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Register a new userKeplers Supernova: An Overluminous Type Ia Event Interacting with a Massive Circumstellar Medium at a Very Late Phase
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We have analyzed XMM-Newton, Chandra, and Suzaku observations of Keplers supernova remnant (SNR) to investigate the properties of both the SN ejecta and the circumstellar medium (CSM). For comparison, we have also analyzed two similarly-aged, ejecta-dominated SNRs: Tychos SNR, thought to be the remnant of a typical Type Ia SN, and SNR 0509-67.5 in the Large Magellanic Cloud, thought to be the remnant of an overluminous Type Ia SN. By simply comparing the X-ray spectra, we find that line intensity ratios of iron-group elements (IGE) to intermediate-mass elements (IME) for Keplers SNR and SNR 0509-67.5 are much higher than those for Tychos SNR. We therefore argue that Kepler is the product of an overluminous Type Ia SN. This inference is supported by our spectral modeling, which reveals the IGE and IME masses respectively to be ~0.95 M_sun and ~0.12 M_sun (Keplers SNR), ~0.75 M_sun and ~0.34 M_sun (SNR 0509-67.5), and ~0.35 M_sun and ~0.70 M_sun (Tychos SNR). We find that the CSM component in Keplers SNR consists of tenuous diffuse gas (~0.3 M_sun) present throughout the entire remnant, plus dense knots (~0.035 M_sun). Both of these components have an elevated N abundance (N/H ~ 4 times the solar value), suggesting that they originate from CNO-processed material from the progenitor system. The mass of the diffuse CSM allows us to infer the pre-SN mass-loss rate to be ~1.5e-5 (V_w/10 km/s) M_sun/yr, in general agreement with results from recent hydrodynamical simulations. Since the dense knots have slow proper motions and relatively small ionization timescales, they were likely located a few pc away from the progenitor system. Therefore, we argue that Keplers SN was an overluminous event that started to interact with massive CSM a few hundred years after the explosion. This supports the possible link between overluminous SNe and the so-called Ia-CSM SNe.
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We present initial results of a 750 ks Chandra observation of the remnant of Keplers supernova of AD 1604. The strength and prominence of iron emission, together with the absence of O-rich ejecta, demonstrate that Kepler resulted from a thermonuclear supernova, even though evidence for circumstellar interaction is also strong. We have analyzed spectra of over 100 small regions, and find that they fall into three classes. (1) The vast majority show Fe L emission between 0.7 and 1 keV and Si and S K alpha emission; we associate these with shocked ejecta. A few of these are found at or beyond the mean blast wave radius. (2) A very few regions show solar O/Fe abundance rations; these we associate with shocked circumstellar medium (CSM). Otherwise O is scarce. (3) A few regions are dominated by continuum, probably synchrotron radiation. Finally, we find no central point source, with a limit about 100 times fainter than the central object in Cas A. The evidence that the blast wave is interacting with CSM may indicate a Ia explosion in a more massive progenitor.
ASASSN-18am/SN 2018gk is a newly discovered member of the rare group of luminous, hydrogen-rich supernovae (SNe) with a peak absolute magnitude of $M_V approx -20$ mag that is in between normal core-collapse SNe and superluminous SNe. These SNe show no prominent spectroscopic signatures of ejecta interacting with circumstellar material (CSM), and their powering mechanism is debated. ASASSN-18am declines extremely rapidly for a Type II SN, with a photospheric-phase decline rate of $sim6.0~rm mag~(100 d)^{-1}$. Owing to the weakening of HI and the appearance of HeI in its later phases, ASASSN-18am is spectroscopically a Type IIb SN with a partially stripped envelope. However, its photometric and spectroscopic evolution show significant differences from typical SNe IIb. Using a radiative diffusion model, we find that the light curve requires a high synthesised $rm ^{56}Ni$ mass $M_{rm Ni} sim0.4~M_odot$ and ejecta with high kinetic energy $E_{rm kin} = (7-10) times10^{51} $ erg. Introducing a magnetar central engine still requires $M_{rm Ni} sim0.3~M_odot$ and $E_{rm kin}= 3times10^{51} $ erg. The high $rm ^{56}Ni$ mass is consistent with strong iron-group nebular lines in its spectra, which are also similar to several SNe Ic-BL with high $rm ^{56}Ni$ yields. The earliest spectrum shows flash ionisation features, from which we estimate a mass-loss rate of $ dot{M}approx 2times10^{-4}~rm M_odot~yr^{-1} $. This wind density is too low to power the luminous light curve by ejecta-CSM interaction. We measure expansion velocities as high as $ 17,000 $ km/s for $H_alpha$, which is remarkably high compared to other SNe II. We estimate an oxygen core mass of $1.8-3.4$ $M_odot$ using the [OI] luminosity measured from a nebular-phase spectrum, implying a progenitor with a zero-age main sequence mass of $19-26$ $M_odot$.
Owing to their utility for measurements of cosmic acceleration, Type Ia supernovae (SNe) are perhaps the best-studied class of SNe, yet the progenitor systems of these explosions largely remain a mystery. A rare subclass of SNe Ia show evidence of strong interaction with their circumstellar medium (CSM), and in particular, a hydrogen-rich CSM; we refer to them as SNe Ia-CSM. In the first systematic search for such systems, we have identified 16 SNe Ia-CSM, and here we present new spectra of 13 of them. Six SNe Ia-CSM have been well-studied previously, three were previously known but are analyzed in-depth for the first time here, and seven are new discoveries from the Palomar Transient Factory. The spectra of all SNe Ia-CSM are dominated by H{alpha} emission (with widths of ~2000 km/s) and exhibit large H{alpha}/H{beta} intensity ratios (perhaps due to collisional excitation of hydrogen via the SN ejecta overtaking slower-moving CSM shells); moreover, they have an almost complete lack of He I emission. They also show possible evidence of dust formation through a decrease in the red wing of H{alpha} 75-100 d past maximum brightness, and nearly all SNe Ia-CSM exhibit strong Na I D absorption from the host galaxy. The absolute magnitudes (uncorrected for host-galaxy extinction) of SNe Ia-CSM are found to be -21.3 <= M_R <= -19 mag, and they also seem to show ultraviolet emission at early times and strong infrared emission at late times (but no detected radio or X-ray emission). Finally, the host galaxies of SNe Ia-CSM are all late-type spirals similar to the Milky Way, or dwarf irregulars like the Large Magellanic Cloud, which implies that these objects come from a relatively young stellar population. This work represents the most detailed analysis of the SN Ia-CSM class to date.
The interaction of a supernova with a circumstellar medium (CSM) can dramatically increase the emitted luminosity by converting kinetic energy to thermal energy. In superluminous supernovae (SLSNe) of Type IIn -- named for narrow hydrogen lines in their spectra -- the integrated emission can reach $sim 10^{51}$ erg, attainable by thermalising most of the kinetic energy of a conventional SN. A few transients in the centres of active galaxies have shown similar spectra and even larger energies, but are difficult to distinguish from accretion onto the supermassive black hole. Here we present a new event, SN2016aps, offset from the centre of a low-mass galaxy, that radiated $gtrsim 5 times 10^{51}$ erg, necessitating a hyper-energetic supernova explosion. We find a total (SN ejecta $+$ CSM) mass likely exceeding 50-100 M$_odot$, with energy $gtrsim 10^{52}$ erg, consistent with some models of pair-instability supernovae (PISNe) or pulsational PISNe -- theoretically-predicted thermonuclear explosions from helium cores $>50$ M$_odot$. Independent of the explosion mechanism, this event demonstrates the existence of extremely energetic stellar explosions, detectable at very high redshifts, and provides insight into dense CSM formation in the most massive stars.
Despite their cosmological utility, the progenitors of Type Ia supernovae (SNe Ia) are still unknown, with many efforts focused on whether accretion from a nondegenerate companion can grow a carbon-oxygen white dwarf to near the Chandrasekhar mass. The association of SNe Ia resembling SN 1991T (91T-like) with circumstellar interaction may be evidence for this single-degenerate channel. However, the observed circumstellar medium (CSM) in these interacting systems is unlike a stellar wind -- of particular interest, it is sometimes detached from the stellar surface, residing at $sim 10^{16}~{rm cm}$. A Hubble Space Telescope (HST) program to discover detached CSM around 91T-like SNe Ia successfully discovered interaction nearly two years after explosion in SN 2015cp (Graham et al., 2018). In this work, we present radio and X-ray follow-up observations of SN 2015cp and analyze them in the framework of Harris, Nugent, & Kasen (2016) to limit the properties of a constant-density CSM shell in this system. Assuming the HST detection was shortly after the shock crossed the CSM, we constrain the total CSM mass in this system to be $< 0.5~{rm M_odot}$. This limit is comparable to the CSM mass of supernova PTF11kx, but does not rule out lower masses predicted for recurrent novae. From lessons learned modeling PTF11kx and SN 2015cp, we suggest a strategy for future observations of these events to increase the sample of known interacting SNe Ia.
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