Superluminous X-rays from a superluminous supernova

The discovery of a population of superluminous supernovae (SLSNe), with peak luminosities a factor of ~100 brighter than normal SNe (typically SLSNe have M_V <-21), has shown an unexpected diversity in core-collapse supernova properties. Numerous models have been postulated for the nature of these events, including a strong interaction of the shockwave with a dense circumstellar environment, a re-energizing of the outflow via a central engine, or an origin in the catastrophic destruction of the star following a loss of pressure due to pair production in an extremely massive stellar core (so-called pair instability supernovae). Here we consider constraints that can be placed on the explosion mechanism of Hydrogen-poor SLSNe (SLSNe-I) via X-ray observations, with XMM-Newton, Chandra and Swift, and show that at least one SLSNe-I is likely the brightest X-ray supernovae ever observed, with L_X ~ 10^45 ergs/s, ~150 days after its initial discovery. This is a luminosity 3 orders of magnitude higher than seen in other X-ray supernovae powered via circumstellar interactions. Such high X-ray luminosities are sufficient to ionize the ejecta and markedly reduce the optical depth, making it possible to see deep into the ejecta and any source of emission that resides there. Alternatively, an engine could have powered a moderately relativistic jet external to the ejecta, similar to those seen in gamma-ray bursts. If the detection of X-rays does require an engine it implies that these SNe do create compact objects, and that the stars are not completely destroyed in a pair instability event. Future observations will determine which, if any, of these mechanisms are at play in superluminous supernovae.

Nickel-Rich Outflows from Accretion Disks Formed by the Accretion-Induced Collapse of White Dwarfs

A white dwarf (WD) approaching the Chandrasekhar mass may in several cases undergo accretion-induced collapse (AIC) to a neutron star (NS) before a thermonuclear explosion ensues. It has generally been assumed that AIC does not produce a detectable supernova (SN). If, however, the progenitor WD is rapidly rotating (as may be expected due to its prior accretion), a centrifugally supported disk forms around the NS upon collapse. We calculate the subsequent evolution of this accretion disk using time-dependent height-integrated simulations with initial conditions taken from the AIC calculations of Dessart et al. (2006). Initially, the disk is cooled by neutrinos and its composition is driven neutron-rich (electron fraction Ye ~ 0.1) by electron captures. However, as the disk viscously spreads, it is irradiated by neutrinos from the central proto-NS, which dramatically alters its neutron-to-proton ratio. We find that electron neutrino captures increase Ye to ~ 0.5 by the time that weak interactions in the disk freeze out. Because the disk becomes radiatively inefficient and begins forming alpha-particles soon after freeze out, powerful winds blow away most of the disks remaining mass. These Ye ~ 0.5 outflows synthesize up to a few times 1e-2 Msun in 56Ni. As a result, AIC may be accompanied by a radioactively powered SN-like transient that peaks on a timescale of ~ 1 day. Since few intermediate mass elements are likely synthesized, these Ni-rich explosions should be spectroscopically distinct from other SNe. PanSTARRs and the Palomar Transient Factory should detect a few AIC transients per year if their true rate is ~1/100 of the Type Ia rate, and LSST should detect hundreds per year. High cadence observations (< 1 day) are optimal for the detection and follow-up of AIC (abridged).

Magnetized Relativistic Jets and Long-Duration GRBs from Magnetar Spindown during Core-Collapse Supernovae

We use ideal axisymmetric relativistic magnetohydrodynamic simulations to calculate the spindown of a newly formed millisecond, B ~ 10^{15} G, magnetar and its interaction with the surrounding stellar envelope during a core-collapse supernova (SN) explosion. The mass, angular momentum, and rotational energy lost by the neutron star are determined self-consistently given the thermal properties of the cooling neutron stars atmosphere and the winds interaction with the surrounding star. The magnetar drives a relativistic magnetized wind into a cavity created by the outgoing SN shock. For high spindown powers (~ 10^{51}-10^{52} ergs/s), the magnetar wind is super-fast at almost all latitudes, while for lower spindown powers (~ 10^{50} erg/s), the wind is sub-fast but still super-Alfvenic. In all cases, the rates at which the neutron star loses mass, angular momentum, and energy are very similar to the corresponding free wind values (<~ 30% differences), in spite of the causal contact between the neutron star and the stellar envelope. In addition, in all cases that we consider, the magnetar drives a collimated (~5-10 deg.) relativistic jet out along the rotation axis of the star. Nearly all of the spindown power of the neutron star escapes via this polar jet, rather than being transferred to the more spherical SN explosion. The properties of this relativistic jet and its expected late-time evolution in the magnetar model are broadly consistent with observations of long duration gamma-ray bursts (GRBs) and their associated broad-lined Type Ic SN.