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

Systematic investigation of the fallback accretion powered model for hydrogen-poor superluminous supernovae

The energy liberated by fallback accretion has been suggested as a possible engine to power hydrogen-poor superluminous supernovae. We systematically investigate this model using the Bayesian light-curve fitting code MOSFiT (Modular Open Source Fitter for Transients), fitting the light curves of 37 hydrogen-poor superluminous supernovae assuming a fallback accretion central engine. We find that this model can yield good fits to their light curves, with a fit quality that rivals the popular magnetar engine models. Examining our derived parameters for the fallback model, we find the total energy requirements from the accretion disk are estimated to be 0.002 - 0.7 Msun c^2. If we adopt a typical conversion efficiency ~ 1e-3, the required mass to accrete is thus 2 - 700 Msun. Many superluminous supernovae, therefore, require an unrealistic accretion mass, and so only a fraction of these events could be powered by fallback accretion unless the true efficiency is much greater than our fiducial value. The superluminous supernovae that require the smallest amounts of fallback mass still remain to be the fallback accretion powered supernova candidates, but they are difficult to be distinguished solely by their light curve properties.Comment: 12 pages, 8 figures, 3 tables, accepted by The Astrophysical Journa

The luminosity function of TDEs from fallback-powered emission: implications for the black hole mass function

Tidal disruption events (TDEs), in which a star is destroyed by the gravitational field of a supermassive black hole (SMBH), are being observed at a high rate owing to the advanced state of survey science. One of the properties of TDEs that is measured with increasing statistical reliability is the TDE luminosity function, $d\dot{N}_{\rm TDE}/dL$, which is the TDE rate per luminosity (i.e., how many TDEs are within a given luminosity range). Here we show that if the luminous emission from a TDE is directly coupled to the rate of return of tidally destroyed debris to the SMBH, then the TDE luminosity function is in good agreement with observations and scales as $\propto L^{-2.5}$ for high luminosities, provided that the SMBH mass function $dN_{\bullet}/dM_{\bullet}$ -- the number of SMBHs ($N_{\bullet}$) per SMBH mass ($M_{\bullet}$) -- is approximately flat in the mass range over which we observe TDEs. We also show that there is a cutoff in the luminosity function at low luminosities that is a result of direct captures, and this cutoff has been tentatively observed. If $dN_{\bullet}/dM_{\bullet}$ is flat, which is in agreement with some observational campaigns, these results suggest that the fallback rate feeds the accretion rate in TDEs. Contrarily, if $dN_{\bullet}/d\log M_{\bullet}$ is flat, which has been found theoretically and is suggested by other observational investigations, then the emission from TDEs is likely powered by another mechanism. Future observations and more TDE statistics, provided by the Rubin Observatory/LSST, will provide additional evidence as to the reality of this tension.Comment: 7 pages, 1 figure, ApJL accepte

LSST Cadence Strategy Evaluations for AGN Time-series Data in Wide-Fast-Deep Field

Machine learning is a promising tool to reconstruct time-series phenomena, such as variability of active galactic nuclei (AGN), from sparsely-sampled data. Here we use three Continuous Auto-Regressive Moving Average (CARMA) representations of AGN variability -- the Damped Random Walk (DRW) and (over/under-)Damped Harmonic Oscillator (DHO) -- to simulate 10-year AGN light curves as they would appear in the upcoming Vera Rubin Observatory Legacy Survey of Space and Time (LSST), and provide a public tool to generate these for any survey cadence. We investigate the impact on AGN science of five proposed cadence strategies for LSST's primary Wide-Fast-Deep (WFD) survey. We apply for the first time in astronomy a novel Stochastic Recurrent Neural Network (SRNN) algorithm to reconstruct input light curves from the simulated LSST data, and provide a metric to evaluate how well SRNN can help recover the underlying CARMA parameters. We find that the light curve reconstruction is most sensitive to the duration of gaps between observing season, and that of the proposed cadences, those that change the balance between filters, or avoid having long gaps in the {g}-band perform better. Overall, SRNN is a promising means to reconstruct densely sampled AGN light curves and recover the long-term Structure Function of the DRW process (SF$_\infty$) reasonably well. However, we find that for all cadences, CARMA/SRNN models struggle to recover the decorrelation timescale ($\tau$) due to the long gaps in survey observations. This may indicate a major limitation in using LSST WFD data for AGN variability science.Comment: accepted by MNRA

The GRB-SLSN Connection: mis-aligned magnetars, weak jet emergence, and observational signatures

Multiple observational lines of evidence support a connection between hydrogen-poor superluminous supernovae (SLSNe) and long duration gamma-ray bursts (GRBs). Both events require a powerful central energy source, usually attributed to a millisecond magnetar or an accreting black hole. The GRB-SLSN link raises several theoretical questions: What distinguishes the engines responsible for these different phenomena? Can a single engine power both a GRB and a luminous SN in the same event? We propose a new unifying model for magnetar thermalization and jet formation: misalignment between the rotation (${\bf \Omega}$) and magnetic dipole (${\bf \mu}$) axes thermalizes a fraction of the spindown power by reconnection in the striped equatorial wind, providing a guaranteed source of "thermal" emission to power the supernova. The remaining un-thermalized power energizes a relativistic jet. In this picture, the GRB-SLSN dichotomy is directly linked to ${\bf \Omega \cdot \mu}$. We extend earlier work to show that even weak relativistic jets of luminosity $\sim10^{46}$ erg s$^{-1}$ can escape the expanding SN ejecta hours after the explosion, implying that escaping relativistic jets may accompany many SLSNe. We calculate the observational signature of these jets. We show that they may produce transient UV cocoon emission lasting a few hours when the jet breaks out of the ejecta surface. A longer-lived optical/UV signal may originate from a mildly-relativistic wind driven from the interface between the jet and the ejecta walls. This provides a new explanation for the secondary early-time maximum observed in some SLSNe light curves, such as LSQ14bdq. This scenario also predicts a population of GRB from on-axis jets with extremely long durations, potentially similar to the population of "jetted tidal disruption events", in coincidence with a small subset of SLSNe.Comment: 17 pages, 7 figures, submitted to MNRA

Unveiling the Engines of Fast Radio Bursts, Super-Luminous Supernovae, and Gamma-Ray Bursts

Young, rapidly spinning magnetars are invoked as central engines behind a diverse set of transient astrophysical phenomena, including gamma-ray bursts (GRB), super-luminous supernovae (SLSNe), fast radio bursts (FRB), and binary neutron star (NS) mergers. However, a barrier to direct confirmation of the magnetar hypothesis is the challenge of directly observing non-thermal emission from the central engine at early times (when it is most powerful and thus detectable) due to the dense surrounding ejecta. We present CLOUDY calculations of the time-dependent evolution of the temperature and ionization structure of expanding supernova or merger ejecta due to photo-ionization by a magnetar engine, in order to study the escape of X-rays (absorbed by neutral gas) and radio waves (absorbed by ionized gas), as well as to assess the evolution of the local dispersion measure due to photo-ionization. We find that ionization breakout does not occur if the engine's ionizing luminosity decays rapidly, and that X-rays typically escape the oxygen-rich ejecta of SLSNe only on $\sim 100 \, {\rm yr}$ timescales, consistent with current X-ray non-detections. We apply these results to constrain engine-driven models for the binary NS merger GW170817 and the luminous transient ASASSN-15lh. In terms of radio transparency and dispersion measure constraints, the repeating FRB 121102 is consistent with originating from a young, $\gtrsim 30-100 \, {\rm yr}$, magnetar similar to those inferred to power SLSNe. We further show that its high rotation measure can be produced within the same nebula that is proposed to power the quiescent radio source observed co-located with FRB 121102. Our results strengthen previous work suggesting that at least some FRBs may be produced by young magnetars, and motivate further study of engine powered transients.Comment: submitted to MNRAS; comments welcom

Mechanisms for high spin in black-hole neutron-star binaries and kilonova emission: inheritance and accretion

Black-hole neutron-star binary mergers, whose existence has been confirmed by gravitational-wave detectors, can lead to an electromagnetic counterpart called a kilonova if the neutron star is disrupted prior to merger. The observability of a kilonova depends crucially on the amount of neutron star ejecta, which is sensitive to the aligned component of the black hole spin. These binaries likely originate from the evolution of isolated stellar binaries. We explore the dependence of the ejected mass on two main mechanisms that provide high black hole spin. When the black hole inherits a high spin from a Wolf-Rayet star that was born with least $\sim 10\%$ of its breakup spin under weak stellar core-envelope coupling, which is relevant for all formation pathways, the median of the ejected mass is $\gtrsim 10^{-2}$ M$_{\odot}$. Though only possible for certain formation pathways, similarly large ejected mass results when the black hole accretes $\gtrsim 20\%$ of its companion's envelope to gain a high spin, and a more massive stellar progenitor provides smaller ejected mass compared to when the black hole inherits high spin. Together, these signatures suggest that a population analysis of black hole masses and spins in black-hole neutron-star binary mergers may help distinguish between mechanisms for spin and possible formation pathways. Using a novel kilonova light curve model we show that current capabilities are unlikely to observe a counterpart, however future facilities such as the Vera Rubin Observatory will likely detect counterparts even if the aligned dimensionless spin of the disrupting black hole is as low as $\sim 0.2$. Our model predicts kilonovae as bright as $M_i \sim -14.5$ for an aligned black hole spin of $\sim 0.9$

Superluminous supernovae in LSST:rates, detection metrics, and light-curve modeling

We explore and demonstrate the capabilities of LSST to study Type I superluminous supernovae (SLSNe). We first fit the light curves of 58 known SLSNe at z~0.1-1.6, using an analytical magnetar spin-down model implemented in MOSFiT. We then use the posterior distributions of the magnetar and ejecta parameters to generate thousands of synthetic SLSN light curves, and we inject those into the OpSim to generate realistic ugrizy light curves. We define simple, measurable metrics to quantify the detectability and utility of the light curve, and to measure the efficiency of LSST in returning SLSN light curves satisfying these metrics. We combine the metric efficiencies with the volumetric rate of SLSNe to estimate the overall discovery rate of LSST, and we find that ~10^4 SLSNe per year with >10 data points will be discovered in the WFD survey at z<3.0, while only ~15 SLSNe per year will be discovered in each DDF at z<4.0. To evaluate the information content in the LSST data, we refit representative output light curves with the same model that was used to generate them. We correlate our ability to recover magnetar and ejecta parameters with the simple light curve metrics to evaluate the most important metrics. We find that we can recover physical parameters to within 30% of their true values from ~18% of WFD light curves. Light curves with measurements of both the rise and decline in gri-bands, and those with at least fifty observations in all bands combined, are most information rich, with ~30% of these light curves having recoverable physical parameters to ~30% accuracy. WFD survey strategies which increase cadence in these bands and minimize seasonal gaps will maximize the number of scientifically useful SLSN light curves. Finally, although the DDFs will provide more densely sampled light curves, we expect only ~50 SLSNe with recoverable parameters in each field in the decade-long survey.Comment: 13 pages, 11 figures, submitted to Ap

Metallicity beats sSFR: The connection between superluminous supernova host galaxy environments and the importance of metallicity for their production

We analyse 33 Type I superluminous supernovae (SLSNe) taken from ZTF's Bright Transient Survey to investigate the local environments of their host galaxies. We use a spectroscopic sample of galaxies from the SDSS to determine the large-scale environmental density of the host galaxy. Noting that SLSNe are generally found in galaxies with low stellar masses, high star formation rates, and low metallicities, we find that SLSN hosts are also rarely found within high-density environments. Only $3\substack{+9 \\-1}$ per cent of SLSN hosts were found in regions with 2 or more bright galaxies within 2 Mpc. For comparison, we generate a sample of 662 SDSS galaxies matched to the photometric properties of the SLSN hosts. This sample is also rarely found within high-density environments, suggesting that galaxies with properties required for SLSN production favour more isolated environments. Furthermore, we select galaxies within the Illustris-TNG simulation to match SLSN host galaxy properties in colour and stellar mass. We find that the fraction of simulated galaxies in high-density environments quantitatively matches the observed SLSN hosts only if we restrict to simulated galaxies with metallicity $12+\log($O/H$) \leq 8.12$. In contrast, limiting to only the highest sSFR galaxies in the sample leads to an overabundance of SLSN hosts in high-density environments. Thus, our measurement of the environmental density of SLSN host galaxies appears to break the degeneracy between low-metallicity or high-sSFR as the driver for SLSN hosts and provides evidence that the most constraining factor on SLSN production is low-metallicity.Comment: Accepted to MNRAS, 10 pages, 6 figures. Re-uploaded for updating reference

The superluminous supernova SN 2017egm in the nearby galaxy NGC 3191: a metal-rich environment can support a typical SLSN evolution

At redshift z=0.03, the recently-discovered SN 2017egm is the nearest Type I superluminous supernova (SLSN) to date, and first near the center of a massive spiral galaxy (NGC 3191). Using SDSS spectra of NGC 3191, we find a metallicity ~2 Z$_\odot$ at the nucleus and ~1.3 Z$_\odot$ for a star forming region at a radial offset similar to SN 2017egm. Archival radio-to-UV photometry reveals a star-formation rate ~15 M$_\odot$ yr$^{-1}$ (with ~70% dust-obscured), which can account for a Swift X-ray detection, and stellar mass ~$10^{10.7}$ M$_\odot$. We model the early UV-optical light curves with a magnetar central-engine model, using the Bayesian light curve fitting tool MOSFiT. The fits indicate ejecta mass 2-4 M$_\odot$, spin period 4-6 ms, magnetic field (0.7-1.7)$\times 10^{14}$G, and kinetic energy 1-2 $\times10^{51}$ erg. These parameters are consistent with the overall distributions for SLSNe, modeled by Nicholl et al (2017), although the derived mass and spin are towards the low end, possibly indicating enhanced loss of mass and angular momentum before explosion. This has two implications: (i) SLSNe can occur at solar metallicity, although with a low fraction ~10%; and (ii) metallicity has at most a modest effect on their properties. Both conclusions are in line with results for long gamma-ray bursts. Assuming a monotonic rise gives an explosion date MJD $57889\pm1$. However, a short-lived excess in the data relative to the best-fitting models may indicate an early-time `bump'. If confirmed, SN 2017egm would be the first SLSN with a spectrum during the bump-phase; this shows the same O II lines seen at maximum light, which may be an important clue for explaining these bumps.Comment: Accepted for publication in ApJ