179 research outputs found
Photoionized HBeta Emission in NGC 5548: It Breathes!
Emission-line regions in active galactic nuclei and other photoionized
nebulae should become larger in size when the ionizing luminosity increases.
This 'breathing' effect is observed for the Hbeta emission in NGC 5548 by using
Hbeta and optical continuum lightcurves from the 13-year 1989-2001 AGN Watch
monitoring campaign. To model the breathing, we use two methods to fit the
observed lightcurves in detail: (i) parameterized models and, (ii) the MEMECHO
reverberation mapping code. Our models assume that optical continuum variations
track the ionizing radiation, and that the Hbeta variations respond with time
delays due to light travel time. By fitting the data using a delay map that is
allowed to change with continuum flux, we find that the strength of the Hbeta
response decreases and the time delay increases with ionizing luminosity. The
parameterized breathing models allow the time delay and the Hbeta flux to
depend on the continuum flux so that, the time delay is proportional to the
continuum flux to the power beta, and the Hbeta flux is proportional to the
continuum flux to the power alpha. Our fits give 0.1 < beta < 0.46 and 0.57 <
alpha < 0.66. alpha is consistent with previous work by Gilbert and Peterson
(2003) and Goad, Korista and Knigge (2004). Although we find beta to be flatter
than previously determined by Peterson et al. (2002) using cross-correlation
methods, it is closer to the predicted values from recent theoretical work by
Korista and Goad (2004).Comment: 13 pages, 13 figures. Accepted for publication in MNRA
Broad iron lines in neutrons stars: dynamical broadening or wind scattering?
Broad iron emission lines are observed in many accreting systems from black
holes in AGN and X-ray binaries to neutron star low-mass X-ray binaries. The
origin of the line broadening is often interpreted as due to dynamical
broadening and relativistic effects. However, alternative interpretations have
been proposed, included broadening due to Compton scattering in a wind or
accretion disk atmosphere. Here we explore the observational signatures
expected from broadening in a wind, in particular that the iron line width
should increase with an increase in the column density of the absorber (due to
an increase in the number of scatterings). We study the data from three neutron
star low-mass X-ray binaries where both a broad iron emission line and
absorption lines are seen simultaneously, and show that there is no significant
correlation between line width and column density. This favors an inner disk
origin for the line broadening rather than scattering in a wind.Comment: 5 pages, 1 table, 5 figures, accepted for publication in Ap
Cooling of the crust in the neutron star low-mass X-ray binary MXB 1659-29
In quasi-persistent neutron star transients, long outbursts cause the neutron
star crust to be heated out of thermal equilibrium with the rest of the star.
During quiescence, the crust then cools back down. Such crustal cooling has
been observed in two quasi-persistent sources: KS 1731-260 and MXB 1659-29.
Here we present an additional Chandra observation of MXB 1659-29 in quiescence,
which extends the baseline of monitoring to 6.6 yr after the end of the
outburst. This new observation strongly suggests that the crust has thermally
relaxed, with the temperature remaining consistent over 1000 days. Fitting the
temperature cooling curve with an exponential plus constant model we determine
an e-folding timescale of 465 +/- 25 days, with the crust cooling to a constant
surface temperature of kT = 54 +/- 2 eV (assuming D=10 kpc). From this, we
infer a core temperature in the range 3.5E7-8.3E7 K (assuming D=10 kpc), with
the uncertainty due to the surface composition. Importantly, we tested two
neutron star atmosphere models as well as a blackbody model, and found that the
thermal relaxation time of the crust is independent of the chosen model and the
assumed distance.Comment: accepted for publication in ApJL, 4 pages, 1 figure
Photoionized H\u3cem\u3eβ\u3c/em\u3e emission in NGC 5548: it breathes!
Emission-line regions in active galactic nuclei (AGNs) and other photoionized nebulae should become larger in size when the ionizing luminosity increases. This `breathing\u27 effect is observed for the Hβ emission in NGC 5548 by using Hβ and optical continuum light curves from the 13-yr (1989-2001) AGN Watch monitoring campaign. To model the breathing, we use two methods to fit the observed light curves in detail: (i) parametrized models and, (ii) the MEMECHO reverberation-mapping code. Our models assume that optical continuum variations track the ionizing radiation, and that the Hβ variations respond with time-delays τ due to light travel-time. By fitting the data using a delay-map Ψ(τ, Fc) that is allowed to change with continuum flux Fc, we find that the strength of the Hβ response decreases and the time-delay increases with ionizing luminosity. The parametrized breathing models allow the time-delay and the Hβ flux to depend on the continuum flux so that, τ~Fβc and FHβ~Fαc. Our fits give 0.1 \u3c β \u3c 0.46 and 0.57 \u3c α \u3c 0.66. α is consistent with previous work by Gilbert and Peterson, and Goad, Korista and Knigge. Although we find β to be flatter than previously determined by Peterson et al. using cross-correlation methods, it is closer to the predicted values from recent theoretical work by Korista and Goad
The Quiescent X-ray Spectrum of Accreting Black Holes
The quiescent state is the dominant accretion mode for black holes on all
mass scales. Our knowledge of the X-ray spectrum is limited due to the
characteristic low luminosity in this state. Herein, we present an analysis of
the sample of dynamically-confirmed stellar-mass black holes observed in
quiescence in the \textit{Chandra/XMM-Newton/Suzaku} era resulting in a sample
of 8 black holes with 570 ks of observations. In contrast to the
majority of AGN where observations are limited by contamination from diffuse
gas, the stellar-mass systems allow for a clean study of the X-ray spectrum
resulting from the accretion flow alone. The data are characterized using
simple models. We find a model consisting of a power-law or thermal
bremsstrahlung to both provide excellent descriptions of the data, where we
measure and
respectively in the 0.3 -- 10 keV bandpass, at a median luminosity of . This result in discussed in the context of our
understanding of the accretion flow onto stellar and supermassive black holes
at low luminosities.Comment: 12 pages, 5 figures, 2 tables, MNRAS accepte
\u3cem\u3eXMM-Newton\u3c/em\u3e discovery of the X-ray transient XMMU J181227.8-181234 in the Galactic plane
We report the discovery of an X-ray transient, observed in outburst with XMM-Newton on 2003 March 20, and with position (J2000, approximate positional error 2arcsec). No known source is present at this position and the source was not detected during published ROSAT or ASCA observations of that region. However, the source may be associated with 1H1812-182 detected by HEAO 1, although the error bars on the HEAO 1 position are very large and the two sources could also be unrelated. Therefore, we name the source XMMU J181227.8-181234. Initially, the source was not detected using the All-Sky Monitor (ASM) on-board the Rossi X-ray Timing Explorer, however, reprocessing of the ASM data shows that the source was in fact detected and it was active for about 50d. The X-ray spectrum of this transient is fitted equally well by an absorbed power law (with a spectral index of 2.5) or multicolour disc blackbody model (with kT ~ 2keV), where we find that the source is highly absorbed. We detect an unabsorbed 0.5-10keV flux in the range (2-5) × 10-9ergcm-2s-1, which at a distance of 8kpc corresponds to a 0.5-10keV luminosity of (1-4) × 1037ergs-1. No pulsations were detected by timing analysis. A colour-colour diagram from ASM data of different accreting objects suggests that the transient is a high-mass X-ray binary, as is also suggested by the high absorption compared to the average interstellar value in the direction of the source. However, the power-law spectral index is far more typical of a low-mass X-ray binary. Thus, we are unable to conclusively identify the nature of the transient. We also report on three sources first detected by the ASCA Galactic Plane Survey that are close to this transient
Correlated X-ray/Ultraviolet/Optical Variability in NGC 6814
We present results of a 3-month combined X-ray/UV/optical monitoring campaign
of the Seyfert 1 galaxy NGC 6814. The object was monitored by Swift from June
through August 2012 in the X-ray and UV bands and by the Liverpool Telescope
from May through July 2012 in B and V. The light curves are variable and
significantly correlated between wavebands. Using cross-correlation analysis,
we compute the time lag between the X-ray and lower energy bands. These lags
are thought to be associated with the light travel time between the central
X-ray emitting region and areas further out on the accretion disc. The computed
lags support a thermal reprocessing scenario in which X-ray photons heat the
disc and are reprocessed into lower energy photons. Additionally, we fit the
lightcurves using CREAM, a Markov Chain Monte Carlo code for a standard disc.
The best-fitting standard disc model yields unreasonably high super-Eddington
accretion rates. Assuming more reasonable accretion rates would result in
significantly under-predicted lags. If the majority of the reprocessing
originates in the disc, then this implies the UV/optical emitting regions of
the accretion disc are farther out than predicted by the standard thin disc
model. Accounting for contributions from broad emission lines reduces the lags
in B and V by approximately 25% (less than the uncertainty in the lag
measurements), though additional contamination from the Balmer continuum may
also contribute to the larger than expected lags. This discrepancy between the
predicted and measured interband delays is now becoming common in AGN where
wavelength-dependent lags are measured.Comment: 11 pages, 8 figures, accepted for publication in MNRA
A comparison of broad iron emission lines in archival data of neutron star low-mass X-ray binaries
Relativistic X-ray disk-lines have been found in multiple neutron star
low-mass X-ray binaries, in close analogy with black holes across the
mass-scale. These lines have tremendous diagnostic power and have been used to
constrain stellar radii and magnetic fields, often finding values that are
consistent with independent timing techniques. Here, we compare CCD-based data
from Suzaku with Fe K line profiles from archival data taken with gas-based
spectrometers. In general, we find good consistency between the gas-based line
profiles from EXOSAT, BeppoSAX and RXTE and the CCD data from Suzaku,
demonstrating that the broad profiles seen are intrinsic to the line and not
broad due to instrumental issues. However, we do find that when fitting with a
Gaussian line profile, the width of the Gaussian can depend on the continuum
model in instruments with low spectral resolution, though when the different
models fit equally well the line widths generally agree. We also demonstrate
that three BeppoSAX observations show evidence for asymmetric lines, with a
relativistic disk-line model providing a significantly better fit than a
Gaussian. We test this by using the posterior predictive p-value method, and
bootstrapping of the spectra to show that such deviations from a Gaussian are
unlikely to be observed by chance.Comment: 13 pages, 9 figures, accepted to Ap
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