280 research outputs found
Magnetic cooling for microkelvin nanoelectronics on a cryofree platform
We present a parallel network of 16 demagnetization refrigerators mounted on
a cryofree dilution refrigerator aimed to cool nanoelectronic devices to
sub-millikelvin temperatures. To measure the refrigerator temperature, the
thermal motion of electrons in a Ag wire -- thermalized by a spot-weld to one
of the Cu nuclear refrigerators -- is inductively picked-up by a
superconducting gradiometer and amplified by a SQUID mounted at 4 K. The noise
thermometer as well as other thermometers are used to characterize the
performance of the system, finding magnetic field independent heat-leaks of a
few nW/mol, cold times of several days below 1 mK, and a lowest temperature of
150 microK of one of the nuclear stages in a final field of 80 mT, close to the
intrinsic SQUID noise of about 100 microK. A simple thermal model of the system
capturing the nuclear refrigerator, heat leaks, as well as thermal and Korringa
links describes the main features very well, including rather high refrigerator
efficiencies typically above 80%.Comment: 4 color figures, including supplementary inf
Near-Infrared Light Curves of the Black Hole Binary A0620-00
We measured the near-infrared orbital light curve of the black hole binary
A0620-00 in 1995 and 1996. The light curves show an asymmetric, double-humped
modulation with extra emission in the peak at orbital phase 0.75. There were no
significant changes in the shape of the light curve over the one-year
observation period. There were no sharp dips in the light curves nor reversals
of the asymmetry between the two peaks as seen in earlier observations. The
light curves are well fit by models incorporating ellipsoidal variations from
the mass-losing K-type star plus a beamed bright spot on the accretion disk
around the compact star. The long-term stability of the light curve shape rules
out superhumps and star spots as sources of asymmetry when we observed
A0620-00. The ellipsoidal variations yield a lower limit i >= 38 deg on the
orbital inclination. The light curves show no eclipse features, which places an
upper limit i <= 75 deg. This range of inclinations constrains the mass of the
compact object to 3.3 < M_1 < 13.6 Msun. The light curves do not further
constrain the orbital inclination because the contribution of the accretion
disk to the observed flux is unknown. We argue that a previous attempt to
measure the near-infrared flux from the accretion disk using the dilution of
the 12CO(2,0) bandhead in the spectrum of the K star is not reliable because
the band strength depends strongly on surface gravity.Comment: Accepted for publication in the Astronomical Journal. 17 pages, 4
figures. Prepared using AASTEX V. 5.
The Mass of the Black Hole in XTE J1118+480
We present contemporaneous, broadband, near-infrared spectroscopy (0.9-2.45 μm) and H-band photometry of the black hole X-ray binary, XTE J1118+480. We determined the fractional dilution of the NIR ellipsoidal light curves of the donor star from other emission sources in the system by comparing the absorption features in the spectrum with field stars of known spectral type. We constrained the donor star spectral type to K7 V-M1 V and determined that the donor star contributed 54% ± 27% of the H-band flux at the epoch of our observations. This result underscores the conclusion that the donor star cannot be assumed to be the only NIR emission source in quiescent X-ray binaries. The H-band light curve shows a double-humped asymmetric modulation with extra flux at orbital phase 0.75. The light curve was fitted with a donor star model light curve, taking into account a constant second flux component based on the dilution analysis. We also fitted models that included emission from the donor star, a constant component from the accretion disk, and a phase-variable component from the bright spot where the mass accretion stream impacts the disk. These simple models with reasonable estimates for the component physical parameters can fully account for the observed light curve, including the extra emission at phase 0.75. From our fits, we constrained the binary inclination to 68° ≤ i ≤ 79°. This leads to a black hole mass of 6.9 M_☉ ≤ M_(BH) ≤ 8.2 M_☉. Long-term variations in the NIR light curve shape in XTE J1118+480 are similar to those seen in other X-ray binaries and demonstrate the presence of continued activity and variability in these systems even when in full quiescence
The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars
The spectral and temporal behavior of exoplanet host stars is a critical
input to models of the chemistry and evolution of planetary atmospheres. At
present, little observational or theoretical basis exists for understanding the
ultraviolet spectra of M dwarfs, despite their critical importance to
predicting and interpreting the spectra of potentially habitable planets as
they are obtained in the coming decades. Using observations from the Hubble
Space Telescope, we present a study of the UV radiation fields around nearby M
dwarf planet hosts that covers both FUV and NUV wavelengths. The combined
FUV+NUV spectra are publically available in machine-readable format. We find
that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ
832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition
region UV emission. No "UV quiet" M dwarfs are observed. The bright stellar
Ly-alpha emission lines are reconstructed, and we find that the Ly-alpha line
fluxes comprise ~37-75% of the total 1150-3100A flux from most M dwarfs; >
10^{3} times the solar value. The F(FUV)/F(NUV) flux ratio, a driver for
abiotic production of the suggested biomarkers O2 and O3, is shown to be ~0.5-3
for all M dwarfs in our sample, > 10^{3} times the solar ratio. For the four
stars with moderate signal-to-noise COS time-resolved spectra, we find UV
emission line variability with amplitudes of 50-500% on 10^{2} - 10^{3} s
timescales. Finally, we observe relatively bright H2 fluorescent emission from
four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832).
Additional modeling work is needed to differentiate between a stellar
photospheric or possible exoplanetary origin for the hot (T(H2) \approx
2000-4000 K) molecular gas observed in these objects.Comment: ApJ, accepted. 16 pages, 10 figures. On-line data at:
http://cos.colorado.edu/~kevinf/muscles.htm
An XMM-Newton observation of the nova-like variable UX UMa: spatially and spectrally resolved two-component X-ray emission
In the optical and ultraviolet regions of the electromagnetic spectrum, UX
Ursae Majoris is a deeply eclipsing cataclysmic variable. However, no soft
X-ray eclipse was detected in ROSAT observations. We have obtained a 38 ksec
XMM-Newton observation to further constrain the origin of the X-rays. The
combination of spectral and timing information allows us to identify two
components in the X-ray emission of the system. The soft component, dominant
below photon energies of 2 keV, can be fitted with a multi-temperature plasma
model and is uneclipsed. The hard component, dominant above 3 keV, can be
fitted with a kT ~ 5 keV plasma model and appears to be deeply eclipsed. We
suggest that the most likely source of the hard X-ray emission in UX UMa, and
other systems in high mass transfer states, is the boundary layer.Comment: To appear in MNRAS Letter
Far Ultraviolet Observations of the Dwarf Nova VW Hyi in Quiescence
We present a 904-1183 A spectrum of the dwarf nova VW Hydri taken with the
Far Ultraviolet Spectroscopic Explorer during quiescence, eleven days after a
normal outburst, when the underlying white dwarf accreter is clearly exposed in
the far ultraviolet. However, model fitting show that a uniform temperature
white dwarf does not reproduce the overall spectrum, especially at the shortest
wavelengths. A better approximation to the spectrum is obtained with a model
consisting of a white dwarf and a rapidly rotating ``accretion belt''. The
white dwarf component accounts for 83% of the total flux, has a temperature of
23,000K, a v sin i = 400 km/s, and a low carbon abundance. The best-fit
accretion belt component accounts for 17% of the total flux, has a temperature
of about 48,000-50,000K, and a rotation rate Vrot sin i around 3,000-4,000
km/s. The requirement of two components in the modeling of the spectrum of VW
Hyi in quiescence helps to resolve some of the differences in interpretation of
ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of
a second component (and its exact nature) in VW Hyi itself is still relatively
uncertain, given the lack of better models for spectra of the inner disk in a
quiescent dwarf nova.Comment: 6 figures, 10 printed page in the journal, to appear in APJ, 1 Sept.
2004 issue, vol. 61
Observations of the SW Sextantis star DW Ursae Majoris with the Far Ultraviolet Spectroscopic Explorer
We present an analysis of the first far-ultraviolet observations of the SW
Sextantis-type cataclysmic variable DW Ursae Majoris, obtained in November 2001
with the Far Ultraviolet Spectroscopic Explorer. The time-averaged spectrum of
DW UMa shows a rich assortment of emission lines (plus some contamination from
interstellar absorption lines including molecular hydrogen). Accretion disk
model spectra do not provide an adequate fit to the far-ultraviolet spectrum of
DW UMa. We constructed a light curve by summing far-ultraviolet spectra
extracted in 60-sec bins; this shows a modulation on the orbital period, with a
maximum near photometric phase 0.93 and a minimum half an orbit later. No other
periodic variability was found in the light curve data. We also extracted
spectra in bins spanning 0.1 in orbital phase; these show substantial variation
in the profile shapes and velocity shifts of the emission lines during an
orbital cycle of DW UMa. Finally, we discuss possible physical models that can
qualitatively account for the observed far-ultraviolet behavior of DW UMa, in
the context of recent observational evidence for the presence of a
self-occulting disk in DW UMa and the possibility that the SW Sex stars may be
the intermediate polars with the highest mass transfer rates and/or weakest
magnetic fields.Comment: accepted by the Astronomical Journal; 36 pages, including 12 figures
and 4 table
Orbital and stochastic far-UV variability in the nova-like system V3885 Sgr
Highly time-resolved time-tagged FUSE satellite spectroscopic data are
analysed to establish the far-ultraviolet (FUV) absorption line characteristics
of the nova-like cataclysmic variable binary, V3885 Sgr. We determine the
temporal behaviour of low (Ly_beta, CIII, NIII) and high (SIV, PV, OVI) ion
species, and highlight corresponding orbital phase modulated changes in these
lines. On average the absorption troughs are blueshifted due to a low velocity
disc wind outflow. Very rapid (~ 5 min) fluctuations in the absorption lines
are isolated, which are indicative of stochastic density changes. Doppler
tomograms of the FUV lines are calculated which provide evidence for structures
where a gas stream interacts with the accretion disc. We conclude that the line
depth and velocity changes as a function of orbital phase are consistent with
an asymmetry that has its origin in a line-emitting, localised disc-stream
interaction region.Comment: Accepted for publication in MNRA
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