1,713 research outputs found
On the anomalous X-ray afterglows of GRB 970508 and GRB 970828
Recently, BeppoSAX and ASCA have reported an unusual resurgence of soft X-ray
emission during the afterglows of GRB 970508 and GRB 970828, together with
marginal evidence for the existence of Fe-lines in both objects. We consider
the implications of the existence of a torus of iron-rich material surrounding
the sites of gamma ray bursts as would be expected in the SupraNova model; in
particular, we show that the fireball will quickly hit this torus, and bring it
to a temperature ~3x10^7 K. Bremsstrahlung emission from the heated up torus
will cause a resurgence of the soft X-ray emission with all expected
characteristics (flux level, duration and spectral hardening with time)
identical to those observed during the reburst. Also, thermal emission from the
torus will account for the observed iron line flux. These events are also
observable, for instance by new missions such as SWIFT, when beaming away from
our line sight makes us miss the main burst, as Fast (soft) X-ray Transients,
with durations ~10^3 s, and fluences ~10^-7-10^-4 erg cm^-2. This model
provides evidence in favor of the SupraNova model for Gamma Ray Bursts.Comment: To appear in MN Pink pages, MN-LateX, no figure
The Turbulent Story of X-ray Bursts: Effects of Shear Mixing on Accreting Neutron Stars
During accretion, a neutron star (NS) is spun up as angular momentum is
transported through its liquid surface layers. We study the resulting
differentially rotating profile, focusing on the impact this has for type I
X-ray bursts. The viscous heating is found to be negligible, but turbulent
mixing can be activated. Mixing has the greatest impact when the buoyancy at
the compositional discontinuity between accreted matter and ashes is overcome.
This occurs preferentially at high accretion rates or low spin frequencies and
may depend on the ash composition from the previous burst. We then find two new
regimes of burning. The first is ignition in a layer containing a mixture of
heavier elements with recurrence times as short as ~5-30 minutes, similar to
short recurrence time bursts. When mixing is sufficiently strong, a second
regime is found where accreted helium mixes deep enough to burn stably,
quenching X-ray bursts altogether. The carbon-rich material produced by stable
helium burning would be important for triggering and fueling superbursts.Comment: 3 pages, 3 figures. To appear in the proceedings of "Forty Years of
Pulsars: Millisecond Pulsars, Magnetars and More" held in Montreal, Canada,
August 12-17, 200
Microcephaly and macrocephaly. A study on anthropometric and clinical data from 308 subjects
Head circumference is the auxological parameter that most correlates with developmental anomalies in childhood. Head circumference (HC) two standard deviations (SD) below or above the mean defines microcephaly and macrocephaly, respectively. The aim of this retrospective study was to explore anthropometric parameters and clinical characteristics among subjects with abnormalities in HC who had been referred for developmental assessment. One hundred and sixty four subjects with microcephaly and 144 subjects with macrocephaly were enrolled from birth to 18 months of age. Head circumference at birth and the association with variables related to maternal health status, gestational age, growth pattern, brain imaging and clinical characteristics were analyzed. In some cases, an etiological diagnosis was made. In the two considered conditions, we found different anthropometric and clinical associations, some of which were statistically significant, with implications for ongoing neurodevelopmental surveillance
The XMM Newton and INTEGRAL observations of the supergiant fast X-ray transient IGR J16328-4726
The accretion mechanism producing the short flares observed from the
Supergiant Fast X-ray Transients (SFXT) is still highly debated and forms a
major part in our attempts to place these X-ray binaries in the wider context
of the High Mass X-ray Binaries.
We report on a 216 ks INTEGRAL observation of the SFXT IGR J16328-4726
(August 24-27, 2014) simultaneous with two fixed-time observations with XMM
Newton (33ks and 20ks) performed around the putative periastron passage, in
order to investigate the accretion regime and the wind properties during this
orbital phase. During these observations, the source has shown luminosity
variations, from 4x10^{34} erg/s to 10^{36} erg/s, linked to spectral
properties changes. The soft X-ray continuum is well modeled by a power law
with a photon index varying from 1.2 up to 1.7 and with high values of the
column density in the range 2-4x10^{23}/cm^2. We report on the presence of iron
lines at 6.8-7.1 keV suggesting that the X-ray flux is produced by accretion of
matter from the companion wind characterized by density and temperature
inhomogeneities
Delayed efficacy of radiofrequency catheter ablation on arrhythmias originating in the interventricular basal septum
Delayed efficacy of radiofrequency energy can suppress ventricular arrhythmias after a failed ablation procedure. The implant of cardiac defibrillator for arrhythmia-induced cardiomyopathy should be procrastinated after a period of follow-up. Waiting for delayed efficacy is a reasonable choice to reduce the risk of complications associated with aggressive ablative approaches
Shear Waves and Giant Flare Oscillations from Soft Gamma-Ray Repeaters
Recent observations of giant flares from soft gamma-ray repeaters have
exhibited multiple 25-150 Hz oscillations. Frequencies in this range are
expected for toroidal shear waves in a neutron star (NS) crust, lending support
to Duncan's proposal that such modes may be excited in these events. This
motivates a reassessment of how these waves reflect the NS structure and what
role the magnetic field plays in setting their frequencies. We calculate the
eigenfrequencies and eigenfunctions of toroidal oscillations for a realistic NS
crust, including a vertical magnetic field at magnetar strengths
(). The lowest radial-order mode has a
red-shifted frequency of , with the
prefactor depending on the NS's mass and radius, and its crust's depth and
composition. This mode is independent of the magnetic field for
, a limit much greater than the inferred
dipole magnetic fields for these objects. Though this is a good fit to the
observed oscillations, only rather loose constraints can be made for the NSs'
properties because all that can be fit is this prefactor (a single parameter).
Modes with shorter radial wavelengths are more sensitive to the magnetic field
starting at and have higher frequencies (600-2000
Hz). The discovery of these modes, coupled with the oscillations observed thus
far, would provide a powerful probe to the NS crustal structure.Comment: Submitted for publication in The Astrophysical Journal Letters, 4
pages, 3 figure
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