Strong Circumstellar Interaction Research Articles

X-Ray-luminous Supernovae: Threats to Terrestrial Biospheres

The spectacular outbursts of energy associated with supernovae (SNe) have long motivated research into their potentially hazardous effects on Earth and analogous environments. Much of this research has focused primarily on the atmospheric damage associated with the prompt arrival of ionizing photons within days or months of the initial outburst, and the high-energy cosmic rays that arrive thousands of years after the explosion. In this study, we turn the focus to persistent X-ray emission, arising in certain SNe that have interactions with a dense circumstellar medium and observed months and/or years after the initial outburst. The sustained high X-ray luminosity leads to large doses of ionizing radiation out to formidable distances. We assess the threat posed by these X-ray-luminous SNe for Earth-like planetary atmospheres; our results are rooted in the X-ray SN observations from Chandra, Swift-XRT, XMM-Newton, NuSTAR, and others. We find that this threat is particularly acute for SNe showing evidence of strong circumstellar interaction, such as Type IIn explosions, which have significantly larger ranges of influence than previously expected and lethal consequences up to ∼50 pc away. Furthermore, X-ray-bright SNe could pose a substantial and distinct threat to terrestrial biospheres and tighten the Galactic habitable zone. We urge follow-up X-ray observations of interacting SNe for months and years after the explosion to shed light on the physical nature and full-time evolution of the emission and to clarify the danger that these events pose for life in our galaxy and other star-forming regions.

A cool and inflated progenitor candidate for the Type Ib supernova 2019yvr at 2.6 yr before explosion

ABSTRACT We present Hubble Space Telescope imaging of a pre-explosion counterpart to SN 2019yvr obtained 2.6 yr before its explosion as a type Ib supernova (SN Ib). Aligning to a post-explosion Gemini-S/GSAOI image, we demonstrate that there is a single source consistent with being the SN 2019yvr progenitor system, the second SN Ib progenitor candidate after iPTF13bvn. We also analysed pre-explosion Spitzer/Infrared Array Camera (IRAC) imaging, but we do not detect any counterparts at the SN location. SN 2019yvr was highly reddened, and comparing its spectra and photometry to those of other, less extinguished SNe Ib we derive $E(B-V)=0.51\substack{+0.27\\ -0.16}$ mag for SN 2019yvr. Correcting photometry of the pre-explosion source for dust reddening, we determine that this source is consistent with a log (L/L⊙) = 5.3 ± 0.2 and $T_{\mathrm{eff}} = 6800\substack{+400\\ -200}$ K star. This relatively cool photospheric temperature implies a radius of 320$\substack{+30\\ -50}~\mathrm{ R}_{\odot}$, much larger than expectations for SN Ib progenitor stars with trace amounts of hydrogen but in agreement with previously identified SN IIb progenitor systems. The photometry of the system is also consistent with binary star models that undergo common envelope evolution, leading to a primary star hydrogen envelope mass that is mostly depleted but still seemingly in conflict with the SN Ib classification of SN 2019yvr. SN 2019yvr had signatures of strong circumstellar interaction in late-time (>150 d) spectra and imaging, and so we consider eruptive mass-loss and common envelope evolution scenarios that explain the SN Ib spectroscopic class, pre-explosion counterpart, and dense circumstellar material. We also hypothesize that the apparent inflation could be caused by a quasi-photosphere formed in an extended, low-density envelope, or circumstellar matter around the primary star.

Late-time Circumstellar Interaction of SN 2017eaw in NGC 6946

Abstract SN 2017eaw, the tenth supernova observed in NGC 6946, was a normal Type II-P supernova with an estimated 11–13 M ⊙ red supergiant progenitor. Here we present nebular-phase spectra of SN 2017eaw at +545 and +900 days post-max, extending approximately 50–400 days past the epochs of previously published spectra. While the +545 day spectrum is similar to spectra taken between days +400 and +493, the +900 day spectrum shows dramatic changes both in spectral features and emission-line profiles. The Hα emission is flat-topped and boxlike with sharp blue and red profile velocities of ≃−8000 and +7500 km s−1. These late-time spectral changes indicate strong circumstellar interaction with a mass-loss shell, expelled ∼1700 yr before explosion. SN 2017eaw’s +900 day spectrum is similar to those seen for SN 2004et and SN 2013ej observed 2–3 yr after explosion. We discuss the importance of late-time monitoring of bright SNe II-P and the nature of presupernova mass-loss events for SN II-P evolution.

One Thousand Days of SN2015bn: HST Imaging Shows a Light Curve Flattening Consistent with Magnetar Predictions

Abstract We present the first observations of a Type I superluminous supernova (SLSN) at ≳1000 days after maximum light. We observed SN 2015bn using the Hubble Space Telescope (HST) Advanced Camera for Surveys in the F475W, F625W and F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and resolved from its compact host, allowing reliable photometry. A galaxy template constructed from these data further enables us to isolate the SLSN flux in deep ground-based imaging. We measure a light curve decline rate at >700 days of 0.19 ± 0.03 mag(100 d)−1, much shallower than the earlier evolution, and slower than previous SLSNe (at any phase) or the decay rate of 56Co. Neither additional radioactive isotopes nor a light echo can consistently account for the slow decline. A spectrum at 1083 days shows the same [O i] λ6300 and [Ca ii] λ7300 lines as seen at ∼300–400 days, with no new features to indicate strong circumstellar interaction. Radio limits with the Very Large Array rule out an extended wind for mass-loss rates yr−1 (where v 10 is the wind velocity in units of 10 km s−1). The optical light curve is consistent with L ∝ t −4, which we show is expected for magnetar spin-down with inefficient trapping; furthermore, the evolution matches predictions from earlier magnetar model fits. The opacity to magnetar radiation is constrained at ∼0.01 cm2 g−1, consistent with photon-matter pair-production over a broad ∼GeV–TeV range. This suggests that the magnetar spectral energy distribution, and hence the “missing energy” leaking from the ejecta, may peak in this range.

SN 2014C: VLBI images of a supernova interacting with a circumstellar shell

We report on VLBI measurements of supernova 2014C at several epochs between $t = 384$ and 1057 days after the explosion. SN 2014C was an unusual supernova that initially had Type Ib optical spectrum, but after $t = 130$ d it developed a Type IIn spectrum with prominent H$\alpha$ lines, suggesting the onset of strong circumstellar interaction. Our first VLBI observation was at $t = 384$ d, and we find that the outer radius of SN 2014C was $(6.40 \pm 0.26) \times 10^{16}$ cm (for a distance of 15.1 Mpc), implying an average expansion velocity of $19300 \pm 790$ \kms\ up to that time. At our last epoch, SN 2014C was moderately resolved and shows an approximately circular outline but with an enhancement of the brightness on the W side. The outer radius of the radio emission at $t = 1057$ d is $(14.9 \pm 0.6) \times 10^{16}$ cm. We find that the expansion between $t = 384$ and 1057 d is well described by a constant velocity expansion with $v = 13600 \pm 650$ \kms. SN 2014C had clearly been substantially decelerated by $t = 384$ d. Our measurements are compatible with a scenario where the expanding shock impacted upon a shell of dense circumstellar material during the first year, as suggested by the observations at other wavelengths, but had progressed through the dense shell by the time of the VLBI observations.

Time Lapse Spectropolarimetry: Constraining the Nature and Progenitors of Interacting CCSNe

AbstractSNe of Type IIn are among the brightest supernova explosions due to strong circumstellar interaction. Examining the geometric and optical properties of the circumstellar material (CSM) can help to identify the progenitors of individual IIn SNe. Polarimetry is the optimal method for constraining CSM characteristics, as polarimetric signals both depend upon and preserve geometric information from unresolved sources. I present the results of fitting an ensemble of simulated polarized Hα emission-line profiles of interacting SNe, created using a three-dimensional Monte Carlo radiative transfer code called SLIP, to the multi-epoch observed polarized spectra of the Type IIn SN 1997eg. Further study of this model ensemble will allow us to investigate relationships among SNe IIn based on viewing angle and consider how the category should be subdivided based on physical properties of the CSM and/or progenitor.

The Persistent Eruption of UGC 2773-OT: finally, a decade-long extragalactic Eta Carinae analogue

While SN impostors resemble the Great Eruption of eta Car in the sense that their spectra show narrow H lines and they have typical peak absolute magnitudes of -13 to -14 mag, most extragalactic events observed so far are quite different from eta Car in duration. Their bright phases typically last for 100~d or less, rather than persisting for several years. The transient object UGC2773-OT had a similar peak absolute magnitude to other SN impostors, but with a gradual 5-yr prediscovery rise. In the 6 yr since discovery, it has faded very slowly (0.26 mag/yr). Overall, we suggest that its decade-long eruption is so far the best known analog of eta Car's 19th century eruption. We discuss extensive spectroscopy of the ongoing eruption. The spectra show interesting changes in velocity and line shape that we discuss in detail, including an asymmetric Halpha emission line that we show is consistent with the ejection of a bipolar nebula that could be very much like the Homunculus of eta Car. Moreover, changes in the line width, line profile, blue excess emission resembling that of Type IIn supernovae, and the intensity of Halpha suggest the presence of strong circumstellar interaction in the eruption at late times. This supports the hypothesis that the extended plateau of eta Car's eruption may have been powered by shock interaction as well. One interesting difference compared to eta Car, however, is that UGC2773-OT so far does not exhibit the repeated brief spikes in luminosity that have been associated with binary periastron events.

DISCRIMINATING THE PROGENITOR TYPE OF SUPERNOVA REMNANTS WITH IRON K-SHELL EMISSION

Supernova remnants (SNRs) retain crucial information about both their parent explosion and circumstellar material left behind by their progenitor. However, the complexity of the interaction between supernova ejecta and ambient medium often blurs this information, and it is not uncommon for the basic progenitor type (Ia or core-collapse) of well-studied remnants to remain uncertain. Here we present a powerful new observational diagnostic to discriminate between progenitor types and constrain the ambient medium density of SNRs solely using Fe K-shell X-ray emission. We analyze all extant Suzaku observations of SNRs and detect Fe K alpha emission from 23 young or middle-aged remnants, including five first detections (IC 443, G292.0+1.8, G337.2-0.7, N49, and N63A). The Fe K alpha centroids clearly separate progenitor types, with the Fe-rich ejecta in Type Ia remnants being significantly less ionized than in core-collapse SNRs. Within each progenitor group, the Fe K alpha luminosity and centroid are well correlated, with more luminous objects having more highly ionized Fe. Our results indicate that there is a strong connection between explosion type and ambient medium density, and suggest that Type Ia supernova progenitors do not substantially modify their surroundings at radii of up to several parsecs. We also detect a K-shell radiative recombination continuum of Fe in W49B and IC 443, implying a strong circumstellar interaction in the early evolutionary phases of these core-collapse remnants.

THE ORIGIN OF KEPLER'S SUPERNOVA REMNANT

It is now well established that Kepler's supernova remnant is the result of a Type Ia explosion. With an age of 407 years, and an angular diameter of ~ 4', Kepler is estimated to be between 3.0 and 7.0 kpc distant. Unlike other Galactic Type Ia supernova remnants such as Tycho and SN 1006, and SNR 0509-67.5 in the Large Magellanic Cloud, Kepler shows evidence for a strong circumstellar interaction. A bowshock structure in the north is thought to originate from the motion of a mass-losing system through the interstellar medium prior to the supernova. We present results of hydrodynamical and spectral modeling aimed at constraining the circumstellar environment of the system and the amount of 56Ni produced in the explosion. Using models that contain either 0.3 M_sun (subenergetic) or 1 M_sun (energetic) of 56Ni, we simulate the interaction between supernova Ia ejecta and various circumstellar density models. Based on dynamical considerations alone, we find that the subenergetic models favor a distance to the SNR of < 6.4 kpc, while the model that produces 1 M_sun of 56Ni requires a distance to the SNR of > 7 kpc. The X-ray spectrum is consistent with an explosion that produced ~ 1 M_sun of 56Ni, ruling out the subenergetic models, and suggesting that Kepler's SNR was a SN 1991T-like event. Additionally, the X-ray spectrum rules out a pure 1/r^2 wind profile expected from isotropic mass loss up to the time of the supernova. Introducing a small cavity around the progenitor system results in modeled X-ray spectra that are consistent with the observed spectrum. If a wind shaped circumstellar environment is necessary to explain the dynamics and X-ray emission from the shocked ejecta in Kepler's SNR, then we require that the distance to the remnant be greater than 7 kpc.

Circumstellar interaction in type Ibn supernovae and SN 2006jc

I analyse peculiar properties of light curve and continua of enigmatic Ibn supernovae, including SN 2006jc, and argue in favour of the early strong circumstellar interaction. This interaction explains the high luminosity and fast flux rise of SN 1999cq, while the cool dense shell formed in shocked ejecta can explain the smooth early continuum of SN 2000er and unusual blue continuum of SN 2006jc. The dust is shown to condense in the cool dense shell at about day 50. Monte Carlo modelling of the He I 7065 \AA line profile affected by the dust occultation supports a picture, in which the dust resides in the fragmented cool dense shell, whereas He I lines originate from circumstellar clouds shocked and fragmented in the forward shock wave.

Supernovae and their Evolution in a Low Metallicity ISM

AbstractObservations of core collapse supernovae and their progenitors generally support expectations of increasing mass loss with increasing initial mass. Mass loss rates are expected to decline at lower metallicity, and there are prospects for directly testing this for the red supergiant progenitors of Type IIP supernovae. However, there are indications that mass loss rates for high mass early type stars may be overestimated and that there are mass loss mechanisms that do not decline at lower metallicity. In this case, there may be supernova emission from strong circumstellar interaction even at low metallicity. Although there is evidence for dust formation in freely expanding ejecta of supernovae, the quantities are relatively small. Another promising site of dust formation is the circumstellar interaction region, but this should occur in only a fraction of supernovae.

The X-ray spectrum and light curve of Supernova 1995N

We report on multi-epoch X-ray observations of the Type IIn (narrow emission line) supernova SN 1995N with the ROSAT and ASCA satellites. The January 1998 ASCA X-ray spectrum is well fitted by a thermal bremsstrahlung (kT~10 keV, N_H~6e20 cm^-2) or power-law (alpha~1.7, N_H~1e21 cm^-2) model. The X-ray light curve shows evidence for significant flux evolution between August 1996 and January 1998: the count rate from the source decreased by 30% between our August 1996 and August 1997 ROSAT observations, and the X-ray luminosity most likely increased by a factor of ~2 between our August 1997 ROSAT and January 1998 ASCA observations, although evolution of the spectral shape over this interval is not ruled out. The high X-ray luminosity, L_X~1e41 erg/sec, places SN 1995N in a small group of Type IIn supernovae with strong circumstellar interaction, and the evolving X-ray luminosity suggests that the circumstellar medium is distributed inhomogeneously.

A Deep Search for Radio Emission from the Type II Supernovae SN 1984E and SN 1986E

We have used the VLA to place deep upper limits on the radio emission from the Type II-L supernovae SN 1984E in NGC 3169 and SN 1986E in NGC 4302. The optical spectra of SN 1984E near maximum light showed evidence of strong circumstellar interaction. The lack of detectable radio emission from SN 1984E at an age of 11 years is consistent with the idea that near its optical maximum SN 1984E was interacting with a circumstellar shell that resulted from a preexplosion mass-loss episode that was of limited duration. However, optical spectra of SN 1986E have shown that it was undergoing strong circumstellar interaction as recently as 1994. Therefore, the lack of detectable radio emission from SN 1986E in 1995 is surprising. SN 1986E is the first old supernova to have been detected in the optical but not in the radio. This calls for further detailed modeling of the optical and radio emission from circumstellar interaction.