How Common are Engines in Ib/c Supernovae?

The association of γ-ray bursts (GRBs) and core-collapse supernovae (SNe) of Type Ib and Ic was motivated by the detection of SN 1998bw in the error box of GRB980425 and the now-secure identification of a SN 1998bw-like event in the cosmological GRB030329. The bright radio emission from SN 1998bw indicated that it possessed some of the unique attributes expected of GRBs, namely a large reservoir of energy in (mildly) relativistic ejecta and variable energy input. Here we discuss the results of a systematic program of radio observations of most reported Type Ib/c SNe accessible to the Very Large Array, designed to determine the fraction of Type Ib/c SNe driven by an engine. We conclude that: (i) the incidence of such events is low, < 3%, and (ii) there appears to be a clear dichotomy between the majority of hydrodynamic explosions (SNe) and engine-driven explosions (GRBs)

The magnetar model for Type I superluminous supernovae I: Bayesian analysis of the full multicolour light curve sample with MOSFiT

We use the new Modular Open Source Fitter for Transients (MOSFiT) to model 38 hydrogen-poor superluminous supernovae (SLSNe). We fit their multicolour light curves with a magnetar spin-down model and present the posterior distributions of magnetar and ejecta parameters. The colour evolution can be well matched with a simple absorbed blackbody. We find the following medians (1$\sigma$ ranges): spin period 2.4 ms (1.2-4 ms); magnetic field $0.8\times 10^{14}$ G (0.2-1.8 $\times 10^{14}$ G); ejecta mass 4.8 Msun (2.2-12.9 Msun); kinetic energy $3.9\times 10^{51}$ erg (1.9-9.8 $\times 10^{51}$ erg). This significantly narrows the parameter space compared to our priors, showing that although the model is flexible, the parameter space relevant to SLSNe is well constrained by existing data. The requirement that the instantaneous engine power is $\sim 10^{44}$ erg at the light curve peak necessitates either a large rotational energy (P<2 ms), or more commonly that the spin-down and diffusion timescales be well-matched. We find no evidence for separate populations of fast- and slow-declining SLSNe, which instead form a continuum both in light curve widths and inferred parameters. Variations in the spectra are well explained through differences in spin-down power and photospheric radii at maximum-light. We find no correlations between any model parameters and the properties of SLSN host galaxies. Comparing our posteriors to stellar evolution models, we show that SLSNe require rapidly rotating (fastest 10%) massive stars (> 20 Msun), and that this is consistent with the observed SLSN rate. High mass, low metallicity, and likely binary interaction all serve to maintain rapid rotation essential for magnetar formation. By reproducing the full set of SLSN light curves, our posteriors can be used to inform photometric searches for SLSNe in future survey data

Exploring the Galaxy Mass-Metallicity Relations at z ~ 3-5

Long-duration gamma-ray bursts (GRBs) provide a premier tool for studying high-redshift star-forming galaxies thanks to their extreme brightness and association with massive stars. Here we use GRBs to study the galaxy stellar mass-metallicity (M_*-Z) relation at z ~ 3-5, where conventional direct metallicity measurements are extremely challenging. We use the interstellar medium metallicities of long GRB hosts derived from afterglow absorption spectroscopy, in conjunction with host galaxy stellar masses determined from deep Spitzer 3.6 μm observations of 20 GRB hosts. We detect about 1/4 of the hosts with M_(AB)(I) ≈ –21.5 to –22.5 mag and place a limit of M_(AB)(I) ≳ –19 mag on the remaining hosts from a stacking analysis. Using these observations, we present the first rest-frame optical luminosity distribution of long GRB hosts at z ≳ 3 and find that it is similar to the distribution of long GRB hosts at z ~ 1. In comparison to Lyman-break galaxies at the same redshift, GRB hosts are generally fainter, but the sample is too small to rule out an overall similar luminosity function. On the other hand, the GRB hosts appear to be more luminous than the population of Lyα emitters at z ~ 3-4. Using a conservative range of mass-to-light ratios for simple stellar populations (with ages of 70 Myr to ~2 Gyr), we infer the host stellar masses and present mass-metallicity measurements at z ~ 3-5 ((z) ≈ 3.5). We find that the detected GRB hosts, with M_* ≈ 2 × 10^(10) M_☉, display a wide range of metallicities, but that the mean metallicity at this mass scale, Z ≈ 0.3 Z_☉, is lower than measurements at z ≾ 3. Combined with stacking of the non-detected hosts with M_* ≾ 3 × 10^9 M_☉ and Z ≾ 0.1 Z_☉, we find tentative evidence for the existence of an M_*-Z relation at z ~ 3.5 and continued evolution of this relation to systematically lower metallicities from z ~ 2