Search results for "Neutron star"
showing 10 items of 343 documents
Searching for pulsed emission from XTE J0929-314 at high radio frequencies
2009
The aim of this work is to search for radio signals in the quiescent phase of accreting millisecond X-ray pulsars, in this way giving an ultimate proof of the recycling model, thereby unambiguously establishing that accreting millisecond X-ray pulsars are the progenitors of radio millisecond pulsars. To overcome the possible free-free absorption caused by matter surrounding accreting millisecond X-ray pulsars in their quiescence phase, we performed the observations at high frequencies. Making use of particularly precise orbital and spin parameters obtained from X-ray observations, we carried out a deep search for radio-pulsed emission from the accreting millisecond X-ray pulsar XTE J0929-31…
Timing Analysis of the 2022 Outburst of the Accreting Millisecond X-Ray Pulsar SAX J1808.4-3658: Hints of an Orbital Shrinking
2022
We present a pulse timing analysis of NICER observations of the accreting millisecond X-ray pulsar SAX J1808.4$-$3658 during the outburst that started on 2022 August 19. Similar to previous outbursts, after decaying from a peak luminosity of $\simeq 1\times10^{36} \, \mathrm{erg \, s^{-1}}$ in about a week, the pulsar entered in a $\sim 1$ month-long reflaring stage. Comparison of the average pulsar spin frequency during the outburst with those previously measured confirmed the long-term spin derivative of $\dot{\nu}_{\textrm{SD}}=-(1.15\pm0.06)\times 10^{-15} \, \mathrm{Hz\,s^{-1}}$, compatible with the spin-down torque of a $\approx 10^{26} \, \mathrm{G \, cm^3}$ rotating magnetic dipole.…
NuSTARandXMM–Newtonbroad-band spectrum of SAX J1808.4–3658 during its latest outburst in 2015
2018
The first discovered accreting millisecond pulsar, SAX J1808.4-3658, went into X-ray outburst in April 2015. We triggered a 100 ks XMM-Newton ToO, taken at the peak of the outburst, and a 55 ks NuSTAR ToO, performed four days apart. We report here the results of a detailed spectral analysis of both the XMM-Newton and NuSTAR spectra. While the XMM-Newton spectrum appears much softer than in previous observations, the NuSTAR spectrum confirms the results obtained with XMM-Newton during the 2008 outburst. We find clear evidence of a broad iron line that we interpret as produced by reflection from the inner accretion disk. For the first time, we use a self-consistent reflection model to fit the…
Stellar structure models in modified theories of gravity: Lessons and challenges
2019
The understanding of stellar structure represents the crossroads of our theories of the nuclear force and the gravitational interaction under the most extreme conditions observably accessible. It provides a powerful probe of the strong field regime of General Relativity, and opens fruitful avenues for the exploration of new gravitational physics. The latter can be captured via modified theories of gravity, which modify the Einstein-Hilbert action of General Relativity and/or some of its principles. These theories typically change the stellar structure equations, thus having a large impact on the astrophysical properties of the corresponding stars and opening a new window to constrain these …
Neutrino pair annihilation ( $$\nu {\bar{\nu }}\rightarrow e^-e^+$$ ν ν ¯ → e - e + ) in the presence of quintessence surrounding a black hole
2021
Quintessence fields, introduced to explain the speed-up of the Universe, might affect the geometry of spacetime surrounding black holes, as compared to the standard Schwarzschild and Kerr geometries. In this framework, we study the neutrino pairs annihilation into electron-positron pairs ($\nu{\bar \nu}\to e^-e^+$) near the surface of a neutron star, focusing, in particular, on the Schwarzschild-like geometry in presence of quintessence fields. The effect of the latter is to increase the minimum photon-sphere radius ($R_{ph}$), increasing in such a way the maximum energy deposition rate near to $R_{ph}$. The rate turns out to be several orders of magnitude greater than the rate computed in …
The force-free twisted magnetosphere of a neutron star
2016
We present a detailed analysis of the properties of twisted, force-free magnetospheres of non-rotating neutron stars, which are of interest in the modelling of magnetar properties and evolution. In our models the magnetic field smoothly matches to a current-free (vacuum) solution at some large external radius, and they are specifically built to avoid pathological surface currents at any of the interfaces. By exploring a large range of parameters, we find a few remarkable general trends. We find that the total dipolar moment can be increased by up to $40\%$ with respect to a vacuum model with the same surface magnetic field, due to the contribution of magnetospheric currents to the global ma…
A new general relativistic magnetohydrodynamics code for dynamical spacetimes
2008
We present a new numerical code which solves the general relativistic magneto-hydrodynamics (GRMHD) equations coupled to the Einstein equations for the evolution of a dynamical spacetime within the conformally-flat approximation. This code has been developed with the main objective of studying astrophysical scenarios in which both, high magnetic fields and strong gravitational fields appear, such as the magneto-rotational collapse of stellar cores, the collapsar model of GRBs, and the evolution of neutron stars. The code is based on an existing and thoroughly tested purely hydrodynamics code and on its extension to accommodate weakly magnetized fluids (passive magnetic field approximation).…
General relativistic simulations of pasive-magneto-rotational core collapse with microphysics
2007
This paper presents results from axisymmetric simulations of magneto-rotational stellar core collapse to neutron stars in general relativity using the passive field approximation for the magnetic field. These simulations are performed using a new general relativistic numerical code specifically designed to study this astrophysical scenario. The code is based on the conformally-flat approximation of Einstein's field equations and conservative formulations of the magneto-hydrodynamics equations. The code has been recently upgraded to incorporate a tabulated, microphysical equation of state and an approximate deleptonization scheme. This allows us to perform the most realistic simulations of m…
Swings between rotation and accretion power in a binary millisecond pulsar
2013
It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods1, 2, 3. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar4, 5 whose emission is powered by the neutron star’s rotating magnetic field6. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars7, 8 and also by the evidence for a past accretion disc in a rotation-powered milli…
Constraining the Equation of State of Neutron Stars with Genral Relativity
2005
When a radio pulsar breakes down by virtue of magnetodipole emission, its gravitational mass decreases accordingly. If the pulsar in hosted in a binary system, this mass loss will increase the orbital period of the system. We show that this relativistic effect can be indeed observable if the NS is fast and magnetized enough and that, if observed, it will help to put tight constraints on the equation of state of ultradense matter.