Rotation-powered pulsars are excellent laboratories for study of particle
acceleration as well as fundamental physics of strong gravity, strong magnetic
fields, high densities and relativity. I will review the outstanding questions
in pulsar physics and the prospects for finding answers with GLAST LAT
observations. LAT observations should significantly increase the number of
detected radio-loud and radio-quiet gamma-ray pulsars, including millisecond
pulsars, giving much better statistics for elucidating population
characteristics, will measure the high-energy spectrum and the shape of
spectral cutoffs and determine pulse profiles for a variety of pulsars of
different age. Further, measurement of phase-resolved spectra and energy
dependent pulse profiles of the brighter pulsars should allow detailed tests of
magnetospheric particle acceleration and radiation mechanisms, by comparing
data with theoretical models that have been developed.Comment: 5 pages, 2 figures, to appear in Proc. of First GLAST Symposium
  (Stanford, Feb. 5-8, 2007), eds. S.Ritz, P.F. Michelson, and C.Meegan, AIP
  Conf. Pro

Harding, Alice K.

English

arXiv

Alice K. Harding

Crossref

Pulsar Physics and GLAST

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Pulsar Physics and GLAST
Alice K. Harding
Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771
Abstract. Rotation-powered pulsars are excellent laboratories for study of particle acceleration as well as fundamental
physics of strong gravity, strong magnetic fields, high densities and relativity. I will review some outstanding questions in
pulsar physics and the prospects for finding answers with GLAST LAT observations. LAT observations should significantly
increase the number of detected radio-loud and radio-quietgamma-ray pulsars, including millisecond pulsars, givingmuch
better statistics for elucidating population characterisics, will measure the high-energy spectrum and the shape ofspectral
cutoffs and determine pulse profiles for a variety of pulsarsof different age. Further, measurement of phase-resolved sp ctra
and energy dependent pulse profiles of the brighter pulsars should allow detailed tests of magnetospheric particle acceleration
and radiation mechanisms, by comparing data with theoretical models that have been developed.
Keywords: γ rays;γ-ray sources; Pulsars
PACS: 97.60.Gb;95.85.Pw;98.70.Rz
INTRODUCTION
The EGRET telescope on the Compton Gamma-Ray Observatory prvided a major increase in our understanding of
rotation-powered pulsars[29]. The years since have witnessed a number of further advances that include the completion
of the Parkes Multibeam survey[20], doubling the number of known radio pulsars, observations of unprecedented
resolution with the Chandra X-ray telescope, and discoveryof new pulsar wind nebulae (PWN) at TeV energies with
HESS. Deep radio observations of newly discovered X-ray PWNs have resulted in discovery of young pulsars with
very low radio luminosities[6]. As a result, there are many more young, and potentialγ-ray, pulsars known today than
at the time of EGRET. In addition, there have been fundamental developments in theoretical models of pulsed high-
energy emission[12] as well as major progress in defining theglobal structure of the pulsar magnetosphere[26]. The
stage is now set for the Gamma-Ray Large Area Space Telescope(GLAST), aγ-ray telescope with thirty times the
sensitivity of EGRET, to detect possibly more than one hundred newγ-ray pulsars and to measure a variety of profiles
and spectral characteristics. It is hoped that some of the these fundamental questions of pulsar physics will finally have
answers.
HOW AND WHERE ARE PARTICLES ACCELERATED IN THE PULSAR
MAGNETOSPHERE?
The location of the pulsed emission site(s) has been very elusiv primarily because we observe only the pulse
profile, the small part of the emission beam that sweeps through our line-of-sight. Constructing the full geometry
of the emission from only the pulse profile is both non-trivial and non-unique. But observing at high-energies has a
distinct advantage that the emission patterns map the acceleration sites, since the accelerated particles radiate most
of their energy very quickly. Thus, there is hope that with GLAST LAT observations we can deduce the geometry of
acceleration by studying both details of individual pulse profiles and patterns of emission in a larger population of
γ-ray pulsars, aided by model predictions.
The theoretical sites of particle acceleration are determined by the distribution of charge in the magnetosphere and
the geometry of pair creation. The electric fieldE|| given by∇·E|| = 4π(ρ−ρGJ)will accelerate particles parallel to the
magnetic field where the local space charge densityρ departs from the Goldreich-Julian change densityρGJ. The high-
energy radiation of the accelerated particles can create electron-positron pairs that can screen theE|| at pair formation
fronts (PFFs), thus limiting the acceleration. The problemis that we do not know the boundary conditions, which
depend both on the surface physics of the neutron star and on the global current flow pattern. Global magnetospheric
models[26] can derive the current flow but assume ideal MHD (ρ = ρGJ everywhere). However, the pair production
that presumably supplies enough charge (at least in the magnetospheres of young pulsars) to provide the ideal MHD
Vela
FIGURE 1. Pulse profile of the Vela pulsar, as observed by EGRET, and computed phase plots of emission in the (ζobs, φ ) plane
and pulse profiles produced by a horizontal cut through the phase plot in polar cap, outer gap and slot gap models[11].α is the
pulsar inclination,φ is the rotational phase andζobs is the viewing angle measured from the rotation axis.
conditions requires a breakdown of this very condition at acceleration sites. Thus, in the absence of magnetospheric
models which include pair physics, the various acceleration m dels must depend on assumed boundary conditions.
Polar cap models[8] assume thatE|| develops along open field lines above the neutron star surface, forming a vacuum
gap[25] or a space-charge limited flow (SCLF) accelerator[3, 22]. Pairs are produced near the polar cap by interaction
of high-energy photons with the strong magnetic field. In thepolar cap SCLF models, acceleration is slower near the
last open field line which is assumed to be a boundary whereE|| = 0. A narrow slot gap forms[2] where particles
cannot produce pairs, theE|| is unscreened and acceleration continues to high altitude[21]. Outer gap accelerator
models [7, 24] focus on regions in the outer magnetosphere that cannot fill with charge, since they lie along open
field lines crossing the null surface,~Ω · ~B = 0, whereρGJ reverses sign. Charges pulled from the polar cap cannot
populate the region between the null surface and the light cylinder, and a vacuum gap forms. Since the magnetic field
is relatively weak in the outer gap, pairs are produced by interaction of high-energy photons with X-rays from the
hot neutron star surface. Both slot gaps and outer gaps can only exist in younger and millisecond pulsars that have
sufficient pair multiplicity[14, 33].
Figure 1 shows predicted sky emission patterns (phase plots) and pulse profiles for polar cap (PC), slot gap (SG) and
outer gap (OG) models. Model profiles are generated by cutting through the phase plots at constant observer angle. All
three models can produce widely spaced, double-peaked profiles similar to that of the Vela pulsar. The PC model can
generate such profiles only for small observer and inclinatio angles (<∼10
◦). SG and OG models generate caustics, due
to relativistic phase shifts of high-altitude emission, that may be seen at larger viewing angles(see [12, 11] for more
details). In the SG model, an observer views caustic emission from both poles, whereas in OG models, an observer can
Energy [MeV]
310 410 510
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 Polar Cap model prediction (Daugherty & Harding 1996).
 Polar Cap model, 1 yr LAT survey 
 Outer Gap model prediction (Romani 1996) 
 Outer Cap model, 1 yr LAT survey 
 EGRET data points (Thompson et al. 1999)
Vela Pulsar: Polar Cap vs. Outer Gap scenario observed by the LAT
FIGURE 2. Simulated spectra of the Vela pulsar observed in 1 year by LATassuming polar cap and outer gap models[23],
compared with observed EGRET spectrum.
view emission from only one pole. Although the model profileslook similar, there are several distinguishing details
that more sensitive measurement may verify. PC and SG modelspredict off-pulse emission throughout the entire pulse
phase, while OG models do not. Recent OG models[28] include emission from both poles, but this adds only a trailing
edge to the peaks. The SG model predicts a possible enhancemet on the trailing edge of the first peak, caused by
overlapping field lines near the light cylinder (an effect which forms the first peak in the OG model).
PC models predict that the pulsed spectrum from polar cap cascade will have a sharp (super-exponential) high-
energy cutoff due to attenuation by magnetic pair production and photon splitting[4]. The energy of these cutoffs is a
function of the local magnetic field strength, pulsar periodan the emission altitude and lies between 1 and 20 GeV.
EGRET detected indications of spectral cutoffs, but did nothave the high-energy sensitivity to distinguish a super-
exponential from the simple exponential cutoffs predictedby high-altitude SG and OG models. GLAST should be
able to measure the shape of the spectral cutoffs well enought make this distinction and therefore place strong limits
on the altitude of emission. Simulations of the Vela pulsar spectrum (Figure 2) show that in 1 - 2 months of observation,
the GLAST LAT will be able to measure the cutoff well enough todistinguish PC from OG model shapes[23]. If sharp
cutoffs are measured in a number of pulsar spectra, then one can also look for the predicted correlation between cutoff
energy and surface field strength.
WHAT ARE THE HIGH-ENERGY RADIATION MECHANISMS?
All models predict that the high-energy spectra are composed f multiple radiation components, although not all
necessarily contribute in the LAT energy range. Curvature radiation (CR) from primary particles, synchrotron radiation
(SR) from pairs and Inverse Compton scattering (ICS) components generally are present for all models. In addition,
there is a thermal X-ray component that may be hidden for Crab-like pulsars. PC pair cascade spectra[8] comprise
a CR component at high energies that is often cutoff by pair production before the pair SR component dominates,
although in pair-starved pulsars the SR component may be much weaker. An ICS component from pairs scattering
thermal X-rays from the neutron star surface may appear at X-r y energies[32]. In OG models, the pair SR component
is dominant for Crab-like pulsars and a transition between CR and SR components may be visible. The predicted
ICS component in OG models comes from primary particles scattering soft photons from various sources and appears
above 100 GeV. Although air Cherenkov telescopes have search d for this component for many years, it was not been
detected[1] and it is therefore very unlikely that GLAST will see it. The radiation from the high-altitude SG consists
of a CR component from primary electrons at the highest energies, and a SR component from both primaries and pairs
from lower altitude cascades that resonantly absorb radio em ssion at higher altitudes[16].
GLAST LAT observations should be able to distinguish multiple spectral components in two ways. Better sensitivity
and energy resolution will enable detection of transitionsin the phase-averaged spectra. In fact, the Crab pulsar
spectrum seems to have a transition between multiple components at an energy around 100 MeV[18] well within
the LAT range. Furthermore, the LAT will obtain phase-resolved spectra having error bars five times smaller than
EGRET. These much improved statistics will enable measurement of spectral components, cutoff energy and shape as
a function of phase to make more detailed comparison with model predictions. For example, PC models predict that
the high-energy cutoffs are sharpest at the pulse peaks[8] and the cutoff energy is lower in the first peak[9].
ARE PROCESSES THE SAME FOR ALL PULSARS?
It is quite possible that accelerator type, spectral formation, radiation processes and emission altitude depend on
pulsar parameters such as period, age or surface field strength. To explore this, one can look for patterns in both
multiwavelength profiles and in broadband spectra. The EGRET pulsars showed an increase in spectral hardness with
age[30], causing the power peak of the broadband spectrum tomove from hard X-rays (in the case of the Crab) to high-
energyγ-rays for the middle-aged pulsars. This indicates that the mechanism(s) that contribute to the non-thermal X-
ray emission are less dominant in older pulsars. Most modelspr dict this behavior, since the pair cascade multiplicity
decreases with age, and it is radiation from pairs that contributes most at X-ray energies. It will be interesting to see if
this trend persists in the larger population of pulsars thatLAT will detect.
The behavior of profiles across energy bands displays fewer clea patterns. For most EGRET pulsars, theγ-ray
peaks do not resemble those at other wavelengths and are not phase-aligned. Only the Crab shows phase-alignment
of the peaks at all wavelengths from radio toγ rays. Although a trend is hard to establish among the EGRET pulsars
themselves, if one examines the profiles of X-ray pulsars onesees that those having short periods (including several
millisecond pulsars) show more phase alignment. This behavior is not simply related to the patterns in the broadband
non-thermal spectra since it also involves the radio profilealignment, and is more probably related to emission
geometry. For example, the smaller magnetospheres of pulsars with shorter periods may put more of the emission at
different wavelengths at high altitudes (relative to the light cylinder) so that caustic effects will align all the emission
in phase[12].
ARE THERE γ-RAY MILLISECOND PULSARS?
A number of studies have predicted that millisecond pulsarshould have a high-energy CR radiation component
originating from polar cap acceleration and extending above 10 GeV[5, 19, 15]. Because of their very low surface
magnetic fields, the pair production attenuation cutoffs are at much higher energies than in normal pulsars and typically
occur above 100 GeV. The sensitivity of the GLAST LAT to thesehigh energies will make millisecond pulsars ideal
sources, but how many will be detectable will depend on whether our viewing angle sees much of the low altitude CR.
The presence of pulsed X-ray emission, as many millisecond pulsars show, may not necessarily be a guide since the
X-rays may originate from higher altitude[15]. EGRET marginally detected the millisecond radio and X-ray pulsar
J0218+4232, but the spectrum is very soft. GLAST LAT observations will certainly confirm this detection and give a
clearer picture of theγ-ray spectrum. CR emission from particles accelerating in the outer gaps of ms pulsars cannot
extend above several GeV in present models (Hirotani, priv.comm.) so that detected emission above 10 GeV from
these sources must originate from near the polar cap. However, the cascades from outer gap-accelerated particles
moving downward toward the polar cap may radiate CR above 10 GeV[31]. But in this case, one would expect that
theγ-ray pulses will be out of phase with the radio pulses.
WHAT IS THE RATIO OF RADIO-LOUD TO RADIO-QUIET γ-RAY PULSARS?
The number of radio-loud and radio-quiet pulsars that GLASTdetects after a 1 to 2 year survey will give us strong
constraints on theγ-ray emission geometry and type of accelerators that are operating. PC, SG and OG models give
very different predictions for the ratio of the radio-loud to radio-quiet populations, so estimating this ratio will be
a sensitive diagnostic. In the population synthesis studies, the term radio-quiet refers toγ-ray sources that are not
detectable by current radio surveys, either because the radio beam is just not visible at our viewing angle or because
the radio emission is absent or very weak. PC models predict the highest ratio of radio-loud to radio-quiet pulsars,
since theγ-ray and radio beams are co-aligned with the magnetic axis. High-altitude emission models predict much
lower ratios and many more radio-quiet pulsars, since theγ-ray beams are most often counter-aligned with the radio
beams. Current predictions are shown in Table 1. The resultsof these studies indicate that if the LAT finds many of
TABLE 1. Predicted GLAST pulsar populations
Normal pulsars Millisecond pulsars
Radio-loud Radio-quiet Radio-loud Radio-quiet
Low Altitude Slot gap [10][27] 81 43 16 99-131
High Altitude Slot gap [13] 5 49
Outer Gap [13] 1 258
[17] 78 740
the EGRET unidentified source near the Galactic plane are radio-loud pulsars, then the emission must come from a
low-altitude accelerator. Many of the simulated radio-quiet millisecond pulsars are located outside the bounds of the
surveys, but are bright enough to possibly be detected by follow-up radio observations.
SUMMARY
GLAST LAT observations promise to provide answers to some fundamental questions about pulsar acceleration and
emission, including the location and type of accelerator and the dominant modes of emission. In addition to detecting
many more radio pulsars inγ rays, GLAST will detect previously unknown radio-quiet pulsars as well as aid in
detecting new millisecond pulsars.
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