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1.
The amount of useful scientific data that a space-borne telescope produces is often limited by the available telemetry of
the platform. General purpose image compression schemes are usually used to compress the image either lossy or losslessly.
These schemes do not take into account the fact that pixel values of typical solar images are only known to within a certain
uncertainty range. We present a preprocessing method to enhance the performance (compression ratio) of any subsequent image
compression scheme. The method uses estimates of the photon shot and thermal noises to compute a recoding look-up table that
maps the initial data into uncertainty intervals. The recoding method is lossy in a mathematical sense but lossless in a physical
sense, since the image alterations are guaranteed to be smaller than the Poisson noise. The performance of any further compression
algorithm is enhanced while achieving a known guaranteed maximum absolute error for each pixel in the case of lossless compression.
This revised version was published online in October 2005 with corrections to one of the authors' names. 相似文献
2.
Noah Brosch David Polishook Avi Shporer Shai Kaspi Assaf Berwald Ilan Manulis 《Astrophysics and Space Science》2008,314(1-3):163-176
We describe the second telescope of the Wise Observatory, a 0.46-m Centurion 18 (C18) installed in 2005, which enhances significantly
the observing possibilities. The telescope operates from a small dome and is equipped with a large-format CCD camera. In the
last two years this telescope was intensively used in a variety of monitoring projects.
The operation of the C18 is now automatic, requiring only start-up at the beginning of a night and close-down at dawn. The
observations are mostly performed remotely from the Tel Aviv campus or even from the observer’s home. The entire facility
was erected for a component cost of about 70k$ and a labor investment of a total of one man-year.
We describe three types of projects undertaken with this new facility: the measurement of asteroid light variability with
the purpose of determining physical parameters and binarity, the following-up of transiting extrasolar planets, and the study
of AGN variability. The successful implementation of the C18 demonstrates the viability of small telescopes in an age of huge
light-collectors, provided the operation of such facilities is very efficient. 相似文献
3.
W. J. Nellis 《Astrophysics and Space Science》2005,298(1-2):141-145
The shock compression curve (Hugoniot) of D2 has been controversial because the two data sets measured previously with a laser (L) and with pulsed currents (PC) differ
substantially. Recently, Hugoniot points of D2 have been measured at shock pressures of 123, 109, 61, 54, and 28 GPa using hemispherically converging, explosively-driven
systems (CS). The CS results are in good agreement with the PC data and the error bars of the CS-PC data are less than half
those of the L data. The limiting compression obtained from the best fit to the CS-PC data is 4.30 ± 0.10 at 100 GPa. The
CS-PC data are in good agreement with PIMC and DFT calculations, which is expected to be the case at higher shock temperatures
and pressures, as well. 相似文献
4.
Properties of a guided MHD-wave propagating in a magnetic field tube with the plasma density differing from the ambient density are studied. Like the Alvén wave this wave propagates along the magnetic field and is connected with the field-aligned currents flowing at the periphery of the oscillating tube. The guided wave is accompanied by the magnetic field compression, nevertheless the wave moves without attenuation. The guided wave velocity is between the Alvén velocities inside and outside the oscillating tube. In a tube of elliptical cross-section the propagation velocity depends on the polarization of the wave. 相似文献
5.
EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disk. EIT waves are
all associated with a flare and a CME and are commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves
could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed
this alternative interpretation. Using two independent 3D MHD codes, we performed nondimensional numerical simulations of
a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast
expansion, as during a CME. We analyse the origins, the development, and the observability in EUV of the narrow electric currents
sheets that appear in the simulations. Both codes give similar results, which we confront with two well-known SOHO/EIT observations
of propagating EIT waves (7 April and 12 May 1997), by scaling the vertical magnetic field components of the simulated bipole
to the line of sight magnetic field observed by SOHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids
observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This
current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields.
It intensifies as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated
over the altitude has the shape of an ellipse, which expands and rotates when viewed from above, reproducing the generic properties
of propagating EIT waves. The timing, orientation, and location of bright and faint patches observed in the two EIT waves
are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating
in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression
inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current
shells. 相似文献
6.
The role of the magnetic field in the confinenment or compression of interstellar gas clouds is reconsidered. The virial theorem for an isolated magnetized cloud in the presence of distant magnetic sources is reformulated in terms of moments of the internal and external currents, and an equilibrium condition is derived. This condition is applied to the interaction between isolated clouds for the simple- and artificial-case in which the field of each cloud is a dipole. With the simplest of statistical assumptions, the probability of any given cloud being compressed is calculated as 10%, the magnetic field acting as a medium which transmits the kinetic pressure between clouds. Even when compression occurs the magnetic pressure 1/2B
2 may decrease on leaving the cloud surface.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
7.
We analyze in detail the statistics of the frequency redistribution of photons during the transfer of gyroresonance radiation
under conditions typical of compact stars. The probabilities of photon escape from a scattering atmosphere of arbitrary optical
thickness in a single scattering have been found. The effects of gyroresonance photon diffusion in space and in frequency
have been simulated. We show that when photons escape from a semi-infinite atmosphere with weak absorption, the frequency
redistribution effects lead to a considerable increase in the probability of photon escape from large optical depths and,
consequently, modify significantly the dynamics of gyroresonance photon transfer for neutron stars and white dwarfs. 相似文献
8.
Distributions of the tangential discontinuity (TD) in the solar wind sector structure are investigated on the basis of the magnetic field data and the ion plasma parameters from the Explorer 33 satellite from 23 January to 23 March 1968. The TD is separated from the observed field fluctuations by calculating the direction of the plasma flow and also the direction of the minimum field fluctuation with respect to the ambient magnetic field direction.It is found that the TD is formed by the thin layered field-aligned currents (the current sheets), and that the TD is predominantly built up in the leading edge of the solar wind where the compression of the plasma and the magnetic field takes place.It is suggested that the current sheets might be locally generated in the leading edge in the turbulent conditions arising from collisions between the fast- and the slow-stream of the solar sector structure. 相似文献
9.
10.
We have studied an electron-photon cascade in an isotropic photon field as one of the possible gamma-ray generation mechanisms in compact sources.A cascade in a monoenergetic photon field and in a field of the power-law spectrum is considered. Two types of interaction of cascade particles with the photon field are taken into account: the inverse Compton scattering of electrons and the photoproduction of electron-positron pairs by gamma-quanta. The cascade characteristics have been obtained by numerical solution of the kinetic equations. A quantitative and qualitative analysis is made of the main regularities of the cascade. We have established simple relations between the spectra of cascade particles and the parameters of the photon field and primary radiation. The magnetic field effect on the cascade characteristics is estimated. The results obtained are compared with the results of calculation of other investigators.In the first part of the paper we outline the method of solution and the results relating to the monoenergetic photon field. The theory of cascades in a photon field of the power-law spectrum and its application to the problem of gamma-rays in compact sources are discussed in the second part of the present paper. 相似文献
11.
12.
Theoretical calculation has shown that the spectrum of the Compton backscattering component in solar hard X-ray flares has a peak around 30 keV for a primary power-law source. Thus the superposition of the Compton backscattering component could cause a photon spectrum received at the Earth to be flattened below the peak energy and steeper above the peak energy. On the other hand, because a thick-target bremsstrahlung photon with a given energy E only could be produced by a nonthermal electron with an energy larger than E, thus if a power-law electron spectrum is cutoff below an energy E c, then the produced photon spectrum will become flattened below E c. In this work we present a calculation of the joint effects of the Compton backscattering and the low-energy cutoff on the spectral characteristics of the received solar hard X-ray in the energy range 10–100 keV. The results show that the flattening caused purely by the Compton backscattering could be comparable with that by the low-energy cutoff for hard spectra. So, it is obvious that the joint effects of the low-energy cutoff and the Compton backscattering could result in the received photon spectra to be much more flattened at lower energies. On the other hand, compared to the primary photon spectrum, the received photon spectral index will increase about 0.15 due to the Compton backscattering at higher energy, which seems independent of the primary spectral index. 相似文献
13.
B. V. Somov 《Astronomy Letters》2012,38(2):128-138
The role of the electric currents distributed over the volume of an active region on the Sun is considered from the standpoint
of solar flare physics. We suggest including the electric currents in a topological model of the magnetic field in an active
region. Typical values of the mutual inductance and the interaction energy of the coronal electric currents flowing along
magnetic loops have been estimated for the M7/1N flare on April 27, 2006. We show that if these currents actually make a significant
contribution to the flare energetics, then they must manifest themselves in the photosphericmagnetic fields. Depending on
their orientation, the distributed currents can both help and hinder reconnection in the current layer at the separator during
the flare. Asymmetric reconnection of the currents is accompanied by their interruption and an inductive change in energy.
The reconnection of currents in flares differs significantly from the ordinary coalescence instability of magnetic islands
in current layers. Highly accurate measurements of the magnetic fields in active regions are needed for a quantitative analysis
of the role of distributed currents in solar flares. 相似文献
14.
N. Solomos 《Astronomische Nachrichten》2008,329(3):252-254
This contribution aims to introduce the idea that a well‐evolved HTN of the far future, with the anticipated addition of very large apertures, could also be made to incorporate the ability to carry out photonic astronomy observations, particularly Optical VLBI in a revived Hanbury‐Brown and Twiss Intensity Interferometry (HBTII) configuration. Such an HTN could exploit its inherent rapid reconfigurational ability to become a multi‐aperture distributed photon‐counting network able to study higher‐order spatiotemporal photon correlations and provide a unique tool for direct diagnostics of astrophysical emission processes. We very briefly review various considerations associated with the switching of the HTN to a special mode in which single‐photon detection events are continuously captured for a posteriori intercorrelation. In this context, photon arrival times should be determined to the highest time resolution possible and extremely demanding absolute time keeping and absolute time distribution schemes should be devised and implemented in the HTN nodes involved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
The recently introduced models of reionization bubbles based on extended Press–Schechter theory (by Furlanetto, Hernquist & Zaldarriaga) are generalized to include mergers of ionization sources. Sources with a recent major merger are taken to have enhanced photon production due to star formation, and accretion on to a central black hole if a black hole is present. This produces a scatter in the number of ionized photons corresponding to a halo of a given mass and a change in photon production over time for any given halo mass. By extending previous methods, photon production histories, bubble distributions and ionization histories are computed for several different parameter and recombination assumptions. The resulting distributions interpolate between previously calculated limiting cases. 相似文献
16.
Magnetic photon splitting γ → γγ, a quantum electrodynamic process that becomes important when magnetic field approaching
the quantum critical value, B
c = 4.413 × 1013
G, may have important effects on pulsar radio emission. According to the standard model, the pulsar radio emission is produced
by coherent curvature radiation of a large amounts of e
± pairs, which are thought to be generated by the pair creation process γ + B → e
±. However, if the magnetic field is strong enough, the photon splitting may dominate the pair creation process, then the amounts
of e
± pairs and the radio luminosity will be strongly suppressed and may be undetectable. Here we use the fitted analytical formula
of the photon splitting attenuation coefficient to study the above process, and find that the photon splitting will strongly
decrease the radio emission when B > 1013
G. We also note that the photon splitting can strongly but not totally suppress the creation of pairs due to the diminishing
dependence of B in the attenuation coefficient. We find that the ratio of the probability of a photon being absorbed by photon splitting
to that by pair creation is no more than about six.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
&#;. Stawarz F. Aharonian S. Wagner M. Ostrowski 《Monthly notices of the Royal Astronomical Society》2006,371(4):1705-1716
Several BL Lac objects are confirmed sources of variable and strongly Doppler-boosted TeV emission produced in the nuclear portions of their relativistic jets. It is more than probable that also many of the Fanaroff–Riley type I (FR I) radio galaxies, believed to be the parent population of BL Lacs, are TeV sources, for which Doppler-hidden nuclear γ-ray radiation may be only too weak to be directly observed. Here we show, however, that about 1 per cent of the total time-averaged TeV radiation produced by the active nuclei of low-power FR I radio sources is inevitably absorbed and re-processed by photon–photon annihilation on the starlight photon field, and the following emission of the created and quickly isotropized electron–positron pairs. In the case of the radio galaxy Centaurus A, we found that the discussed mechanism can give a distinctive observable feature in the form of an isotropic γ-ray halo. It results from the electron–positron pairs injected to the interstellar medium of the inner parts of the elliptical host by the absorption process, and upscattering starlight radiation via the inverse-Compton process mainly to the GeV–TeV photon energy range. Such a galactic γ-ray halo is expected to possess a characteristic spectrum peaking at ∼0.1 TeV photon energies, and the photon flux strong enough to be detected by modern Cherenkov Telescopes and, in the future, by GLAST. These findings should apply as well to the other nearby FR I sources. 相似文献
18.
R. A. Krikorian 《Astrophysics》1999,42(3):338-343
The field-to-particle method of H. P. Robertson as applied by Noonan, in order to obtain the general relativistic equations
describing the trajectory of a photon in a refractive medium, is compared with Synge’s general relativistic Hamiltonian theory
of waves and rays. For a photon in vacuum it is known that both approaches yield the same equation for the trajectory, i.e.,
a null geodesic. However for a photon in a medium, in contradistinction to the Hamiltonian theory, the field-to-particle method
(a) yields equations of the photon trajectory valid only in a nondispersive medium, (b) the time component u0
of the tangent to the ray remains an undetermined quantity, (c) agreement with the Hamiltonian theory is achieved by substituting
into Noonan’s equations the Hamiltonian expression for u
0.
Published in Astrofizika, Vol. 42, No. 3, pp. 449–455, July–September, 1999. 相似文献
19.
Having analyzed high-resolution observations of extragalactic compact radio sources with modern systems of radio telescopes, we obtained an estimate of the upper limit for the photon electric charge, eγ ? 3×10?33 of the elementary charge (assuming the photon charge to be energy independent). This is three orders of magnitude better than the limit obtained from radio pulsar timing. We also set a limit on the charge of a gamma-ray (energy ～0.1 MeV) photon. In the future, the estimate based on extragalactic sources can be improved significantly. 相似文献
20.
Photon counting strategies with low-light-level CCDs 总被引：1，自引：0，他引：1
A. G. Basden C. A. Haniff C. D. Mackay 《Monthly notices of the Royal Astronomical Society》2003,345(3):985-991
Low light level charge-coupled devices (L3CCDs) have recently been developed, incorporating on-chip gain. They may be operated to give an effective readout noise of much less than one electron by implementing an on-chip gain process allowing the detection of individual photons. However, the gain mechanism is stochastic and so introduces significant extra noise into the system. In this paper we examine how best to process the output signal from an L3CCD so as to minimize the contribution of stochastic noise, while still maintaining photometric accuracy.
We achieve this by optimizing a transfer function that translates the digitized output signal levels from the L3CCD into a value approximating the photon input as closely as possible by applying thresholding techniques. We identify several thresholding strategies and quantify their impact on the photon counting accuracy and the effective signal-to-noise ratio.
We find that it is possible to eliminate the noise introduced by the gain process at the lowest light levels. Reduced improvements are achieved as the light level increases up to about 20 photon pixel−1 and above this there is negligible improvement. Operating L3CCDs at very high speeds will keep the photon flux low, giving the best improvements in signal-to-noise ratio. 相似文献
We achieve this by optimizing a transfer function that translates the digitized output signal levels from the L3CCD into a value approximating the photon input as closely as possible by applying thresholding techniques. We identify several thresholding strategies and quantify their impact on the photon counting accuracy and the effective signal-to-noise ratio.
We find that it is possible to eliminate the noise introduced by the gain process at the lowest light levels. Reduced improvements are achieved as the light level increases up to about 20 photon pixel