3,249 research outputs found
Exploring the role of X-ray reprocessing and irradiation in the anomalous bright optical outbursts of A0538-66
In 1981, the Be/X-ray binary (Be/XRB) A0538-66 showed outbursts characterized
by high peak luminosities in the X-ray and optical bands. The optical outbursts
were qualitatively explained as X-ray reprocessing in a gas cloud surrounding
the binary system. Since then, further important information about A0538-66
have been obtained, and sophisticated photoionization codes have been developed
to calculate the radiation emerging from a gas nebula illuminated by a central
X-ray source. In the light of the new information and tools available, we
studied again the enhanced optical emission displayed by A0538-66 to understand
the mechanisms responsible for these unique events among the class of Be/XRBs.
We performed about 10^5 simulations of a gas envelope photoionized by an X-ray
source. We assumed for the shape of the gas cloud either a sphere or a
circumstellar disc observed edge-on. We studied the effects of varying the main
properties of the envelope and the influence of different input X-ray spectra
on the optical/UV emission emerging from the photoionized cloud. We compared
the computed spectra with the IUE spectrum and photometric UBV measurements
obtained during the outburst of 29 April 1981. We also explored the role played
by the X-ray heating of the surface of the donor star irradiated by the X-ray
emission of the neutron star (NS). We found that reprocessing in a spherical
cloud with a shallow radial density distribution can reproduce the optical/UV
emission. To our knowledge, this configuration has never been observed either
in A0538-66 during other epochs or in other Be/XRBs. We found, contrary to the
case of most other Be/XRBs, that the optical/UV radiation produced by the X-ray
heating of the surface of the donor star irradiated by the NS is
non-negligible, due to the particular orbital parameters of this system that
bring the NS very close to its companion.Comment: Accepted for publication in Astronomy & Astrophysics. Abstract
abridged to meet arXiv requirement
Spectral and temporal properties of the supergiant fast X-ray transient IGR J18483-0311 observed by INTEGRAL
IGR J18483-0311 is a supergiant fast X-ray transient whose compact object is
located in a wide (18.5 d) and eccentric (e~0.4) orbit, which shows sporadic
outbursts that reach X-ray luminosities of ~1e36 erg/s. We investigated the
timing properties of IGR J18483-0311 and studied the spectra during bright
outbursts by fitting physical models based on thermal and bulk Comptonization
processes for accreting compact objects. We analysed archival INTEGRAL data
collected in the period 2003-2010, focusing on the observations with IGR
J18483-0311 in outburst. We searched for pulsations in the INTEGRAL light
curves of each outburst. We took advantage of the broadband observing
capability of INTEGRAL for the spectral analysis. We observed 15 outbursts,
seven of which we report here for the first time. This data analysis almost
doubles the statistics of flares of this binary system detected by INTEGRAL. A
refined timing analysis did not reveal a significant periodicity in the
INTEGRAL observation where a ~21s pulsation was previously detected. Neither
did we find evidence for pulsations in the X-ray light curve of an archival
XMM-Newton observation of IGR J18483-0311. In the light of these results the
nature of the compact object in IGR J18483-0311 is unclear. The broadband X-ray
spectrum of IGR J18483-0311 in outburst is well fitted by a thermal and bulk
Comptonization model of blackbody seed photons by the infalling material in the
accretion column of a neutron star. We also obtained a new measurement of the
orbital period using the Swift/BAT light curve.Comment: Accepted for publication in Astronomy and Astrophysics. 8 page
Properties and observability of glitches and anti-glitches in accreting pulsars
Several glitches have been observed in young, isolated radio pulsars, while a
clear detection in accretion-powered X-ray pulsars is still lacking. We use the
Pizzochero snowplow model for pulsar glitches as well as starquake models to
determine for the first time the expected properties of glitches in accreting
pulsars and their observability. Since some accreting pulsars show
accretion-induced long-term spin-up, we also investigate the possibility that
anti-glitches occur in these stars. We find that glitches caused by quakes in a
slow accreting neutron star are very rare and their detection extremely
unlikely. On the contrary, glitches and anti-glitches caused by a transfer of
angular momentum between the superfluid neutron vortices and the non-superfluid
component may take place in accreting pulsars more often. We calculate the
maximum jump in angular velocity of an anti-glitch and we find that it is
expected to be about 1E-5 - 1E-4 rad/s. We also note that since accreting
pulsars usually have rotational angular velocities lower than those of isolated
glitching pulsars, both glitches and anti-glitches are expected to have long
rise and recovery timescales compared to isolated glitching pulsars, with
glitches and anti-glitches appearing as a simple step in angular velocity.
Among accreting pulsars, we find that GX 1+4 is the best candidate for the
detection of glitches with currently operating X-ray instruments and future
missions such as the proposed Large Observatory for X-ray Timing (LOFT).Comment: Accepted for publication in Astronomy & Astrophysics. 6 pages. Minor
changes to match the final A&A versio
Broadband observations of the X-ray burster 4U 1705-44 with BeppoSAX
4U 1705-44 is one of the most-studied type I X-ray burster and Atoll sources.
This source represents a perfect candidate to test different models proposed to
self-consistently track the physical changes occurring between different
spectral states because it shows clear spectral state transitions. The
broadband coverage, the sensitivity and energy resolution of the BeppoSAX
satellite offers the opportunity to disentangle the components that form the
total X-ray spectrum and to study their changes according to the spectral
state. Using two BeppoSAX observations carried out in August and October 2000,
respectively, for a total effective exposure time of about 100 ks, we study the
spectral evolution of the source from a soft to hard state. Energy spectra are
selected according to the source position in the color-color diagram (CCD)
Results. We succeeded in modeling the spectra of the source using a physical
self-consistent scenario for both the island and banana branches (the double
Comptonization scenario). The components observed are the soft Comptonization
and hard Comptonization, the blackbody, and a reflection component with a broad
iron line. When the source moves from the banana state to the island state, the
parameters of the two Comptonization components change significantly and the
blackbody component becomes too weak to be detected. We interpret the soft
Comptonization component as emission from the hot plasma surrounding the
neutron star, hard Comptonization as emission from the disk region, and the
blackbody component as emission from the inner accretion disk. The broad
feature in the iron line region is compatible with reflection from the inner
accretion disk.Comment: 8 pages, 10 figures, accepted for publication by A&
A smooth cascade of wrinkles at the edge of a floating elastic film
The mechanism by which a patterned state accommodates the breaking of
translational symmetry by a phase boundary or a sample wall has been addressed
in the context of Landau branching in type-I superconductors, refinement of
magnetic domains, and compressed elastic sheets. We explore this issue by
studying an ultrathin polymer sheet floating on the surface of a fluid,
decorated with a pattern of parallel wrinkles. At the edge of the sheet, this
corrugated profile meets the fluid meniscus. Rather than branching of wrinkles
into generations of ever-smaller sharp folds, we discover a smooth cascade in
which the coarse pattern in the bulk is matched to fine structure at the edge
by the continuous introduction of discrete, higher wavenumber Fourier modes.
The observed multiscale morphology is controlled by a dimensionless parameter
that quantifies the relative strength of the edge forces and the rigidity of
the bulk pattern.Comment: 4 pages, 4 figure
Disentangling the jet emission from protostellar systems. The ALMA view of VLA1623
Context: High-resolution studies of class 0 protostars represent the key to
constraining protostar formation models. VLA16234-2417 represents the prototype
of class 0 protostars, and it has been recently identified as a triple
non-coeval system. Aim: We aim at deriving the physical properties of the jets
in VLA16234-2417 using tracers of shocked gas. Methods: ALMA Cycle 0 Early
Science observations of CO(2-1) in the extended configuration are presented in
comparison with previous SMA CO(3-2) and Herschel-PACS [OI}] 63 micron
observations. Gas morphology and kinematics were analysed to constrain the
physical structure and origin of the protostellar outflows. Results: We reveal
a collimated jet component associated with the [OI] 63 micron emission at about
8'' (about 960 AU) from source B. This newly detected jet component is
inversely oriented with respect to the large-scale outflow driven by source A,
and it is aligned with compact and fast jet emission very close to source B
(about 0.3'') rather than with the direction perpendicular to the A disk. We
also detect a cavity-like structure at low projected velocities, which
surrounds the [OI] 63 micron emission and is possibly associated with the
outflow driven by source A. Finally, no compact outflow emission is associated
with source W. Conclusions: Our high-resolution ALMA observations seem to
suggest there is a fast and collimated jet component associated with source B.
This scenario would confirm that source B is younger than A, that it is in a
very early stage of evolution, and that it drives a faster, more collimated,
and more compact jet with respect to the large-scale slower outflow driven by
A. However, a different scenario of a precessing jet driven by A cannot be
firmly excluded from the present observations.Comment: Accepted for publication in Astronomy & Astrophysic
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