We present observations of the Crab Nebula above 20 keV by the SPI/INTEGRAL telescope during more than 5 years of operations. Our study demonstrates the stability of the instrument with time and allows a detailed analysis of the emission observed from the Crab Nebula between 20 keV and 1 MeV. The flux stability is discussed and serves a robust spectral shape analysis. We find that a single power law is clearly excluded since the photon spectrum presents a curvature in the considered energy domain. We have modelled it by a broken power law with the energy break fixed to 100 keV and determined the two photon indices together with the 100 keV flux for 9 periods between 2004 and 2008. The spectral shape of the Crab nebula is very stable as well as its intensity and connects nicely with previous measurements, at lower (X-rays) or higher (MeV) energies.
The radiation mechanisms contributing to formation of the nonthermal spectrum of the Crab Nebula, as well as the information that could be derived from future observations in different energy bands, are discussed.
We investigate the consequences of the acceleration of heavy nuclei (e.g. iron nuclei) by the Crab pulsar. Accelerated nuclei can photodisintegrate in collisions with soft photons produced in the pulsar's outer gap, injecting energetic neutrons which decay either inside or outside the Crab Nebula. The protons from neutron decay inside the nebula are trapped by the Crab Nebula magnetic field, and accumulate inside the nebula producing gamma-rays and neutrinos in collisions with the matter in the nebula. Neutrons decaying outside the Crab Nebula contribute to the Galactic cosmic rays. We compute…
A model of an accelerated expansion of the Crab Nebula powered by the spinning-down Crab pulsar is proposed, in which time dependence of the acceleration is connected with evolution of pulsar luminosity. Using recent observational data, we derive estimates of the Crab neutron-star moment of inertia. Correlations between the neutron star moment of inertia and its mass and radius allow for rough estimates of the Crab neutron-star radius and mass. In contrast to the previously used constant-acceleration approximation, even for the expanding nebula mass ~7 M_sun results obtained within our model d…
The formation of the toroidal and jet-like structures in the central part of the Crab Nebula is explained in the framework of Kennel & Coroniti theory. The only new element introduced by us in this theory is the initial anisotropy of the energy flux in the wind. We estimate the X-ray surface brightness of the Crab Nebula from the region of interaction of this wind with the interstellar medium and compare it with observations.
We present results of new photoionization calculations for investigating gaseous regions that represent potentially expected stages of nuclear processing in the Crab Nebula supernova remnant. In addition to gas resulting from CNO-processing and oxygen-burning, as previously reported, a large component of the nebula appears to be carbon-rich. These results suggest that the precursor star had an initial mass of 9.5 solar masses or more.
The HEGRA system of imaging atmospheric Cherenkov telescopes provides for specially selected classes of events an angular resolution of better than 3'. By comparing the measured angular distribution of TeV gamma rays from the Crab Nebula with the distribution expected on the basis of Monte Carlo simulations, and with measurements of gamma rays from the point source Mrk 501, we conclude that the rms size of the VHE gamma-ray emission region in the Crab Nebula is less than 1.5'.
Long-term measurements have been carried out of the Crab Nebula radio emission flux density relative to Orion A at 927 MHz and relative to Cygnus A and Virgo A at 151.5 MHz. As a result the mean rates have been found of the secular change of the Crab Nebula radio emission flux density: $d_m (927\hbox{MHz})=-(0.18\pm0.10)$% year$^{-1}$ (over the period 1977--2000), $d_m (151.5\hbox{MHz})=- (0.32\pm0.08)$% year$^{-1}$ (over the period 1980--2003).
The recent Chandra image of the Crab nebula shows a striking, axisymmetric polar jet. It is shown that jets are formed in axisymmetric, magnetized pulsar winds and that the jet luminosity scales relative to the total as (γ_0σ_{eq})^{-4/3}, where σ_{eq} is the ratio of Poynting flux to particle kinetic energy output at the equator at the base of the flow and γ_0 the initial Lorentz factor of the flow. The results are applied to the image of the Crab nebula, and the limit is set for the Crab pulsar of σ_{eq} \leq 100. It is argued that conventional pulsar theory needs to be reexamined in light o…
Recent discoveries of the intriguing ``jet-torus'' structure in the Crab Nebula and other pulsar nebulae prompted calls for re-examining of their theory. The most radical proposals involve abolishing of the MHD approximation altogether and developing of purely electromagnetic models. However, the classical MHD models of the Crab Nebula were hampered by the assumption of spherical symmetry made in order to render the flow equations easily integrable. The impressive progress in computational relativistic magnetohydrodynamics in recent years has made it possible to study the Crab nebula via numer…
Observations of the Crab Nebula have proven to be the best tool to calibrate and to characterize the performance of a Cherenkov telescope. Scientifically, it is interesting to measure the energy spectrum of the Crab Nebula close to the inverse-Compton peak where a deviation is expected from the power law seen at energies above 300 GeV. Additionally, it is important to search for pulsed emission from the Crab Pulsar at energies beyond the 10 GeV upper limit of the EGRET pulsar detection. Since current models predict a cut-off in pulsed emission between 10 and 100 GeV, measurements at energies c…
Routine observations of the Crab Nebula for a total of about 250 hrs, performed with the HEGRA stereoscopic system of 5 imaging atmospheric Cerenkov telescopes in the standard operational mode, have proven the energy threshold of the system to be 500 GeV for small zenith angles (< 20 deg). A topological trigger applied along with the convergent observational mode allows to reduce noticeably the energy threshold of the system down to 350 GeV. Here we present the relevant Monte Carlo simulations as well as the analysis results of 15 hrs Crab Nebula data taken in such an observational mode. From …
The HI environment of the Crab Nebula is investigated using 2.75' and 9' resolution data from (respectively) the DRAO Synthesis, and Effelsberg 100 m, radio telescopes. No clear evidence for an interaction between the Crab and the surrounding HI is found; the Crab probably lies within the boundaries of a large scale, low-density void in the HI distribution. The presence of a wind-blown HI bubble near the Crab is confirmed, and it is suggested that the unidentified star which powers this bubble is responsible for the stellar wind material detected along the line of sight towards the Crab.
The Crab Nebula has been observed by the HEGRA stereoscopic system of 4 imaging air Cherenkov telescopes (IACTs) for about 100 hours from 1997 September to 1998 March. The recent detailed studies on the system performance give an energy threshold and an energy resolution for gamma-rays of 500 GeV and \~20 %, respectively. The Crab spectrum was measured with the IACTs system in a very broad energy range from 500 GeV to 25 TeV, using the observations at zenith angles up to 50 degree. The pure power-law spectrum in TeV gamma-rays constrains the physics parameters of the nebula environment as well…
Observations of the Crab Nebula at 850 micron made with the "Submillimetre Common-User Bolometer Array" (SCUBA) on the James Clerk Maxwell Telescope are presented. A variety of chop-throws and scanning directions was used in these observations, to well sample structure over a range of scales and directions. The resulting image, which was restored using a MEM algorithm, has a resolution of 17 arcsec, and has been compared with a 20-cm VLA image of the Crab Nebula to study spectral index variations. The 850-micron and 20-cm images are very similar, implying that there is little variation in spec…
High-quality spatially resolved spectra of synchrotron emission from plerions represent a powerful diagnostic tool for the physical conditions in these objects. A fundamental question is to what extent spatial variations of the synchrotron spectrum originate from the evolution of a single population of injected electrons, and when instead they imply the coexistence of different populations of electrons. Here I shall discuss observations of the Crab Nebula at millimetric wavelengths which, compared with a radio map, show evidence for the emergence of a second synchrotron component, characterize…
The kinematic and morphological properties of a string of [Ar III] bright knots in the Crab Nebula are examined using 1994 - 1999 HST WFPC-2 images of the remnant. We find that five southern [Ar III] bright knots exhibit ordinary radial motions away from the nebula's center of expansion with magnitudes consistent with their projected radial displacements. These results do not support the suggestion by MacAlpine et al.(1994) that these knots might be moving rapidly away from the Crab pulsar due to a collimated wind. The HST images also do not show that the [Ar III] knots have unusual morphologi…
We present the results of a new, comprehensive investigation of the radio spectral index of the Crab Nebula supernova remnant. New data at 74 MHz are combined with data at 327 MHz, 1.5 GHz and 5 GHz. In contrast to previous claims, little spatial variation in the spectral index is seen. In particular, between 327 MHz and 5 GHz we see no evidence of spectral steepening near the edge of the nebula, the ``jet'' or the ionized filaments. The rms limits on any spectral index variations in these regions amount to no more than 0.01. We believe that earlier reports of large steepening were the result …
We have analyzed five ROSAT HRI images of the Crab Nebula spanning the years 1991 to 1997 and have found significant changes in the emission structure of the X-ray torus surrounding the pulsar. Certain regions increase in brightness by about 20% over the six years, while others show decreases in surface brightness. The origin of these changes is unclear, but a possible explanation is that the bulk velocity of the synchrotron radiating electrons has decreased on the order of 20% as well.
Whithin the framework of the Planck satellite polarisation calibration, we present a study of the Crab Nebula spectral energy distribution (SED) over more than 6 decades in frequency ranging from 1 to $\rm 10^6 GHz$. The Planck satellite mission observes the sky from 30 to 857 GHz and therefore we focus on the millimetre region. We use radio and submillimetre data from the WMAP satellite between 23 and 94 GHz (from 13 to 3.18 mm) and from the Archeops balloon experiment between 143 (2.1 mm) and 545 GHz (0.55 mm), and a compendium of other Crab Nebula observations. The Crab SED is compared to m…
The Crab nebula has proved to be the nearest to a standard candle in VHE $γ-$ ray astronomy. Results on the gamma ray emission from the nebula at various energies have come in the last decade mostly from imaging telescopes. The aim of the new Pachmarhi Atmospheric Cerenkov Telescope (PACT) array has been to use the temporal and spatial distribution of Cerenkov photons in distinguishing between proton and gamma ray showers. The array, with timing information from 175 mirrors, is ideally suited for precise estimation of the arrival direction. Preliminary results from the recent data taken on Cra…
The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray Astronomy) stereoscopic system of imaging air Cherenkov telescopes (IACTs) for a total of about 200 hrs during two observational campaigns: from September 1997 to March 1998 and from August 1998 to April 1999. The recent detailed studies of system performance give an energy threshold and an energy resolution for gamma-rays of 500 GeV and ~ 18%, respectively. The Crab energy spectrum was measured with the HEGRA IACT system in a very broad energy range up to 20 TeV, using observations at zenith angles up to 65 degrees. The Cra…
We construct an axisymmetric model for the Crab Nebula. The flow dyamics is based on the model by Kennel and Coroniti (1984), but we assume that the kinetic-energy-dominant wind is confined in the equatorial region. We reproduce spacially resolved spectra which agree well with the Chandra results. However, the reproduced image is not a ring but lip-shaped. In addition, brightness contrast between fore and back sides of the ring cannot be reproduced if we assumes that the sigma-parameter is as small as 10**-3. We suggest that the nebula field is highly disordered due to for instance magnetic re…
We extend Pacini & Salvati's model for the synchrotron emission from plerions to allow more realistic predictions of the surface brightness profiles of the Crab Nebula at radio, optical and X-ray frequencies. Abandoning the assumption of a uniform particle distribution, we assume that particles are injected into the synchrotron nebula only in the vicinity of the pulsar. Their distribution is then determined, to zeroth order, by MHD propagation in an azimuthal magnetic field of spatially constant but time-dependent intensity. A good agreement with observations across the electromagnetic spectru…
In this article we discuss the possibility of using the observations by GLAST of standard gamma sources, as the Crab Nebula, to calibrate Imaging Air Cherenkov detectors, MAGIC in particular, and optimise their energy resolution. We show that at around 100 GeV the absolute energy calibration uncertainty of Cherenkov telescopes can be reduced to <10% by means of such cross-calibration procedure.
We present a deep [O III] 4959,5007 image of the northern filamentary jet in the Crab Nebula taken with the 8.2m Subaru telescope. Using this image and an image taken with the KPNO 4m in 1988 (Fesen & Staker 1993), we have computed proper motions for 35 locations in the jet. The results suggest that when compared to the main body of the remnant, the jet experienced less outward acceleration from the central pulsar's rapidly expanding synchrotron nebula. The jet's apparent expansion rate yields an undecelerated explosion date for the Crab Nebula of 1055 plus or minus 24 C.E., a date much closer…
The Chandra X-ray Observatory observed the Crab Nebula and Pulsar during orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) read-out by the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of x-ray structure, at a resolution comparable to that of the best ground-based visible-light observations. The HETG--ACIS-S images reveal, for the first time, an x-ray inner ring within the x-ray torus, the suggestion of a hollow-tube structure for the torus, and x-ray knots along the inner ring and (perhaps) along the inward exten…
The origin of relativistic electrons in the Crab nebula which are producing the broad-band flat radio spectrum of this prototype plerion has proved difficult to understand. Here I show that these electrons can be naturally explained as a relic population of the pulsar wind electrons that have lost most of their energy in the expanding nebula because of intensive radiative and adiabatic cooling in the past. The observed radio spectrum suggests that the initial slowing-down time of the pulsar was about 30 yrs (or less), which implies that it has been born with a spin period P_0 ~ 3-5 ms, several…
The Crab Nebula has become established as the standard candle for TeV gamma-ray astronomy using the atmospheric Cherenkov technique. No evidence for variability has been seen. The spectrum of gamma rays from the Crab Nebula has been measured in the energy range 500 GeV to 8 TeV at the Whipple Observatory by the atmospheric Cherenkov imaging technique. Two methods of analysis involving independent Monte Carlo simulations and two databases of observations (1988-89 and 1995-96) were used and gave close agreement. Using the complete spectrum of the Crab Nebula, the spectrum of relativistic electro…
The Tibet experiment, operating at Yangbajing (4,300 m above sea level), is the lowest energy air shower array and the new high density array constructed in 1996 has sensitivity to $γ$-ray air showers at energies as low as 3 TeV. With this new array, the Crab Nebula was observed in multi-TeV $γ$-rays and a signal was detected at the 5.5 $σ$ level. We also obtained the energy spectrum of $γ$-rays in the energy region above 3 TeV which partially overlaps those observed with imaging atmospheric Cherenkov telescopes. This is the first observation of $γ$-ray signals from point sources with a conven…
We present year-scale morphological variations of the Crab Nebula revealed by Chandra X-ray observatory. Observations have been performed about every 1.7 years over the 3 years from its launch. The variations are clearly recognized at two sites: the torus and the southern jet. The torus, which had been steadily expanding until 1.7 years ago, now appears to have shrunk in the latest observation. Additionally, the circular structures seen to the northeast of the torus have decayed into several arcs. On the other hand, the southern jet shows the growth of its overall kinked-structure. We discuss …
We present evidence of wavelength-dependent variations within the infrared, optical, and ultraviolet pulse profile peaks of the Crab Nebula pulsar. The leading and trailing edge half-width half-maxima of the peaks display clear differences in their wavelength dependences. In addition, phase-resolved infrared-to-ultraviolet color spectra show significant variations from the leading to trailing edges of the peaks. The color variations between the leading and trailing edges remain significant over phase differences smaller than 0.0054, corresponding to timescales of $<180 μ$s. These results are n…
We report on a Chandra observation of the Crab Nebula that gives the first clear view of the faint boundary of the Crab's X-ray-emitting Pulsar Wind Nebula, or PWN. There is structure in all directions. Fingers, loops, bays, and the South Pulsar Jet all indicate that either filamentary material or the magnetic field are controlling the relativistic electrons. In general, spectra soften as distance from the pulsar increases but do not change rapidly along linear features. This is particularly true for the Pulsar Jet. The termination of the Jet is abrupt; the E side is close to an [O {\small III…
Spectroscopy of the Crab nebula along different slit directions reveals the 3 dimensional structure of the optical nebula. On the basis of the linear radial expansion result first discovered by Trimble (1968), we make a 3D model of the optical emission. Results from a limited number of slit directions suggest that optical lines originate from a complicated array of wisps that are located in a rather thin shell, pierced by a jet. The jet is certainly not prominent in optical emission lines, but the direction of the piercing is consistent with the direction of the X-ray and radio jet. The shell'…
We present three high resolution radio images of the Crab nebula, taken in 1998.6, 1998.8 and 2000.1 with the VLA. These are the best radio images of the Crab to date. We show that, near the pulsar, there are significant changes between our three observing epochs. These changes have an elliptical geometry very similar to that of the optical wisps. One radio wisp in particular can be unambiguously identified between two of our observing epochs, and moves outward with an apparent velocity of ~0.24. The similarity in both morphology and behavior of the present radio wisps to the optical wisps sug…
We present a series of monitoring observations of the Crab Nebula with the Chandra X-ray Observatory, focusing on the temporal evolution of the structure. This series of 8 observations, spanning a period of approximately six months, shows the dynamic nature of the inner X-ray structures. We detected outward moving ``wisps'' from the recently discovered inner ring seen in optical observations. We also find that the inner ring itself shows temporal variations in structure. The torus also appears to be expanding. Such temporal variations generally match the canonical scenario that an expanding sy…
The deeper and more extended survey of the central parts of the Galactic Plane by H.E.S.S. during 2005-2007 has revealed a number of new point-like, as well as, extended sources. Two point-like sources can be associated to two remarkable objects around "Crab-like" young and energetic pulsars in our Galaxy: G21.5-0.9 and Kes 75. The characteristics of each of the sources are presented and possible interpretations are briefly discussed.
We have searched the region surrounding the Crab Nebula for the existence of a shock wave with the imaging instruments of the Einstein Observatory. The search is complicated by the scattering of nebula and pulsar X-rays from the imperfectly polished surfaces of the telescope mirror, as well as from interstellar grains along the line of sight. Both of these effects lead to the appearance of X-ray emission, in the form of an X-ray halo, beyond the boundaries of the nebula filaments. We show that the size, shape, and intensity of the halo around the Crab Nebula, above the contribution of mirror s…
We present a {\it Chandra} ACIS observation of SNR G54.1+0.3. This supernova remnant is resolved into several distinct X-ray emitting components: a central bright point-like source (CXOU J193030.13+185214.1), a surrounding ring, bipolar elongations, plus low surface brightness diffuse emission. The spectra of these components are all well described by power-law models; the spectral index steepens with increasing distance from the point-like source. We find no evidence for any thermal plasma emission that would correspond to shocked interstellar medium or ejecta. The observed morphological and …
Saturn's largest satellite, Titan, transited the Crab Nebula on 5 January 2003. We observed this astronomical event with the {\it Chandra} X-ray Observatory. An ``occultation shadow'' has clearly been detected and is found to be larger than the diameter of Titan's solid surface. The difference gives a thickness for Titan's atmosphere of 880 $\pm$ 60 km. This is the first measurement of Titan's atmospheric extent at X-ray wavelengths. The value measured is consistent with or slightly larger than those estimated from earlier Voyager observations at other wavelengths. We discuss the possibility o…
We report observations of Crab giant pulses made with the Australia Telescope Compact Array and a baseband recorder system, made simultaneously at two frequencies, 1300 and 1470 MHz. These observations were sensitive to pulses with amplitudes \ga 3 kJy and widths \ga 0.5 $μ$s. Our analysis led to the detection of more than 700 such bright giant pulses over 3 hours, and using this large sample we investigate their amplitude, width, arrival time and energy distributions. The brightest pulse detected in our data has a peak amplitude of $\sim$ 45 kJy and a width of $\sim$ 0.5 $μ$s, and therefore a…
Neutron stars generate powerful winds of relativistic particles that form bright synchrotron nebulae around them. Polarimetry provides a unique insight into the geometry and magnetic configuration of the wind, but high-energy measurements have failed until recently. The Integral-IBIS telescope has been used in its Compton mode to search for linearly polarized emission for energies above 200 keV from the Crab nebula. The asymmetries in the instrument response are small and we obtain evidences for a strongly polarized signal at an angle parallel to the pulsar rotation axis. This result confirms …
We present deep images of a region around the Crab nebula made with the VLA, utilizing new imaging and deconvolution algorithms in a search for a faint radio shell. The existence of a high-velocity, hydrogen-rich envelope has been predicted to account for the low total mass and kinetic energy of the observed nebula. No radio emission was detected from an extended source outside the Crab nebula. Our limits on surface brightness are sufficiently low to rule out the existence of a shell around the Crab whose brightness is at least two orders of magnitude below SN\thinspace{1006}, the faintest his…
The recent advances in TeV gamma-ray astronomy are largely the result of the ability to differentiate between extensive air showers generated by gamma rays and hadronic cosmic rays. Air Cherenkov telescopes have developed and perfected the "imaging" technique over the past several decades. However until now no background rejection method has been successfully used in an air shower array to detect a source of TeV gamma rays. We report on a method to differentiate hadronic air showers from electromagnetic air showers in the Milagro gamma ray observatory, based on the ability to detect the energe…
I propose that the moving `wisps' near the center of the Crab Nebula result from nonlinear Kelvin-Helmholtz instabilities in the equatorial plane of the shocked pulsar wind. Recent observations suggest that the wisps trace out circular wavefronts in this plane, expanding radially at speeds approximately less than c/3. Instabilities could develop if there is sufficient velocity shear between a faster-moving equatorial zone and a slower moving shocked pulsar wind at higher latitudes. The development of shear could be related to the existence of a neutral sheet -- with weak magnetic field -- in t…
We present the results of 1.3 mm observations of the Crab Nebula, performed with the MPIfR bolometer arrays at the IRAM 30-m telescope. The maps obtained, of unprecedented quality at these wavelengths, allow a direct comparison with high-resolution radio maps. Although the spatial structure of the Crab Nebula does not change much from radio to millimetre wavelengths, we have detected significant spatial variations of the spectral index between 20 cm and 1.3 mm. The main effect is a spectral flattening in the inner region, which can be hardly explained just in terms of the evolution of a single…
Due to the strong and steady TeV gamma-ray emission from the Crab Nebula supernova remnant, its measured flux and energy spectrum can be used to verify the calibration and data reduction methods applied to IACT data acquired over many observing seasons. This gives us confidence in the results obtained on variable TeV sources observed over the same period and in relating the sensitivity of new instruments to historical datasets. Here we present the results of an analysis of 65.3 hours of good quality data taken on the Crab Nebula between October 2000 and March 2006 with the Whipple 10m telescop…
Two Chandra observations have been used to search for thermal X-ray emission from within and around the Crab Nebula. Dead-time was minimized by excluding the brightest part of the Nebula from the field of view. A dust-scattered halo comprising 5% of the strength of the Crab is clearly detected with surface brightness measured out to a radial distance of 18 arcminutes. Coverage is 100% at 4 arcminutes, 50% at 12 arcminutes, and 25% at 18 arcminutes. The observed halo is compared with predictions based on 3 different interstellar grain models and one can be adjusted to fit the observation. This …
In separate series of observations of the Crab pulsar, pulse broadening due to scattering was measured at 111 MHz, and variations of dispersion due to pulse delay were measured at higher radio frequencies. In a remarkable event lasting 200 days a large increase occurred in both parameters and with similar time signatures. The increases in scattering and dispersion measure observed over the 200 days MJD 53950 - 54150 are attributable to the effects of an ionised cloud or filament crossing the line of sight. The cloud would be $10^11 - 10^12 m across, with electron density 10^3 - 10^4 cm^-3. The…
We report about very high energy (VHE) gamma-ray observations of the Crab Nebula with the MAGIC telescope. The gamma-ray flux from the nebula was measured between 60 GeV and 9 TeV. The energy spectrum can be described with a curved power law dF/dE=f_0 (E/300GeV)^(a+b log10(E/300GeV)) with a flux normalization f_0 of (6.0+-0.2stat)*10^-10 cm^-2 s^-1 TeV^-1, a=-2.31+-0.06stat and b=-0.26+-0.07stat. The position of the IC-peak is determined at 77+-47 GeV. Within the observation time and the experimental resolution of the telescope, the gamma-ray emission is steady and pointlike. The emission's ce…
We report about very high energy (VHE) gamma-ray observations of the Crab Nebula with the MAGIC telescope. The gamma-ray flux from the nebula was measured between 60 GeV and 9 TeV. The energy spectrum can be described with a curved power law dF/dE=f0 (E/300 GeV)^(a+b*log10(E/300 GeV)) with a flux normalization f0 of(6.0+-0.2)*10^-10 1/(cm^2 s TeV), a=-2.31+-0.06 and b=-0.26+-0.07. The position of the IC-peak is determined at (77+-47) GeV. Within the observation time and the experimental resolution of the telescope, the gamma-ray emission is steady and pointlike. The emission's center of gravit…
We report on a study of the gamma-ray continuum emission from the Crab supernova nebula and on a search for nuclear de-excitation gamma-ray lines. Crab is the brightest continuum source in the 1-10 MeV gamma-ray sky, and its continuum radiation is most likely of synchrotron origin. It is a likely source of cosmic rays through shock acceleration and thus a potential candidate for gamma-ray line emission from nuclear interactions. Five years of COMPTEL observations enable a fine spectral binning to investigate the behaviour of the 0.75-30 MeV emission in detail and to search for nuclear de-excit…
We present here the X-ray observation of a Crab-like supernova remnant (SNR) 3C58 with ASCA. We find that the integrated energy spectrum over the nebula is consistent with previous results, showing a power-law spectrum with the photon index $γ= 2.2-2.4$ modified by interstellar absorption of about $(3-4)\times 10^{21}{\rm cm^{-2}}$. Inclusion of a blackbody component which is attributable to the central compact source significantly improves the spectral fit. Stringent upper limits for any line emitting thin hot plasma are established. We find for the first time that the nebular spectrum is har…
Pulsar wind driven synchrotron nebulae are offering a unique view on the connection of the pulsar wind and the surrounding medium. The Crab nebula is particularly well suited for detailed studies of the different emission regions. As inferred from the observed synchrotron emission extending beyond MeV energies, the Crab is a unique and extreme accelerator. In the framework of the synchrotron/inverse Compton emission model, the same electrons with energies exceeding $10^{15}$ eV that are responsible for the MeV synchrotron emission produce via inverse Compton scattering 10-50 TeV radiation whic…
An axisymmetric model for the Crab Nebula is constructed to examine the flow dynamics in the nebula. The model is based on that of Kennel and Coroniti (1984), although we assume that the kinetic-energy-dominant wind is confined in an equatorial region. The evolution of the distribution function of the electron-positron plasma flowing out in the nebula is calculated. Given viewing angles, we reproduce an image of the nebula and compare it with Chandra observation. The reproduced image is not a ring-like but rather 'lip-shaped'. It is found that the assumption of toroidal field does not reproduc…
We present four new, high-resolution VLA radio images of the Crab nebula, taken between 2001 Feb. and Apr. The radio images show systematic variability in the Crab's radio emission throughout the region near the pulsar. The principal geometry of the variable features is that of elliptical ripples very similar to the optical wisps. The radio wisps are seen to move systematically outward with projected speeds of up to 0.3c. Comparing the new radio images to our earlier ones from 1998 and 2000, we show there are also more slowly moving features somewhat farther away from the pulsar. In particular…
We present and discuss correlations for optical and near-infrared (5500-10030 angstroms) line intensity measurements at many positions in the Crab Nebula. These correlations suggest the existence of gas produced by a range of nuclear processing, from material in which synthesis ended with the CNO-cycle, to some helium-burning and nitrogen depletion, to regions containing enriched products of oxygen-burning. The latter exhibit a gradual, linear rise of [Ni II] emission with increasing argon enrichment, whereas gas with less nuclear processing shows markedly different [Ni II] emission characteri…
The CAT imaging telescope, recently built on the site of the former solar plant Themis (French Pyrenees), observed gamma-rays from the Crab nebula from October 1996 to March 1997. This steady source, often considered as the standard candle of very-high-energy gamma-ray astronomy, is used as a test-beam to probe the performances of the new telescope, particularly its energy threshold (220 GeV at 20 degrees zenith angle) and the stability of its response. Due to the fine-grain camera, an accurate analysis of the longitudinal profiles of shower images is performed, yielding the source position in…
(Abridged) We present high-resolution Chandra X-ray observations of PSR B0833-45, the 89 ms pulsar associated with the Vela supernova remnant. We have acquired two observations separated by one month to search for changes in the pulsar and its environment following an extreme glitch in its rotation frequency. We find a well-resolved nebula with a toroidal morphology remarkably similar to that observed in the Crab Nebula, along with an axial Crab-like jet. Between the two observations the flux from the pulsar is found to be steady to within 0.75%; the 3 sigma limit on the fractional increase in…
The results of our observational campaigns on the two extragalactic sources Mkn 501 and Mkn 421, carried out with the Imaging Element of the TACTIC array, during March-May, 1998 and April-May, 1999, are presented. The results indicate that the two BL Lac objects (Mkn 501 and Mkn 421) were in a `low' gamma-ray emission state during both epochs of our observations.
We set constraints on O(E/M) Lorentz Violation in QED in an effective field theory framework. A major consequence of such assumptions is the modification of the dispersion relations for electrons/positrons and photons, which in turn can affect the electromagnetic output of astrophysical objects. We compare the information provided by multiwavelength observations with a full and self-consistent computation of the broad-band spectrum of the Crab Nebula. We cast constraints of order 10^{-5} at 95% confidence level on the lepton Lorentz Violation parameters.
