2,064 research outputs found
Long-term Properties of Accretion Disks in X-ray Binaries: II. Stability of Radiation-Driven Warping
A significant number of X-ray binaries are now known to exhibit long-term
``superorbital'' periodicities on timescales of 10 - 100 days. Several
physical mechanisms have been proposed that give rise to such periodicities, in
particular warping and/or precession of the accretion disk. Recent theoretical
work predicts the stability to disk warping of X-ray binaries as a function of
the mass ratio, binary radius, viscosity and accretion efficiency, and here we
examine the constraints that can be placed on such models by current
observations.
In paper I we used a dynamic power spectrum (DPS) analysis of long-term X-ray
datasets (CGRO, RXTE), focusing on the remarkable, smooth variations in the
superorbital period exhibited by SMC X-1. Here we use a similar DPS analysis to
investigate the stability of the superorbital periodicities in the neutron star
X-ray binaries Cyg X-2, LMC X-4 and Her X-1, and thereby confront stability
predictions with observation. We find that the period and nature of
superorbital variations in these sources is consistent with the predictions of
warping theory.
We also use a dynamic lightcurve analysis to examine the behaviour of Her X-1
as it enters and leaves the 1999 Anomalous Low State (ALS). This reveals a
significant phase shift some 15 cycles before the ALS, which indicates a change
in the disk structure or profile leading into the ALS.Comment: 12 pages, 14 figures, Re-submitted to MNRAS after referee's comment
Creation of collective many-body states and single photons from two-dimensional Rydberg lattice gases
The creation of collective many-body quantum states from a two-dimensional
lattice gas of atoms is studied. Our approach relies on the van-der-Waals
interaction that is present between alkali metal atoms when laser excited to
high-lying Rydberg s-states. We focus on a regime in which the laser driving is
strong compared to the interaction between Rydberg atoms. Here energetically
low-lying many-particle states can be calculated approximately from a quadratic
Hamiltonian. The potential usefulness of these states as a resource for the
creation of deterministic single-photon sources is illustrated. The properties
of these photon states are determined from the interplay between the particular
geometry of the lattice and the interatomic spacing.Comment: 12 pages, 8 figure
The Great Pretenders Among the ULX Class
The recent discoveries of pulsed X-ray emission from three ultraluminous
X-ray (ULX) sources have finally enabled us to recognize a subclass within the
ULX class: the great pretenders, neutron stars (NSs) that appear to emit X-ray
radiation at isotropic luminosities ~erg~s~erg~s only because their emissions are strongly beamed toward
our direction and our sight lines are offset by only a few degrees from their
magnetic-dipole axes. The three known pretenders appear to be stronger emitters
than the presumed black holes of the ULX class, such as Holmberg II \& IX X-1,
IC10 X-1, and NGC300 X-1. For these three NSs, we have adopted a single
reasonable assumption, that their brightest observed outbursts unfold at the
Eddington rate, and we have calculated both their propeller states and their
surface magnetic-field magnitudes. We find that the results are not at all
different from those recently obtained for the Magellanic Be/X-ray pulsars: the
three NSs reveal modest magnetic fields of about 0.3-0.4~TG and beamed
propeller-line X-ray luminosities of ~erg~s,
substantially below the Eddington limit.Comment: To appear in Research in Astronomy and Astrophysic
Optical studies of two LMC X-ray transients : RX J0544.1-7100 and RX J0520.5-6932
We report observations which confirm the identities of the optical
counterpart to the transient sources RX J0544.1-7100 and RX J0520.5-6932. The
counterparts are suggested to be a B-type stars. Optical data from the
observations carried out at ESO and SAAO, together with results from the OGLE
data base, are presented. In addition, X-ray data from the RXTE all-sky monitor
are investigated for long term periodicities. A strong suggestion for a binary
period of 24.4d is seen in RX J0520.5-6932 from the OGLE data.Comment: 6 pages, 7 figure
Constraining the Nature of the Galactic Center X-ray Source Population
We searched for infrared counterparts to the cluster of X-ray point sources
discovered by Chandra in the Galactic Center Region (GCR). While the sources
could be white dwarfs, neutron stars, or black holes accreting from stellar
companions, their X-ray properties are consistent with magnetic Cataclysmic
Variables, or High Mass X-ray Binaries (HMXB) at low accretion-rates. A direct
way to decide between these possibilities and hence between alternative
formation scenarios is to measure or constrain the luminosity distribution of
the companions. Using infrared (J, H, K, Br-gamma) imaging, we searched for
counterparts corresponding to typical HMXB secondaries: spectral type B0V with
K<15 at the GCR. We found no significant excess of bright stars in Chandra
error circles, indicating that HMXBs are not the dominant X-ray source
population, and account for fewer than 10% of the hardest X-ray sources.Comment: 4 pages, 3 figures, 1 table, accepted in ApJ Letters for publicatio
Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit
Massive X-ray binaries are usually classified depending on the properties of
the donor star in classical, supergiant and Be X-ray binaries. The massive
X-ray binary 4U 2206+54 does not fit in any of these groups, and deserves a
detailed study to understand how the transfer of matter and the accretion on to
the compact object take place. To this end we study an IUE spectrum of the
donor and obtain a wind terminal velocity (v_inf) of ~350 km/s, which is
abnormally slow for its spectral type. We also analyse here more than 9 years
of available RXTE/ASM data. We study the long-term X-ray variability of the
source and find it to be similar to that observed in the wind-fed supergiant
system Vela X-1, reinforcing the idea that 4U 2206+54 is also a wind-fed
system. We find a quasi-period decreasing from ~270 to ~130 d, noticed in
previous works but never studied in detail. We discuss possible scenarios and
conclude that long-term quasi-periodic variations in the mass-loss rate of the
primary are probably driving such variability in the measured X-ray flux. We
obtain an improved orbital period of 9.5591 d with maximum X-ray flux at MJD
51856.6. Our study of the orbital X-ray variability in the context of wind
accretion suggests a moderate eccentricity around 0.15. Moreover, the low value
of v_inf solves the long-standing problem of the relatively high X-ray
luminosity for the unevolved nature of the donor, BD +53 2790, which is
probably an O9.5 V star. We note that changes in v_inf and/or the mass-loss
rate of the primary alone cannot explain the diferent patterns displayed by the
orbital X-ray variability. We finally emphasize that 4U 2206+54, together with
LS 5039, could be part of a new population of wind-fed HMXBs with main sequence
donors, the natural progenitors of supergiant X-ray binaries. (Abridged)Comment: 12 pages, 9 figures; to appear in A&A; corrected typos, updated
references; matches published versio
Constraints in the Context of Induced-gravity Inflation
Constraints on the required flatness of the scalar potential for a
cousin-model to extended inflation are studied. It is shown that, unlike
earlier results, Induced-gravity Inflation can lead to successful inflation
with a very simple lagrangian and , rather than
as previously reported. A second order phase transition further
enables this model to escape the \lq big bubble' problem of extended inflation,
while retaining the latter's motivations based on the low-energy effective
lagrangians of supergravity, superstring, and Kaluza-Klein theories.Comment: 19 pp; 3 figures (not included -- available from author). Plain
LaTeX. In press in Physical Review
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