Pulsar Wind Nebulae in the SKA era

0.19 MB·English

by  J. D. Gelfand

Pulsar Wind Nebulae in the SKA era
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n
a J. D.Gelfand∗ab, R.P.Bretoncf, C.-Y.Ngd, J.W.T.Hesselse, B.Stappersf,M.S. E.
J
Robertsa, A.Possentig

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aNewYorkUniversityAbuDhabi,POBox129188,AbuDhabi,UnitedArabEmirates;E-mail:

] [email protected],[email protected]
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bCenterforCosmologyandParticlePhysics,NewYorkUniversity,MeyerHallofPhysics,4
H
WashingtonPlace,NewYork,NY10003
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h cSchoolofPhysicsandAstronomy,UniversityofSouthampton,Southampton,UnitedKingdom;
p
E-mail: [email protected]
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o dDepartmentofPhysics,TheUniversityofHongKong,PokfulamRoad,HongKong;E-mail:
r
[email protected]
t
s
eASTRON,Postbus2,7990AADwingeloo,TheNetherlands/UniversityofAmsterdam;E-mail:
a
[ [email protected]

1 fTheSchoolofPhysicsandAstronomy,TheUniversityofManchester,Manchester,United
v Kingdom;E-mail: [email protected]
4 gAstronomicalObservatoryofCagliari,Selargius,Italy;E-mail:
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[email protected]
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Neutron stars lose the bulk of their rotational energy in the form of a pulsar wind: an ultra-
.
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relativisticoutflowofpredominantlyelectronsandpositrons. Thispulsarwindsignificantlyim-
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5 pactstheenvironmentandpossiblebinarycompanionoftheneutronstar,andstudyingtheresul-
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tantpulsarwindnebulaeiscriticalforunderstandingtheformationofneutronstarsandmillisec-
:
v
ondpulsars,thephysicsoftheneutronstarmagnetosphere,theaccelerationofleptonsuptoPeV
i
X
energies,andhowtheseparticlesimpacttheinterstellarmedium. WiththeSKA1andtheSKA2,
r
a itcouldbepossibletostudyliterallyhundredsofPWNe indetail,criticalforunderstandingthe
manyopenquestionsinthetopicslistedabove.
AdvancingAstrophysicswiththeSquareKilometreArray
June8-13,2014
GiardiniNaxos,Italy
∗
Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
PulsarWindNebulaeintheSKAera J.D.Gelfand
1. Introduction
Roughly every 100 years, a neutron star is born in the Milky Way (e.g., Faucher-Giguère &
Kaspi 2006). Therotational energy of aneutron star powers ahighly relativistic, magnetized out-
flowcalled a“pulsar wind”(e.g.,Goldreich &Julian 1969), whichcreates a“pulsar windnebula”
(PWN) as it expands into its surroundings. The properties of the PWN depend on how this wind
is generated inside the neutron star’s magnetosphere, how PeV and higher energy particles are
generated insidethisoutflow,andthewind’sinteraction withitssurroundings.
This is true for neutron stars young and old, isolated and in binaries. When the neutron star
is young (. 104−105 years old), it is still inside the supernova remnant (SNR) created by the
progenitor explosion, and the expansion of the PWN inside the SNR creates a “composite SNR”
(Helfand & Becker 1987). As described in §2, not only do such systems allow us to study the
generation andproperties ofthepulsar windforthemostenergetic neutron stars, theycanbeused
to determine the neutron star’s initial spin period and the mass and initial kinetic energy of the
supernova ejecta – important quantities for understanding how neutron stars are formed in these
explosions. Asdescribedin§3,inbinarysystemsthePWNisproducedbytheinteractionbetween
the neutron star’s pulsar wind and its companion. By studying such PWNe, one can measure the
properties of the pulsar wind under very different conditions than in composite SNRs and gain
valuable insight into the last stages of the formation of millisecond pulsars (MSPs). Last, but not
least,asdescribedin§4,olderneutronstarsmovingsupersonically throughtheinterstellarmedium
(ISM)alsoproducePWNe. StudyingthesePWNeisimportantforunderstandingthemagneticfield
structure ofthepulsar, andhowitinteracts withitssurroundings.
Therefore, PWNeplay an important role in many areas of astrophysics – from the explosion
mechanism of core-collapse supernovae to the physics of magnetized plasmas to the acceleration
ofparticlesinmanydifferentphysicalconditions. Asdescribedin§5,thesignificantimprovements
incollectingareaandobservingcapabilitiespromisedbytheSKA1andSKA2hasthepotentialto
revolutionize thisfieldbyincreasing thenumberofwell-studied PWNeby∼10−100×,allowing
forthestatistical studiesneeded toanswertheabovequestions.
2. CompositeSupernova Remnants
When the pulsar wind leaves the neutron star magnetosphere, it is thought to be a primar-
ily equatorial outflow comprised of regions of alternating magnetic field polarity whose energy is
mainly intheform ofmagnetic fields(e.g.,Bogovalov 1999). Theconfinement ofthepulsarwind
by the surrounding medium creates a “termination shock,” where the “cold” pulsar wind is con-
vertedtoa“hot”outflow(e.g.,Kennel&Coroniti1984). ThePWNisthenformedbytheexpansion
oftheshocked, nowparticledominated, pulsarwindintothesurrounding medium. Thesignificant
fraction of Galactic TeV g -ray sources associated with PWNe (∼ 40%; Wakely & Horan 2008)
requires that these objects produce extremely high energy particles. While observations support
this general picture, many basic questions remain unanswered: How is the pulsar wind generated
in the magnetosphere? What is responsible for converting the pulsar wind from a magnetically
dominated toaparticledominatedoutflow? Howareparticles accelerated intheseobjects?
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PulsarWindNebulaeintheSKAera J.D.Gelfand
Photon Energy [eV]
10-5 100 105 1010
10-11
10-12
10-13
-2m]
-1 [ergs s c 1100--1154
Fn
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10-16
10-17
1010 1015 1020 1025
Frequency n [Hz]
Figure 1: Left: Broadband SED of PWN G54.1+0.3 overlaid with the best fit model SED. Right: The
model-predicted 60 MHz flux density S of G54.1+0.3 as a function of the initial spin period P of the
60 0
central pulsar. The color bar indicates the c 2 of the model fit to the observed data, with red (lower c 2)
indicatingabetterfit(Gelfandetal.submitted).
Answering these questions requires studying, in detail, the PWNeproduced by the youngest,
and most energetic, neutron stars. Understanding how particles are created in the neutron star’s
magnetosphere requires measuring thetotalnumberofparticles produced byapulsaroveritslife-
time. This is possible only for pulsars whose PWN is detected at both radio and g -ray energies
(e.g., de Jager 2007; Gelfand et al. submitted). Currently, only ∼10 pulsars meet these criteria
(e.g., Roberts 2004), with the SKA and the Cerenkov Telescope Array (CTA), this number could
increase byafactorof∼5−10×(§5). Ionsinthepulsarwindcanexplainboththehighaccelera-
tionefficiencyofPWNe(e.g.,Amato&Arons2006)andthedetectionofvariable“wisps”nearthe
termination shock of several PWNe (e.g., Spitkovsky & Arons 2004), as can magnetic reconnec-
tion in the pulsar wind before (e.g., Kirk & Skjæraasen 2003), at (e.g., Lyubarsky 2003), or after
the termination shock (e.g., Porth etal. 2013). Distinguishing between these models requires sen-
sitivemeasurements ofaPWN’sradiopolarization structureandspectrum (e.g.,Olmietal.2014).
Currently, thisispossibleformaybeahandfulofPWNe–toofewtodrawanystrongconclusions.
WiththeSKA,itshouldbepossible fordozens(§5).
As mentioned in §1, when a neutron star is young and most energetic, it is likely inside the
SNR produced by the progenitor explosion. In this case, the evolution of a PWN is sensitive to
the initial spin period of the central neutron star as well as the mass and initial kinetic energy of
the supernova ejecta (e.g., Chevalier 2005; Gelfand et al. 2009). This can be done using models
for the evolution of a PWN inside a SNR (Figure 1, e.g., Gelfand et al. submitted), but requires
measuring the current spin-down luminosity and characteristic age of the central neutron star, the
radio, X-ray, and g -ray spectrum of the PWN, and the radius of the surrounding SNR. Currently,
only ∼ 10 composite SNRs meet this criteria (e.g., Bucciantini et al. 2011; Torres et al. 2014),
since only ∼50% of all PWNe are associated with a pulsar (e.g., Roberts 2004), <50% of these
are associated with a SNR, and many of the remaining PWNe detected at g -ray energies (e.g.,
Ferrand&Safi-Harb2012)areundetectedintheradio. Evenworse,apreciseestimateoftheinitial
spin-period requires measuring the braking index of the central pulsar (e.g., Gelfand et al. 2014),
currently accomplished for <5 sources. As described in §5, the SKA could increase this number
by ∼5−10×, enabling one to directly test different models for the formation of neutron stars in
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PulsarWindNebulaeintheSKAera J.D.Gelfand
core-collapse supernovae (e.g.,Watts&Andersson2002;Blondin&Mezzacappa 2007).
Lastly,asmentionedabove,PWNedominatetheluminousTeVg -raypopulation oftheMilky
Way(Carriganetal.2013),withTeVPWNetypicallyassociated withpulsarswhosecharacteristic
ages are t . 105 years (Wakely & Horan 2008). Therefore, the majority of these pulsars, and
ch
PWNe,willbeinsidetheSNRoftheirprogenitor explosion evenifthisSNRisnotdetected. With
the SKA1 and SKA2, we will determine if this is true for the considerable number of currently
unidentified Milky Way TeV sources, by discovering a coincident “young” radio pulsar, diffuse,
flat spectrum radio emission characteristic of a PWN, and/or a surrounding steep-spectrum shell
suggestive of a SNR. Additionally, the SKA1 and SKA2 will discover PWNe around energetic
pulsars whose radio beams donot point towards the Earth. Forallthese objects, thefluxdensities
measured by SKA1-LOWwill allow us to estimate their initial periods even if pulsed emission is
notdetected.
3. Neutron Star Binaries
If a neutron star is in a binary system, the highly relativistic “pulsar wind” powered by its
rotationalenergywillinteractwithitscompanion. Theinteractionbetweentheneutronstar’spulsar
wind and its stellar companion leads to an intrabinary shock which possibly accelerates particles
tohigh energies (e.g., Bogdanov etal. 2011), fillingthesystem withplasma. Evidence forplasma
production inpulsarbinarieshasbeenobservedfromhigh-massX-raybinaries(e.g.,Moldónetal.
2014) to “black widow” and “redback” systems (e.g., Roberts et al. 2014), where the pulsar wind
actually ablates material from the companion star (Figure 2, e.g., Fruchter et al. 1988). Studying
theintrabinaryplasmaprobesthepulsarwindmuchclosertotheneutronstarmagnetosphere(Pétri
&Dubus2011)thanthesystemsdescribedin§2,whereextremelydifferentconditionsmayprevail.
Infact,analysisofonebinarypulsarsystemsuggeststhewindisstronglymagnetizedinthisregime
(Bogdanovetal.2011),whilethestudyofthePWNedescribedin§2requireaweaklymagnetized
windfurtherfromtheneutronstar. Ifcorrect,thisplacesstrongconstraintsonmodelsformagnetic
reconnection inthepulsar wind(§2), thecurrently leading theoryforparticle acceleration inthese
systems(e.g. Kirk&Skjæraasen2003).
Typically,suchstudiesrequiredetectingorbitally-modulatednon-thermalX-rayemissionfrom
the binary (e.g., Bogdanov etal. 2011), not achievable witha radio telescope like the SKA.How-
ever, as described in §5, the collecting area of the SKA2 could increase by > 10× the number
of binary pulsars – providing much needed targets for these X-ray studies. Additionally, inhomo-
geneities and/or free-free absorption in the cloud of plasma generated at the intrabinary shock is
believed to be responsible for the “eclipses” in the pulsed radio emission (Fig. 2) observed from
∼50 binaries pulsars (Roberts 2013; Freire 2013). By simultaneously measuring the pulsed and
unpulsed continuum radio flux of these systems, it is possible to determine the density, geometry,
and filling factor of this plasma as well as the structure of its magnetic field – especially if there
are simultaneous or contemporaneous g -ray observations. With the SKA1 and SKA2, we expect
to make such measurements for ∼300 eclipsing binaries (§5), allowing one to determine how the
properties of the plasma produced in this interaction depend on the characteristics of the neutron
starandbinarysystem(e.g.,neutronstarspin-downluminosity,binaryseparation,companionmass
andradius).
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PulsarWindNebulaeintheSKAera J.D.Gelfand
Figure2: Eclipsesin thepulsedradioemissionfromPSR J1023+0038(Hessels, priv. comm.) aswellas
an artist’s impressionof the cloudofplasma resultingfromthe ablationofits low-masscompanionby its
pulsarwind(BillSaxton,NRAO).
Lastly, the recent discovery of several millisecond pulsars (MSPs) / low mass X-ray binaries
(LMXBs) transitioning between an “accretion”-dominated phase where no radio pulsations are
detected and a MSP phase which shows no evidence for accretion (e.g., Archibald et al. 2009;
Bassa et al. 2014; Papitto et al. 2013; Stappers et al. 2014) promises important insight into the
formationofMSPsaswellasunderstandingaccretionontomagnetizedneutronstars. Sincestudies
of intermittent pulsars strongly suggest that the emission of radio pulses is connected to their loss
ofrotationalenergy(e.g.,Youngetal.2013;Lietal.2014),thephysicalmechanismresponsiblefor
generatingapulsarwindlikelyplaysanimportantroleinthistransition. Onlybystudyingtheradio
and higher energy (X-ray, g -ray) properties of such systems will it be possible to understand the
interplay between accretion and aneutron star’s magnetosphere. Asdescribed in §5, the potential
multi-beam and high-frequency capabilities of the SKA are critical for understanding the physics
of these transitions which may comprise the final stage in the formation of a MSP (though some
systemsmightbestucktransitioning back-and-forth toaLMXBstate).
4. IsolatedPulsarsinthe InterstellarMedium
Thepulsarwindsofisolated, olderneutron starswillalsosignificantly impacttheirsurround-
ing. In this case, the pulsar wind is confined by the ram pressure created by the neutron star’s
supersonic motion. The morphology (e.g., Vigelius et al. 2007) and the magnetic field structure
(e.g., Bucciantini et al. 2005) of such PWNe are sensitive to the density structure of the ISM,
the speed and direction of the neutron star’s spatial velocity, and the geometry of its pulsar wind.
Additionally, X-ray observations suggest such PWNe efficiently inject high-energy particles into
the surrounding ISM, often in directions misaligned with the neutron star’s proper motion (e.g.,
Bandiera(2008); DeLucaetal.(2013); Marellietal.(2013);Pavanetal.(2014)).
Understanding the structure of these pulsar winds, and how high energy particles escape the
PWN, requires mapping the polarized radio emission for a range of neutron star and ISM proper-
ties. Currently, this ispossible for.5−10 systems, but eventhis small sample showssignificant
diversity intheirmagneticfieldstructure –forexample,the“FryingPan”(G315.9−0.0)PWNhas
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PulsarWindNebulaeintheSKAera J.D.Gelfand
Figure3: TheradionebulaeG315.9−0.0(left;Ngetal.2012)andG319.9−0.7(right;Ngetal.2010. The
~
locationoftheassociatedpulsarsareindicatedbythegreencrosses,whilethepolarizationB-vectorisshown
bytheredlines.
amagnetic fieldaligned withthepulsar’s proper motion (Ngetal.2012), whilethemagnetic field
of G319.9−0.7 has a helical structure Ng et al. (2010) (Figure 3) – suggesting a dependence on
theflowconditions, pulsarspeed,and/ormisalignmentbetweenthepulsarrotationaxisandproper
motion. Understanding therelationship betweenthesedifferent physical parameters requires mea-
suring the magnetic field structure of many more PWN which, as described in §5, will become
possible withtheSKA.
5. Requirements forSKA1and SKA2
Thankstotheconsiderable improvement insensitivity promised bytheSKA1andtheSKA2,
theseinstrumentsshouldbecapableofmeasuringtheradiomorphology,spectrum,andpolarization
propertiesof∼100PWNe–asignificantimprovementoverthe∼10forwhichsuchmeasurements
exist, and increase the number of eclipsing binary pulsars from ∼ 50 to ∼ 300. Such sample
sizes are needed to determine how a neutron star’s spin-down luminosity, age, and environment
affects the evolution of its PWN. Additionally, the SKA1 and especially SKA2 will be able to
measure properties of a PWN never before possible. These studies depend less on the collecting
areaoftheSKA1,andthereforearemoreimmunetoa50%reductionofitscapabilities, butonthe
configuration andcapabilities ofitstelescopes asdescribed below:
Discover pulsars: Currently, ∼40−50% of PWN candidates are unassociated with a pulsar in
anywaveband(Roberts2004),problematicfortheanalysesdiscussedin§2and§4. Whilebeaming
explains some of the missing pulsars, for many PWNe it likely results from the limitations of
current observatories. Thelarge collecting area oftheSKA1-LOWshould increase thenumber of
pulsars detected in nearby PWN,but the high dispersion measure (DM)and scattering timescales
expected for pulsars on the far side of the Milky Way require that the SKA1-MID also be able to
detectpulsars.
Monitor a PWN’s pulsed and unpulsed radio emission: Monitoring the timing properties of
young, isolated pulsars is needed to measure their braking indices which, as described in §2, is
critical forestimating their initial spin period. Foreclipsing binary pulsars, monitoring the pulsed
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PulsarWindNebulaeintheSKAera J.D.Gelfand
emission allows one to detect changes in the orbital phase and length of radio eclipses changes,
and monitoring the pulsed emission of MSP/LMXBis critical for determining if the pulsed radio
emissiondisappearsbeforeoraftertheresumptionofaccretionontotheneutronstar(§3). Boththe
SKA1-LOWandSKA1-MIDrequirethesecapabilitiessincethepossiblepresenceofdenseplasma
in these systems willresult infrequency dependent behavior. Additionally, observations of young
PWNe every few months are required to measure the variability of “wisps” near the termination
shock,criticalfordeterminingifionsarepresentinthepulsarwind(§2). Sincethesewispsareonly
afewarcseconds insize(e.g.,Bietenholz etal.2004),thisisbestdonewiththehigherfrequencies
ofSKA1-MID.
Conduct pulsar gating and VLBI observations: While all pulsars are believed to generate a
PWN,radioPWNearedetected around<10%ofyoung, energetic pulsars. Onepossibility isthat
the radio PWNismasked incontinuum observations bythe pulsar’s pulsed emission. With pulsar
gating, it is possible to image the region when the pulsar is “off”, making it possible to detect
faint radio PWNe(Gaensler etal.1998). Thisisparticularly important forSKA1-LOW,duetoits
lower angular resolution and increased brightness ofthe pulsar relative to thePWN.Pulsargating
of eclipsing binary pulsars allows simultaneous measurements of its pulsed and unpulsed flux,
criticalfordeterminingtheeclipsingmechanism(§3),andsignificantlyimprovesmeasurementsof
a pulsar’s parallax and proper motion in VLBI observations – important for measuring the initial
spin periods of young pulsars in composite SNRs(e.g., Gelfand et al. submitted) and interpreting
themorphology of“bowshock”PWNe(§4).
ObservePWNeacrossabroadrangeoffrequencies: ThebroadbandradiospectrumofaPWN
insideaSNRisexpectedtohavenumerousfeatures(e.g.,Gelfandetal.2009). Theminimumparti-
cleenergyinjectedattheterminationshockwillresultina“break”atlow(n .1GHz)frequencies,
andmeasuringitsfluxdensitybelowthisbreakiscriticalforestimatingtheinitialspinperiodofthe
central neutron star (e.g., Gelfand et al. submitted; Figure 1). Ions in thepulsar windand/or mag-
neticreconnection downstreamofthetermination shock(§2)isexpectedtoleadtobroad“bumps”
inthespectrum athigher frequencies (n &10GHz;Olmietal.2014). Continuous frequency cov-
erage between the different SKA1 and SKA2 observing bands would improve measurements of
curvature inthePWN’sradiospectrum.
Detectemissiononlargeangularscales: Thenon-detection ofmanyTeVPWNeatradiowave-
lengths (e.g., Ferrand & Safi-Harb 2012) and SNRs around young pulsars / PWNe (e.g., Roberts
2004) likely results from the large angular size (∼ 30′− .1◦) and low radio surface brightness
of these objects. Current radio interferometers do not have the short baselines needed to detect
emission on these angular scales, and single-dish telescopes do not have the sensitivity needed
to detect this emission over the Galactic background. While SKA1-LOW and SKA1-MID will
have the needed sensitivity, a dense core is required to detect emission on the needed angular
scales.instantaneous SKA1-LOWsinglebeam
Measurethepolarization propertiesofPWNe: Last,butnotleast,mappingthemagnetic field
structureofyoung(§2)andold(§4)PWNerequiresspatiallyresolvedmeasurementsoftheirpolar-
izedintensity anddirection atmultiplefrequencies tobothcorrect forforeground Faradayrotation
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PulsarWindNebulaeintheSKAera J.D.Gelfand
and detect changes inits rotation measure (e.g., Ngetal. 2010). Furthermore, measuring changes
in the DM during the “eclipse” of a binary pulsar (e.g., Archibald et al. 2013) are important for
measuring the density and magnetic field structure of the intervening plasma (§3). These obser-
vations require high polarization purity of both SKA1-LOW and SKA1-MID, and sensitivity to
polarized emission over a small bandwidth or at higher (n >1 GHz) frequencies to avoid band-
width depolarization, especially for SKA1-MID. Furthermore, since a PWN’s radio emission can
behighlylinearlypolarized,anall-skypolarizationsurveywiththeSKA1-SURcouldidentifynew
PWNe,particularly aroundolder, isolatedpulsars.
ACKNOWLEDGMENTS: CYN is supported by a ECS grant under HKU 709713P. J.W.T.H. ac-
knowledgesfundingfromanNWOVidifellowshipandERCStartingGrant"DRAGNET"(337062).
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