415 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
Cardiovascular Magnetic Resonance Parametric Mapping Techniques: Clinical Applications and Limitations
Purpose of Review: Parametric mapping represents a significant innovation in cardiovascular magnetic resonance (CMR) tissue characterisation, allowing the quantification of myocardial changes based on changes on T1, T2 and T2* relaxation times and extracellular volume (ECV). Its clinical use is rapidly expanding, but it requires availability of dedicated equipment as well as expertise in image acquisition and analysis. This review focuses on the principles of CMR parametric mapping, its current clinical applications, important limitations, as well as future directions of this technique in cardiovascular medicine. Recent Findings: There is increasing evidence that CMR parametric mapping techniques provide accurate diagnostic and prognostic tools that can be applied to and support the clinical management of patients with a range of cardiovascular disease. Summary: The unique capability of CMR myocardial tissue characterisation in cardiovascular diseases has further expanded by the introduction of parametric mapping. Its use in clinical practice presents opportunities but has also limitations
GK Persei and EX Hydrae: Intermediate polars with small magnetospheres
Observed hard X-ray spectra of intermediate polars are determined mainly by the accretion flow velocity at the white dwarf surface, which is normally close to the free-fall velocity. This allows us to estimate the white dwarf masses as the white dwarf mass-radius relation M − R and the expected free-fall velocities at the surface are well known. This method is widely used. However, derived white dwarf masses M can be systematically underestimated because the accretion flow is stopped at, and re-accelerates from, the magnetospheric boundary R_m and, therefore, its velocity at the surface is lower than free fall. To avoid this problem, we computed a two-parameter set of model hard X-ray spectra, which allows us to constrain a degenerate M – R_m dependence. Previous works showed that power spectra of accreting X-ray pulsars and intermediate polars exhibit breaks at frequencies corresponding to Keplerian frequencies at the magnetospheric boundary. Therefore, the break frequency ν_b in an intermediate polar power spectrum gives another relation in the M – R_m plane. The intersection of the two dependences allows us, therefore, to determine the white dwarf mass and magnetospheric radius simultaneously. To verify the method, we analysed the archival Suzaku observation of EX Hya, obtaining M/M⊙ = 0.73 ± 0.06 and R_m/R = 2.6 ± 0.4, which is consistent with the values determined by other authors. Subsequently, we applied the same method to a recent NuSTAR observation of another intermediate polar GK Per performed during an outburst and found M/M⊙ = 0.86 ± 0.02 and R_m/R = 2.8 ± 0.2. The long duration observations of GK Per in quiescence performed by Swift/BAT and INTEGRAL observatories indicate increase of magnetosphere radius R_m at lower accretion rates
Swift/XRT monitoring of the Supergiant Fast X-ray Transient IGR J18483-0311 for an entire orbital period
IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging
to the new class of High Mass X-ray Binaries called Supergiant Fast X-ray
Transients. This system is one of the two members of this class, together with
IGR J11215-5952, where both the orbital (18.52d) and spin period (21s) are
known. We report on the first complete monitoring of the X-ray activity along
an entire orbital period of a Supergiant Fast X-ray Transient. These Swift
observations, lasting 28d, cover more than one entire orbital phase
consecutively. They are a unique data-set, which allows us to constrain the
different mechanisms proposed to explain the nature of this new class of X-ray
transients. We applied the new clumpy wind model for blue supergiants developed
by Ducci et al. (2009), to the observed X-ray light curve. Assuming an
eccentricity of e=0.4, the X-ray emission from this source can be explained in
terms of the accretion from a spherically symmetric clumpy wind, composed of
clumps with different masses, ranging from 10^{18}g to 5x 10^{21}g.Comment: Accepted for publication in MNRAS. 7 pages, 7 figures, 2 table
Cyclotron line energy in Hercules X-1: stable after the decay
We summarize the results of a dedicated effort made between 2012 and 2019 to follow the evolution of the cyclotron line in Her X-1 through repeated NuSTAR observations. The previously observed nearly 20-year-long decay of the cyclotron line energy has ended in 2012: from then on, the pulse-phase-averaged flux-corrected cyclotron line energy has remained stable and constant at an average value of E_(cyc) = (37.44 ± 0.07) keV (normalized to a flux level of 6.8 RXTE/ASM-cts s⁻¹). The flux dependence of E_(cyc) discovered in 2007 is now measured with high precision, giving a slope of (0.675 ± 0.075) keV/(ASM-cts s⁻¹), corresponding to an increase of 6.5% of E_(cyc) for an increase in flux by a factor of two. We also find that all line parameters as well as the continuum parameters show a correlation with X-ray flux. While a correlation between E_(cyc) and X-ray flux (both positive and negative) is now known for several accreting binaries with various suggestions for the underlying physics, the phenomenon of a long-term decay has so far only been seen in Her X-1 and Vela X-1, with far less convincing explanations
Use of portable air purifiers to reduce aerosols in hospital settings and cut down the clinical backlog
SARS-CoV-2 has severely affected capacity in the NHS, and waiting lists are markedly
increasing due to downtime of up to 50 minutes between patient consultations/procedures,
to reduce the risk of infection. Ventilation accelerates this air cleaning, but retroactively
installing built-in mechanical ventilation is often cost-prohibitive. We investigated the effect
of using portable air cleaners (PAC), a low-energy and low-cost alternative, to reduce the
concentration of aerosols in typical patient consultation/procedure environments. The
experimental setup consisted of an aerosol generator, which mimicked the subject affected
by SARS-CoV-19, and an aerosol detector, representing a subject who could potentially
contract SARS-CoV-19. Experiments of aerosol dispersion and clearing were undertaken in
situ in a variety of rooms with 2 different types of PAC in various combinations and
positions. Correct use of PAC can reduce the clearance half-life of aerosols by 82%
compared to the same indoor-environment without any ventilation, and at a broadly
equivalent rate to built-in mechanical ventilation. In addition, the highest level of aerosol
concentration measured when using PAC remains at least 46% lower than that when no
mitigation is used, even if the PAC’s operation is impeded due to placement under a table.
The use of PAC leads to significant reductions in the level of aerosol concentration,
associated with transmission of droplet-based airborne diseases. This could enable NHS
departments to reduce the downtime between consultations/procedures
Response to: ‘Use of portable air purifiers to reduce aerosols in hospital settings and cut down the clinical backlog’
How do psychiatrists address delusions in first meetings in acute care? A qualitative study
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1009 Myocardial injury following percutaneous coronary intervention in complex lesion: a cardiovascular magnetic resonance imaging and cardiac marker study
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