Day 761

Thursday.  Friday.

Nature
An ultraluminous X-ray source powered by an accreting neutron star

The majority of UL X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass BHs.  Their X-ray L in the 0.5-10 keV energy range from 1e39 to 41 ergs/s.  Because higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit, theoretical models have focused on the BH rather than neutron star systems.  The most challenging sources to explain are those at the luminous end of the range (more than 1e40 ergs/s), which require BH masses of 50-100 times the solar value or significant departures from the standard thin disk accretion that powers bright galactic X-ray binaries, or both.  Here report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 S and a 2.5 day sinusoidal modulation.  The pulsations results from the rotation of a magnetized NS, and the modulation arises from its binary orbit.  The pulsed flux alone corresponds to an X-ray luminosity in the 3-30 keV range of 4.9e39 ergs/s.  The pulsating sources is spatially coincident with a variable source that can reach an X-ray luminosity in the 0.3-10 keV range of 1.8e40 ergs/s.  This association implies a luminosity of about 100 x the Eddington limit for a 1.4 Msun object, or more than 10x brighter than any known accreting pulsar.  This implies that NSs may not be rare in the UL X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects.

1410.1874
Neutrino viscosity and drag: impact on the magnetorotational instability in proto-neutron stars
Guilet, Mueller, Janka

The magneto-rotational instability (MRI) is a promising mechanism to amplify the B-field in fast rotation proto-NSs (PNS).  The diffusion of neutrinos trapped in the PNS induces a transport of momentum, which can be modeled as a viscosity on length scales longer than the neutrino mean free path.  This neutrino-viscosity can slow down the growth of MRI modes to such an extent that a minimum initial B-field strength of >1e12 G is needed for the MRI to grow on a sufficiently short timescale to potentially affect the explosion.  It is uncertain whether the B-field of fast rotating progenitor cores is strong enough to yield such an initial B-field in PNSs.  At MRI wavelengths shorter than the neutrino mean free path, on the other hand, neutrino radiation does not act as a viscosity but rather induces a drag on the velocity with a damping rate independent of the wavelength.  Perform a linear analysis of the MRI in this regime, and apply the analytical results to the proto-NS structure from a one-dimensional numerical simulation.  Show that in the outer layers of the PNS, the MIR can grow from weak B-fields at wavelengths shorter than the neutrino mean free path, while deeper in the PNS MRI growth takes place in the viscous regime and requires a minimum B-field strength.

1410.1893
Origin of magnetic field in the intracluster medium: primordial or astrophysical?
Cho

The origin of B-fields in clusters of galaxies is still and unsolved problem, which is largely due to the poor understanding of initial seed B-fields.  If the seed magnetic fields have primordial origins, it is likely that large-scale pervasive B-fields were present before the formation of the large-scale structure.  On the other hand, if they were ejected from astrophysical bodies, they were highly localized in space at the time of injection.  In this paper, using turbulence dynamo models for high magnetic Prandtl number fluids, find constraints on the seed B-fields.  The hydrodynamic Reynolds number based on the Spitzer viscosity in the ICM is believed to be less than O(1e2), while the magnetic Reynolds number can be much larger than that.  IN this case, if the seed magnetic fields have primordial origins, they should be stronger than O(1e-11) G, which is very close to the upper limit of O(1e-9)G set by the CMB observations.  On the other hand, if the seed B-fields were ejected from astrophysical bodies, any seed B-fields stronger than O(1e-9) G can safely magnetize the ICM.  Therefore, it is less likely that primordial B-fields are the direct origin of present-day B-fields in the ICM.