GeV Emission from Pulsars

Zaven ArzoumanianCRESST/GSFC

[channeling Alice Harding, Dave Thompson,and others]

DC2 Gamma-raySky

Why do we care?

• Gamma-rays trace properties of the highest-energyparticles ⇒ probe the acceleration mechanism. In mostcases, lion’s share of luminosity is in gamma-rays.Particles go on to sculpt the surrounding medium.

• Explore how particles interact with the extremeenvironment: strong magnetic and electric fields,frame-dragging gravity, high currents, etc.

• What roles do neutron star properties (temperature,magnetic configuration) and viewing geometry (light-bending, aberration) play in the observed radiation?

• Handful of pulsed detections by EGRET have had aninordinate impact on our understanding of neutronstar magnetospheres.

Multiwavelength profiles of γ-ray pulsars

• Power peaked in γ-rays

• No pulsations detectedabove 20 GeV

• High-energy turnover

• Increase in hardnesswith age

• Thermal componentappears in olderpulsars

Broad-band spectra

High-energy emission models

!

B"Light

Cylinder

closed field

region

polar

cap

null charge surface

! . B = 0

outer

gap

slotgap

• Different pro-cesses canaccelerateparticles in themagnetosphere

• Pair-creationcascades,interactions withthermal surfacephotons likely

• Differentbeaming solidangles, spectralproperties

POLARCAPMODEL

α =100,ζobs=160

VelaSLOT GAPMODELα =450,ζobs=1130

OUTER GAPMODELα =650,ζobs=820

Model light-curves

PSR B1055-52Photon-counting limited:EGRET provided insufficientstatistics to comparemodels. In 2 years, LAT willprovide 25-30 times asmany photons.

Pulse Profiles

• Pulse shapes, peakseparations, phaserelations to other wave-lengths test models

• Sensitive to possiblemicrostructure and off-pulse emission

• Probe to 5 times greaterdistances

One year LAT simulation of the Crabpulsar, including sky background, andspectral indices in various phaseranges. Top: Input profile. Bottom:Results from a likelihood analysis forthe spectral index and flux in the samephase ranges. (Courtesy T. Reposeur)

Phase-resolvedspectroscopy

Measurement of spectralproperties as a function ofpulse phase (currentlylimited by largeuncertainties) will provideimportant diagnostics ofemission physics.

Courtesy M. Razzano

Spectral Cutoffs

No pulsed emission seenat TeV energies. Spectralgap between EGRET andCerenkov telescopes isimportant for constrainingemission models. GLASTwill close this gap.

Above 10 GeV, LAT is atleast 100 times moresensitive than EGRET.

Instead of the handful ofphotons seen by EGRET(red bars), LAT will seehundreds of photons atthese energies.

Pulsars at GeV energies

Undetected Undetected

How many pulsars will GLAST detect?

Population synthesis predicts several hundred � ��������in 1 year.Relative numbers of radio-loud/quiet are model-sensitive.LAT will allow blind periodicity searches of brightest unID-ed EGRET sources.

Does trend measuredby EGRET hold forother pulsars?

What happens asobserved luminosityapproaches total spin-down luminosity?

High-confidencegamma-ray pulsars

Lower-confidencegamma-ray pulsars

Luminosity vs. Voltage

Pulsar Population

With LAT sensitivity,detection thresholdmoves well into the bulkof the pulsar population.

In particular, millisecondpulsars now lie almostentirely to the left of theline, in the potentiallyobservable region, if thistrend remains valid.

B1821-24: screened acceleration

10-4

10-2

100

102

104

106

Energy (MeV)

10-10

10-8

10-6

10-4

MeV

/(cm

2 s

)B0 = 4.4 x 10

9 G

P = 3 ms

! = 1.0

PCA

HRI

EGRET

HEXTE

OSSE

GLAST

PSR B1821-24

H.E.S.S.

MAGIC

SR

ICS

CR

•Different processes can acceleratechargedparticles above NS surface, depending onthetemperature of the NS surface (SpaceChargeLimited Flow if T > Ti or Vacuum gap ifviceversa);•Accelerated charges initiate cascade atfewstellar radii above polar caps;•The Curvature Radiation-inducedcascadesare more stable and much important forhighenergyemission; IC is not stable;•Gamma-ray absorption from magnetic-fieldinduced pair production•Recent update based on high-altitude SlotGaps (1983)Model predictions:•Sharp cutoff (super-exponential due tophoton absorption);•Cutoff energy decreases with increasing B;•Luminosity proportional to the currentoutflow from poles;•Radio Quiet pulsars >> Radio Loudpulsars;

Polar cap models

LAT Pulsar Science: Phase-Resolved Spectra

Models have calculated phase-resolved spectra for some gamma-ray pulsars. Comparisonwith the data is largely limited by the data uncertainties.

LAT will provide high-quality phase-resolved spectra for the bright pulsars, with error barssmaller than these by about a factor of five.

Left: Polar Capmodel for Vela(Daugherty &Harding, 1996)Right: OuterGap model forCrab (Cheng,2004)

Multiwavelength supporting observations

-3 -2 -1 0 1 2

Log[Period (s)]

-20

-19

-18

-17

-16

-15

-14

-13

-12

-11

-10

-9L

og

(P

erio

d d

eriva

tive

)

B0 = 10 12

G

Radio pulsar

AXP

SGR

Radio quiet pulsar

HE pulsar

10 11 G

10 9 G

10 14 G

10 15 G

10 13 G

10 10 G

• ~1700 radio pulsars

• ~10 γ-ray pulsars

• ~30 X-ray pulsars

• 7 AXPs

• 5 SGRs

CR - screened

ICS - unscreened

Detections of Detections of MSPs MSPs at high energyat high energy

1 20 6>100Millisecondpulsars

8 12 16 1400Normalpulsars

γ-ray

steadyX-raypulsed

Radio

Most Most MSPs MSPs havehaveunscreened accelerationunscreened acceleration