21 research outputs found
High throughput microbalance and methods of using same
The method and apparatus is particularly adapted for providing microbalance measurement of solid materials as part of a combinatorial research program. The method and apparatus contemplate monitoring the response of a resonator holding a sample and correlating the response with mass change in the samples
High throughput microbalance and methods of using the same
A method and apparatus for measurement of mass of small sample sizes. The method and apparatus is particularly adapted for providing microbalance measurement of solid materials as part of a combinatorial research program. The method and apparatus contemplate monitoring the response of a resonator holding a sample and correlating the response with mass change in the samples
Magnetic fields and cosmic rays in GRBs. A self-similar collisionless foreshock
Cosmic rays accelerated by a shock form a streaming distribution of outgoing
particles in the foreshock region. If the ambient fields are negligible
compared to the shock and cosmic ray energetics, a stronger magnetic field can
be generated in the shock upstream via the streaming (Weibel-type) instability.
Here we develop a self-similar model of the foreshock region and calculate its
structure, e.g., the magnetic field strength, its coherence scale, etc., as a
function of the distance from the shock. Our model indicates that the entire
foreshock region of thickness , being comparable
to the shock radius in the late afterglow phase when ,
can be populated with large-scale and rather strong magnetic fields (of
sub-gauss strengths with the coherence length of order )
compared to the typical interstellar medium magnetic fields. The presence of
such fields in the foreshock region is important for high efficiency of Fermi
acceleration at the shock. Radiation from accelerated electrons in the
foreshock fields can constitute a separate emission region radiating in the
UV/optical through radio band, depending on time and shock parameters. We also
speculate that these fields being eventually transported into the shock
downstream can greatly increase radiative efficiency of a gamma-ray burst
afterglow shock.Comment: 10 pages, 1 figure. Submitted to Ap
Angular Dependence of Jitter Radiation Spectra from Small-Scale Magnetic Turbulence
Jitter radiation is produced by relativistic electrons moving in turbulent
small-scale magnetic fields such as those produced by streaming Weibel-type
instabilities at collisionless shocks in weakly magnetized media. Here we
present a comprehensive study of the dependence of the jitter radiation spectra
on the properties of, in general, anisotropic magnetic turbulence. We have
obtained that the radiation spectra do reflect, to some extent, properties of
the magnetic field spatial distribution, yet the radiation field is anisotropic
and sensitive to the viewing direction with respect to the field anisotropy
direction. We explore the parameter space of the magnetic field distribution
and its effect on the radiation spectrum. Some important results include: the
presence of the harder-than-synchrotron segment below the peak frequency at
some viewing angles, the presence of the high-frequency power-law tail even for
a monoenergetic distribution of electrons, the dependence of the peak frequency
on the field correlation length rather than the field strength, the strong
correlation of the spectral parameters with the viewing angle. In general, we
have found that even relatively minor changes in the magnetic field properties
can produce very significant effects upon the jitter radiation spectra. We
consider these results to be important for accurate interpretation of prompt
gamma-ray burst spectra and possibly other sources.Comment: 75 pages, 29 figures, submitted to Ap
Radiative cooling in relativistic collisionless shocks. Can simulations and experiments probe relevant GRB physics?
We address the question of whether numerical particle-in-cell (PIC)
simulations and laboratory laser-plasma experiments can (or will be able to, in
the near future) model realistic gamma-ray burst (GRB) shocks. For this, we
compare the radiative cooling time, t_cool, of relativistic electrons in the
shock magnetic fields to the microscopic dynamical time of collisionless
relativistic shocks -- the inverse plasma frequency of protons, omega_pp^{-1}.
We obtain that for t_cool*omega_pp^{-1}\lesssim ~few hundred, the electrons
cool efficiently at or near the shock jump and are capable of emitiing away a
large fraction of the shock energy. Such shocks are well-resolved in existing
PIC simulations; therefore, the microscopic structure can be studied in detail.
Since most of the emission in such shocks would be coming from the vicinity of
the shock, the spectral power of the emitted radiation can be directly obtained
from finite-length simulations and compared with observational data. Such
radiative shocks correspond to the internal baryon-dominated GRB shocks for the
conventional range of ejecta parameters. Fermi acceleration of electrons in
such shocks is limited by electron cooling, hence the emitted spectrum should
be lacking a non-thermal tail, whereas its peak likely falls in the multi-MeV
range. Incidentally, the conditions in internal shocks are almost identical to
those in laser-produced plasmas; thus, such GRB-like plasmas can be created and
studied in laboratory experiments using the presently available Petawatt-scale
laser facilities. An analysis of the external shocks shows that only the highly
relativistic shocks, corresponding to the extremely early afterglow phase, can
have efficient electron cooling in the shock transition. We emphasize the
importance of radiative PIC simulations for further studies.Comment: 15 pages, submitted to Ap
Jitter radiation images, spectra, and light curves from a relativistic spherical blastwave
We consider radiation emitted by the jitter mechanism in a Blandford-McKee
self-similar blastwave. We assume the magnetic field configuration throughout
the whole blastwave meets the condition for the emission of jitter radiation
and we compute the ensuing images, light curves and spectra. The calculations
are performed for both a uniform and a wind environment. We compare our jitter
results to synchrotron results. We show that jitter radiation produces slightly
different spectra than synchrotron, in particular between the self-absorption
and the peak frequency, where the jitter spectrum is flat, while the
synchrotron spectrum grows as \nu^{1/3}. The spectral difference is reflected
in the early decay slope of the light curves. We conclude that jitter and
synchrotron afterglows can be distinguished from each other with good quality
observations. However, it is unlikely that the difference can explain the
peculiar behavior of several recent observations, such as flat X-ray slopes and
uncorrelated optical and X-ray behavior.Comment: 11 pages, 7 postscript figures. Accepted for publication in MNRA
Radiation of electrons in Weibel-generated fields: a general case
Weibel instability turns out to be the a ubiquitous phenomenon in High-Energy
Density environments, ranging from astrophysical sources, e.g., gamma-ray
bursts, to laboratory experiments involving laser-produced plasmas.
Relativistic particles (electrons) radiate in the Weibel-produced magnetic
fields in the Jitter regime. Conventionally, in this regime, the particle
deflections are considered to be smaller than the relativistic beaming angle of
1/ ( being the Lorentz factor of an emitting particle) and the
particle distribution is assumed to be isotropic. This is a relatively
idealized situation as far as lab experiments are concerned. We relax the
assumption of the isotropy of radiating particle distribution and present the
extension of the jitter theory amenable for comparisons with experimental data.Comment: Proceedings of International Conference on HEDP/HEDLA-0
Modeling Spectral Variability of Prompt GRB Emission within the Jitter Radiation Paradigm
The origin of rapid spectral variability and certain spectral correlations of
the prompt gamma-ray burst emission remains an intriguing question. The
recently proposed theoretical model of the prompt emission is build upon unique
spectral properties of jitter radiation -- the radiation from small-scale
magnetic fields generated at a site of strong energy release (e.g., a
relativistic collisionless shock in baryonic or pair-dominated ejecta, or a
reconnection site in a magnetically-dominated outflow). Here we present the
results of implementation of the model. We show that anisotropy of the jitter
radiation pattern and relativistic shell kinematics altogether produce effects
commonly observed in time-resolved spectra of the prompt emission, e.g., the
softening of the spectrum below the peak energy within individual pulses in the
prompt light-curve, the so-called "tracking" behavior (correlation of the
observed flux with other spectral parameters), the emergence of hard,
synchrotron-violating spectra at the beginning of individual spikes. Several
observational predictions of the model are discussed.Comment: ApJL, in pres
Radiation from sub-Larmor scale magnetic fields
Spontaneous rapid growth of strong magnetic fields is rather ubiquitous in
high-energy density environments ranging from astrophysical sources (e.g.,
gamma-ray bursts and relativistic shocks), to reconnection, to laser-plasma
interaction laboratory experiments, where they are produced by kinetic
streaming instabilities of the Weibel type. Relativistic electrons propagating
through these sub-Larmor-scale magnetic fields radiate in the jitter regime, in
which the anisotropy of the magnetic fields and the particle distribution have
a strong effect on the produced radiation. Here we develop the general theory
of jitter radiation, which includes (i) anisotropic magnetic fields and
electron velocity distributions, (ii) the effects of trapped electrons and
(iii) extends the description to large deflection angles of radiating particles
thus establishing a cross-over between the classical jitter and synchrotron
regimes. Our results are in remarkable agreement with the radiation spectra
obtained from particle-in-cell simulations of the classical Weibel instability.
Particularly interesting is the onset of the field growth, when the transient
hard synchrotron-violating spectra are common as a result of the dominant role
of the trapped population. This effect can serve as a distinct observational
signature of the violent field growth in astrophysical sources and lab
experiments. It is also interesting that a system with small-scale fields tends
to evolve toward the small-angle jitter regime, which can, under certain
conditions, dominate the overall emission of a source.Comment: 13 pages, 12 figures, matches published versio
Jitter radiation in gamma-ray bursts and their afterglows: emission and self-absorption
Relativistic electrons moving into a highly tangled magnetic field emit
jitter radiation. We present a detailed computation of the jitter radiation
spectrum, including self-absorption, for electrons inside Weibel-like shock
generated magnetic fields. We apply our results to the case of the prompt and
afterglow emission of gamma-ray bursts. We show that jitter emission can
reproduce most of the observed features with some important differences with
respect to standard synchrotron, especially in the frequency range between the
self-absorption and the peak frequency. We discuss the similarities and
differences between jitter and synchrotron and discuss experiments that can
disentangle the two mechanisms.Comment: 12 pages, 7 postscript figures. Figures, discussion, and references
updated. Accepted for publication in MNRA