1,118 research outputs found
Continued cooling of the crust in the neutron star low-mass X-ray binary KS 1731-260
Some neutron star low-mass X-ray binaries have very long outbursts (lasting
several years) which can generate a significant amount of heat in the neutron
star crust. After the system has returned to quiescence, the crust then
thermally relaxes. This provides a rare opportunity to study the thermal
properties of neutron star crusts, putting constraints on the thermal
conductivity and hence the structure and composition of the crust. KS 1731-260
is one of only four systems where this crustal cooling has been observed. Here,
we present a new Chandra observation of this source approximately 8 years after
the end of the last outburst, and 4 years since the last observation. We find
that the source has continued to cool, with the cooling curve displaying a
simple power-law decay. This suggests that the crust has not fully thermally
relaxed yet, and may continue to cool further. A simple power law decay is in
contrast to theoretical cooling models of the crust, which predict that the
crust should now have cooled to the same temperature as the neutron star core.Comment: Accepted to ApJ Letter
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
On Neutral Absorption and Spectral Evolution in X-ray Binaries
Current X-ray observatories make it possible to follow the evolution of
transient and variable X-ray binaries across a broad range in luminosity and
source behavior. In such studies, it can be unclear whether evolution in the
low energy portion of the spectrum should be attributed to evolution in the
source, or instead to evolution in neutral photoelectric absorption. Dispersive
spectrometers make it possible to address this problem. We have analyzed a
small but diverse set of X-ray binaries observed with the Chandra High Energy
Transmission Grating Spectrometer across a range in luminosity and different
spectral states. The column density in individual photoelectric absorption
edges remains constant with luminosity, both within and across source spectral
states. This finding suggests that absorption in the interstellar medium
strongly dominates the neutral column density observed in spectra of X-ray
binaries. Consequently, evolution in the low energy spectrum of X-ray binaries
should properly be attributed to evolution in the source spectrum. We discuss
our results in the context of X-ray binary spectroscopy with current and future
X-ray missions.Comment: Accepted for publication in ApJ Letter
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
Super-Eddington fluxes during thermonuclear X-ray bursts
It has been known for nearly three decades that the energy spectra of
thermonuclear X-ray bursts are often well-fit by Planck functions with
temperatures so high that they imply a super-Eddington radiative flux at the
emitting surface, even during portions of bursts when there is no evidence of
photospheric radius expansion. This apparent inconsistency is usually set aside
by assuming that the flux is actually sub-Eddington and that the fitted
temperature is so high because the spectrum has been distorted by the
energy-dependent opacity of the atmosphere. Here we show that the spectra
predicted by currently available conventional atmosphere models appear
incompatible with the highest-precision measurements of burst spectra made
using the Rossi X-ray Timing Explorer, such as during the 4U 1820-30 superburst
and a long burst from GX 17+2. In contrast, these measurements are well-fit by
Bose-Einstein spectra with high temperatures and modest chemical potentials.
Such spectra are very similar to Planck spectra. They imply surface radiative
fluxes more than a factor of three larger than the Eddington flux. We find that
segments of many other bursts from many sources are well-fit by similar
Bose-Einstein spectra, suggesting that the radiative flux at the emitting
surface also exceeds the Eddington flux during these segments. We suggest that
burst spectra can closely approximate Bose-Einstein spectra and have fluxes
that exceed the Eddington flux because they are formed by Comptonization in an
extended, low-density radiating gas supported by the outward radiation force
and confined by a tangled magnetic field.Comment: 5 pages, 1 figure. Analyzed additional data, adjusted text, figure,
and references following referee response. Accepted for publication in
ApJLetter
Online Versus Face-to-Face Education
This paper explores the benefits and disadvantages of online and face-to-face modes of education. Throughout the paper, I discuss whether online courses provide the same level of education as face-to-face courses without the nurturing, attentive, and physical aspects. Based on extensive reading on the topic, as well drawing on my personal experience, I argue that online and face-to-face education can have both positive and negative effects on students’ learning. However, I conclude that online courses do not provide the same level of education that face-to-face courses do, based on the nurturing, attentive, and often physical aspects that traditional education provides. I provide what I hope is an argument that will help others appreciate that face-to-face courses provide a more effective and helpful way for students to have a successful academic career
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