19 research outputs found
Pulsed radiation from neutron star winds
The radiation of a pulsar wind is computed assuming that at roughly 10 to 100
light cylinder radii from the star, magnetic energy is dissipated into particle
energy. The synchrotron emission of heated particles appears periodic, with, in
general, both a pulse and an interpulse. The predicted spacing agrees well with
the Crab and Vela pulse profiles.Using parameters appropriate for the Crab
pulsar (magnetisation parameter at the light cylinder ,
Lorentz factor ) agreement is found with the observed total pulsed
luminosity. This suggests that the high-energy pulses from young pulsars
originate not in the corotating magnetosphere within the light cylinder (as in
all other models) but from the radially directed wind well outside it.Comment: 4 pages, 2 figures, accepted for publication in A&A Letter
Discovery of the Binary Pulsar PSR B1259-63 in Very-High-Energy Gamma Rays around Periastron with H.E.S.S
We report the discovery of very-high-energy (VHE) gamma-ray emission of the
binary system PSR B1259-63/SS 2883 of a radio pulsar orbiting a massive,
luminous Be star in a highly eccentric orbit. The observations around the 2004
periastron passage of the pulsar were performed with the four 13 m Cherenkov
telescopes of the H.E.S.S. experiment, recently installed in Namibia and in
full operation since December 2003. Between February and June 2004, a gamma-ray
signal from the binary system was detected with a total significance above 13
sigma. The flux was found to vary significantly on timescales of days which
makes PSR B1259-63 the first variable galactic source of VHE gamma-rays
observed so far. Strong emission signals were observed in pre- and
post-periastron phases with a flux minimum around periastron, followed by a
gradual flux decrease in the months after. The measured time-averaged energy
spectrum above a mean threshold energy of 380 GeV can be fitted by a simple
power law F_0(E/1 TeV)^-Gamma with a photon index Gamma =
2.7+-0.2_stat+-0.2_sys and flux normalisation F_0 = (1.3+-0.1_stat+-0.3_sys)
10^-12 TeV^-1 cm^-2 s^-1. This detection of VHE gamma-rays provides unambiguous
evidence for particle acceleration to multi-TeV energies in the binary system.
In combination with coeval observations of the X-ray synchrotron emission by
the RXTE and INTEGRAL instruments, and assuming the VHE gamma-ray emission to
be produced by the inverse Compton mechanism, the magnetic field strength can
be directly estimated to be of the order of 1 G.Comment: 10 pages, 8 figures, accepted in Astronomy and Astrophysics on 2 June
2005, replace: document unchanged, replaced author field in astro-ph entry -
authors are all members of the H.E.S.S. collaboration and three additional
authors (99+3, see document
Electric current circuits in astrophysics
Cosmic magnetic structures have in common that they are anchored
in a dynamo, that an external driver converts kinetic energy into internal
magnetic energy, that this magnetic energy is transported as Poynting fl ux across the magnetically dominated structure, and that the magnetic energy
is released in the form of particle acceleration, heating, bulk motion,
MHD waves, and radiation. The investigation of the electric current system is
particularly illuminating as to the course of events and the physics involved.
We demonstrate this for the radio pulsar wind, the solar flare, and terrestrial
magnetic storms
Proxy Measures of Fitness Suggest Coastal Fish Farms Can Act as Population Sources and Not Ecological Traps for Wild Gadoid Fish
Background: Ecological traps form when artificial structures are added to natural habitats and induce mismatches between
habitat preferences and fitness consequences. Their existence in terrestrial systems has been documented, yet little evidence suggests they occur in marine environments. Coastal fish farms are widespread artificial structures in coastal ecosystems and are highly attractive to wild fish. Methodology/Principal Findings: To investigate if coastal salmon farms act as ecological traps for wild Atlantic cod (Gadus morhua) and saithe (Pollachius virens), we compared proxy measures of fitness between farm-associated fish and control fish caught distant from farms in nine locations throughout coastal Norway, the largest coastal fish farming industry in the world. Farms modified wild fish diets in both quality and quantity, thereby providing farm-associated wild fish with a strong trophic subsidy. This translated to greater somatic (saithe: 1.06–1.12 times; cod: 1.06–1.11 times) and liver condition indices (saithe: 1.4–1.8 times; cod: 2.0–2.8 times) than control fish caught distant from farms. Parasite loads of farm-associated wild fish were modified from control fish, with increased external and decreased internal parasites, however the strong effect of the trophic subsidy overrode any effects of altered loads upon condition. Conclusions and Significance: Proxy measures of fitness provided no evidence that salmon farms function as ecological traps for wild fish. We suggest fish farms may act as population sources for wild fish, provided they are protected from fishing while resident at farms to allow their increased condition to manifest as greater reproductive output.Funding was provided by the Norwegian Research Council Havet og kysten program to the CoastACE project (no: 173384)
Dissipation in Poynting-flux-dominated flows: The sigma-problem of the Crab Pulsar wind
Flows in which energy is transported predominantly as Poynting flux are thought to occur in pulsars, gamma-ray bursts, and relativistic jets from compact objects. The fluctuating component of the magnetic field in such a flow can in principle be dissipated by magnetic reconnection and used to accelerate the flow. We investigate how rapidly this transition can take place by implementing into a global MHD model that uses a thermodynamic description of the plasma, explicit, physically motivated prescriptions for the dissipation rate: a lower limit on this rate is given by limiting the maximum drift speed of the current carriers to that of light, an upper limit follows from demanding that the dissipation zone expand only subsonically in the comoving frame, and a further prescription is obtained by assuming that the expansion speed is limited by the growth rate of the relativistic tearing mode. In each case, solutions are presented that give the Lorentz factor of a spherical wind containing a transverse, oscillating magnetic field component as a function of radius. In the case of the Crab Pulsar, we find that the Poynting flux can be dissipated before the wind reaches the inner edge of the Nebula if the pulsar emits electron-positron pairs at a rate (N)over dot(+/-) >10(40) s(-1), thus providing a possible solution to the "sigma-problem.
The Sigma Problem of the Crab Pulsar Wind
We investigate how rapidly Poynting flux can be converted into kinetic energy in the wind of the Crab pulsar by implementing into a global MHD model explicit prescriptions for the dissipation rate. A lower limit is given by limiting the maximum drift speed of the current carriers to that of light an upper limit follows from demanding that the dissipation zone expand only subsonically in the comoving frame. A further prescription is obtained by assuming that the expansion speed is limited by the growth rate of the relativistic tearing mode. In each case solutions are presented which give the Lorentz factor of the wind containing a transverse oscillating magnetic field component as a function of radius. We find that the Poynting flux can be dissipated before the wind reaches the inner edge of the Nebula if the pulsar emits electron positron pairs at a rate greater than 1.E40 per secon