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Register a new userA fallback accretion model for the unusual type II-P supernova iPTF14hls
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The Intermediate Palomar Transient Factory reported the discovery of an unusual type II-P supernova iPTF14hls. Instead of a ~100-day plateau as observed for ordinary type II-P supernovae, the light curve of iPTF14hls has at least five distinct peaks, followed by a steep decline at ~1000 days since discovery. Until 500 days since discovery, the effective temperature of iPTF14hls is roughly constant at 5000-6000K . In this paper we propose that iPTF14hls is likely powered by intermittent fallback accretion. It is found that the light curve of iPTF14hls can be well fit by the usual t^{-5/3} accretion law until ~1000 days post discovery when the light curve transitions to a steep decline. To account for this steep decline, we suggest a power-law density profile for the late accreted material, rather than the constant profile as appropriated for the t^{-5/3} accretion law. Detailed modeling indicates that the total fallback mass is ~0.2M_{sun}, with an ejecta mass M_{ej}~21M_{sun}. We find the third peak of the light curve cannot be well fit by the fallback model, indicating that there could be some extra rapid energy injection. We suggest that this extra energy injection may be a result of a magnetic outburst if the central object is a neutron star. These results indicate that the progenitor of iPTF14hls could be a massive red supergiant.
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We study iPTF14hls, a luminous and extraordinary long-lived Type II supernova, which lately has attracted much attention and disparate interpretation. We present new optical photometry that extends the light curves until more than 3 yr past discovery. We also obtained optical spectroscopy over this period, and furthermore present additional space-based observations using Swift and HST. After an almost constant luminosity for hundreds of days, the later light curve of iPTF14hls finally fades and then displays a dramatic drop after about 1000 d, but the supernova is still visible at the latest epochs presented. The spectra have finally turned nebular, and the very last optical spectrum likely displays signatures from the deep and dense interior of the explosion. The high-resolution HST image highlights the complex environment of the explosion in this low-luminosity galaxy. We provide a large number of additional late-time observations of iPTF14hls, which are (and will continue to be) used to assess the many different interpretations for this intriguing object. In particular, the very late (+1000 d) steep decline of the optical light curve, the lack of very strong X-ray emission, and the emergence of intermediate-width emission lines including of [S II] that likely originate from dense, processed material in the core of the supernova ejecta, are all key observational tests for existing and future models.
A new component was reported in the X-ray counterpart to the binary neutron-star merger and gravitational wave event GW170817, exceeding the afterglow emission from an off-axis structured jet. The afterglow emission from the kilonova/macronova ejecta may explain the X-ray excess but exceeds the radio observations if the spectrum is the same. We propose a fallback accretion model that a part of ejecta from the neutron star merger falls back and forms a disk around the central compact object. In the super-Eddington accretion phase, the X-ray luminosity stays near the Eddington limit of a few solar masses and the radio is weak, as observed. This will be followed by a power law decay. The duration of the constant luminosity phase conveys the initial fallback timescale $t_0$ in the past. The current multi-year duration requires $t_0 > 3$--$30$ sec, suggesting that the disk wind rather than the dynamical ejecta falls back after the jet launch. Future observations in the next decades will probe the timescale of $t_0 sim 10$--$10^4$ sec, around the time of extended emission in short gamma-ray bursts. The fallback accretion has not been halted by the $r$-process heating, implying that fission is weak on the year scale. We predict that the X-ray counterpart will disappear in a few decades due to the $r$-process halting or the depletion of fallback matter.
We present high-cadence ultraviolet (UV), optical, and near-infrared (NIR) data on the luminous Type II-P supernova SN 2017gmr from hours after discovery through the first 180 days. SN 2017gmr does not show signs of narrow, high-ionization emission lines in the early optical spectra, yet the optical lightcurve evolution suggests that an extra energy source from circumstellar medium (CSM) interaction must be present for at least 2 days after explosion. Modeling of the early lightcurve indicates a ~500R$_{odot}$ progenitor radius, consistent with a rather compact red supergiant, and late-time luminosities indicate up to 0.130 $pm$ 0.026 M$_{odot}$ of $^{56}$Ni are present, if the lightcurve is solely powered by radioactive decay, although the $^{56}$Ni mass may be lower if CSM interaction contributes to the post-plateau luminosity. Prominent multi-peaked emission lines of H$alpha$ and [O I] emerge after day 154, as a result of either an asymmetric explosion or asymmetries in the CSM. The lack of narrow lines within the first two days of explosion in the likely presence of CSM interaction may be an example of close, dense, asymmetric CSM that is quickly enveloped by the spherical supernova ejecta.
We present extensive observations of SN 2018zd covering the first $sim450$,d after the explosion. This SN shows a possible shock-breakout signal $sim3.6$,hr after the explosion in the unfiltered light curve, and prominent flash-ionisation spectral features within the first week. The unusual photospheric temperature rise (rapidly from $sim 12,000$,K to above 18,000,K) within the earliest few days suggests that the ejecta were continuously heated. Both the significant temperature rise and the flash spectral features can be explained with the interaction of the SN ejecta with the massive stellar wind ($0.18^{+0.05}_{-0.10}, rm M_{odot}$), which accounts for the luminous peak ($L_{rm max} = [1.36pm 0.63] times 10^{43}, rm erg,s^{-1}$) of SN 2018zd. The luminous peak and low expansion velocity ($v approx 3300$ km s$^{-1}$) make SN 2018zd to be like a member of the LLEV (luminous SNe II with low expansion velocities) events originated due to circumstellar interaction. The relatively fast post-peak decline allows a classification of SN 2018zd as a transition event morphologically linking SNe~IIP and SNe~IIL. In the radioactive-decay phase, SN 2018zd experienced a significant flux drop and behaved more like a low-luminosity SN~IIP both spectroscopically and photometrically. This contrast indicates that circumstellar interaction plays a vital role in modifying the observed light curves of SNe~II. Comparing nebular-phase spectra with model predictions suggests that SN 2018zd arose from a star of $sim 12,rm M_{odot}$. Given the relatively small amount of $^{56}$Ni ($0.013 - 0.035 rm M_{odot}$), the massive stellar wind, and the faint X-ray radiation, the progenitor of SN 2018zd could be a massive asymptotic giant branch star which collapsed owing to electron capture.
It is shown that the H$alpha$ luminosity and the Thomson optical depth of the iPTF14hls on day 600 after the detection provide us with the estimate of the envelope age which turns to be about 1000 days. I propose a model that suggests an explosion of a massive star with the radius of $sim 2times10^{13}$ cm at 450 days prior to the discovery. For the optimal model the ejected mass is $30,M_{odot}$, and the kinetic energy is $8times10^{51}$ erg. The energy source at the dominant luminosity stage is presumably related to the relativistic bipolar outflow originated from a disk accretion onto the black hole. The [O,I] 6300, 6364 AA doublet in the spectrum on day 600 is shown to be the result of the emission of at least $1-3,M_{odot}$ of oxygen in the ejecta inner zone. The oxygen distribution is non-spherical and can be represented either by two components with blue and red shifts (in the optically thin case), or by one blue shifted component, in the case of optically thick lines for the filling factor of $sim 2times10^{-3}$.
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