Long duration gamma-ray bursts (GRBs) mark 1 the explosive death of some massive stars and are a rare sub-class of Type Ibc supernovae (SNe Ibc). They are distinguished by the production of an energetic and collimated relativistic outflow powered 2 by a central engine (an accreting black hole or neutron star).Observationally, this outflow is manifested 3 in the pulse of gamma-rays and a long-lived radio afterglow. To date, central engine-driven SNe have been discovered exclusively through their gamma-ray emission, yet it is expected 4 that a larger population goes undetected due to limited satellite sensitivity or beaming of the collimated emission away from our line-of-sight. In this framework, 2 Soderberg et al.the recovery of undetected GRBs may be possible through radio searches 5,6 for SNe Ibc with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary Type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. The lack of a coincident GRB makes SN 2009bb the first engine-driven SN discovered without a detected gamma-ray signal. A comparison with our extensive radio survey of SNe Ibc reveals that the fraction harboring central engines is low, ∼ 1%, measured independently from, but consistent with, the inferred 46 rate of nearby GRBs. Our study demonstrates that upcoming optical and radio surveys will soon rival gamma-ray satellites in pinpointing the nearest engine-driven SNe.A similar result for a different supernova is reported 8 independently. A Relativistic SN 3Unlike the optical emission from SNe which traces only the slowest explosion debris, radio observations uniquely probe 35 the fastest ejecta as the expanding blastwave (velocity, v) shocks and accelerates electrons in amplified magnetic fields. The resulting synchrotron emission is suppressed by self-absorption (SSA) producing a low frequency radio turnover that defines the spectral peak frequency, ν p . Combining our observations from the VLA and the Giant Meterwave Radio Telescope (GMRT), the radio spectra of SN 2009bbare well described by an SSA model across multiple epochs ( Figure 2). From our earliest spectrum on Apr 8 UT (∆t ≈ 20 days), we infer ν p ≈ 6 GHz and a spectral peak luminosity,Making the conservative assumption that the energy of the radio emitting material is partitioned equally into accelerating electrons and amplifying magnetic fields (equipartition), the properties of the SSA radio spectrum enable 13,35 a robust estimate of the blastwave radius, R ≈ 2.9 × 10 16 (L ν,p /10 28 erg ssynchrotron sources with a low spectral peak frequency thus require larger sizes (Figure 3).For SN 2009bb, we infer R ≈ 4.4 × 10 16 cm at ∆t ≈ 20 days and thus the mean expansion velocity is R/∆t = 0.85 ± 0.02c, where c is the speed of light. The transverse expansion speed, Γβc = R/∆t indicates that the blastwave is relativistic, Γ 1.3, at this time [bulk Lorentz factor Γ = (1 − β 2 ) −1/2 with β = v/c]. This is a lower limit on the initial velocity since th...
We present multi-wavelength observations of SN 2014C during the first 500 days. These observations represent the first solid detection of a young extragalactic stripped-envelope SN out to high-energy X-rays ∼40 keV. SN 2014C shows ordinary explosion parameters (E k ∼1.8×10 51 erg and M ej ∼1.7 M e ). However, over an ∼1 year timescale, SN 2014C evolved from an ordinary hydrogen-poor supernova into a strongly interacting, hydrogen-rich supernova, violating the traditional classification scheme of type-I versus type-II SNe. Signatures of the SN shock interaction with a dense medium are observed across the spectrum, from radio to hard X-rays, and revealed the presence of a massive shell of ∼1 M e of hydrogen-rich material at ∼6×10 16 cm. The shell was ejected by the progenitor star in the decades to centuries before collapse. This result challenges current theories of massive star evolution, as it requires a physical mechanism responsible for the ejection of the deepest hydrogen layer of H-poor SN progenitors synchronized with the onset of stellar collapse. Theoretical investigations point at binary interactions and/or instabilities during the last nuclear burning stages as potential triggers of the highly time-dependent mass loss. We constrain these scenarios utilizing the sample of 183 SNe Ib/c with public radio observations. Our analysis identifies SN 2014C-like signatures in ∼10% of SNe. This fraction is reasonably consistent with the expectation from the theory of recent envelope ejection due to binary evolution if the ejected material can survive in the close environment for 10 3 -10 4 years. Alternatively, nuclear burning instabilities extending to core C-burning might play a critical role.
We present extensive radio and millimeter observations of the unusually bright GRB 130427A at z = 0.340, spanning 0.67 to 12 d after the burst. Taken in conjunction with detailed multi-band UV, optical, NIR, and X-ray observations we find that the broad-band afterglow emission is composed of distinct reverse shock and forward shock contributions. The reverse shock emission dominates in the radio/millimeter and at 0.1 d in the UV/optical/NIR, while the forward shock emission dominates in the X-rays and at 0.1 d in the UV/optical/NIR. We further find that the optical and X-ray data require a Wind circumburst environment, pointing to a massive star progenitor. Using the combined forward and reverse shock emission we find that the parameters of the burst are an isotropic kinetic energy of E K,iso ≈ 2 × 10 53 erg, a mass loss rate ofṀ ≈ 3 × 10 −8 M ⊙ yr −1 (for a wind velocity of 1, 000 km s −1 ), and a Lorentz factor at the deceleration time of Γ(200 s) ≈ 130. Due to the low density and large isotropic energy, the absence of a jet break to ≈ 15 d places only a weak constraint on the opening angle, θ j 2.5 • , and therefore a total energy of E γ + E K 1.2 × 10 51 erg, similar to other GRBs. The reverse shock emission is detectable in this burst due to the low circumburst density, which leads to a slow cooling shock. We speculate that this is a required property for the detectability of reverse shocks in the radio and millimeter bands. Following on GRB 130427A as a benchmark event, observations of future GRBs with the exquisite sensitivity of VLA and ALMA, coupled with detailed modeling of the reverse and forward shock contributions will test this hypothesis.
We present extensive multi-wavelength observations of the extremely rapidly declining Type Ic supernova, SN 2005ek. Reaching a peak magnitude of M R = −17.3 and decaying by ∼ 3 mag in the first 15 days post-maximum, SN 2005ek is among the fastest Type I supernovae observed to date. The spectra of SN 2005ek closely resemble those of normal SN Ic, but with an accelerated evolution. There is evidence for the onset of nebular features at only nine days post-maximum. Spectroscopic modeling reveals an ejecta mass of ∼ 0.3 M ⊙ that is dominated by oxygen (∼ 80%), while the pseudo-bolometric light curve is consistent with an explosion powered by ∼ 0.03 M ⊙ of radioactive 56 Ni. Although previous rapidly evolving events (e.g., SN 1885A, SN 1939B, SN 2002bj, SN 2010X) were hypothesized to be produced by the detonation of a helium shell on a white dwarf, oxygendominated ejecta are difficult to reconcile with this proposed mechanism. We find that the properties of SN 2005ek are consistent with either the edge-lit double detonation of a low-mass white dwarf or the iron-core collapse of a massive star, stripped by binary interaction. However, if we assume that the strong spectroscopic similarity of SN 2005ek to other SN Ic is an indication of a similar progenitor channel, then a white-dwarf progenitor becomes very improbable. SN 2005ek may be one of the lowest mass stripped-envelope core-collapse explosions ever observed. We find that the rate of such rapidly declining Type I events is at least 1-3% of the normal SN Ia rate.
Deep late-time X-ray observations of the relativistic, engine-driven, type Ic SN 2012ap allow us to probe the nearby environment of the explosion and reveal the unique properties of relativistic SNe. We find that on a local scale of ∼ 0.01 pc the environment was shaped directly by the evolution of the progenitor star with a pre-explosion mass-loss rateṀ < 5 × 10 −6 M yr −1 , in line with GRBs and the other relativistic SN2009bb. Like sub-energetic GRBs, SN 2012ap is characterized by a bright radio emission and evidence for mildly relativistic ejecta. However, its late time (δt ≈ 20 d) X-ray emission is ∼ 100 times fainter than the faintest sub-energetic GRB at the same epoch, with no evidence for late-time central engine activity. These results support theoretical proposals that link relativistic SNe like 2009bb and 2012ap with the weakest observed engine-driven explosions, where the jet barely fails to breakout. Furthermore, our observations demonstrate that the difference between relativistic SNe and sub-energetic GRBs is intrinsic and not due to line-of-sight effects. This phenomenology can either be due to an intrinsically shorter-lived engine or to a more extended progenitor in relativistic SNe.
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