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Register a new userA mildly relativistic radio jet from the otherwise normal Type Ic Supernova 2007gr
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Z. Paragi
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The class of type Ic supernovae have drawn increasing attention since 1998 owing to their sparse association (only four so far) with long duration gamma-ray bursts. Although both phenomena originate from the core collapse of a massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit mostly in soft gamma-rays or hard X-rays. Though the GRB central engine generates ultra-relativistic jets, which beam the early emission into a narrow cone, no relativistic outflows have hitherto been found in type Ib/c supernovae explosions, despite theoretical expectations and searches. Here we report radio (interferometric) observations that reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days apart, we detect expansion of the source and establish a conservative lower limit for the average apparent expansion velocity of 0.6c. Independently, a second mildly relativistic supernova has been reported. Contrary to the radio data, optical observations of SN 2007gr indicate a typical type Ic supernova with ejecta velocities ~6000 km/s, much lower than in GRB-associated supernovae. We conclude that in SN 2007gr a small fraction of the ejecta produced a low-energy mildly relativistic bipolar radio jet, while the bulk of the ejecta were slower and, as shown by optical spectro-polarimetry, mildly aspherical.
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We report our attempts to locate the progenitor of the peculiar type Ic SN 2007gr in HST pre-explosion images of the host galaxy, NGC 1058. Aligning adaptive optics Altair/NIRI imaging of SN 2007gr from the Gemini (North) Telescope with the pre-explosion HST WFPC2 images, we identify the SN position on the HST frames with an accuracy of 20 mas. Although nothing is detected at the SN position we show that it lies on the edge of a bright source, 134+/-23 mas (6.9 pc) from its nominal centre. Based on its luminosity we suggest that this object is possibly an unresolved, compact and coeval cluster and that the SN progenitor was a cluster member, although we note that model profile fitting favours a single bright star. We find two solutions for the age of this assumed cluster; 7-/+0.5 Myrs and 20-30 Myrs, with turn-off masses of 28+/-4 Msun and 12-9 Msun respectively. Pre-explosion ground-based K-band images marginally favour the younger cluster age/higher turn-off mass. Assuming the SN progenitor was a cluster member, the turn-off mass provides the best estimate for its initial mass. More detailed observations, after the SN has faded, should determine if the progenitor was indeed part of a cluster, and if so allow an age estimate to within ~2 Myrs thereby favouring either a high mass single star or lower mass interacting binary progenitor.
We present an analysis of late-time Hubble Space Telescope Wide Field Camera 3 and Wide Field Planetary Camera 2 observations of the site of the Type Ic SN 2007gr in NGC 1058. The SN is barely recovered in the late-time WFPC2 observations, while a possible detection in the later WFC3 data is debatable. These observations were used to conduct a multiwavelength study of the surrounding stellar population. We fit spatial profiles to a nearby bright source that was previously proposed to be a host cluster. We find that, rather than being an extended cluster, it is consistent with a single point-like object. Fitting stellar models to the observed spectral energy distribution of this source, we conclude it is A1-A3 Yellow Supergiant, possibly corresponding to a star with $M_{ZAMS} = 40M_{odot}$. SN 2007gr is situated in a massive star association, with diameter of $approx 300,mathrm{pc}$. We present a Bayesian scheme to determine the properties of the surrounding massive star population, in conjunction with the Padova isochrones. We find that the stellar population, as observed in either the WFC3 and WFPC2 observations, can be well fit by two age distributions with mean ages: ~6.3 Myr and ~50 Myr. The stellar population is clearly dominated by the younger age solution (by factors of 3.5 and 5.7 from the WFPC2 and WFC3 observations, respectively), which corresponds to the lifetime of a star with $M_{ZAMS} sim 30M_{odot}$. This is strong evidence in favour of the hypothesis that SN 2007gr arose from a massive progenitor star, possibly capable of becoming a Wolf-Rayet star.
Only a few cases of type Ic supernovae (SNe) with high-velocity ejecta have been discovered and studied. Here we present our analysis of radio and X-ray observations of a Type Ic SN, PTF12gzk. The radio emission rapidly declined less than 10 days after explosion, suggesting SN ejecta expanding at high velocity (~0.3c). The radio data also indicate that the density of the circumstellar material (CSM) around the supernova is lower by a factor of ~10 than the CSM around normal Type Ic SNe. Our observations of this rapidly declining radio SN at a distance of 58 Mpc demonstrates the potential to detect many additional radio SNe, given the new capabilities of the VLA (improved sensitivity and dynamic scheduling), that are currently missed, leading to a biased view of radio SNe Ic. Early optical discovery followed by rapid radio observations would provide a full description of the ejecta velocity distribution and CSM densities around stripped massive star explosions, as well as strong clues about the nature of their progenitor stars.
We present the discovery of a light echo from SN 2007af, a normal Type Ia supernova (SN Ia) in NGC 5584. Hubble Space Telescope (HST) images taken three years post explosion reveal two separate echoes; an outer echo and extended central region, which we propose as an unresolved inner echo. Multiple images were obtained in the F160W, F350LP, F555W, and F814W using the Wide Field Camera 3. If the outer echo is produced by an interstellar dust sheet perpendicular to the line of sight, it is located ~800 pc in front of the SN. The dust for the inner echo is 0.45 pc < d < 90 pc away from the SN. The inner echo color is consistent with typical interstellar dust wavelength-dependent scattering cross-sections, while the outer echo color does not match the predictions. Both dust sheets, if in the foreground, are optically thin for scattering, with the outer echo sheet thickness consistent with the inferred extinction from peak brightness. Whether the inner echo is from interstellar or circumstellar dust is ambiguous. Overall, the echo characteristics are quite similar to previously observed SN Ia echoes.
We present extensive optical observations of the normal Type Ic supernova (SN) 2007gr, spanning from about one week before maximum light to more than one year thereafter. The optical light and color curves of SN 2007gr are very similar to those of the broad-lined Type Ic SN 2002ap, but the spectra show remarkable differences. The optical spectra of SN 2007gr are characterized by unusually narrow lines, prominent carbon lines, and slow evolution of the line velocity after maximum light. The earliest spectrum (taken at t=-8 days) shows a possible signature of helium (He~I 5876 at a velocity of ~19,000 km s{-1}). Moreover, the larger intensity ratio of the [O I] 6300 and 6364 lines inferred from the early nebular spectra implies a lower opacity of the ejecta shortly after the explosion. These results indicate that SN 2007gr perhaps underwent a less energetic explosion of a smaller-mass Wolf-Rayet star (~ 8--9 Msun) in a binary system, as favored by an analysis of the progenitor environment through pre-explosion and post-explosion Hubble Space Telescope images. In the nebular spectra, asymmetric double-peaked profiles can be seen in the [O~I] 6300 and Mg~I] 4571 lines. We suggest that the two peaks are contributed by the blueshifted and rest-frame components. The similarity in velocity structure and the different evolution of the strength of the two components favor an aspherical explosion with the ejecta distributed in a torus or disk-like geometry, but inside the ejecta the O and Mg have different distributions.
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