Evidence for a Compact Object in the Aftermath of the Extra-Galactic Transient AT2018cow

Fast Blue Optical Transients (FBOTs) are mysterious extragalactic explosions that may represent a new class of astrophysical phenomena. Their fast time to maximum brightness of less than 10 days and decline over less than 2 months and unusual optical spectra and evolution are difficult to explain within the context of core-collapse of massive stars which are powered by radioactive decay of Nickel-56 and evolve slowly on months timescales. AT2018cow (at a redshift of 0.014) is an extreme FBOT both in terms of rapid evolution and high X-ray and bolometric luminosities. Several alternative hypotheses have been proposed to explain its unusual properties. These include shock interactions with dense circumstellar medium, tidal disruption of a star by a 10,000−million solar mass black hole, failed supernova with fallback accretion onto a newborn black hole, neutron star formed in a supernova or from merging compact objects, etc. Here, we present evidence for a high-amplitude (fractional root-mean-squared amplitude of>30%) quasi-periodic oscillation (QPO) of AT2018cow’s soft X-rays with a centroid frequency of roughly 225 Hz (statistically significant at the 3.7-sigma level, or a false alarm probability of 0.02%). This signal is found in the average power density spectrum of data taken over the entire outburst lasting roughly 60 days and thus suggests that the signal is highly persistent over several hundreds of millions of cycles (60 daysx225 Hz). This high frequency (rapid timescale) of 225 Hz (4.4 ms) argues for the presence of a compact object in AT2018cow which can either be a neutron star or a black hole, and disfavors circumstellar medium interactions for the origin of X-ray emission. Also, the QPO’s timescale sets an upper limit on the compact object's mass to be 850 solar masses, and thus disfavors models with a heavier black hole. If the QPO represents the spin period of a neutron star we can set upper limits on its magnetic field under different scenarios. This work highlights a new way of using high time-resolution X-ray observations to study FBOTs.

Non-thermal radio supernova remnants of exiled Wolf–Rayet stars

ABSTRACT A signification fraction of Galactic massive stars (${\ge}8\, \rm M_{\odot }$) are ejected from their parent cluster and supersonically sail away through the interstellar medium (ISM). The winds of these fast-moving stars blow asymmetric bubbles thus creating a circumstellar environment in which stars eventually die with a supernova explosion. The morphology of the resulting remnant is largely governed by the circumstellar medium of the defunct progenitor star. In this paper, we present 2D magneto-hydrodynamical simulations investigating the effect of the ISM magnetic field on the shape of the supernova remnants of a $35\, \mathrm{M}_{\odot }$ star evolving through a Wolf–Rayet phase and running with velocity 20 and $40\, \rm km\, \rm s^{-1}$, respectively. A $7\, \mu \rm G$ ambient magnetic field is sufficient to modify the properties of the expanding supernova shock front and in particular to prevent the formation of filamentary structures. Prior to the supernova explosion, the compressed magnetic field in the circumstellar medium stabilizes the wind/ISM contact discontinuity in the tail of the wind bubble. A consequence is a reduced mixing efficiency of ejecta and wind materials in the inner region of the remnant, where the supernova shock wave propagates. Radiative transfer calculations for synchrotron emission reveal that the non-thermal radio emission has characteristic features reflecting the asymmetry of exiled core-collapse supernova remnants from Wolf–Rayet progenitors. Our models are qualitatively consistent with the radio appearance of several remnants of high-mass progenitors, namely the bilateral G296.5+10.0 and the shell-type remnants CTB109 and Kes 17, respectively.

RINGO3 polarimetry of very young ZTF supernovae

The early phases of the observed evolution of the supernovae (SNe) are expected to be dominated by the shock breakout and ‘flash’ ionization of the surrounding circumstellar medium. This material arises from the last stages of the evolution of the progenitor, such that photometry and spectroscopy of SNe at early times can place vital constraints on the latest and fastest evolutionary phases leading up to stellar death. These signatures are erased by the expansion of the ejecta within ∼5 days after explosion. Here we present the earliest constraints, to date, on the polarization of ten transients discovered by the Zwicky Transient Facility (ZTF), between June 2018 and August 2019. Rapid polarimetric followup was conducted using the Liverpool Telescope RINGO3 instrument, including 3 SNe observed within <1 day of detection by the ZTF. The limits on the polarization within the first 5 days of explosion, for all SN types, is generally $<2{{\ \rm per\ cent}}$, implying early asymmetries are limited to axial ratios >0.65 (assuming an oblate spheroidal configuration). We also present polarimetric observations of the Type I Superluminous SN 2018bsz and Type II SN 2018hna, observed around and after maximum light.