We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN 2017egm and analyze these with a larger ...sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 e-folding times after light-curve peak, with the rate of spectroscopic evolution correlated to the light-curve timescale. Emission lines are identified with well-known transitions of oxygen, calcium, magnesium, sodium, and iron. SLSNe are differentiated from other SNe Ic by a prominent O i λ7774 line and higher ionization states of oxygen. The iron-dominated region around 5000 is more similar to broad-lined SNe Ic than to normal SNe Ic. Principal component analysis shows that five "eigenspectra" capture 70% of the variance, while a clustering analysis shows no clear evidence for multiple SLSN subclasses. Line velocities are 5000-8000 km s−1 and show stratification of the ejecta. O i λ7774 likely arises in a dense inner region that also produces calcium emission, while O i λ6300 comes from farther out until 300-400 days. The luminosities of O i λ7774 and Ca ii suggest significant clumping, in agreement with previous studies. Ratios of Ca ii λ7300/O i λ6300 favor progenitors with relatively massive helium cores, likely 6 , though more modeling is required here. SLSNe with broad light curves show the strongest O i λ6300, suggesting larger ejecta masses. We show how the inferred velocity, density, and ionization structure point to a central power source.
Despite indications that superluminous supernovae (SLSNe) originate from massive progenitors, the lack of a uniformly analyzed statistical sample has so far prevented a detailed view of the ...progenitor mass distribution. Here we present and analyze the pre-explosion mass distribution of hydrogen-poor SLSN progenitors as determined from uniformly modeled light curves of 62 events. We construct the distribution by summing the ejecta mass posteriors of each event, using magnetar light-curve models presented in our previous works (and using a nominal neutron star remnant mass). The resulting distribution spans 3.6-40 M , with a sharp decline at lower masses, and is best fit by a broken power law described by at 3.6-8.6 M and at 8.6-40 M . We find that observational selection effects cannot account for the shape of the distribution. Relative to Type Ib/c SNe, the SLSN mass distribution extends to much larger masses and has a different power-law shape, likely indicating that the formation of a magnetar allows more massive stars to explode as some of the rotational energy accelerates the ejecta. Comparing the SLSN distribution with predictions from single and binary star evolution models, we find that binary models for a metallicity of Z 1/3 Z are best able to reproduce its broad shape, in agreement with the preference of SLSNe for low metallicity environments. Finally, we uncover a correlation between the pre-explosion mass and the magnetar initial spin period, where SLSNe with low masses have slower spins, a trend broadly consistent with the effects of angular momentum transport evident in models of rapidly rotating carbon-oxygen stars.
Some reports of supernova (SN) discoveries turn out not to be true core-collapse explosions. One such case was SN 2009ip, which was recognized to be the eruption of a luminous blue variable (LBV) ...star. This source had a massive (50-80 M), hot progenitor star identified in pre-explosion data; it had documented evidence of pre-outburst variability and it was subsequently discovered to have a second outburst in 2010. In 2012, the source entered its third known outburst. Initial spectra showed the same narrow-line profiles as before, suggesting another LBV-like eruption. We present new photometry and spectroscopy of SN 2009ip, indicating that the 2012 outburst transitioned into a genuine SN explosion. The most striking aspect of these data is that unlike any previous episodes, the spectrum developed Balmer lines with very broad P-Cygni profiles characteristic of normal Type II supernovae (SNe II), in addition to overlying narrow emission components. The emission lines exhibit unprecedented (for any known non-terminal LBV-like eruption) full width at half-maximum intensity values of ∼8000 km s−1, while the absorption components seen just before the main brightening had blue wings extending out to −13 000 km s−1. These velocities are typical of core-collapse SN explosions, but have never been associated with emission lines from a non-terminal LBV-like eruption. SN 2009ip is the first object to have both a known massive blue progenitor star and LBV-like eruptions with accompanying spectra observed a few years prior to becoming a SN. Immediately after the broad lines first appeared, the peak absolute magnitude of M
V
−14.5 was fainter than that of normal SNe II. However, after a brief period of fading, the source quickly brightened again to M
R
= −17.5 mag in ∼2 d, suggesting a causal link to the prior emergence of the broad-line spectrum. Once the bright phase began, the broad lines mostly disappeared, and the spectrum resembled the early optically thick phases of luminous SNe IIn. The source reached a peak brightness of −18 mag about 2 weeks later, after which broad emission lines again developed in the spectrum as the source faded. We conclude that the most recent 2012 outburst of SN 2009ip was the result of a true core-collapse SN IIn that occurred when the progenitor star was in an LBV-like outburst phase, and where the SN was initially faint and then rapidly brightened due to interaction with circumstellar material. The pulsational pair instability, LBV-like eruptions or other instabilities due to late nuclear burning phases in massive stars may have caused the multiple pre-SN eruptions.
ABSTRACT We present the results of an extensive Hubble Space Telescope imaging study of 105, mostly Swift, long-duration gamma-ray bursts (LGRBs) spanning , which were localized using relative ...astrometry from ground- and space-based afterglow observations. We measure the distribution of LGRB offsets from their host centers and their relation to the underlying host light distribution. We find that the host-normalized offsets of LGRBs are more centrally concentrated than expected for an exponential disk profile, = 0.63, and in particular they are more concentrated than the underlying surface brightness profiles of their host galaxies and more concentrated than supernovae. The fractional flux distribution, with a median of 0.78, indicates that LGRBs prefer some of the brightest locations in their host galaxies but are not as strongly correlated as previous studies indicated. Importantly, we find a clear correlation between offset and fractional flux, where bursts at offsets exclusively occur at fractional fluxes , while bursts at have a uniform fractional flux distribution. This indicates that the spatial correlation of LGRBs with bright star-forming regions seen in the full sample is dominated by the contribution from bursts at small offset and that LGRBs in the outer parts of galaxies show no preference for unusually bright regions. We conclude that LGRBs strongly prefer the bright, inner regions of their hosts, indicating that the star formation taking place there is more favorable for LGRB progenitor production. This indicates that environmental factors beyond metallicity, such as binary interactions or IMF differences, may operate in the central regions of LGRB hosts.
At redshift z = 0.03, the recently discovered SN 2017egm is the nearest Type I superluminous supernova (SLSN) to date and first near the center of a massive spiral galaxy (NGC 3191). Using SDSS ...spectra of NGC 3191, we find a metallicity ∼2 at the nucleus and ∼1.3 for a star-forming region at a radial offset similar to SN 2017egm. Archival radio-to-UV photometry reveals a star formation rate of ∼15 yr−1 (with ∼70% dust obscured), which can account for a Swift X-ray detection and a stellar mass of . We model the early UV-optical light curves with a magnetar central-engine model, using the Bayesian light curve fitting tool MOSFiT. The fits indicate an ejecta mass of 2-4 , a spin period of 4-6 ms, a magnetic field of G, and a kinetic energy of erg. These parameters are consistent with the overall distributions for SLSNe, modeled by Nicholl et al., although the derived mass and spin are toward the low end, possibly indicating an enhanced loss of mass and angular momentum before explosion. This has two implications: (i) SLSNe can occur at solar metallicity, although with a low fraction of ∼10%, and (ii) metallicity has at most a modest effect on their properties. Both conclusions are in line with results for long gamma-ray bursts. Assuming a monotonic rise gives an explosion date of MJD 57889 1. However, a short-lived excess in the data relative to the best-fitting models may indicate an early-time "bump." If confirmed, SN 2017egm would be the first SLSN with a spectrum during the bump phase; this shows the same O ii lines seen at maximum light, which may be an important clue for explaining these bumps.
Abstract
We present optical photometry and spectroscopy of SN 2019stc (=ZTF19acbonaa), an unusual Type Ic supernova (SN Ic) at a redshift of
z
= 0.117. SN 2019stc exhibits a broad double-peaked light ...curve, with the first peak having an absolute magnitude of
M
r
= −20.0 mag, and the second peak, about 80 rest-frame days later,
M
r
= −19.2 mag. The total radiated energy is large,
E
rad
≈ 2.5 × 10
50
erg. Despite its large luminosity, approaching those of Type I superluminous supernovae (SLSNe), SN 2019stc exhibits a typical SN Ic spectrum, bridging the gap between SLSNe and SNe Ic. The spectra indicate the presence of Fe-peak elements, but modeling of the first light-curve peak with radioactive heating alone leads to an unusually high nickel mass fraction of
f
Ni
≈ 0.31 (
M
Ni
≈ 3.2
M
⊙
). Instead, if we model the first peak with a combined magnetar spin-down and radioactive heating model we find a better match with
M
ej
≈ 4
M
⊙
, a magnetar spin period of
P
spin
≈ 7.2 ms, and magnetic field of
B
≈ 10
14
G, and
f
Ni
≲ 0.2 (consistent with SNe Ic). The prominent second peak cannot be naturally accommodated with radioactive heating or magnetar spin-down, but instead can be explained as circumstellar interaction with ≈0.7
M
⊙
of hydrogen-free material located ≈400 au from the progenitor. Accounting for the ejecta mass, circumstellar shell mass, and remnant neutron star mass, we infer a CO core mass prior to explosion of ≈6.5
M
⊙
. The host galaxy has a metallicity of ≈0.26
Z
⊙
, low for SNe Ic but consistent with SLSNe. Overall, we find that SN 2019stc is a transition object between normal SNe Ic and SLSNe.
Abstract
We present an extensive Hubble Space Telescope rest-frame UV imaging study of the locations of Type I superluminous supernovae (SLSNe) within their host galaxies. The sample includes 65 ...SLSNe with detected host galaxies in the redshift range
z
≈ 0.05–2. Using precise astrometric matching with SN images, we determine the distributions of the physical and host-normalized offsets relative to the host centers, as well as the fractional flux distribution relative to the underlying UV light distributions. We find that the host-normalized offsets of SLSNe roughly track an exponential disk profile, but exhibit an overabundance of sources with large offsets of 1.5–4 times their hosts' half-light radii. The SLSNe normalized offsets are systematically larger than those of long gamma-ray bursts (LGRBs), and even Type Ib/c and Type II SNe. Furthermore, we find from a Monte Carlo procedure that about
37
−
8
+
6
%
of SLSNe occur in the dimmest regions of their host galaxies, with a median fractional flux value of 0.16, in stark contrast to LGRBs and Type Ib/c and Type II SNe. We do not detect any significant trends in the locations of SLSNe as a function of redshift, or as a function of explosion and magnetar engine parameters inferred from modeling of their optical light curves. The significant difference in SLSN locations compared to LGRBs (and normal core-collapse SNe) suggests that at least some of their progenitors follow a different evolutionary path. We speculate that SLSNe arise from massive runaway stars from disrupted binary systems, with velocities of ∼10
2
km s
−1
.
Abstract
Stripped-envelope core-collapse supernovae can be divided into two broad classes: the common Type Ib/c supernovae (SNe Ib/c), powered by the radioactive decay of
56
Ni, and the rare ...superluminous supernovae (SLSNe), most likely powered by the spin-down of a magnetar central engine. Up to now, the intermediate regime between these two populations has remained mostly unexplored. Here, we present a comprehensive study of 40
luminous supernovae
(LSNe), SNe with peak magnitudes of
M
r
= −19 to −20 mag, bound by SLSNe on the bright end and by SNe Ib/c on the dim end. Spectroscopically, LSNe appear to form a continuum between Type Ic SNe and SLSNe. Given their intermediate nature, we model the light curves of all LSNe using a combined magnetar plus radioactive decay model and find that they are indeed intermediate, not only in terms of their peak luminosity and spectra, but also in their rise times, power sources, and physical parameters. We subclassify LSNe into distinct groups that are either as fast evolving as SNe Ib/c or as slow evolving as SLSNe, and appear to be either radioactively or magnetar powered, respectively. Our findings indicate that LSNe are powered by either an overabundant production of
56
Ni or by weak magnetar engines, and may serve as the missing link between the two populations.
We present optical photometry and spectroscopy of SN 2016iet (=Gaia16bvd=PS17brq), an unprecedented Type I supernova (SN I) at z = 0.0676 with no obvious analog in the existing literature. SN 2016iet ...exhibits a peculiar light curve, with two roughly equal brightness peaks ( −19 mag) separated by about 100 days, and a subsequent slow decline by about 5 mag in 650 rest-frame days. The spectra are dominated by strong emission lines of calcium and oxygen, with a width of only 3400 km s−1, superposed on a strong blue continuum in the first year. There is no clear evidence for hydrogen or helium associated with the SN at any phase. The nebular spectra exhibit a ratio of , much larger than for core-collapse SNe and Type I superluminous SNe. We model the light curves with several potential energy sources: radioactive decay, a central engine, and ejecta-circumstellar medium (CSM) interaction. Regardless of the model, the inferred progenitor mass near the end of its life (i.e., the CO core mass) is 55 M and potentially up to 120 M , clearly placing the event in the regime of pulsational pair instability supernovae (PPISNe) or pair instability supernovae (PISNe). The models of CSM interaction provide the most consistent explanation for the light curves and spectra, and require a CSM mass of 35 M ejected in the final decade before explosion. We further find that SN 2016iet is located at an unusually large projected offset (16.5 kpc, 4.3 effective radii) from its low-metallicity dwarf host galaxy (Z 0.1 Z , L 0.02 L*, M 108.5 M ), supporting the interpretation of a PPISN/PISN explosion. In our final spectrum at a phase of about 770 rest-frame days we detect weak and narrow H emission at the location of the SN, corresponding to a star formation rate of 3 × 10−4 M yr−1, which is likely due to a dim underlying galaxy host or an H ii region. Despite the overall consistency of the SN and its unusual environment with PPISNe and PISNe, we find that the inferred properties of SN 2016iet challenge existing models of such events.
Abstract
We present the discovery of the Type II supernova SN 2023ixf in M101 and follow-up photometric and spectroscopic observations, respectively, in the first month and week of its evolution. Our ...discovery was made within a day of estimated first light, and the following light curve is characterized by a rapid rise (≈5 days) to a luminous peak (
M
V
≈ − 18.2 mag) and plateau (
M
V
≈ − 17.6 mag) extending to 30 days with a fast decline rate of ≈0.03 mag day
−1
. During the rising phase,
U
−
V
color shows blueward evolution, followed by redward evolution in the plateau phase. Prominent flash features of hydrogen, helium, carbon, and nitrogen dominate the spectra up to ≈5 days after first light, with a transition to a higher ionization state in the first ≈2 days. Both the
U
−
V
color and flash ionization states suggest a rise in the temperature, indicative of a delayed shock breakout inside dense circumstellar material (CSM). From the timescales of CSM interaction, we estimate its compact radial extent of ∼(3–7) × 10
14
cm. We then construct numerical light-curve models based on both continuous and eruptive mass-loss scenarios shortly before explosion. For the continuous mass-loss scenario, we infer a range of mass-loss history with 0.1–1.0
M
⊙
yr
−1
in the final 2−1 yr before explosion, with a potentially decreasing mass loss of 0.01–0.1
M
⊙
yr
−1
in ∼0.7–0.4 yr toward the explosion. For the eruptive mass-loss scenario, we favor eruptions releasing 0.3–1
M
⊙
of the envelope at about a year before explosion, which result in CSM with mass and extent similar to the continuous scenario. We discuss the implications of the available multiwavelength constraints obtained thus far on the progenitor candidate and SN 2023ixf to our variable CSM models.
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