100 research outputs found
The X-ray dust scattered rings of the black hole low mass binary V404 Cyg
We report on the first detection of X-ray dust scattered rings from the
Galactic low mass X-ray binary V404 Cyg. The observation of the system with
Swift/XRT on June 30 2015 revealed the presence of five concentric ring-like
structures centred at the position of V404 Cyg. Follow-up Swift/XRT
observations allowed a time-dependent study of the X-ray rings. Assuming that
these are the result of small-angle, single X-ray scattering by dust grains
along the line of sight, we find that their angular size scales as in agreement with theoretical predictions. The dust grains are
concentrated in five dust layers located at about 2.12, 2.05, 1.63, 1.50 and
1.18 kpc from the observer. These coincide roughly with locations of enhanced
extinction as determined by infrared photometry. Assuming that the grain size
distribution is described by a generalized Mathis-Rumpl-Nordsieck model, we
find that the power-law index of the most distant cloud is , while
in all other clouds. We constrain at a level the
maximum grain size of the intermediate dust layers in the range
m and set a lower limit of m in the other
clouds. Hints of an exponential cutoff at the angular intensity profile of the
outermost X-ray ring suggest that the smallest grains have sizes m. Based on the relative ratios of dust
column densities we find the highest dust concentration at kpc. Our
results indicate a gradient in the dust properties within 1 kpc from V404 Cyg.Comment: 17 pages, 13 figures, accepted by MNRA
Local and non-local mathematical modelling of signalling during embryonic development
Embryonic development requires cells to communicate as they arrange into the adult
organs and tissues. The ability of cells to sense their environment, respond to signals
and self-organise is of crucial importance. Patterns of cells adopting distinct states of
differentiation arise in early development, as a result of cell signalling. Furthermore,
cells interact with each other in order to form aggregations or rearrange themselves
via cell-cell adhesion. The distance over which cells can detect their surroundings
plays an important role to the form of patterns to be developed, as well as the time
necessary for developmental processes to complete. Cells achieve long range communication
through the use of extensions such as filopodia. In this work we formulate
and analyse various mathematical models incorporating long-range signalling. We
first consider a spatially discrete model for juxtacrine signalling extended to include
filopodial action. We show that a wide variety of patterns can arise through this
mechanism, including single isolated cells within a large region or contiguous blocks
of cells selected for a specific fate. Cell-cell adhesion modelling is addressed in this
work. We propose a variety of discrete models from which continuous models are
derived. We examine the models’ potential to describe cell-cell adhesion and the associated
phenomena such as cell aggregation. By extending these models to consider
long range cell interactions we were able to demonstrate their ability to reproduce
biologically relevant patterns. Finally, we consider an application of cell adhesion
modelling by attempting to reproduce a specific developmental event, the formation
of sympathetic ganglia
The TeV emission of Ap Librae: a hadronic interpretation and prospects for CTA
Ap Librae is one out of a handful of low-frequency peaked blazars to be
detected at TeV -rays and the only one with an identified X-ray jet.
Combined observations of Fermi-LAT at high energies (HE) and of H.E.S.S at very
high energies (VHE) revealed a striking spectral property of Ap Librae; the
presence of a broad high-energy component that extends more than nine orders of
magnitude in energy and is, therefore, hard to be explained by the usual
single-zone synchrotron self-Compton model. We show that the superposition of
different emission components related to photohadronic interactions can explain
the -ray emission of Ap Librae without invoking external radiation
fields. We present two indicative model fits to the spectral energy
distribution of Ap Librae where the VHE emission is assumed to originate from a
compact, sub-pc scale region of the jet. A robust prediction of our model is
VHE flux variability on timescales similar to those observed at X-rays and HE
-rays, which can be further used to distinguish between a sub-pc or kpc
scale origin of the TeV emission. We thus calculate the expected variability
signatures at X-rays, HE and VHE -rays and show that quasi-simultaneous
flares are expected, with larger amplitude flares appearing at -rays.
We assess the detectability of VHE variability from Ap Librae with CTA, next
generation of IACTs. We show that hr timescale variability at
TeV could be detectable at high significance with shorter
exposure times than current Cherenkov telescopes.Comment: 11 pages, 4 figures, submitted to MNRA
Detection of a cyclotron line in SXP 15.3 during its 2017 outburst
We report the results of AstroSat and NuSTAR observations of the Be/X-ray
binary pulsar SXP 15.3 in the Small Magellanic Cloud during its outburst in
late 2017, when the source reached a luminosity level of ~ 10^{38} erg s^{-1},
close to the Eddington limit. The unprecedented broadband coverage of the
source allowed us to perform timing and spectral analysis between 3 and 80 keV.
The pulse profile exhibits a significant energy dependence, and morphs from a
double peaked profile to a single broad pulse at energies >15 keV. This can be
explained by a spectral hardening during an intensity dip seen between the two
peaks of the pulse profile. We detect a Cyclotron Resonance Scattering Feature
(CRSF) at ~5 keV in the X-ray spectrum, independent of the choice of the
continuum model. This indicates a magnetic field strength of 6x10^{11} G for
the neutron star.Comment: 5 pages, 6 figures, accepted for publication in MNRAS Letter
Point-source and diffuse high-energy neutrino emission from Type IIn supernovae
Type IIn supernovae (SNe), a rare subclass of core collapse SNe, explode in
dense circumstellar media that have been modified by the SNe progenitors at
their last evolutionary stages. The interaction of the freely expanding SN
ejecta with the circumstellar medium gives rise to a shock wave propagating in
the dense SN environment, which may accelerate protons to multi-PeV energies.
Inelastic proton-proton collisions between the shock-accelerated protons and
those of the circumstellar medium lead to multi-messenger signatures. Here, we
evaluate the possible neutrino signal of type IIn SNe and compare with IceCube
observations. We employ a Monte Carlo method for the calculation of the diffuse
neutrino emission from the SN IIn class to account for the spread in their
properties. The cumulative neutrino emission is found to be ~ 10 per cent of
the observed IceCube neutrino flux above 60 TeV. Type IIn SNe would be the
dominant component of the diffuse astrophysical flux, only if 4 per cent of all
core collapse SNe were of this type and 20 to 30 per cent of the shock energy
was channeled to accelerated protons. Lower values of the acceleration
efficiency are accessible by the observation of a single type IIn SN as a
neutrino point source with IceCube using up-going muon neutrinos. Such an
identification is possible in the first year following the SN shock breakout
for sources within 20 Mpc.Comment: 13 pages, 9 figures, MNRAS accepte
Spectral and temporal properties of RX J0520.5-6932 (LXP 8.04) during a type-I outburst
We observed RX J0520.5-6932 in the X-rays and studied the optical light curve
of its counterpart to verify it as a Be/X-ray binary. We performed an
XMM-Newton anticipated target of opportunity observation in January 2013 during
an X-ray outburst of the source in order to search for pulsations and derive
its spectral properties. We monitored the source with Swift to follow the
evolution of the outburst and to look for further outbursts to verify the
regular pattern seen in the optical light curve with a period of ~24.4 d. The
XMM-Newton EPIC light curves show coherent X-ray pulsations with a period of
8.035331(15) s (1 sigma). The X-ray spectrum can be modelled by an absorbed
power law with photon index of ~0.8, an additional black-body component with
temperature of ~0.25 keV and an Fe K line. Phase-resolved X-ray spectroscopy
reveals that the spectrum varies with pulse phase. We confirm the
identification of the optical counterpart within the error circle of XMM-Newton
at an angular distance of ~0.8 arcsec, which is an O9Ve star with known Halpha
emission. By analyzing the combined data from three OGLE phases we derived an
optical period of 24.43 d.The X-ray pulsations and long-term variability, as
well as the properties of the optical counterpart, confirm that RX J0520.5-6932
is a Be/X-ray binary pulsar in the Large Magellanic Cloud. Based on the X-ray
monitoring of the source we conclude that the event in January 2013 was a
moderately bright type-I X-ray outburst, with a peak luminosity of 1.79e36
erg/s.Comment: 10 pages, 9 figures, accepted A&
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