We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient (FBOT), AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity L pk ∼ 4 × 10 44 erg s −1 exceeding those of superluminous supernovae (SNe), before declining as L ∝ t −2 . Initial spectra at δt 15 days were mostly featureless and indicated large expansion velocities v ∼ 0.1 c and temperatures arXiv:1810.10720v1 [astro-ph.HE] 25 Oct 2018 2 MARGUTTI ET AL. reaching T ∼ 3 × 10 4 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v ∼ 4000 km s −1 with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at E ≥ 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blastwave with v sh ∼ 0.1 c with a dense environment (Ṁ ∼ 10 −3 − 10 −4 M yr −1 for v w = 1000 km s −1 ). While these properties exclude 56 Ni-powered transients, our multi-wavelength analysis instead indicates that AT 2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼ 10 50 − 10 51.5 erg over ∼ 10 3 − 10 5 s and resides within lowmass fast-moving material with equatorial-polar density asymmetry (M ej,fast 0.3 M ). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations.
Growing evidence for shocks in nova outflows include (1) multiple velocity components in the optical spectra; (2) hard X-ray emission starting weeks to months after the outburst;(3) an early radio flare on timescales of months, in excess of that predicted from the freely expanding photo-ionized gas; and, perhaps most dramatically, (4) ∼ GeV gamma-ray emission. We present a one dimensional model for the shock interaction between the fast nova outflow and a dense external shell (DES) and its associated thermal X-ray, optical, and radio emission. The lower velocity DES could represent an earlier stage of mass loss from the white dwarf or ambient material not directly related to the thermonuclear runaway. The forward shock is radiative initially when the density of shocked gas is highest, at which times radio emission originates from the dense cooling layer immediately downstream of the shock. Our predicted radio light curve is characterized by sharper rises to maximum and later peak times at progressively lower frequencies, with a peak brightness temperature that is approximately independent of frequency. We apply our model to the recent gamma-ray producing classical nova V1324 Sco, obtaining an adequate fit to the early radio maximum for reasonable assumptions about the fast nova outflow and assuming the DES possesses a characteristic velocity ∼ 10 3 km s −1 and mass ∼ few 10 −4 M ⊙ ; the former is consistent with the velocities of narrow line absorption systems observed previously in nova spectra, while the total ejecta mass of the DES and fast outflow is consistent with that inferred independently by modeling the late radio peak as uniformly expanding photo-ionized gas. Rapid evolution of the early radio light curves require the DES to possess a steep outer density profile, which may indicate that the onset of mass loss from the white dwarf was rapid, providing indirect evidence that the DES was expelled as the result of the thermonuclear runaway event. Reprocessed X-rays from the shock absorbed by the DES at early times are found to contribute significantly to the optical/UV emission, which we speculate may be responsible for the previously unexplained 'plateaus' and secondary maxima in nova optical light curves.
The Fermi LAT discovery that classical novae produce ∼ > 100 MeV gamma-rays establishes that shocks and relativistic particle acceleration are key features of these events. These shocks are likely to be radiative due to the high densities of the nova ejecta at early times coincident with the gamma-ray emission. Thermal X-rays radiated behind the shock are absorbed by neutral gas and reprocessed into optical emission, similar to Type IIn (interacting) supernovae. Gamma-rays are produced by collisions between relativistic protons with the nova ejecta (hadronic scenario) or Inverse Compton/bremsstrahlung emission from relativistic electrons (leptonic scenario), where in both scenarios the efficiency for converting relativistic particle energy into LAT gamma-rays is at most a few tens of per cent. The measured ratio of gamma-ray and optical luminosities, L γ /L opt , thus sets a lower limit on the fraction of the shock power used to accelerate relativistic particles, nth . The measured value of L γ /L opt for two classical novae, V1324 Sco and V339 Del, constrains nth ∼ > 10 −2 and ∼ > 10 −3 , respectively. Leptonic models for the gamma-ray emission are disfavored given the low electron acceleration efficiency, nth ∼ 10 −4 − 10 −3 , inferred from observations of Galactic cosmic rays and particle-in-cell (PIC) numerical simulations. A fraction f sh ∼ > 100( nth /0.01) −1 and ∼ > 10( nth /0.01) −1 per cent of the optical luminosity is powered by shocks in nova Sco and nova Del, respectively. Such high fractions challenge standard models that instead attribute all nova optical emission to the direct outwards transport of thermal energy released near the white dwarf surface. We predict hard ∼ 10 − 100 keV X-ray emission coincident with the LAT emission, which should be detectable by NuSTAR or ASTRO-H, even at times when softer ∼ < 10 keV emission is absorbed by neutral gas ahead of the shocks.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.