69 research outputs found
Gamma-Rays from Intergalactic Shocks
Structure formation in the intergalactic medium (IGM) produces large-scale,
collisionless shock waves, where electrons can accelerate to highly
relativistic energies. Such electrons can Compton scatter cosmic microwave
background photons up to gamma-ray energies. We study the radiation emitted in
this process using a hydrodynamic cosmological simulation of a LCDM universe.
This radiation, extending beyond TeV energies, has roughly constant energy flux
per decade in photon energy, in agreement with the predictions of Loeb & Waxman
(2000). Assuming that a fraction xi_e=0.05 of the shock thermal energy is
transferred to relativistic electrons, as inferred from collisionless
non-relativistic shocks in the interstellar medium, we find that the radiation
energy flux, e^2(dJ/de)~ 50-160 eV cm^-2 s^-1 sr^-1, constitutes ~10% of the
extragalactic gamma-ray background. The associated point-sources are too faint
to account for the ~60 unidentified EGRET gamma-ray sources, but GLAST should
resolve several sources associated with large-scale IGM structures for
xi_e~0.03, and many more sources for larger xi_e. The intergalactic origin of
the radiation can be verified through a cross-correlation with, e.g., the
galaxy distribution that traces the same structure. Its shock-origin may be
tested by a cross-correlation with radio synchrotron radiation, emitted as the
same electrons gyrate in post-shock magnetic fields. We predict that GLAST and
Cherenkov telescopes such as MAGIC, VERITAS and HESS should resolve gamma-rays
from nearby (redshifts z < 0.01) rich galaxy clusters, perhaps in the form of a
\~5-10 Mpc diameter ring-like emission tracing the cluster accretion shock,
with luminous peaks at its intersections with galaxy filaments detectable even
at z~0.025.Comment: 55 pages, 13 figures, accepted ApJ, added discussion to clarify some
points, for high resolution:
http://www.weizmann.ac.il/~keshet/IGM_Shocks.htm
Age-Dependent Remarkable Regenerative Potential of the Dentate Gyrus Provided by Intrinsic Stem Cells
Multiple insults to the brain lead to neuronal cell death, thus raising the question to what extent can lost neurons be replenished by adult neurogenesis. Here we focused on the hippocampus and especially the dentate gyrus (DG), a vulnerable brain region and one of the two sites where adult neuronal stem cells (NSCs) reside. While adult hippocampal neurogenesis was extensively studied with regard to its contribution to cognitive enhancement, we focused on their underestimated capability to repair a massively injured, nonfunctional DG. To address this issue, we inflicted substantial DG-specific damage in mice of either sex either by diphtheria toxin-based ablation of >50% of mature DG granule cells (GCs) or by prolonged brain-specific VEGF overexpression culminating in extensive, highly selective loss of DG GCs (thereby also reinforcing the notion of selective DG vulnerability). The neurogenic system promoted effective regeneration by increasing NSCs proliferation/survival rates, restoring a nearly original DG mass, promoting proper rewiring of regenerated neurons to their afferent and efferent partners, and regaining of lost spatial memory. Notably, concomitantly with the natural age-related decline in the levels of neurogenesis, the regenerative capacity of the hippocampus also subsided with age. The study thus revealed an unappreciated regenerative potential of the young DG and suggests hippocampal NSCs as a critical reservoir enabling recovery from catastrophic DG damage.Peer reviewe
Self-Similar Collisionless Shocks
Observations of gamma-ray burst afterglows suggest that the correlation
length of magnetic field fluctuations downstream of relativistic non-magnetized
collisionless shocks grows with distance from the shock to scales much larger
than the plasma skin depth. We argue that this indicates that the plasma
properties are described by a self-similar solution, and derive constraints on
the scaling properties of the solution. For example, we find that the scaling
of the characteristic magnetic field amplitude with distance from the shock is
B \propto D^{s_B} with -1<s_B<=0, that the spectrum of accelerated particles is
dn/dE \propto E^{-2/(s_B+1)}, and that the scaling of the magnetic correlation
function is \propto x^{2s_B} (for x>>D). We show that the
plasma may be approximated as a combination of two self-similar components: a
kinetic component of energetic particles and an MHD-like component representing
"thermal" particles. We argue that the latter may be considered as infinitely
conducting, in which case s_B=0 and the scalings are completely determined
(e.g. dn/dE \propto E^{-2} and B \propto D^0). Similar claims apply to non-
relativistic shocks such as in supernova remnants, if the upstream magnetic
field can be neglected. Self-similarity has important implications for any
model of particle acceleration and/or field generation. For example, we show
that the diffusion function in the angle \mu of momentum p in diffusive shock
acceleration models must satisfy D_{\mu\mu}(p,D) = D^{-1}D'_{\mu\mu}(p/D), and
that a previously suggested model for the generation of large scale magnetic
fields through a hierarchical merger of current-filaments should be
generalized. A numerical experiment testing our analysis is outlined
(Abridged).Comment: 16 pages, 1 figure, accepted for publication in Ap
Long wavelength unstable modes in the far upstream of relativistic collisionless shocks
The growth rate of long wavelength kinetic instabilities arising due to the
interaction of a collimated beam of relativistic particles and a cold
unmagnetized plasma are calculated in the ultra relativistic limit. For
sufficiently culminated beams, all long wave-length modes are shown to be
Weibel-unstable, and a simple analytic expression for their growth rate is
derived. For large transverse velocity spreads, these modes become stable. An
analytic condition for stability is given. These analytic results, which
generalize earlier ones given in the literature, are shown to be in agreement
with numerical solutions of the dispersion equation and with the results of
novel PIC simulations in which the electro-magnetic fields are restricted to a
given k-mode. The results may describe the interaction of energetic cosmic
rays, propagating into the far upstream of a relativistic collisionless shock,
with a cold unmagnetized upstream. The long wavelength modes considered may be
efficient in deflecting particles and could be important for diffusive shock
acceleration. It is shown that while these modes grow in relativistic shocks
propagating into electron-positron pair plasmas, they are damped in
relativistic shocks propagating into electron-proton plasmas with moderate
Lorenz factors \Gamma_{sh}\lesssim 100. If these modes dominate the deflection
of energetic cosmic rays in electron-positron shocks, it is argued that
particle acceleration is suppressed at shock frame energies that are larger
than the downstream thermal energy by a factor greater than the shock Lorentz
factor.Comment: 8 pages, 4 figure
The spectrum of particles accelerated in relativistic, collisionless shocks
We analytically study diffusive particle acceleration in relativistic,
collisionless shocks. We find a simple relation between the spectral index s
and the anisotropy of the momentum distribution along the shock front. Based on
this relation, we obtain s = (3beta_u - 2beta_u*beta_d^2 + beta_d^3) / (beta_u
- beta_d) for isotropic diffusion, where beta_u (beta_d) is the upstream
(downstream) fluid velocity normalized to the speed of light. This result is in
agreement with previous numerical determinations of s for all (beta_u,beta_d),
and yields s=38/9 in the ultra-relativistic limit. The spectrum-anisotropy
connection is useful for testing numerical studies and for constraining
non-isotropic diffusion results. It implies that the spectrum is highly
sensitive to the form of the diffusion function for particles travelling along
the shock front.Comment: 4 pages, 1 figur
On The GeV & TeV Detections of the Starburst Galaxies M82 & NGC 253
The GeV and TeV emission from M82 and NGC 253 observed by Fermi, HESS, and
VERITAS constrains the physics of cosmic rays (CRs) in these dense starbursts.
We argue that the gamma rays are predominantly hadronic in origin, as expected
by previous studies. The measured fluxes imply that pionic losses are efficient
for CR protons in both galaxies: we show that a fraction F_cal ~ 0.2 - 0.4 of
the energy injected in high energy primary CR protons is lost to inelastic
proton-proton collisions (pion production) before escape, producing gamma rays,
neutrinos, and secondary electrons and positrons. We discuss the factor ~2
uncertainties in this estimate, including supernova rate and leptonic
contributions to the GeV-TeV emission. We argue that gamma-ray data on ULIRGs
like Arp 220 can test whether M82 and NGC 253 are truly calorimetric, and we
present upper limits on Arp 220 from the Fermi data. We show that the observed
ratio of the GeV to GHz fluxes of the starbursts suggests that non-synchrotron
cooling processes are important for cooling the CR electron/positron
population. We briefly reconsider previous predictions in light of the
gamma-ray detections, including the starburst contribution to the gamma-ray
background and CR energy densities. Finally, as a guide for future studies, we
list the brightest star-forming galaxies on the sky and present updated
predictions for their gamma-ray and neutrino fluxes.Comment: 15 pages, emulateapj format, accepted to ApJ, Table 1 fixe
The Case for a Low Extragalactic Gamma-ray Background
Measurements of the diffuse extragalactic gamma-ray background (EGRB) are
complicated by a strong Galactic foreground. Estimates of the EGRB flux and
spectrum, obtained by modeling the Galactic emission, have produced a variety
of (sometimes conflicting) results. The latest analysis of the EGRET data found
an isotropic flux I_x=1.45+-0.05 above 100 MeV, in units of 10^-5 s^-1 cm^-2
sr^-1. We analyze the EGRET data in search for robust constraints on the EGRB
flux, finding the gamma-ray sky strongly dominated by Galactic foreground even
at high latitudes, with no conclusive evidence for an additional isotropic
component. The gamma-ray intensity measured towards the Galactic poles is
similar to or lower than previous estimates of I_x. The high latitude profile
of the gamma-ray data is disk-like for 40<|b[deg]|<70, and even steeper for
|b|>70; overall it exhibits strong Galactic features and is well fit by a
simple Galactic model. Based on the |b|>40 data we find that I_x<0.5 at a 99%
confidence level, with evidence for a much lower flux. We show that
correlations with Galactic tracers, previously used to identify the Galactic
foreground and estimate I_x, are not satisfactory; the results depend on the
tracers used and on the part of the sky examined, because the Galactic emission
is not linear in the Galactic tracers and exhibits spectral variations across
the sky. The low EGRB flux favored by our analysis places stringent limits on
extragalactic scenarios involving gamma-ray emission, such as radiation from
blazars, intergalactic shocks and production of ultra-high energy cosmic rays
and neutrinos. We suggest methods by which future gamma-ray missions such as
GLAST and AGILE could indirectly identify the EGRB.Comment: Accepted for publication in JCAP. Increased sizes of polar regions
examined, and added discussion of spectral data. Results unchange
A Transgenic Model for Conditional Induction and Rescue of Portal Hypertension Reveals a Role of VEGF-Mediated Regulation of Sinusoidal Fenestrations
Portal hypertension (PH) is a common complication and a leading cause of death in patients with chronic liver diseases. PH is underlined by structural and functional derangement of liver sinusoid vessels and its fenestrated endothelium. Because in most clinical settings PH is accompanied by parenchymal injury, it has been difficult to determine the precise role of microvascular perturbations in causing PH. Reasoning that Vascular Endothelial Growth Factor (VEGF) is required to maintain functional integrity of the hepatic microcirculation, we developed a transgenic mouse system for a liver-specific-, reversible VEGF inhibition. The system is based on conditional induction and de-induction of a VEGF decoy receptor that sequesters VEGF and preclude signaling. VEGF blockade results in sinusoidal endothelial cells (SECs) fenestrations closure and in accumulation and transformation of the normally quiescent hepatic stellate cells, i.e. provoking the two processes underlying sinusoidal capillarization. Importantly, sinusoidal capillarization was sufficient to cause PH and its typical sequela, ascites, splenomegaly and venous collateralization without inflicting parenchymal damage or fibrosis. Remarkably, these dramatic phenotypes were fully reversed within few days from lifting-off VEGF blockade and resultant re-opening of SECs' fenestrations. This study not only uncovered an indispensible role for VEGF in maintaining structure and function of mature SECs, but also highlights the vasculo-centric nature of PH pathogenesis. Unprecedented ability to rescue PH and its secondary manifestations via manipulating a single vascular factor may also be harnessed for examining the potential utility of de-capillarization treatment modalities
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