447 research outputs found
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
A method to calculate the Galactic cosmic ray density with high spatial resolution, new developments
Neutral Pions with Large Transverse Momentum in d+Au and Au+Au Collisions
Measurements of transverse-momentum p_T spectra of neutral pions in Au+Au and
d+Au collisions at sqrt{s_NN}=200 GeV and 62.4 GeV by the PHENIX experiment at
RHIC in comparison to p+p reference spectra at the same sqrt{s_NN} are
presented. In central Au+Au collisions at sqrt{s_NN}=200 GeV a factor 4-5
suppression for neutral pions and charged hadrons with p_T > 5 GeV/c is found
relative to the p+p reference scaled by the nuclear overlap function .
In contrast, such a suppression of high-p_T particles is absent in d+Au
collisions independent of the centrality of the collision. To study the
sqrt{s_NN} dependence of the suppression Au+Au collisions at sqrt{s_NN}=200 GeV
and 62.4 GeV are compared.Comment: 7 pages, 5 figures, presented at Hot Quarks 2004, Taos, N
A Cosmic Ray Positron Anisotropy due to Two Middle-Aged, Nearby Pulsars?
Geminga and B0656+14 are the closest pulsars with characteristic ages in the
ran ge of 100 kyr to 1 Myr. They both have spindown powers of the order 3e34
erg/s at present. The winds of these pulsars had most probably powered pulsar
wind nebulae (PWNe) that broke up less than about 100 kyr after the birth of
the pulsars. Assuming that leptonic particles accelerated by the pulsars were
confined in th e PWNe and were released into the interstellar medium (ISM) on
breakup of the PW Ne, we show that, depending on the pulsar parameters, both
pulsars make a non-ne gligible contribution to the local cosmic ray (CR)
positron spectrum, and they m ay be the main contributors above several GeV.
The relatively small angular dist ance between Geminga and B0656+14 thus
implies an anisotropy in the local CR po sitron flux at these energies. We
calculate the contribution of these pulsars to the locally observed CR electr
on and positron spectra depending on the pulsar birth period and the magnitude
o f the local CR diffusion coefficient. We further give an estimate of the
expecte d anisotropy in the local CR positron flux. Our calculations show that
within the framework of our model, the local CR posit ron spectrum imposes
constraints on pulsar parameters for Geminga and B0656+14, notably the pulsar
period at birth, and also the local interstellar diffusion co efficient for CR
leptons.Comment: accepted for publication in ApJ
Motor Imagery Impairment in Postacute Stroke Patients
Not much is known about how well stroke patients are able to perform motor imagery (MI) and which MI abilities are preserved after stroke. We therefore applied three different MI tasks (one mental chronometry task, one mental rotation task, and one EEG-based neurofeedback task) to a sample of postacute stroke patients () and age-matched healthy controls () for addressing the following questions: First, which of the MI tasks indicate impairment in stroke patients and are impairments restricted to the paretic side? Second, is there a relationship between MI impairment and sensory loss or paresis severity? And third, do the results of the different MI tasks converge? Significant differences between the stroke and control groups were found in all three MI tasks. However, only the mental chronometry task and EEG analysis revealed paresis side-specific effects. Moreover, sensitivity loss contributed to a performance drop in the mental rotation task. The findings indicate that although MI abilities may be impaired after stroke, most patients retain their ability for MI EEG-based neurofeedback. Interestingly, performance in the different MI measures did not strongly correlate, neither in stroke patients nor in healthy controls. We conclude that one MI measure is not sufficient to fully assess an individual’s MI abilities
Spatially resolved XMM-Newton analysis and a model of the nonthermal emission of MSH 15-52
We present an X-ray analysis and a model of the nonthermal emission of the
pulsar wind nebula (PWN) MSH15-52. We analyzed XMM-Newton data to obtain the
spatially resolved spectral parameters around the pulsar PSRB1509-58. A
steepening of the fitted power-law spectra and decrease in the surface
brightness is observed with increasing distance from the pulsar. In the second
part of this paper, we introduce a model for the nonthermal emission, based on
assuming the ideal magnetohydrodynamic limit. This model is used to constrain
the parameters of the termination shock and the bulk velocity of the leptons in
the PWN. Our model is able to reproduce the spatial variation of the X-ray
spectra. The parameter ranges that we found agree well with the parameter
estimates found by other authors with different approaches. In the last part of
this paper, we calculate the inverse Compton emission from our model and
compare it to the emission detected with the H.E.S.S. telescope system. Our
model is able to reproduce the flux level observed with H.E.S.S., but not the
spectral shape of the observed TeV {\gamma}-ray emission.Comment: Accepted for publication in A&A, 9 pages, 15 figure
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