41,001 research outputs found
Is SGR 1900+14 a Magnetar?
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
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
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
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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