Bipolar Jets Research Articles

Appearance of Jet-Driving Poynting Flux in Hot, Tenuous Accretion Disks Threaded by an Ordered Magnetic Field

In a series of our previous studies, a model of radiatively inefficient accretion flows (RIAFs) in a global magnetic field (so-called resistive RIAF model) has proved its ability to account for many physical processes taking place in such accretion flows, as realized in the nuclei of galaxies believed to be accreting at a very small fraction of each Eddington accretion rate. Within the present status of this model, however, the model cannot describe the launch of a self-confined bipolar jet from the vicinity of the disk's inner edge, although it allows the existence of a thermal wind widely distributed over the disk surfaces. This is because the electric field (and hence the Poynting flux) vanishes everywhere in the disk, whereas such a jet in a globally ordered magnetic field is most likely to be accelerated electrodynamically. We show in the present paper that this defect can be overcome naturally if we reformulate the problem so as to admit a quasi-stationary change of the magnetic field (and hence the appearance of a non-irrotational electric field), and also restore all of the terms of order $ \epsilon$$ \equiv$ ($ v_r/v_{\varphi}$ )$ ^2$$ \lesssim$ 1 (where $ v_r$ and $ v_{\varphi}$ denote the radial and azimuthal components, respectively, of the fluid velocity), which have been neglected altogether in our previous scheme. The restored effects are the inertial and magnetic draggings on the infalling matter. As an illustrative example, a model solution that is correct up to $ {\cal O}(\epsilon)$ is derived under a set of plausible restrictions. The new solution predicts the appearance of a localized Poynting flux in a region near the disk inner edge, strongly suggesting that a jet is launched from this region. Another interesting prediction is the appearance of a rapid change of the magnetic field, also localized to this region.

MAGNETOROTATIONALLY DRIVEN SUPERNOVAE AS THE ORIGIN OF EARLY GALAXY r -PROCESS ELEMENTS?

We examine magnetorotationally driven supernovae as sources of $r$-process elements in the early Galaxy. On the basis of thermodynamic histories of tracer particles from a three-dimensional magnetohydrodynamical core-collapse supernova model with approximated neutrino transport, we perform nucleosynthesis calculations with and without considering the effects of neutrino absorption reactions on the electron fraction ($Y_{e}$) during post-processing. We find that the peak distribution of $Y_{e}$ in the ejecta is shifted from $\sim0.15$ to $\sim0.17$ and broadened toward higher $Y_{e}$ due to neutrino absorption. Nevertheless, in both cases the second and third peaks of the solar $r$-process element distribution can be well reproduced. The rare progenitor configuration that was used here, characterized by a high rotation rate and a large magnetic field necessary for the formation of bipolar jets, could naturally provide a site for the strong $r$-process in agreement with observations of the early galactic chemical evolution.

JET ROTATION INVESTIGATED IN THE NEAR-ULTRAVIOLET WITH THEHUBBLE SPACE TELESCOPEIMAGING SPECTROGRAPH

We present results of the second phase of our near-ultraviolet investigation into protostellar jet rotation using HST/STIS. We obtain long-slit spectra at the base of five T Tauri jets to determine if there is a difference in radial velocity between the jet borders which may be interpreted as a rotation signature. These observations are extremely challenging and push the limits of current instrumentation, but have the potential to provide long-awaited observational support for the magneto-centrifugal mechanism of jet launching in which jets remove angular momentum from protostellar systems. We successfully detect all five jet targets (from RW Aur, HN Tau, DP Tau and CW Tau) in several near-ultraviolet emission lines, including the strong Mg II doublet. However, only RW Aur's bipolar jet presents sufficient signal-to-noise for analysis. The approaching jet lobe shows a difference of 10 km/s in a direction which agrees with the disk rotation sense, but is opposite to previously published optical measurements for the receding jet. The near-ultraviolet difference is not found six months later, nor is it found in the fainter receding jet. Overall, in the case of RW Aur, differences are not consistent with a simple jet rotation interpretation. Indeed, given the renowned complexity and variability of this system, it now seems likely that any rotation signature is confused by other influences, with the inevitable conclusion that RW Aur is not suited to a jet rotation study.

AN OBSERVATIONAL IMPRINT OF THE COLLAPSAR MODEL OF LONG GAMMA-RAY BURSTS

The Collapsar model provides a theoretical framework for the well known association between long gamma-ray bursts (GRBs) and collapsing massive stars. A bipolar relativistic jet, launched at the core of a collapsing star, drills its way through the stellar envelope and breaks out of the surface before producing the observed gamma-rays. While a wealth of observations associate GRBs with the death of massive stars, as yet there is no direct evidence for the Collapsar model itself. Here we show that a distinct signature of the Collapsar model is the appearance of a plateau in the duration distribution of the prompt GRB emission at times much shorter than the typical breakout time of the jet. This plateau is evident in the data of all three major satellites. These findings provide an evidence that directly supports the Collapsar model. Additionally, it suggests the existence of a large population of choked (failed) GRBs and that the 2 s duration commonly used to separate Collapsars and non-Collapasars is inconsistent with the duration distributions of {\it Swift} and Fermi GRBs and only holds for BATSE GRBs.

Discovery of a compact gas-rich damped Lyman-αgalaxy atz = 2.2: evidence of a starburst-driven outflow

We present the detection of Ly-alpha, [OIII] and H-alpha emission associated with an extremely strong DLA system (N(HI) = 10^22.10 cm^-2) at z=2.207 towards the quasar SDSS J113520-001053. This is the largest HI column density ever measured along a QSO line of sight, though typical of what is seen in GRB-DLAs. This absorption system also classifies as ultrastrong MgII system with W2796_r=3.6 A. The mean metallicity of the gas ([Zn/H]=-1.1) and dust depletion factors ([Zn/Fe]=0.72, [Zn/Cr]=0.49) are consistent with (and only marginally larger than) the mean values found in the general QSO-DLA population. The [OIII]-Ha emitting region has a very small impact parameter with respect to the QSO line of sight, b=0.1", and is unresolved. From the Ha line, we measure SFR=25 Msun/yr. The Ly-a line is double-peaked and is spatially extended. More strikingly, the blue and red Ly-a peaks arise from distinct regions extended over a few kpc on either side of the star-forming region. We propose that this is the consequence of Ly-a transfer in outflowing gas. The presence of starburst-driven outflows is also in agreement with the large SFR together with a small size and low mass of the galaxy (Mvir~10^10 Msun). From the stellar UV continuum luminosity of the galaxy, we estimate an age of at most a few 10^7 yr, again consistent with a recent starburst scenario. We interpret the data as the observation of a young, gas rich, compact starburst galaxy, from which material is expelled through collimated winds powered by the vigorous star formation activity. We substantiate this picture by modelling the radiative transfer of Ly-a photons in the galactic counterpart. Though our model (a spherical galaxy with bipolar outflowing jets) is a simplistic representation of the true gas distribution and velocity field, the agreement between the observed and simulated properties is particularly good. [abridged]

CAUSE AND EFFECT OF FEEDBACK: MULTIPHASE GAS IN CLUSTER CORES HEATED BY AGN JETS

Multiwavelength data indicate that the X-ray emitting plasma in the cores of galaxy clusters is not cooling catastrophically. To large extent, cooling is offset by heating due to active galactic nuclei (AGN) via jets. The cool-core clusters, with cooler/denser plasmas, show multiphase gas and signs of some cooling in their cores. These observations suggest that the cool core is locally thermally unstable while maintaining global thermal equilibrium. Using high-resolution, three-dimensional simulations we study the formation of multiphase gas in cluster cores heated by highly-collimated bipolar AGN jets. Our key conclusion is that spatially extended multiphase filaments form only when the instantaneous ratio of the thermal instability and free-fall timescales (t_TI/t_ff) falls below a critical threshold of \approx 10. When this happens, dense cold gas decouples from the hot ICM phase and generates inhomogeneous and spatially extended Halpha filaments. These cold gas clumps and filaments `rain' down onto the central regions of the core, forming a cold rotating torus and in part feeding the supermassive black hole. Consequently, the self-regulated feedback enhances AGN heating and the core returns to a higher entropy level with t_TI/t_ff > 10. Eventually the core reaches quasi-stable global thermal equilibrium, and cold filaments condense out of the hot ICM whenever t_TI/t_ff \lesssim 10. This occurs despite the fact that the energy from AGN jets is supplied to the core in a highly anisotropic fashion. The effective spatial redistribution of heat is enabled in part by the turbulent motions in the wake of freely-falling cold filaments. Increased AGN activity can locally reverse the cold gas flow, launching cold filamentary gas away from the cluster center. Our criterion for the condensation of spatially extended cold gas is in agreement with observations and previous idealized simulations.

SN 2010jp (PTF10aaxi): a jet in a Type II supernova

We present photometry and spectroscopy of the peculiar TypeII supernova (SN) 2010jp, also named PTF10aaxi. The light curve exhibits a linear decline with a relatively low peak absolute magnitude of only -15.9, and a low radioactive decay luminosity at late times that suggests a nickel mass below 0.003 $M_{\odot}$. Spectra of SN2010jp display an unprecedented triple-peaked H$\alpha$ line profile, showing: (1) a narrow (800 km/s) central component that suggests shock interaction with dense CSM; (2) high-velocity blue and red emission features centered at -12600 and +15400 km/s; and (3) broad wings extending from -22000 to +25000 km/s. These features persist during 100 days after explosion. We propose that this line profile indicates a bipolar jet-driven explosion, with the central component produced by normal SN ejecta and CSM interaction at mid latitudes, while the high-velocity bumps and broad line wings arise in a nonrelativistic bipolar jet. Two variations of the jet interpretation seem plausible: (1) A fast jet mixes 56Ni to high velocities in polar zones of the H-rich envelope, or (2) the reverse shock in the jet produces blue and red bumps in Balmer lines when a jet interacts with dense CSM. Jet-driven SNeII are predicted for collapsars resulting from a wide range of initial masses above 25 $M_{\odot}$ at sub-solar metallicity. This seems consistent with the SN host environment, which is either an extremely low-luminosity dwarf galaxy or very remote parts of an interacting pair of star-forming galaxies. It also seems consistent with the low 56Ni mass that may accompany black hole formation. We speculate that the jet survives to produce observable signatures because the star's H envelope was mostly stripped away by previous eruptive mass loss.

Present-day star formation: From molecular cloud cores to protostars and protoplanetary disks

Essential physical processes in the formation of protostars and protoplanetary disks are described. Recent advances in non-ideal magnetohydrodynamics simulations, which cover a huge dynamic range from molecular cloud core density (104/cc) to stellar density (1022/cc) in a self-consistent manner, enable us to study the realistic evolution of the magnetic field and rotation of protostars and the dynamics of outflows and jets. First we emphasize the importance of radiative heating and cooling, and describe thermal evolution in a self-gravitationally collapsing cloud. The increased pressure at the center creates the first hydrostatic core, which consists of molecular gas. After the dissociation of molecular hydrogen triggers the second gravitational collapse at the center of the first core, a protostar is quickly formed and the first core gradually transforms into a circumstellar disk that eventually accretes onto the central protostar. The importance of the short-lived first core formed in the early collapsing phase is emphasized in the contexts of driving magnetohydrodynamical bipolar outflows and self-gravitational fragmentation into binary or multiple stars. When the central density becomes sufficiently high (1012/cc), ohmic dissipation largely removes the magnetic flux from a collapsing cloud core, and the strongly twisted magnetic field lines are straightened. The magnetic field lines are twisted and amplified again for much higher density (1016/cc) where the magnetic field is recoupled with warm gas (∼103 K). Finally, protostars at their formation epoch have magnetic fields of 0.1–1 kG, which is comparable to observed values of pre-main-sequence stars. A substantially reduced magnetic flux at the center results in passively wound-up magnetic field lines just after the formation of a protostar. This is followed by driving of a fast bipolar jet along the rotation axis by the resultant magnetic pressure due to excessive winding. Strong collimation of the jet is due to the hoop stress of piled-up toroidal field lines. The angular momentum in a collapsing cloud is removed by magnetohydrodynamical effects such as magnetic braking and driving of outflows and jets. The rotation velocity of the protostar tends to be on the order of break-up speed at its formation epoch, and thus, a further removal mechanism for the angular momentum, such as through the interactions between the protostar, disk, jets, and winds, should be important in its long-term evolution. The circumstellar disk is born in the “dead zone,” a region that is decoupled from the magnetic field. The outer radius of the disk increases with that of the dead zone during accretion from the envelope of the molecular cloud core. A rapid increase in the disk size occurs after depletion of the envelope. The circumstellar disks remain massive in their formation phase, and are subject to gravitational instability, even at 10 AU from the central stars. The further long-term evolution of massive disks is also described. This may provide an improved description for the realistic initial condition and environments for planet formation in gaseous protoplanetary disks.

Multipolar young planetary nebulae modelled as a precessing and orbiting jet with time-dependent ejection velocity

We have carried out 3D hydrodynamic simulations of precessing, variable ejection velocity bipolar jets in order to model multipolar protoplanetary (or planetary) nebulae. For these nebulae, we assume a binary source, with an asymptotic giant branch primary star which ejects an isotropic wind, and a companion which ejects the bipolar jet system. We find that it is possible to relate the large-scale morphological characteristics of these nebulae (lobe size, semi-aperture angle, number of observed lobes) to some of the parameters of the binary system, such as the ratio between the orbital and precession periods, the ratio between the masses of the binary components and the major axis of the elliptical orbit. Our results show that synthetic nebulae with well-defined lobe morphologies (resembling many of the observed multipolar planetary nebulae) are obtained from our models.

Short gamma-ray bursts with extended emission from magnetar birth: jet formation and collimation

Approximately 1/4-1/2 of short duration Gamma-Ray Bursts are followed by variable X-ray emission lasting ~ 100 s with a fluence comparable or exceeding that of the initial burst itself. The long duration and significant energy of this `extended emission'(EE) poses a major challenge to the standard binary neutron star merger model. metzger08 recently proposed that the EE is powered by the spin-down of a strongly magnetized neutron star. However, the effects of surrounding material on the magnetar outflow have not yet been considered. Here we present time-dependent axisymmetric relativistic magnetohydrodynamic simulations of the interaction of the relativistic proto-magnetar wind with a surrounding 10^-1 10^-3 M_\odot envelope, which represents material ejected during the merger; the supernova following AIC; or via outflows from the accretion disk. The collision between the relativistic magnetar wind and the expanding ejecta produces a magnetized nebula inside the ejecta. A strong toroidal magnetic field builds up in the nebula, which drives a bipolar jet out through the ejecta, similar to the magnetar model developed in the case of long duration GRBs. We quantify the `break-out' time and opening angle of the jet theta_j as a function of the wind energy flux dot{E} and ejecta mass M_ej. We show that dot{E} and theta_j are inversely correlated, such that the beaming-corrected (isotropic) luminosity of the jet is primarily a function of M_ej. Both variability arguments, and the lower limit on the power of magnetar outflows capable of producing bright emission, suggest that the true opening angle of the magnetar jet must be relatively large. The model thus predicts a class of events for which the EE is observable with no associated short GRB. These may appear as long-duration GRBs or X-Ray Flashes, which may be detected by future all-sky X-ray survey missions.

GALAXY FORMATION WITH SELF-CONSISTENTLY MODELED STARS AND MASSIVE BLACK HOLES. I. FEEDBACK-REGULATED STAR FORMATION AND BLACK HOLE GROWTH

There is mounting evidence for the coevolution of galaxies and their embedded massive black holes (MBHs) in a hierarchical structure formation paradigm. To tackle the nonlinear processes of galaxy-MBH interaction, we describe a self-consistent numerical framework which incorporates both galaxies and MBHs. The high-resolution adaptive mesh refinement (AMR) code Enzo is modified to model the formation and feedback of molecular clouds at their characteristic scale of 15.2 pc and the accretion of gas onto a MBH. Two major channels of MBH feedback, radiative feedback (X-ray photons followed through full 3D adaptive ray tracing) and mechanical feedback (bipolar jets resolved in high-resolution AMR), are employed. We investigate the coevolution of a 9.2e11 Msun galactic halo and its 1e5 Msun embedded MBH at redshift 3 in a cosmological LCDM simulation. The MBH feedback heats the surrounding ISM up to 1e6 K through photoionization and Compton heating and locally suppresses star formation in the galactic inner core. The feedback considerably changes the stellar distribution there. This new channel of feedback from a slowly growing MBH is particularly interesting because it is only locally dominant, and does not require the heating of gas globally on the disk. The MBH also self-regulates its growth by keeping the surrounding ISM hot for an extended period of time.

CHANDRAEVIDENCE FOR EXTENDED X-RAY STRUCTURE IN RY Tau

We report results of a sensitive Chandra ACIS-S observation of the classical T Tauri star RY Tau. Previous studies have shown that it drives a spectacular bipolar jet whose blueshifted component is traced optically along P.A. approximately 295 degrees at separations of 1.5 - 31 arcseconds from the star. Complex X-ray emission is revealed, including a very soft non-variable spectral component (some of which may originate in shocks), a superhot flaring component (T >= 100 MK), and faint extended structure near the star. The structure is visible in deconvolved images and extends northwestward out to a separation of 1.7 arcseconds, overlapping the inner part of the optical jet. Image analysis suggests that most of the extension is real, but some contamination by PSF-induced structure within the central arcsecond may be present. The predicted temperature for a shock-heated jet based on jet speed and shock speed estimates from optical measurements is too low to explain the extended X-ray structure. Either higher speed material within the jet has escaped optical detection or other mechanisms besides shock-heating are involved. Alternative mechanisms that could produce higher temperature plasma at small offsets to the northwest of RY Tau include magnetic heating in the jet, hot plasmoids ejected at high speeds, or X-ray emission from a putative close companion whose presence has been inferred from Hipparcos variations.

A Cat's Eye view of the Eskimo from Saturn

AbstractThe 3-D and kinematic structure of the Eskimo nebula, NGC 2392, has been notoriously difficult to interpret given its complex morphology, multiple kinematic components and its nearly pole-on orientation along the line of sight. Here we present the most comprehensive high resolution spectroscopic mapping of the Eskimo planetary nebula to date. The data consist of 21 spatially resolved, long-slit echelle spectra tightly spaced over the Eskimo and along its bipolar jets. This data set allowed us to construct a velocity-resolved [NII] channel map of the nebula with a resolution of 10 km/s that disentagles the differente kinematic components of the nebula and reveals clearly for the first time the kinematic expansion pattern for each of the components. The spectroscopic information is combined with a HST image to construct the first detailed three dimensional model of the Eskimo with the code SHAPE. With this model we demostrate that the Eskimo is nearly a twin to the Saturn nebula, but rotated 90° to the line sight. Furthermore, we show that the main characteristics of our model apply to the general properties of the group of elliptical planetary nebulae with ansae, once the orientation is considered.

A complete survey of mm line emission from CO and 13CO in water fountain stars

Abstract“Water fountains” (WF) are post-AGB stars characterized by very fast and often bipolar jets in the water maser line at 22 GHz. We conducted a survey of the CO and 13CO line emission using the IRAM 30m radio telescope of all the sources visible from the observatory. Over a total of 10 WFs observed, we identified CO and 13CO associated to IRAS 18460-0151 and IRAS 18596+0315. As the CO line emission is ubiquitous in the Galactic plane, the confirmed detections have met strict criteria to consider these cases as confirmed detections. The velocity components associated to the WFs are ~ 40 km s−1 wide, centered at the star velocity, present only at the star position, and detected in the four observed transitions, the CO and 13CO J=1→0 and 2→1 lines. A preliminary study, based on line ratios, indicates that this molecular component has a high opacity, and a very low 12C/13C isotopic ratio.

Trigonometric parallax measurements of post-AGB stars with VERA

AbstractA trigonometric parallax distance is measurable for a planetary nebula if it harbors H2O (or OH) maser spots. We have demonstrated it for H2O maser sources, K3-35, IRAS 19312+1950 and IRAS 18286-0959 using the VLBI Exploration of Radio Astrometry (VERA). They are post-AGB stars and exhibit peculiar morpho-kinematical structures like a bipolar planetary nebula or a bipolar molecular jet (water fountain). The luminosities and secular motions of these sources derived from the trigonometric distances and proper motions of the H2O masers suggest that their parental stars may be intermediate-mass (2M⊙ < M* < 8 M⊙) evolved stars.

VISIR-VLT images of the water maser: Emitting planetary nebula K3-35

AbstractK3-35 is an extremely young bipolar planetary nebula that contains a precessing bipolar jet and a small (radius 80 AU) water maser equatorial ring. We have obtained VISIR-VLT images of K3-35 in the PAH1 (λ=8.6 μm), [S iv] (λ=10.6 μm), and SiC (λ=11.85 μm) filters to analize the mid-IR morphology and the temperature structure of its dust emission. The images show the innermost nebular regions undetected at optical wavelegths and the precessing bipolar jets. The temperature map shows variations in the temperature in the equatorial zone and in regions associated to its jets.

Herschel’sview into Mira’s head

Herschel's PACS instrument observed the environment of the binary system Mira Ceti in the 70 and 160 micron bands. These images reveal bright structures shaped as five broken arcs and fainter filaments in the ejected material of Mira's primary star. The overall shape of the IR emission around Mira deviates significantly from the expected alignment with Mira's exceptionally high space velocity. The observed broken arcs are neither connected to each other nor are they of a circular shape; they stretch over angular ranges of 80 to 100 degrees. By comparing Herschel and GALEX data, we found evidence for the disruption of the IR arcs by the fast outflow visible in both Halpha and the far UV. Radial intensity profiles are derived, which place the arcs at distances of 6-85" (550 - 8000 AU) from the binary. Mira's IR environment appears to be shaped by the complex interaction of Mira's wind with its companion, the bipolar jet, and the ISM.

GLOBAL SELF-SIMILAR PROTOSTELLAR DISK/WIND MODELS

The magnetocentrifugal disk wind mechanism is the leading candidate for producing the large-scale, bipolar jets commonly seen in protostellar systems. I present a detailed formulation of a global, radially self-similar model for a non-ideal disk that launches a magnetocentrifugal wind. This formulation generalizes the conductivity tensor formalism previously used in radially localized disk models. The model involves matching a solution of the equations of non-ideal MHD describing matter in the disk to a solution of the equations of ideal MHD describing a "cold" wind. The disk solution must pass smoothly through the sonic point, the wind solution must pass smoothly through the Alfv\'en point, and the two solutions must match at the disk/wind interface. This model includes for the first time a self-consistent treatment of the evolution of magnetic flux threading the disk, which can change on the disk accretion timescale. The formulation presented here also allows a realistic conductivity profile for the disk to be used in a global disk/wind model for the first time. The physical constraints on the model solutions fix the distribution of the magnetic field threading the disk, the midplane accretion speed, and the midplane migration speed of flux surfaces. I present a representative solution that corresponds to a disk in the ambipolar conductivity regime with a nominal neutral-matter--magnetic-field coupling parameter that is constant along field lines, matched to a wind solution. I conclude with a brief discussion of the importance of self-similar disk/wind models in studying global processes such as dust evolution in protostellar systems.

SN 2010jp (PTF10aaxi): A Jet-driven Type II Supernova

AbstractWe present photometry and spectroscopy of the peculiar Type II supernova SN 2010jp, also named PTF10aaxi. The light curve exhibits a linear decline with a relatively low peak absolute magnitude of only −15.9 (unfiltered), and a low radioactive decay luminosity at late times that suggests a low synthesized nickel mass of about 0.003 M⊙ or less. Spectra of SN 2010jp display an unprecedented triple-peaked Hα line profile, showing: (1) a narrow central component that suggests shock interaction with a dense circumstellar medium (CSM); (2) high-velocity blue and red emission features centered at −12,600 and +15,400 km s−1; and (3) very broad wings extending from −22,000 to +25,000 km s−1. We propose that this line profile indicates a bipolar jet-driven explosion, with the central component produced by normal SN ejecta and CSM interaction at mid and low latitudes, while the high-velocity bumps and broad line wings arise in a nonrelativistic bipolar jet. Jet-driven SNe II are predicted for collapsars resulting from a wide range of initial masses above 25 M⊙, especially at the sub-solar metallicity consistent with the SN host environment. It also seems consistent with the apparently low 56Ni mass that may accompany black hole formation. We speculate that the jet survives to produce observable signatures because the star's H envelope was very low mass, having been mostly stripped away by the previous eruptive mass loss.

The protomagnetar model for gamma-ray bursts

Long duration Gamma-Ray Bursts (GRBs) originate from the core collapse of massive stars, but the identity of the central engine remains elusive. Previous work has shown that rapidly spinning, strongly magnetized proto-neutron stars (`millisecond proto-magnetars') produce outflows with energies, timescales, and magnetizations sigma_0 (maximum Lorentz factor) that are consistent with those required to produce long GRBs. Here we extend this work in order to construct a self-consistent model that directly connects the properties of the central engine to the observed prompt emission. Just after the launch of the supernova shock, a wind heated by neutrinos is driven from the proto-magnetar. The outflow is collimated into a bipolar jet by its interaction with the star. As the magnetar cools, the wind becomes ultra-relativistic and Poynting-flux dominated (sigma_0 >> 1) on a timescale comparable to that required for the jet to clear a cavity through the star. Although the site and mechanism of the prompt emission are debated, we calculate the emission predicted by two models: magnetic dissipation and internal shocks. Our results favor the magnetic dissipation model in part because it predicts a relatively constant `Band' spectral peak energy E_peak with time during the GRB. The jet baryon loading decreases abruptly when the neutron star becomes transparent to neutrinos at t ~ 10-100 seconds. Jets with ultra-high magnetization cannot effectively accelerate and dissipate their energy, suggesting this transition ends the prompt emission and may explain the steep decay phase that follows. We assess several phenomena potentially related to magnetar birth, including low luminosity GRBs, thermal-rich GRBs/X-ray Flashes, very luminous supernovae, and short duration GRBs with extended emission.