arXiv subjects

David Eichler

Publications and source records attributed to David Eichler.

At least 37 records · Page 2Linked to original sources

arXiv2016

Ultrahigh Energy Cosmic Rays: A Galactic Origin?

It is suggested that essentially all of the UHECRs we detect, including those at the highest energy, originate in our Galaxy. It is shown that even if the density of sources decreases with Galactic radius, then the anisotropy and composition can be understood. Inward anisotropy, as recently reported by the Auger collaboration can be understood as drift along the current sheet of UHECRs originating outside the solar circle, as predicted in Kumar and Eichler (2014), while those originating within the solar circle exit the Galaxy at high latitudes.

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arXiv2016

On ultra high energy cosmic rays and their resultant gamma rays

The Fermi Large Area Telescope (Fermi LAT) collaboration has recently reported on 50 months of measurements of the isotropic Extragalactic Gamma Ray Background (EGRB) spectrum between $100\mbox{MeV}$ and $820\mbox{GeV}$. Ultrahigh Energy Cosmic Ray (UHECR) protons interact with the Cosmic Microwave Background (CMB) photons and produce cascade photons of energies $10\mbox{MeV}\mbox{-}1\mbox{TeV}$ that contribute to the EGRB flux. We examine seven possible evolution models for UHECRs and find that UHECR sources that evolve as the Star Formation Rate (SFR), medium low luminosity active galactic nuclei type-1 ($L = 10^{43.5}\mbox{erg sec}^{-1}$ in the $[0.5\mbox{-}2]\mbox{KeV}$ band), and BL Lacertae objects (BL Lacs) are the most acceptable given the constraints imposed by the observed EGRB. Other possibilities produce too much secondary $γ$-radiation. In all cases, the decaying dark matter contribution improves the fit at high energy, but the contribution of still unresolved blazars, which would leave the smallest role for decaying dark matter, may yet provide an alternative improvement. The possibility that the entire EGRB can be fitted with resolvable but not-yet-resolved blazars, as recently claimed by Ajello et al. (2015), would leave little room in the EGRB to accommodate $γ$-rays from extragalactic UHECR production, even for many source evolution rates that would otherwise be acceptable. We find that, under the assumption of UHECRs being mostly protons, there is not enough room for producing extragalactic UHECRs with AGN, GRB, or even SFR source evolution. Sources that evolve as BL Lacs on the other hand, would produce much less secondary $γ$-radiation and would remain a viable source of UHECRs, provided that they dominate.

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arXiv2015

Electron Heating in a Relativistic, Weibel-Unstable Plasma

The dynamics of two initially unmagnetized relativistic counter-streaming homogeneous ion-electron plasma beams are simulated in two dimensions using the particle-in-cell (PIC) method. It is shown that current filaments, which form due to the Weibel instability, develop a large scale longitudinal electric field in the direction opposite to the current carried by the filaments as predicted by theory. Fast moving ions in the current filaments decelerate due to this longitudinal electric field. The same longitudinal electric field, which is partially inductive and partially electrostatic, is identified as the main source of acceleration of electrons in the current filaments. The transverse electric field, though larger than the longitudinal one, is shown to play a smaller role in heating electrons, contrary to previous claims. It is found that, in 1D, the electrons become strongly magnetized and are \textit{not} accelerated beyond their initial kinetic energy. Rather, the heating of the electrons is enhanced by the bending and break-up of the filaments, which releases electrons that would otherwise be trapped within a single filament and hence slow the development of the Weibel instability (i.e. the magnetic field growth) via induction as per Lenz's law. In 2D simulations electrons are heated to about one quarter of the initial kinetic energy of ions. The magnetic energy at maximum is about 4 percent, decaying to less than 1 percent by the end of the simulation. Most of the heating of electrons takes place while the longitudinal electric field is still growing while only a small portion of the heating is a result of subsequent magnetic field decay. The ions are found to gradually decelerate until the end of the simulation by which time they retain a residual anisotropy less than 10 percent.

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arXiv2014

Cloaked Gamma Ray Bursts

It is suggested that many $γ$-ray bursts (GRBs) are cloaked by an ultra-relativistic baryonic shell that has high optical depth when the photons are manufactured. Such a shell would not fully block photons reflected or emitted from its inner surface, because the radial velocity of the photons can be less than that of the shell. This avoids the standard problem associated with GRBs that the thermal component should be produced where the flow is still obscured by high optical depth. The radiation that escapes high optical depth obeys the Amati relation. Observational implications may include a) anomalously high ratios of afterglow to prompt emission, such as may have been the case in the recently discovered PTF 11agg, and b) ultrahigh-energy neutrino pulses that are non-coincident with detectable GRB. It is suggested that GRB 090510, a short, very hard GRB with very little afterglow, was an {\it exposed} GRB, in contrast to those cloaked by baryonic shells. \end{abstract}

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arXiv2013

The isotropy problem of Sub-ankle Ultra-high energy cosmic rays

We study the time dependent propagation of sub-ankle ultra-high energy cosmic rays (UHECRs) originating from point-like Galactic sources. We show that drift in the Galactic magnetic field (GMF) may play an important role in the propagation of UHECRs and their measured anisotropy, particularly when the transport is anisotropic. To fully account for the discreteness of UHECR sources in space and time, a Monte Carlo method is used to randomly place sources in the Galaxy. The low anisotropy measured by Auger is not generally characteristic of the theoretical models, given that the sources are distributed in proportion to the star formation rate, but it can possibly be understood as a) intermittency effects due to the discrete nature of the sources or, with extreme parameters, b) a cancellation of drift current along a current sheet with the outward radial diffusive flux. We conclude that it is possible to interpret the Galactic sub-ankle CR flux as being due entirely to intermittent discrete Galactic sources distributed in proportion to star formation, but only with probability of roughly 35 percent, of which the spectrum is in accord with observations about 30 percent of the time. An alternative explanation for the low anisotropy may be that they are mostly extragalactic and/or heavy.

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arXiv2013

Is the Galactic Cosmic Ray Spectrum Constant in Time?

The hypothesis is considered that the present Galactic cosmic ray spectrum is at present softer than its time average due to source intermittency. Measurements of muogenic nuclides underground could provide an independent measurement of the time averaged spectrum. Source intermittency could also account for the surprising low anisotropy reported by the IceCube collaboration. Predictions for Galactic emission of ultrahigh-energy quanta, such as UHE gamma rays and neutrinos, might be higher or lower than previously estimated.

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arXiv2012

Comet Encounters and Carbon 14

The $^{14}$C production of shock-accelerated particles is calculated in terms of the total energy released in energetic particles. The recently reported 1.2% jump in the $^{14}$C content of the atmosphere in the year C.E. 775, it is found, would require $\gtrsim 10^{34}$ erg in energetic particles, less than first estimates but far more than any known solar flare on record. It is noted that the superflare from a large comet (comparable to C/Hale-Bopp) colliding with the sun could produce shock-accelerated GeV cosmic rays in the solar corona and/or solar wind, and possibly account for the CE 775 event. Several additional predictions of cometary encounters with the sun and other stars may be observable in the future.

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arXiv2012

Understanding TeV-band cosmic-ray anisotropy

We investigate the temporal and spectral correlations between flux and anisotropy fluctuations of TeV-band cosmic rays in the light of recent data taken with IceCube. We find that for a conventional distribution of cosmic-ray sources the dipole anisotropy is higher than observed, even if source discreteness is taken into account. Moreover, even for a shallow distribution of galactic cosmic-ray sources and a reacceleration model, fluctuations arising from source discreteness provide a probability only of the order of 10% that the cosmic-ray anisotropy limits of the recent IceCube analysis are met. This probability estimate is nearly independent of the exact choice of source rate, but generous for a large halo size. The location of the intensity maximum far from the Galactic Center is naturally reproduced.

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arXiv2012

Cosmic ray current-driven turbulence and mean-field dynamo effect

We show that an alpha effect is driven by the cosmic ray Bell instability exciting left-right asymmetric turbulence. Alfven waves of a preferred polarization have maximally helical motion, because the transverse motion of each mode is parallel to its curl. We show how large-scale Alfven modes, when rendered unstable by cosmic ray streaming, can create new net flux over any finite region, in the direction of the original large-scale field. We perform direct numerical simulations (DNS) of an MHD fluid with a forced cosmic ray current and use the test-field method to determine the alpha effect and the turbulent magnetic diffusivity. As follows from DNS, the dynamics of the instability has the following stages: (i) in the early stage, the small-scale Bell instability that results in a production of small-scale turbulence is excited; (ii) in the intermediate stage, there is formation of larger-scale magnetic structures; (iii) finally, quasi-stationary large-scale turbulence is formed at a growth rate that is comparable to that expected from the dynamo instability, but its amplitude over much longer timescales remains unclear. The results of DNS are in good agreement with the theoretical estimates. It is suggested that this dynamo is what gives weakly magnetized relativistic shocks such as those from gamma ray bursts a macroscopic correlation length. It may also be important for large-scale magnetic field amplification associated with cosmic ray production and diffusive shock acceleration in supernova remnants (SNR) and blast waves from gamma ray bursts. Magnetic field amplification by Bell turbulence in SNR is found to be significant, but it is limited owing to the finite time available to the super-Alfvenicly expanding remnant. The effectiveness of the mechanisms is shown to be dependent on the shock velocity.

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arXiv2011

Origin of Ultrahigh Energy Galactic Cosmic Rays: The Isotropy Problem

We study the propagation of ultra-high-energy cosmic rays (UHECR) in the Galaxy, concentrating on the energy range below the ankle in the spectrum at 4 EeV. A Monte-Carlo method, based on analytical solutions to the time-dependent diffusion problem, is used to account for intermittency by placing sources at random locations. Assuming a source population that scales with baryon mass density or star formation (e.g. long GRB), we derive constraints arising from intermittency and the observational limits on the composition and anisotropy. It is shown that the composition and anisotropy at 1e18 eV are difficult to reproduce and require that either a) the particle mean free path is much smaller than a gyroradius, implying the escape time is very long, b) the composition is heavier than suggested by recent Auger data, c) the ultrahigh-energy sub-ankle component is mostly extragalactic, or d) we are living in a rare lull in the ultrahigh-energy cosmic-ray production, and the current ultrahigh-energy cosmic-ray intensity is far below the Galactic time average. We therefore recommend a strong observational focus on determining the UHECR composition around 1e18 eV.

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arXiv2011

Can Ultrahigh Energy Cosmic Rays Come from Gamma-Ray Bursts? II: Cosmic Rays Below the Ankle and Galactic GRB

The maximum cosmic ray energy achievable by acceleration by a relativistic blast wave is derived. It is shown that forward shocks from long GRB in the interstellar medium are powerful enough to produce the Galactic cosmic-ray component up to the ankle at $4\times 10^{18}$eV, as per an earlier suggestion (Levinson and Eichler, 1993). It is further argued that, were extragalactic long GRB responsible for the component {\it above} the ankle as well, the contribution from an occasional Galactic GRB within the solar circle would yield more than the observational limits on the outward flux from the solar circle, unless intermittency and/or beaming causes the present-day contribution to be less than $10^{-3}$ times the time average, and difficulties with these avoidance scenarios are also noted.

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arXiv2011

Stream instabilities in relativistically hot plasma

The instabilities of relativistic ion beams in a relativistically hot electron background are derived for general propagation angles. It is shown that the Weibel instability in the direction perpendicular to the streaming direction is the fastest growing mode, and probably the first to appear, consistent with the aligned filaments that are seen in PIC simulations. Oblique, quasiperpendicular modes grow almost as fast, as the growth rate varies only moderately with angle, and they may distort or corrugate the filaments after the perpendicular mode saturates.

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arXiv2011

Standard GRB, Dirty Fireballs, and the Excluded Middle

It is shown that the contribution of faint gamma-ray bursts (GRB) to the total allsky GRB energy flux is small. The allsky flux of GRB appears to be $5.3 \times 10^{-3}$ erg/$\rm cm^2 y$, with little additional component hidden within weak or otherwise undetectable GRB. This significantly constrains physical models of GRB and dirty fireballs, suggesting a rather sharp dichotomy between them should the latter exist.

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arXiv2011

Long Term Radiative Behavior of SGR 1900+14

The prolific magnetar SGR 1900+14 showed two outbursts in the last decade and has been closely monitored in the X-rays to track the changes in its radiative properties. We use archival Chandra and XMM-Newton observations of SGR 1900+14 to construct a history of its spectrum and persistent X-ray flux spanning a period of about seven years. We show that the decline of its X-ray flux in these two outburst episodes follows the same trend. The flux begins to decline promptly and rapidly subsequent to the flares, then decreases gradually for about 600 days, at which point it resumes a more rapid decline. Utilizing the high quality spectral data in each epoch, we also study the spectral coevolution of the source with its flux. We find that neither the magnetic field strength nor the magnetospheric properties change over the period spanned by the observations, while the surface temperature as well as the inferred emitting area both decline with time following both outbursts. We also show that the source reached the same minimum flux level in its decline from these two subsequent outbursts, suggesting that this flux level may be its steady quiescent flux.

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arXiv2010

How unusual is XRF 060218 ?

Light curves are calculated for an off-axis observer due the scattering of primary radiation by extended baryonic material. The unusually long duration and the chromaticity of the light curves above several KeV of XRF 060218 can be explained as a result of the acceleration of the baryonic scattering material by the primary radiation. The observed light curves by our model and detailed fits to the data are presented. The model predicts that $\sim 4\times 10^{48}$ ergs are put into accelerated, mildly relativistic baryons by the radiation pressure at large radii from the central engine. It is suggested that the emission below 3 KeV, which lies {\it below} the Amati relation, is a baryon contaminated fireball.

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arXiv2010

The High Energy Budget Allocations in Shocks and GRB

The statistical distribution of energies among particles responsible for long Gamma Ray Burst (GRB) emission is analyzed in light of recent results of the Fermi Observatory. The allsky flux, $F_γ$, recorded by the Gamma Ray Burst Monitor (GBM) is shown, despite its larger energy range, to be not significantly larger than that reported by the Burst and Transient Explorer (BATSE), suggesting a relatively small flux in the 3 - 30 MeV energy range. The present-day energy input rate in $γ$-rays recorded by the GBM from long GRB is found, assuming star-formation rates in the literature, to be $\dot W(0)=0.5 F_γ H/c = 5 \times 10^{42}\ \rm{erg/Mpc^3 yr}$. The Large Area Telescope (LAT) fluence, when observed, is about 5-10\% per decade of the total, in good agreement with the predictions of saturated, non-linear shock acceleration. The high-energy component of long GRBs, as measured by Fermi, is found to contain only $\sim 10^{-2.5}$ of the energy needed to produce ultrahigh-energy cosmic rays (UHECR) above 4 Eev, assuming the latter to be extragalactic, when various numerical factors are carefully included, if the cosmic ray source spectrum has a spectral index of -2. The observed $γ$-ray fraction of the required UHECR energy is even smaller if the source spectrum is softer than $E^{-2}$. The AMANDA II limits rule out such a GRB origin for UHECR if much more than $10^{-2}$ of the cosmic ray energy goes into neutrinos that are within, and simultaneous with, the $γ$-ray beam. It is suggested that "orphan" neutrinos out of the $γ$-ray beam might be identifiable via orphan afterglow { or other wide angle signatures of GRB in lieu of coincidence with prompt $γ$-rays}, and it is recommended that feasible single neutrino trigger criteria be established to search for such coincidences.

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arXiv2010

Fermi Constrains Dark Matter Origin of High Energy Positron Anomaly

Fermi measurements of the high-latitude gamma-ray background strongly constrain a decaying-dark-matter origin for the 1--100 GeV Galactic positron anomaly measured with PAMELA. Inverse-Compton scattering of the microwave background by the emergent positrons produces a bump in the diffuse 100-200 MeV gamma-ray background that would protrude from the observed background at these energies. The positrons are thus constrained to emerge from the decay process at a typical energy between ~100 GeV and ~250 GeV. By considering only gamma-ray emission of the excess positrons and electrons, we derive a minimum diffuse gamma-ray flux that, apart from the positron spectrum assumed, is independent of the actual decay modes. Any gamma-rays produced directly by the dark-matter decay leads to an additional signal that make the observational limits more severe. A similar constraint on the energy of emergent positrons from annihilation in dark-matter substructures is argued to exist, according to recent estimates of enhancement in low-mass dark-matter substructures, and improved simulations of such substructure will further sharpen this constraint.

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arXiv2010

Dynamical Oscillations and Glitches in AXPs

Recently reported observations of magnetar glitches and coincident X-ray long term brightening events establish enough of a data base to indicate that the brightening events are accompanied by glitches at their outset, and that they are probably triggered by the same event that triggers the glitch. We suggest, on the basis of various observational clues, that a) these are caused by energy releases at depths below 100 meters, and b) the unpinning is due to global mechanical motion triggered by the energy release, not by heat. Because mechanical triggering of a glitch requires less energy than a detectable long term X-ray brightening, the latter does not necessarily accompany every glitch, but it is predicted, when it does occur, to be heralded by a glitch. Crustal oscillation associated with the mechanical energy release may cause short ($\ll 10^3$s) flares.

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