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Search Results: 1 - 10 of 264512 matches for " O. de Jager "
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TeV emission from SN 1006
A. Mastichiadis,O. C. de Jager
Physics , 1996,
Abstract: Supernova 1006 is the first shell type supernova remnant to show evidence of particle acceleration to TeV energies. In the present paper we examine this possibility by modeling the observed X-ray non-thermal emission in terms of synchrotron radiation from Fermi accelerated electrons. The predicted synchrotron spectrum fits the radio and non-thermal component of the observed soft X-ray to hard X-ray emission quite well. These particles can produce TeV gamma rays by inverse Compton scattering on the microwave radiation and other ambient fields, and the derived electron distribution is also used to calculate the expected inverse Compton flux. We find that if the remnant is characterised by a magnetic field strength lower than $\sim 7\mu$G, then the TeV flux can be higher than that of the Crab Nebula. About 75\% of the TeV emission from SN 1006 is expected to be concentrated in the synchrotron bright NE and SW rims (the ``hard aegis") of the remnant, which would allow a sensitive search if the Atmospheric Imaging Cherenkov Technique is used.
Absorption of Very High Energy Gamma-Rays by Intergalactic Infrared Radiation: A New Determination
F. W. Stecker,O. C. De Jager
Physics , 1998,
Abstract: We present a new calculation of the intergalactic gamma-ray absorption coefficient as a function of both energy and redshift. In reexamining this problem, we make use of a new, empirically based calculation (as opposed to previous model calculations) of the intergalactic infrared radiation field. We find smaller opacities than those given previously (Stecker & De Jager 1997). We apply our results to the new observations of the flaring gamma-ray spectra of Mrk421 and Mrk501, both at a redshift of apx. 0.03. Our new calculations indicate that there should be no significant curvature in the spectra of these sources for energies below 10 TeV, as indicated by recent observations. However, the intrinsic spectra of these sources should be harder by apx. 0.2 to 0.45 in the spectral index in the 1 to 10 TeV range with an intergalactic absorption cutoff above apx. 20 TeV.
Absorption of High Energy Gamma-Rays by Low Energy Intergalactic Photons
F. W. Stecker,O. C. De Jager
Physics , 1995, DOI: 10.1007/BF00195048
Abstract: Following our previously proposed technique, we have used the recent gamma-ray observations of Mrk 421 to place theoretically significant constraints on and possible estimates of the intergalactic infrared radiation field (IIRF) which are consistent with normal galactic IR production by stars and dust and rule out exotic mechanisms proposed to produce a larger IIRF. Using models for the low energy intergalactic photon spectrum from microwave to UV energies, we calculate the opacity of inter- galactic space to gamma-rays as a function of energy and redshift. These calculations indicate that the GeV gamma-ray burst recently observed by the EGRET experiment on CGRO originates at a redshift less than approximately 1.5.
On the Absorption of High Energy Gamma-Rays by Intergalactic Infrared Radiation
F. W. Stecker,O. C. de Jager
Physics , 1996, DOI: 10.1086/303668
Abstract: We present a new calculation of the intergalactic $\gamma$-ray pair-production absorption coefficient as a function of both energy and redshift up to the redshift of 3C279, z = 0.54. In reexamining this problem, we make use of new observational data on the intergalactic infrared radiation field (IIRF), together with recent theoretical models of the galactic spectral energy distributions of the IIRF from stars and dust reradiation and estimates of the IIRF from galaxy counts and {\it COBE} results. We present our results for two fairly well defined IIRF spectral energy distributions, one of which is within $1 \sigma $ of our previous estimate of the IIRF at $ \sim 20$ $\mu$m. We then apply our results to the $\gamma$-ray spectrum of Mrk 421, and obtain good agreement with the observational data, including the recent results of the {\it HEGRA} group.
The H-test probability distribution revisited: Improved sensitivity
O. C. de Jager,I. Büsching
Physics , 2010,
Abstract: Aims: To provide a significantly improved probability distribution for the H-test for periodicity in X-ray and $\gamma$-ray arrival times, which is already extensively used by the $\gamma$-ray pulsar community. Also, to obtain an analytical probability distribution for stacked test statistics in the case of a search for pulsed emission from an ensemble of pulsars where the significance per pulsar is relatively low, making individual detections insignificant on their own. This information is timely given the recent rapid discovery of new pulsars with the Fermi-LAT t $\gamma$-ray telescope. Methods: Approximately $10^{14}$ realisations of the H-statistic ($H$) for random (white) noise is calculated from a random number generator for which the repitition cycle is $\gg 10^{14}$. From these numbers the probability distribution $P(>H)$ is calculated. Results: The distribution of $H$ is is found to be exponential with parameter $\lambda=0.4$ so that the cumulative probability distribution $P(>H)=\exp{(-\lambda H)}$. If we stack independent values for $H$, the sum of $K$ such values would follow the Erlang-K distribution with parameter $\lambda$ for which the cumulative probability distribution is also a simple analytical expression. Conclusion: Searches for weak pulsars with unknown pulse profile shapes in the Fermi-LAT, Agile or other X-ray data bases should benefit from the {\it H-test} since it is known to be powerful against a broad range of pulse profiles, which introduces only a single statistical trial if only the {\it H-test} is used. The new probability distribution presented here favours the detection of weaker pulsars in terms of an improved sensitivity relative to the previously known distribution.
Absorption of Intergalactic TeV Gamma-Rays
F. W. Stecker,O. C. De Jager
Physics , 1997,
Abstract: We discuss the problem of the absorption of very high-energy gamma-rays by pair production interactions with extragalactic photons which originate from stellar emission in the near IR-UV and reradiation of starlight in the mid- and far-IR. The absorption of gamma-rays above 1 TeV is dominated by interactions with infrared photons. We make a new determination of the optical depth of the universe to multi-TeV photons as a function of energy and redshift and use the results to compare with recent spectral data of Mrk 421 and Mrk 501, sources that have been observed in the flaring state up to apx. 10 TeV energy. For the optical depth calculations, we have made use of a new, empirically based calculation of the intergalactic radiation field by Malkan & Stecker which we consider to be more accurate than that based on previous theoretical modeling. We also discuss the absorption of sub-TeV gamma-rays by starlight photons at high redshifts.
Extragalactic Gamma-ray Absorption and the Intrinsic Spectrum of Mkn 501 During the 1997 Flare
O. C. De Jager,F. W. Stecker
Physics , 2001, DOI: 10.1086/338275
Abstract: Using the recent models of Malkan & Stecker (2001) for the infrared background radiation and extrapolating them into the optical and UV range using recent galaxy count data, we rederive the optical depth of the Universe to high energy gamma-rays as a function of energy and redshift for energies between 50 GeV and 100 TeV and redshifts between 0.03 and 0.3. We then use these results to derive the intrinsic gamma-ray spectrum of Mkn 501 during its 1997 high state. We find that the time averaged spectral energy distribution of Mkn 501 while flaring had a broad, flat peak in the 5 to 10 TeV range which corresponds to a broad, flat time averaged X-ray peak in the 50 to 100 keV range observed during the flare. The spectral index of our derived intrinsic differential photon spectrum for Mkn 501 at energies below about 2 TeV was found to be apx. 1.6 to 1.7. This corresponds to a time averaged spectral index of 1.76 found in soft X-rays at energies below the X-ray (synchrotron) peak. These results appear to favor a synchrotron-self Compton origin for the TeV emission together wuth jet parameters which are consistent with time variability constraints within the context of a simple SSC model.
Estimates for Very High Energy Gamma Rays from Globular Cluster Pulsars
C. Venter,O. C. de Jager
Physics , 2009, DOI: 10.1063/1.3076659
Abstract: Low-Mass X-ray Binaries (LMXRBs), believed to be the progenitors of recycled millisecond pulsars (MSPs), occur abundantly in globular clusters (GCs). GCs are therefore expected to host large numbers of MSPs. This is also confirmed observationally. The MSPs continuously inject relativistic electrons into the ambient region beyond their light cylinders, and these relativistic particles produce unpulsed radiation via the synchrotron and inverse Compton (IC) processes. It is thus possible, in the context of General Relativistic (GR) frame-dragging MSP models, to predict unpulsed very high energy radiation expected from nearby GCs. We use a period-derivative cleaned sample of MSPs in 47 Tucanae, where the effects of the cluster potential on the individual period derivatives have been removed. Using a Monte Carlo process to include effects of pulsar geometry, we obtain average injection spectra (with relatively small errors) of particles leaving the MSPs. These spectra are next used to predict cumulative synchrotron and IC spectra expected from 47 Tucanae, which is a lower limit, as no reacceleration is assumed. We find that the IC radiation from 47 Tucanae may be visible for H.E.S.S., depending on the nebular field B as well as the number of MSPs N in the GC. Telescopes such as Chandra and Hubble may find it difficult to test the SR component prediction of diffuse radiation if there are many unresolved sources in the field of view. These results may be rescaled for other GCs where less information is available, assuming universal GC MSP characteristics.
Predicted Extragalactic TeV Gamma-Ray Sources
F. W. Stecker,O. C. De Jager,M. H. Salamon
Physics , 1996, DOI: 10.1086/310407
Abstract: We suggest that low-redshift XBLs (X-ray selected BL Lacertae objects) may be the only extragalactic gamma-ray sources observable at TeV energies. We use simple physical considerations involving synchrotron and Compton component spectra for blazars to suggest why the observed TeV sources are XBLs, whereas mostly RBLs and FSRQs are seen at GeV energies. These considerations indicate that the differences between XBLs and RBLs cannot be explained purely as relativistic jet orientation effects. We note that the only extragalactic TeV sources which have been observed are XBLs and that a nearby RBL with a very hard spectrum in the GeV range has not been seen at TeV energies. We also note that of the 14 BL Lacs observed by EGRET, 12 are RBLs, whereas only 2 are XBLs. We give a list of nearby XBLs which we consider to be good candidate TeV sources and predict estimated TeV fluxes for these objects.
Are Galactic Gamma-Ray Bursters the Main Source of Hadronic Non-Solar Cosmic Rays at all Energies?
R. Plaga,O. C. de Jager,A. Dar
Physics , 1999,
Abstract: We propose a new hypothesis for the origin of non-solar hadronic cosmic rays (CRs) at all energies: Highly relativistic, narrowly collimated jets from the birth or collapse of neutron stars (NSs) in our Galaxy accelerate ambient disk and halo matter to CR energies and disperse it in ``hot spots'' which they form when they stop in the Galactic halo. Such events - ``Galactic Gamma-Ray Bursters'' (GGRBs) - are proposed to cause cosmological gamma-ray bursts (GRBs) in other galaxies when their beamed radiation happens to point in our direction. Our hypothesis naturally explains some observations which are difficult to understand with the currently popular ideas about CR origin - e.g. the small Galacto-centric gradient of the cosmic-ray density and the absence of the Greisen-Zatsepin-Kuzmin cutoff. Our idea stands or falls with the existence of the ``hot spots'' (``GGRB remnants'') in the Galactic halo. We discuss their expected observational signatures and find that they could appear as EGRET unidentified high-latitude sources.
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