In the last decade ground-based Imaging Atmospheric Cherenkov Telescopes have discovered roughly 30 pulsar wind nebulae at energies above 100 GeV. We present first results from a leptonic emission code that models the spectral energy density of a pul
sar wind nebula by solving the Fokker-Planck transport equation and calculating inverse Compton and synchrotron emissivities. Although models such as these have been developed before, most of them model the geometry of a pulsar wind nebula as that of a single sphere. We have created a time-dependent, multi-zone model to investigate changes in the particle spectrum as the particles diffuse through the pulsar wind nebula, as well as predict the radiation spectrum at different positions in the nebula. We calibrate our new model against a more basic previous model and fit the observed spectrum of G0.9+0.1, incorporating data from the High Energy Stereoscopic System as well as radio and X-ray experiments.
We previously obtained constraints on the viewing geometries of 6 Fermi LAT pulsars using a multiwavelength approach (Seyffert et al., 2011). To obtain these constraints we compared the observed radio and $gamma$-ray light curves (LCs) for those 6 pu
lsars by eye to LCs predicted by geometric models detailing the location and extent of emission regions in a pulsar magnetosphere. As a precursor to obtaining these constraints, a parameter study was conducted to reinforce our qualitative understanding of how the underlying model parameters effect the LCs produced by the geometric models. Extracting useful trends from the $gamma$-ray model LCs proved difficult though due to the increased complexity of the geometric models for the $gamma$-ray emission relative to those for the radio emission. In this paper we explore a second approach to investigating the interplay between the model parameters and the LC atlas. This approach does not attempt to understand how the set of model parameters influences the LC shapes directly, but rather, more fundamentally, investigates how the set of model parameters effects the sky maps from which the latter are extracted. This allows us to also recognise structure within the atlas itself, as we are now able to attribute certain features of the LCs to specific features on the sky map, meaning that we not only understand how the structure of single LCs come about, but also how their structure changes as we move through the geometric solution space.
The second pulsar catalogue of the Fermi Large Area Telescope (LAT) will contain in excess of 100 gamma-ray pulsars. The light curves (LCs) of these pulsars exhibit a variety of shapes, and also different relative phase lags with respect to their rad
io pulses, hinting at distinct underlying emission properties (e.g., inclination and observer angles) for the individual pulsars. Detailed geometric modelling of the radio and gamma-ray LCs may provide constraints on the B-field structure and emission geometry. We used different B-field solutions, including the static vacuum dipole and the retarded vacuum dipole, in conjunction with an existing geometric modelling code, and constructed radiation sky maps and LCs for several different pulsar parameters. Standard emission geometries were assumed, namely the two-pole caustic (TPC) and outer gap (OG) models. The sky maps and LCs of the various B-field and radiation model combinations were compared to study their effect on the resulting LCs. As an application, we compared our model LCs with Fermi LAT data for the Vela pulsar, and inferred the most probable configuration in this case, thereby constraining Velas high-altitude magnetic structure and system geometry.
The pulsar wind nebula (PWN) associated with the Vela pulsar is a bright source in the radio, X-ray and gamma-ray bands, but not in the optical. This source is very near, lying at a distance of 290 pc, as inferred from the radio and optical parallax
measurements of the pulsar. Knowledge of the brightness and structure of the Vela PWN in optical is important in order to constrain the underlying particle spectrum (and possibly the B-field properties and particle losses) associated with this extended source. We use results from the Digital Sky Survey, as well as results obtained using the SAAO 1.0 m telescope equipped with an imaging CCD (STE4) and BV filters, in an attempt to measure optical radiation from Vela X. To enlarge our field of view, we constructed a mosaic consisting of 3 x 3 frames around the pulsar position. We present spectral measurements from the High Energy Stereoscopic System (H.E.S.S.), Fermi Large Area Telescope (LAT), ASCA, Hubble Space Telescope (HST), Very Large Telescope (VLT), New Technology Telescope (NTT), and Wilkinson Microwave Anisotropy Probe (WMAP), in addition to our optical results. Using these data, we investigate whether or not the radio synchrotron component can be smoothly extrapolated to the optical band. This would constrain the electron population to consist of either a single or multiple components, representing a significant advancement in our understanding of this complex multiwavelength source.
Globular clusters (GCs) are astronomical tapestries embroidered with an abundance of exotic stellar-type objects. Their high age promises a rich harvest of evolved stellar products, while the deep potential wells and high mass densities at their cent
res probably facilitate the formation of multiple-member stellar systems via increased stellar encounter rates. The ubiquity of low-mass X-ray binaries, thought to be the progenitors of millisecond pulsars (MSPs), explain the large number of observed GC radio pulsars and X-ray counterparts. The Fermi Large Area Telescope (LAT) recently unveiled the first gamma-ray GC pulsar (PSR J1823-3021A). The first observations of GCs in the GeV and TeV bands furthermore created much excitement, and in view of the above, it seems natural to explain these high-energy lanterns by investigating an MSP origin. An MSP population is expected to radiate several pulsed spectral components in the radio through gamma-ray wavebands, in addition to being sources of relativistic particles. The latter may interact with background photons in the clusters producing TeV excesses, while they may also radiate synchrotron photons as they traverse the cluster magnetic field. We present multiwavelength modelling results for Terzan 5. We also briefly discuss some alternative interpretations for the observed GC gamma-ray signals.
The light curves of gamma-ray pulsars detected by the Fermi Large Area Telescope show great variety in profile shape and position relative to their radio profiles. Such diversity hints at distinct underlying magnetospheric and/or emission geometries
for the individual pulsars. We implemented an offset-dipole magnetic field in an existing geometric pulsar modelling code which already includes static and retarded vacuum dipole fields. In our model, this offset is characterised by a parameter epsilon (with epsilon = 0 corresponding to the static dipole case). We constructed sky maps and light curves for several pulsar parameters and magnetic fields, studying the effect of an offset dipole on the resulting light curves. A standard two-pole caustic emission geometry was used. As an application, we compared our model light curves with Fermi data for the bright Vela pulsar.
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.
Millisecond pulsars (MSPs) are a growing class of gamma-ray emitters. Pulsed gamma-ray signals have been detected from more than 40 MSPs with the Fermi Large Area Telescope (LAT). The wider radio beams and more compact magnetospheres of MSPs enable s
tudies of emission geometries over a broader range of phase space than non-recycled radio-loud gamma-ray pulsars. We have modeled the gamma-ray light curves of 40 LAT-detected MSPs using geometric emission models assuming a vacuum retarded-dipole magnetic field. We modeled the radio profiles using a single-altitude hollow-cone beam, with a core component when indicated by polarimetry; however, for MSPs with gamma-ray and radio light curve peaks occurring at nearly the same rotational phase we assume that the radio emission is co-located with the gamma rays and caustic in nature. The best-fit parameters and confidence intervals are determined using a maximum likelihood technique. We divide the light curves into three model classes, with gamma-ray peaks trailing (Class I), aligned (Class II) or leading (Class III) the radio peaks. Outer gap and slot gap (two-pole caustic) models best fit roughly equal numbers of Class I and II, while Class III are exclusively fit with pair-starved polar cap models. Distinguishing between the model classes based on typical derived parameters is difficult. We explore the evolution of magnetic inclination angle with period and spin-down power, finding possible correlations. While the presence of significant off-peak emission can often be used as a discriminator between outer gap and slot gap models, a hybrid model may be needed.
Since the launch of the Large Area Telescope (LAT) on board the Fermi spacecraft in June 2008, the number of observed gamma-ray pulsars has increased dramatically. A large number of these are also observed at radio frequencies. Constraints on the vie
wing geometries of 5 of 6 gamma-ray pulsars exhibiting single-peaked gamma-ray profiles were derived using high-quality radio polarization data (Weltevrede et al., 2010). We obtain independent constraints on the viewing geometries of 6 by using a geometric emission code to model the Fermi LAT and radio light curves (LCs). We find fits for the magnetic inclination and observer angles by searching the solution space by eye. Our results are generally consistent with those previously obtained (Weltevrede et al., 2010), although we do find small differences in some cases. We will indicate how the gamma-ray and radio pulse shapes as well as their relative phase lags lead to constraints in the solution space. Values for the flux correction factor corresponding to the fits are also derived (with errors).
The Fermi Large Area Telescope (LAT) has recently reported the detection of pulsed gamma-rays from 6 young pulsars (J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850, and J1718-3825), all exhibiting single-peaked pulse profiles (Weltevrede e
t al., 2010). High-quality radio polarization data are also available for 5 of these pulsars, allowing derivation of constraints on their viewing geometries. We obtain independent constraints on the viewing geometries of these pulsars by using a geometric pulsar emission code to model the Fermi LAT and radio light curves. We find fits for the magnetic inclination and observer angles alpha and zeta with typical errors of ~ 5deg. Our results are generally consistent with those obtained by Weltevrede et al. (2010), although we do find differences in some cases. Our model may lastly provide a framework to constrain the radio emission altitude.
