672 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
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
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
Early X-ray and optical observations of the soft gamma-ray repeater SGR 0418+5729
Emission of two short hard X-ray bursts on 2009 June 5 disclosed the
existence of a new soft gamma-ray repeater, now catalogued as SGR 0418+5729.
After a few days, X-ray pulsations at a period of 9.1 s were discovered in its
persistent emission. SGR 0418+5729 was monitored almost since its discovery
with the Rossi X-ray Timing Explorer (2-10 keV energy range) and observed many
times with Swift (0.2-10 keV). The source persistent X-ray emission faded by a
factor 10 in about 160 days, with a steepening in the decay about 19 days after
the activation. The X-ray spectrum is well described by a simple absorbed
blackbody, with a temperature decreasing in time. A phase-coherent timing
solution over the 160 day time span yielded no evidence for any significant
evolution of the spin period, implying a 3-sigma upper limit of 1.1E-13 s/s on
the period derivative and of 3E+13 G on the surface dipole magnetic field.
Phase-resolved spectroscopy provided evidence for a significant variation of
the spectrum as a function of the stellar rotation, pointing to the presence of
two emitting caps, one of which became hotter during the outburst. Finally, a
deep observation of the field of SGR 0418+5729 with the new Gran Telescopio
Canarias 10.4-m telescope allowed us to set an upper limit on the source
optical flux of i'>25.1 mag, corresponding to an X-ray-to-optical flux ratio
exceeding 10000, consistent with the characteristics of other magnetars.Comment: The paper (10 pages) contains 6 colour figures and 2 tables; accepted
for publication in MNRA
Recent Progress on Anomalous X-ray Pulsars
I review recent observational progress on Anomalous X-ray Pulsars, with an
emphasis on timing, variability, and spectra. Highlighted results include the
recent timing and flux stabilization of the notoriously unstable AXP 1E
1048.1-5937, the remarkable glitches seen in two AXPs, the newly recognized
variety of AXP variability types, including outbursts, bursts, flares, and
pulse profile changes, as well as recent discoveries regarding AXP spectra,
including their surprising hard X-ray and far-infrared emission, as well as the
pulsed radio emission seen in one source. Much has been learned about these
enigmatic objects over the past few years, with the pace of discoveries
remaining steady. However additional work on both observational and theoretical
fronts is needed before we have a comprehensive understanding of AXPs and their
place in the zoo of manifestations of young neutron stars.Comment: 10 pages, 6 figures; to appear in proceedings of the conference
"Isolated Neutron Stars: From the Interior to the Surface" eds. S. Zane, R.
Turolla, D. Page; Astrophysics & Space Science in pres
X-ray emission from isolated neutron stars
X-ray emission is a common feature of all varieties of isolated neutron stars
(INS) and, thanks to the advent of sensitive instruments with good
spectroscopic, timing, and imaging capabilities, X-ray observations have become
an essential tool in the study of these objects. Non-thermal X-rays from young,
energetic radio pulsars have been detected since the beginning of X-ray
astronomy, and the long-sought thermal emission from cooling neutron star's
surfaces can now be studied in detail in many pulsars spanning different ages,
magnetic fields, and, possibly, surface compositions. In addition, other
different manifestations of INS have been discovered with X-ray observations.
These new classes of high-energy sources, comprising the nearby X-ray Dim
Isolated Neutron Stars, the Central Compact Objects in supernova remnants, the
Anomalous X-ray Pulsars, and the Soft Gamma-ray Repeaters, now add up to
several tens of confirmed members, plus many candidates, and allow us to study
a variety of phenomena unobservable in "standard'' radio pulsars.Comment: Chapter to be published in the book of proceedings of the 1st Sant
Cugat Forum on Astrophysics, "ICREA Workshop on the high-energy emission from
pulsars and their systems", held in April, 201
Long-term spectral and timing properties of the soft gamma-ray repeater SGR 1833-0832 and detection of extended X-ray emission around the radio pulsar PSR B1830-08
SGR 1833-0832 was discovered on 2010 March 19 thanks to the Swift detection
of a short hard X-ray burst and follow-up X-ray observations. Since then, it
was repeatedly observed with Swift, Rossi X-ray Timing Explorer, and
XMM-Newton. Using these data, which span about 225 days, we studied the
long-term spectral and timing characteristics of SGR 1833-0832. We found
evidence for diffuse emission surrounding SGR 1833-0832, which is most likely a
halo produced by the scattering of the point source X-ray radiation by dust
along the line of sight, and we show that the source X-ray spectrum is well
described by an absorbed blackbody, with temperature kT=1.2 keV and absorbing
column nH=(10.4+/-0.2)E22 cm^-2, while different or more complex models are
disfavoured. The source persistent X-ray emission remained fairly constant at
about 3.7E-12 erg/cm^2/s for the first 20 days after the onset of the bursting
episode, then it faded by a factor 40 in the subsequent 140 days, following a
power-law trend with index alpha=-0.5. We obtained a phase-coherent timing
solution with the longest baseline (225 days) to date for this source which,
besides period P=7.5654084(4) s and period derivative dP/dt=3.5(3)E-12 s/s,
includes higher order period derivatives. We also report on our search of the
counterpart to the SGR at radio frequencies using the Australia Telescope
Compact Array and the Parkes radio telescope. No evidence for radio emission
was found, down to flux densities of 0.9 mJy (at 1.5 GHz) and 0.09 mJy (at 1.4
GHz) for the continuum and pulsed emissions, respectively, consistently with
other observations at different epochs.Comment: 12 pages, 7 colour figures and 3 tables, accepted for publication in
MNRAS. Figure 6 in reduced quality and abstract abridged for astro-ph
submissio
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
A Swift gaze into the 2006 March 29th burst forest of SGR 1900+14
We report on the intense burst ``forest'' recorded on 2006 March 29 which
lasted for ~30s. More than 40 bursts were detected both by BAT and by XRT,
seven of which are rare intermediate flares (IFs): several times 10^{42} ergs
were released. The BAT data were used to carry out time-resolved spectroscopy
in the 14-100keV range down to 8ms timescales.
This unique dataset allowed us to test the magnetar model predictions such as
the magnetically trapped fireball and the twisted magnetosphere over an
unprecedented range of fluxes and with large statistics (in terms of both
photons and IFs). We confirmed that a two blackbody component fits adequately
the time-resolved and integrated spectra of IFs. However, Comptonization models
give comparable good reduced chi^2. Moreover, we found: i) a change of
behavior, around ~10^{41} erg/s, above which the softer blackbody shows a sort
of saturation while the harder one still grows to a few times 10^{41} erg/s;
ii) a rather sharp correlation between temperature and radii of the blackbodies
(R^2 prop kT^{-3}), which holds for the most luminous parts of the flares
(approximately for L_{tot} > 10^{41} erg/s). Within the magnetar model, the
majority of these findings can be accounted for in terms of thermalised
emission from the E-mode and O-mode photospheres. Interestingly, the maximum
observed luminosity coming from a region of ~15km matches the magnetic
Eddington luminosity at the same radius, for a surface dipole field of ~8 x
10^{14} G (virtually equal to the one deduced from the spindown of SGR
1900+14).Comment: Accepted for publication on ApJ, 15 pages, 10 figure
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