710 research outputs found
Generation of strong magnetic fields by r-modes in millisecond accreting neutron stars: induced deformations and gravitational wave emission
Differential rotation induced by the r-mode instability can generate very
strong toroidal fields in the core of accreting, millisecond spinning neutron
stars. We introduce explicitly the magnetic damping term in the evolution
equations of the r-modes and solve them numerically in the Newtonian limit, to
follow the development and growth of the internal magnetic field. We show that
the strength of the latter can reach large values, G, in the
core of the fastest accreting neutron stars. This is strong enough to induce a
significant quadrupole moment of the neutron star mass distribution,
corresponding to an ellipticity |\epsilon_B}| \sim 10^{-8}. If the symmetry
axis of the induced magnetic field is not aligned with the spin axis, the
neutron star radiates gravitational waves. We suggest that this mechanism may
explain the upper limit of the spin frequencies observed in accreting neutron
stars in Low Mass X-Ray Binaries. We discuss the relevance of our results for
the search of gravitational waves.Comment: 11 pages, 8 figure
GRB Afterglows with Energy Injection from a spinning down NS
We investigate a model for the shallow decay phases of Gamma-ray Burst (GRB)
afterglows discovered by Swift/XRT in the first hours following a GRB event. In
the context of the fireball scenario, we consider the possibility that
long-lived energy injection from a millisecond spinning, ultramagnetic neutron
star (magnetar) powers afterglow emission during this phase. We consider the
energy evolution in a relativistic shock subject to both radiative losses and
energy injection from a spinning down magnetar in spherical symmetry. We model
the energy injection term through magnetic dipole losses and discuss an
approximate treatment for the dynamical evolution of the blastwave. We obtain
an analytic solution for the energy evolution in the shock and associated
lightcurves. To fully illustrate the potential of our solution we calculate
lightcurves for a few selected X-ray afterglows observed by Swift and fit them
using our theoretical lightcurves. Our solution naturally describes in a single
picture the properties of the shallow decay phase and the transition to the
so-called normal decay phase. In particular, we obtain remarkably good fits to
X-ray afterglows for plausible parameters of the magnetar. Even though
approximate, our treatment provides a step forward with respect to previously
adopted approximations and provides additional support to the idea that a
millisecond spinning (1-3 ms), ultramagnetic (B G)
neutron star loosing spin energy through magnetic dipole radiation can explain
the luminosity, durations and shapes of X-ray GRB afterglows.Comment: 7 pages, 2 figures, submitted to Astronomy & Astrophysics - referee's
comments include
Gravitational wave emission from a magnetically deformed non-barotropic neutron star
A strong candidate for a source of gravitational waves is a highly
magnetised, rapidly rotating neutron star (magnetar) deformed by internal
magnetic stresses. We calculate the mass quadrupole moment by perturbing a
zeroth-order hydrostatic equilibrium by an axisymmetric magnetic field with a
\emph{linked poloidal-toroidal structure}. In this work, we do \emph{not}
require the model star to obey a barotropic equation of state (as a realistic
neutron star is not barotropic), allowing us to explore the hydromagnetic
equilibria with fewer constraints. We derive the relation between the ratio of
poloidal-to-total field energy and ellipticity and briefly
compare our results to those obtained using the barotropic assumption. Then, we
present some examples of how our results can be applied to astrophysical
contexts. First, we show how our formulae, in conjunction with current
gravitational wave (non-)detections of the Crab pulsar and the Cassiopeia A
central compact object (Cas A CCO), can be used to constrain the strength of
the internal toroidal fields of those objects. We find that, for the Crab
pulsar (whose canonical equatorial dipole field strength, inferred from spin
down, is T) to emit detectable gravitational radiation, the
neutron star must have a strong toroidal field component, with maximum internal
toroidal field strength T; for gravitational
waves to be detected from the Cas A CCO at 300 Hz, T, whereas detection at 100 Hz would require T. Using our results, we also show how the gravitational wave signal
emitted by a magnetar immediately after its birth (assuming it is born rapidly
rotating, with ) makes such a newborn magnetar a stronger
candidate for gravitational wave detection than, for example, an SGR giant
flare.Comment: 15 pages, 8 figures, 2 table
A Gemini Observation of the Anomalous X-ray Pulsar 1RXS J170849-400910
The anomalous X-ray pulsars (AXPs) represent a growing class of neutron stars
discovered at X-ray energies. While the nature of their multi-wavelength
emission mechanism is still under debate, evidence has been recently
accumulating in favor of their magnetar nature. Their study in the optical and
infrared (IR) wavelengths has recently opened a new window to constrain the
proposed models. We here present a brief overview of AXPs and our Gemini-South
observation of 1RXS J170849-400910, which is a relatively bright AXP discovered
with ROSAT and later found to be an 11 s X-ray pulsar by ASCA. The observation
was taken with the near-IR imager Flamingos in J (1.25 um), H (1.65 um), and Ks
(2.15 um). We confirm the recent detection by Israel et al. (2003) of a source
coincident with the Chandra source (candidate `A'). Our derived magnitudes of J
= 20.6 (0.2), H = 18.6 (0.2), and Ks = 17.1 (0.2) are consistent with those
derived by Israel et al. (2003), and indicate that if this source is indeed the
IR counterpart to 1RXS J170849-400910, then there is no evidence of variability
from this AXP. However, given the lack of IR variability and the relatively
high IR to X-ray flux of this source when compared to the other AXPs, we
conclude that this source is unlikely the counterpart of the AXP, and that the
other source (candidate `B') within the Chandra error circle should not be
ruled out as the counterpart. Further monitoring of these sources and a deep
observation of this complex field are needed to confirm the nature of these
sources and their association with the AXP.Comment: A refereed publication that includes a brief overview of AXPs.
Submitted to Elsevier Science 29 Oct. 2004, accepted 24 Dec. 2004. To appear
in Advances in Space Research. 10 pages including 2 tables and one color jpg
figure. The paper with the high-resolution eps color figure can be downloaded
from: http://aurora.physics.umanitoba.ca/~samar/cospar2004/AXP
Magnetic field decay in neutron stars: from Soft Gamma Repeaters to "weak field magnetars"
The recent discovery of the "weak field, old magnetar", the soft gamma
repeater SGR 0418+5729, whose dipole magnetic field is less than 7.5 \times
10^{12} G, has raised perplexing questions: How can the neutron star produce
SGR-like bursts with such a low magnetic field? What powers the observed X-ray
emission when neither the rotational energy nor the magnetic dipole energy are
sufficient? These observations, that suggest either a much larger energy
reservoir or a much younger true age (or both), have renewed the interest in
the evolutionary sequence of magnetars. We examine, here, a phenomenological
model for the magnetic field decay: B_dip} \propto (B_dip)^{1+a} and compare
its predictions with the observed period, P,the period derivative, \dot{P}, and
the X-ray luminosity, L_X, of magnetar candidates. We find a strong evidence
for a dipole field decay on a timescale of \sim 10^3 yr for the strongest (\sim
10^{15} G) field objects, with a decay index within the range 1 \leq a < 2 and
more likely within 1.5\lesssim a \lesssim 1.8. The decaying field implies a
younger age than what is implied by the spinown age. Surprisingly, even with
the younger age, the energy released in the dipole field decay is insufficient
to power the X-ray emission, suggesting the existence of a stronger internal
field, B_int. Examining several models for the internal magnetic field decay we
find that it must have a very large (> 10^{16} G) initial value. Our findings
suggest two clear distinct evolutionary tracks -- the SGR/AXP branch and the
transient branch, with a possible third branch involving high-field radio
pulsars that age into low luminosity X-ray dim isolated neutron stars.Comment: 47 pages, 11 figures, accepted for publication on MNRAS, in pres
The post-burst awakening of the anomalous x-ray pulsar in Westerlund
On 2006 September 21, an intense (~10^39 erg s^-1) and short (20 ms) burst was detected by Swift BAT at a position
consistent with that of the candidate anomalous X-ray pulsar (AXP) CXOU J164710.2-455216, discovered by
Chandra in 2005. Swift follow-up observations began ~13 hr after the event and found the source at a 1â10 keV flux
level of about 4.5 x 10^-11 erg cm^-2 s^-1, i.e., ~300 times brighter than measured 5 days earlier by XMM-Newton. We
report the results obtained from Swift BAT observations of the burst and subsequent Swift XRT observations carried
out during the first 4 months after the burst. These data are complemented with those from two XMM-Newton observations (carried out just before and after the BAT event) and four archival Chandra observations carried out
between 2005 and 2007. We find a phase-coherent solution for the source pulsations after the burst. The evolution of
the pulse phase comprises an exponential component decaying with timescale of 1.4 days, which we interpret as the
recovery stage following a large glitch (Îv/v ~ 6 x 10^-5). We also detect a quadratic component corresponding to a
spin-down rate of P ~ 9 x 10^-13 s s^-1, implying a magnetic field strength of 10^14 G. During the first Swift XRT
observation taken 0.6 days after the burst, the spectrum showed a kT ~0.65 keV blackbody (R_(BB) ~ 1.5 km) plus a
Î ~ 2.3 power law accounting for about 60% of the 1â10 keV observed flux. Analysis of Chandra archival data,
taken during 2005 when the source was in quiescence, reveal that the modulation in quiescence is 100% pulsed at
energies above ~4 keV and consistent with the (unusually small-sized) blackbody component being occulted by the
neutron star as it rotates. These findings demonstrate that CXOU J164710.2-455216 is indeed an AXP; we compare
them with the properties of three other AXPs which displayed similar behavior in the past
Spatial interactivity. Sensoriality and design of digital data in the space
This paper explores the thesis according to which the relationship between urban space, digital data and the user is biunivocal in nature and plays a strategic role in defining a new trajectory of development of the way users inhabit and transform the city. As digital is now invisibly integrated into the activities we perform daily, networks are extending into the real space, demonstrating how technology does not eliminate the human need for physical presence. Architecture must grapple with a non-tectonic reality, and ask itself not how to use the digital to manage data apparatuses to generate dynamic architectural forms, but rather how to turn information into an engine for spaces that can dynamically respond to the needs of the inhabitants
The 2006 Outburst of the Magnetar CXOU J164710.2-455216
We report on data obtained with the Chandra, XMM-Newton, Suzaku and Swift
X-ray observatories, following the 2006 outburst of the Anomalous X-ray Pulsar
CXO J164710.2-455216. We find no evidence for the very large glitch and rapid
exponential decay as was reported previously for this source. We set a 3 sigma
upper limit on any fractional frequency increase at the time of the outburst of
Delta nu/nu < 1.5 x 10^{-5}. Our timing analysis, based on the longest time
baseline yet, yields a spin-down rate for the pulsar that implies a surface
dipolar magnetic field of ~9 x 10^{13} G, although this could be biased high by
possible recovery from an undetected glitch. We also present an analysis of the
source flux and spectral evolution, and find no evidence for long-term spectral
relaxation post-outburst as was previously reported.Comment: Submitted to Ap
Ultra-compact (X-ray) binaries
A short review of ultra-compact binaries, focused on ultra-compact X-ray
binaries, is followed by a discussion of recent results of our VLT campaign to
obtain optical spectra of (candidate) ultra-compact X-ray binaries. We find
evidence for carbon/oxygen as well as helium/nitrogen discs and no evidence for
(traces) of hydrogen. This suggests that the donors in the observed systems are
white dwarfs. However, we also find large differences between the two C/O discs
of which we have good spectra, which highlights the need for a better
understanding of the optical spectra.Comment: To appear in proceedings of "A life with stars" a conference in
honour of Ed van den Heuvel's 60th birthday, New Ast. Re
An assessment of the tsunami risk in Muscat and Salalah, Oman, based on estimations of probable maximum loss
We present a method for determining an initial assessment of tsunami risk, with application for two coastal areas of Oman. Using open source GIS and seismic databases we carry out a tsunami risk assessment using a deterministic and probabilistic approach based on worst-case scenarios. A quick and effective method for estimating tsunami run-up without the use of complex modelling software is an important step in disaster risk reduction efforts as many government and emergency response organisations do not possess the expertise to carry out or interpret tsunami inundation numerical models. Estimates of probable maximum loss were calculated using a simple method of building identification and a revised building damage assessment technique. A series of tsunami risk maps were created for the coastal settlements of Muscat and Salalah, with the aim of improving tsunami response. We find Muscat to be at far greater risk of tsunami damage than Salalah; this is due in part to Muscat's proximity to potential tsunamigenic sources and the cities current level of urban infrastructure. Whilst much of the infrastructure in Salalah is currently at low risk from tsunami, development pressures could lead to increased risk within the region. It is hoped that the assessment of risk may go some way to a government led disaster risk reduction strategy being implemented in coastal Oman. The methods detailed provide a cheap and efficient means to quantify tsunami risk in many coastal Middle Eastern countries, of which several have poor disaster risk reduction strategies
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