66 research outputs found
Twisted-torus configurations with large toroidal magnetic fields in relativistic stars
Understanding the properties of the internal magnetic field of neutron stars
remains a theoretical challenge. Over the last years, twisted-torus geometries
have been considered both in Newtonian and general-relativistic equilibrium
models, as they represent a potentially good description of neutron star
interiors. All of these works have found an apparent intrinsic limitation to
geometries that are poloidal-field-dominated, with a toroidal-to-poloidal
energy ratio inside the star that are <10%, unless surface currents are
included and magnetic fields are allowed to be discontinuous. This limitation
is in stark contrast with the general expectation that much higher toroidal
fields should be present in the stellar interior and casts doubt about the
stability and hence realism of these configurations. We here discuss how to
overcome this limitation by adopting a new prescription for the azimuthal
currents that leads to magnetized equilibria where the toroidal-to-total
magnetic-field energy ratio can be as high as 90%, thus including geometries
that are toroidal-field-dominated. Moreover, our results show that for a fixed
exterior magnetic-field strength, a higher toroidal-field energy implies a much
higher total magnetic energy stored in the star, with a potentially strong
impact on the expected electromagnetic and gravitational-wave emission from
highly magnetized neutron stars.Comment: 5 pages, 3 figures, 1 tabl
Magnetically-induced outflows from binary neutron star merger remnants
Recent observations by the Swift satellite have revealed long-lasting (), "plateau-like" X-ray afterglows in the vast majority
of short gamma-ray bursts events. This has put forward the idea of a long-lived
millisecond magnetar central engine being generated in a binary neutron star
(BNS) merger and being responsible for the sustained energy injection over
these timescales ("magnetar model"). We elaborate here on recent simulations
that investigate the early evolution of such a merger remnant in
general-relativistic magnetohydrodynamics. These simulations reveal very
different conditions than those usually assumed for dipole spin-down emission
in the magnetar model. In particular, the surrounding of the newly formed NS is
polluted by baryons due to a dense, highly magnetized and isotropic wind from
the stellar surface that is induced by magnetic field amplification in the
interior of the star. The timescales and luminosities of this wind are
compatible with early X-ray afterglows, such as the "extended emission". These
isotropic winds are a generic feature of BNS merger remnants and thus represent
an attractive alternative to current models of early X-ray afterglows. Further
implications to BNS mergers and short gamma-ray bursts are discussed.Comment: 4 pages, 2 figures. To appear in proceedings of "Swift: 10 Years of
Discovery
Electromagnetic emission from long-lived binary neutron star merger remnants II: lightcurves and spectra
Recent observations indicate that in a large fraction of binary neutron star
(BNS) mergers a long-lived neutron star (NS) may be formed rather than a black
hole. Unambiguous electromagnetic (EM) signatures of such a scenario would
strongly impact our knowledge on how short gamma-ray bursts (SGRBs) and their
afterglow radiation are generated. Furthermore, such EM signals would have
profound implications for multimessenger astronomy with joint EM and
gravitational-wave (GW) observations of BNS mergers, which will soon become
reality with the ground-based advanced LIGO/Virgo GW detector network starting
its first science run this year. Here we explore such EM signatures based on
the model presented in a companion paper, which provides a self-consistent
evolution of the post-merger system and its EM emission starting from an early
baryonic wind phase and resulting in a final pulsar wind nebula that is
confined by the previously ejected material. Lightcurves and spectra are
computed for a wide range of post-merger physical properties and particular
attention is paid to the emission in the X-ray band. In the context of SGRB
afterglow modeling, we present X-ray lightcurves corresponding to the
'standard' and the recently proposed 'time-reversal' scenario (SGRB prompt
emission produced at the time of merger or at the time of collapse of the
long-lived NS). The resulting afterglow lightcurve morphologies include, in
particular, single and two-plateau features with timescales and luminosities
that are in good agreement with the observations by the Swift satellite.
Furthermore, we compute the X-ray signal that should precede the SGRB in the
time-reversal scenario. If found, such a signal would represent smoking-gun
evidence for this scenario. Finally, we find a bright, highly isotropic EM
transient signal peaking in the X-ray band ...Comment: 20 pages, 16 figure
Magnetic field amplification in hypermassive neutron stars via the magnetorotational instability
Mergers of binary neutron stars likely lead to the formation of a
hypermassive neutron star (HMNS), which is metastable and eventually collapses
to a black hole. This merger scenario is thought to explain the phenomenology
of short gamma-ray bursts (SGRBs). The very high energies observed in SGRBs
have been suggested to stem from neutrino-antineutrino annihilation and/or from
very strong magnetic fields created during or after the merger by mechanisms
like the magnetorotational instability (MRI). Here, we report on results that
show for the first time the development of the MRI in HMNSs in
three-dimensional, fully general-relativistic magnetohydrodynamic simulations.
This instability amplifies magnetic fields exponentially and could be a vital
ingredient in solving the SGRB puzzle.Comment: 6 pages, 3 figures. Proceedings of the Karl Schwarzschild Meeting
201
Electromagnetic emission from long-lived binary neutron star merger remnants I: formulation of the problem
Binary neutron star (BNS) mergers are the leading model to explain the
phenomenology of short gamma-ray bursts (SGRBs), which are among the most
luminous explosions in the universe. Recent observations of long-lasting X-ray
afterglows of SGRBs challenge standard paradigms and indicate that in a large
fraction of events a long-lived neutron star (NS) may be formed rather than a
black hole. Understanding the mechanisms underlying these afterglows is
necessary in order to address the open questions concerning the nature of SGRB
central engines. However, recent theoretical progress has been hampered by the
fact that the timescales of interest for the afterglow emission are
inaccessible to numerical relativity simulations. Here we present a detailed
model to bridge the gap between numerical simulations of the merger process and
the relevant timescales for the afterglows, assuming that the merger results in
a long-lived NS. This model is formulated in terms of a set of coupled
differential equations that follow the evolution of the post-merger system and
predict its electromagnetic (EM) emission in a self-consistent way, starting
from initial data that can be extracted from BNS merger simulations and taking
into account the most relevant radiative processes. Moreover, the model can
accomodate the collapse of the remnant NS at any time during the evolution as
well as different scenarios for the prompt SGRB emission. A second major reason
of interest for BNS mergers is that they are considered the most promising
source of gravitational waves (GWs) for detection with the advanced
ground-based detector network LIGO/Virgo coming online this year.
Multimessenger astronomy with joint EM and GW observations of the merger and
post-merger phase can greatly enhance the scientific output of either type of
observation. However, the actual benefit depends on ...Comment: 27 pages, 3 figures, 4 appendice
Binary neutron star mergers after GW170817
The first combined detection of gravitational waves and electromagnetic
signals from a binary neutron star (BNS) merger in August 2017 (event named
GW170817) represents a major landmark for the ongoing investigation on these
extraordinary systems. In this short review, we introduce BNS mergers as events
of the utmost importance for astrophysics and fundamental physics and discuss
the main discoveries enabled by this first multimessenger observation, which
include compelling evidence that such mergers produce a copious amount of heavy
r-process elements and can power short gamma-ray bursts. We further discuss key
open questions left behind on this event and BNS mergers in general, focussing
the attention on the current status and limitations of theoretical models and
numerical simulations.Comment: 5 pages, 2 figures; accepted on Frontiers in Astronomy and Space
Sciences, invited review for the article collection "Gravitational Waves: A
New Window to the Universe" (hosted by R. Perna and B. Giacomazzo
Collimated outflows from long-lived binary neutron star merger remnants
The connection between short gamma-ray bursts (SGRBs) and binary neutron star
(BNS) mergers was recently confirmed by the association of GRB 170817A with the
merger event GW170817. However, no conclusive indications were obtained on
whether the merger remnant that powered the SGRB jet was an accreting black
hole (BH) or a long-lived massive neutron star (NS). Here, we explore the
latter case via BNS merger simulations covering up to 250 ms after merger. We
report, for the first time in a full merger simulation, the formation of a
magnetically-driven collimated outflow along the spin axis of the NS remnant.
For the system at hand, the properties of such an outflow are found largely
incompatible with a SGRB jet. With due consideration of the limitations and
caveats of our present investigation, our results favour a BH origin for GRB
170817A and SGRBs in general. Even though this conclusion needs to be confirmed
by exploring a larger variety of physical conditions, we briefly discuss
possible consequences of all SGRB jets being powered by accreting BHs.Comment: 6 pages, 4 figures; accepted for publication in MNRAS Letter
Short gamma-ray burst central engines
Growing evidence connects the progenitor systems of the short-hard subclass
of gamma-ray bursts (GRBs) to the merger of compact object binaries composed by
two neutron stars (NSs) or by a NS and a black hole (BH). The recent
observation of the binary NS (BNS) merger event GW170817 associated with GRB
170817A brought a great deal of additional information and provided further
support to the above connection, even though the identification of this burst
as a canonical short GRB (SGRB) remains uncertain. Decades of observational
constraints and theoretical models consolidated the idea of a jet origin for
the GRB prompt emission, which can also explain the multiwavelength afterglow
radiation observed in most of the events. However, the mechanisms through which
a BNS or NS-BH merger remnant would power a collimated outflow are much less
constrained. Understanding the properties of the remnant systems and whether
they can provide the right conditions for jet production has been a main driver
of the great effort devoted to study BNS and NS-BH mergers, and still
represents a real challenge from both the physical and the computational point
of view. One fundamental open question concerns the nature of the central
engine itself. While the leading candidate system is a BH surrounded by a
massive accretion disk, the recent observation of plateau-shaped X-ray
afterglows in some SGRBs would suggest a longer-lived engine, i.e. a metastable
(or even stable) massive NS, which would also exclude NS-BH progenitors. Here
we elaborate on this key aspect, considering three different scenarios to
explain the SGRB phenomenology based on different hypotheses on the nature of
the merger remnant. Then, we discuss the basic properties of GRB 170817A and
how this event would fit within the different frameworks of the above
scenarios, under the assumption that it was or was not a canonical SGRB.Comment: Submitted to IJMPD, Invited contribution for the Special Issue
"Gamma-Ray Bursts in the Multi-Messenger Era
The key role of magnetic fields in binary neutron star mergers
The first multimessenger observation of a binary neutron star (BNS) merger in
August 2017 demonstrated the huge scientific potential of these extraordinary
events. This breakthrough led to a number of discoveries and provided the best
evidence that BNS mergers can launch short gamma-ray burst (SGRB) jets and are
responsible for a copious production of heavy r-process elements. On the other
hand, the details of the merger and post-merger dynamics remain only poorly
constrained, leaving behind important open questions. Numerical relativity
simulations are a powerful tool to unveil the physical processes at work in a
BNS merger and as such they offer the best chance to improve our ability to
interpret the corresponding gravitational wave (GW) and electromagnetic
emission. Here, we review the current theoretical investigation on BNS mergers
based on general relativistic magnetohydrodynamics simulations, paying special
attention to the magnetic field as a crucial ingredient. First, we discuss the
evolution, amplification, and emerging structure of magnetic fields in BNS
mergers. Then, we consider their impact on various critical aspects: (i) jet
formation and the connection with SGRBs, (ii) matter ejection, r-process
nucleosynthesis, and radiocatively-powered kilonova transients, and (iii)
post-merger GW emission.Comment: Invited review for the Topical Collection on BNS mergers of the
journal General Relativity and Gravitation; 30 pages, 7 figures, accepted
versio
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