760 research outputs found
The Nature of the Secondary Star in the Black Hole X-Ray Transient V616 Mon (=A0620-00)
We have used NIRSPEC on Keck II to obtain -band spectroscopy of the low
mass X-ray binary V616 Mon (= A062000). V616 Mon is the proto-typical soft
x-ray transient containing a black hole primary. As such it is important to
constrain the masses of the binary components. The modeling of the infrared
observations of ellipsoidal variations in this system lead to a derived mass of
11.0 M_{\sun} for the black hole. The validity of this derivation has been
called into question due to the possiblity that the secondary star's spectral
energy distribution is contaminated by accretion disk emission (acting to
dilute the variations). Our new -band spectrum of V616 Mon reveals a
late-type K dwarf secondary star, but one that has very weak CO
absorption features. Comparison of V616 Mon with SS Cyg leads us to estimate
that the accretion disk supplies only a small amount of -band flux, and the
ellipsoidal variations are not seriously contaminated. If true, the derived
orbital inclination of V616 Mon is not greatly altered, and the mass of the
black hole remains large. A preliminary stellar atmosphere model for the
-band spectrum of V616 Mon reveals that the carbon abundance is
approximately 50% of the solar value. We conclude that the secondary star in
V616 Mon has either suffered serious contamination from the accretion of
supernova ejecta that created the black hole primary, or it is the stripped
remains of a formerly more massive secondary star, one in which the CNO cycle
had been active.Comment: 20 pages, 5 figure
On the secondary star of the cataclysmic variable 1RXS J094432.1+035738
We present V and Rc band photometry and optical near-infrared spectroscopy of
the cataclysmic variable 1RXS J094432.1+035738. We detected features of a cool
secondary star, which can be modeled with a red dwarf of spectral type M2 (+0.5
-1.0) V at a distance of 433 +- 100 pc.Comment: Accepted for publication in Astronomy and Astrophysic
Detection of Pulsed X-ray Emission from XMM-Newton Observations of PSR J0538+2817
We report on the XMM-Newton observations of the 143 ms pulsar PSR J0538+2817.
We present evidence for the first detections of pulsed X-rays from the source
at a frequency which is consistent with the predicted radio frequency. The
pulse profile is broad and asymmetric, with a pulse fraction of 18 +/- 3%. We
find that the spectrum of the source is well-fit with a blackbody with
T^{infty} = (2.12^{+0.04}_{-0.03}) x 10^6 K and N_{H} = 2.5 x 10^21 cm^{-2}.
The radius determined from the model fit of 1.68 +/- 0.05 km suggests that the
emission is from a heated polar cap. A fit to the spectrum with an atmospheric
model reduces the inferred temperature and hence increases the radius of the
emitting region, however the pulsar distance determined from the fit is then
smaller than the dispersion distance.Comment: 24 pages, 6 figures, 3 tables, accepted for publication in ApJ. Error
in radius calculation corrected, discussion and conclusions remain unchange
Probing the Pulsar Wind Nebula of PSR B0355+54
We present XMM-Newton and Chandra X-ray observations of the middle-aged radio
pulsar PSR B0355+54. Our X-ray observations reveal emission not only from the
pulsar itself, but also from a compact diffuse component extending ~50'' in the
opposite direction to the pulsar's proper motion. There is also evidence for
the presence of fainter diffuse emission extending ~5' from the point source.
The compact diffuse feature is well-fitted with a power-law, the index of which
is consistent with the values found for other pulsar wind nebulae. The
morphology of the diffuse component is similar to the ram-pressure confined
pulsar wind nebulae detected for other sources. The X-ray emission from the
pulsar itself is described well by a thermal plus power-law fit, with the
thermal emission most likely originating in a hot polar cap.Comment: 9 pages (uses emulateapj.cls), 8 figures, 2 tables, accepted for
publication in Ap
XMM-Newton Observations of PSR B1706-44
We report on the XMM-Newton observations of the young, 102 ms pulsar PSR
B1706-44. We have found that both a blackbody plus power-law and a magnetized
atmospheric model plus power-law provide an excellent fit to the EPIC spectra.
The two scenarios are therefore indistinguishable on a statistical basis,
although we are inclined to prefer the latter on physical grounds. In this
case, assuming a source distance of ~2.3 kpc, the size of the region
responsible for the thermal emission is R~13 km, compatible with the surface of
a neutron star. A comparison of the surface temperature of PSR B1706-44
obtained from this fit with cooling curves favor a medium mass neutron star
with M~1.45 solar masses or M~1.59 solar masses, depending on two different
models of proton superfluidity in the interior. The large collecting area of
XMM-Newton allows us to resolve a substructure in the broad soft X-ray
modulation detected by Chandra, revealing the presence of two separate peaks
with pulsed fractions of 7 +/- 4% and 15 +/- 3%, respectively.Comment: 21 pages, 5 figures, 2 tables, accepted for publication in Ap
The X-ray Spectra of VY Scl Stars Are Not Blackbodies
Using ASCA data, we find, contrary to other researchers using ROSAT data,
that the X-ray spectra of the VY Scl stars TT Ari and KR Aur are poorly fit by
an absorbed blackbody model but are well fit by an absorbed thermal plasma
model. The different conclusions about the nature of the X-ray spectrum of KR
Aur may be due to differences in the accretion rate, since this star was in a
high optical state during the ROSAT observation, but in an intermediate optical
state during the ASCA observation. TT Ari, on the other hand, was in a high
optical state during both observations, so directly contradicts the hypothesis
that the X-ray spectra of VY Scl stars in their high optical states are
blackbodies. Instead, based on theoretical expectations and the ASCA, Chandra,
and XMM spectra of other nonmagnetic cataclysmic variables, we believe that the
X-ray spectra of VY Scl stars in their low and high optical states are due to
hot thermal plasma in the boundary layer between the accretion disk and the
surface of the white dwarf, and appeal to the acquisition of Chandra and XMM
grating spectra to test this prediction.Comment: 9 pages including 1 table and 1 encapsulated postscript figure; LaTeX
format, uses aastex.cls; accepted on 2002 January 4 for publication in The
Astrophysical Journal Letter
Resonance line-profile calculations based on hydrodynamical models of cataclysmic variable winds
We present synthetic line profiles as predicted by the models of 2-D line-
driven disk winds due to Proga, Stone & Drew. We compare the model line
profiles with HST observations of the cataclysmic variable IX Vel. The model
wind consists of a slow outflow that is bounded on the polar side by a fast
stream. We find that these two components of the wind produce distinct spectral
features. The fast stream produces profiles which show features consistent with
observations. These include the appearance of the P-Cygni shape for a range of
inclinations, the location of the maximum depth of the absorption component at
velocities less than the terminal velocity, and the transition from absorption
to emission with increasing inclination. However the model profiles have too
little absorption or emission equivalent width. This quantitative difference
between our models and observations is not a surprise because the line-driven
wind models predict a mass loss rate that is lower than the rate required by
the observations. We note that the model profiles exhibit a double-humped
structure near the line center which is not echoed in observations. We identify
this structure with a non-negligible redshifted absorption which is formed in
the slow component of the wind where the rotational velocity dominates over
expansion velocity. We conclude that the next generation of disk wind models,
developed for application to CVs, needs to yield stronger wind driving out to
larger disk radii than do the present models.Comment: LaTeX, 19 pages, to appear in Ap
BeppoSAX observations of the three Gamma-ray pulsars PSR B0656+14, PSR B1055-52 and PSR B1706-44
We report the results of the observations of the three gamma-ray pulsars PSR
B0656+14, PSR B1055-52 and PSR B1706-44 performed with BeppoSAX. We detected a
pulsed emission only for PSR B1055-52: in the range 0.1-6.5 keV the pulse
profile is sinusoidal and the statistical significance is 4.5 sigma. The pulsed
fraction was estimated 0.64+/-0.17. This pulsation was detected also at
energies greater than 2.5 keV suggesting either a non-thermal origin or a quite
high temperature region on the neutron star surface. Spectral analysis showed
that only the X-ray spectrum of PSR B1706-44 can be fitted by a single
power-law component, while that of PSR B1055-52 requires also a blackbody
component (kT = 0.075 keV) and that of PSR B0656+14 two blackbody components
(kT_1 = 0.059, kT_2 = 0.12 keV).Comment: 8 pages, 5 figures. Accepted for publication in A&
VV Pup in a low state: secondary-star irradiation or stellar activity?
Aims. Emission lines in polars show complex profiles with multiple components
that are typically ascribed to the accretion stream, threading region,
accretion spot, and the irradiated secondary-star. In low-state polars the
fractional contribution by the accretion stream, and the accretion spot is
greatly reduced offering an opportunity to study the effect of the
secondary-star irradiation or stellar activity. We observed VV Pup during an
exceptional low-state to study and constrain the properties of the line-forming
regions and to search for evidence of chromospheric activity and/or
irradiation. Methods. We obtained phase-resolved optical spectra at the ESO
VLT+FORS1 with the aim of analyzing the emission line profile and radial
velocity as a function of the orbital period. We also tailored irradiated
secondary-star models to compare the predicted and the observed emission lines
and to establish the nature of the line-forming regions. Results. Our
observations and data analysis, when combined with models of the irradiated
secondary-star, show that, while the weak low ionization metal lines (FeI and
MgI) may be consistent with irradiation processes, the dominant Balmer H
emission lines, as well as NaI and HeI, cannot be reproduced by the irradiated
secondary-star models. We favor the secondary-star chromospheric activity as
the main forming region and cause of the observed H, NaI, and He emission
lines, though a threading region very close to the L1 point cannot be excluded.Comment: 10 pages, 9 figures, in press on A&
Coronal X-Ray Emission from the Stellar Companions to Transiently Accreting Black Holes
Observations of soft X-ray transients (SXTs) in quiescence have found that
the binaries harboring black holes are fainter than those that contain a
neutron star. Narayan and collaborators postulated that the faint X-ray
emission from black hole binaries was powered by an advection dominated
accretion flow (ADAF). We explore an alternative explanation for the quiescent
X-ray emission from the black hole systems: coronal emission from the rapidly
rotating optical companion. This is commonly observed and well studied in other
tidally locked binaries, such as the RS CVns. We show that two of the three
X-ray detected black hole binaries (A0620-00 and GRO J1655-40) exhibit X-ray
fluxes entirely consistent with coronal emission. The X-ray spectra of these
objects should be best fit with thermal Raymond-Smith models rich in lines when
coronal emission predominates. One black hole system (V404 Cyg) is too X-ray
bright to be explained as coronal emission. The quiescent X-ray emission from
the neutron star binaries is far too bright for coronal emission. It might be
that all SXT's have variable accretion rates in quiescence and that the basal
quiescent X-ray flux is set by either coronal emission from the companion or --
when present -- by thermal emission from the neutron star. We also show that
the lithium abundances in the black hole systems are comparable to those in the
RS CVns, reducing the need for production mechanisms that involve the compact
object.Comment: ApJ, accepted (v541; Oct 1, 2000); Changes to figures and tables,
minor modifications to text. Uses emulateapj.sty. 14 pages, 3 figure
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