41,001 research outputs found

    Is SGR 1900+14 a Magnetar?

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    We present RXTE observations of the soft gamma--ray repeater SGR 1900+14 taken September 4-18, 1996, nearly 2 years before the 1998 active period of the source. The pulsar period (P) of 5.1558199 +/- 0.0000029 s and period derivative (Pdot) of (6.0 +/- 1.0) X 10^-11 s/s measured during the 2-week observation are consistent with the mean Pdot of (6.126 +/- 0.006) X 10^-11 s/s over the time up to the commencement of the active period. This Pdot is less than half that of (12.77 +/- 0.01) X 10^-11 s/s observed during and after the active period. If magnetic dipole radiation were the primary cause of the pulsar spindown, the implied pulsar magnetic field would exceed the critical field of 4.4 X 10^13 G by more than an order of magnitude, and such field estimates for this and other SGRs have been offered as evidence that the SGRs are magnetars, in which the neutron star magnetic energy exceeds the rotational energy. The observed doubling of Pdot, however, would suggest that the pulsar magnetic field energy increased by more than 100% as the source entered an active phase, which seems very hard to reconcile with models in which the SGR bursts are powered by the release of magnetic energy. Because of this, we suggest that the spindown of SGR pulsars is not driven by magnetic dipole radiation, but by some other process, most likely a relativistic wind. The Pdot, therefore, does not provide a measure of the pulsar magnetic field strength, nor evidence for a magnetar.Comment: 14 pages, aasms4 latex, figures 1 & 2 changed, accepted by ApJ letter

    A Survey of the Northern Sky for TeV Point Sources

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    A search for steady TeV point sources anywhere in the northern sky has been made with data from the Milagrito air-shower-particle detector. Over 3 x 10**9 events collected from 1997 February to 1998 May have been used in this study. No statistically significant excess above the background from the isotropic flux of cosmic rays was found for any direction of the sky with declination between -5 degrees and 71.7 degrees. Upper limits are derived for the photon flux above 1 TeV from any steady point source in the northern sky.Comment: 2 Figure

    Statistical correlation studies of astrophysical objects with H.E.S.S. data

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    Recent advances in the instrumentation to observe very high energy (VHE) gamma rays have made the discovery of many new sources possible, most of them being discovered in the Galactic plane survey of H.E.S.S., an array of imaging atmospheric Cherenkov telescopes in Namibia. Of these sources, a significant number can be identified as pulsar wind nebulae, but supernova remnants and high mass X-ray binaries were also detected. However, a large fraction of the sources remains with no clear counterpart. In this work a study is presented searching for statistical correlations between different types of astrophysical objects and all the H.E.S.S. sources in the Galactic plane, rather than looking for counterparts for individual sources. The results of this work show that HII regions, OB stars and Galactic bubbles do not correlate significantly with VHEgamma -ray sources, while the expected correlations with supernova remnants and high mass X-ray binaries are limited due to the low statistics. Pulsar wind nebulae, regions of star formation and star forming complexes seem to correlate with VHE gamma-ray sources

    Searching for high-energy neutrinos in coincidence with gravitational waves with the ANTARES and VIRGO/LIGO detectors

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    Cataclysmic cosmic events can be plausible sources of both gravitational waves (GW) and high-energy neutrinos (HEN). Both GW and HEN are alternative cosmic messengers that may escape very dense media and travel unaffected over cosmological distances, carrying information from the innermost regions of the astrophysical engines. For the same reasons, such messengers could also reveal new, hidden sources that were not observed by conventional photon astronomy. Requiring the consistency between GW and HEN detection channels shall enable new searches as one has significant additional information about the common source. A neutrino telescope such as ANTARES can determine accurately the time and direction of high energy neutrino events, while a network of gravitational wave detectors such as LIGO and VIRGO can also provide timing/directional information for gravitational wave bursts. By combining the information from these totally independent detectors, one can search for cosmic events that may arrive from common astrophysical sources.Comment: 4 pages, 2 figures. Prepared for the Proceedings of the 31st ICRC, Lodz (Poland), July 7-15, 200
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