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تسجيل مستخدم جديدCalibration of statistical methods used to constrain pulsar geometries via multiband light curve modelling
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Since its launch in 2008, the Fermi Large Area Telescope (LAT) has detected over 200 -ray pulsars above 100 MeV. This population of pulsars is characterised by a rich diversity of light curve morphologies. Researchers have been using both the radio and -ray light curves to constrain the inclination and observer angles for each of these pulsars. At first, this was done using a by-eye technique and later via statistical approaches. We have also developed two novel statistical approaches that place the radio and -ray data on equal footing despite their disparate relative flux errors. We chose eleven pulsars from the Second Fermi Pulsar Catalog, both old and young, and applied these new techniques as well as the by-eye technique to constrain their geometric parameters using standard pulsar models. We present first results on our comparison of the best-fit parameters yielded by each of the aforementioned techniques. This will assist us in determining the utility of our new statistical approaches, and gauge the overlap of the best-fit parameters (plus errors) from each of the different methods. Such a statistical fitting approach will provide the means for further pulsar magnetospheric model development using light curve data.
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The Large Area Telescope aboard the Fermi spacecraft has detected more than 200 $gamma$-ray pulsars since its launch in 2008. By concurrently fitting standard geometric model light curves onto Fermi and radio data, researchers have constrained the in
clination and observer angles of a number of pulsars. At first this was done by comparing observed and modelled light curves by eye, and later via statistical approaches. We fit modelled light curves of 16 pulsars to radio and $gamma$-ray data by optimising a custom test statistic that we have developed for combining light curves across the two wavebands, taking their disparate errors into account. We present geometrical constraints found using this process, and compare them with results found by eye or using other statistical methods.
Thanks to the huge amount of gamma-ray pulsar photons collected by the Fermi Large Area Telescope since June 2008, it is now possible to constrain gamma-ray geometrical models by comparing simulated and observed light-curve morphological characterist
ics. We assumed vacuum-retarded dipole pulsar magnetic field and tested simulated and observed morphological light-curve characteristics in the framework of two pole emission geometries, Polar Cap (PC), radio, and Slot Gap (SG), and Outer Gap (OG)/One Pole Caustic (OPC) emission geometries. We compared simulated and observed/estimated light-curve morphological parameters as a function of observable and non-observable pulsar parameters. The PC model gives the poorest description of the LAT pulsar light-curve morphology. The OPC best explains both the observed gamma-ray peak multiplicity and shape classes. The OPC and SG models describe the observed gamma-ray peak-separation distribution for low- and high-peak separations, respectively. This suggests that the OPC geometry best explains the single-peak structure but does not manage to describe the widely separated peaks predicted in the framework of the SG model as the emission from the two magnetic hemispheres. The OPC radio-lag distribution shows higher agreement with observations suggesting that assuming polar radio emission, the gamma-ray emission regions are likely to be located in the outer magnetosphere. The larger agreement between simulated and LAT estimations in the framework of the OPC suggests that the OPC model best predicts the observed variety of profile shapes. The larger agreement between observations and the OPC model jointly with the need to explain the abundant 0.5 separated peaks with two-pole emission geometries, calls for thin OPC gaps to explain the single-peak geometry but highlights the need of two-pole caustic emission geometry to explain widely separated peaks.
This paper reports a detailed analysis of the optical light curve of PSR B0540-69, the second brightest pulsar in the visible band, obtained in 2009 (Jan. 18 and 20, and Dec. 14, 15, 16, 18) with the very high speed photon counting photometer Iqueye
mounted at the ESO 3.6-m NTT in La Silla (Chile). The optical light curve derived by Iqueye shows a double structure in the main peak, with a raising edge steeper than the trailing edge. The double peak can be fitted by two Gaussians with the same height and FWHM of 13.3 and 15.5 ms respectively. Our new values of spin frequencies allow to extend by 3.5 years the time interval over which a reliable estimate of frequency first and second derivatives can be performed. A discussion of implications on the braking index and age of the pulsar is carried out. A value of n = 2.087 +/- 0.007 for the overall braking index from 1987 to 2009 is derived. The braking index corrected age is confirmed around 1700 years.
Guillemot et al. recently reported the discovery of $gamma$-ray pulsations from the 22.7ms pulsar (pulsar A) in the famous double pulsar system J0737-3039A/B. The $gamma$-ray light curve (LC) of pulsar A has two peaks separated by approximately half
a rotation, and these are non-coincident with the observed radio and X-ray peaks. This suggests that the $gamma$-ray emission originates in a part of the magnetosphere distinct from where the radio and X-ray radiation is generated. Thus far, three different methods have been applied to constrain the viewing geometry of pulsar A (its inclination and observer angles $alpha$ and $zeta$): geometric modelling of the radio and $gamma$-ray light curves, modelling of the position angle sweep in phase seen in the radio polarisation data, and independent studies of the time evolution of the radio pulse profile of pulsar A. These three independent, complementary methods have yielded consistent results: pulsar As rotation axis is likely perpendicular to the orbital plane of the binary system, and its magnetic axis close to lying in the orbital plane (making this pulsar an orthogonal rotator). The observer is furthermore observing emission close to the magnetic axis. Thus far, however, current models could not reproduce all the characteristics of the radio and $gamma$-ray light curves, specifically the large radio-to-$gamma$ phase lag. In this paper we discuss some preliminary modelling attempts to address this problem, and offer ideas on how the LC fits may be improved by adapting the standard geometric models in order to reproduce the profile positions more accurately.
In more than four years of observation the Large Area Telescope on board the Fermi satellite has identified pulsed $gamma$-ray emission from more than 80 young pulsars, providing light curves with high statistics. Fitting the observations with geomet
rical models can provide estimates of the magnetic obliquity $alpha$ and aspect angle $zeta$, yielding estimates of the radiation beaming factor and luminosity. Using $gamma$-ray emission geometries (Polar Cap, Slot Gap, Outer Gap, One Pole Caustic) and radio emission geometry, we fit $gamma$-ray light curves for 76 young pulsars and we jointly fit their $gamma$-ray plus radio light curves when possible. We find that a joint radio plus $gamma$-ray fit strategy is important to obtain ($alpha$, $zeta$) estimates that can explain simultaneous radio and $gamma$-ray emission. The intermediate-to-high altitude magnetosphere models, Slot Gap, Outer Gap, and One pole Caustic, are favoured in explaining the observations. We find no evolution of $alpha$ on a time scale of a million years. For all emission geometries our derived $gamma$-ray beaming factors are generally less than one and do not significantly evolve with the spin-down power. A more pronounced beaming factor vs. spin-down power correlation is observed for Slot Gap model and radio-quiet pulsars and for the Outer Gap model and radio-loud pulsars. For all models, the correlation between $gamma$-ray luminosity and spin-down power is consistent with a square root dependence. The $gamma$-ray luminosities obtained by using our beaming factors not exceed the spin-down power. This suggests that assuming a beaming factor of one for all objects, as done in other studies, likely overestimates the real values. The data show a relation between the pulsar spectral characteristics and the width of the accelerator gap that is consistent with the theoretical prediction for the Slot Gap model.
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