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Nir J. Shaviv

Publications and source records attributed to Nir J. Shaviv.

54 records · Page 3Linked to original sources
arXiv2002

Cosmic Ray Diffusion from the Galactic Spiral Arms, Iron Meteorites, and a possible climatic connection?

We construct a Galactic cosmic ray (CR) diffusion model while considering that CR sources reside predominantly in the Galactic spiral arms. We find that the CR flux (CRF) reaching the solar system should periodically increase each crossing of a Galactic spiral arm. We search for this signal in the CR exposure age record of Iron meteorites and confirm this prediction. We then check the hypothesis that climate, and in particular the temperature, is affected by the CRF to the extent that glaciations can be induced or completely hindered by possible climatic variations. We find that although the geological evidence for the occurrence of IAEs in the past Eon is not unequivocal, it appears to have a nontrivial correlation with the spiral arm crossings--agreeing in period and phase. Thus, a better timing study of glaciations could either confirm this result as an explanation to the occurrence of IAEs or refute a CRF climatic connection.

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arXiv2002

QED and the High Polarization of the Thermal Radiation from Neutron Stars

The thermal emission of strongly magnetized neutron-star atmospheres is thought to be highly polarized. However, because of the different orientations of the magnetic field over the surface of the neutron star (NS), it is commonly assumed that the net observed polarization will be significantly reduced as the polarization from different regions will cancel each other. We show that the birefringence of the magnetized QED vacuum decouples the polarization modes in the magnetosphere; therefore, the direction of the polarization follows the direction of the magnetic field up to a large distance from the stellar surface. At this distance, the rays that leave the surface and are destined for our detectors pass through only a small solid angle; consequently, the polarization direction of the emission originating in different regions will tend to align together. The net observed polarization of the thermal radiation of NSs should therefore be very large. Measurement of this polarization will be the first direct evidence of the birefringence of the magnetized vacuum due to QED and a direct probe of behavior of the vacuum at magnetic fields of order of and above the critical QED field of 4.4 x 10 13 G. The large observable polarization will also help us learn more about the atmospheric properties of NSs.

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arXiv2001

The Theory of Steady State Super-Eddington Winds and its Application to Novae

We present a model for steady state winds of systems with super-Eddington luminosities. These radiatively driven winds are expected to be optically thick and clumpy as they arise from an instability driven porous atmosphere. The model is then applied to derive the mass loss observed in bright classical novae. The main results are: 1) A general relation between the mass loss rate and the total luminosity in super-Eddington systems. 2) A quantitative agreement between the observed luminosity evolution which is used to predict both the mass loss and temperature evolution, and their observations. 3) An agreement between the predicted average integrated mass loss of novae as a function of WD mass and its observations. 4) A natural explanation for the `transition phase' of novae. 5) Agreement with eta Carinae which was used to double check the theory. The prediction for the mass shed in the star's great eruption agrees with observations to within the measurement error.

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arXiv2000

Super Eddington Atmospheres and their Winds

We present a model for the steady state winds of super-Eddington systems. These radiatively driven winds are expected to be optically thick and clumpy as they arise from a porous atmosphere. The model is then used to derive the mass loss of bright classical novae. The long duration super-Eddington outflows that are clearly observed in at least two cases (Nova LMC 1988 #1 & Nova FH Ser) are naturally explained. Moreover, the predicted mass loss and temperature evolution agree nicely with the observations, as do additional features. eta-Car is then used to double check the theory which predicts the observed mass shed in the great eruption.

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arXiv2000

The Nature of the Radiative Hydrodynamic Instabilities in Radiatively Supported Thomson Atmospheres

Atmospheres having a significant radiative support are shown to be intrinsically unstable at luminosities above a critical fraction Gamma_crit ~ 0.5-0.85 of the Eddington limit, with the exact value depending on the boundary conditions. Two different types of absolute radiation-hydrodynamic instabilities of acoustic waves are found to take place even in the electron scattering dominated limit. Both instabilities grow over dynamical time scales and both operate on non radial modes. One is stationary and arises only after the effects of the boundary conditions are taken into account, while the second is a propagating wave and is insensitive to the boundary conditions. Although a significant wind can be generated by these instabilities even below the classical Eddington luminosity limit, quasi-stable configurations can exist beyond the Eddington limit due to the generally reduced effective opacity. The study is done using a rigorous numerical linear analysis of a gray plane parallel atmosphere under the Eddington approximation. We also present more simplified analytical explanations.

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arXiv2000

The porous atmosphere of eta Carinae

We analyze the wind generated by the great 20 year long super-Eddington outburst of eta-Carinae. We show that using classical stellar atmospheres and winds theory, it is impossible to construct a consistent wind model in which a sufficiently small amount of mass, like the one observed, is shed. One expects the super-Eddington luminosity to drive a thick wind with a mass loss rate substantially higher than the observed one. The easiest way to resolve the inconsistency is if we alleviate the implicit notion that atmospheres are homogeneous. An inhomogeneous atmosphere, or "porous", allows more radiation to escape while exerting a smaller average force. Consequently, such an atmosphere yields a considerably lower mass loss rate for the same total luminosity. Moreover, all the applications of the Eddington Luminosity as a strict luminosity limit should be revised, or at least reanalyzed carefully.

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arXiv1999

Polarization Evolution in Strong Magnetic Fields

Extremely strong magnetic fields change the vacuum index of refraction. Although this polarization dependent effect is small for typical neutron stars, it is large enough to decouple the polarization states of photons traveling within the field. The photon states evolve adiabatically and follow the changing magnetic field direction. The combination of a rotating magnetosphere and a frequency dependent state decoupling predicts polarization phase lags between different wave bands, if the emission process takes place well within the light cylinder. This QED effect may allow observations to distinguish between different pulsar emission mechanisms and to reconstruct the structure of the magnetosphere.

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arXiv1999

Magnetic Lensing near Ultramagnetized Neutron Stars

Extremely strong magnetic fields change the vacuum index of refraction. This induces a lensing effect that is not unlike the lensing phenomenon in strong gravitational fields. The main difference between the two is the polarization dependency of the magnetic lensing, a behaviour that induces a handful of interesting effects. The main prediction is that the thermal emission of neutron stars with extremely strong magnetic fields is polarized - up to a few percent for the largest fields known. This potentially allows a direct method for measuring their magnetic fields.

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arXiv1998

Can Nonlinear Structure Form at the Era of Decoupling?

The effects that large scale fluctuations had on small scale isothermal modes at the epoch of recombination are analysed. We find that: (a) Albeit the fact that primordial fluctuations were at this epoch still well in the linear regime, a significant nonlinear radiation hydrodynamic interaction could have taken place. (b) Short wavelength isothermal fluctuations are unstable. Their growth rate is exponential in the amplitude of the large scale fluctuations and is therefore very sensitive to the initial conditions. (c) The observed CMBR fluctuations are of order the limit above which the effect should be significant. Thus, according to their exact value, the effect may be negligible or lead to structure formation out of isothermal fluctuations within the period of recombination. (d) If the cosmological parameters are within the prescribed regime, the effect should be detectable through induced deviations in the Planck spectrum. (e) The sensitivity of the effect to the initial conditions provides a tool to set limits on various cosmological parameters with emphasis on the type and amplitude of the primordial fluctuation spectrum. (f) Under proper conditions, the effect may be responsible for the formation of sub-globular cluster sized objects at particularly high red shifts. (g) Under certain circumstances, it can also affect horizon sized large scale structure.

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arXiv1997

Life Extinctions By Cosmic Ray Bursts

High energy cosmic ray jets from nearby mergers or accretion induced collapse (AIC) of neutron stars (NS) that hit the atmosphere can produce lethal fluxes of atmospheric muons at ground level, underground and underwater, destroy the ozone layer and radioactivate the environment. They could have caused most of the massive life extinctions on planet Earth in the past 600 My. Biological mutations due to ionizing radiations could have caused the fast appearance of new species after the massive extinctions. An early warning of future extinctions due to NS mergers may be obtained by identifying, mapping and timing all the nearby binary neutron stars systems. A warning of an approaching cosmic ray burst from a nearby NS merger/AIC may be provided by a very intense gamma ray burst which preceeds it.

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arXiv1996

Life Extinction Due To Neutron Star Mergers

Cosmic ray bursts (CRBs) from mergers or accretion induced collapse of neutron stars that hit an Earth-like planet closer than $\sim 1 kpc$ from the explosion produce lethal fluxes of atmospheric muons at ground level, underground and underwater. These CRBs also destroy the ozone layer and radioactivate the environment. The mean rate of such life devastating CRBs is one in 100 million years (Myr), consistent with the observed 5 ``great'' extinctions in the past 600 Myr. Unlike the previously suggested extraterrestrial extinction mechanisms the CRBs explain massive life extinction on the ground, underground and underwater and the higher survival levels of radiation resistant species and of terrain sheltered species. More distant mergers can cause smaller extinctions. Biological mutations caused by ionizing radiation produced by the CRB may explain a fast appearance of new species after mass extinctions. The CRB extinction predicts detectable enrichment of rock layers which formed during the extinction periods with cosmogenically produced radioactive nucleides such as $^{129}$I, $^{146}$Sm, $^{205}$Pb with and $^{244}$Pu. Tracks of high energy particles in rock layers on Earth and on the moon may also contain records of intense CRBs. An early warning of future extinctions due to neutron star mergers can be obtained by identifying, mapping and timing all the nearby binary neutron stars systems. A final warning of an approaching CRB from a nearby neutron stars merger will be provided by a gamma ray burst a few days before the arrival of the CRB.

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arXiv1996

Solution To The Gamma Ray Burst Mystery?

Photoexcitation and ionization of partially ionized heavy atoms in highly relativistic flows by interstellar photons, followed by their reemission in radiative recombination and decay, boost star-light into beamed $γ$ rays along the flow direction. Repeated excitation/decay of highly relativistic baryonic ejecta from merger or accretion induced collapse of neutron stars in dense stellar regions (DSRs), like galactic cores, globular clusters and super star-clusters, can convert enough kinetic energy in such events in distant galaxies into cosmological gamma ray bursts (GRBs). The model predicts remarkably well all the main observed temporal and spectral properties of GRBs. Its success strongly suggests that GRBs are $γ$ ray tomography pictures of DSRs in galaxies at cosmological distances with unprecedented resolution: A time resolution of $dt\sim 1~ms$ in a GRB can resolve stars at a Hubble distance which are separated by only $D\sim 10^{10}cm$. This is equivalent to the resolving power of an optical telescope with a diameter larger than one Astronomical Unit!

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arXiv1996

On The Origin of Gamma Ray Bursts

We propose that repeated photoexcitation/ionization of high Z atoms of highly relativistic flows by star light in dense stellar regions followed by emission of decay/recombination photons, which are beamed and boosted to gamma ray energies in the observer frame, produce gamma ray bursts (GRBs). We show that this overlooked mechanism, which is able to convert efficiently baryonic kinetic energy release in merger or accretion induced collapse of neutron stars into cosmological GRBs, reproduces remarkably well all the main observed properties of GRBs.

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arXiv1995

Fireballs in Dense Stellar Regions as an explanation of Gamma Ray Bursts

We study a cosmological scenario for gamma ray bursts (GRBs) where relativistic flows interact with dense radiation fields. It is shown that this scenario is plausible in very dense stellar regions which are known to exist in collapsed cores of globular clusters or dense nuclei of galaxies. It yields a correct quantitative description of the temporal behavior of GRBs. Several other properties of GRBs are easily explained.

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arXiv1995

The Extragalactic Neutrino Background Radiations From Blazars and Cosmic Rays

Blazar emission of gamma rays and cosmic ray production of gamma rays in gas-rich clusters have been proposed recently as alternative sources of the high energy extragalactic diffuse gamma ray background radiation. We show that these sources also produce a very different high energy extragalactic diffuse neutrino background radiation. This neutrino background may be detected by the new generation of large neutrino telescopes under construction and may be used to trace the origin of the extragalactic gamma radiation.

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arXiv1995

On the Source and Location of Temporal Variability in Fireballs

Most cosmological models for gamma-ray bursts invoke the production of a ``fireball'' in a compact region, as indicated by the short time variability of the observed GRBs. The high density of $e^+e^-$ pairs in such fireballs inevitably makes them opaque to gamma-rays and requires the gamma-ray emission to take place only after the fireball has expanded and became optically thin to gamma's. Many of the ``traditional'' scenarios explain the temporal variability of GRBs as being formed by growing instabilities in the expanding fireball. Here we explore this possiblity and examine its validity.

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arXiv1994

Gamma Ray Bursts from Minijets

Striking similarities exist between high energy gamma ray emission from active galactic nuclei (AGN) and gamma ray bursts (GRBs). They suggest that GRBs are generated by inverse Compton scattering from highly relativistic electrons in transient jets. Such jets may be produced along the axis of an accretion disk formed around stellar black holes (BH) or neutron stars (NS) in BH-NS and NS-NS mergers and in accretion induced collapse of magnetized white dwarfs (WD) or neutron stars in close binary systems. Such events may produce the cosmological GRBs. Transient jets formed by single old magnetized neutron stars in an extended Galactic halo may produce a local population of GRBs. Here we show that jet production of GRBs by inverse Compton scattering can explain quite simply the striking correlations that exist between various temporal features of GRBs, their duration histogram, the power spectrum of their complex multipeak light curves, their power-law high energy spectra and other features of GRBs. Some additional predictions are made including the expected polarization of gamma-rays in the bursts.

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