Light Curve Fitting Research Articles

Cosmological foundations revisited with Pantheon+

Abstract We reanalyse the Pantheon+ supernova catalogue to compare a cosmology with non-FLRW evolution, the timescape cosmology, with the standard ΛCDM cosmology. To this end, we analyse the Pantheon+ for a geometric comparison between the two models. We construct a covariance matrix to be as independent of cosmology as possible, including independence from the FLRW geometry and peculiar velocity with respect to FLRW average evolution. This framework goes far beyond most other definitions of model independence. We introduce new statistics to refine Type Ia supernova (SNe Ia) light-curve analysis. In addition to conventional galaxy correlation functions used to define the scale of statistical homogeneity we introduce empirical statistics which enables refined analysis of the distribution biases of SNe Ia light-curve parameters βc and $\alpha {x_{\lower2pt\hbox{$\scriptstyle 1$}}}$. For lower redshifts, the Bayesian analysis highlights important features attributable to the increased number of low-redshift supernovae, the artefacts of model-dependent light-curve fitting and the cosmic structure through which we observe supernovae. This indicates the need for cosmology-independent data reduction to conduct a stronger investigation of the emergence of statistical homogeneity and to compare alternative cosmologies in light of recent challenges to the standard model. Dark energy is generally invoked as a place-holder for new physics. For the first time, we find evidence that the timescape cosmology may provide a better overall fit than ΛCDM and that its phenomenology may help disentangle other astrophysical puzzles. Our from-first-principles reanalysis of Pantheon+ supports future deeper studies between the interplay of matter and nonlinear spacetime geometry in a data-driven setting.

Variational inference for acceleration of SN Ia photometric distance estimation with BayeSN

Abstract Type Ia supernovae (SNe Ia) are standarizable candles whose observed light curves can be used to infer their distances, which can in turn be used in cosmological analyses. As the quantity of observed SNe Ia grows with current and upcoming surveys, increasingly scalable analyses are necessary to take full advantage of these new datasets for precise estimation of cosmological parameters. Bayesian inference methods enable fitting SN Ia light curves with robust uncertainty quantification, but traditional posterior sampling using Markov Chain Monte Carlo (MCMC) is computationally expensive. We present an implementation of variational inference (VI) to accelerate the fitting of SN Ia light curves using the BayeSN hierarchical Bayesian model for time-varying SN Ia spectral energy distributions (SEDs). We demonstrate and evaluate its performance on both simulated light curves and data from the Foundation Supernova Survey with two different forms of surrogate posterior – a multivariate normal and a custom multivariate zero-lower-truncated normal distribution – and compare them with the Laplace Approximation and full MCMC analysis. To validate of our variational approximation, we calculate the pareto-smoothed importance sampling (PSIS) diagnostic, and perform variational simulation-based calibration (VSBC). The VI approximation achieves similar results to MCMC but with an order-of-magnitude speedup for the inference of the photometric distance moduli. Overall, we show that VI is a promising method for scalable parameter inference that enables analysis of larger datasets for precision cosmology.

Separating the causes of O–C variations in an RR Lyrae star with the Blazhko effect

Context. To date, puzzlingly few bona fide RR Lyrae stars have been identified in binaries. Binarity in pulsating stars can be revealed through well-timed photometric data over sufficiently long time bases because of the light travel time effect (LTTE) on the pulsations, which manifests as variation in the timings of maximum light in the O−C (observed minus calculated) diagram. However, O−C variations can also have other causes, such as the Blazhko effect or a sudden or gradual change in the main pulsation period. Aims. We approach this challenge by disentangling the Blazhko effect and period changes from the potential LTTE on V1109 Cas, an RR Lyrae star suspected to be part of a binary system based on its O−C data in the GEOS database. In doing so, we aim to uncover the subtler signals indicative of a companion (LTTE). Methods. We analysed nine years of ground-based photometric data, using Fourier analysis to model the pulsation modulated by the Blazhko effect. From the fit to the observed light curves, we constructed a refined O−C diagram without the scatter caused by Blazhko modulation. Subsequently, we considered different possible scenarios, because distinguishing intrinsic period changes or breaks from the LTTE is challenging. Results. If the remaining O−C variation is due to a period break, refining the O−C diagram can almost entirely remove the trends. If we interpret the variation as an LTTE, we can consider possible configurations. While we currently favour a period break as the explanation for the residual O−C variations, more data on the star in the forthcoming years will help reveal whether the O−C variations are due to the LTTE or intrinsic period changes. Conclusions. Our study, based on a detailed light-curve fitting of V1109 Cas, offers insights into the discernment between Blazhko modulation, intrinsic period breaks, and the LTTE in binary systems. Our results highlight the complexity of determining binarity from O−C data traditionally used for detecting binary motion, calling for caution in this process.

Biases in Exoplanet Transmission Spectra Introduced by Limb-darkening Parametrization

One of the main endeavors of the field of exoplanetary sciences is the characterization of exoplanet atmospheres on a population level. The current method of choice to accomplish this task is transmission spectroscopy, where the apparent radius of a transiting exoplanet is measured at multiple wavelengths in search of atomic and molecular absorption features produced by the upper atmosphere constituents. To extract the planetary radius from a transit light curve, it is necessary to account for the decrease in luminosity away from the center of the projected stellar disk, known as limb darkening. Physically motivated parametrizations of limb darkening, in particular of the quadratic form, are commonly used in exoplanet transit light-curve fitting. Here, we show that such parametrizations can introduce significant wavelength-dependent biases in the transmission spectra currently obtained with all instrument modes of the JWST, and thus have the potential to affect atmospheric inferences. To avoid such biases, we recommend the use of standard limb-darkening parametrizations with wide uninformative priors that allow for nonphysical stellar intensity profiles in the transit fits, and thus for a complete and symmetrical exploration of the parameter space. We further find that fitting the light curves at the native resolution results in errors on the measured transit depths that are significantly smaller compared to light curves that are binned in wavelength before fitting, thus potentially maximizing the amount of information that can be extracted from the data.

Exomod: A Python Library for Exoplanet Transit Light Curve Fitting and Stacking with Background Simulation and Optimization

Abstract From the light curve taken during exoplanet transit, important parameters such as the radius ratio of the star-planet, impact parameter, inclination, and relative tangential velocity could be derived. A module was developed in the form of a Python library based on modeling using background simulation and fitting by utilizing global and local optimization, such as differential evolution and the Broyden–Fletcher–Goldfarb–Shanno (BFGS) method. For more accurate fitting, the effects of linear limb darkening on exoplanets are also included. The performance testing for the program was done by using dummy data to test its accuracy, followed by validation tests using WASP-12b transit curves to find out any discrepancies between our model and real exoplanet data. While performing a characterization test on WASP-12b, we found an interesting result regarding its impact parameter and the exoplanet’s velocity, which might be an interesting subject for further analysis.

Addressing type Ia supernova color variability with a linear spectral template

Type Ia Supernovae (SNeIa) provided the first evidence of an accelerated expansion of the universe and remain a valuable probe to cosmology. They are deemed standardizable candles due to the observed correlations between their luminosity and photometric quantities. This characteristic can be exploited to estimate cosmological distances after accounting for the observed variations. There is however a remaining dispersion unaccounted for in the current state-of-the-art standardization methods. In an attempt to explore this issue, we propose a simple linear 3-component rest-frame flux description for a light-curve fitter. Since SNIa intrinsic color index variations are expected to be time-dependent, our description builds upon the mathematical expression of the well-known Spectral Adaptive Light Curve Template 2 (SALT2) for rest-frame flux, whilst we drop the exponential factor and add an extra model component with time and wavelength dependencies. The model components are obtained by performing either Principal Component Analysis (PCA) or Factor Analysis (FA) onto a representative training set. The constraining power of the model dubbed Pure Expansion Template for Supernovae (PETS) is evaluated and we found compatible results with SALT2 for Ωm0 and ΩΛ0 within 68% uncertainty between the two models, with PETS’ fit parameters exhibiting non-negligible linear correlations with SALT2’ parameters. For both PCA and FA model versions we verified that the first component mainly describes color index variations, proving it is a dominant effect on SNIa spectra. The model nuisance parameter which multiplies the color index variation-like fit parameter shows evolution with redshift in an initial binned cosmology analysis. This behavior can be due to selection effects and should be further investigated with higher redshift SNeIa samples. Overall, our model shows promise, as there are still a few aspects to be refined; however, it still falls short in reducing the unaccounted dispersion.

Observations of type Ia supernova SN 2020nlb up to 600 days after explosion, and the distance to M85

The type Ia supernova (SN Ia) SN 2020nlb was discovered in the Virgo Cluster galaxy M85 shortly after explosion. Here we present observations that include one of the earliest high-quality spectra and some of the earliest multi-colour photometry of a SN Ia to date. We calculated that SN 2020nlb faded 1.28 ± 0.02 mag in the B band in the first 15 d after maximum brightness. We independently fitted a power-law rise to the early flux in each filter, and found that the optical filters all give a consistent first light date estimate. In contrast to the earliest spectra of SN 2011fe, those of SN 2020nlb show strong absorption features from singly ionised metals, including Fe II and Ti II, indicating lower-excitation ejecta at the earliest times. These earliest spectra show some similarities to maximum-light spectra of 1991bg-like SNe Ia. The spectra of SN 2020nlb then evolve to become hotter and more similar to SN 2011fe as it brightens towards peak. We also obtained a sequence of nebular spectra that extend up to 594 days after maximum light, a phase out to which SNe Ia are rarely followed. The [Fe III]/[Fe II] flux ratio (as measured from emission lines in the optical spectra) begins to fall around 300 days after peak; by the +594 d spectrum, the ionisation balance of the emitting region of the ejecta has shifted dramatically, with [Fe III] by then being completely absent. The final spectrum is almost identical to SN 2011fe at a similar epoch. Comparing our data to other SN Ia nebular spectra, there is a possible trend where SNe that were more luminous at peak tend to have a higher [Fe III]/[Fe II] flux ratio in the nebular phase, but there is a notable outlier in SN 2003hv. Finally, using light-curve fitting on our data, we estimate the distance modulus for M85 to be μ0 = 30.99 ± 0.19 mag, corresponding to a distance of 15.8+1.4-1.3 Mpc.

The Next Generation Virgo Cluster Survey. XXXVII. Distant RR Lyrae Stars and the Milky Way Stellar Halo Out to 300 kpc

RR Lyrae stars are standard candles with characteristic photometric variability and serve as powerful tracers of Galactic structure, substructure, accretion history, and dark matter content. Here we report the discovery of distant RR Lyrae stars, including some of the most distant stars known in the Milky Way halo, with Galactocentric distances of ∼300 kpc. We use time-series u*g′i′z′ Canada–France–Hawaii Telescope/MegaCam photometry from the Next Generation Virgo Cluster Survey (NGVS). We use a template light-curve fitting method based on empirical Sloan Digital Sky Survey Stripe 82 RR Lyrae data to identify RR Lyrae candidates in the NGVS data set. We eliminate several hundred suspected quasars and identify 180 RR Lyrae candidates with heliocentric distances of ∼20–300 kpc. The halo stellar density distribution is consistent with an r −4.09±0.10 power-law radial profile over most of this distance range with no signs of a break. The distribution of ab-type RR Lyrae in a period–amplitude plot (Bailey diagram) suggests that the mean metallicity of the halo decreases outward. Compared to other recent RR Lyrae surveys, like Pan-STARRS1, the High Cadence Transient Survey, and the Dark Energy Survey, our NGVS study has better single-epoch photometric precision and a comparable number of epochs but smaller sky coverage. At large distances, our RR Lyrae sample appears to be relatively pure and complete, with well-measured periods and amplitudes. These newly discovered distant RR Lyrae stars are important additions to the few secure stellar tracers beyond 150 kpc in the Milky Way halo.

Tracing back the birth environments of Type Ia supernova progenitor stars: a pilot study based on 44 early-type host galaxies

ABSTRACT The environmental dependence of Type Ia supernova (SN Ia) luminosities is well established, and efforts are being made to find its origin. Previous studies typically use the currently observed status of the host galaxy. However, given the delay time between the birth of the progenitor star and the SN Ia explosion, the currently observed status may differ from the birth environment of the SN Ia progenitor star. In this paper, employing the chemical evolution and accurately determined stellar population properties of 44 early-type host galaxies, we, for the first time, estimate the SN Ia progenitor star birth environment, specifically [Fe/H]Birth and [α/Fe]Birth. We show that [α/Fe]Birth has a $30.4^{\text{+10.6}}_{-10.1}{{\ \rm per\ cent}}$ wider range than the currently observed [α/Fe]Current, while the range of [Fe/H]Birth is not statistically different ($17.9^{\text{+26.0}}_{-27.1}{{\ \rm per\ cent}}$) to that of [Fe/H]Current. The birth and current environments of [Fe/H] and [α/Fe] are sampled from different populations (p-values of the Kolmogorov–Smirnov test <0.01). We find that light-curve fit parameters are insensitive to [Fe/H]Birth (<0.9σ for the non-zero slope), while a linear trend is observed with Hubble residuals (HRs) at the 2.4σ significance level. With [α/Fe]Birth, no linear trends (<1.1σ) are observed. Interestingly, we find that [α/Fe]Birth clearly splits the SN Ia sample into two groups: SN Ia exploded in [α/Fe]Birth-rich or [α/Fe]Birth-poor environments. SNe Ia exploded in different [α/Fe]Birth groups have different weighted-means of light-curve shape parameters: 0.81 ± 0.33 (2.5σ). They are thought to be drawn from different populations (p-value = 0.01). Regarding SN Ia colour and HRs, there is no difference (<1.0σ) in the weighted-means and distribution (p-value > 0.27) of each [α/Fe]Birth group.

Stellar Cycle and Evolution of Polar Spots in an M+WD Binary

Stellar activity cycles reveal continuous relaxation and induction of magnetic fields. The activity cycle is typically traced through the observation of cyclic variations in total brightness or Ca H&K emission flux of stars, as well as cyclic variations in the orbital periods of binary systems. In this work, we report the identification of a semidetached binary system (TIC 16320250) consisting of a white dwarf (0.67 M ⊙) and an active M dwarf (0.56 M ⊙). The long-term multiband optical light curves spanning twenty years revealed three repeated patterns, suggestive of a possible activity cycle of about 10 years of the M dwarf. Light-curve fitting indicates the repeated variation is caused by the evolution, particularly the motion, of polar spots. The significant Ca H&K, Hα, ultra-violet, and X-ray emissions imply that the M dwarf is one of the most magnetically active stars. We propose that in the era of large time-domain photometric sky surveys (e.g., ASAS-SN, Zwicky Transient Facility, LSST, Sitian), long-term light-curve modeling can be a valuable tool for tracing and revealing stellar activity cycle, especially for stars in binary systems.

RAINBOW: A colorful approach to multipassband light-curve estimation

Context. Time series generated by repeatedly observing astronomical transients are generally sparse, irregularly sampled, noisy, and multidimensional (obtained through a set of broad-band filters). In order to fully exploit their scientific potential, it is necessary to use this incomplete information to estimate a continuous light-curve behavior. Traditional approaches use ad hoc functional forms to approximate the light curve in each filter independently (hereafter, the MONOCHROMATIC method). Aims. We present RAINBOW, a physically motivated framework that enables simultaneous multiband light-curve fitting. It allows the user to construct a 2D continuous surface across wavelength and time, even when the number of observations in each filter is significantly limited. Methods. Assuming the electromagnetic radiation emission from the transient can be approximated by a blackbody, we combined an expected temperature evolution and a parametric function describing its bolometric light curve. These three ingredients allow the information available in one passband to guide the reconstruction in the others, thus enabling a proper use of multisurvey data. We demonstrate the effectiveness of our method by applying it to simulated data from the Photometric LSST Astronomical Time-series Classification Challenge (PLAsTiCC) as well as to real data from the Young Supernova Experiment (YSE DR1). Results. We evaluate the quality of the estimated light curves according to three different tests: goodness of fit, peak-time prediction, and ability to transfer information to machine-learning (ML) based classifiers. The results confirm that RAINBOW leads to an equivalent goodness of fit (supernovae II) or to a goodness of fit that is better by up to 75% (supernovae Ibc) than the MONOCHROMATIC approach. Similarly, the accuracy improves for all classes in our sample when the RAINBOW best-fit values are used as a parameter space in a multiclass ML classification. Conclusions. Our approach enables a straightforward light-curve estimation for objects with observations in multiple filters and from multiple experiments. It is particularly well suited when the light-curve sampling is sparse. We demonstrate its potential for characterizing supernova-like events here, but the same approach can be used for other classes by changing the function describing the light-curve behavior and temperature representation. In the context of the upcoming large-scale sky surveys and their potential for multisurvey analysis, this represents an important milestone in the path to enable population studies of photometric transients.

Delayed and Fast-rising Radio Flares from an Optical and X-Ray-detected Tidal Disruption Event in the Center of a Dwarf Galaxy

AT 2018cqh is a unique tidal disruption event (TDE) discovered in a dwarf galaxy. Both the light-curve fitting and galaxy scaling relationships suggest a central black hole mass in the range of 5.9 < logM BH/M ☉ < 6.4. The r-band peak luminosity is ∼ 1043 erg s−1, making AT 2018cqh relatively faint among known optical TDEs. A delayed X-ray brightening was found around 590 days after the optical discovery but shows an unusually long time rising to peak over at least 558 days, which could be coming from delayed accretion of a newly forming debris disk. We report the discovery of delayed radio flares around 1105 days since its discovery, characterized by an initial steep rise of ≳175 days, a flattening lasting about 544 days, and a phase with another steep rise. The rapid rise in radio flux coupled with the slow decay in the X-ray emission points to a delayed launching of outflow, perhaps due to a transition in the accretion state. However, known accretion models can hardly explain the origins of the secondary radio flare that is rising even more rapidly in comparison with the initial one. If confirmed, AT 2018cqh would be a rare faint TDE in a dwarf galaxy exhibiting optical, X-ray, and radio flares. We call for continued multifrequency radio observations to monitor its spectral and temporal evolution, which may help to reveal new physical processes that are not included in standard TDE models.

Collapsars as Sites of r-process Nucleosynthesis: Systematic Photometric Near-infrared Follow-up of Type Ic-BL Supernovae

One of the open questions following the discovery of GW170817 is whether neutron star (NS) mergers are the only astrophysical sites capable of producing r-process elements. Simulations have shown that 0.01–0.1 M ⊙ of r-process material could be generated in the outflows originating from the accretion disk surrounding the rapidly rotating black hole that forms as a remnant to both NS mergers and collapsing massive stars associated with long-duration gamma-ray bursts (collapsars). The hallmark signature of r-process nucleosynthesis in the binary NS merger GW170817 was its long-lasting near-infrared (NIR) emission, thus motivating a systematic photometric study of the light curves of broad-lined stripped-envelope (Ic-BL) supernovae (SNe) associated with collapsars. We present the first systematic study of 25 SNe Ic-BL—including 18 observed with the Zwicky Transient Facility and 7 from the literature—in the optical/NIR bands to determine what quantity of r-process material, if any, is synthesized in these explosions. Using semi-analytic models designed to account for r-process production in SNe Ic-BL, we perform light curve fitting to derive constraints on the r-process mass for these SNe. We also perform independent light curve fits to models without the r-process. We find that the r-process-free models are a better fit to the light curves of the objects in our sample. Thus, we find no compelling evidence of r-process enrichment in any of our objects. Further high-cadence infrared photometric studies and nebular spectroscopic analysis would be sensitive to smaller quantities of r-process ejecta mass or indicate whether all collapsars are completely devoid of r-process nucleosynthesis.

The Hidden Companion in J1527: A 0.69 Solar-mass White Dwarf?

Finding nearby neutron stars can probe the supernova and metal-enrichment histories near our solar system. Recently, Lin et al. reported an exciting neutron star candidate, Two Micron All Sky Survey J15274848+3536572 (hereafter J1527), with a small Gaia distance of 118 pc. They claim that J1527 harbors an unseen neutron star candidate with an unusually low mass of 0.98 ± 0.03 M ⊙. In this work, we use the Canada–France–Hawaii Telescope high-resolution spectrum to measure J1527's orbital inclination independently. Our spectral fitting suggests an orbital inclination of 63° ± 2°. Instead, by fitting a complex hybrid variability model consisting of the ellipsoidal-variation component and the starspot modulation to the observed light curve, Lin et al. obtain an orbital inclination of 45.2−0.20+0.13 degrees. We speculate that the orbital inclination obtained by the light-curve fitting is underestimated since J1527's light curves are obviously not pure ellipsoidal variations. According to our new inclination (i ∼ 63°), the mass of the unseen compact object is reduced to 0.69 ± 0.02 M ⊙, which is as massive as a typical white dwarf.

The first analysis of three long-period low mass-ratio contact binaries

ABSTRACT The photometric and spectroscopic studies of three contact binaries, ASASSN-V J052036.28+144711.0, ASASSN-V J064846.22+241709.9, and ASASSN-V J073441.02+555833.0 were performed for the first time. The periods of all the three targets are longer than 0.5 d, and we discovered that their mass ratios are all smaller than 0.25. So, they are long-period low mass-ratio contact binaries. ASASSN-V J052036.28+144711.0 and ASASSN-V J064846.22+241709.9 are medium contact binaries. ASASSN-V J073441.02+555833.0 is a deep contact binary, which is also the only one existing O’Connell effect. We attached a dark spot on its secondary component for a better fit of light curves featuring asymmetry. From orbital period analysis, two of the three targets are believed to demonstrate a secular period increase, while one shows no long-term variation. We employed the spectral subtraction approach to investigate the LAMOST spectra, while all of the three binaries show no H α emission line, implying no chromospheric activity. Their initial masses, current average densities, and mass transfer rates between the two components were calculated. We obtained that the three targets are stable for the moment.

Three is the magic number: Distance measurement of NGC 3147 using SN 2021hpr and its siblings

Context. The nearby spiral galaxy NGC 3147 hosted three Type Ia supernovae (SNe Ia) in the past decades that have been the subjects of intense follow-up observations. Simultaneous analysis of their data provides a unique opportunity for testing different methods of light curve fitting and distance estimation. Aims. The detailed optical follow-up of SN 2021hpr allows us to revise the previous distance estimations to NGC 3147 and compare the widely used light curve fitting algorithms to each other. After the combination of the available and newly published data of SN 2021hpr, its physical properties can also be estimated with higher accuracy. Methods. We present and analyse new BV griz and Swift photometry of SN 2021hpr to constrain its general physical properties. Together with its siblings, SNe 1997bq and 2008fv, we cross-compared the individual distance estimates of these three SNe given by the Spectral Adaptive Lightcurve Template (SALT) code, and we also checked their consistency with the results from the Multi-Color Light Curve Shape (MLCS) code. The early spectral series of SN 2021hpr was also fit with the radiative spectral code TARDIS to verify the explosion properties and constrain the chemical distribution of the outer ejecta. Results. After combining the distance estimates for the three SNe, the mean distance to their host galaxy, NGC 3127, is 42.5 ± 1.0 Mpc, which matches with the distance inferred by the most up-to-date light curve fitters, SALT3 and BayeSN. We confirm that SN 2021hpr is a Branch-normal Type Ia SN that ejected ~1.12 ± 0.28 M⊙ from its progenitor white dwarf and synthesized ~0.44 ± 0.14 M⊙ of radioactive 56Ni.

Connecting Infrared Surface Brightness Fluctuation Distances to Type Ia Supernova Hosts: Testing the Top Rung of the Distance Ladder

We compare infrared surface brightness fluctuation (IR SBF) distances measured in galaxies that have hosted type Ia supernovae (SNe Ia) to distances estimated from SNe Ia light-curve fits. We show that the properties of the SNe Ia found in IR SBF hosts are very different from those exploding in Cepheid calibrators, therefore this is a direct test of systematic uncertainties on the estimation of the Hubble constant (H 0) using SNe. The IR SBF results from Jensen et al. provide a large and uniformly measured sample of IR SBF distances which we directly compare with the distances to 25 SN Ia host galaxies. We divide the Hubble flow SNe Ia into subsamples that best match the divergent SN properties seen in the IR SBF hosts and Cepheid hosts. We further divide the SNe Ia into a sample with light-curve widths and host masses that are congruent to those found in the SBF-calibrated hosts. We refit the light-curve stretch and color correlations with luminosity, and use these revised parameters to calibrate a sample of “Hubble flow” SNe Ia with IR SBF calibrators. Relative to the Hubble flow sample, the average calibrator distance moduli vary by 0.03 mag depending on the SN Ia subsample examined and this adds a 1.8% systematic uncertainty to our Hubble constant estimate. Based on the IRSBF calibrators, H 0 = 74.6 ± 0.9(stat) ± 2.7(syst) km s−1 Mpc−1, which is consistent with the Hubble constant derived from SNe Ia calibrated from Cepheid variables. We conclude that IR SBFs provide reliable calibration of SNe Ia with a precision comparable to Cepheid calibrators, and with a significant saving in telescope time.

Growth-rate measurement with type-Ia supernovae using ZTF survey simulations

Measurements of the growth rate of structures at z &lt; 0.1 with peculiar velocity surveys have the potential of testing the validity of general relativity on cosmic scales. In this work, we present growth-rate measurements from realistic simulated sets of type-Ia supernovae (SNe Ia) from the Zwicky Transient Facility (ZTF). We describe our simulation methodology, the light-curve fitting, and peculiar velocity estimation. Using the maximum likelihood method, we derived constraints on fσ8 using only ZTF SN Ia peculiar velocities. We carefully tested the method and we quantified biases due to selection effects (photometric detection, spectroscopic follow-up for typing) on several independent realizations. We simulated the equivalent of 6 years of ZTF data, and considering an unbiased spectroscopically typed sample at z &lt; 0.06, we obtained unbiased estimates of fσ8 with an average uncertainty of 19% precision. We also investigated the information gain in applying bias correction methods. Our results validate our framework, which can be used on real ZTF data.

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 μm. 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 × 10−4 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.

Follow-up Survey for the Binary Black Hole Merger GW200224_222234 Using Subaru/HSC and GTC/OSIRIS

The LIGO/Virgo detected a gravitational wave (GW) event, named GW200224_222234 (also known as S200224ca) and classified as a binary-black hole coalescence, on 2020 February 24. Given its relatively small localization skymap (71 deg2 for a 90% credible region; revised to 50 deg2 in GWTC-3), we performed target-of-opportunity observations using the Subaru/Hyper Suprime-Cam (HSC) in the r2 and z bands. Observations were conducted on 2020 February 25 and 28 and March 23, with the first epoch beginning 12.3 hr after the GW detection. The survey covered the highest-probability sky area of 56.6 deg2, corresponding to a 91% probability. This was the first deep follow-up (m r ≳ 24, m z ≳ 23) for a binary-black hole merger covering >90% of the localization. By performing image subtraction and candidate screening including light-curve fitting with transient templates and examples, we found 22 off-nucleus transients that were not ruled out as the counterparts of GW200224_222234 with our Subaru/HSC data alone. We also performed GTC/OSIRIS spectroscopy of the probable host galaxies for five candidates; two are likely to be located within the 3D skymap, whereas the others are not. In conclusion, 19 transients remain as possible optical counterparts of GW200224_222234; but we could not identify a unique promising counterpart. If there are no counterparts in the remaining candidates, the upper limits of the optical luminosity are erg s−1 and erg s−1 in the r2 and z bands, respectively, at ∼12 hr after GW detection. We also discuss improvements in the strategies of optical follow-ups for future GW events.