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Register a new userHypercritical Accretion onto a Newborn Neutron Star and Magnetic Field Submergence
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We present magnetohydrodynamic numerical simulations of the late post-supernova hypercritical accretion to understand its effect on the magnetic field of the new-born neutron star. We consider as an example the case of a magnetic field loop protruding from the stars surface. The accreting matter is assumed to be non magnetized and, due to the high accretion rate, matter pressure dominates over magnetic pressure. We find that an accretion envelope develops very rapidly and once it becomes convectively stable the magnetic field is easily buried and pushed into the newly forming neutron star crust. However, for low enough accretion rates the accretion envelope remains convective for an extended period of time and only partial submergence of the magnetic field occurs due to a residual field that is maintained at the interface between the forming crust and the convective envelope. In this latter case, the outcome should be a weakly magnetized neutron star with a likely complicated field geometry. In our simulations we find the transition from total to partial submergence to occur around dotM ~ 10 M_sun/yr. Back-diffusion of the submerged magnetic field toward the surface, and the resulting growth of the dipolar component, may result in a delayed switch-on of a pulsar on time-scales of centuries to millenia.
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We numerically investigate the dynamics of a supernova fallback accretion confronting with a relativistic wind from a newborn neutron star (NS). The time evolution of the accretion shock in the radial direction is basically characterized by the encounter radius of the flow $r_mathrm{enc}$ and a dimensionless parameter $zeta equiv L/dot M_mathrm{fb}c^2$, where $L$ is the NS wind luminosity and $dot M_mathrm{fb}$ is the fallback mass accretion rate. We find that the critical condition for the fallback matter to reach near the NS surface can be simply described as $zeta < zeta_mathrm{min} equiv GM_*/c^2r_mathrm{enc}$ or $r_mathrm{enc}L/G M_* dot M_mathrm{fb} < 1$ independent of the wind Lorentz factor, where $M_*$ is the NS mass. With combining the condition for the fallback matter to bury the surface magnetic field under the NS crust, we discuss the possibility that the trifurcation of NSs into rotation-powered pulsars, central compact objects (CCOs), and magnetars can be induced by supernova fallback.
The problem of interaction of the rotating magnetic field, frozen to a star, with a thin well conducting accretion disk is solved exactly. It is shown that a disk pushes the magnetic field lines towards a star, compressing the stellar dipole magnetic field. At the point of corotation, where the Keplerian rotation frequency coincides with the frequency of the stellar rotation, the loop of the electric current appears. The electric currents flow in the magnetosphere only along two particular magnetic surfaces, which connect the corotation region and the inner edge of a disk with the stellar surface. It is shown that the closed current surface encloses the magnetosphere. Rotation of a disk is stopped at some distance from the stellar surface, which is 0.55 of the corotation radius. Accretion from a disk spins up the stellar rotation. The angular momentum transferred to the star is determined.
Studying the physical processes occurring in the region just above the magnetic poles of strongly magnetized, accreting binary neutron stars is essential to our understanding of stellar and binary system evolution. Perhaps more importantly, it provides us with a natural laboratory for studying the physics of high temperature and high density plasmas exposed to extreme radiation, gravitational, and magnetic fields. Observations over the past decade have shed new light on the manner in which plasma falling at velocities near the speed of light onto a neutron star surface is halted. Recent advances in modeling these processes have resulted in direct measurement of the magnetic fields and plasma properties. On the other hand, numerous physical processes have been identified that challenge our current picture of how the accretion process onto neutron stars works. Observation and theory are our essential tools in this regime because the extreme conditions cannot be duplicated on Earth. This white paper gives an overview of the current theory, the outstanding theoretical and observational challenges, and the importance of addressing them in contemporary astrophysics research.
Neutron star mergers are unique laboratories of accretion, ejection, and r-process nucleosynthesis. We used 3D general relativistic magnetohydrodynamic simulations to study the role of the post-merger magnetic geometry in the evolution of merger remnant discs around stationary Kerr black holes. Our simulations fully capture mass accretion, ejection, and jet production, owing to their exceptionally long duration exceeding $4$ s. Poloidal post-merger magnetic field configurations produce jets with energies $E_mathrm{jet} sim (4{-}30)times10^{50}$ erg, isotropic equivalent energies $E_mathrm{iso}sim(4{-}20)times10^{52}$ erg, opening angles $theta_mathrm{jet}sim6{-}13^circ$, and durations $t_jlesssim1$ s. Accompanying the production of jets is the ejection of $f_mathrm{ej}sim30{-}40%$ of the post-merger disc mass, continuing out to times $> 1$ s. We discover that a more natural, purely toroidal post-merger magnetic field geometry generates large-scale poloidal magnetic flux of alternating polarity and striped jets. The first stripe, of $E_mathrm{jet}simeq2times10^{48},mathrm{erg}$, $E_mathrm{iso}sim10^{51}$ erg, $theta_mathrm{jet}sim3.5{-}5^circ$, and $t_jsim0.1$ s, is followed by $gtrsim4$ s of striped jet activity with $f_mathrm{ej}simeq27%$. The dissipation of such stripes could power the short gamma-ray burst (sGRB) prompt emission. Our simulated jet energies and durations span the range of sGRBs. We find that although the blue kilonova component is initially hidden from view by the red component, it expands faster, outruns the red component, and becomes visible to off-axis observers. In comparison to GW 170817/GRB 170817A, our simulations under-predict the mass of the blue relative to red component by a factor of few. Including the dynamical ejecta and neutrino absorption may reduce this tension.
We present a detailed, broadband X-ray spectral analysis of the ULX pulsar NGC 7793 P13, a known super-Eddington source, utilizing data from the $XMM$-$Newton$, $NuSTAR$ and $Chandra$ observatories. The broadband $XMM$-$Newton+NuSTAR$ spectrum of P13 is qualitatively similar to the rest of the ULX sample with broadband coverage, suggesting that additional ULXs in the known population may host neutron star accretors. Through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures $sim$0.5 and $sim$1.5 keV are required to fit the data below 10 keV, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. The characteristic radii of the thermal components appear to be similar, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form, but is terminated by the neutron stars magnetic field soon after its onset, implying a limit of $B lesssim 6 times 10^{12}$ G for the dipolar component of the central neutron stars magnetic field. Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs. Finally, we examine the spectrum exhibited by P13 during one of its unusual off states. These data require both a hard powerlaw component, suggesting residual accretion onto the neutron star, and emission from a thermal plasma, which we argue is likely associated with the P13 system.
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