ThePolariza,onFron,erKeithJahodaonbehalfoftheGEMSteam

ElihuBoldtMemorialSymposium6November2009

Fromanobserva,onalpointofview,X‐raypolarimetrystartswithanearly

completelyblankslate

KNOWNSINCE1978

• Probeblackholespinandaccre,ondiskgeometry

• directradia,onispolarizedparalleltothediskanddominatesatlowerenergies• returnradia,onispolarizedperpendiculartothedisk• polariza,onmagnitudeandorienta,onisdiagnos,c

• Probethehighmagne,cfieldsnearthesurfaceofneutronstars

• X‐raysarehighlypolarizedperpendiculartok‐Bplane• Propaga,oninhighfieldsprobesGReffects

• Characterizethemagne,cfieldsinsupernovaremnants

TheorypredictsthatX‐raypolarimetrywill

Photoelectric X-ray Polarimetry •  Exploits: strong correlation between the X-ray electric field vector and

the photoelectron emission direction

•  Advantages: dominates interaction cross section below 100keV

•  Challenge: •  Photoelectron range < 1% X-ray absorption depth (λx) •  Photoelectron scattering mfp < e- range

•  Requirements: •  Accurate emission direction measurement •  Good quantum efficiency

•  Ideal polarimeter: 2d imager with: •  resolution elements σx,y < e- mfp •  Active depth ~ λx •  => σx,y < depth/103 Auger

electron

X-ray

Photoelectron

φ

sin2θcos2φ distribution

E

µ= Nmax‐Nmin

Nmax+Nmin

N=A+Bcos2( φ ‒ φ0 )

µ =B/(2A+B)

Inprac,ce,thedistribu,onofes,matedtrackdirec,ons,evenforpurelypolarizedinput,ismorecomplicatedthanaprojec,onofthesin2θcos2φprobabilitydistribu,on.

Modulation and sensitivity

Elihu’sPolarimeter

Elihu’sPolarimeter–Physicaleffects

•  Photoelectricabsorp,on–  Efficiency~0.03at20keV

•  Scaeeringinthedetector–  Energylossandanglechangeviamul,plescaeers

•  Taggingthesignal•  Scin,llatortags9keVCu‐Kphoton

•  Distribu,onoftracks–  Angularresolu,onprovidedbyanelectroncollimator

Elihu’ssensi,vity

Elihu’ses,mateddetectorrateis~0.3ct/secfor1Crabsource.(assumes5cmaperture)

IfS>B,µ>0.1

Polariza,onoftheCrabnebulacouldhavebeendetectedinafewhours

TPC Polarimeter Concept •  Drift direction is perpendicular to X-ray propagation so efficiency is

decoupled from other considerations •  One dimensional images using strip readout •  Pixels are formed by time projection, coordinates [arrival time,strip location] •  Drift height determined by collimation of beam

GEM

ReadoutStripsX‐ray

DriiElectrode

Digi,zedWaveforms

Differen,atedWaveforms

Image

CSAs

Trigger

x

y

z

x

y

Photoelectron

e‐Drii

10

•  TheTimeProjec,onPolarimeteristheheartoftheGravityandExtremeMagne,smSmallExplorer

•  CurrentlyinPhaseB•  Launchin2014

•  Rota,onofthree‐axisstabilizedspacecraiforlowfalsemodula,onduetoinstrumentalsystema,cerror

•  Fullskyvisibility;~300sourcesaccessible,eachfor~8weeksevery6months

•  Straighmorwardopera,onsconcept•  9monthprogramof35targets

•  Noconsumables,life,me≥2yr

Gravity and Extreme Magnetism Small Explorer Concept

Benefits of Rotation

Blackholesbendx‐raysandrotatethepolariza,onaccordingtotheirspin

Accre,ondisk

Howtheimageofthediskwouldlookifwecouldresolvetheimageforablackholewithhighspin

12

KNOWNSINCE1978

NEWTERRITORY

Targetswithknownfluxesandpolariza,ones,mates

Blackholes

Neutronstars

Supernovaremnants

•  GEMSwillopenabrand‐newwindowonthesky.

•  Polariza,onintheX‐raybandisessen,allyunexplored:thepolariza,onofonlyoneobjectoutsidethesolarsystemhasbeenpreviouslydetected.

•  GEMSwillbe~100xmoresensi,vethananypreviousinstrumentandwillbeabletostudywelldozensofobjects.

• Discoveriesareexpectedfromblackholes,neutronstars,supernovaremnants.

GEMSwillopenthefron,erofX‐raypolarimetry