Interstellar-medium mapping in M82 through light echoes around supernova 2014J

We present multiple-epoch measurements of the size and surface brightness of the light echoes from supernova (SN) 2014J in the nearby starburst galaxy M82. Hubble Space Telescope (HST) ACS/WFC images were taken ~277 and ~416 days after B-band maximum in the filters F475W, F606W, and F775W. Observations with HST WFC3/UVIS images at epochs ~216 and ~365 days are included for a more complete analysis. The images reveal the temporal evolution of at least two major light-echo components. The first one exhibits a filled ring structure with position-angle-dependent intensity. This radially extended, diffuse echo indicates the presence of an inhomogeneous interstellar dust cloud ranging from ~100 to ~500 pc in the foreground of the SN. The second echo component appears as an unresolved luminous quarter-circle arc centered on the SN. The wavelength dependence of scattering measured in different dust components suggests that the dust producing the luminous arc favors smaller grain sizes, while that causing the diffuse light echo may have sizes similar to those of the Milky Way dust. Smaller grains can produce an optical depth consistent with that along the supernova-Earth line of sight measured by previous studies around maximum light. Therefore, it is possible that the dust slab from which the luminous arc arises is also responsible for most of the extinction toward SN 2014J. The optical depths determined from the Milky Way-like dust in the scattering matters are lower than the optical depth produced by the dust slab

Mapping Circumstellar Matter with Polarized Light: The Case of Supernova 2014J in M82

Optical polarimetry is an effective way of probing the environment of a supernova for dust. We acquired linear HST ACS/WFC polarimetry in bands F W 475 , F606W, and F775W of the supernova (SN) 2014J in M82 at six epochs from ∼277 days to ∼1181 days after the B-band maximum. The polarization measured at day 277 shows conspicuous deviations from other epochs. These differences can be attributed to at least ∼10-6 M of circumstellar dust located at a distance of ~ ´5 10 c 17 m from the SN. The scattering dust grains revealed by these observations seem to be aligned with the dust in the interstellar medium that is responsible for the large reddening toward the supernova. The presence of this circumstellar dust sets strong constraints on the progenitor system that led to the explosion of SN 2014J; however, it cannot discriminate between single- and double-degenerate models

Late-time Flattening of Type Ia Supernova Light Curves: Constraints from SN 2014J in M82

The very nearby Type Ia supernova 2014J in M82 offers a rare opportunity to study the physics of thermonuclear supernovae at extremely late phases (gsim800 days). Using the Hubble Space Telescope, we obtained 6 epochs of high-precision photometry for SN 2014J from 277 days to 1181 days past the B-band maximum light. The reprocessing of electrons and X-rays emitted by the radioactive decay chain ${}^{57}\mathrm{Co}\to {}^{57}\mathrm{Fe}$ is needed to explain the significant flattening of both the F606W-band and the pseudo-bolometric light curves. The flattening confirms previous predictions that the late-time evolution of type Ia supernova luminosities requires additional energy input from the decay of 57Co. By assuming the F606W-band luminosity scales with the bolometric luminosity at ~500 days after the B-band maximum light, a mass ratio ${}^{57}\mathrm{Ni}{/}^{56}\mathrm{Ni}\sim {0.065}_{-0.004}^{+0.005}$ is required. This mass ratio is roughly ~3 times the solar ratio and favors a progenitor white dwarf with a mass near the Chandrasekhar limit. A similar fit using the constructed pseudo-bolometric luminosity gives a mass ratio ${}^{57}\mathrm{Ni}{/}^{56}\mathrm{Ni}\sim {0.066}_{-0.008}^{+0.009}$. Astrometric tests based on the multi-epoch HST ACS/WFC images reveal no significant circumstellar light echoes in between 0.3 and 100 pc from the supernova

Heavy element production in a compact object merger observed by JWST

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs) 1, sources of high-frequency gravitational waves (GWs) 2 and likely production sites for heavy-element nucleosynthesis by means of rapid neutron capture (the r-process) 3. Here we present observations of the exceptionally bright GRB 230307A. We show that GRB 230307A belongs to the class of long-duration GRBs associated with compact object mergers 4–6 and contains a kilonova similar to AT2017gfo, associated with the GW merger GW170817 (refs. 7–12). We obtained James Webb Space Telescope (JWST) mid-infrared imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns, which we interpret as tellurium (atomic mass A = 130) and a very red source, emitting most of its light in the mid-infrared owing to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy-element nucleosynthesis across the Universe

End-to-end study of the host galaxy and genealogy of the first binary neutron star merger

Binary neutron star mergers are one of the ultimate events of massive binary star evolution, and our understanding of their parent systems is still in its infancy. Upcoming gravitational wave detections, coupled with multi-wavelength follow-up observations, will allow us to study an increasing number of these events by characterizing their neighbouring stellar populations and searching for their progenitors. Stellar evolution simulations are essential to this work, but they are also based on numerous assumptions. Additionally, the models used to study the host galaxies differ from those used to characterize the progenitors and are typically based on single-star populations. Here we introduce a framework to perform an end-to-end analysis and deploy it to the first detected binary neutron star merger event, GW170817. With the Binary Population And Spectral Synthesis codes we are able to retrieve the physical properties of the host galaxy NGC 4993 as well as infer progenitor candidates. In our simulations, there is a >98% chance that GW170817 originated from a stellar population with metallicity Z = 0.010, born between 5 and 12.5 Gyr ago. By carefully weighing the stellar genealogies, we find that GW170817 most likely came from a binary system born with a 13–24 M⊙ primary and 10–12 M⊙ secondary that underwent 2 or 3 common envelope events over their lifetime

NEural Engine for Discovering Luminous Events (NEEDLE): identifying rare transient candidates in real time from host galaxy images

Known for their efficiency in analysing large data sets, machine learning-based classifiers have been widely used in wide-field sky survey pipelines. The upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will generate millions of real-time alerts every night, enabling the discovery of large samples of rare events. Identifying such objects soon after explosion will be essential to study their evolution. Using ∼5400 transients from the Zwicky Transient Facility (ZTF) Bright Transient Survey as training and test data, we develop NEEDLE (NEural Engine for Discovering Luminous Events), a novel hybrid (convolutional neural network + dense neural network) classifier to select for two rare classes with strong environmental preferences: superluminous supernovae (SLSNe) preferring dwarf galaxies, and tidal disruption events (TDEs) occurring in the centres of nucleated galaxies. The input data includes (i) cutouts of the detection and reference images, (ii) photometric information contained directly in the alert packets, and (iii) host galaxy magnitudes from Pan-STARRS (Panoramic Survey Telescope and Rapid Response System). Despite having only a few tens of examples of the rare classes, our average (best) completeness on an unseen test set reaches 73 per cent (86 per cent) for SLSNe and 80 per cent (87 per cent) for TDEs. While very encouraging for completeness, this may still result in relatively low purity for the rare transients, given the large class imbalance in real surveys. However, the goal of NEEDLE is to find good candidates for spectroscopic classification, rather than to select pure photometric samples. Our system will be deployed as an annotator on the UK alert broker, Lasair, to provide predictions of real-time alerts from ZTF and LSST to the community.<br/

Linear spectropolarimetry of 35 Type Ia supernovae with VLT/FORS: an analysis of the Si II line polarization

Spectropolarimetry enables us to measure the geometry and chemical structure of the ejecta in supernova explosions, which is fundamental for the understanding of their explosion mechanism(s) and progenitor systems. We collected archival data of 35 Type Ia supernovae (SNe Ia), observed with Focal Reducer and Low-Dispersion Spectrograph (FORS) on the Very Large Telescope at 127 epochs in total. We examined the polarization of the Si II λ6355 Å line (pSi II) as a function of time, which is seen to peak at a range of various polarization degrees and epochs relative to maximum brightness. We reproduced the m15–pSi II relationship identified in a previous study, and show that subluminous and transitional objects display polarization values below the m15–pSi II relationship for normal SNe Ia. We found a statistically significant linear relationship between the polarization of the Si II λ6355 Å line before maximum brightness and the Si II line velocity and suggest that this, along with the m15–pSi II relationship, may be explained in the context of a delayed-detonation model. In contrast, we compared our observations to numerical predictions in the m15–vSi II plane and found a dichotomy in the polarization properties between Chandrasekhar and sub-Chandrasekhar mass explosions, which supports the possibility of two distinct explosion mechanisms. A subsample of SNe displays evolution of loops in the q–u plane that suggests a more complex Si structure with depth. This insight, which could not be gleaned from total flux spectra, presents a new constraint on explosion models. Finally, we compared our statistical sample of the Si II polarization to quantitative predictions of the polarization levels for the double-detonation, delayed-detonation, and violent-merger models

The impact of pre-supernova feedback and its dependence on environment

Integral field units enable resolved studies of a large number of star-forming regions across entire nearby galaxies, providing insight on the conversion of gas into stars and the feedback from the emerging stellar populations over unprecedented dynamic ranges in terms of spatial scale, star-forming region properties, and environments. We use the Very Large Telescope (VLT) MUSE (Multi Unit Spectroscopic Explorer) legacy data set covering the central 35 arcmin2 (∼12 kpc2) of the nearby galaxy NGC 300 to quantify the effect of stellar feedback as a function of the local galactic environment. We extract spectra from emission line regions identified within dendrograms, combine emission line ratios and line widths to distinguish between HII regions, planetary nebulae, and supernova remnants, and compute their ionized gas properties, gas-phase oxygen abundances, and feedback-related pressure terms. For the HII regions, we find that the direct radiation pressure (Pdir) and the pressure of the ionized gas (⁠PHII⁠) weakly increase towards larger galactocentric radii, i.e. along the galaxy’s (negative) abundance and (positive) extinction gradients. While the increase of PHII with galactocentric radius is likely due to higher photon fluxes from lower-metallicity stellar populations, we find that the increase of Pdir is likely driven by the combination of higher photon fluxes and enhanced dust content at larger galactocentric radii. In light of the above, we investigate the effect of increased pre-supernova feedback at larger galactocentric distances (lower metallicities and increased dust mass surface density) on the ISM, finding that supernovae at lower metallicities expand into lower-density environments, thereby enhancing the impact of supernova feedback

Heavy element production in a compact object merger observed by JWST

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs) , sources of high-frequency gravitational waves (GW) and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process) . Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers , and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW170817 . We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe. [Abstract copyright: © 2023. The Author(s), under exclusive licence to Springer Nature Limited.