1,023 research outputs found
A jet model for Galactic black-hole X-ray sources: Some constraining correlations
Some recent observational results impose significant constraints on all the
models that have been proposed to explain the Galactic black-hole X-ray sources
in the hard state. In particular, it has been found that during the hard state
of Cyg X-1 the power-law photon number spectral index is correlated with the
average time lag between hard and soft X-rays. Furthermore, the peak
frequencies of the four Lorentzians that fit the observed power spectra are
correlated with both the photon index and the time lag. We performed Monte
Carlo simulations of Compton upscattering of soft, accretion-disk photons in
the jet and computed the time lag between hard and soft photons and the
power-law index of the resulting photon number spectra. We demonstrate that our
jet model naturally explains the above correlations, with no additional
requirements and no additional parameters
Correlated Timing and Spectral Variations of the Soft X-ray Transient Aquila X-1: Evidence for an Atoll classification
Based on Rossi X-ray Timing Explorer data, we discuss the classification of
the soft X-ray transient Aquila X-1 in the Z/atoll scheme, and the relation of
its kilohertz quasi-periodic oscillations (kHz QPO) properties to the X-ray
colors. The color-color diagram shows one elongated ("banana") structure and
several "islands" of data points. The power spectra of the island are best
represented by a broken power-law, whereas those of the banana by a power-law
below ~ 1 Hz plus an exponentially cut-off component at intermediate
frequencies (30-60 Hz). The parameters of these two components change in
correlation with the position of the source in the color-color diagram. Based
on the pattern that the source shows in the color-color diagram and its
aperiodic variability we conclude that Aquila X-1 is an atoll source. We have
also investigated the possible correlation between the frequency of the kHz QPO
and the position of the source in the color-color diagram. The complexity seen
in the frequency versus count rate diagram is reduced to a single track when
the frequency is plotted against hard or soft color.Comment: 9 pages, 5 figures. Accepted for publication in Ap
Identification of the optical counterparts of high-mass X-ray binaries through optical photometry and spectroscopy
We present the results of our search for optical counterparts to high-mass
X-ray transient sources discovered by various X-ray missions. We obtained CCD
images of the X-ray fields through BVR and Halpha filters in order to identify
early-type stars in the R-Halpha versus B-V colour-colour diagram. We also
obtained medium-resolution spectroscopy of the candidates in order to confirm
the presence of H\alpha emission and perform spectral classification. We report
on the discovery of the optical counterparts to two X-ray sources: XTE
J1858+034 and IGR J01363+6610, and the follow-up observations of another two,
newly identified by our group: SAX J2103.5+4545 and GRO J2058+42. For another
source, IGR J00370+6122, we present the first detailed optical spectral
analysis. The optical photometry and spectroscopy reveal B-type companions in
all five sources; GRO J2058+42, SAX J2103.5+4545 and IGR J01363+6610 are
positively identified with Be/X-ray binaries, IGR J00370+6122 with a supergiant
X-ray binary, while the nature of XTE J1858+034 is uncertain. We also study the
relationship between the optical and X-ray emission during quiescent states.Comment: 10 pages, to be published in A&
Surface collective modes in the topological insulators BiSe and BiSbTeSe
We used low-energy, momentum-resolved inelastic electron scattering to study
surface collective modes of the three-dimensional topological insulators
BiSe and BiSbTeSe. Our goal was to
identify the "spin plasmon" predicted by Raghu and co-workers [S. Raghu, et
al., Phys. Rev. Lett. 104, 116401 (2010)]. Instead, we found that the primary
collective mode is a surface plasmon arising from the bulk, free carrers in
these materials. This excitation dominates the spectral weight in the bosonic
function of the surface, , at THz energy scales, and
is the most likely origin of a quasiparticle dispersion kink observed in
previous photoemission experiments. Our study suggests that the spin plasmon
may mix with this other surface mode, calling for a more nuanced understanding
of optical experiments in which the spin plasmon is reported to play a role.Comment: 5 pages, 4 figure
The X-ray binary 2S0114+650=LSI+65 010:A slow pulsar or tidally-induced pulsations?
The X-ray source 2S0114+650=LSI+65 010 is a binary system containing a B-type
primary and a low mass companion believed to be a neutron star. The system has
three reported periodicities: the orbital period, P{orb}~11.6 d, X-ray flaring
with P{flare}~2.7 hr, and a "superorbital" X-ray periodicity P{super}~30.7 d.
The objective of this paper is to show that the puzzling periodicities in the
system may be explained in the context of scenarios in which tidal interactions
drive oscillations in the B-supergiant star. We calculate the solution of the
equations of motion for one layer of small surface elements distributed along
the equator of the star, as they respond to the forces due to gas pressure,
centrifugal, coriolis, viscous forces, and the gravitational forces of both
stars. This calculation provides variability timescales that can be compared
with the observations. In addition, we use observational data obtained at the
Observatorio Astron\'omico Nacional en San Pedro M\'artir (OAN/SPM) between
1993-2004 to determine which of the periodicities may be present in the optical
region. We suggest that the tidal oscillations lead to a structured stellar
wind which, when fed to the neutron star, produces the X-ray modulations. The
connection between the stellar oscillations and the modulation of the mass
ejection may lie in the shear energy dissipation generated by the tangential
motions that are produced by the tidal interaction, particularly in the tidal
bulge region. The tidal oscillation scenario weakens the case for 2S0114+650
containing a magnetar descendent.Comment: 12 pages, 14 figure
Bright flares from the X-ray pulsar SWIFT J1626.6-5156
We have performed a timing and spectral analysis of the X-ray pulsar SWIFT
J1626.6-5156 during a major X-ray outburst in order to unveil its nature and
investigate its flaring activity. Epoch- and pulse-folding techniques were used
to derive the spin period. Time-average and pulse-phase spectroscopy were
employed to study the spectral variability in the flare and out-of-flare states
and energy variations with pulse phase. Power spectra were obtained to
investigate the periodic and aperiodic variability. Two large flares, with a
duration of ~450 seconds were observed on 24 and 25 December 2005. During the
flares, the X-ray intensity increased by a factor of 3.5, while the
peak-to-peak pulsed amplitude increased from 45% to 70%. A third, smaller flare
of duration ~180 s was observed on 27 December 2005. The flares seen in SWIFT
J1626.6-5156 constitute the shortest events of this kind ever reported in a
high-mass X-ray binary. In addition to the flaring activity, strong X-ray
pulsations with Pspin=15.3714+-0.0003 s characterise the X-ray emission in
SWIFT J1626.6-5156. After the major outburst, the light curve exhibits strong
long-term variations modulated with a 45-day period. We relate this modulation
to the orbital period of the system or to a harmonic. Power density spectra
show, in addition to the harmonic components of the pulsation, strong
band-limited noise with an integrated 0.01-100 Hz fractional rms of around 40%
that increased to 64% during the flares. A weak QPO (fractional rms 4.7%) with
characteristic frequency of 1 Hz was detected in the non-flare emission. The
timing (short X-ray pulsations, long orbital period) and spectral (power-law
with cut off energy and neutral iron line) properties of SWIFT J1626.6-5156 are
characteristic of Be/X-ray binaries
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