23 research outputs found
Constraining supermassive primordial black holes with magnetically induced gravitational waves
Primordial black holes (PBHs) can answer a plethora of cosmic conundra, among
which the origin of the cosmic magnetic fields. In particular, supermassive
PBHs with masses and furnished with a
plasma-disk moving around them can generate through the Biermann battery
mechanism a seed primordial magnetic field which can later be amplified so as
to provide the magnetic field threading the intergalactic medium. In this
Letter, we derive the gravitational wave (GW) signal induced by the magnetic
anisotropic stress of such a population of magnetised PBHs. Interestingly
enough, by using GW constraints from Big Bang Nucleosynthesis (BBN) and an
effective model for the galactic/turbulent dynamo amplification of the magnetic
field, we set a conservative upper bound constraint on the abundances of
supermassive PBHs at formation time, as a function of
the their masses, namely that . Remarkably, these
constraints are comparable, and, in some mass ranges, even tighter compared to
the constraints on from large-scale structure (LSS)
probes; hence promoting the portal of magnetically induced GWs as a new probe
to explore the enigmatic nature of supermassive PBHs.Comment: 5 pages, 2 figures (comments are welcome
Dynamical Friction due to fuzzy dark matter on satellites described by axisymmetric logarithmic potentials
A plausible dark matter candidate is an ultralight bosonic particle referred
to as fuzzy dark matter. The equivalent mass-energy of the fuzzy dark matter
boson is eV and has a corresponding de Broglie wavelength of
kiloparsec scale, thus exhibiting wave behaviour in scales comparable to a
galactic core, which could not appear in conventional cold dark matter models.
The presence of fuzzy dark matter in galactic clusters will impact the motion
of their members through dynamical friction. In this work, we present
simulations of the dynamical friction on satellites traversing an initially
uniform fuzzy dark matter halo. We focus on the satellites whose shapes are
beyond spherical symmetry described by ellipsoidal and logarithmic potentials.
We find that the wakes created on the fuzzy dark matter halo due to the passage
of such satellites are qualitatively different from those generated by
spherically symmetric ones. Furthermore, we quantify the dynamical friction
coefficient for such systems, finding that the same satellite may experience a
drag differing by a factor of depending on its ellipticity and the
direction of motion. Finally, we find that the dynamical friction time-scale is
close to Hubble time, assuming a satellite of M traversing
at km/s a FDM halo whose mean density is Mkpc.Comment: 17 pages, 7 figures, Accepted by JCA
Coupled axisymmetric pulsar magnetospheres
We present solutions of force-free pulsar magnetospheres coupled with a uniform external magnetic field aligned with the dipole magnetic moment of the pulsar. The inclusion of the uniform magnetic field has the following consequences: the equatorial current sheet is truncated to a finite length, the fraction of field lines that are open increases, and the open field lines are confined within a cylindrical surface instead of becoming radial. A strong external magnetic field antiparallel to the dipole allows for solutions where the pulsar magnetic field is fully enclosed within an ellipsoidal surface. Configurations of fully enclosed or confined magnetospheres may appear in a double neutron star (DNS) where one of the components is a slowly rotating, strongly magnetized pulsar and the other a weakly magnetized millisecond pulsar. Depending on the geometry, twisted field lines from the millisecond pulsar could form an asymmetric wind. More dramatic consequences could appear closer to merger: there, the neutron star with the weaker magnetic field may undergo a stage where it alternates between an open and a fully enclosed magnetosphere releasing up 1037 erg. Under favourable combinations of magnetic fields DNSs can spend up to 103 yr in the coupled phase, implying a Galactic population of 0.02 systems. Potential observational signatures of this interaction are discussed including the possibility of powering recurring fast radio bursts. We also note that the magnetic interaction cannot have any significant effect on the spin evolution of the two pulsars
Magnetic inhibition of centrifugal instability
Recently, it was shown that the centrifugal instability may be important in the dynamics of astrophysical jets undergoing reconfinement by external pressure. However, these studies were limited to the case of unmagnetized flows. Here, we explore the role of the magnetic field within both the Newtonian and relativistic frameworks. Since the jet problem is rather complicated, we focus instead on the simpler problem of cylindrical rotation and axial magnetic field, which shares significant similarity with the jet problem, and consider only axisymmetric perturbations. The studied equilibrium configurations involve a cylindrical interface and they are stable to non-magnetic centrifugal and magnetorotational instabilities everywhere except this interface. We use a heuristic approach to derive the local stability criterion for the interface in the magnetic case and numerical simulations to verify the role of the magnetic field. The theory and simulations agree quite well for Newtonian models but indicate a potential discrepancy for the relativistic models in the limit of high Lorentz factor of the rotational motion at the interface. In general, the magnetic field sets a critical wavelength below which the centrifugal modes are stabilized. We discuss the implication of our findings for the astrophysical jets, which suggest that the centrifugal instability develops only in jets with relatively low magnetization. Namely, the magnetic pressure has to be below the thermal one and for the relativistic case the jets have to be kinetic-energy dominated
Magnetar Activity via the Density-Shear Instability in Hall-MHD
We investigate the density-shear instability in Hall-MHD via numerical simulation of the full non-linear problem, in the context of magnetar activity. We confirm the development of the instability of a plane-parallel magnetic field with an appropriate intensity and electron density profile, in accordance with analytic theory. We find that the instability also appears for a monotonically decreasing electron number density and magnetic field, a plane-parallel analogue of an azimuthal or meridional magnetic field in the crust of a magnetar. The growth rate of the instability depends on the Hall properties of the field (magnetic field intensity, electron number density and the corresponding scale-heights), while being insensitive to weak resistivity. Since the Hall effect is the driving process for the evolution of the crustal magnetic field of magnetars, we argue that this instability is critical for systems containing strong meridional or azimuthal fields. We find that this process mediates the formation of localised structures with much stronger magnetic field than the average, which can lead to magnetar activity and accelerate the dissipation of the field and consequently the production of Ohmic heating. Assuming a 5 × 1014G magnetic field at the base of crust, we anticipate that magnetic field as strong as 1015G will easily develop in regions of typical size of a few 102 meters, containing magnetic energy of 1043erg, sufficient to power magnetar bursts. These active regions are more likely to appear in the magnetic equator where the tangential magnetic field is stronger
Dynamics of rising magnetized cavities and UHECR acceleration in clusters of galaxies
We study the expansion of low density cavities produced by Active Galactic
Nuclei jets in clusters of galaxies. The long term stability of these cavities
requires the presence of linked magnetic fields. We find solutions describing
the self-similar expansion of structures containing large-scale electromagnetic
fields. Unlike the force-free spheromak-like configurations, these solutions
have no surface currents and, thus, are less susceptible to resistive decay.
The cavities are internally confined by external pressure, with zero gradient
at the surface. If the adiabatic index of the plasma within the cavity is
, the expansion ultimately leads to the formation of large-scale
current sheets. The resulting dissipation of the magnetic field can only
partially offset the adiabatic and radiative losses of radio emitting
electrons. We demonstrate that if the formation of large-scale current sheets
is accompanied by explosive reconnection of the magnetic field, the resulting
reconnection layer can accelerate cosmic rays to ultra high energies. We
speculate that the enhanced flux of UHECRs towards Centaurus A originates at
the cavities due to magnetic reconnection.Comment: 9 page