arXiv subjects

Eli Waxman

Publications and source records attributed to Eli Waxman.

At least 37 records · Page 2Linked to original sources

arXiv2024

Shock cooling emission from explosions of red super-giants: II. An analytic model of deviations from blackbody emission

Light emission in the first hours and days following core-collapse supernovae (SNe) is dominated by the escape of photons from the expanding shock heated envelope. In a preceding paper, Paper I, we provided a simple analytic description of the time dependent luminosity, $L$, and color temperature, $T_{\rm col}$, valid up to H recombination ($T\approx0.7$ eV), for explosions of red supergiants with convective polytropic envelopes without significant circum-stellar medium (CSM). The analytic description was calibrated against "gray" (frequency-independent) photon diffusion numeric calculations. Here we present the results of a large set of 1D multi-group (frequency-dependent) calculations, for a wide range of progenitor parameters (mass, radius, core/envelope mass ratios, metalicity) and explosion energies, using opacity tables that we constructed (and made publicly available), including the contributions of bound-bound and bound-free transitions. We provide an analytic description of the small, $\simeq10\%$ deviations of the spectrum from blackbody at low frequencies, $hν< 3T_{\rm col}$, and an improved (over Paper I) description of `line dampening' for $hν> 3T_{\rm col}$. We show that the effects of deviations from initial polytropic density distribution are small, and so are the effects of `expansion opacity' and deviations from LTE ionization and excitation (within our model assumptions). A recent study of a large set of type II SN observations finds that our model accounts well for the early multi-band data of more than 50\% of observed SNe (the others are likely affected by thick CSM), enabling the inference of progenitor properties, explosion velocity, and relative extinction.

astro-ph.HE↗

arXiv2023

A 12.4 day periodicity in a close binary system after a supernova

Neutron stars and stellar-mass black holes are the remnants of massive star explosions. Most massive stars reside in close binary systems, and the interplay between the companion star and the newly formed compact object has been theoretically explored, but signatures for binarity or evidence for the formation of a compact object during a supernova explosion are still lacking. Here we report a stripped-envelope supernova, SN 2022jli, which shows 12.4-day periodic undulations during the declining light curve. Narrow H$α$ emission is detected in late-time spectra with concordant periodic velocity shifts, likely arising from hydrogen gas stripped from a companion and accreted onto the compact remnant. A new Fermi/LAT $γ$-ray source is temporally and positionally consistent with SN 2022jli. The observed properties of SN 2022jli, including periodic undulations in the optical light curve, coherent H$α$ emission shifting, and evidence for association with a $γ$-ray source, point to the explosion of a massive star in a binary system leaving behind a bound compact remnant. Mass accretion from the companion star onto the compact object powers the light curve of the supernova and generates the $γ$-ray emission.

astro-ph.HE↗

arXiv2023

Shock cooling emission from explosions of red super-giants: I. A numerically calibrated analytic model

Supernova light curves are dominated at early time, hours to days, by the escape of photons from the expanding shock heated envelope. We provide a simple analytic description of the time dependent luminosity, $L$, and color temperature, $T_{\rm col}$, for explosions of red supergiants (with convective polytropic envelopes), valid up to H recombination ($T\approx0.7$ eV). The analytic description is based on an interpolation between earlier analytic expressions valid at different (initial planar and later spherical) stages of the expansion, calibrated against the results of numerical hydrodynamic diffusion calculations for a wide range of progenitor parameters (mass, radius, core/envelope mass and radius ratios, metalicity), and explosion energies. The numerically derived $L$ and $T_{\rm col}$ are described by the analytic expressions with 10\% and 5\% accuracy respectively. $T_{\rm col}$ is inferred from the hydrodynamic profiles using (time and space dependent) effective "gray" (frequency independent) opacity, based on opacity tables that we have constructed for this purpose (and will be made publicly available) including the contributions of bound-bound and bound-free transitions. In an accompanying paper (Paper II) we show, using a large set of multi-group photon diffusion calculations, that the spectral energy distribution is well described by a Planck spectrum with $T=T_{\rm col}$, except at UV frequencies (beyond the spectral peak at $3T_{\rm col}$), where the flux is significantly suppressed due to the presence of strong line absorption. We defer the full discussion of the multi-group results to paper II, but provide here for completeness an analytic description also of the UV suppression. Our analytic results are a useful tool for inferring progenitor properties, explosion velocity, and also relative extinction based on early multi-band shock cooling observations of supernovae.

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arXiv2023

The scientific payload of the Ultraviolet Transient Astronomy Satellite (ULTRASAT)

The Ultraviolet Transient Astronomy Satellite (ULTRASAT) is a space-borne near UV telescope with an unprecedented large field of view (200 sq. deg.). The mission, led by the Weizmann Institute of Science and the Israel Space Agency in collaboration with DESY (Helmholtz association, Germany) and NASA (USA), is fully funded and expected to be launched to a geostationary transfer orbit in Q2/3 of 2025. With a grasp 300 times larger than GALEX, the most sensitive UV satellite to date, ULTRASAT will revolutionize our understanding of the hot transient universe, as well as of flaring galactic sources. We describe the mission payload, the optical design and the choice of materials allowing us to achieve a point spread function of ~10arcsec across the FoV, and the detector assembly. We detail the mitigation techniques implemented to suppress out-of-band flux and reduce stray light, detector properties including measured quantum efficiency of scout (prototype) detectors, and expected performance (limiting magnitude) for various objects.

astro-ph.IM↗

arXiv2022

Non-thermal emission from mildly relativistic dynamical ejecta of neutron star mergers

Binary neutron star mergers are expected to produce fast dynamical ejecta, with mildly relativistic velocities extending to $β=v/c>0.6$. We consider the radio to X-ray synchrotron emission produced by collisionless shocks driven by such fast ejecta into the interstellar medium. Analytic expressions are given for spherical ejecta with broken power-law mass (or energy) distributions, $M(>γβ)\propto(γβ)^{-s}$ with $s=s_{\rm KN}$ at $γβ<γ_0β_0$ and $s=s_{\rm ft}$ at $γβ>γ_0β_0$ (where $γ$ is the Lorentz factor). For parameter values characteristic of merger calculation results -- a "shallow" mass distribution, $1 5$, "fast tail" mass distribution -- our model provides an accurate (to 10's of percent) description of the evolution of the flux, including at the phase of deceleration to sub-relativistic expansion. This is a significant improvement over earlier results, based on extrapolations of results valid for $γβ\gg1$ or $\ll1$ to $γβ\approx1$, which overestimate the flux by an order of magnitude for typical parameter values. It will enable a more reliable inference of ejecta parameters from future measurements of the non-thermal emission. For the merger event GW170817, the existence of a "fast tail" is expected to produce detectable radio and X-ray fluxes over a time scale of $\sim10^4$days.

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arXiv2021

Sensor characterization for the ULTRASAT space telescope

The Ultraviolet Transient Astronomical Satellite is a scientific space mission carrying an astronomical telescope. The mission is led by the Weizmann Institute of Science in Israel and the Israel Space Agency, while the camera in the focal plane is designed and built by Deutsches Elektronen Synchrotron in Germany. Two key science goals of the mission are the detection of counterparts to gravitational wave sources and supernovae. The launch to geostationary orbit is planned for 2024. The telescope with a field-of-view of $\approx200$deg$^2$, is optimized to work in the near-ultraviolet band between $220$ and $280$nm. The focal plane array is composed of four $22.4$-megapixel, backside-illuminated CMOS sensors with a total active area of 90x90mm$^2$. Prior to sensor production, smaller test sensors have been tested to support critical design decisions for the final flight sensor. These test sensors share the design of epitaxial layer and anti-reflective coatings (ARC) with the flight sensors. Here, we present a characterization of these test sensors. Dark current and read noise are characterized as a function of the device temperature. A temperature-independent noise level is attributed to on-die infrared emission and the read-out electronics` self-heating. We utilize a high-precision photometric calibration setup to obtain the test sensors` quantum efficiency (QE) relative to PTB/NIST-calibrated transfer standards ($220$-$1100$nm), the quantum yield for $λ< 300$nm, the non-linearity of the system, and the conversion gain. The uncertainties are discussed in the context of the newest results on the setup`s performance parameters. From three ARC options, Tstd, T1 and T2, the latter optimizes out-of-band rejection and peaks in the mid of the ULTRASAT operational waveband (max. QE $\approx80\%$ at $245\mathrm{nm}$). We recommend ARC option T2 for the final ULTRASAT UV sensor.

astro-ph.IM↗

arXiv2021

Design of the ULTRASAT UV camera

The Ultraviolet Transient Astronomical Satellite (ULTRASAT) is a scientific UV space telescope that will operate in geostationary orbit. The mission, targeted to launch in 2024, is led by the Weizmann Institute of Science (WIS) in Israel and the Israel Space Agency (ISA). Deutsches Elektronen Synchrotron (DESY) in Germany is tasked with the development of the UV-sensitive camera at the heart of the telescope. The camera's total sensitive area of ~90mm x 90mm is built up by four back-side illuminated CMOS sensors, which image a field of view of ~200 deg2. Each sensor has 22.4 megapixels. The Schmidt design of the telescope locates the detector inside the optical path, limiting the overall size of the assembly. As a result, the readout electronics is located in a remote unit outside the telescope. The short focal length of the telescope requires an accurate positioning of the sensors within +-50 mu along the optical axis, with a flatness of +-10 mu. While the telescope will be at around 295K during operations, the sensors are required to be cooled to 200K for dark current reduction. At the same time, the ability to heat the sensors to 343K is required for decontamination. In this paper, we present the preliminary design of the UV sensitive ULTRASAT camera.

astro-ph.IM↗

arXiv2020

The GALEX-PTF experiment: II. supernova progenitor radius and energetics via shock-cooling modeling

The radius and surface composition of an exploding massive star, as well as the explosion energy per unit mass, can be measured using early ultraviolet (UV) observations of core-collapse supernovae (CC SNe). We present the results from a simultaneous \GALEX and Palomar Transient Factory (PTF) search for early UV emission from SNe. We analyze five CC SNe for which we obtained $NUV$ measurements before the first ground-based $R$-band detection. We introduce SOPRANOS, a new maximum likelihood fitting tool for models with variable temporal validity windows, and use it to fit the \citet{SapirWaxman2017} shock cooling model to the data. We report four Type II SNe with progenitor radii in the range of $R_*\approx600-1100R_\odot$ and a shock velocity parameter in the range of $v_{s*}\approx 2700-6000 \,\rm km\,s^{-1}$ ($E/M\approx2-8\times10^{50}\,\rm erg/M_\odot$) and one type IIb SN with $R_*\approx210R_\odot$ and $v_{s*}\approx11000 \rm\, km\,s^{-1}$ ($E/M\approx1.8\times10^{51}\,\rm erg/M_\odot$). Our pilot GALEX/PTF project thus suggests that a dedicated, systematic SN survey in the $NUV$ band, such as the wide-field UV explorer \textit{ULTRASAT} mission, is a compelling method to study the properties of SN progenitors and SN energetics.

astro-ph.HE↗

arXiv2020

SN 2018fif: The Explosion of a Large Red Supergiant Discovered in Its Infancy by the Zwicky Transient Facility

High cadence transient surveys are able to capture supernovae closer to their first light than before. Applying analytical models to such early emission, we can constrain the progenitor stars properties. In this paper, we present observations of SN2018fif (ZTF18abokyfk). The supernova was discovered close to first light and monitored by the Zwicky Transient Facility (ZTF) and the Neil Gehrels Swift Observatory. Early spectroscopic observations suggest that the progenitor of SN2018fif was surrounded by relatively small amounts of circumstellar material (CSM) compared to all previous cases. This particularity, coupled with the high cadence multiple-band coverage, makes it a good candidate to investigate using shock-cooling models. We employ the SOPRANOS code, an implementation of the model by Sapir & Waxman and its extension to early times by Morag, Sapir & Waxman. Compared with previous implementations, SOPRANOS has the advantage of including a careful account of the limited temporal validity domain of the shock-cooling model as well as allowing usage of the entirety of the early UV data. We find that the progenitor of SN2018fif was a large red supergiant, with a radius of R=744.0_{-128.0}^{+183.0} solar radii and an ejected mass of Mej=9.3_{-5.8}^{+0.4} solar masses. Our model also gives information on the explosion epoch, the progenitor inner structure, the shock velocity and the extinction. The distribution of radii is double-peaked, with lower radii corresponding to lower values of the extinction, earlier recombination times and better match to the early UV data. If these correlations persist in future objects, denser spectroscopic monitoring constraining the time of recombination, as well as accurate UV observations (e.g. with ULTRASAT), will help break the radius-extinction degeneracy and independently determine both.

astro-ph.HE↗

arXiv2020

Implications of CHIME repeating fast radio bursts

CHIME has now detected 18 repeating fast radio bursts (FRBs). We explore what can be learned about the energy distribution and activity level of the repeaters by fitting realistic FRB population models to the data. For a power-law energy distribution dN/dE ~ E^{-γ} for the repeating bursts, there is a critical index γ_c that controls whether the dispersion measure (DM, a proxy for source distance) distribution of repeaters is bottom or top-heavy. We find γ_c = 7/4 for Poisson wait-time distribution of repeaters in Euclidean space and further demonstrate how it is affected by temporal clustering of repetitions and cosmological effects. It is especially interesting that two of the CHIME repeaters (FRB 181017 and 190417) have large DM ~ 1000 pc/cm^3. These can be understood if: (i) the energy distribution is shallow γ= 1.7^{+0.3}_{-0.1} (68% confidence) or (ii) a small fraction of sources are extremely active. In the second scenario, these two high-DM sources should be repeating more than 100 times more frequently than FRB 121102, and the energy index is constrained to be γ= 1.9^{+0.3}_{-0.2} (68% confidence). In either case, this γis consistent with the energy dependence of the non-repeating ASKAP sample, which suggests that they are drawn from the same population. Finally, our model predicts how the CHIME repeating fraction should decrease with redshift, and this can be compared with observations to infer the distribution of activity level in the whole population.

astro-ph.HE↗

arXiv2020

Constraints on the density distribution of type Ia supernovae ejecta inferred from late-time light-curve flattening

The finite time, $τ_{\rm dep}$, over which positrons from $β^{+}$ decays of $^{56}$Co deposit energy in type Ia supernovae ejecta lead, in case the positrons are trapped, to a slower decay of the bolometric luminosity compared to an exponential decline. Significant light-curve flattening is obtained when the ejecta density drops below the value for which $τ_{\rm dep}$ equals the $^{56}$Co life-time. We provide a simple method to accurately describe this "delayed deposition" effect, which is straightforward to use for analysis of observed light curves. We find that the ejecta heating is dominated by delayed deposition typically from 600 to 1200~day, and only later by longer lived isotopes $^{57}$Co and $^{55}$Fe decay (assuming solar abundance). For the relatively narrow $^{56}$Ni velocity distributions of commonly studied explosion models, the modification of the light curve depends mainly on the $^{56}$Ni mass-weighted average density, $\langle ρ\rangle t^{3}$. Accurate late-time bolometric light curves, which may be obtained with JWST far-infrared (far-IR) measurements, will thus enable to discriminate between explosion models by determining $\langle ρ\rangle t^3$ (and the $^{57}$Co and $^{55}$Fe abundances). The flattening of light curves inferred from recent observations, which is uncertain due to the lack of far-IR data, is readily explained by delayed deposition in models with $\langle ρ\rangle t^{3} \approx 0.2\,M_{\odot}\,(10^{4}\, \textrm{km}\,\textrm{s}^{-1})^{-3}$, and does not imply supersolar $^{57}$Co and $^{55}$Fe abundances.

astro-ph.HE↗

arXiv2019

Screening of fusion reactions from the principle of detailed balance and application to the $pep$ reaction

Dewitt et al. (1973) derived a useful relation between the plasma screening factor for a reaction of two fusing ions and their chemical potentials, based on the plasma pair distribution functions. We show that their result can be derived in a simpler, more straightforward way, by applying the principle of detailed balance, which also enables us to generalize the relation to reactions involving $N$ fusing ions. In order to demonstrate the usefulness of applying the principle of detailed balance, we calculate the screening factor for the $pep$ reaction, $p+e+p\rightarrow ^{2}$D $+ν_{e}$. For the plasma conditions near the centre of the Sun, the reaction is suppressed by roughly the same amount by which the reaction $p+p\rightarrow ^{2}$D $+$ $e^{+}+ν_{e}$ is enhanced. This effect may be measured in the near future.

astro-ph.SR↗

arXiv2019

Late time kilonova light curves and implications to GW 170817

We discuss the late time (tens of days) emission from the radioactive ejecta of mergers involving neutron stars, when the ionization energy loss time of beta-decay electrons and positrons exceeds the expansion time. We show that if the e$^\pm$ are confined to the plasma (by magnetic fields), then the time dependence of the plasma heating rate, $\dot{\varepsilon}_d$, and hence of the bolometric luminosity $L=\dot{\varepsilon}_d$, are given by $d\log L/d\log t\simeq-2.8$, nearly independent of the composition and of the instantaneous radioactive energy release rate, $\dot{\varepsilon}$. This universality of the late time behavior is due to the weak dependence of the ionization loss rate on composition and on e$^\pm$ energy. The late time IR and optical measurements of GW 170817 are consistent with this expected behavior provided that the ionization loss time exceeds the expansion time at $t>t_\varepsilon\approx 7$~d, as predicted based on the early (few day) electromagnetic emission.

astro-ph.HE↗

arXiv2019

Maser and other instabilities in a weakly magnetized relativistic plasma: Theory and the astrophysical relevance of the maser

A sufficient condition for maser instability in a weakly magnetized relativistic plasma with an isotropic particle distribution function is given. The maser growth rates and polarizations are computed starting from the exact dielectric permittivity tensor of a magnetized plasma. For very weak magnetic fields, our results confirm the approximate validity of the 'standard maser theory', which is based on the Einstein coefficients method, with one significant exception. For inclined propagation and realistic (small but finite) field, the growth rates of the two (nearly circular) polarizations differ significantly, while the standard theory predicts two (nearly circular) polarizations with similar growth rates. We show that this deviation is due to circularly polarized synchrotron emission, which is neglected in the standard theory. The maser is shown to grow slower than Langmuir waves. Nevertheless, significant generation of EM waves is seen in (highly simplified) direct numerical simulations. We study the nonlinear saturation of the maser instability and find that it offers a mechanism for the conversion of a significant fraction of the plasma energy into radio waves. We briefly discuss the conditions under which the maser instability may operate in astrophysical sources, and provide rough estimates that may be used as a guidance when studying particular astrophysical sources/phenomena.

astro-ph.HE↗

arXiv2018

Constraints on the ejecta of the GW170817 neutron-star merger from its electromagnetic emission

We present a simple analytic model, that captures the key features of the emission of radiation from material ejected by the merger of neutron stars (NS), and construct the multi-band and bolometric luminosity light curves of the transient associated with GW170817, AT\,2017gfo, using all available data. The UV to IR emission is shown to be consistent with a single $\approx0.05$\,M$_\odot$ component ejecta, with a power-law velocity distribution between $\approx 0.1\,c$ and $>0.3\,c$, a low opacity, {$κ<1$\,cm$^2$\,g$^{-1}$}, and a radioactive energy release rate consistent with an initial $Y_{\rm e}<0.4$. The late time spectra require an opacity of $κ_ν\approx0.1$\,cm$^2$\,g$^{-1}$ at 1 to $2μ$m. If this opacity is provided entirely by Lanthanides, their implied mass fraction is $X_{\rm Ln}\approx10^{-3}$, approximately 30 times below the value required to account for the solar abundance. The inferred value of $X_{\rm Ln}$ is uncertain due to uncertainties in the estimates of IR opacities of heavy elements, which also do not allow the exclusion of a significant contribution to the opacity by other elements (the existence of a slower ejecta rich in Lanthanides, that does not contribute significantly to the luminosity, can also not be ruled out). The existence of a relatively massive, $\approx 0.05$\,M$_\odot$, ejecta with high velocity and low opacity is in tension with the results of numerical simulations of NS mergers.

astro-ph.HE↗

arXiv2017

Preliminary evidence for a virial shock around the Coma galaxy cluster

Galaxy clusters, the largest gravitationally bound objects in the Universe, are thought to grow by accreting mass from their surroundings through large-scale virial shocks. Due to electron acceleration in such a shock, it should appear as a $γ$-ray, hard X-ray, and radio ring, elongated towards the large-scale filaments feeding the cluster, coincident with a cutoff in the thermal Sunyaev-Zel'dovich (SZ) signal. However, no such signature was found until now, and the very existence of cluster virial shocks has remained a theory. We find preliminary evidence for a large, $\sim 5$ Mpc minor axis $γ$-ray ring around the Coma cluster, elongated towards the large scale filament connecting Coma and Abell 1367, detected at the nominal $2.7σ$ confidence level ($5.1σ$ using control signal simulations). The $γ$-ray ring correlates both with a synchrotron signal and with the SZ cutoff, but not with Galactic tracers. The $γ$-ray and radio signatures agree with analytic and numerical predictions, if the shock deposits $\sim 1\%$ of the thermal energy in relativistic electrons over a Hubble time, and $\sim 1\%$ in magnetic fields. The implied inverse-Compton and synchrotron cumulative emission from similar shocks can significantly contribute to the diffuse extragalactic $γ$-ray and low frequency radio backgrounds. Our results, if confirmed, reveal the prolate structure of the hot gas in Coma, the feeding pattern of the cluster, and properties of the surrounding large scale voids and filaments. The anticipated detection of such shocks around other clusters would provide a powerful new cosmological probe.

astro-ph.CO↗

arXiv2017

Cosmic-ray Antimatter

In recent years, space-born experiments have delivered new measurements of high energy cosmic-ray (CR) $\bar p$ and $e^+$. In addition, unprecedented sensitivity to CR composite anti-nuclei anti-d and anti-He is expected to be achieved in the near future. We report on the theoretical interpretation of these measurements. While CR antimatter is a promising discovery tool for new physics or exotic astrophysical phenomena, an irreducible background arises from secondary production by primary CR collisions with interstellar matter. Understanding this irreducible background or constraining it from first principles is an interesting challenge. We review the attempt to obtain such understanding and apply it to CR $\bar p,\, e^+,$ anti-d and anti-He. Based on state of the art Galactic cosmic ray measurements, dominated currently by the AMS-02 experiment, we show that: (i) CR $\bar p$ most likely come from CR-gas collisions; (ii) $e^+$ data is consistent with, and suggestive of the same secondary astrophysical production mechanism responsible for $\bar p$ and dominated by proton-proton collisions. In addition, based on recent accelerator analyses we show that the flux of secondary high energy anti-He may be observable with a few years exposure of AMS-02. We highlight key open questions, as well as the role played by recent and upcoming space and accelerator data in clarifying the origins of CR antimatter.

astro-ph.HE↗

arXiv2017

On the origin of fast radio bursts (FRBs)

We derive stringent constraints on the persistent source associated with FRB 121102: Size $10^{17}$ cm $<R<10^{18}$ cm, age $<300$ yr, characteristic electron energy $\varepsilon_e\sim0.3$ GeV, total energy $\sim10^{49}$ erg. The hot radiating plasma is confined by a cold plasma of mass $M_c<0.03 (R/10^{17.5}{\rm cm})^4 M_\odot$. The source is nearly resolved, and may be resolved by 10 GHz observations. The fact that $\varepsilon_e\sim m_p c^2$ suggests that the hot plasma was created by the ejection of a mildly relativistic, $M\sim10^{-5} M_\odot$ shell, which propagated into an extended ambient medium or collided with a pre-ejected shell of mass $M_c$. The inferred plasma properties are inconsistent with typical "magnetar wind nebulae" model predictions. We suggest a physical mechanism for the generation of FRBs (independent of the persistent source model): Ejection from an underlying compact object, $R_s\sim10^{6}$ cm, of highly relativistic shells, with energy $E_s=10^{41}$ erg and Lorentz factor $γ_s$~$10^3$, into a surrounding e-p plasma with density $n\sim0.1/cm^3$ (consistent with that inferred for the plasma producing the persistent emission associated with FRB 121102). Such shell ejections with energy typical for FRB events lead to plasma conditions appropriate for strong synchrotron maser emission at the GHz range, $ν_{ coh.}\sim0.5(E/10^{41}erg)^{1/4}$ GHz. In this model, a significant fraction of the deposited energy is converted to an FRB with duration $R_s/c$, accompanied by ~10 MeV photons carrying less energy than the FRB. The inferred energy and mass associated with the source are low compared to those of typical supernova ejecta. This may suggest some type of a "weak stellar explosion", where a neutron star is formed with relatively low mass and energy ejection. However, the current upper limit on R does not allow one to rule out $M_c\sim1M_\odot$.

astro-ph.HE↗