291 research outputs found
A Dynamical Study of the Black Hole X-ray Binary Nova Muscae 1991
We present a dynamical study of the Galactic black hole binary system Nova
Muscae 1991 (GS/GRS 1124-683). We utilize 72 high resolution Magellan
Echellette (MagE) spectra and 72 strictly simultaneous V-band photometric
observations; the simultaneity is a unique and crucial feature of this
dynamical study. The data were taken on two consecutive nights and cover the
full 10.4-hour orbital cycle. The radial velocities of the secondary star are
determined by cross-correlating the object spectra with the best-match template
spectrum obtained using the same instrument configuration. Based on our
independent analysis of five orders of the echellette spectrum, the
semi-amplitude of the radial velocity of the secondary is measured to be K_2 =
406.8+/-2.7 km/s, which is consistent with previous work, while the uncertainty
is reduced by a factor of 3. The corresponding mass function is f(M) =
3.02+/-0.06 M_\odot. We have also obtained an accurate measurement of the
rotational broadening of the stellar absorption lines (v sin i = 85.0+/-2.6
km/s) and hence the mass ratio of the system q = 0.079+/-0.007. Finally, we
have measured the spectrum of the non-stellar component of emission that veils
the spectrum of the secondary. In a future paper, we will use our
veiling-corrected spectrum of the secondary and accurate values of K_2 and q to
model multi-color light curves and determine the systemic inclination and the
mass of the black hole.Comment: ApJ accepted version; minor revision; added a subsection about
systematic uncertaintie
A Sequence of Declining Outbursts from GX339-4
The flux and spectrum of the black hole candidate GX339-4 has been monitored
by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-ray
Observatory (CGRO) since the observatory became operational in May 1991.
Between the summer of 1991 and the fall of 1996, eight outbursts from GX339-4
were observed. The history of these outbursts is one of declining fluence or
total energy release, as well as a shortening of the time between outbursts. A
rough linear correlation exists between the fluence emitted during an outburst
and the time elapsed between the end of the previous outburst and the beginning
of the current one. The peak flux is also roughly linearly correlated with
outburst fluence. The lightcurves of the earlier, more intense, outbursts
(except for the second one) can be modeled by a fast exponential (time constant
~ 10 days) followed by a slower exponential (~ 100 days) on the rise and a fast
exponential decay (~ 5 days) on the fall. The later, weaker, outbursts are
modeled with a single rising time constant (~ 20 days) and a longer decay on
the fall (~ 50 days). An exponential model gives a marginally better fit than a
power law to the rise/decay profiles. GX339-4 is a unique source in having more
frequent outbursts than other low mass x-ray binary black hole candidates.
These observations can be used to constrain models of the behavior of the
accretion disk surrounding the compact object.Comment: Accepted for Publication in the Astrophysical Journal Letters, AASTE
Multiwavelength Observations of GX 339-4 in 1996. III. Keck Spectroscopy
As part of our multiwavelength campaign of observations of GX 339-4 in 1996
we present our Keck spectroscopy performed on May 12 UT. At this time, neither
the ASM on the RXTE nor BATSE on the CGRO detected the source. The optical
emission was still dominated by the accretion disk with V approximately 17 mag.
The dominant emission line is H alpha, and for the first time we are able to
resolve a double peaked profile. The peak separation Delta v = 370 +/- 40 km/s.
Double peaked H alpha emission lines have been seen in the quiescent optical
counterparts of many black hole X-ray novae. However, we find that the peak
separation is significantly smaller in GX 339-4, implying that the optical
emission comes from a larger radius than in the novae. The H alpha emission
line may be more akin to the one in Cygnus X-1, where it is very difficult to
determine if the line is intrinsically double peaked because absorption and
emission lines from the companion star dominate.Comment: Submitted to Astrophysical Journal. 10 pages. 2 figure
A Catalog of Transient X-ray Sources in M31
From October 1999 to August 2002, 45 transient X-ray sources were detected in
M31 by Chandra and XMM-Newton. We have performed spectral analysis of all
XMM-Newton and Chandra ACIS detections of these sources, as well as flux
measurements of Chandra HRC detections. The result is absorption-corrected
X-ray lightcurves for these sources covering this 2.8 year period, along with
spectral parameters for several epochs of the outbursts of most of the
transient sources. We supply a catalog of the locations, outburst dates, peak
observed luminosities, decay time estimates, and spectral properties of the
transient sources, and we discuss similarities with Galactic X-ray novae. Duty
cycle estimates are possible for 8 of the transients and range from 40% to 2%;
upper limits to the duty cycles are estimated for an additional 15 transients
and cover a similar range. We find 5 transients which have rapid decay times
and may be ultra-compact X-ray binaries. Spectra of three of the transients
suggest they may be faint Galactic foreground sources. If even one is a
foreground source, this suggests a surface density of faint transient X-ray
sources of >~1 deg.Comment: 63 pages, 22 figures, 3 tables, accepted for publication in Ap
The Temperature and Cooling Age of the White-Dwarf Companion to the Millisecond Pulsar PSR B1855+09
We report on Keck and {\em Hubble Space Telescope} observations of the binary
millisecond pulsar PSR B1855+09. We detect its white-dwarf companion and
measure \mv=25.90\pm0.12 and \mi=24.19\pm0.11 (Vega system). From the
reddening-corrected color, (\mv-\mi)_0=1.06\pm0.21, we infer a temperature
\Teff=4800\pm800 K. The white-dwarf mass is known accurately from
measurements of the Shapiro delay of the pulsar signal,
\Mcomp=0.258^{+0.028}_{-0.016} \Msun. Hence, given a cooling model, one can
use the measured temperature to determine the cooling age. The main uncertainty
in the cooling models for such low-mass white dwarfs is the amount of residual
nuclear burning, which is set by the thickness of the hydrogen layer
surrounding the helium core. From the properties of similar systems, it has
been inferred that helium white dwarfs form with thick hydrogen layers, with
mass \simgt3\times10^{-3} \Msun, which leads to significant additional
heating. This is consistent with expectations from simple evolutionary models
of the preceding binary evolution. For PSR B1855+09, though, such models lead
to a cooling age of Gyr, which is twice the spin-down age of the
pulsar. It could be that the spin-down age were incorrect, which would call the
standard vacuum dipole braking model into question. For two other pulsar
companions, however, ages well over 10 Gyr are inferred, indicating that the
problem may lie with the cooling models. There is no age discrepancy for models
in which the white dwarfs are formed with thinner hydrogen layers
(\simlt3\times10^{-4} \Msun).Comment: 7 pages, 1 figure, aas4pp2.sty. Accepted for publication in ApJ
Selective expansion of viral variants following experimental transmission of a reconstituted feline immunodeficiency virus quasispecies
Following long-term infection with virus derived from the pathogenic GL8 molecular clone of feline immunodeficiency virus (FIV), a range of viral variants emerged with distinct modes of interaction with the viral receptors CD134 and CXCR4, and sensitivities to neutralizing antibodies. In order to assess whether this viral diversity would be maintained following subsequent transmission, a synthetic quasispecies was reconstituted comprising molecular clones bearing envs from six viral variants and its replicative capacity compared in vivo with a clonal preparation of the parent virus. Infection with either clonal (Group 1) or diverse (Group 2) challenge viruses, resulted in a reduction in CD4+ lymphocytes and an increase in CD8+ lymphocytes. Proviral loads were similar in both study groups, peaking by 10 weeks post-infection, a higher plateau (set-point) being achieved and maintained in study Group 1. Marked differences in the ability of individual viral variants to replicate were noted in Group 2; those most similar to GL8 achieved higher viral loads while variants such as the chimaeras bearing the B14 and B28 Envs grew less well. The defective replication of these variants was not due to suppression by the humoral immune response as virus neutralising antibodies were not elicited within the study period. Similarly, although potent cellular immune responses were detected against determinants in Env, no qualitative differences were revealed between animals infected with either the clonal or the diverse inocula. However, in vitro studies indicated that the reduced replicative capacity of variants B14 and B28 in vivo was associated with altered interactions between the viruses and the viral receptor and co-receptor. The data suggest that viral variants with GL8-like characteristics have an early, replicative advantage and should provide the focus for future vaccine development
RXTE Studies of X-ray Spectral Variations with Accretion Rate in 4U 1915-05
We present the results of detailed spectral studies of the ultra-compact low
mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing
Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to
exhibit a ~199-day modulation in its 2--12 keV flux. Observations were
performed with the PCA and HEXTE instruments on RXTE at roughly one-month
intervals to sample this long-term period and study accretion rate-related
spectral changes. We obtain good fits with a model consisting of a blackbody
and an exponentially cut-off power law. The spectral parameters are strongly
correlated with both the broad-band (2--50 keV) luminosity and the position in
the color-color diagram, with the source moving from a low hard state to a high
soft state as the accretion rate increases. The blackbody component appears to
drive the spectral evolution. Our results are consistent with a geometry in
which the soft component arises from an optically thick boundary layer and the
hard component from an extended Comptonizing corona. Comparing our results with
those of a similar study of the brighter source 4U 1820-30 (Bloser et al.
2000), we find that the two ultra-compact LMXBs occupy similar spectral states
even though the transitions occur at very different total luminosities.Comment: 27 pages LaTeX, 8 figures, accepted to the Astrophysical Journa
Thermodynamic properties of tungsten ditelluride (WTe2) I. The preparation and lowtemperature heat capacity at temperatures from 6 K to 326 K
The heat capacity of the dichalcogenide: tungsten ditelluride, WTe2, was measured over the temperature range 5.5 T/K Cp m occurs in the region 92 T/K Sm = (0.10±0.02) · R. The anomaly coincides with the temperature range where all the translational, librational, and internal vibrational modes become fully excited. The electronic molar heat capacity Tγm = (5.99±1.83) mJ · K-1·mol-1 and, for the lattice, the Debye characteristic temperature ΘD = (133.8 ± 0.06) K. Standard molar thermodynamic functions are presented at selected temperatures from 5 K to 335 K.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30021/1/0000389.pd
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