We derive stringent constraints on the persistent source associated with FRB 121102: Size 0.3 < R 17.5 = (R/10 17.5 cm) < 3, age < 10 2.5 yr, energy E ≈ 10 49 (ε e /0.2 GeV) 3 erg, characteristic electron energy 0.1 ≤ ε e /1 GeV ≤ 0.5; The radiating plasma is confined by a cold plasma of mass M c < 10 −1.5 R 4 17.5 M ⊙ . These properties are inconsistent with typical "magnetar wind nebulae" model predictions. The fact that ε e ∼ m p c 2 suggests that the hot plasma was created by the ejection of a mildly relativistic, M ≈ E/c 2 ≈ 10 −5 M ⊙ shell, which propagated into an extended ambient medium or collided with a pre-ejected shell.Independent of the persistent source model, we suggest a physical mechanism for the generation of FRBs: Ejection from an underlying compact object, R s = 10 6 R s,6 cm, of highly relativistic shells, with energy E s = 10 41 E 41 erg and Lorentz factor γ s = 10 3 E 1/8 41 R −3/8 s,6 , into a surrounding e − p plasma with density n ∼ 10 −1 cm −3 (consistent with that inferred for the persistent source). For E s similar to observed FRB energies, plasma conditions appropriate for strong synchrotron maser emission at ν coh. ≈ 0.5Es,6 GHz are formed. A significant fraction of the deposited energy is converted to an FRB with duration R s /c, accompanied by ∼ 10 MeV gamma-rays carrying less energy than the FRB.The inferred energy and mass associated with the source suggest some type of a "weak stellar explosion", where a neutron star is formed with relatively low mass and energy ejection. However, the current upper limit on R does not allow one to rule out M c ∼ 1M ⊙ , or the ejection of larger mass well before the ejection of the confining shell.
We discuss the late time (tens of days) emission from the radioactive ejecta of mergers involving neutron stars, when the ionization energy loss time of beta-decay electrons and positrons exceeds the expansion time. We show that if the e ± are confined to the plasma (by magnetic fields), then the time dependence of the plasma heating rate,ε d , and hence of the bolometric luminosity L =ε d , are given by d log L/d log t ≃ −2.8, nearly independent of the composition and of the instantaneous radioactive energy release rate,ε. This universality of the late time behavior is due to the weak dependence of the ionization loss rate on composition and on e ± energy. The late time IR and optical measurements of GW 170817 are consistent with this expected behavior provided that the ionization loss time exceeds the expansion time at t > t ε ≈ 7 d, as predicted based on the early (few day) electromagnetic emission.
High cadence transient surveys are able to capture supernovae closer to their first light than before. Applying analytical models to such early emission, we can constrain the progenitor stars properties. In this paper, we present observations of SN 2018 fif (ZTF18abokyfk). The supernova was discovered close to first light and monitored by the Zwicky Transient Facility (ZTF) and the Neil Gehrels Swift Observatory. Early spectroscopic observations suggest that the progenitor of SN 2018 fif was surrounded by relatively small amounts of circumstellar material (CSM) compared to all previous cases. This particularity, coupled with the high cadence multiple-band coverage, makes it a good candidate to investigate using shock-cooling models. We employ the SOPRANOS code, an implementation of the model by Sapir & Waxman (2017). Compared with previous implementations, SOPRANOS has the advantage of including a careful account of the limited temporal validity domain of the shockcooling model. We find that the progenitor of SN 2018 fif was a large red supergiant, with a radius of R = 1174 +208 −81 R and an ejected mass of M ej = 5.6 +9.1 −1.0 M. Our model also gives information on the explosion epoch, the progenitor inner structure, the shock velocity and the extinction. The large radius differs from previously modeled objects, and the difference could be either intrinsic or due to the relatively small amount of CSM around SN 2018 fif, perhaps making it a "cleaner" candidate for applying shock-cooling analytical models.
We present visible-light and ultraviolet (U V ) observations of the supernova PTF 12glz. The SN was discovered and monitored in near-U V and R bands as part of a joint GALEX and Palomar Transient Factory campaign. It is among the most energetic Type IIn supernovae observed to date (≈ 10 51 erg). If the radiated energy mainly came from the thermalization of the shock kinetic energy, we show that PTF 12glz was surrounded by ∼ 1 M of circumstellar material (CSM) prior to its explosive death. PTF 12glz shows a puzzling peculiarity: at early times, while the freely expanding ejecta are presumably masked by the optically thick CSM, the radius of the blackbody that best fits the observations grows at ≈ 8000 km s −1 . Such a velocity is characteristic of fast moving ejecta rather than optically thick CSM. This phase of radial expansion takes place before any spectroscopic signature of expanding ejecta appears in the spectrum and while both the spectroscopic data and the bolometric luminosity seem to indicate that the CSM is optically thick. We propose a geometrical solution to this puzzle, involving an aspherical structure of the CSM around PTF 12glz. By modeling radiative diffusion through a slab of CSM, we show that an aspherical geometry of the CSM can result in a growing effective radius. This simple model also allows us to recover the decreasing blackbody temperature of PTF 12glz. SLAB-Diffusion, the code we wrote to model the radiative diffusion of photons through a slab of CSM and evaluate the observed radius and temperature, is made available on-line.
We report on the discovery of AT 2018lqh (ZTF 18abfzgpl)—a rapidly evolving extragalactic transient in a star-forming host at 242 Mpc. The transient g-band light curve’s duration above a half-maximum light is about 2.1 days, where 0.4/1.7 days are spent on the rise/decay, respectively. The estimated bolometric light curve of this object peaked at about 7 × 1042erg s−1—roughly 7 times brighter than the neutron star (NS)–NS merger event AT 2017gfo. We show that this event can be explained by an explosion with a fast (v ∼ 0.08 c) low-mass (≈0.07 M ⊙) ejecta, composed mostly of radioactive elements. For example, ejecta dominated by 56Ni with a timescale of t 0 ≅ 1.6 days for the ejecta to become optically thin for γ-rays fits the data well. Such a scenario requires burning at densities that are typically found in the envelopes of neutron stars or the cores of white dwarfs. A combination of circumstellar material (CSM) interaction power at early times and shock cooling at late times is consistent with the photometric observations, but the observed spectrum of the event may pose some challenges for this scenario. We argue that the observations are not consistent with a shock breakout from a stellar envelope, while a model involving a low-mass ejecta ramming into low-mass CSM cannot explain both the early- and late-time observations.
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