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Supernovae (SNe) are stellar explosions driven by gravitational or thermonuclear energy, observed as electromagnetic radiation emitted over weeks or more. In all known SNe, this radiation comes from internal energy deposited in the outflowing ejecta by either radioactive decay of freshly-synthesized elements (typically 56Ni), stored heat deposited by the explosion shock in the envelope of a supergiant star, or interaction between the SN debris and slowly-moving, hydrogen-rich circumstellar material. Here we report on a new class of luminous SNe whose observed properties cannot be explained by any of these known processes. These include four new SNe we have discovered, and two previously unexplained events (SN 2005ap; SCP 06F6) that we can now identify as members. These SNe are all ~10 times brighter than SNe Ia, do not show any trace of hydrogen, emit significant ultra-violet (UV) flux for extended periods of time, and have late-time decay rates which are inconsistent with radioactivity. Our data require that the observed radiation is emitted by hydrogen-free material distributed over a large radius (~10^15 cm) and expanding at high velocities (>10^4 km s^-1). These long-lived, UV-luminous events can be observed out to redshifts z>4 and offer an excellent opportunity to study star formation in, and the interstellar medium of, primitive distant galaxies.
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A number of Type I (hydrogenless) superluminous supernova (SLSN) events have been discovered recently. However, their nature remains debatable. One of the most promising ideas is the shock-interaction mechanism, but only simplified semi-analytical models have been applied so far. We simulate light curves for several Type I SLSN (SLSN-I) models enshrouded by dense, non-hydrogen circumstellar envelopes, using a multi-group radiation hydrodynamics code that predicts not only bolometric, but also multicolor light curves. We demonstrate that the bulk of SLSNe-I including those with relatively narrow light curves like SN 2010gx or broad ones like PTF09cnd can be explained by the interaction of the SN ejecta with he CS envelope, though the range of parameters for these models is rather wide. Moderate explosion energy ($sim (2 - 4)cdot 10^{51}$ ergs) is sufficient to explain both narrow and broad SLSN-I light curves, but ejected mass and envelope mass differ for those two cases. Only 5 to 10 $M_odot$ of non-hydrogen material is needed to reproduce the light curve of SN 2010gx, while the best model for PTF09cnd is very massive: it contains almost $ 50 M_odot $ in the CS envelope and only $ 5 M_odot $ in the ejecta. The CS envelope for each case extends from 10 $R_odot$ to $sim 10^5R_odot$ ($7cdot 10^{15} $ cm), which is about an order of magnitude larger than typical photospheric radii of standard SNe near the maximum light. We briefly discuss possible ways to form such unusual envelopes.
We present observations and analysis of PS1-10bzj, a superluminous supernova (SLSN) discovered in the Pan-STARRS Medium Deep Survey at a redshift z = 0.650. Spectroscopically, PS1-10bzj is similar to the hydrogen-poor SLSNe 2005ap and SCP 06F6, though with a steeper rise and lower peak luminosity (M_bol = -21.4 mag) than previous events. We construct a bolometric light curve, and show that while PS1-10bzjs energetics were less extreme than previous events, its luminosity still cannot be explained by radioactive nickel decay alone. We explore both a magnetar spin-down and circumstellar interaction scenario and find that either can fit the data. PS1-10bzj is located in the Extended Chandra Deep Field South and the host galaxy is imaged in a number of surveys, including with the Hubble Space Telescope. The host is a compact dwarf galaxy (M_B ~ -18 mag, diameter < 800 pc), with a low stellar mass (M_* ~ 2.4 * 10^7 M_sun), young stellar population (tau_* ~ 5 Myr), and a star formation rate of ~ 2-3 M_sun/yr. The specific star formation rate is the highest seen in a SLSN host so far (~ 100 Gyr^{-1}). We detect the [O III]lambda 4363 line, and find a low metallicity: 12+(O/H) = 7.8 +/- 0.2 (~ 0.1 Z_sun). Together, this indicates that at least some of the progenitors of SLSNe come from young, low-metallicity populations.
We present the light curves of the hydrogen-poor superluminous supernovae (SLSNe-I) PTF12dam and iPTF13dcc, discovered by the (intermediate) Palomar Transient Factory. Both show excess emission at early times and a slowly declining light curve at late times. The early bump in PTF12dam is very similar in duration (~10 days) and brightness relative to the main peak (2-3 mag fainter) compared to those observed in other SLSNe-I. In contrast, the long-duration (>30 days) early excess emission in iPTF13dcc, whose brightness competes with that of the main peak, appears to be of a different nature. We construct bolometric light curves for both targets, and fit a variety of light-curve models to both the early bump and main peak in an attempt to understand the nature of these explosions. Even though the slope of the late-time light-curve decline in both SLSNe is suggestively close to that expected from the radioactive decay of $^{56}$Ni and $^{56}$Co, the amount of nickel required to power the full light curves is too large considering the estimated ejecta mass. The magnetar model including an increasing escape fraction provides a reasonable description of the PTF12dam observations. However, neither the basic nor the double-peaked magnetar model is capable of reproducing the iPTF13dcc light curve. A model combining a shock breakout in an extended envelope with late-time magnetar energy injection provides a reasonable fit to the iPTF13dcc observations. Finally, we find that the light curves of both PTF12dam and iPTF13dcc can be adequately fit with the circumstellar medium (CSM) interaction model.
We investigate the light-curve properties of a sample of 26 spectroscopically confirmed hydrogen-poor superluminous supernovae (SLSNe-I) in the Palomar Transient Factory (PTF) survey. These events are brighter than SNe Ib/c and SNe Ic-BL, on average, by about 4 and 2~mag, respectively. The peak absolute magnitudes of SLSNe-I in rest-frame $g$ band span $-22lesssim M_g lesssim-20$~mag, and these peaks are not powered by radioactive $^{56}$Ni, unless strong asymmetries are at play. The rise timescales are longer for SLSNe than for normal SNe Ib/c, by roughly 10 days, for events with similar decay times. Thus, SLSNe-I can be considered as a separate population based on photometric properties. After peak, SLSNe-I decay with a wide range of slopes, with no obvious gap between rapidly declining and slowly declining events. The latter events show more irregularities (bumps) in the light curves at all times. At late times, the SLSN-I light curves slow down and cluster around the $^{56}$Co radioactive decay rate. Powering the late-time light curves with radioactive decay would require between 1 and 10${rm M}_odot$ of Ni masses. Alternatively, a simple magnetar model can reasonably fit the majority of SLSNe-I light curves, with four exceptions, and can mimic the radioactive decay of $^{56}$Co, up to $sim400$ days from explosion. The resulting spin values do not correlate with the host-galaxy metallicities. Finally, the analysis of our sample cannot strengthen the case for using SLSNe-I for cosmology.
SN2017egm is the closest (z=0.03) H-poor superluminous supernova (SLSN-I) detected to date, and a rare example of an SLSN-I in a massive and metal-rich galaxy. Here we present the HST UV & optical spectra covering (1000 - 5500)A taken at +3 day relative to the peak. Our data reveal two sets of absorption systems, separated by 235 km/s, at redshifts matching the host galaxy, NGC3191 and its companion galaxy 73 arcsec apart. Weakly damped Lyman-alpha absorption lines are detected at these two redshifts, with HI column densities of $(3.0pm0.8)times10^{19}$ and $(3.7pm0.9)times10^{19}$,cm$^{-2}$ respectively. This is an order of magnitude smaller than HI column densities in the disks of nearby galaxies ($>10^{10}M_odot$) and suggests that SN2017egm is on the near side of NGC3191 and has a low host extinction (E(B-V)=0.007). Using unsaturated metal absorption lines and taking into account of H ionization and dust depletion corrections, we find that the host of SN2017egm probably has a solar or higher metallicity and is unlikely to be a dwarf companion to NGC3191. Comparison of early-time UV spectra of SN2017egm, Gaia16apd, iPTF13ajg and PTF12dam finds that the continuum at wavelength > 2800A is well fit by a blackbody, whereas the continuum at wavelength < 2800A is considerably below the model. The degree of UV suppression varies from source to source, with the 1400A to 2800A continuum flux ratio of 1.5 for Gaia16apd and 0.4 for iPTF13ajg. This can not be explained by the differences in magnetar power or blackbody temperature (i.e. color temperature). Finally, the UV spectra reveal a common set of seven broad absorption features and their equivalent widths are similar (within a factor of 2) among the four events. These seven features bode well for future high-z SLSN-I spectral classifications.
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