arXiv subjects

Christopher Bilinski

Publications and source records attributed to Christopher Bilinski.

14 recordsLinked to original sources

arXiv2025

Spectropolarimetric Evolution Reveals Dual-Axis Ejecta in the Atypical Magnetar-Powered SN 2012au

We present six epochs of optical spectropolarimetric observations of the unique and slow-evolving Type Ib supernova (SN) 2012au, between 0 and 295 days post R-band maximum. The polarization levels seen throughout our observations are on average 0.87% +/- 0.05% higher than those of any Type Ib SN~yet studied, suggesting either that it is the most asymmetric of the sample, or if all SNe Ib have similar asymmetry, that it is viewed at a more optimum angle. Significant continuum polarization indicates that the photosphere exhibited a global departure from spherical symmetry at the level of 10%-40% at the earliest times (days 0--40), which decreased to 0%-20% by days 57--90. During the early photospheric phase, the ejecta maintained a near-constant orientation of 12{\deg}-20{\deg} on the sky, as shown by the dominant axis in the Stokes q-u plane. Polarization signatures in the Fe II {\lambda}{\lambda}{\lambda}4924, 5018, 5169 lines shared this axis. Meanwhile, high levels of polarization associated with the He I lines traced distinct q-u loops with a dramatic rotation away from the dominant axis, indicating that the early-time ejecta were also characterized by hot, fast, helium-rich material concentrated near the poles. At day 295, during the transition to the nebular phase, a new, highly elongated structure became prominent in the ejecta, with an axis orthogonal to the dominant axis that defined the photospheric phase. This dual-axis geometry may link SN 2012au's high luminosity and asymmetric structure to a magnetar powering mechanism.

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arXiv2024

Spectropolarimetry of SN 2023ixf reveals both circumstellar material and helium core to be aspherical

We present multi-epoch optical spectropolarimetric and imaging polarimetric observations of the nearby Type II supernova (SN) 2023ixf discovered in M101 at a distance of 6.85 Mpc. The first imaging polarimetric observations were taken +2.33 days (60085.08 MJD) after the explosion, while the last imaging polarimetric data points (+73.19 and +76.19 days) were acquired after the fall from the light curve plateau. At +2.33 days there is strong evidence of circumstellar material (CSM) interaction in the spectra and the light curve. A significant level of intrinsic polarization $p_r = 1.02\pm 0.07 \% $ is seen during this phase which indicates that this CSM is aspherical. We find that the polarization evolves with time toward the interstellar polarization level during the photospheric phase, which suggests that the recombination photosphere is spherically symmetric. There is a jump in polarization ($p_r =0.45 \pm 0.08 \% $ and $p_r =0.62 \pm 0.08 \% $) at +73.19 and +76.19 days when the light curve falls from the plateau. This is a phase where polarimetric data is sensitive to non-spherical inner ejecta or a decrease in optical depth into the single scattering regime. We also present spectropolarimetric data that reveal line (de)polarization during most of the observed epochs. In addition, at +14.50 days we see an ``inverse P Cygni" profile in the H and He line polarization, which clearly indicates the presence of asymmetrically distributed material overlying the photosphere. The overall temporal evolution of polarization is typical for Type II SNe, but the high level of polarization during the rising phase has only been observed in SN 2023ixf.

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arXiv2023

Record-breaking polarization from the interacting superluminous supernova 2017hcc

We present multiepoch spectropolarimetry of the superluminous interacting Type IIn supernova SN2017hcc, covering 16 to 391 days after explosion. In our first epoch we measure continuum polarization as high as 6%, making SN 2017hcc the most intrinsically polarized SN ever reported. During the first 29 days of coverage, when the polarization is strongest, the continuum polarization has a wavelength dependence that rises toward blue wavelengths, and becomes wavelength independent by day 45. The polarization strength drops rapidly during the first month, even as the SN flux is still climbing to peak brightness. Nonetheless, record-high polarization is maintained until day 68, at which point the source polarization declines to 1.9%, comparable to peak levels in previous well-studied SNe IIn. Thereafter the SN continues in polarization decline, while exhibiting only minor changes in position angle on the sky. The blue slope of the polarized continuum during the first month, accompanied by short-lived polarized flux for Balmer emission, suggests that an aspherical distribution of dust grains in pre-shock circumstellar material (CSM) is echoing the SN IIn spectrum and strongly influencing the polarization, while the subsequent decline during the wavelength-independent phase appears broadly consistent with electron scattering near the SN/CSM interface. The persistence of the polarization position angle between these two phases suggests that the pre-existing CSM responsible for the dust scattering at early times is part of the same geometric structure as the electron-scattering region that dominates the polarization at later times. SN2017hcc appears to be yet another, but much more extreme, case of aspherical yet well-ordered CSM in Type IIn SNe, possibly resulting from pre-SN mass loss shaped by a binary progenitor system.

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arXiv2023

Multi-Epoch Spectropolarimetry for a Sample of Type IIn Supernovae: Persistent Asymmetry in Dusty Circumstellar Material

We present multi-epoch spectropolarimetry and spectra for a sample of 14 Type IIn supernovae (SNe IIn). We find that after correcting for likely interstellar polarization, SNe IIn commonly show intrinsic continuum polarization of 1--3% at the time of peak optical luminosity, although a few show weaker or negligible polarization. While some SNe IIn have even stronger polarization at early times, their polarization tends to drop smoothly over several hundred days after peak. We find a tendency for the intrinsic polarization to be stronger at bluer wavelengths, especially at early times. While polarization from an electron scattering region is expected to be grey, scattering of SN light by dusty circumstellar material (CSM) may induce such a wavelength-dependent polarization. For most SNe IIn, changes in polarization degree and wavelength dependence are not accompanied by changes in the position angle, requiring that asymmetric pre-SN mass loss had a persistent geometry. While 2--3% polarization is typical, about 30% of SNe IIn have very low or undetected polarization. Under the simplifying assumption that all SN IIn progenitors have axisymmetric CSM (i.e. disk/torus/bipolar), then the distribution of polarization values we observe is consistent with similarly asymmetric CSM seen from a distribution of random viewing angles. This asymmetry has very important implications for understanding the origin of pre-SN mass loss in SNe IIn, suggesting that it was shaped by binary interaction.

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arXiv2020

The electron-capture origin of supernova 2018zd

In the transitional mass range ($\sim$ 8-10 solar masses) between white dwarf formation and iron core-collapse supernovae, stars are expected to produce an electron-capture supernova. Theoretically, these progenitors are thought to be super-asymptotic giant branch stars with a degenerate O+Ne+Mg core, and electron capture onto Ne and Mg nuclei should initiate core collapse. However, no supernovae have unequivocally been identified from an electron-capture origin, partly because of uncertainty in theoretical predictions. Here we present six indicators of electron-capture supernovae and show that supernova 2018zd is the only known supernova having strong evidence for or consistent with all six: progenitor identification, circumstellar material, chemical composition, explosion energy, light curve, and nucleosynthesis. For supernova 2018zd, we infer a super-asymptotic giant branch progenitor based on the faint candidate in the pre-explosion images and the chemically-enriched circumstellar material revealed by the early ultraviolet colours and flash spectroscopy. The light-curve morphology and nebular emission lines can be explained with the low explosion energy and neutron-rich nucleosynthesis produced in an electron-capture supernova. This identification provides insights into the complex stellar evolution, supernova physics, cosmic nucleosynthesis, and remnant populations in the transitional mass range.

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arXiv2020

SN 2014ab: An Aspherical Type IIn Supernova with Low Polarization

We present photometry, spectra, and spectropolarimetry of supernova (SN) 2014ab, obtained through $\sim 200$ days after peak brightness. SN 2014ab was a luminous Type IIn SN ($M_V < -19.14$ mag) discovered after peak brightness near the nucleus of its host galaxy, VV 306c. Prediscovery upper limits constrain the time of explosion to within 200 days prior to discovery. While SN 2014ab declined by $\sim 1$ mag over the course of our observations, the observed spectrum remained remarkably unchanged. Spectra exhibit an asymmetric emission-line profile with a consistently stronger blueshifted component, suggesting the presence of dust or a lack of symmetry between the far side and near side of the SN. The Pa$β$ emission line shows a profile very similar to that of H$α$, implying that this stronger blueshifted component is caused either through obscuration by large dust grains, occultation by optically thick material, or a lack of symmetry between the far side and near side of the interaction region. Despite these asymmetric line profiles, our spectropolarimetric data show that SN 2014ab has little detected polarization after accounting for the interstellar polarization. This suggests that we are seeing emission from a photosphere that has only small deviation from circular symmetry face-on. We are likely seeing a SN IIn with nearly circular symmetry in the plane normal to our line of sight, but with either large-grain dust or significant asymmetry in the density of circumstellar material or SN ejecta along our line of sight. We suggest that SN 2014ab and SN 2010jl (as well as other SNe IIn) may be similar events viewed from different directions.

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arXiv2020

A new and unusual LBV-like outburst from a Wolf-Rayet star in the outskirts of M33

MCA-1B (also called UIT003) is a luminous hot star in the western outskirts of M33, classified over 20yr ago with a spectral type of Ofpe/WN9 and identified then as a candidate luminous blue variable (LBV). Palomar Transient Factory data reveal that this star brightened in 2010, with a light curve resembling that of the classic LBV star AFAnd in M31. Other Ofpe/WN9 stars have erupted as LBVs, but MCA-1B was unusual because it remained hot. It showed a WN-type spectrum throughout its eruption, whereas LBVs usually get much cooler. MCA-1B showed an almost four-fold increase in bolometric luminosity and a doubling of its radius, but its temperature stayed around 29kK. As it faded, it shifted to even hotter temperatures, exhibiting a WN7/WN8-type spectrum, and doubling its wind speed. MCA-1B is reminiscent of some supernova impostors, and its location resembles the isolated environment of SN 2009ip. It is most similar to HD5980 (in the SMC) and GR 290 (also in M33). Whereas these two LBVs exhibited B-type spectra in eruption, MCA-1B is the first clear case where a Wolf-Rayet (WR) spectrum persisted at all times. Together, MCA-1B, HD 5980, and GR 290 constitute a class of WN-type LBVs, distinct from S Doradus LBVs. They are most interesting in the context of LBVs at low metallicity, a possible post-LBV/WR transition in binaries, and as likely Type~Ibn supernova progenitors.

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arXiv2018

ASASSN-15nx: A luminous Type II supernova with a "perfect" linear decline

We report a luminous Type II supernova, ASASSN-15nx, with a peak luminosity of M_V=-20 mag, that is between typical core-collapse supernovae and super-luminous supernovae. The post-peak optical light curves show a long, linear decline with a steep slope of 2.5 mag/100 days (i.e., an exponential decline in flux), through the end of observations at phase ~260 days. In contrast, the light curves of hydrogen rich supernovae (SNe II-P/L) always show breaks in their light curves at phase ~100 days, before settling onto Co56 radioactive decay tails with a decline rate of about 1 mag/100 days. The spectra of ASASSN-15nx do not exhibit the narrow emission-line features characteristic of Type IIn SNe, which can have a wide variety of light-curve shapes usually attributed to strong interactions with a dense circumstellar medium (CSM). ASASSN-15nx has a number of spectroscopic peculiarities, including a relatively weak and triangularly-shaped H-alpha emission profile with no absorption component. The physical origin of these peculiarities is unclear, but the long and linear post-peak light curve without a break suggests a single dominant powering mechanism. Decay of a large amount of Ni56 (M_Ni56 = 1.6 +/- 0.2 M_sun) can power the light curve of ASASSN-15nx, and the steep light-curve slope requires substantial gamma-ray escape from the ejecta, which is possible given a low-mass hydrogen envelope for the progenitor. Another possibility is strong CSM interactions powering the light curve, but the CSM needs to be sculpted to produce the unique light-curve shape and to avoid producing SN IIn-like narrow emission lines.

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arXiv2018

SN2013fs and SN2013fr: Exploring the circumstellar-material diversity in Type II supernovae

We present photometry and spectroscopy of SN2013fs and SN2013fr in the first 100 days post-explosion. Both objects showed transient, relatively narrow H$α$ emission lines characteristic of SNeIIn, but later resembled normal SNeII-P or SNeII-L, indicative of fleeting interaction with circumstellar material (CSM). SN2013fs was discovered within 8hr of explosion. Its light curve exhibits a plateau, with spectra revealing strong CSM interaction at early times. It is a less luminous version of the transitional SNIIn PTF11iqb, further demonstrating a continuum of CSM interaction intensity between SNeII-P and IIn. It requires dense CSM within 6.5$\times$10$^{14}$~cm of the progenitor, from a phase of advanced pre-SN mass loss shortly before explosion. Spectropolarimetry of SN2013fs shows little continuum polarization, but noticeable line polarization during the plateau phase. SN2013fr morphed from a SNIIn at early times to a SNII-L. After the first epoch its narrow lines probably arose from host-galaxy emission, but the bright, narrow H$α$ emission at early times may be intrinsic. As for SN2013fs, this would point to a short-lived phase of strong CSM interaction if proven to be intrinsic, suggesting a continuum between SNeIIn and II-L. It is a low-velocity SNII-L, like SN2009kr but more luminous. SN2013fr also developed an IR excess at later times, due to warm CSM dust that require a more sustained phase of strong pre-SN mass loss.

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arXiv2017

SN2012ab: A Peculiar Type IIn Supernova with Aspherical Circumstellar Material

We present photometry, spectra, and spectropolarimetry of supernova (SN) 2012ab, mostly obtained over the course of $\sim 300$ days after discovery. SN 2012ab was a Type IIn (SN IIn) event discovered near the nucleus of spiral galaxy 2MASXJ12224762+0536247. While its light curve resembles that of SN 1998S, its spectral evolution does not. We see indications of CSM interaction in the strong intermediate-width emission features, the high luminosity (peak at absolute magnitude $M=-19.5$), and the lack of broad absorption features in the spectrum. The H$α$ emission undergoes a peculiar transition. At early times it shows a broad blue emission wing out to $-14{,}000$ km $\mathrm{s^{-1}}$ and a truncated red wing. Then at late times ($>$ 100$\,$days) it shows a truncated blue wing and a very broad red emission wing out to roughly $+20{,}000$ km $\mathrm{s^{-1}}$. This late-time broad red wing probably arises in the reverse shock. Spectra also show an asymmetric intermediate-width H$α$ component with stronger emission on the red side at late times. The evolution of the asymmetric profiles requires a density structure in the distant CSM that is highly aspherical. Our spectropolarimetric data also suggest asphericity with a strong continuum polarization of $\sim 1-3$% and depolarization in the H$α$ line, indicating asphericity in the CSM at a level comparable to that in other SNe IIn. We estimate a mass-loss rate of $\dot{M} = 0.050\, {\rm M}_{\odot}\,\mathrm{yr^{-1}}$ for $v_{\rm pre} = 100$$\,$km$\,$$\mathrm{s^{-1}}$ extending back at least 75$\,$yr prior to the SN. The strong departure from axisymmetry in the CSM of SN 2012ab may suggest that the progenitor was an eccentric binary system undergoing eruptive mass loss.

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arXiv2016

Asymmetries in SN 2014J Near Maximum Light Revealed Through Spectropolarimetry

We present spectropolarimetric observations of the nearby Type Ia SN 2014J in M82 over six epochs: +0, +7, +23, +51, +77, +109, and +111 days with respect to B-band maximum. The strong continuum polarization, which is constant with time, shows a wavelength dependence unlike that produced by linear dichroism in Milky Way dust. The observed polarization may be due entirely to interstellar dust or include a circumstellar scattering component. We find that the polarization angle aligns with the magnetic field of the host galaxy, arguing for an interstellar origin. Additionally, we confirm a peak in polarization at short wavelengths that would imply $R_V < 2 $ along the light of sight, in agreement with earlier polarization measurements. For illustrative purposes, we include a two component fit to the continuum polarization of our +51 day epoch that combines a circumstellar scattering component with interstellar dust where scattering can account for over half of the polarization at $4000$ Å. Upon removal of the interstellar polarization signal, SN 2014J exhibits very low levels of continuum polarization. Asymmetries in the distribution of elements within the ejecta are visible through moderate levels of time-variable polarization in accordance with the Si II 6355 Åabsorption line. At maximum light, the line polarization reaches $\sim0.6$% and decreases to $\sim0.4\%$ one week later. This feature also forms a loop on the $q_{RSP}$-$u_{RSP}$ plane illustrating that the ion does not have an axisymmetric distribution. The observed polarization properties suggest the explosion geometry of SN 2014J is generally spheroidal with a clumpy distribution of silicon.

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arXiv2015

Constraints on Type IIn Supernova Progenitor Outbursts from the Lick Observatory Supernova Search

We searched through roughly 12 years of archival survey data acquired by the Katzman Automatic Imaging Telescope (KAIT) as part of the Lick Observatory Supernova Search (LOSS) in order to detect or place limits on possible progenitor outbursts of Type IIn supernovae (SNe~IIn). The KAIT database contains multiple pre-SN images for 5 SNe~IIn (plus one ambiguous case of a SN IIn/imposter) within 50 Mpc. No progenitor outbursts are found using the false discovery rate (FDR) statistical method in any of our targets. Instead, we derive limiting magnitudes (LMs) at the locations of the SNe. These limiting magnitudes (typically reaching $m_R \approx 19.5\,\mathrm{mag}$) are compared to outbursts of SN 2009ip and $η$ Car, plus additional simulated outbursts. We find that the data for SN 1999el and SN 2003dv are of sufficient quality to rule out events $\sim40$ days before the main peak caused by initially faint SNe from blue supergiant (BSG) precursor stars, as in the cases of SN 2009ip and SN 2010mc. These SNe~IIn may thus have arisen from red supergiant progenitors, or they may have had a more rapid onset of circumstellar matter interaction. We also estimate the probability of detecting at least one outburst in our dataset to be $\gtrsim60\%$ for each type of the example outbursts, so the lack of any detections suggests that such outbursts are either typically less luminous (intrinsically or owing to dust) than $\sim -13\,\mathrm{mag}$, or not very common among SNe~IIn within a few years prior to explosion.

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arXiv2013

Investigation of Kepler Objects of Interest Stellar Parameters from Observed Transit Durations

The Kepler mission discovery of candidate transiting exoplanets (KOIs) enables a plethora of ensemble analysis of the architecture and properties of exoplanetary systems. We compare the observed transit durations of KOIs to a synthetic distribution generated from the known eccentricities of radial velocity (RV) discovered exoplanets. We find that the Kepler and RV distributions differ at a statistically significant level. We identify three related systematic trends that are likely due to errors in stellar radii, which in turn affect the inferred exoplanet radii and the distribution thereof, and prevent a valid analysis of the underlying ensemble eccentricity distribution. First, 15% of KOIs have transit durations >20% longer than the transit duration expected for an edge-on circular orbit, including 92 KOIs with transit durations >50% longer, when only a handful of such systems are expected. Second, the median transit duration is too long by up to ~25%. Random errors of <50% in the stellar radius are not adequate to account for these two trends, and they are present for all spectral types in the Kepler sample. We identify that incorrect estimates of stellar metallicity and extinction could account for these anomalies, rather than astrophysical effects such as eccentric exoplanets improbably transiting near apastron. Third, we find that the median transit duration is correlated with stellar radius, when no such trend is expected. All three effects are still present, although less pronounced, when considering only multiple transiting KOI systems which are thought to have a low false positive rate. Improved stellar parameters for KOIs are necessary for the validity of future ensemble tests of exoplanetary systems found by Kepler.

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arXiv2013

Stars Don't Eat Their Young Migrating Planets - Empirical Constraints On Planet Migration Halting Mechanisms

Abridged: The discovery of "hot Jupiters" very close to their parent stars confirmed that Jovian planets migrate inward via several potential mechanisms. We present empirical constraints on planet migration halting mechanisms. We compute model density functions of close-in exoplanets in the orbital semi-major axis - stellar mass plane to represent planet migration that is halted via several mechanisms, including the interior 1:2 resonance with the magnetospheric disk truncation radius, the interior 1:2 resonance with the dust sublimation radius, and several scenarios for tidal halting. The models differ in the predicted power law dependence of the exoplanet orbital semi-major axis as a function stellar mass, and thus we also include a power law model with the exponent as a free parameter. We use a Bayesian analysis to assess the model success in reproducing empirical distributions of confirmed exoplanets and Kepler candidates that orbit interior to 0.1 AU. Our results confirm a correlation of the halting distance with stellar mass. Tidal halting provides the best fit to the empirical distribution of confirmed Jovian exoplanets at a statistically robust level, consistent with the Kozai mechanism and the spin-orbit misalignment of a substantial fraction of hot Jupiters. For Kepler candidates, which have a more restricted range in stellar mass compared to confirmed planets, we are unable to discern between the tidal dissipation and magnetospheric disk truncation braking mechanisms at a statistically significant level. The power law model favors exponents in the range of 0.38-0.9. This is larger than that predicted for tidal halting (0.23-0.33), which suggests that additional physics may be missing in the tidal halting theory.

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