulated postscript figures.
Accepted for publication in the Astrophysical Journa
- …