arXiv subjects

Ehud Nakar

Publications and source records attributed to Ehud Nakar.

At least 91 records · Page 5Linked to original sources

arXiv2016

The B/C and sub-Iron/Iron Cosmic ray ratios - further evidence in favor of the spiral arm diffusion model

The Boron to Carbon (B/C) and sub-Fe/Fe ratios provides an important clue on Cosmic Ray (CR) propagation within the Galaxy. These ratios estimate the grammage that the CR traverse as they propagate from their sources to Earth. Attempts to explain these ratios within the standard CR propagation models require ad hoc modifications and even with those these models necessitate inconsistent grammages to explain both ratios. As an alternative, physically motivated model, we have proposed that CR originate preferably within the galactic spiral arms. CR propagation from dynamic spiral arms has important imprints on various secondary to primary ratios, such as the B/C ratio and the positron fraction. We use our spiral arm diffusion model with the spallation network extended up to Nickel to calculate the sub-Fe/Fe ratio. We show that without any additional parameters the spiral arm model consistently explains both ratios with the same grammage, providing further evidence in favor of this model.

astro-ph.HE↗

arXiv2016

The importance of 56Ni in shaping the light curves of type II supernovae

What intrinsic properties shape the light curves of Type II supernovae (SNe)? To address this question we derive observational measures that are robust (i.e., insensitive to detailed radiative transfer) and constrain the contribution from $^{56}$Ni, as well as a combination of the envelope mass, progenitor radius, and explosion energy. By applying our methods to a sample of type II SNe from the literature we find that $^{56}$Ni contribution is often significant. In our sample its contribution to the time weighted integrated luminosity during the photospheric phase ranges between 8% and 72% with a typical value of 30%. We find that the $^{56}$Ni relative contribution is anti-correlated with the luminosity decline rate. When added to other clues, this in turn suggests that the flat plateaus often observed in type II SNe are not a generic feature of the cooling envelope emission, and that without $^{56}$Ni many of the SNe that are classified as II-P would have shown a decline rate that is steeper by up to 1 mag/100 d. Nevertheless, we find that the cooling envelope emission, and not $^{56}$Ni contribution, is the main driver behind the observed range of decline rates. Furthermore, contrary to previous suggestions, our findings indicate that fast decline rates are not driven by lower envelope masses. We therefore suggest that the difference in observed decline rates is mainly a result of different density profiles of the progenitors.

astro-ph.HE↗

arXiv2016

The dynamics of radiation driven, optically thick winds

Recent observation of some luminous transient sources with low color temperatures suggests that the emission is dominated by optically thick winds driven by super-Eddington accretion. We present a general analytical theory of the dynamics of radiation pressure-driven, optically thick winds. Unlike the classical adiabatic stellar wind solution whose dynamics are solely determined by the sonic radius, here the loss of the radiation pressure due to photon diffusion also plays an important role. We identify two high mass loss rate regimes ($\dot{M} > L_{\rm Edd\,}/c^2$). In the large total luminosity regime the solution resembles an adiabatic wind solution. Both the radiative luminosity, $L$, and the kinetic luminosity, $L_k$, are super-Eddington with $L < L_k$ and $L \propto L_k^{1/3}$. In the lower total luminosity regime most of the energy is carried out by the radiation with $L_k < L \approx L_{\rm Edd\,}$. In a third, low mass loss regime ($\dot{M} < L_{\rm Edd\,}/c^2$), the wind becomes optically thin early on and, unless gas pressure is important at this stage, the solution is very different from the adiabatic one. The results are independent from the energy generation mechanism at the foot of the wind, therefore they are applicable to a wide range of mass ejection systems, from black hole accretion, to planetary nebulae, and to classical novae.

astro-ph.HE↗

arXiv2016

Type II supernovae progenitor and ejecta properties from the total emitted light, ET

It was recently shown that the bolometric light curves of type II supernovae (SNe) allow an accurate and robust measurement of the product of the radiation energy in the ejecta, $E_r$, and the time since the explosion, $t$, at early phases ($t\lesssim 10d$) of the homologous expansion. This observable, denoted here $ET \equiv E_rt$ is constant during that time and depends only on the progenitor structure and explosion energy. We use a 1D hydrodynamic code to find $ET$ of simulated explosions of 145 red supergiant progenitors obtained using the stellar evolution code MESA, and relate this observable to the properties of the progenitor and the explosion energy. We show that $ET$ probes only the properties of the envelope (velocity, mass and initial structure), similarly to other observables that rely on the photospheric phase emission. Nevertheless, for explosions where the envelope dominates the ejected mass, $M_{env}/M_{ej} \gtrsim 0.6$, $ET$ is directly related to the explosion energy $E_{exp}$ and ejected mass $M_{ej}$ through the relation $ET \approx 0.15 E_{exp}^{1/2} R_* M_{ej}^{1/2}$, where $R_*$ is the progenitor radius, to an accuracy better than $30\%$. We also provide relations between $ET$ and the envelope properties that are accurate (to within 20\%) for all the progenitors in our sample, including those that lost most of their envelope. We show that when the envelope velocity can be reasonably measured by line shifts in observed spectra, the envelope is directly constrained from the bolometric light curve (independent of $E_{exp}$). We use that to compare observations of 11 SNe with measured $ET$ and envelope velocity to our sample of numerical progenitors. This comparison suggests that many SNe progenitors have radii that are $\lesssim 500~R_\odot$. In the framework of our simulations this indicates, most likely, a rather high value of the mixing length parameter.

astro-ph.HE↗

arXiv2016

Testing the Magnetar Model via Late Time Radio Observations of Two Macronova Candidates

Compact binary mergers may have already been observed as they are the leading model for short gamma-ray bursts (sGRBs). Radioactive decay within the ejecta from these mergers is expected to produce an infra-red flare, dubbed macronova (or kilonova), on a time scale of a week. Recently two such macronova candidates were identified in followup observations of sGRBs, strengthening the possibility that those indeed arise from mergers. The same ejecta will also produce a long term (months to years) radio emission due to its interaction with the surrounding ISM. In search for this emission, we observed the two macronova candidates, GRB 130603B and GRB 060614 with the Jansky very large array (VLA) and the Australia Telescope Compact Array (ATCA). Our observations resulted in null-detections, putting strong upper limits on the kinetic energy and mass of the ejecta. A possible outcome of a merger is a highly magnetized neutron star (a magnetar), which has been suggested as the central engine for GRBs. Such a magnetar will deposit a significant fraction of its energy into the ejecta leading to a brighter radio flare. Our results, therefore, rule out magnetars in these two events.

astro-ph.HE↗

arXiv2015

Linear and Circular polarization in ultra-relativistic synchrotron sources - implications to GRB afterglows

Polarization measurements from relativistic outflows are a valuable tool to probe the geometry of the emission region and the microphysics of the particle distribution. Indeed, the polarization level depends on: (i) the local magnetic field orientation, (ii) the geometry of the emitting region with respect to the line of sight, and (iii) the electron pitch-angle distribution. Here we consider optically thin synchrotron emission and we extend the theory of circular polarization from a point source to an extended radially expanding relativistic jet. We present numerical estimates for both linear and circular polarization in such systems. We consider different configurations of the magnetic field, spherical and jetted outflows, isotropic and anisotropic pitch-angle distributions, and outline the difficulty in obtaining the reported high level of circular polarization observed in the afterglow of GRB 121024A. We conclude that the origin of the observed polarization cannot be intrinsic to an optically thin synchrotron process, even when the electron pitch-angle distribution is extremely anisotropic.

astro-ph.HE↗

arXiv2015

X-ray-powered macronovae

A macronova (or kilonova) was observed as an infrared excess several days after short gamma-ray burst, GRB 130603B. Although the $r$-process radioactivity is widely discussed as an energy source, it requires huge mass of ejecta from a neutron star (NS) binary merger. We propose a new model that the X-ray excess gives rise to the simultaneously observed infrared excess via thermal re-emission and explore what constraints this would place on the mass and velocity of the ejecta. This X-ray-powered model explains both the X-ray and infrared excesses with a single energy source by the central engine like a black hole, and allows for broader parameter region, in particular smaller ejecta mass $\sim10^{-3}-10^{-2}M_{\odot}$ with iron mixed as suggested by general relativistic simulations for typical NS-NS mergers, than the previous models. We also discuss the other macronova candidates in GRB 060614 and GRB 080503, and implications for the search of electromagnetic counterparts to gravitational waves.

astro-ph.HE↗

arXiv2015

Linear and circular polarization in GRB afterglows

A certain degree of linear polarization has been measured in several GRB afterglows. More surprisingly, circular polarization has been recently measured in GRB121024A. For synchrotron emission, the polarization level depends on: (i) the local magnetic field orientation (ii) the geometry of the emitting region with respect to the line of sight and (iii) the electron pitch-angle distribution. For this reason, polarization measurements are a valuable tool to probe afterglow micro-physics. We present numerical estimates of linear and circular polarization for different configurations (i.e., magnetic fields, geometries and pitch-angle distributions). For each different scenario, we study the conditions for reaching the maximum and minimum linear and circular polarization and provide their values. We discuss the implication of our results to the micro-physics of GRB afterglows in view of recent polarization measurements.

astro-ph.HE↗

arXiv2015

A unified picture for low-luminosity and long gamma-ray bursts based on the extended progenitor of llgrb 060218/SN 2006aj

The relation between long gamma-ray bursts (LGRBs) and low-luminosity GRBs (llgrbs) is a long standing puzzle -- on the one hand their high energy emission properties are fundamentally different, implying a different gamma-ray source, yet both are associated with similar supernovae of the same peculiar type (broad-line Ic), pointing at a similar progenitor and a similar explosion mechanism. Here we analyze the multi-wavelength data of the particularly well-observed SN 2006aj, associated with llgrb 060218, finding that its progenitor star is sheathed in an extended ($>100R_\odot$), low-mass ($\sim 0.01M_\odot$) envelope. This progenitor structure implies that the gamma-ray emission in this llgrb is generated by a mildly relativistic shock breakout. It also suggests a unified picture for llgrbs and LGRBs, where the key difference is the existence of an extended low-mass envelope in llgrbs and its absence in LGRBs. The same engine, which launches a relativistic jet, can drive the two explosions, but, while in LGRBs the ultra-relativistic jet emerges from the bare progenitor star and produces the observed gamma-rays, in llgrbs the extended envelope smothers the jet and prevents the generation of a large gamma-ray luminosity. Instead, the jet deposits all its energy in the envelope, driving a mildly relativistic shock that upon breakout produces a llgrb. In addition for giving a unified view of the two phenomena, this model provides a natural explanation to many observed properties of llgrbs. It also implies that llgrbs are a viable source of the observed extra-galactic diffuse neutrino flux and that they are promising sources for future gravitational wave detectors.

astro-ph.HE↗

arXiv2015

The afterglow of a relativistic shock breakout and low-luminosity GRBs

The prompt emission of low-luminosity gamma-ray bursts (llGRBs) indicates that these events originate from a relativistic shock breakout. In this case we can estimate, based on the properties of the prompt emission, the energy distribution of the ejecta. We develop a general formalism to estimate the afterglow produced by synchrotron emission from the forward shock resulting from the interaction of this ejecta with the circum-burst matter. We assess whether this emission can produce the observed radio and X-ray afterglows of the available sample of 4 llGRBs. All 4 radio afterglows can be explained within this model, providing further support for shock breakouts being the origin of llGRBs. We find that in one of the llGRBs (GRB 031203) the predicted X-ray emission, using the same parameters that fit the radio, can explain the observed one. In another one (GRB 980425) the observed X-rays can be explained if we allow for a slight modification of the simplest model. For the last two cases (GRBs 060218 and 100316D), we find that, as is the case for previous attempts to model these afterglows, the simplest model that fits the radio emission underpredicts the observed X-ray afterglows. Using general arguments, we show that the most natural location of the X-ray source is, like the radio source, within the ejecta-external medium interaction layer but that emission is due to a different population of electrons or to a different emission process.

astro-ph.HE↗

arXiv2014

The Nature of ULX Source M101 X-1: Optically Thick Outflow from A Stellar Mass Black Hole

The nature of ultra-luminous X-ray sources (ULXs) has long been plagued by an ambiguity about whether the central compact objects are intermediate-mass (IMBH, >~ 10^3 M_sun) or stellar-mass (a few tens M_sun) black holes (BHs). The high luminosity (~ 10^39 erg/s) and super-soft spectrum (T ~ 0.1 keV) during the high state of the ULX source X-1 in the galaxy M101 suggest a large emission radius (>~ 10^9 cm), consistent with being an IMBH accreting at a sub-Eddington rate. However, recent kinematic measurement of the binary orbit of this source and identification of the secondary as a Wolf-Rayet star suggest a stellar-mass BH primary with a super-Eddington accretion. If that is the case, a hot, optically thick outflow from the BH can account for the large emission radius and the soft spectrum. By considering the interplay of photons' absorption and scattering opacities, we determine the radius and mass density of the emission region of the outflow and constrain the outflow mass loss rate. The analysis presented here can be potentially applied to other ULXs with thermally dominated spectra, and to other super-Eddington accreting sources.

astro-ph.HE↗

arXiv2014

Photometric and Spectroscopic Properties of Type II-P Supernovae

We study a sample of 23 Type II Plateau supernovae (SNe II-P), all observed with the same set of instruments. Analysis of their photometric evolution confirms that their typical plateau duration is 100 days with little scatter, showing a tendency to get shorter for more energetic SNe. The rise time from explosion to plateau does not seem to correlate with luminosity. We analyze their spectra, measuring typical ejecta velocities, and confirm that they follow a well behaved power-law decline. We find indications of high-velocity material in the spectra of six of our SNe. We test different dust extinction correction methods by asking the following -- does the uniformity of the sample increase after the application of a given method? A reasonably behaved underlying distribution should become tighter after correction. No method we tested made a significant improvement.

astro-ph.HE↗

arXiv2014

Supernovae with Two Peaks in the Optical Light Curve and the Signature of Progenitors with Low-mass Extended Envelopes

Early observations of supernova light curves are powerful tools for shedding light on the pre-explosion structures of their progenitors and their mass-loss histories just prior to explosion. Some core-collapse supernovae that are detected during the first days after the explosion prominently show two peaks in the optical bands, including the $R$ and $I$ bands, where the first peak appears to be powered by the cooling of shocked surface material and the second peak is clearly powered by radioactive decay. Such light curves have been explored in detail theoretically for SN 1993J and 2011dh, where it was found that they may be explained by progenitors with extended, low-mass envelopes. Here we generalize these results. We explore, first, whether any double-peaked light curve of this type can be generated by a progenitor with a "standard" density profile, such as a red supergiant or a Wolf-Rayet star. We show that a standard progenitor (1) cannot produce a double-peaked light curve in the $R$ and $I$ bands, and (2) cannot exhibit a fast drop in the bolometric luminosity as is seen after the first peak. We then explore the signature of a progenitor with a compact core surrounded by extended, low-mass material. This may be a hydrostatic low-mass envelope or material ejected just prior to the explosion. We show that it naturally produces both of these features. We use this result to provide simple formulae to estimate (1) the mass of the extended material from the time of the first peak, (2) the extended material radius from the luminosity of the first peak, and (3) an upper limit on the core radius from the luminosity minimum between the two peaks.

astro-ph.HE↗

arXiv2014

Recombination Effects on Supernovae Light-Curves

Supernovae of type IIP are marked by the long plateau seen in their optical light curves. The plateau is believed to be the result of a recombination wave that propagates through the outflowing massive hydrogen envelope. Here, we analytically investigate the transition from a fully ionized envelope to a partially recombined one and its effects on the SN light curve. The motivation is to establish the underlying processes which dominate the evolution at late times when recombination takes place in the envelope, yet early enough so that $^{56}$Ni decay is a negligible source of energy. We assume a simple, yet adequate, hydrodynamic profile of the envelope and study the mechanisms which dominate the energy emission and the observed temperature. We consider the diffusion of photons through the envelope while analyzing the ionization fraction and the coupling between radiation and gas. We find that once recombination starts, the observed temperature decreases slowly in time. However, in a typical red supergiant (RSG) explosion, the recombination wave does not affect the bolometric luminosity immediately. Only at later times, the cooling wave may reach layers that are deep enough to affect the luminosity. We find that the plateau is not a generic result of a recombination process in expanding gas. Instead it depends on the density profile of the parts of the envelope which undergo recombination. Our results are useful to investigate the light curves of RSG explosions. We show the resulting light curves of two examples of RSG explosions according to our model and discuss their compatibility with observations. In addition, we improve the analytical relations between the plateau luminosity and plateau duration to the properties of the pre-explosion progenitor (Arnett 1980; Popov 1993).

astro-ph.HE↗

arXiv2014

SN 2008D: A Wolf-Rayet explosion through a thick wind

Supernova (SN) 2008D/XRT 080109 is considered to be the only direct detection of a shock breakout from a regular SN to date. While a breakout interpretation was favored by several papers, inconsistencies remain between the observations and current SN shock breakout theory. Most notably, the duration of the luminous X-ray pulse is considerably longer than expected for a spherical breakout through the surface of a type Ibc SN progenitor, and the X-ray radiation features, mainly its flat spectrum and its luminosity evolution, are enigmatic. We apply a recently developed theoretical model for the observed radiation from a Wolf-Rayet SN exploding through a thick wind and show that it naturally explains all the observed features of SN 2008D X-ray emission, including the energetics, the spectrum and the detailed luminosity evolution. We find that the inferred progenitor and SN parameters are typical for an exploding Wolf-Rayet. A comparison of the wind density found at the breakout radius to the density at much larger radii, as inferred by late radio observations, suggests an enhanced mass loss rate taking effect about ten days or less prior to the SN explosion. This finding joins accumulating evidence for a possible late phase in the stellar evolution of massive stars, involving vigorous mass loss a short time before the SN explosion.

astro-ph.HE↗

arXiv2014

Spectrum and light curve of a supernova shock breakout through a thick Wolf-Rayet wind

Wolf-Rayet stars are known to eject winds. Thus, when a Wolf-Rayet star explodes as a supernova, a fast, $>30,000$ km/s, shock is expected to be driven through a wind. We study the signal expected from a fast supernova shock propagating through an optically thick wind, and find that the electrons behind the shock driven into the wind are cooled efficiently, by inverse Compton over soft photons that were deposited by the radiation mediated shock that crossed the star. Therefore, the bolometric luminosity is comparable to the kinetic energy flux through the shock, and the spectrum is found to be a power-law, which slope and frequency range depend on the number flux of soft photons available for cooling. Wolf-Rayet supernovae that explode through a thick wind have a high flux of soft photons, producing a flat spectrum, $νF_ν=Const$, in the X-ray range $0.1\lesssim T\lesssim 50$ keV. As the shock expands into an optically thin wind, the soft photons are no longer able to cool the shock that plows through the wind, and the bulk of the emission takes the form of a standard core-collapse supernova (without a wind). However, a small fraction of the soft photons is upscattered by the shocked wind and produces a transient unique X-ray signature.

astro-ph.HE↗

arXiv2014

Constraints on Shallow 56Ni from the Early Lightcurves of Type Ia Supernovae

Ongoing transient surveys are presenting an unprecedented account of the rising lightcurves of Type Ia supernovae (SNe Ia). This early emission probes the shallowest layers of the exploding white dwarf, which can provide constraints on the progenitor star and the properties of the explosive burning. We use semi-analytic models of radioactively-powered rising lightcurves to analyze these observations. As we have summarized in previous work, the main limiting factor in determining the surface distribution of 56Ni is the lack of an unambiguously identified time of explosion, as would be provided by detection of shock breakout or shock-heated cooling. Without this the SN may in principle exhibit a "dark phase" for a few hours to days, where the only emission is from shock-heated cooling that is too dim to be detected. We show that by assuming a theoretically-motivated time-dependent velocity evolution, the explosion time can be better constrained, albeit with potential systematic uncertainties. This technique is used to infer the surface 56Ni distributions of three recent SNe Ia that were caught especially early in their rise. In all three we find fairly similar 56Ni distributions. Observations of SN 2011fe and SN 2012cg probe shallower depths than SN 2009ig, and in these two cases 56Ni is present merely ~0.01Msun from the WDs' surfaces. The uncertainty in this result is up to an order of magnitude given the difficulty of precisely constraining the explosion time. We also use our conclusions about the explosion times to reassess radius constraints for the progenitor of SN 2011fe, as well as discuss the roughly t^2 power law that is inferred for many observed rising lightcurves.

astro-ph.HE↗

arXiv2013

A Wolf in Sheepskin: Extraordinary Supernova 2012au Veiled Behind Ordinary Radio Emission

We present extensive radio and X-ray observations of SN\, 2012au, the energetic radio luminous supernova of type Ib that may be a link between subsets of hydrogen-poor superluminous and normal core-collapse supernovae. The observations closely follow models of synchrotron emission from shock heated circum-burst medium that has a wind density profile ($ρ\propto r^{-2}$). We infer a sub-relativistic velocity for the shock wave $v \approx 0.2\,c$ and a radius of $r \approx 1.4 \times 10^{16} \rm cm$ at 25 days after the estimated date of explosion. For a constant wind velocity of 1000 km/s we determine the constant mass loss rate of the progenitor to be $\dot{M} = 3.6 \times 10^{-6} \rm M_{\odot} yr^{-1}$, consistent with the estimates from X-ray observations. We estimate the total internal energy of the radio emitting material to be $E \approx 10^{47} \rm erg$, which is intermediate to SN\,1998bw and SN\,2002ap. Evolution of the radio light curves of SN\,2012au is consistent with interaction with a smoothly distributed circum-burst medium and absence of stellar shells ejected from previous outbursts out to $r \approx 10^{17} \rm cm$ from the supernova site. Based on this we conclude that the evolution of the SN\,2012au progenitor star was relatively quiet during the final years preceding explosion. We find that the bright radio emission from SN2012au was not dissimilar from other core collapse supernovae despite it's extraordinary optical properties. We speculate that it was the nature of the explosion that led to the unusual demise of the SN2012au progenitor star.

astro-ph.HE↗