arXiv subjects

Smaranika Banerjee

Publications and source records attributed to Smaranika Banerjee.

17 recordsLinked to original sources

arXiv2026

Free Neutron Decay in Kilonova Ejecta: X-ray/UV Flashes with Non-Thermal Effects

Neutron star-bearing compact-object mergers can create a kilonova, a transient powered by the radioactive decay of material synthesized via rapid neutron capture ($r$-process). During the merger, a fraction of the ejecta is launched at high speeds ($\gtrsim$ 0.4c) that can lead to neutrons evading capture onto seed nuclei, resulting in free neutrons. The free neutrons then decay and inject additional energy that power early-time ($\lesssim$1 day) emission. Here, using \texttt{Sedona}, we present a grid of the first non-local thermodynamic equilibrium and frequency-dependent opacity radiative transfer simulations to predict early X-ray/UV/optical emission from free neutron decay that span the expected theoretical range of free neutron mass, mixing with $r$-process material, and extent of the high-velocity ejecta tail. The emission can be characterized by an SED that rapidly shifts from X-rays of $\sim\rm{few} \times10^{41}-10^{42}$ erg s$^{-1}$ in the first $\sim$ minutes to a far-UV and near-UV peak on the timescale of $\sim$ minutes to hours, followed by enhanced optical and IR emission for sufficiently large free neutron masses. The properties of the free neutron ejecta are most distinguishable, in principle, at extreme-UV and far-UV wavelengths, with SED peak flux and wavelength determined by the maximum velocity of the ejecta and the mixing. In the far-UV and near-UV, free neutron ejecta masses as small as 10$^{-7}$ M$_\odot$ exhibit a unique bump compared to neutron-free models, demonstrating the importance of upcoming UV missions like UVEX and ULTRASAT to constraining the nucleosynthetic environment of neutron star mergers.

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arXiv2026

Recombination Rates for $5f^N$ Uranium Ions Using the Analogous $4f^4$ Nd III as a Guideline and Their Effect on Kilonova Nebular SpectraAn Exploration of Recombination of Uranium with application to Kilonovae Spectra

Dielectronic recombination (DR) is expected to be the dominant recombination process during the non-local thermodynamic equilibrium (non-LTE) phase of kilonovae, yet reliable DR data remain unavailable for most heavy ions. Current spectral models therefore rely on simplified recombination prescriptions, introducing significant uncertainties into predicted spectra. We present an optimization strategy for open f-shell ions using \texttt{AUTOSTRUCTURE}, targeting uranium ions U II--U IV relevant to actinide-producing kilonovae. As a benchmark case, calculations are performed for Nd III to validate the treatment of the f-shell structure and its impact on DR. The resulting DR rate coefficients are of order $10^{-10}$--$10^{-12}$ cm$^{3}$\,s$^{-1}$ over temperatures relevant to kilonova plasmas. The optimized rates are implemented into the radiative-transfer code \texttt{SUMO}, and their impact on kilonova spectra studied. The Nd III benchmark demonstrates that refinements to the atomic structure can produce measurable changes in spectral features supporting the atomic structure optimisation performed for U II--IV.

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arXiv2026

Ultraviolet Signatures of Jet-Ejecta Interaction in Early Kilonovae: Prediction from Realistic Atomic Opacities

We investigate the signature of the jet-ejecta interaction in early kilonova (t < 1day) using detailed atomic opacities developed in Banerjee et al. (2020, 2024), appropriate for early times (t~1hour after merger). We explore jets with different powers and opening angles. We find that the presence of the jet shifts the spectral peak to longer wavelengths, with the strongest effect near the polar viewing angle. This occurs because the jet creates a thin, low-density outer layer ahead of the bulk ejecta. The opacity of this layer can be as high as kappa ~200 cm2/g, causing photons to escape from cooler, faster-moving outer layer rather than from the hot inner ejecta. The bolometric light curves likewise exhibit a clear imprint of the jet-ejecta interaction, showing suppressed early-time luminosity near polar viewing angles compared to the equatorial one, as the photosphere resides in this thin layer where radioactive heating is lower than in the bulk ejecta. These signatures are also evident in multi-color light curves, particularly in the ultraviolet and u-bands. In the Swift-UVW2 band at t~= 0.15 days for a source at 100 Mpc, the ultraviolet luminosities can reach ~ 19.5 mag if no jet is present, while the presence of the jet can make it fainter by ~ 2.5 mag. The strongest observational signature occurs in the UVEX-NUV, Swift-UVW2, and UVM2 bands, which remains detectable out to viewing angles of ~ 60 deg for t <=1 days. Rapid follow-up with future ultraviolet facilities, such as ULTRASAT and UVEX, will provide powerful probes of jet-ejecta interaction through early-time kilonova observations.

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arXiv2025

ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Sub-threshold Sub-Solar Gravitational Wave Trigger

On August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported gravitational waves from a sub-threshold binary neutron star merger. If astrophysical, this event would have a surprisingly low chirp mass, suggesting that at least one neutron star was below a solar mass. The Zwicky Transient Facility mapped the coarse localization and discovered a transient, ZTF25abjmnps (AT2025ulz), that was spatially and temporally coincident with the gravitational wave trigger. The first week of follow-up suggested properties reminiscent of a GW170817-like kilonova. Subsequent follow-up suggests properties most similar to a young, stripped-envelope, Type IIb supernova. Although we cannot statistically rule out chance coincidence, we undertake due diligence analysis to explore the possible association between ZTF25abjmnps and S250818k. Theoretical models have been proposed wherein sub-solar neutron star(s) may form (and subsequently merge) via accretion disk fragmentation or core fission inside a core-collapse supernova i.e. a ``superkilonova". Here, we qualitatively discuss our multi-wavelength dataset in the context of the superkilonova picture. Future higher significance gravitational wave detections of sub-solar neutron star mergers with extensive electromagnetic follow-up would conclusively resolve this tantalizing multi-messenger association.

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arXiv2025

Infrared spectral signatures of light r-process elements in kilonovae

A central question regarding neutron star mergers is whether they are able to produce all the r-process elements, from first to third peak. The high abundances of first-peak elements (atomic number $Z \sim 31-40$) in the solar composition means they may dominate the ejecta mass in kilonovae. We here study theoretical infrared signatures of such light elements with spectral synthesis modelling. By combining state-of-the-art NLTE physics with new radiative and collisional data for these elements, we identify several promising diagnostic lines from Ge, As, Se, Br, Kr and Zr. The models give self-consistent line luminosities and indicate specific features that probe emission volumes at early phases ($\sim$10d), the product of ion mass and electron density in late phases ($\gtrsim$75d), and in some cases direct ionic masses at intermediate phases. Emission by [Se I] 5.03 \mum\ + [Se III] 4.55 \mum\ can produce satisfactory fits to the Spitzer photometry of AT2017gfo. However, the models show consistently that with a Kr/Te and Se/Te ratio following the solar r-process pattern, Kr + Se emission is dominant over Te for the blend at 2.1 \mum\ observed in both AT2017gfo and AT2023vfi. The somewhat better line profile fit with [Te III] may suggest that both AT2017gfo and AT2023vfi had a strongly sub-solar production of the light r-process elements. An alternative scenario could be that Kr + Se in an asymmetric morphological distribution generates the feature. Further JWST spectral data, in particular covering the so far unobserved $>5$ \mum\ region, holds promise to determine the light r-process production of kilonovae, and in particular whether the light elements are made in a slow disk wind or in a fast proto-NS outflow. We identify specific needs for further atomic data on recombination rates and collision strengths for $Z=31-40$ elements.

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arXiv2025

SN 2023xgo: Helium-rich Type Icn or Carbon-Flash Type Ibn supernova?

We present observations of SN~2023xgo, a transitional Type Ibn/Icn supernova, from $-5.6$ to $+63$~days relative to the $r$-band peak. Early spectra show C~III $λ5696$ emission reminiscent of Type~Icn SNe, which later gives way to Type~Ibn features. The He~I velocities ($1800$--$10{,}000$~km~s$^{-1}$) and pseudo-equivalent widths are among the highest in the Ibn/Icn class. The light curve declines at $0.14$~mag~d$^{-1}$ until $+30$~days, consistent with SNe~Ibn/Icn and slower than fast transients. SN~2023xgo is the faintest in our SN~Ibn sample ($M_r=-17.65\pm0.04$) but shows typical color and host properties. Semi-analytical modeling of the light curve suggests a compact CSM shell ($\sim 10^{12}$--$10^{13}$~cm) and a mass-loss rate of $10^{-4}$--$10^{-3}$~$M_{\odot}$~yr$^{-1}$, with CSM and ejecta masses of $\sim 0.22$ and $0.12$~$M_{\odot}$, respectively. Post-maximum light-curve and spectral modeling favor a $\sim 3$~$M_{\odot}$ helium-star progenitor with extended ($\sim 10^{15}$~cm), stratified CSM (density exponent $n=2.9$) and a mass-loss rate of $0.1$--$2.7$~$M_{\odot}$~yr$^{-1}$. These two mass-loss regimes imply a radially varying CSM, shaped by asymmetry or temporal changes in the progenitor's mass loss. This behavior is compatible with both binary and single-star evolution. We argue that the early Icn-like features arise from hot carbon ionization and fade to Ibn-like signatures as the ejecta and CSM cool, making SN~2023xgo a rare probe of the connection between SNe~Icn, SNe~Ibn, and Ibn events with ejecta signatures.

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arXiv2025

Nebular spectra of kilonovae with detailed recombination rates -- I. Light r-process composition

To investigate spectra of kilonovae in the NLTE phase (t>= 1 week), we perform atomic calculations for dielectronic recombination (DR) rates for the light r-process elements Se (Z = 34), Rb (Z = 37), Sr (Z = 38), Y (Z = 39), and Zr (Z = 40) using the HULLAC code. For the different elements, our results for the DR rate coefficients for recombining from the ionization states of II to I, III to II, and IV to III vary between 2x10^{-12} - 5x10^{-11} cm^3/s, 10^{-13} - 5x10^{-11} cm^3/s and 2x10^{-15} - 10^{-11} cm^3/s, respectively, at a temperature of T = 10,000 K. Using this new atomic data (DR), we study the impact on kilonova model spectra at phases of t = 10 days and t = 25 days after the merger using the spectral synthesis code SUMO. Compared to models using the previous treatment of recombination as a constant rate, the new models show significant changes in ionization and temperature, and correspondingly, in emergent spectra. With the new rates, we find that Zr (Z = 40) plays a yet more dominant role in kilonova spectra for light r-process compositions. Further, we show that previously predicted mid-infrared (e.g. [Se III] 4.55 mum) and optical (e.g. Rb I 7802, 7949 Å) lines weaken in the new model. Instead [Se I] 5.03 mum emerges as a signature. These results demonstrate the importance of considering the detailed microphysics for modelling and interpreting the late-time kilonova spectra.

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arXiv2025

IIb or not IIb: A Catalog of ZTF Kilonova Imposters

Among the various classes of fast optical transients (FOTs), kilonovae (KNe), which can emerge as a result of neutron star mergers, are extremely challenging to observe because of not only the rapid timescale on which they fade (on the order of days), but also due to the relative scarcity of their occurrence. This scarcity is compounded by the large number of other FOTs that may initially resemble the characteristic rise of a KNe. While these objects can be ruled out as candidate KNe by taking spectroscopy, a method of confidently ruling out transients based on photometric analysis alone would be incredibly valuable. We describe the compilation of various "imposter" transients, including a plurality of IIb SNe, and investigate a number of comparative metrics by which one might be able to remove transients from consideration without the use of spectroscopy. We provide a list of these objects and their classifications as well as a glossary of the transient types included in the sample.

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arXiv2025

LIGO/Virgo/KAGRA neutron star merger candidate S250206dm: Zwicky Transient Facility observations

We present the searches conducted with the Zwicky Transient Facility (ZTF) in response to S250206dm, a bona fide event with a false alarm rate of one in 25 years, detected by the International Gravitational Wave Network (IGWN). Although the event is significant, the nature of the compact objects involved remains unclear, with at least one likely neutron star. ZTF covered 68% of the localization region, though we did not identify any likely optical counterpart. We describe the ZTF strategy, potential candidates, and the observations that helped rule out candidates, including sources circulated by other collaborations. Similar to Ahumada et al. 2024, we perform a frequentist analysis, using simsurvey, as well as Bayesian analysis, using nimbus, to quantify the efficiency of our searches. We find that, given the nominal distance to this event of 373$\pm$104 Mpc, our efficiencies are above 10% for KNe brighter than $-17.5$ absolute magnitude. Assuming the optical counterpart known as kilonova (KN) lies within the ZTF footprint, our limits constrain the brightest end of the KN parameter space. Through dedicated radiative transfer simulations of KNe from binary neutron star (BNS) and black hole-neutron star (BHNS) mergers, we exclude parts of the BNS KN parameter space. Up to 35% of the models with high wind ejecta mass ($M_{\rm wind} \approx 0.13$ M$_{\odot}$) are ruled out when viewed face-on ($\cosθ_{\rm obs} = 1.0$). Finally, we present a joint analysis using the combined coverage from ZTF and the Gravitational Wave Multimessenger Dark Energy Camera Survey (GW-MMADS). The joint observations cover 73% of the localization region, and the combined efficiency has a stronger impact on rising and slowly fading models, allowing us to rule out 55% of the high-mass KN models viewed face-on.

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arXiv2024

Searching for gravitational wave optical counterparts with the Zwicky Transient Facility: summary of O4a

During the first half of the fourth observing run (O4a) of the International Gravitational Wave Network (IGWN), the Zwicky Transient Facility (ZTF) conducted a systematic search for kilonova (KN) counterparts to binary neutron star (BNS) and neutron star-black hole (NSBH) merger candidates. Here, we present a comprehensive study of the five high-significance (FAR < 1 per year) BNS and NSBH candidates in O4a. Our follow-up campaigns relied on both target-of-opportunity observations (ToO) and re-weighting of the nominal survey schedule to maximize coverage. We describe the toolkit we have been developing, Fritz, an instance of SkyPortal, instrumental in coordinating and managing our telescope scheduling, candidate vetting, and follow-up observations through a user-friendly interface. ZTF covered a total of 2841 deg$^2$ within the skymaps of the high-significance GW events, reaching a median depth of g~20.2 mag. We circulated 15 candidates, but found no viable KN counterpart to any of the GW events. Based on the ZTF non-detections of the high-significance events in O4a, we used a Bayesian approach, nimbus, to quantify the posterior probability of KN model parameters that are consistent with our non-detections. Our analysis favors KNe with initial absolute magnitude fainter than -16 mag. The joint posterior probability of a GW170817-like KN associated with all our O4a follow-ups was 64%. Additionally, we use a survey simulation software, simsurvey, to determine that our combined filtered efficiency to detect a GW170817-like KN is 36%, when considering the 5 confirmed astrophysical events in O3 (1 BNS and 4 NSBH), along with our O4a follow-ups. Following Kasliwal et al. (2020), we derived joint constraints on the underlying KN luminosity function based on our O3 and O4a follow-ups, determining that no more than 76% of KNe fading at 1 mag/day can peak at a magnitude brighter than -17.5 mag.

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arXiv2024

Science with the Daksha High Energy Transients Mission

We present the science case for the proposed Daksha high energy transients mission. Daksha will comprise of two satellites covering the entire sky from 1~keV to $>1$~MeV. The primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave source; and to study Gamma Ray Bursts (GRBs). Daksha is a versatile all-sky monitor that can address a wide variety of science cases. With its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. Daksha can make key strides in GRB research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. Daksha will provide continuous monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy counterparts to Fast Radio Bursts. Using Earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard X-ray sources including active galactic nuclei, X-ray binaries, and slow transients like Novae. Correlation studies between the two satellites can be used to probe primordial black holes through lensing. Daksha will have a set of detectors continuously pointing towards the Sun, providing excellent hard X-ray monitoring data. Closer to home, the high sensitivity and time resolution of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray Flashes.

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arXiv2024

Joint gravitational wave-short GRB detection of Binary Neutron Star mergers with existing and future facilities

We explore the joint detection prospects of short gamma-ray bursts (sGRBs) and their gravitational wave (GW) counterparts by the current and upcoming high-energy GRB and GW facilities from binary neutron star (BNS) mergers. We consider two GW detector networks: (1) A four-detector network comprising LIGO Hanford, Livingston, Virgo, and Kagra, (IGWN4) and (2) a future five-detector network including the same four detectors and LIGO India (IGWN5). For the sGRB detection, we consider existing satellites Fermi and Swift and the proposed all-sky satellite Daksha. Most of the events for the joint detection will be off-axis, hence, we consider a broad range of sGRB jet models predicting the off-axis emission. Also, to test the effect of the assumed sGRB luminosity function, we consider two different functions for one of the emission models. We find that for the different jet models, the joint sGRB and GW detection rates for Fermi and Swift with IGWN4 (IGWN5) lie within 0.07-0.62$\mathrm{\ yr^{-1}}$ (0.8-4.0$\mathrm{\ yr^{-1}}$) and 0.02-0.14$\mathrm{\ yr^{-1}}$ (0.15-1.0$\mathrm{\ yr^{-1}}$), respectively, when the BNS merger rate is taken to be 320$\mathrm{\ Gpc^{-3}~yr^{-1}}$. With Daksha, the rates increase to 0.2-1.3$\mathrm{\ yr^{-1}}$ (1.3-8.3$\mathrm{\ yr^{-1}}$), which is 2-9 times higher than the existing satellites. We show that such a mission with higher sensitivity will be ideal for detecting a higher number of fainter events observed off-axis or at a larger distance. Thus, Daksha will boost the joint detections of sGRB and GW, especially for the off-axis events. Finally, we find that our detection rates with optimal SNRs are conservative, and noise in GW detectors can increase the rates further.

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arXiv2023

Diversity of early kilonova with the realistic opacities of highly ionized heavy elements

We investigate the early (t < 1 day) kilonova from the neutron star merger by deriving atomic opacities for all the elements from La to Ra (Z = 57 - 88) ionized to the states V - XI. The opacities at high temperatures for the elements with open f-shells (e.g., lanthanides) are exceptionally high, reaching kappa_{exp} ~ 10^4 cm2/g at lambda < 1000 A at T ~ 70,000 K, whereas, the opacities at the same temperature and wavelengths for the elements with the open d-, p-, and s-shells reach kappa_{exp} ~ 1 cm2/g, 0.1 cm2/g, and 0.01 cm2/g, respectively. Using the new opacity dataset, we derive the early kilonovae for various compositions and density structures expected for neutron star merger ejecta. The bolometric luminosity for the lanthanide-rich ejecta shows distinct signatures and is fainter than that for the lanthanide-free ejecta. The early luminosity is suppressed by the presence of a thin outer layer, agreeing with the results of Kasen et al. (2017) and Banerjee et al. (2020). The early brightness in Swift UVOT filters and in the optical g-, r-, i-, z-filters for a source at 100 Mpc are ~ 22 - 20 mag and ~ 21 - 19 mag, respectively, at t ~ 0.1 days. Such kilonovae are ideal targets for the upcoming UV satellites, such as ULTRASAT, UVEX, and DORADO, and the upcoming surveys, e.g., Vera Rubin Observatory. We suggest the gray opacities to reproduce the bolometric light curves with and without lanthanides are ~ 1 - 20 cm2/g and ~ 0.8 - 1 cm2/g.

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arXiv2023

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 (a.k.a. S200224ca) and classified as a binary-black-hole coalescence, on February 24, 2020. Given its relatively small localization skymap (71 deg$^2$ for a 90% credible region; revised to 50 deg$^2$ 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 February 25 and 28 and March 23, 2020, with the first epoch beginning 12.3 h after the GW detection. The survey covered the highest probability sky area of 56.6 deg$^2$, corresponding to a 91% probability. This was the first deep follow-up ($m_{r}\gtrsim24, m_{z}\gtrsim23$) 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 only our Subaru/HSC data. 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; however, we could not identify a unique promising counterpart. If there are no counterparts in the remaining candidates, the upper limits of optical luminosity are $νL_ν < 5.2^{+2.4}_{-1.9}\times 10^{41}$ erg s$^{-1}$ and $νL_ν < 1.8^{+0.8}_{-0.6}\times 10^{42}$ erg s$^{-1}$ in the $r2$- and $z$-bands, respectively, at $\sim$12 h after GW detection. We also discuss improvements in the strategies of optical follow-ups for future GW events.

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arXiv2022

Opacity of the highly ionized lanthanides and the effect on the early kilonova

We investigate the effect of the presence of lanthanides (Z = 57- 71) on the kilonova at t~hours after the neutron star merger for the first time. For this purpose, we calculate the atomic structures and the opacities for selected lanthanides: Nd (Z = 60), Sm (Z = 62), and Eu (Z = 63). We consider the ionization degree up to tenth (XI), applicable for the ejecta at t ~ a few hours after the merger, when the temperature is T ~ 10^5 K. We find that the opacities for the highly ionized lanthanides are exceptionally high, reaching k_exp~1000 cm^2/g for Eu, due to the highly dense energy levels. Using the new opacity, we perform radiative transfer simulations to show that the early light curves become fainter by a (maximum) factor of four, in comparison to lanthanide-free ejecta at t~0.1 day. However, the period at which the light curves are affected is relatively brief due to the rapid time evolution of the opacity in the outermost layer of the ejecta. We predict that for a source at a distance of ~100 Mpc, UV brightness for lanthanide-rich ejecta shows a drop to ~21-22 mag at t~0.1 day and the UV peaks around t~0.2 day with a magnitude of ~19 mag. Future detection of such a kilonova by the existing UV satellite like Swift or the upcoming UV satellite ULTRASAT will provide useful constraints on the abundance in the outer ejecta and the corresponding nucleosynthesis conditions in the neutron star mergers.

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arXiv2021

Optical follow-up observation for GW event S190510g using Subaru/Hyper Suprime-Cam

A gravitational wave event, S190510g, which was classified as a binary-neutron-star coalescence at the time of preliminary alert, was detected by LIGO/Virgo collaboration on May 10, 2019. At 1.7 hours after the issue of its preliminary alert, we started a target-of-opportunity imaging observation in Y-band to search for its optical counterpart using the Hyper Suprime-Cam (HSC) on the Subaru Telescope. The observation covers a 118.8 deg$^2$ sky area corresponding to 11.6% confidence in the localization skymap released in the preliminary alert and 1.2% in the updated skymap. We divided the observed area into two fields based on the availability of HSC reference images. For the fields with the HSC reference images, we applied an image subtraction technique; for the fields without the HSC reference images, we sought individual HSC images by matching a catalog of observed objects with the PS1 catalog. The search depth is 22.28 mag in the former method and the limit of search depth is 21.3 mag in the latter method. Subsequently, we performed visual inspection and obtained 83 candidates using the former method and 50 candidates using the latter method. Since we have only the 1-day photometric data, we evaluated probability to be located inside the 3D skymap by estimating their distances with photometry of associated extended objects. We found three candidates are likely located inside the 3D skymap and concluded they could be an counterpart of S190510g, while most of 133 candidates were likely to be supernovae because the number density of candidates was consistent with the expected number of supernova detections. By comparing our observational depth with a light curve model of such a kilonova reproducing AT2017gfo, we show that early-deep observations with the Subaru/HSC can capture the rising phase of blue component of kilonova at the estimated distance of S190510g (~230 Mpc).

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arXiv2020

Simulations of early kilonova emission from neutron star mergers

We present radiative transfer simulations for blue kilonovae hours after neutron star (NS) mergers by performing detailed opacity calculations for the first time. We calculate atomic structures and opacities of highly ionized elements (up to the tenth ionization) with atomic number Z = 20 - 56. We find that the bound-bound transitions of heavy elements are the dominant source of the opacities in the early phase (t < 1 day after the merger), and that the ions with a half-closed electron shell provide the highest contributions. The Planck mean opacity for lanthanide-free ejecta (with electron fraction of Ye = 0.30 - 0.40) can only reach around kappa ~ 0.5 - 1 cm^2 g^-1 at t = 0.1 day, whereas that increases up to kappa ~ 5 - 10 cm^2 g^-1 at t = 1 day. The spherical ejecta model with an ejecta mass of Mej = 0.05Msun gives the bolometric luminosity of ~ 2 x 10^42 erg s^-1 at t ~ 0.1 day. We confirm that the existing bolometric and multi-color data of GW170817 can be naturally explained by the purely radioactive model. The expected early UV signals reach 20.5 mag at t ~ 4.3 hours for sources even at 200 Mpc, which is detectable by the facilities such as Swift and the Ultraviolet Transient Astronomy Satellite (ULTRASAT). The early-phase luminosity is sensitive to the structure of the outer ejecta, as also pointed out by Kasen et al. (2017). Therefore, the early UV observations give strong constraints on the structure of the outer ejecta as well as the presence of a heating source besides r-process nuclei.

astro-ph.HE↗