Searcharxiv⌕ Search

arXiv subjects

David Eichler

Publications and source records attributed to David Eichler.

At least 55 records · Page 3Linked to original sources
arXiv2010

Wide Angle X-ray Sky Monitoring for Corroborating non-Electromagnetic Cosmic Transients

Gravitational waves (GW) can be emitted from coalescing neutron star (NS) and black hole-neutron star (BH-NS) binaries, which are thought to be the sources of short hard gamma ray bursts (SHBs). The gamma ray fireballs seem to be beamed into a small solid angle and therefore only a fraction of detectable GW events is expected to be observationally coincident with SHBs. Similarly ultrahigh energy (UHE) neutrino signals associated with gamma ray bursts (GRBs) could fail to be corroborated by prompt gamma-ray emission if the latter is beamed in a narrower cone than the neutrinos. Alternative ways to corroborate non-electromagnetic signals from coalescing neutron stars are therefore all the more desirable. It is noted here that the extended X-ray tails (XRT) of SHBs are similar to X-ray flashes (XRFs), and that both can be attributed to an off-axis line of sight and thus span a larger solid angle than the hard emission. It is proposed that a higher fraction of detectable GW events may be coincident with XRF/XRT than with hard gamma-rays, thereby enhancing the possibility to detect it as a GW or neutrino source. Scattered gamma-rays, which may subtend a much larger solid angle that the primary gamma ray jet, are also candidates for corroborating non-electromagnetic signals.

astro-ph.HE↗
arXiv2008

Spectral Lags Explained as Scattering from Accelerated Scatterers

A quantitative theory of spectral lags for $γ$-ray bursts (GRBs) is given. The underlying hypothesis is that GRB subpulses are photons that are scattered into our line of sight by local concentrations of baryons that are accelerated by radiation pressure. For primary spectra that are power laws with exponential cutoffs, the width of the pulse and its fast rise, slow decay asymmetry is found to increase with decreasing photon energy, and the width near the exponential cutoff scales approximately as $E_{ph}^{-η}$, with $η\sim 0.4$, as observed. The spectral lag time is naturally inversely proportional to luminosity, all else being equal, also as observed.

astro-ph↗
arXiv2008

A Universal Central Engine Hypothesis for Short and Long GRBs

It is noted that X-ray tails (XRTs) of short, hard $γ$-ray bursts (SHBs) are similar to X-ray flashes (XRFs). We suggest a universal central engine hypothesis, as a way of accounting for this curiosity, in which SHBs differ from long $γ$-ray bursts (GRBs) in prompt emission because of the differences in the host star and attendant differences in the environment they present to the compact central engine (as opposed to differences in the central engine itself). Observational constraints and implications are discussed, especially for confirming putative detections of gravitational waves from merging compact objects.

astro-ph↗
arXiv2007

A Model for Fast Rising, Slowly Decaying Subpulses in Gamma-Ray Bursts

Gamma ray bursts (GRB's) often feature subpulses that have a distinctively asymmetric profile -- they rise quickly and decay much more slowly, while their spectrum softens slightly with observer time. It is suggested that these subpulses are caused by slow baryonic clouds embedded within a primary $γ$-ray beam, which scatter the $γ$-radiation into our line of sight as they accelerate. Good quantitative agreement is obtained with observed light curves and spectral evolution. The kinetic energy that the baryonic component of GRB jets receives from the primary $γ$-radiation is predicted to be about equal to the amount of $γ$-radiation that is scattered, consistent with observations of afterglow. Several other observational consequences are briefly discussed. The possibility is raised that the time scale of short GRB is established by radiative acceleration and/or baryon injection rather than the time scale of the central engine.

astro-ph↗
arXiv2007

On the corona of magnetars

Slow dissipation of non-potential magnetic fields in the magnetosphere of the magnetar is assumed to accelerate particles to hundreds MeV along the magnetic field lines. We consider interaction of fast particles with the surface of the magnetar. We argue that the collisionless dissipation does not work in the atmosphere of the neutron star because the two-stream instability is stabilized by the inhomogeneity of the atmosphere. Rather, the dominant dissipation mechanism is collisional Landau levelexcitations followed by pair production via the deexcitation gamma-rays ultimately leading to electrons with the energy below the Landau energy. We show that, because of the effects of the superstrong magnetic field, these electrons could emerge from the surface carrying most of the original energy so that a hot corona arises with the temperature of 1 - 2 MeV. This extended corona is better suited than a thin atmosphere to convert most of the primary beam energy to non-thermal radiation and, as we show, most of the coronal energy release is radiated away in the hard X-ray and the soft gamma-ray bands by Comptonization and bremsstrahlung. The radiation spectrum is a power-law with the photon index $1<α<2$. The model may account for the persistent hard X-ray emission discovered recently from the soft gamma-ray repeaters and anomalous X-ray pulsars and predicts that the radiation spectrum is extended into the MeV band.

astro-ph↗
arXiv2006

Long Term Cooling of Magnetar Crusts

X-ray emission following giant flares of magnetars can be categorized into three categories of time scales (a) short term afterglow (b) medium term afterglow and (c) long term afterglow. Short term afterglow, which declines over several hours, seems to correspond to gravitational resettling of uplifted material. Medium term afterglow, which declines over several weeks or months, appears to be the cooling of the heated outer crust, and long term afterglow, which declines over a period of many years, can be understood to be the cooling of the inner crust. The long term afterglow profile may be a very sensitive indicator of neutron star mass.

astro-ph↗
arXiv2006

Are Gamma-ray Bursts Universal?

It is noted that the Liang-Zhang correlation can be accounted for with the viewing angle interpretation proposed earlier. The Ghirlanda correlation, recently generalized by Nava et al (2006) to a wind profile, can be accounted for by the viewing angle interpretation accordingly generalized to a wind profile. Most of the scatter in the spectra and time-integrated brightness in $γ$-ray bursts (GRB) can thus be accounted for by variation in two parameters, 1) the viewing angle and 2) the jet opening angle, with very little variation in any other intrinsic parameters. The scatter in apparent isotropic equivalent fluence and other parameters is reduced by a factor of order 30 when each of these parameters is considered. Possible difficulties with alternative explanations are briefly discussed. It is also noted that the relative scatter in the Amati and Ghirlanda correlations suggests certain conclusions about the inner engine.

astro-ph↗
arXiv2005

The Case for Anisotropic Afterglow Efficiency within Gamma-Ray Burst Jets

Early X-ray afterglows recently detected by {\it Swift} frequently show a phase of very shallow flux decay lasting from $\sim 10^{2.5} $s up to $\sim 10^4 $s, followed by a steeper, more familiar decay. We suggest that the flat early part of the light curve may be a combination of the decaying tail of the prompt emission and the delayed onset of the afterglow emission observed from viewing angles slightly outside the edge of the region within the jet with prominent afterglow emission, as predicted previously. This would imply that a significant fraction of viewers get very little external shock energy along their line of sight, and, therefore, see a very high $γ$-ray to kinetic energy ratio at early times. The early flat phase in the afterglow light curve implies, in a rather robust and model independent manner, a very large $γ$-ray efficiency, typically $\gtrsim 90%$, which is very difficult to extract from baryons by internal shocks.

astro-ph↗
arXiv2005

Are GRB shocks mediated by the Weibel instability?

It is estimated that the Weibel instability is not generally an effective mechanism for generating ultrarelativistic astrophysical shocks. Even if the upstream magnetic field is as low as in the interstellar medium, the shock is mediated not by the Weibel instability but by the Larmor rotation of protons in the background magnetic field. Future simulations should be able to verify or falsify our conclusion.

astro-ph↗
arXiv2005

The Difference Between the Amati and Ghirlanda Relations

It is pointed out that the beaming correction commonly inferred from the achromatic breaks in the afterglow light curve is biased in situations where the isotropic equivalent energy is affected by factors other than the spread in opening angles. In particular, it underestimates the beaming factor of sources observed off-axis. Here we show that both the slopes and scatters in the Amati and Ghirlanda relations, and the difference between them, are quantitatively consistent with a model proposed recently, in which the $E_{iso}-ν_{peak}$ relation, as originally derived by Amati et al., is due to viewing angle effects. The quantitative difference between them confirms the relations between opening angle and break time suggested by Frail et al.

astro-ph↗
arXiv2005

Ultrahigh Energy Activity in Giant Magnetar Outbursts

The recent superflare of 27 December 2004 from the magnetar SGR 1806-20 was the brightest extrasolar flash ever recorded in the modern era. The chances for seeing exotic ultrahigh energy (UHE) radiation - neutrons, neutrinos, gamma rays and charged cosmic rays - from it are far better from an energetic point of view than from cosmological gamma ray bursts (GRBs). The chances for detecting the various components are discussed in light of recent data from the 27 December event.

astro-ph↗
arXiv2005

The efficiency of electron acceleration in collisionless shocks and GRB energetics

Afterglow observations are commonly used to determine the parameters of GRB explosions, the energy E, surrounding density n, post-shock magnetic field equipartition fraction ε_B and electron equipartition fraction ε_e, under the frequently made assumption that the efficiency of electron "injection" into relativistic shock acceleration is high, i.e. that the fraction f of electrons which undergo acceleration is f~1. We show that the value of f can not be determined by current observations, since currently testable model predictions for a parameter choice {E'=E/f,n'=n/f,ε'_B=fε_B,ε'_e=fε_e} are independent of the value of f for m_e/m_p< f< 1. Current observations imply that the efficiency f is similar for highly relativistic and for sub relativistic shocks, and plausibly suggest that f~1, quite unlike the situation in the Crab Nebula. However, values m_e/m_p< f<< 1 can not be ruled out, implying a factor m_e/m_p uncertainty in determination of model parameters. We show that early, <10 hr, radio afterglow observations, which will be far more accessible in the SWIFT era, may provide constraints on f. Such observations will therefore provide a powerful diagnostic of GRB explosions and of the physics of particle acceleration in collisionless shocks.

astro-ph↗
arXiv2005

Delayed Afterglow Onset Interpreted as Baryon-Poor Viewing Angle

We have suggested previously that baryons in GRB fireballs infiltrate from the surrounding walls that collimate the fireball. The efficiency $ε_b$ for generating blast energy can then be angle dependent. Delayed onset of afterglow can be interpreted as being due to a baryon-poor viewing angle.

astro-ph↗
arXiv2005

Ongoing Space Physics - Astrophysics Connections

I review several ongoing connections between space physics and astrophysics: a) Measurements of energetic particle spectra have confirmed theoretical prediction of the highest energy to which shocks can accelerate particles, and this has direct bearing on the origin of the highest energy cosmic rays. b) Mass ejection in solar flares may help us understand photon ejection in the giant flares of magnetar outbursts. c) Measurements of electron heat fluxes in the solar wind can help us understand whether heat flux in tenuous astrophysical plasma is in accordance with the classical Spitzer-Harm formula or whether it is reduced well below this value by plasma instabilities.

astro-ph↗
arXiv2005

Galactic Positrons From Localized Sources

The anomalous bump in the cosmic ray positron to electron ratio at $10 GeV$ can be explained as being a component from a point source that was originally harder than the primary electron background and degrades due to synchrotron and inverse Compton losses in the Galaxy while propagating to the Earth's vicinity. The fit is better than can be obtained with homogeneous injection and is attributed to a minimum age threshold. Annihilating neutralinos can provide a fair fit to the data if they have a mass just above 1/2 the mass of the $Z^o$ and if they annihilate primarily in distant density concentrations in the Galaxy. A possible observational consequence of this scenario would be intense inverse Comptonization of starlight at the Galactic center, with a sharp energy cutoff in the emergent photons as a possible signature of the neutralino mass.

astro-ph↗
arXiv2004

Compton Sailing and Strong Polarization

It is noted that a surface layer of matter in contact with a sufficiently super-Eddington, radially combed photon flux typically attains a relativistic coasting state whereby the radiation does not accelerate the matter. Radiation that scatters off this layer is most likely to be observed along the velocity vector of the matter, where it would be most strongly polarized.

astro-ph↗