7,395 research outputs found
Hubble Space Telescope Observations of the Black Hole X-ray Transient GRO J0422+32 Near Quiescence
We present HST/FOS ultraviolet and optical spectroscopy of the black hole
X-ray transient GRO J0422+32 shortly before the system reached quiescence. We
find that the accretion spectrum from 2500-9000A can be very well fit by a
self-absorbed synchrotron model, with superposed HI and MgII emission lines.
The explanations we suggest for this spectrum are that it is either due to
active coronal regions above a geometrically thin accretion disc, or that the
disc is evaporated into an advective flow.Comment: 7 pages with 7 postscript figures included, uses mn.sty. Accepted for
publication in MNRA
The 1996 outburst of GRO J1655-40: disc irradiation and enhanced mass transfer
We show that the 1996 outburst of the X-ray binary transient system GRO
J1655-40 can be explained by the standard dwarf-nova type disc instability,
followed by an episode of enhanced mass transfer from the secondary if the mass
transfer rate in GRO J1655-40 is within a factor < 10 of the stability limit.
We argue that irradiation of the secondary during the onset of the outburst
driven by the thermal instability in the outer disc can increase the mass
transfer rate above the minimum value required for stable accretion. This will
then produce the period of near-constant X-ray emission seen in this system.
This scenario can also explain the observed anti-correlation between the
optical and X-ray fluxes. It is generally accepted that optical emission in
low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays.
There is also strong circumstantial evidence that in order for the outer disc
to see the irradiating flux, it must be warped. Depending on the warp
propagation mechanism, either a burst of mass from the secondary or viscous
decay are likely to decrease the degree of warping, thereby causing the
decrease in the observed optical flux while the X-ray flux remains constant or
even increases, exactly as observed in GRO J1655-40. Finally, the decrease of
the disc warping and, therefore, irradiation will cause the disc to become
unstable once again, terminating the outburst.Comment: Astronomy and Astrophysics - in pres
Doppler Tomography of XTE J2123-058 and Other Neutron Star LMXBs
We describe Doppler tomography obtained in the 1998 outburst of the neutron
star low mass X-ray binary (LMXB) XTE J2123-058. This analysis, and other
aspects of phase-resolved spectroscopy, indicate similarities to SW Sex
systems, except that anomalous emission kinematics are seen in HeII, whilst
phase 0.5 absorption is confined to H alpha. This separation of these effects
may provide tighter constraints on models in the LMXB case than is possible for
SW Sex systems. We will compare results for other LMXBs which appear to show
similar kinematics and discuss how models for the SW Sex phenomenon can be
adapted to these systems. Finally we will summarise the limited Doppler
tomography performed on the class of neutron star LMXBs as a whole, and discuss
whether any common patterns can yet be identified.Comment: 8 pages, 5 postscript figures. To appear in Proceedings of
Astro-Tomography Workshop, Brussels, July 2000, Eds. H. Boffin, D. Steeghs,
Springer-Verlag Lecture Notes in Physic
The X-ray Transient XTE J2012+381
We present optical and infrared observations of the soft X-ray transient
(SXT) XTE J2012+381 and identify the optical counterpart with a faint red star
heavily blended with a brighter foreground star. The fainter star is coincident
with the radio counterpart and appears to show weak H alpha emission and to
have faded between observations. The RXTE/ASM lightcurve of XTE J2012+381 is
unusual for an SXT in that after an extended linear decay, it settled into a
plateau state for about 40 days before undergoing a weak mini-outburst. We
discuss the nature of the object and suggest similarities to long orbital
period SXTs.Comment: 5 pages, 7 postscript figures included, uses mn.sty. Accepted for
publication in MNRA
A Qualitative Study of English Language Learners in the Portland Public Schools
The purpose of this research is to identify the demographic characteristics of English Language Learner (ELL) students who successfully complete secondary education in the Portland Public Schools. The first phase of this project involves identifying ELL educators and staff to participate in interviews about this demographic and the curriculum used. This information is analyzed using Qualitative Data Analysis(QDA) to determine if there are any similar concepts or themes in the information gathered. The school and home environment and prior education experience were examined. Additionally, protective factors in the school or community were explored to see these factors made on the academic success of students. Identifying patterns of success allows us to gain some understanding of how to best accommodate the growing number of ELL students in the community
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