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 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 the results of a broadband simultaneous campaign on the nearby low-luminosity active galactic nucleus M81*. From February through August 2005, we observed M81* five times using the Chandra X-ray Observatory with the High-Energy Transmission Grating Spectrometer, complemented by groundbased observations with the Giant Meterwave Radio Telescope, the Very Large Array and Very Large Baseline Array, the Plateau de Bure Interferometer at IRAM, the Submillimeter Array and Lick Observatory. We discuss how the resulting spectra vary over short and longer timescales compared to previous results, especially in the X-rays where this is the first ever longer-term campaign at spatial resolution high enough to nearly isolate the nucleus (17pc). We compare the spectrum to our Galactic center weakly active nucleus Sgr A*, which has undergone similar campaigns, as well as to weakly accreting X-ray binaries in the context of outflow-dominated models. In agreement with recent results suggesting that the physics of weakly-accreting black holes scales predictably with mass, we find that the exact same model which successfully describes hard state X-ray binaries applies to M81*, with very similar physical parameters.

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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.

We present optical photometric and spectroscopic observations of supernova 2013ej. It is one of the brightest type II supernovae exploded in a nearby (∼ 10 Mpc) galaxy NGC 628. The light curve characteristics are similar to type II SNe, but with a relatively shorter (∼ 85 day) and steeper (∼ 1.7 mag (100 d) −1 in V ) plateau phase. The SN shows a large drop of 2.4 mag in V band brightness during plateau to nebular transition. The absolute ultraviolet (UV) light curves are identical to SN 2012aw, showing a similar UV plateau trend extending up to 85 days. The radioactive 56 Ni mass estimated from the tail luminosity is 0.02M ⊙ which is significantly lower than typical type IIP SNe. The characteristics of spectral features and evolution of line velocities indicate that SN 2013ej is a type II event. However, light curve characteristics and some spectroscopic features provide strong support in classifying it as a type IIL event. A detailed synow modelling of spectra indicates the presence of some high velocity components in Hα and Hβ profiles, implying possible ejecta-CSM interaction. The nebular phase spectrum shows an unusual notch in the Hα emission which may indicate bipolar distribution of 56 Ni. Modelling of the bolometric light curve yields a progenitor mass of ∼ 14M ⊙ and a radius of ∼ 450R ⊙ , with a total explosion energy of ∼ 2.3 × 10 51 erg.

This paper presents the investigations of SN 1993J using low-frequency observations with the Giant Metrewave Radio Telescope (GMRT). We analyze the light curves of SN 1993J at 1420, 610, 325, and 243 MHz during 7.5-10 yr after the explosion. The supernova has become optically thin early on in the 1420 and 610 MHz bands, while it has only recently entered the optically thin phase in the 325 MHz band. The radio light curve in the 235 MHz band is more or less flat. This indicates that the supernova is undergoing a transition from an optically thick to optically thin limit in this frequency band. In addition, we analyze the supernova radio spectra at five epochs on days 3000, 3200, 3266, 3460, and 3730 after the explosion. The spectral break in the day 3200 composite spectrum from the GMRT and Very Large Array implies that the plasma is dominated by a magnetic field, and the latter is far from being in equipartition with relativistic particles. SN 1993J is the only young supernova for which the magnetic field and the size of the radio-emitting region are determined through unrelated methods. Thus, the mechanism that controls the evolution of the radio spectra can be identified. We suggest that at all epochs, the synchrotron self-absorption mechanism is primarily responsible for the turnover in the spectra. Light-curve models based on free-free absorption in homogeneous or inhomogeneous media at high frequencies overpredict the flux densities at low frequencies. The discrepancy is increasingly larger at lower and lower frequencies. We suggest that an extra opacity, sensitively dependent on frequency, is likely to account for the difference at lower frequencies. The evolution of the magnetic field (determined from synchrotron self-absorption turnover) is roughly consistent with B / t À1 . The radio spectral index in the optically thin part evolves from $ 0:8 1:0 at a few tens of days to $0.6 in about 10 yr.

We present X‐ray, broad‐band optical and low‐frequency radio observations of the bright type IIP supernova SN 2004et. The Chandra X‐ray Observatory observed the supernova at three epochs, and the optical coverage spans a period of ∼470 d since explosion. The X‐ray emission softens with time, and we characterize the X‐ray luminosity evolution as LX∝t−0.4. We use the observed X‐ray luminosity to estimate a mass‐loss rate for the progenitor star of ∼2 × 10−6 M⊙ yr−1. The optical light curve shows a pronounced plateau lasting for about 110 d. Temporal evolution of photospheric radius and colour temperature during the plateau phase is determined by making blackbody fits. We estimate the ejected mass of 56Ni to be 0.06 ± 0.03 M⊙. Using the expressions of Litvinova & Nadëzhin we estimate an explosion energy of (0.98 ± 0.25) × 1051 erg. We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier, Fransson & Nymark. These multiwavelength studies suggest a main‐sequence progenitor mass of ∼20 M⊙ for SN 2004et.