arXiv subjects

Santiago González-Gaitán

Publications and source records attributed to Santiago González-Gaitán.

At least 19 recordsLinked to original sources

arXiv2026

Examining extinction distributions for type Ia supernovae in simulated 3D galaxies

Dust extinction and reddening greatly contribute to type Ia supernovae (SNe Ia) observed color and magnitude variations. The models used to describe the extinction probability density function (PDF) are often simplistic, which can negatively impact SN simulations and cosmology. We present an analysis of simulated SN Ia extinction in galaxies along realistic lines of sight and investigate the parameterization of its PDF, as well as its dependence on host properties. We employed SKIRT, a radiative transfer code, to simulate observations of SNe Ia in different environments and generate synthetic extinction distributions. To parameterize and fit these distributions, we used both the commonly assumed single-parameter exponential PDF and some of its two-parameter generalizations. We find that the standard exponential PDF does not adequately describe simulated SN extinction: It underestimates low-extinction events and overestimates high-extinction ones. 2D KS tests show significant differences between the simulated extinction distributions for SNe in different environments, which the exponential parameterization cannot properly distinguish. In contrast, the two-parameter PDFs parameterize SN extinction distributions more accurately across all simulated environments. Variations in host morphology or dust mass relate to variations in different PDF parameters, meaning that the two effects can effectively be disentangled. We conclude that the two-parameter Weibull or exponentiated exponential PDFs offer the best parameterizations of SN Ia extinction for a wide range of simulated environments. Analyzing observed SN colors from the literature and assuming a Gaussian distribution for the intrinsic component, we conclude that a two-parameter extinction PDF results in intrinsically redder SNe, with their mean intrinsic color shifted ~2$σ$ in relation to the standard exponential extinction PDF.

astro-ph.GA↗

arXiv2026

Narrow absorption lines from intervening material in supernovae. IV. Type Ia supernovae: Na I D line strength relating to external material and intrinsic properties

Type Ia supernovae (SNe Ia) are thermonuclear runaways of some white dwarfs in binary systems. They have been extensively studied, yet their progenitor and explosion mechanisms remain poorly understood. We study a large sample of SNe Ia comparing the narrow interstellar absorption features in their spectra with various photometric and spectroscopic supernova properties, as well as environmental characteristics. The sodium absorption is significantly stronger in younger, more star-forming and more centrally located SNe Ia, as expected. However, we also show that there is a relation with intrinsic properties that is independent of the environment. In fact, there is substantial evidence for two environmental SN Ia populations, an old and a young one, with the young population showing significantly different distributions of sodium strength when divided according to the Si II ejecta velocity, nebular velocity, extinction, E(B-V), and reddening curve, RV. Performing a clustering of the SNe Ia, we recover an old population of SNe with low extinction and normal ejecta velocity, while the young population can be indeed subdivided into a group of highly-extincted, high-velocity SNe Ia with much stronger blueshifted sodium absorption, and another of low-extincted, normal-velocity objects with little sodium absorption. We interpret this relation of intervening material with intrinsic properties as evidence for the young SN Ia population, occurring in young and star-forming environments, to have asymmetric radiation that interacts with nearby material, and whose observables depend thus on the viewing angle. Finally, we show that the cosmological mass-step is consistent with these populations.

astro-ph.SR↗

arXiv2026

On the impact of the supernova subsamples in reducing the Hubble tension

The persistent 4-6$σ$ difference between early- and late-time Hubble constant ($H_{0}$) measurements, known as the "Hubble tension", is a major problem in modern cosmology. We study how differences in colour ($c$), stretch ($x_{1}$), and host galaxy properties-stellar mass ($M$) and specific star formation rate (sSFR)-between calibration and Hubble Flow (HF) Type Ia supernova (SN Ia) samples used by SH0ES affect SN luminosity standardization and $H_{0}$ estimates. We generate subsamples from both, estimating $H_{0}$, $M_{B}$, $α$, $β$, $Δ_{host}$, and $σ_{int}$. We use Kolmogorov-Smirnov to assess the consistency between subsamples and reveal how parameter estimates change as sample matching improves. The calibration sample is not fully representative of the HF sample, especially in $M$ and sSFR. Improving sample consistency leads to changes in $H_{0}$, $M_{B}$ and $σ_{int}$, though overall values remain broadly stable. Better-matched subsamples tend to yield a mass step consistent with zero within 1$σ$. By disentangling SN subpopulations, we find persistent differences in $H_{0}$ ($\sim$2-3$σ$) and $M_{B}$ ($\sim2σ$) between low- and high-stretch SNe: $H_{0} = 75.27 \pm 1.18$ km s$^{-1}$ Mpc$^{-1}$ for low-stretch and $H_{0} = 71.25 \pm 1.59$ km s$^{-1}$ Mpc$^{-1}$ for high-stretch, resulting in Hubble tensions of 6.07$σ$ and 2.52$σ$. These differences suggest SNe Ia subpopulations with varying dust and intrinsic colour not captured by a single $β$, impacting cosmology. Estimating a single $H_{0}$ for both subpopulations yields $H_{0} = 73.78 \pm 2.17$ km s$^{-1}$ Mpc$^{-1}$, with a much larger uncertainty that lowers the Hubble tension from $5.87σ$ to $\sim2.86σ$. Our results suggest that the mass step may arise from an over-correction of more than one SN subpopulation associated to different environments.

astro-ph.CO↗

arXiv2026

Narrow absorption lines from intervening material in supernovae: III. Supernovae and their environments

Narrow interstellar absorption features in supernova (SN) spectra serve as valuable diagnostics for probing dust extinction and the presence of circumstellar or interstellar material. In this third paper in a series, we investigate how the strength of narrow interstellar absorption lines in low-resolution spectra varies with SN type and host galaxy properties, both on local and global scales. Using a dataset of over 10000 spectra from $\sim1800$ low-redshift SNe, we find that Type Ia SNe (SNe Ia) in passive galaxies exhibit significantly weaker narrow absorption features compared to CC-SNe and SNe Ia in star-forming hosts (SNe Ia-SF), suggesting lower interstellar gas content in quiescent environments. Within the star-forming hosts, the Na I D equivalent-width distribution of SNe II is much lower than that of both SNe Ia-SF and stripped-envelope SNe (SE-SNe). This result is somewhat unexpected, since CC-SNe are generally associated with star-forming regions and occur deeper within galactic disks, where stronger line-of-sight extinction would be anticipated. This suggests that the observed behaviour cannot be explained solely by absorption from the integrated interstellar medium (ISM) along the line of sight. Instead, if part of the absorption arises from material near the explosion, the similarity between the Na I D EW distributions of SNe Ia-SF and SE-SNe implies that comparable absorption signatures can emerge from distinct progenitor pathways. Possible explanations include (a) circumstellar material (CSM) expelled by the progenitor system before explosion, or (b) interaction of SN radiation with nearby patchy ISM clouds. Our results highlight the diagnostic power of interstellar absorption features in revealing the diverse environments and progenitor pathways of SNe.

astro-ph.GA↗

arXiv2025

Exploring the origins of high-velocity features in SNe Ia with the spectral synthesis code TARDIS

Appearing as secondary higher-velocity absorption components, high-velocity features (HVFs) have been observed in several absorption lines in many Type Ia supernovae (SNe Ia). The frequency and ubiquity of these components in silicon and calcium features specifically indicates that the mechanism through which they form must be common occurrence among the majority of SNe Ia. Here we present modelling of the HVF evolution in a sample of six well observed SNe Ia with the radiative-transfer code tardis. A base model is derived for each of the SNe to reproduce the photospheric velocity components, followed by a grid of simulations with Gaussian enhancements to the density profile at high velocities. We train a set of neural networks to emulate the impact of these density enhancements upon the simulated silicon line profile. These networks are subsequently used within a Markov-Chain Monte Carlo (MCMC) framework to infer the density enhancement parameters that most closely reproduce the HVF evolution. While we obtain good matches for the silicon profile, we find that a single density enhancement alone cannot simultaneously produce the observed silicon and calcium HVF evolution. Our findings indicate that neither the delayed-detonation mechanism, nor the double-detonation mechanism can produce these HVFs, suggesting that something may be missing from the models.

astro-ph.HE↗

arXiv2025

Narrow absorption lines from intervening material in supernovae. II. Galaxy properties

The interstellar medium (ISM) has a number of tracers such as the Na I D 5890, 5896 AA absorption lines that are evident in the spectra of galaxies but also in those of individual astrophysical sources such as stars, novae or quasars. Here, we investigate narrow absorption features in the spectra of nearby supernovae (SNe) and compare them to local (< 0.5 kpc) and global host galaxy properties. With a large and heterogeneous sample of spectra, we are able to recover the known relations of ISM with galaxy properties: larger columns of ISM gas are found in environments that are more massive, more actively star-forming, younger and viewed from a more inclined angle. Most trends are stronger for local than global properties, and we find that the ISM column density decreases exponentially with the offset from the host galaxy centre, as expected for a gas distribution following an exponential radial profile. We also confirm trends for the velocity of galactic outflows increasing with radius. The current study demonstrates the capability of individual light sources to serve as ubiquitous tracers of ISM properties across various environments and galaxies.

astro-ph.GA↗

arXiv2025

Statistical Analysis of Early Spectra in Type II and IIb Supernovae

We present a comprehensive analysis of the early spectra of type II and type IIb supernovae (SNe) to explore their diversity and distinguishable characteristics. Using 866 publicly available spectra from 393 SNe, 407 from type IIb SNe (SNe IIb) and 459 from type II SNe (SNe II), we analysed H$α$ and He~I 5876 A at early phases ($<40$ days from the explosion) to identify possible differences between these two SN types. By comparing the pseudo-equivalent width (pEW) and full width at half maximum (FWHM), we find that the strength of the absorption component of these lines serves as a quantitative discriminator, with SNe IIb exhibiting stronger lines at all times. The most significant differences emerge within the first 10-20 days. To assess the statistical significance of these differences, we apply statistical methods and machine-learning techniques. Population density evolution reveals a clear distinction in both pEW and FWHM. Quadratic Discriminant Analysis confirms distinct evolutionary patterns, particularly in pEW, while FWHM variations are less pronounced. A combination of t-distributed Stochastic Neighbour Embedding and Linear Discriminant Analysis effectively separates the two SN types. Additionally, a Random Forest Classifier demonstrates the robustness of pEW and FWHM as classification criteria, allowing for accurate classification of newly observed SNe II and IIb based on computed classification probabilities. Applying our method to low-resolution spectra obtained from the Zwicky Transient Facility Bright Transient, a magnitude-limited survey, we identified 34 misclassified SNe. This revision increases the estimated fraction of SNe IIb from 4.0% to 7.26%. This finding suggests that misclassification significantly impacts the estimated core-collapse SN rate. Our approach enhances classification accuracy and provides a valuable tool for future supernova studies.

astro-ph.HE↗

arXiv2025

Infant Core-collapse Supernovae with Circumstellar Interactions from KMTNet I: Luminous Transitional Case of KSP-SN-2022c

We present $BVi$ multi-band high-cadence observations of a Type II supernova (SN) KSP-SN-2022c from a star-forming galaxy at $z$ $\simeq$ 0.041 from its infant to nebular phase. Early light curve fitting with a single power-law is consistent with the first detection of roughly 15 minutes after shock breakout. The SN light curves feature a rapid rise and decline across its luminous ($V$ $\simeq$ -18.41 mag) peak together with a short plateau. The presence of the short plateau and rapid post-peak decline place the SN within a small group of transitional type between Type II-P and II-L subtypes. Its (i) broad and asymmetric H profiles with large emission-to-absorption ratios and (ii) near-peak luminosity in excess of predictions from SN shock cooling models both point to circumstellar interactions in this SN. Early colour evolution exhibits a short-lived blueward motion in $B-V$ within the first few days and continuous reddening in $V-i$, inconsistent with simple blackbody heating. Our simulations of SN light curves estimate 13 $M_\odot$ and 680 $R_\odot$ for the mass and radius of the progenitor, respectively, together with CSM of 0.73 $M_\odot$ to account for the excess luminosity and rapid post-peak declines. We discuss the origin of its short plateau and early colour evolution in the context of partial envelope stripping of the progenitor star and a delayed SN shock breakout near the edge of the CSM, respectively, as indicated by our simulations. We establish a correlation between post-peak decline rates and CSM mass in Type II SNe, highlighting that CSM interactions play a major role in shaping the post-peak evolution of transitional types.

astro-ph.HE↗

arXiv2025

$R_V$ from multi-waveband galaxy polarimetry in supernovae vicinity

Peculiar dust extinction laws have been reported for some type Ia supernovae (SNe) with the parameter $R_V$ much lower than the average value for the Milky Way (MW) of 3.1. Using optical photopolarimetry of supernova (SN) host galaxies, a few years after the explosion, we estimate $R_V$ in the vicinity of each SN and compare it with the extinction law calculated directly from SN observations. Multiband photopolarimetric data of nine galaxies, hosts of eleven SNe, acquired with VLT-FORS2 in IPOL mode, are used to map the polarization angle and the polarization degree in each galaxy. Data are processed with a custom-built reduction pipeline that corrects for instrumental, background, and MW interstellar polarization effects. The validity of Serkowski relations is tested at different locations in the galaxy to extract the wavelength of the maximum polarization λmax and obtain 2D maps for RV . When the fit to λmax at the SN location is poor, or impossible, an approximate Bayesian spatial inference method is employed to obtain an estimate of λmax using well-fitted neighboring locations. The estimated local $R_V$ for each SN is compared with published values from the SN light curves. We find $R_V$ values from optical photopolarimetry at SNe locations consistent with the average MW value and a median difference of > 3σ with the low peculiar $R_V$ obtained from the analysis of some reddened SN Ia light curves. The $R_V$ estimates obtained with BVRI photopolarimetry for the SNe vicinity are statistically similar to the hosts global $R_V$. Conclusions. The discrepancy between the local $R_V$, inferred from photopolarimetry in the SN vicinity, and RV obtained from SNe light curves suggests that the extinction laws obtained directly from the SNe may be driven by more local effects, perhaps from the interaction of light from the SN with very nearby material.

astro-ph.GA↗

arXiv2024

CSS161010: a luminous, fast blue optical transient with broad blueshifted hydrogen lines

We present ultraviolet, optical and near-infrared photometric and optical spectroscopic observations of the luminous, fast blue optical transient (LFBOT), CSS161010:045834-081803 (CSS161010). The transient was found in a low-redshift (z=0.033) dwarf galaxy. The light curves of CSS161010 are characterized by an extremely fast evolution and blue colours. The V-band light curve shows that CSS161010 reaches an absolute peak of M$_{V}^{max}=-20.66\pm0.06$ mag in 3.8 days from the start of the outburst. After maximum, CSS161010 follows a power-law decline $\propto t^{-2.8\pm0.1}$ in all optical bands. These photometric properties are comparable to those of well-observed LFBOTs such as AT 2018cow, AT 2020mrf and AT 2020xnd. However, unlike these objects, the spectra of CSS161010 show a remarkable transformation from a blue and featureless continuum to spectra dominated by very broad, entirely blueshifted hydrogen emission lines of velocities of up to 10% of the speed of light. The persistent blueshifted emission and the lack of any emission at the rest wavelength of CSS161010 are unique features not seen in any transient before CSS161010. The combined observational properties of CSS161010 and its M$_{*}\sim10^{8}$ M$_\odot$ dwarf galaxy host favour the tidal disruption of a star by an intermediate-mass black hole as its origin.

astro-ph.HE↗

arXiv2024

A Novel Optimal Transport-Based Approach for Interpolating Spectral Time Series: Paving the Way for Photometric Classification of Supernovae

This paper introduces a novel method for creating spectral time series, which can be used for generating synthetic light curves for photometric classification but also for applications like K-corrections and bolometric corrections. This approach is particularly valuable in the era of large astronomical surveys, where it can significantly enhance the analysis and understanding of an increasing number of SNe, even in the absence of extensive spectroscopic data. methods: By employing interpolations based on optimal transport theory, starting from a spectroscopic sequence, we derive weighted average spectra with high cadence. The weights incorporate an uncertainty factor for penalizing interpolations between spectra that show significant epoch differences and lead to a poor match between the synthetic and observed photometry. results: Our analysis reveals that even with phase difference of up to 40 days between pairs of spectra, optical transport can generate interpolated spectral time series that closely resemble the original ones. Synthetic photometry extracted from these spectral time series aligns well with observed photometry. The best results are achieved in the V band, with relative residuals of less than 10% for 87% and 84% of the data for type Ia and II, respectively. For the B, g, R and r bands, the relative residuals are between 65% and 87% within the previously mentioned 10% threshold for both classes. The worse results correspond to the i and I bands where, in the case, of SN~Ia the values drop to 53% and 42%, respectively. conclusions: We introduce a new method for constructing spectral time series for individual SNe starting from a sparse spectroscopic sequence, and demonstrate its capability to produce reliable light curves that can be used for photometric classification.

astro-ph.HE↗

arXiv2024

Narrow absorption lines from intervening material in supernovae I. Measurements and temporal evolution

Narrow absorption features in nearby supernova (SN) spectra are a powerful diagnostic of the slow-moving material in the line of sight: they are extensively used to infer dust extinction from the host galaxies, and they can also serve in the detection of circumstellar material originating from the SN progenitor and present in the vicinity of the explosion. Despite their wide use, very few studies have examined the biases of the methods to characterize narrow lines, and not many statistical analyses exist. This is the first paper of a series in which we present a statistical analysis of narrow lines of SN spectra of various resolutions. We develop a robust automated methodology to measure the equivalent width (EW) and velocity of narrow absorption lines from intervening material in the line of sight of SNe, including Na I D , Ca II H&K, K i and diffuse interstellar bands (DIBs). We carefully study systematic biases in heterogeneous spectra from the literature by simulating different signal-to-noise, spectral resolution, slit size and orientation and present the real capabilities and limitations of using low- and mid-resolution spectra to study these lines. In particular, we find that the measurement of the equivalent width of the narrow lines in low-resolution spectra is highly affected by the evolving broad P-Cygni profiles of the SN ejecta, both for core-collapse and type Ia SNe, inducing a conspicuous apparent evolution. We present thus an easy way to detect and exclude those cases to obtain more robust and reliable measurements. Finally, after considering all possible effects, we analyse the temporal evolution of the narrow features in a large sample of nearby SNe to detect any possible variation in their EWs over time. We find no time evolution of the narrow line features in our large sample for all SN types

astro-ph.IM↗

arXiv2023

A graph-based spectral classification of Type II supernovae

Given the ever-increasing number of time-domain astronomical surveys, employing robust, interpretative, and automated data-driven classification schemes is pivotal. Based on graph theory, we present new data-driven classification heuristics for spectral data. A spectral classification scheme of Type II supernovae (SNe II) is proposed based on the phase relative to the maximum light in the $V$ band and the end of the plateau phase. We utilize a compiled optical data set that comprises 145 SNe and 1595 optical spectra in 4000-9000 $\overset{\circ}{\mathrm {A}}$. Our classification method naturally identifies outliers and arranges the different SNe in terms of their major spectral features. We compare our approach to the off-the-shelf umap manifold learning and show that both strategies are consistent with a continuous variation of spectral types rather than discrete families. The automated classification naturally reflects the fast evolution of Type II SNe around the maximum light while showcasing their homogeneity close to the end of the plateau phase. The scheme we develop could be more widely applicable to unsupervised time series classification or characterisation of other functional data.

astro-ph.IM↗

arXiv2023

Carnegie Supernova Project-II: Near-infrared spectral diversity and template of Type Ia Supernovae

We present the largest and most homogeneous collection of near-infrared (NIR) spectra of Type Ia supernovae (SNe Ia): 339 spectra of 98 individual SNe obtained as part of the Carnegie Supernova Project-II. These spectra, obtained with the FIRE spectrograph on the 6.5 m Magellan Baade telescope, have a spectral range of 0.8--2.5 $μ$m. Using this sample, we explore the NIR spectral diversity of SNe Ia and construct a template of spectral time series as a function of the light-curve-shape parameter, color stretch $s_{BV}$. Principal component analysis is applied to characterize the diversity of the spectral features and reduce data dimensionality to a smaller subspace. Gaussian process regression is then used to model the subspace dependence on phase and light-curve shape and the associated uncertainty. Our template is able to predict spectral variations that are correlated with $s_{BV}$, such as the hallmark NIR features: Mg II at early times and the $H$-band break after peak. Using this template reduces the systematic uncertainties in K-corrections by ~90% compared to those from the Hsiao template. These uncertainties, defined as the mean K-correction differences computed with the color-matched template and observed spectra, are on the level of $4\times10^{-4}$ mag on average. This template can serve as the baseline spectral energy distribution for light-curve fitters and can identify peculiar spectral features that might point to compelling physics. The results presented here will substantially improve future SN~Ia cosmological experiments, for both nearby and distant samples.

astro-ph.HE↗

arXiv2022

EmulART: Emulating Radiative Transfer -- A pilot study on autoencoder based dimensionality reduction for radiative transfer models

Dust is a major component of the interstellar medium. Through scattering, absorption and thermal re-emission, it can profoundly alter astrophysical observations. Models for dust composition and distribution are necessary to better understand and curb their impact on observations. A new approach for serial and computationally inexpensive production of such models is here presented. Traditionally these models are studied with the help of radiative transfer modelling, a critical tool to understand the impact of dust attenuation and reddening on the observed properties of galaxies and active galactic nuclei. Such simulations present, however, an approximately linear computational cost increase with the desired information resolution. Our new efficient model generator proposes a denoising variational autoencoder (or alternatively PCA), for spectral compression, combined with an approximate Bayesian method for spatial inference, to emulate high information radiative transfer models from low information models. For a simple spherical dust shell model with anisotropic illumination, our proposed approach successfully emulates the reference simulation starting from less than 1% of the information. Our emulations of the model at different viewing angles present median residuals below 15% across the spectral dimension, and below 48% across spatial and spectral dimensions. EmulART infers estimates for ~85% of information missing from the input, all within a total running time of around 20 minutes, estimated to be 6x faster than the present target high information resolution simulations, and up to 50x faster when applied to more complicated simulations.

astro-ph.IM↗

arXiv2022

Dissecting the active galactic nucleus in Circinus -- III. VLT/FORS2 polarimetry confirms dusty cone illuminated by a tilted accretion disc

We present polarimetric maps of the Circinus galaxy nucleus in the $BVRI$ bands, obtained with VLT/FORS2. Circinus is the closest Seyfert 2 galaxy and harbours an archetypal obscured active galactic nucleus (AGN). Recent high angular resolution imaging revealed that a major fraction of its mid-infrared (MIR) emission is coming from the polar region. Previously, we demonstrated that these observations are consistent with a model of a compact dusty disc and a hyperboloid shell, resembling a hollow cone on larger scales. Here we focus on the AGN core, up to 40 pc from the central engine, and compare the observations to the radiative transfer models. Polarization maps reveal a conical structure, coinciding with the ionization cone. The wavelength-dependence of the polarization degree indicates that scattering on dust grains is producing polarization. The observed polarization degree ($\approx1-3\%$) is lower than predicted by the models; however, this is only a lower limit, since stellar emission dominates the total flux in the optical. The observed polarization angle ($\approx30$ degrees) is reproduced by the model of a dusty disc with a hollow cone that is illuminated by a tilted anisotropic central source. An accretion disc aligned with the ionization cone axis, and alternative dust geometries, such as a paraboloid shell, or a torus enveloped by ambient dust, are inconsistent with the data. We conclude that the optical polarimetric imaging supports earlier evidence for the presence of dust in the polar region, tentatively associated with dusty outflows.

astro-ph.GA↗

arXiv2022

Spatial field reconstruction with INLA: Application to simulated galaxies

Aims. Monte Carlo Radiative Transfer (MCRT) simulations are a powerful tool for understanding the role of dust in astrophysical systems and its influence on observations. However, due to the strong coupling of the radiation field and medium across the whole computational domain, the problem is non-local and non-linear and such simulations are computationally expensive in case of realistic 3D inhomogeneous dust distributions. We explore a novel technique for post-processing MCRT output to reduce the total computational run time by enhancing the output of computationally less expensive simulations of lower-quality. Methods. We combine principal component analysis (PCA) and non-negative matrix factorization (NMF) as dimensionality reduction techniques together with Gaussian Markov random fields and the Integrated nested Laplace approximation (INLA), an approximate method for Bayesian inference, to detect and reconstruct the non-random spatial structure in the images of lower signal-to-noise or with missing data. Results. We test our methodology using synthetic observations of a galaxy from the SKIRT Auriga project - a suite of high resolution magneto-hydrodynamic Milky Way-sized galaxies simulated in cosmological environment by 'zoom-in' technique. With this approach, we are able to reproduce high photon number reference images $\sim5$ times faster with median residuals below $\sim20\%$.

astro-ph.IM↗

arXiv2021

SN 2018agk: A Prototypical Type Ia Supernova with a Smooth Power-law Rise in Kepler (K2)

We present the 30-min cadence Kepler/K2 light curve of the Type Ia supernova (SN Ia) SN 2018agk, covering approximately one week before explosion, the full rise phase and the decline until 40 days after peak. We additionally present ground-based observations in multiple bands within the same time range, including the 1-day cadence DECam observations within the first $\sim$5 days after the first light. The Kepler early light curve is fully consistent with a single power-law rise, without evidence of any bump feature. We compare SN 2018agk with a sample of other SNe~Ia without early excess flux from the literature. We find that SNe Ia without excess flux have slowly-evolving early colors in a narrow range ($g-i\approx -0.20\pm0.20$ mag) within the first $\sim 10$ days. On the other hand, among SNe Ia detected with excess, SN 2017cbv and SN 2018oh tend to be bluer, while iPTF16abc's evolution is similar to normal SNe Ia without excess in $g-i$. We further compare the Kepler light curve of SN 2018agk with companion-interaction models, and rule out the existence of a typical non-degenerate companion undergoing Roche-lobe overflow at viewing angles smaller than $45^{\circ}$.

astro-ph.HE↗