Jan Staff

Quark Deconfinement in Neutron Star Cores: The Effects of Spin-down

We study the role of spin-down in driving quark deconfinement in the high-density cores of isolated neutron stars. Assuming spin-down to be solely due to magnetic braking, we obtain typical timescales to quark deconfinement for neutron stars that are born with Keplerian frequencies. Employing different equations of state (EOSs), we determine the minimum and maximum neutron star masses that will allow for deconfinement via spin-down only. We find that the time to reach deconfinement is strongly dependent on the magnetic field and that this time is least for EOSs that support the largest minimum mass at zero spin, unless rotational effects on stellar structure are large. For a fiducial critical density of 5ρ0 for the transition to the quark phase (ρ0 = 2.5 × 1014 g cm-3 is the saturation density of nuclear matter), we find that neutron stars lighter than 1.5 M☉ cannot reach a deconfined phase. Depending on the EOS, neutron stars of more than 1.5 M☉ can enter a quark phase only if they are spinning faster than about 3 ms as observed now, whereas larger spin periods imply either that they are already quark stars or that they will never become them.

A three-stage model for the inner engine of gamma-ray bursts : Prompt emission and early afterglow

We propose a new model within the quark nova scenario to interpret the recent observations of early afterglows of long gamma-ray bursts (GRBs) with the Swift satellite. This is a three-stage model within the context of a core-collapse supernova. Stage 1 is an accreting (proto-)neutron star leading to a possible delay between the core collapse and the GRB. Stage 2 is an accreting quark star, generating the prompt GRB. Stage 3, which occurs only if the quark star collapses to form a black hole, consists of an accreting black hole. The jet launched in this accretion process interacts with the ejecta from stage 2, and could generate the flaring activity frequently seen in X-ray afterglows. This model may be able to account for both the energies and the timescales of GRBs, in addition to the newly discovered early X-ray afterglow features.

Quark-Nova Explosion inside a Collapsar: Application to Gamma Ray Bursts

If a quark-nova occurs inside a collapsar, the interaction between the quark-nova ejecta (relativistic iron-rich chunks) and the collapsar envelope leads to features indicative of those observed in Gamma Ray Bursts. The quark-nova ejecta collides with the stellar envelope creating an outward moving cap ( 1–10) above the polar funnel. Prompt gamma-ray burst emission from internal shocks in relativistic jets (following accretion onto the quark star) becomes visible after the cap becomes optically thin. Model features include (i) precursor activity (optical, X-ray, -ray), (ii) prompt -ray emission, and (iii) afterglow emission. We discuss SN-less long duration GRBs, short hard GRBs (including association and nonassociation with star forming regions), dark GRBs, the energetic X-ray flares detected in Swift GRBs, and the near-simultaneous optical and -ray prompt emission observed in GRBs in the context of our model.

Outflow-confined H ii Regions. II. The Early Break-out Phase

© 2017. The American Astronomical Society. All rights reserved. In this series of papers, we model the formation and evolution of the photoionized region and its observational signatures during massive star formation. Here, we focus on the early breakout of the photoionized region into the outflow cavity. Using results of 3D magnetohydrodynamic-outflow simulations and protostellar evolution calculations, we perform a post-processing radiative transfer. The photoionized region first appears at a protostellar mass of m∗ = 10 M ⊙ in our fiducial model and is confined to within 10-100 au by the dense inner outflow, which is similar to some of the observed very small hypercompact H II regions. Since the ionizing luminosity of the massive protostar increases dramatically as the Kelvin-Helmholtz (KH) contraction proceeds, the photoionized region breaks out to the entire outflow region in ≲10,000 year. Accordingly, the radio free-free emission brightens significantly in this stage. In our fiducial model, the radio luminosity at 10 GHz changes from 0.1 mJy kpc 2 at m∗ = 11 M ⊙ to 100 mJy kpc 2 at m∗ = 16 M ⊙ , while the infrared luminosity increases by less than a factor of two. The radio spectral index also changes in the break-out phase from the optically thick value of ∼2 to the partially optically thin value of ∼0.6. Additionally, we demonstrate that short-timescale variation in the free-free flux would be induced by an accretion burst. The outflow density is enhanced in the accretion burst phase, which leads to a smaller ionized region and weaker free-free emission. The radio luminosity may decrease by one order of magnitude during such bursts, while the infrared luminosity is much less affected because internal protostellar luminosity dominates over accretion luminosity after the KH contraction starts. Such a variability may be observable on timescales as short 10-100 year if accretion bursts are driven by disk instabilities.

Cannonballs in the context of gamma ray bursts. Formation sites

We investigate possible formation sites of the cannonballs (in the gamma ray bursts context) by calculating their physical parameters, such as density, magnetic field, and temperature close to the origin. Our results favor scenarios where the cannonballs form as instabilities (knots) within magnetized jets from hyperaccreting disks. These instabilities would most likely set in beyond the light cylinder where flow velocity with Lorentz factors as high as 2000 can be achieved. The cannonball model for gamma ray bursts requires that cannonballs form inside core-collapse supernovae. Our findings challenge the cannonball model of gamma ray bursts, unless hyperaccreting disks and the corresponding jets are common occurrences in core-collapse SNe.