1,717 research outputs found

    Estimating the birth period of pulsars through GLAST/LAT observations of their wind nebulae

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    In this paper we show that the high energy γ\gamma-ray flux in the GeV domain from mature pulsar wind nebulae (PWN) scales as the change in rotational kinetic energy I(Ω02Ω2)/2I(\Omega_0^2-\Omega^2)/2 since birth, rather than the present day spindown power IΩΩ˙I\Omega\dot{\Omega}. This finding holds as long as the lifetime of inverse Compton emitting electrons exceeds the age of the system. For a typical γ2\gamma^{-2} electron spectrum, the predicted flux depends mostly on the pulsar birth period, conversion efficiency of spindown power to relativistic electrons and distance to the PWN, so that first order estimates of the birth period can be assessed from {\it GLAST/LAT} observations of PWN. For this purpose we derive an analytical expression. The associated (``uncooled'') photon spectral index in the GeV domain is expected to cluster around 1.5\sim 1.5, which is bounded at low energies by an intrinsic spectral break, and at higher energies by a second spectral break where the photon index steepens to 2\sim 2 due to radiation losses. Mature PWN are expected to have expanded to sizes larger than currently known PWN, resulting in relatively low magnetic energy densities and hence survival of GeV inverse Compton emitting electrons. Whereas such a PWN may be radio and X-ray quiet in synchrotron radiation, it may still be detectable as a {\it GLAST/LAT} source as a result of the relic electrons in the PWN.Comment: 10 pages, no figures. To appear in Astrophysical Journal Letter

    Constraining A General-Relativistic Frame-Dragging Model for Pulsed Radiation from a Population of Millisecond Pulsars in 47 Tucanae using GLAST/LAT

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    Although only 22 millisecond pulsars (MSPs) are currently known to exist in the globular cluster (GC) 47 Tucanae, this cluster may harbor 30-60 MSPs, or even up to ~200. In this Letter, we model the pulsed curvature radiation (CR) gamma-ray flux expected from a population of MSPs in 47 Tucanae. These MSPs produce gamma-rays in their magnetospheres via accelerated electron primaries which are moving along curved magnetic field lines. A GC like 47 Tucanae containing a large number of MSPs provides the opportunity to study a randomized set of pulsar geometries. Geometry-averaged spectra make the testing of the underlying pulsar model more reliable, since in this case the relative flux uncertainty is reduced by one order of magnitude relative to the variation expected for individual pulsars (if the number of visible pulsars N=100). Our predicted spectra violate the EGRET upper limit at 1 GeV, constraining the product of the number of visible pulsars N and the average integral flux above 1 GeV per pulsar. GLAST/LAT should place even more stringent constraints on this product, and may also limit the maximum average accelerating potential by probing the CR spectral tail. For N=22-200, a GLAST/LAT non-detection will lead to the constraints that the average integral flux per pulsar should be lower by factors 0.03-0.003 than current model predictions.Comment: 10 pages, 2 figures, to appear in the Astrophysical Journal Letter

    The H-test probability distribution revisited: Improved sensitivity

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    Aims: To provide a significantly improved probability distribution for the H-test for periodicity in X-ray and γ\gamma-ray arrival times, which is already extensively used by the γ\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 γ\gamma-ray telescope. Methods: Approximately 101410^{14} realisations of the H-statistic (HH) for random (white) noise is calculated from a random number generator for which the repitition cycle is 1014\gg 10^{14}. From these numbers the probability distribution P(>H)P(>H) is calculated. Results: The distribution of HH is is found to be exponential with parameter λ=0.4\lambda=0.4 so that the cumulative probability distribution P(>H)=exp(λH)P(>H)=\exp{(-\lambda H)}. If we stack independent values for HH, the sum of KK such values would follow the Erlang-K distribution with parameter λ\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.Comment: 4 pages, two figures, to appear in Astronomy and Astrophysics, Letter

    Absorption of High Energy Gamma-Rays by Low Energy Intergalactic Photons

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    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.Comment: 12 pg., uuencoded, Z-compressed ps file (includes figures), To be published in Space Sci. Re

    HESS J1825-137: A pulsar wind nebula associated with PSR B1823-13?

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    HESS J1825-137 was detected with a significance of 8.1 σ\sigma in the Galactic Plane survey conducted with the H.E.S.S. instrument in 2004. Both HESS J1825-137 and the X-ray pulsar wind nebula G18.0--0.7 (associated with the Vela-like pulsar PSR B1823-13) are offset south of the pulsar, which may be the result of the SNR expanding into an inhomogeneous medium. The TeV size (35\sim 35 pc, for a distance of 4 kpc) is 6\sim 6 times larger than the X-ray size, which may be the result of propagation effects as a result of the longer lifetime of TeV emitting electrons, compared to the relatively short lifetime of keV synchrotron emitting electrons. The TeV photon spectral index of 2.4\sim 2.4 can also be related to the extended PWN X-ray synchrotron photon index of 2.3\sim 2.3, if this spectrum is dominated by synchrotron cooling. The anomalously large size of the pulsar wind nebula can be explained if the pulsar was born with a relatively large initial spindown power and braking index n2n\sim 2, provided that the SNR expanded into the hot ISM with relatively low density (0.003\sim 0.003 cm3^{-3}).Comment: 4 pages, 4 figures, to appear in the Proc. of the 29th International Cosmic Ray Conference, OG Sessio

    Probing the Nature of the Vela X Cocoon

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    Vela X is a pulsar wind nebula (PWN) associated with the active pulsar B0833-45 and contained within the Vela supernova remnant (SNR). A collimated X-ray filament ("cocoon") extends south-southwest from the pulsar to the center of Vela X. VLA observations uncovered radio emission coincident with the eastern edge of the cocoon and H.E.S.S. has detected TeV γ\gamma-ray emission from this region as well. Using XMM-\textit{Newton} archival data, covering the southern portion of this feature, we analyze the X-ray properties of the cocoon. The X-ray data are best fit by an absorbed nonequilibrium plasma model with a powerlaw component. Our analysis of the thermal emission shows enhanced abundances of O, Ne, and Mg within the cocoon, indicating the presence of ejecta-rich material from the propagation of the SNR reverse shock, consistent with Vela X being a disrupted PWN. We investigate the physical processes that excite the electrons in the PWN to emit in the radio, X-ray and γ\gamma-ray bands. The radio and non-thermal X-ray emission can be explained by synchrotron emission. We model the γ\gamma-ray emission by Inverse Compton scattering of electrons off of cosmic microwave background (CMB) photons. We use a 3-component broken power law to model the synchrotron emission, finding an intrinsic break in the electron spectrum at 5×106\sim5 \times 10^{6} keV and a cooling break at \sim 5.5 ×1010\times 10^{10} keV. This cooling break along with a magnetic field strength of 5 ×106\times 10^{-6} G indicate that the synchrotron break occurs at \sim1 keV.Comment: accepted for publication to ApJ
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