766 research outputs found
An X-Ray Dip in the X-Ray Transient 4U 1630-47
An x-ray dip was observed during a 1996 Rossi X-Ray Timing Explorer
observation of the recurrent x-ray transient 4U 1630-47. During the dip, the
2-60 keV x-ray flux drops by a factor of about three, and, at the lowest point
of the dip, the x-ray spectrum is considerably softer than at non-dip times. We
find that the 4U 1630-47 dip is best explained by absorption of the inner part
of an accretion disk, while the outer part of the disk is unaffected. The
spectral evolution during the dip is adequately described by the variation of a
single parameter, the column density obscuring the inner disk.Comment: 13 pages, 4 figures, Accepted for publication in Ap
A Method of Mass Measurement in Black Hole Binaries Using Timing and High Resolution X-ray Spectroscopy
In X-ray binaries, several percent of the compact object luminosity is
intercepted by the surface of the normal companion and re-radiated through
Compton reflection and the K-fluorescence. This reflected emission follows the
variability of the compact object with a delay approximately equal to the
orbital radius divided by the speed of light. This provides the possibility of
measuring the orbital radius and thus substantially refining the compact object
mass determination compared to using optical data alone. We demonstrate that it
may be feasible to measure the time delay between the direct and reflected
emission using cross-correlation of the light curves observed near the Kalpha
line and above the K-edge of neutral iron. In the case of Cyg X-1, the time
delay measurement is feasible with a 300--1000 ksec observation by a telescope
with a 1000 cm^2 effective area near 6.4 keV and with a ~5eV energy resolution.
With longer exposures, it may be possible to obtain mass constraints even if an
X-ray source in the binary system lacks an optical counterpart.Comment: ApJ Letters, in press. 4 pages, 3 figures, uses emulateapj.st
Inclination Effects and Beaming in Black Hole X-ray Binaries
We investigate the dependence of observational properties of black hole X-ray
binaries on the inclination angle i of their orbits. We find the following: (1)
Transient black hole binaries show no trend in their quiescent X-ray
luminosities as a function of i, suggesting that the radiation is not
significantly beamed. This is consistent with emission from an accretion disk.
If the X-rays are from a jet, then the Lorentz factor gamma of the jet is less
than 1.24 at the 90% confidence level. (2) The X-ray binary 4U1543-47 with i of
order 21 degrees has a surprisingly strong fluorescent iron line in the high
soft state. Quantifying an earlier argument by Park et al. (2004), we conclude
that if the continuum X-ray emission in this source is from a jet, then gamma <
1.04. (3) None of the known binaries has cos i 75 degrees. This
fact, plus the lack of eclipses among the 20 black hole binaries in our sample,
strongly suggests at the 99.5% confidence level that systems with large
inclination angles are hidden from view. The obscuration could be the result of
disk flaring, as suggested by Milgrom (1978) for neutron star X-ray binaries.
(4) Transient black hole binaries with i ~ 70-75 degrees have significantly
more complex X-ray light curves than systems with i < 65 degrees. This may be
the result of variable obscuration and/or variable height above the disk of the
radiating gas.Comment: 26 pages, to appear in The Astrophysical Journal, vol. 624, May 1,
200
Complex outburst behaviour from the black-hole candidate 4U 1630-47
We present data from different epochs in 1978, 1987-1991 and 1996 from the
black-hole candidate 4U1630-47. For the first time we present almost complete
coverage of the outbursts which started in 1987, 1988 and 1996. We find that
the outburst behaviour of 4U1630-47 is more complex than previously realized.
The source shows outbursts with durations on the order of ~100-200 days and
sometimes intervals of long-term X-ray activity.
The relatively short outbursts which occurred in 1987 and 1996 exhibited
different colour behaviour: the outburst in 1987 showed softening of the X-ray
spectrum, whereas the outburst in 1996 showed hardening of the X-ray spectrum,
as the outbursts proceeded.
The outburst which started in 1977 may have lasted for up to ~10 months,
whereas the outburst which started in 1988 showed activity for up to ~2.4
years. Such long-term activity is reminiscent of that seen in GRS1716-249 and
in the galactic superluminal sources GRS1915+105 and GROJ1655-40.
We refine the outburst ephemeris of 4U\1630-47 and find that the outburst
recurrence time scale may have changed from ~600 days to ~690 days between the
1984 and 1987 outbursts. If the recurrence interval of ~690 days continues, the
next outburst of 4U1630-47 is predicted to occur in 1998 January.Comment: 30 pages including 9 figures accepted for publication in MNRA
Mobile target tracking using a reconfigurable low earth orbit constellation
Traditional Earth-observing satellite constellations must preselect orbits to match established mission objectives, inherently limiting the capability of the system to respond to dynamic events. A more flexible approach to constellation design which maneuvers satellites to new orbits throughout the mission lifetime can enable the tracking of mobile targets; such a concept will be explored and evaluated in the following paper. Currently, an existing approach for creating a reconfigurable constellation of satellites (ReCon) demonstrates some increased functionality. However, the repeating ground track (RGT) orbits used are only well-suited for static ground targets (i.e. particular latitude and longitude points) as they are defined by their repetitious passes over the particular points on the globe; inherently these orbits are not ideal for tracking mobile targets such as ships, oil spills, hurricanes, and other weather events. By addressing this gap and designing the constellation to follow these mobile targets, data return can be of higher quality and quantity leading toward improved predictions of future movements. Traditional non-maneuverable satellites may of course capture mobile feature data; however, the quality and quantity of data returned is driven by the satellites’ orbits which were not designed for the particular mobile target. A proposed concept of operations that models possible sequential satellite maneuvers as a graph has shown promise for tracking mobile targets in the case of a single satellite. In the following paper, this concept is further explored by the addition of an optimizer to this method, applied to both a single satellite and a constellation of maneuverable satellites, like ReCon. Results for a tropical storm case study indicate that with relatively low amounts of delta-V per satellite, many possible maneuver combinations are found which result in improved accesses with greater total access time and closer passes. Trade-offs are shown between total target access time, mean distance to targets upon access, and total delta-V used. The performance of increased constellation size is also explored. By having satellites dynamically respond to mobile targets such as hurricanes, satellite imagery and remote sensing can be of better quality and even contribute to better predictions for developing storms
Changes in the long term intensity variations in Cyg X-2 and LMC X-3
We report the detection of changes in the long-term intensity variations in
two X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used the
long-term light curves obtained with the All-Sky Monitors (ASMs) of the Rossi
X-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanning
modulation collimator of HEAO 1. It is found that in the light curves of both
the sources, obtained with these instruments at various times over the last 30
years, more than one periodic or quasi-periodic component is always present.
The multiple prominent peaks in the periodograms have frequencies unrelated to
each other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days,
and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks are
also observed in LMC X-3. The various strong peaks in the periodograms of LMC
X-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and
328 days (observed with Ginga-ASM). The present results, when compared with the
earlier observations of periodicities in these two systems, demonstrate the
absence of any stable long period. The 78 day periodicity detected earlier in
Cyg X-2 was probably due to the short time base in the RXTE data that were
used, and the periodicity of 198 days in LMC X-3 was due to a relatively short
duration of observation with HEAO 1.Comment: 11 pages, 7 postscript figures include
Evidence for Doppler-Shifted Iron Emission Lines in Black Hole Candidate 4U 1630-47
We report the first detection of a pair of correlated emission lines in the
X-ray spectrum of black hole candidate 4U 1630-47 during its 1996 outburst,
based on RXTE observations of the source. At the peak plateau of the outburst,
the emission lines are detected, centered mostly at 5.7 keV and 7.7
keV, respectively, while the line energies exhibit random variability 5%.
Interestingly, the lines move in a concerted manner to keep their separation
roughly constant. The lines also vary greatly in strength, but with the
lower-energy line always much stronger than the higher-energy one. The measured
equivalent width ranges from 50 eV to 270 eV for the former, and
from insignificant detection to 140 eV for the latter; the two are
reasonably correlated.
The correlation between the lines implies a causal connection --- perhaps
they share a common origin. Both lines may arise from a single
line of highly ionized iron that is Doppler-shifted either in a Keplerian
accretion disk or in a bi-polar outflow or even both. In both scenarios, a
change in the line energy might simply reflect a change in the ionization state
of line-emitting matter. We discuss the implication of the results and also
raise some questions about such interpretations.Comment: To appear in Ap
Orbital dynamics of Cygnus X-3
Orbital-phased-resolved infrared spectra of Cygnus X-3 in outburst and
quiescence, including tomographic analysis, are presented. We confirm the
phasing of broad HeII and NV lines in quiescence, such that maximum blue shift
corresponds to the X-ray minimum at phase = 0.00 +/- 0.04. In outburst,
double-peaked HeI structures show a similar phasing with two significant
differences: (a) although varying in relative strength, there is continuous
line emission in blue and red peaks around the orbit, and (b) an absorption
component, ~1/4 of an orbit out of phase with the emission features, is
discerned. Doppler tomograms of the double-peaked profiles are consistent with
a disk-wind geometry, rotating at velocities of 1000 km/s. Regrettably, the
tomography algorithm will produce a similar ring structure from alternative
line sources if contaminated by overlying P Cygni profiles. This is certainly
the case in the strong 2.0587 micron HeI line, leading to an ambiguous solution
for the nature of double-peaked emission. The absorption feature, detected 1/4
of an orbit out of phase with the emission features, is consistent with an
origin in the He star wind and yields for the first time a plausible radial
velocity curve for the system. We directly derive the mass function of the
system, 0.027 M_sun. If we assume a neutron star accretor and adopt a high
orbital inclination, i > 60 degrees, we obtain a mass range for the He star of
5 M_sun < M_WR < 11 M_sun. Alternatively if the compact object is a black hole,
we estimate M_BH < 10 M_sun. We discuss the implications of these masses for
the nature and size of the binary system.Comment: Accepted for publication in ApJ main journa
Cygnus X-3 and the problem of the missing Wolf-Rayet X-ray binaries
Cygnus X-3 is a strong X-ray source (L_X about 10^38 erg/s) which is thought
to consist of a compact object, accreting matter from a helium star. We find
analytically that the estimated ranges of mass-loss rate and orbital-period
derivative for Cyg X-3 are consistent with two models: i) the system is
detached and the mass loss from the system comes from the stellar wind of a
massive helium star, of which only a fraction that allows for the observed
X-ray luminosity is accreted, or ii) the system is semidetached and a
Roche-lobe-overflowing low- or moderate-mass helium donor transfers mass to the
compact object, followed by ejection of its excess over the Eddington rate from
the system. These analytical results appear to be consistent with evolutionary
calculations. By means of population synthesis we find that currently in the
Galaxy there may exist ~1 X-ray binary with a black hole that accretes from a
>~ 7 MSun Wolf-Rayet star and ~1 X-ray binary in which a neutron star accretes
matter from a Roche-lobe-overflowing helium star with mass <~ 1.5 MSun. Cyg X-3
is probably one of these systems.Comment: 12 pages, 6 figures, accepted by A&
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