1,730 research outputs found
The evolution of the high energy tail in the quiescent spectrum of the soft X-ray transient Aql X-1
A moderate level of variability has been detected in the quiescent luminosity
of several neutron star soft X-ray transients. Spectral variability was first
revealed by Chandra observations of Aql X-1 in the four months that followed
the 2000 X-ray outburst. By adopting the canonical model for quiescent spectrum
of soft X-ray transients, i.e. an absorbed neutron star atmosphere model plus a
power law tail, Rutledge et al. (2002a) concluded that the observed spectral
variations can be ascribed to temperature variations of the neutron star
atmosphere. These results can hardly be reconciled with the neutron star
cooling that is expected to take place in between outbursts (after deep crustal
heating in the accretion phase). Here we reanalyse the Chandra spectra of Aql
X-1, together with a long BeppoSAX observation in the same period, and propose
a different interpretation of the spectral variability: that this is due to
correlated variations of the power law component and the column density (>5, a
part of which might be intrinsic to the source), while the temperature and flux
of the neutron star atmospheric component remained unchanged. This lends
support to the idea that the power law component arises from emission at the
shock between a radio pulsar wind and inflowing matter from the companion star.Comment: 6 pages, 2 figures. Accepted for publication on Ap
A Radial Velocity Study of CTCV J1300-3052
We present time-resolved spectroscopy of the eclipsing, short period
cataclysmic variable CTCV J1300-3052. Using absorption features from the
secondary star, we determine the radial velocity semi-amplitude of the
secondary star to be K2 = 378 \pm 6 km/s, and its projected rotational velocity
to be v sin i = 125 \pm 7 km/s. Using these parameters and Monte Carlo
techniques, we obtain masses of M1 = 0.79 \pm 0.05 MSun for the white dwarf
primary and M2 = 0.198 \pm 0.029 MSun for the M-type secondary star. These
parameters are found to be in excellent agreement with previous mass
determinations found via photometric fitting techniques, supporting the
accuracy and validity of photometric mass determinations in short period CVs.Comment: Accepted for publication in MNRAS (24th January 2012). 10 pages, 9
figures (black and white
The mass of the neutron star in Cyg X-2 (V1341 Cyg)
Cygnus X-2 is one of the brightest and longest known X-ray sources. We
present high resolution optical spectroscopy of Cyg X-2 obtained over 4 years
which gives an improved mass function of 0.69 +/- 0.03 Msun (1 sigma error). In
addition, we resolve the rotationally broadened absorption features of the
secondary star for the first time, deriving a rotation speed of vsin(i) = 34.2
+/- 2.5 km per s (1 sigma error) which leads to a mass ratio of q = M_c/M_x =
0.34 +/- 0.04 (1 sigma error), assuming a tidally-locked and Roche lobe-filling
secondary). Hence with the lack of X-ray eclipses (i.e. i <~ 73 degrees) we can
set firm 95% confidence lower limits to the neutron star mass of M_x > 1.27
Msun and to the companion star mass of M_c > 0.39 Msun. However, by
additionally requiring that the companion must exceed 0.75 Msun (as required
theoretically to produce a steady low-mass X-ray binary), then M_x > 1.88 Msun
and i < 61 degrees (95% confidence lower and upper limit, respectively),
thereby making Cyg X-2 the highest mass neutron star measured to date. If
confirmed this would set significant constraints on the equation of state of
nuclear matter.Comment: 16 pages, 4 figures, ApJ Letters, accepted, LaTeX, aasms4.st
The X-ray eclipse of the dwarf nova HT CAS observed by the XMM-Newton satellite: spectral and timing analysis
A cataclysmic variable is a binary system consisting of a white dwarf that
accretes material from a secondary object via the Roche-lobe mechanism. In the
case of long enough observation, a detailed temporal analysis can be performed,
allowing the physical properties of the binary system to be determined. We
present an XMM-Newton observation of the dwarf nova HT Cas acquired to resolve
the binary system eclipses and constrain the origin of the X-rays observed. We
also compare our results with previous ROSAT and ASCA data. After the spectral
analysis of the three EPIC camera signals, the observed X-ray light curve was
studied with well known techniques and the eclipse contact points obtained.
The X-ray spectrum can be described by thermal bremsstrahlung of temperature
keV plus a black-body component (upper limit) with
temperature eV. Neglecting the black-body, the bolometric
absorption corrected flux is erg
s cm, which, for a distance of HT Cas of 131 pc, corresponds to a
bolometric luminosity of erg s.
The study of the eclipse in the EPIC light curve permits us to constrain the
size and location of the X-ray emitting region, which turns out to be close to
the white dwarf radius. We measure an X-ray eclipse somewhat smaller (but only
at a level of ) than the corresponding optical one. If this
is the case, we have possibly identified the signature of either high latitude
emission or a layer of X-ray emitting material partially obscured by an
accretion disk.Comment: Accepted for publication on Astronomy and Astrophysics, 200
Doppler tomography of the transient X-ray binary Centaurus X-4 in quiescence
We present ESO-NTT low resolution spectroscopy of the neutron star X-ray
transient Cen X-4 in quiescence over a complete orbital cycle. Our data reveal
the presence of a K3-7 V companion which contributes 63% to the 5600-6900A flux
and orbits the neutron star with a velocity semi-amplitude of K_2=145.8 +/- 1.0
km s^{-1}. This, combined with a previous determination of the inclination
angle and mass ratio, yields a neutron star and companion mass of M_1=1.5 +/-
1.0 M_Sun and M_2=0.31 +/- 0.27 M_Sun, respectively. The mass donor is thus
undermassive for the inferred spectral type indicating it is probably evolved,
in agreement with previous studies. Doppler tomography of the H_alpha line
shows prominent emission located on the companion and a slightly asymmetric
accretion disc distribution similar to that seen in systems with precessing
eccentric discs. Strong H_alpha emission from the companion can be explained by
X-ray irradiation from the primary. No evidence is found for a hot spot in
H_alpha, whereas one is revealed via Doppler tomography of the HeI lines. This
can be interpreted as the hot spot and outer regions of the disc being at a
higher temperature than in other systems.Comment: 9 pages, 5 figures, accepted for publication in A&
Three-dimensional topologically gauged N=6 ABJM type theories
In this paper we construct the conformal supergravity in three
dimensions from a set of Chern-Simons-like terms one for each of the graviton,
gravitino, and R-symmetry gauge field and then couple this theory to the
superconformal ABJM theory. In a first step part of the coupled
Lagrangian for this topologically gauged ABJM theory is derived by demanding
that all terms of third and second order in covariant derivatives cancel in the
supersymmtry variation of the Lagrangian. To achieve this the transformation
rules of the two separate sectors must be augmented by new terms. In a second
step we analyze all terms in that are of first order in covariant
derivatives. The cancelation of these terms require additional terms in the
transformation rules as well as a number of new terms in the Lagrangian. As a
final step we check that all remaining terms in which are bilinear
in fermions cancel which means that the presented Lagrangian and transformation
rules constitute the complete answer. In particular we find in the last step
new terms in the scalar potential containing either one or no structure
constant. The non-derivative higher fermion terms in that have not
yet been completely analyzed are briefly discussed.Comment: 26 pages, v.2 minor corrections, comment on relation to chiral
gravity added
Observations of the Hot Horizontal-Branch Stars in the Metal-Rich Bulge Globular Cluster NGC 6388 - Indications of Helium Enrichment and a Lesson in Crowded Field Spectroscopy
(abridged) The metal-rich bulge globular cluster NGC6388 shows a distinct
blue horizontal-branch tail with a strong upward tilt in its colour-magnitude
diagram. Several noncanonical scenarios have been proposed to explain these
puzzling observations. In order to test the predictions of these scenarios, we
have obtained medium resolution spectra to determine the atmospheric parameters
of a sample of the blue HB stars in NGC 6388. Using the medium resolution
spectra, we determine effective temperatures, surface gravities and helium
abundances by fitting the observed Balmer and helium lines with appropriate
theoretical stellar spectra. As we know the distance to the cluster, we can
verify our results by determining masses for the stars. During the data
reduction we took special care in subtracting the background, which is
dominated by the overlapping spectra of cool stars. The physical parameters of
the blue HB stars near 10,000 K support the helium pollution scenario. The low
gravities and masses found for the hot blue tail stars, however, are probably
caused by problems with the data reduction, most likely due to remaining
background light in the spectra, which would affect the fainter hot blue tail
stars much more strongly than the brighter cool blue tail stars. Our study of
the hot blue tail stars in NGC 6388 illustrates the obstacles which are
encountered when attempting to determine the atmospheric parameters of hot HB
stars in very crowded fields using ground-based observations. We discuss these
obstacles and offer possible solutions for future projects.Comment: 11 pages, 9 figures (finding charts as png only), uses aa.cls;
accepted for publication in A&
Irradiation of the secondary star in X-ray Nova Scorpii 1994 (=GRO J1655--40)
We have obtained intermediate resolution optical spectra of the black-hole
candidate Nova Sco 1994 in June 1996, when the source was in an X-ray/optical
active state (R~15.05). We measure the radial velocity curve of the secondary
star and obtain a semi-amplitude of 279+/-10 km/s; a value which is 30 per cent
larger than the value obtained when the source is in quiescence. Our large
value for K_2 is consistent with 60 +9,-7 per cent of the secondary star's
surface being heated; compared to 35 per cent, which is what one would expect
if only the inner face of the secondary star were irradiated. Effects such as
irradiation-induced flows on the secondary star may be important in explaining
the observed large value for K_2.Comment: 5 pages, 2 figures, accepted by MNRA
Observational evidence for matter propagation in accretion flows
We study simultaneous X-ray and optical observations of three intermediate
polars EX Hya, V1223 Sgr and TV Col with the aim to understand the propagation
of matter in their accretion flows. We show that in all cases the power spectra
of flux variability of binary systems in X-rays and in optical band are similar
to each other and the majority of X-ray and optical fluxes are correlated with
time lag <1 sec. These findings support the idea that optical emission of
accretion disks, in these binary systems,largely originates as reprocessing of
X-ray luminosity of their white dwarfs. In the best obtained dataset of EX Hya
we see that the optical lightcurve unambiguously contains some component, which
leads the X-ray emission by ~7 sec. We interpret this in the framework of the
model of propagating fluctuations and thus deduce the time of travel of the
matter from the innermost part of the truncated accretion disk to the white
dwarf surface. This value agrees very well with the time expected for matter
threaded onto the magnetosphere of the white dwarf to fall to its surface. The
datasets of V1223 Sgr and TV Col in general confirm these findings,but have
poorer quality.Comment: 7 pages, 6 figures. Accepted for publication in MNRA
Discovery of 5.16s pulsations from the isolated neutron star RBS 1223
The isolated neutron star candidate RBS 1223 was observed with the Advanced
CCD Imaging Spectrometer aboard the Chandra X-ray observatory on 2000 June 24.
A timing analysis of the data yielded a periodic modulation with a period
P=5.1571696^(+1.57*10^(-4) -1.36*10^(-4)s. Using ROSAT HRI archived
observations we detected a period P=5.1561274 \pm 4.4*10^(-4)s and determined
period derivative dP/dt=(0.7 - 2.0)*10^(-11) s*s^(-1). The detection of this
period and dP/dt indicates that RBS 12223 has a ``characteristic'' age of
6000-12000 years and huge magnetic field at the surface (B(dipole)~(1.7-
3.2)*10^(+14) G) typical for anomalous X-ray pulsars (AXPs).Comment: 7 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
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