1,717 research outputs found
Estimating the birth period of pulsars through GLAST/LAT observations of their wind nebulae
In this paper we show that the high energy -ray flux in the GeV
domain from mature pulsar wind nebulae (PWN) scales as the change in rotational
kinetic energy since birth, rather than the present
day spindown power . This finding holds as long as the
lifetime of inverse Compton emitting electrons exceeds the age of the system.
For a typical 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 , 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 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
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
Aims: To provide a significantly improved probability distribution for the
H-test for periodicity in X-ray and -ray arrival times, which is
already extensively used by the -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 -ray telescope. Methods:
Approximately realisations of the H-statistic () for random
(white) noise is calculated from a random number generator for which the
repitition cycle is . From these numbers the probability
distribution is calculated. Results: The distribution of is is
found to be exponential with parameter so that the cumulative
probability distribution . If we stack independent
values for , the sum of such values would follow the Erlang-K
distribution with parameter 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
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?
HESS J1825-137 was detected with a significance of 8.1 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 ( pc, for a distance of 4 kpc) is 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 can also be related to the extended PWN X-ray synchrotron photon index of
, 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
, provided that the SNR expanded into the hot ISM with relatively low
density ( cm).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
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 -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 -ray
bands. The radio and non-thermal X-ray emission can be explained by synchrotron
emission. We model the -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 keV and a
cooling break at 5.5 keV. This cooling break along with
a magnetic field strength of 5 G indicate that the synchrotron
break occurs at 1 keV.Comment: accepted for publication to ApJ
- …