946 research outputs found
Neutron star bulk viscosity, "spin-flip" and GW emission of newly born magnetars
The viscosity-driven "spin-flip" instability in newborn magnetars with
interior toroidal magnetic fields is re-examined. We calculate the bulk
viscosity coefficient () of cold, matter in neutron stars
(NS), for selected values of the nuclear symmetry energy and in the regime
where -equilibration is slower than characteristic oscillation periods.
We show that: i) is larger than previously assumed and the instability
timescale correspondingly shorter; ii) for a magnetically-induced ellipticity
, typically expected in newborn
magnetars, spin-flip occurs for initial spin periods ms, with
some dependence on the NS equation of state (EoS). We then calculate the
detectability of GW signals emitted by newborn magnetars subject to
"spin-flip", by accounting also for the reduction in range resulting from
realistic signal searches. For an optimal range of , and birth spin period ms, we estimate an horizon
of Mpc, and Mpc, for Advanced and third generation
interferometers at design sensitivity, respectively. A supernova (or a
kilonova) is expected as the electromagnetic counterpart of such GW events.
Outside of the optimal range for GW emission, EM torques are more efficient in
extracting the NS spin energy, which may power even brighter EM transients.Comment: 10 pages, 4 figures, accepted for publication in MNRA
NuSTAR J095551+6940.8: a highly magnetised neutron star with super-Eddington mass accretion
The identification of the Ultraluminous X-ray source (ULX) X-2 in M82 as an
accreting pulsar has shed new light on the nature of a subset of ULXs, while
rising new questions on the nature of the super-Eddington accretion. Here, by
numerically solving the torque equation of the accreting pulsar within the
framework of the magnetically threaded-disk scenario, we show that three
classes of solutions, corresponding to different values of the magnetic field,
are mathematically allowed. We argue that the highest magnetic field one,
corresponding to B G, is favoured based on physical
considerations and the observed properties of the source. In particular, that
is the only solution which can account for the observed variations in
(over four time intervals) without requiring major changes in , which
would be at odds with the approximately constant X-ray emission of the source
during the same time. For this solution, we find that the source can only
accomodate a moderate amount of beaming, 0.5 . Last, we show
that the upper limit on the luminosity, L erg s
from archival observations, is consistent with a highly-magnetized neutron star
being in the propeller phase at that time.Comment: 8 pages, 3 figures, accepted for publication on MNRA
Gravitational Radiation from Newborn Magnetars
There is growing evidence that two classes of high-energy sources, the Soft
Gamma Repeaters and the Anomalous X-ray Pulsars contain slowly spinning
``magnetars'', i.e. neutron stars whose emission is powered by the release of
energy from their extremely strong magnetic fields (>10^15 G. We show here that
the enormous energy liberated in the 2004 December 27 giant flare from
SGR1806-20 (~5 10^46 erg), together with the likely recurrence time of such
events, requires an internal field strength of > 10^16 G. Toroidal magnetic
fields of this strength are within an order of magnitude of the maximum fields
that can be generated in the core of differentially-rotating neutron stars
immediately after their formation, if their initial spin period is of a few
milliseconds. A substantial deformation of the neutron star is induced by these
magnetic fields and, provided the deformation axis is offset from the spin
axis, a newborn fast-spinning magnetar would radiate for a few weeks a strong
gravitational wave signal the frequency of which (0.5-2 kHz range) decreases in
time. The signal from a newborn magnetar with internal field > 10^16.5 G could
be detected with Advanced LIGO-class detectors up to the distance of the Virgo
cluster (characteristic amplitude h_c about 10^-21). Magnetars are expected to
form in Virgo at a rate approx. 1/yr. If a fraction of these have sufficiently
high internal magnetic field, then newborn magnetars constitute a promising new
class of gravitational wave emitters.Comment: Accepted for publication on ApJ Letter
Magnetar central engines in gamma-ray busts follow the universal relation of accreting magnetic stars
Gamma-ray bursts (GRBs), both long and short, are explosive events whose
inner engine is generally expected to be a black hole or a highly magnetic
neutron star (magnetar) accreting high density matter. Recognizing the nature
of GRB central engines, and in particular the formation of neutron stars (NSs),
is of high astrophysical significance. A possible signature of NSs in GRBs is
the presence of a plateau in the early X-ray afterglow. Here we carefully
select a subset of long and short GRBs with a clear plateau, and look for an
additional NS signature in their prompt emission, namely a transition between
accretion and propeller in analogy with accreting, magnetic compact objects in
other astrophysical sources. We estimate from the prompt emission the minimum
accretion luminosity below which the propeller mechanism sets in, and the NS
magnetic field and spin period from the plateau. We demonstrate that these
three quantities obey the same universal relation in GRBs as in other accreting
compact objects switching from accretion to propeller. This relation provides
also an estimate of the radiative efficiency of GRBs, which we find to be
several times lower than radiatively efficient accretion in X-ray binaries and
in agreement with theoretical expectations. These results provide additional
support to the idea that at least some GRBs are powered by magnetars surrounded
by an accretion disc.Comment: 15 pages, 5 figures, accepted for publication in The Astrophysical
Journal Letter
GRAVITATIONAL WAVES FROM MASSIVE MAGNETARS FORMED IN BINARY NEUTRON STAR MERGERS
Binary neutron star (NS) mergers are among the most promising sources of gravitational waves (GWs), as well as candidate progenitors for short gamma-ray bursts (SGRBs). Depending on the total initial mass of the system and the NS equation of state (EOS), the post-merger phase can be characterized by a prompt collapse to a black hole or by the formation of a supramassive NS, or even a stable NS. In the latter cases of post-merger NS (PMNS) formation, magnetic field amplification during the merger will produce a magnetar and induce a mass quadrupole moment in the newly formed NS. If the timescale for orthogonalization of the magnetic symmetry axis with the spin axis is smaller than the spindown time, the NS will radiate its spin down energy primarily via GWs. Here we study this scenario for the various outcomes of NS formation: we generalize the set of equilibrium states for a twisted torus magnetic configuration to include solutions that, for the same external dipolar field, carry a larger magnetic energy reservoir; we hence compute the magnetic ellipticity for such configurations, and the corresponding strength of the expected GW signal as a function of the relative magnitude of the dipolar and toroidal field components. The relative number of GW detections from PMNSs and from binary NSs is a very strong function of the NS EOS, being higher (~1%) for the stiffest EOSs and negligibly small for the softest ones. For intermediate-stiffness EOSs, such as the n = 4/7 polytrope recently used by Giacomazzo and Perna or the GM1 used by Lasky et al., the relative fraction is ~0.3%; correspondingly, we estimate a GW detection rate from stable PMNSs of ~0.1-1 yr-1 with advanced detectors, and of ~100-1000 yr-1 with detectors of third generation such as the Einstein Telescope. Measurement of such GW signals would provide constraints on the NS EOS and, in connection with an SGRB, on the nature of the binary progenitors giving rise to these events
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