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Register a new userGRB 081007 and GRB 090424: the surrounding medium, outflows and supernovae
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We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum luminosity is only about half as large as that of SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a GRB clearly associated with a massive star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be Gamma ~ 200, while for GRB 090424 a lower limit of Gamma > 170 is derived. We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux dominated outflow or to a dissipative photosphere.
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Twenty years ago, GRB 980425/SN 1998bw revealed that long Gamma-Ray Bursts (GRBs) are physically associated with broad-lined type Ic supernovae. Since then more than 1000 long GRBs have been localized to high angular precision, but only in about 50 cases the underlying supernova (SN) component was identified. Using the multi-channel imager GROND (Gamma-Ray Burst Optical Near-Infrared Detector) at ESO/La Silla, during the last ten years we have devoted a substantial amount of observing time to reveal and to study SN components in long-GRB afterglows. Here we report on four more GRB-SNe (associated with GRBs 071112C, 111228A, 120714B, and 130831A) which were discovered and/or followed-up with GROND and whose redshifts lie between z=0.4 and 0.8. We study their afterglow light curves, follow the associated SN bumps over several weeks, and characterize their host galaxies. Using SN 1998bw as a template, the derived SN explosion parameters are fully consistent with the corresponding properties of the so-far known GRB-SN ensemble, with no evidence for an evolution of their properties as a function of redshift. In two cases (GRB 120714B/SN 2012eb at z=0.398 and GRB 130831A/SN 2013fu at z=0.479) additional Very Large Telescope (VLT) spectroscopy of the associated SNe revealed a photospheric expansion velocity at maximum light of about 40 000 and 20 000 km/s, respectively. For GRB 120714B, which was an intermediate-luminosity burst, we find additional evidence for a blackbody component in the light of the optical transient at early times, similar to what has been detected in some GRB-SNe at lower redshifts.
Relativistic supernovae constitute a sub-class of type Ic supernovae (SNe). Their non-thermal, radio emission differs notably from that of regular type Ic supernovae as they have a fast expansion speed (with velocities $sim$ 0.6-0.8 c) which can not be explained by a standard, spherical SN explosion but advocates for a quickly evolving, mildly relativistic ejecta associated with the SN. In this paper, we compute the synchrotron radiation emitted by the cocoon of a long gamma-ray burst jet (GRB). We show that the energy and velocity of the expanding cocoon, and the radio non-thermal light curves and spectra are consistent with those observed in relativistic SNe. Thus, the radio emission from this events is not coming from the SN shock front, but from the mildly relativistic cocoon produced by the passage of a GRB jet through the progenitor star. We also show that the cocoon radio emission dominates the GRB emission at early times for GRBs seen off-axis, and the flux can be larger at late times compared with on-axis GRBs if the cocoon energy is at least comparable with respect to the GRB energy.
Every GRB model where the progenitor is assumed to be a highly relativistic hadronic jet whose pions, muons and electron pair secondaries are feeding the gamma jets engine, necessarily (except for very fine-tuned cases) leads to a high average neutrino over photon radiant exposure (radiance), a ratio well above unity, though the present observed average IceCube neutrino radiance is at most comparable to the gamma in the GRB one. Therefore no hadronic GRB, fireball or hadronic thin precessing jet, escaping exploding star in tunneled or penetrarting beam, can fit the actual observations. A new model is shown here, based on a purely electronic progenitor jet, fed by neutrons (and relics) stripped from a neutron star (NS) by tidal forces of a black hole or NS companion, showering into a gamma jet. Such thin precessing spinning jets explain unsolved puzzles such as the existence of the X-ray precursor in many GRBs. The present pure electron jet model, disentangling gamma and (absent) neutrinos, explains naturally why there is no gamma GRB correlates with any simultaneous TeV IceCube astrophysical neutrinos. Rare unstable NS companion stages while feeding the jet may lead to an explosion simulating a SN event. Recent IceCube-160731A highest energy muon neutrino event with absent X-gamma traces confirms the present model expectations.
We present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A / SN 2013ez and GRB 130831A / SN 2013fu. In the case of GRB 130215A / SN 2013ez, we also present optical spectroscopy at t-t0=16.1 d, which covers rest-frame 3000-6250 Angstroms. Based on Fe II (5169) and Si (II) (6355), our spectrum indicates an unusually low expansion velocity of 4000-6350 km/s, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A / SN 2013fu, we use our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we take advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Meszaros (2001), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P=12 ms and a magnetic field of B=1.1 x 10^15 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs.
Long duration gamma-ray bursts are commonly associated with the deaths of massive stars. Spectroscopic studies using the afterglow as a light source provide a unique opportunity to unveil the medium surrounding it, probing the densest region of their galaxies. This material is usually in a low ionisation state and at large distances from the burst site, hence representing the normal interstellar medium in the galaxy. Here we present the case of GRB 090426 at z=2.609, whose optical spectrum indicates an almost fully ionised medium together with a low column density of neutral hydrogen. For the first time, we also observe variations in the Ly alpha absorption line. Photoionisation modeling shows that we are probing material from the vicinity of the burst (~80 pc). The host galaxy is a complex of two luminous interacting galaxies, which might suggest that this burst could have occurred in an isolated star-forming region outside its host galaxy created in the interaction of the two galaxies.
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