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Milisavljevic, Danny

Super-luminous supernovae that radiate more than 10(^{44}) ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1–4. Some evolve slowly, resembling models of ‘pair-instability’ supernovae. Such models involve stars with original masses 140–260 times that of the Sun that now have carbon–oxygen cores of 65–130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron–positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of (^{56})Ni are synthesized; this isotope decays to (^{56})Fe via (^{56})Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10–16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10(^{−6}) times that of the core-collapse rate.

We present the Pan-STARRS1 discovery of PS1-10afx, a unique hydrogen-deficient superluminous supernova (SLSN) at redshift z = 1.388. The light curve peaked at z P1 = 21.7 mag, making PS1-10afx comparable to the most luminous known SNe, with Mu = –22.3 mag. Our extensive optical and near-infrared observations indicate that the bolometric light curve of PS1-10afx rose on the unusually fast timescale of ~12 days to the extraordinary peak luminosity of 4.1 × 1044 erg s–1 (M bol = –22.8 mag) and subsequently faded rapidly. Equally important, the spectral energy distribution is unusually red for an SLSN, with a color temperature of ~6800 K near maximum light, in contrast to previous hydrogen-poor SLSNe, which are bright in the ultraviolet (UV). The spectra more closely resemble those of a normal SN Ic than any known SLSN, with a photospheric velocity of ~11, 000 km s–1 and evidence for line blanketing in the rest-frame UV. Despite the fast rise, these parameters imply a very large emitting radius (gsim 5 × 1015 cm). We demonstrate that no existing theoretical model can satisfactorily explain this combination of properties: (1) a nickel-powered light curve cannot match the combination of high peak luminosity with the fast timescale; (2) models powered by the spindown energy of a rapidly rotating magnetar predict significantly hotter and faster ejecta; and (3) models invoking shock breakout through a dense circumstellar medium cannot explain the observed spectra or color evolution. The host galaxy is well detected in pre-explosion imaging with a luminosity near L*, a star formation rate of ~15 M ☉ yr–1, and is fairly massive (~2 × 1010 M ☉), with a stellar population age of ~108 yr, also in contrast to the young dwarf hosts of known hydrogen-poor SLSNe. PS1-10afx is distinct from known examples of SLSNe in its spectra, colors, light-curve shape, and host galaxy properties, suggesting that it resulted from a different channel than other hydrogen-poor SLSNe.

We present the detection of persistent soft X-ray radiation with ${L}_{x}\sim {10}^{41}$–1042 erg s–1 at the location of the extremely luminous, double-humped transient ASASSN-15lh as revealed by Chandra and Swift. We interpret this finding in the context of observations from our multiwavelength campaign, which revealed the presence of weak narrow nebular emission features from the host-galaxy nucleus and clear differences with respect to superluminous supernova optical spectra. Significant UV flux variability on short timescales detected at the time of the rebrightening disfavors the shock interaction scenario as the source of energy powering the long-lived UV emission, while deep radio limits exclude the presence of relativistic jets propagating into a low-density environment. We propose a model where the extreme luminosity and double-peaked temporal structure of ASASSN-15lh is powered by a central source of ionizing radiation that produces a sudden change in the ejecta opacity at later times. As a result, UV radiation can more easily escape, producing the second bump in the light curve. We discuss different interpretations for the intrinsic nature of the ionizing source. We conclude that, if the X-ray source is physically associated with the optical–UV transient, then ASASSN-15lh most likely represents the tidal disruption of a main-sequence star by the most massive spinning black hole detected to date. In this case, ASASSN-15lh and similar events discovered in the future would constitute the most direct probes of very massive, dormant, spinning, supermassive black holes in galaxies. Future monitoring of the X-rays may allow us to distinguish between the supernova hypothesis and the hypothesis of a tidal disruption event.

We present optical observations of supernova SN 2014C, which underwent an unprecedented slow metamorphosis from H-poor type Ib to H-rich type IIn over the course of one year. The observed spectroscopic evolution is consistent with the supernova having exploded in a cavity before encountering a massive shell of the progenitor star's stripped hydrogen envelope. Possible origins for the circumstellar shell include a brief Wolf–Rayet fast wind phase that overtook a slower red supergiant wind, eruptive ejection, or confinement of circumstellar material by external influences of neighboring stars. An extended high velocity Hα absorption feature seen in near-maximum light spectra implies that the progenitor star was not completely stripped of hydrogen at the time of core collapse. Archival pre-explosion Subaru Telescope Suprime-Cam and Hubble Space Telescope Wide Field Planetary Camera 2 images of the region obtained in 2009 show a coincident source that is most likely a compact massive star cluster in NGC 7331 that hosted the progenitor system. By comparing the emission properties of the source with stellar population models that incorporate interacting binary stars we estimate the age of the host cluster to be 30–300 Myr, and favor ages closer to 30 Myr in light of relatively strong Hα emission. SN 2014C is the best observed member of a class of core-collapse supernovae that fill the gap between events that interact strongly with dense, nearby environments immediately after explosion and those that never show signs of interaction. Better understanding of the frequency and nature of this intermediate population can contribute valuable information about the poorly understood final stages of stellar evolution.

We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor superluminous supernova (SLSN) at redshift z = 0.5215 discovered in the last months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out by its extremely slow evolution, with an observed rise of ∼ > 125 restframe days, and exponential decline out to ∼ 250 days past peak at a measured rate of 0.01 mag day−1, consistent with fully-trapped 56Co decay. This is the longest rise time measured in a SLSN to date, and the first SLSN to show a rise time consistent with pair-instability supernova (PISN) models. Compared to other slowly-evolving SLSNe, it is spectroscopically similar to the prototype SN 2007bi at maximum light, though lower in luminosity (Lpeak ≃ 4.6×1043ergs−1) and with a flatter peak than previous events. PS1-14bj shows a number of peculiar properties, including a near-constant color temperature for > 200 days past peak, and strong emission lines from [O III] λ5007 and [O III] λ4363 with a velocity width of ∼3400 km s−1 in its late-time spectra. These both suggest there is a sustained source of heating over very long timescales, and are incompatible with a simple 56Ni-powered/PISN interpretation. A modified magnetar model including emission leakage at late times can reproduce the light curve, in which case the blue continuum and [O III] features are interpreted as material heated and ionized by the inner pulsar wind nebula becoming visible at late times. Alternatively, the late-time heating could be due to interaction with a shell of H-poor circumstellar material.

We present the first spectroscopic study of the host environments of Type Ibc supernovae (SN Ibc) discovered exclusively by untargeted SN searches. Past studies of SN Ibc host environments have been biased towards high-mass, high-metallicity galaxies by focusing on SNe discovered in galaxytargeted SN searches. Our new observations more than double the total number of spectroscopic stellar population age and metallicity measurements published for untargeted SN Ibc host environments. For the 12 SNe Ib and 21 SNe Ic in our metallicity sample, we find median metallicities of 0.62 Z⊙ and 0.83 Z⊙, respectively, but determine that the discrepancy in the full distribution of metallicities is not statistically significant. This median difference would correspond to only a small difference in the mass loss via metal-line driven winds (. 30%), suggesting this does not play the dominant role in distinguishing SN Ib and Ic progenitors. However, the median metallicity of the 7 broad-lined SN Ic (SN Ic-BL) in our sample is significantly lower, 0.45 Z⊙. The age of the young stellar population of SN Ic-BL host environments also seems to be lower than for SN Ib and Ic, but our age sample is small. Combining all SN Ibc host environment spectroscopy from the literature to date does not reveal a significant difference in SN Ib and Ic metallicities, but reinforces the significance of the lower metallicities for SN Ic-BL. This combined sample demonstrates that galaxy-targeted SN searches introduce a significant bias for studies seeking to infer the metallicity distribution of SN progenitors, and we identify and discuss other systematic effects that play smaller roles. We discuss the path forward for making progress on SN Ibc progenitor studies in the LSST era. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

A significant fraction (~30%) of the γ-ray sources listed in the second Fermi/LAT (2FGL) catalog is still of unknown origin, being not yet associated with counterparts at lower energies. Using the available information at lower energies and optical spectroscopy on the selected counterparts of these γ-ray objects, we can pinpoint their exact nature. Here, we present a pilot project pointing to assess the effectiveness of the several classification methods developed to select γ-ray blazar candidates. To this end, we report optical spectroscopic observations of a sample of five γ-ray blazar candidates selected on the basis of their infrared Wide-field Infrared Survey Explorer (WISE) colors or of their low-frequency radio properties. Blazars come in two main classes, BL Lac objects and FSRQs, showing similar optical spectra except for the stronger emission lines of the latter. For three of our sources, the almost featureless optical spectra obtained confirm their BL Lac nature, while for the source WISEJ022051.24+250927.6 we observe emission lines with equivalent width EW ~ 31 Å, identifying it as a FSRQ with z = 0.48. The source WISEJ064459.38+603131.7, although not featuring a clear radio counterpart, shows a blazar-like spectrum with weak emission lines with EW ~ 7 Å, yielding a redshift estimate of z = 0.36. In addition, we report optical spectroscopic observations of four WISE sources associated with known γ-ray blazars without a firm classification or redshift estimate. For the latter sources, we confirm a BL Lac classification, with a tentative redshift estimate for the source WISEJ100800.81+062121.2 of z = 0.65.

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 (Mbol ≃ −21.4 mag) than previous events. We construct a bolometric light curve, and show that while PS1-10bzj’s 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 (MB ≈ −18 mag, diameter. 800 pc), with a low stellar mass (M∗ ≈ 2.4 × 107 M⊙), young stellar population (τ∗ ≈ 5 Myr), and a star formation rate of ∼ 2 − 3 M⊙ yr−1 . The specific star formation rate is the highest seen in an SLSN host so far (∼ 100 Gyr−1). We detect the [O III] λ4363 line, and find a low metallicity: 12+(O/H) = 7.8 ± 0.2 (≃ 0.1Z⊙). Together, this indicates that at least some of the progenitors of SLSNe come from young, low-metallicity populations.

We present optical and near-infrared observations of SN 2012au, a slow-evolving supernova (SN) with properties that suggest a link between subsets of energetic and H-poor SNe and superluminous SNe. SN 2012au exhibited conspicuous SN Ib-like He I lines and other absorption features at velocities reaching ≈ 2×104 km s−1 in its early spectra, and a broad light curve that peaked at MB = −18.1 mag. Models of these data indicate a large explosion kinetic energy of ∼ 1052 erg and 56Ni mass ejection of MNi ≈ 0.3M⊙ on par with SN 1998bw. SN 2012au’s spectra almost one year after explosion show a blend of persistent Fe II P-Cyg absorptions and nebular emissions originating from two distinct velocity regions. These late-time emissions include strong [Fe II], [Ca II], [O I], Mg I], and Na I lines at velocities & 4500 km s−1, as well as O I and Mg I lines at noticeably smaller velocities . 2000 km s−1. Many of the late-time properties of SN 2012au are similar to the slow-evolving hypernovae SN 1997dq and SN 1997ef, and the superluminous SN 2007bi. Our observations suggest that a single explosion mechanism may unify all of these events that span −21 . MB . −17 mag. The aspherical and possibly jetted explosion was most likely initiated by the core collapse of a massive progenitor star and created substantial high-density, low-velocity Ni-rich material.

We report on our discovery and observations of the Pan-STARRS1 supernova (SN) PS1-12sk, a transient with properties that indicate atypical star formation in its host galaxy cluster or pose a challenge to popular progenitor system models for this class of explosion. The optical spectra of PS1- 12sk classify it as a Type Ibn SN (c.f. SN 2006jc), dominated by intermediate-width (3×103 km s−1) and time variable He I emission. Our multi-wavelength monitoring establishes the rise time dt ∼ 9 − 23 days and shows an NUV-NIR SED with temperature & 17 × 103 K and a peak magnitude of Mz = −18.88 ± 0.02 mag. SN Ibn spectroscopic properties are commonly interpreted as the signature of a massive star (17 − 100 M⊙) explosion within a He-enriched circumstellar medium. However, unlike previous Type Ibn supernovae, PS1-12sk is associated with an elliptical brightest cluster galaxy, CGCG 208-042 (z = 0.054) in cluster RXC J0844.9+4258. The expected probability of an event like PS1-12sk in such environments is low given the measured infrequency of core-collapse SNe in red sequence galaxies compounded by the low volumetric rate of SN Ibn. Furthermore, we find no evidence of star formation at the explosion site to sensitive limits (ΣHα . 2×10−3 M⊙ yr−1 kpc−2). We therefore discuss white dwarf binary systems as a possible progenitor channel for SNe Ibn. We conclude that PS1-12sk represents either a fortuitous and statistically unlikely discovery, evidence for a top-heavy IMF in galaxy cluster cooling flow filaments, or the first clue suggesting an alternate progenitor channel for Type Ibn SNe.