Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star

Abstract

Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.

Fermi Large Area Telescope Detection of Gamma-Ray Emission from the Direction of Supernova iPTF14hls

“…The γ-ray flux of the source is about 1.5 × 10 −9 cm −2 s −1 between 0.2 and 500 GeV. For a distance of 156 Mpc (Arcavi et al 2017), it corresponds to a γ-ray luminosity of 1.0 × 10 43 erg s −1 . This value is comparable with the peak bolometric luminosity of iPTF14hls (Arcavi et al 2017).…”

“…For a distance of 156 Mpc (Arcavi et al 2017), it corresponds to a γ-ray luminosity of 1.0 × 10 43 erg s −1 . This value is comparable with the peak bolometric luminosity of iPTF14hls (Arcavi et al 2017). Assuming a 3 year emission time, the total energy released in γ-rays is estimated to be about 10 51 erg.…”

“…Assuming that the Fermi-LAT source is associated with the supernova, we discuss possible physical implications of the γ-ray emission. The ejected mass by the supernova iPTF14hls was estimated to be several tens of solar masses, which corresponds to a total kinetic energy of where M ej is the ejected mass, and v sn is the velocity of the ejecta (Arcavi et al 2017). The corresponding gas density of the ejecta is estimated as n gas ∼ M ej 4πm p R 2 ∆R , where m p is the proton mass, R ∼ v sn t ∼ 1.8 × 10 16 (t/1 yr) cm is the radius and ∆R is the width of the ejecta.…”

“…A very peculiar supernova, iPTF14hls, was discovered by the Intermediate Palomar Transient Factory on 2014 September 22.53 UT (Arcavi et al 2017). This event has the identical spectra compared with a typical hydrogen-rich core-collapse supernova, but shows a very different light curve.…”

“…Multi-wavelength observations are particularly important for understanding the nature of such a peculiar event. The observation in the X-ray band by Swift/XRT on 2015 May 23.05, which was about 244 days after the discovery, showed no detection of the source and gave an upper limit of luminosity of L X < 2.5 × 10 41 erg s −1 in the 0.3 − 10.0 keV band assuming a photon index of Γ = 2 and a neutral hydrogen column density of 1.4 × 10 20 cm −2 (Arcavi et al 2017). In 2015 May and 2016 June, the source was observed by the Arcminute Microkelvin Imager Large Array and the Very Large Array in the radio bands, resulting in no detection in all those observations (Arcavi et al 2017).…”