LOFAR: A powerful tool for pulsar studies

0.29 MB·English

by  B. W. Stappers

Proceedingsofthe363.WE-HeraeusSeminaron:“NeutronStars and Pulsars”(Postersand contributedtalks)
Physikzentrum BadHonnef, Germany,May.14−19,2006, eds.W.Becker,H.H.Huang, MPEReport291,pp.100-103
LOFAR: A Powerful Tool for Pulsar Studies
B. W. Stappers1, A. G. J. van Leeuwen2, M. Kramer3, D. Stinebring4, J. Hessels5
1 StichtingASTRON,Postbus2, 7990 AA Dwingeloo, TheNetherlands
2 Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, B.C. V6T 1Z1,
Canada.
3 University of Manchester, Jodrell Bank Observatory,Macclesfield, Cheshire SK11 9DL, UK.
7 4 Department of Physics and Astronomy,Oberlin College, Oberlin, OH44074
0 5 Department of Physics, McGill University,Montreal, QCH3A 2T8, Canada.
0
2

n
a
Abstract. The LOw Frequency ARray, LOFAR, will Table 1. LOFAR Sensitivity for Pulsar observationsa
J
havethe sensitivity,bandwidth,frequency rangeandpro-
9
cessing power to revolutionise low-frequency pulsar stud-

1 ies. We present results of simulations that indicate that a
Frequency Sensitivity BeamSize
v LOFAR survey will find approximately1500 new pulsars.
(MHz) (mJy)
9 These new pulsars willgiveus a muchbetter understand-
2 ′
ing of the low end of the luminosity function and thus al- 30 0.48 21
2 ′
1 lowforamuchmorepreciseestimateofthetruetotallocal 75 0.33 8.3
′
0 pulsarpopulation.Thesurveywillalsobeverysensitiveto 120 0.017 5.2
7 the ultra-steep spectrum pulsars, RRATs, and the pulsed 200 0.015 3.1′
0 radio emission from objects like Geminga and AXPs. We
h/ willalsoshowthatbyenablingus toobservesinglepulses a Integration times of 1 h, 2 polarisations, 32 MHz of band-
p fromhundredsofpulsars,includingmanymillisecondpul- width, 10% dutycycle and only using theLOFAR core.
-
sars, LOFAR opens up new possibilities for the study of
o
r emission physics.
t
s
of 4 MHz of bandwidth. This multibeaming capability al-
a
: lows for extremely wide fields of view to be monitored
v
1. Introduction and/or it allows for multiple experiments to be carried
i
X out simultaneously. Estimates of the LOFAR sensitivity
LOFAR will provide a low frequency radio telescope con-
r for pulsar observations using the core are given in Table
a centrated in the Netherlands with extensions into other
1 and more information about LOFAR in general can be
European countries. It consists of a core, an extended
found at www.lofar.org
array and a long baseline component which will have
maximum baselines of 2 km, 100 km and ∼1000 km re-
spectively. It will operate in the frequency range of 30
2. Pulsar Survey:
– 240 MHz which will be split into two bands, the low
band (optimised for 30–80 MHz) and the high band (op- Pulsarsaresteepspectrumobjectswhosepulsedfluxden-
timised for 115-240 MHz). Bandwidths of either 100 or sityusuallypeaksinthe100-200MHzrange(e.g.Malofeev
80MHz willbe digitisedwith 12-bitresolutionandit will etal1994).AsLOFARwillhaveunprecedentedsensitivity
bepossibletoselectatotalof32MHzofbandwidthtobe in exactly this frequency range this makes it an excellent
processed. There will be a total of 7700 low-band anten- instrument for carryingout an all-sky pulsar survey.How
nae and 7700 high-band tiles where each tile is made up many pulsars LOFAR will discover depends on the low
of a grid of 16 antennae. The antennae and tiles will be end of the pulsar luminosity function. There are indica-
dividedupintoatotalof77stations.Inthecoretherewill tions that the luminosity function turns overin the range
be 32 stations and the remaining 45 will be arranged in 0.3-1mJykpc2 (Lyne et al. 1998)howeveritislikelythat
a logarithmic spiral-like distribution outside of the core. surveyshavesofarbeenincompletealreadyatthe10mJy
All the antennae in a station will be combined to form a kpc2 level.LOFARwillbe able to measurethe low-endof
so-calledstation beam and it will be possible to exchange the pulsar luminosity function significantly better than
bandwidth for beams. For example one could decide to any previous survey allowing, for the first time, a much
have 8 station beams (each with an independent look di- more precise estimate of the true total local pulsar popu-
rectionwithinthefieldofviewoftheantennaortile)each lation.
Stappers et al.: LOFAR: A Powerful Tool for Pulsar Studies 101
Fig.1. Simulated detections for the LOFAR and Parkes Multibeam surveys, for 1-hour LOFAR pointings. Left)
projected onthe Galactic plane. Right) projected on the plane through the Galactic centre and sun, perpendicular to
the disk.
The apparent detection of a few neutron stars in- (suchasthepulsarperioddistribution)onecanstudy the
cluding Anomalous X-ray Pulsars and Magnetars only birth properties of neutrons stars, core collapse physics,
at frequencies near or below 100 MHz (e.g. Mal- the velocities and spatial distribution of pulsars, and the
ofeev et al 2005, Kuzmin & Losovsky 1997, Mal- physics of neutron stars in general.We note also that the
ofeev & Malov 1997r, Shitov & Pugachev 1997), the long pointings possible, because of the wide field of view,
existence of pulsars like B0943+10, which have flux in a LOFAR pulsar survey,makesit particularly sensitive
density spectra with a spectral index steeper than - to the recently discovered classes of infrequently emit-
3.0 (Deshpande & Radhakrishnan 1994), and millisecond ting neutron stars like RRATs (McLaughlin et al. 2006)
pulsars which have steep spectra which do not show and pulsars that are on for 10% or less of the time
a turn-over even at frequencies as low as 30 MHz (Kramer et al. 2006).
(e.g. Navarro et al. 1995), suggest that a large number of
pulsars is detectable only at low frequencies. Moreover,
3. A survey for extragalactic pulsars.
the radio beam of pulsars is knownto broadenat low fre-
quencies,increasingtheilluminatedskyandhencethede-
Spiralandirregulargalaxieswillhostyoung,bright,Crab-
tection probability at Earth. Indeed, the non-detection of
like pulsars,and due to its sensitivity, LOFARcan be the
Geminga at cm-radio wavelengths, a prominent rotation-
first telescope to find such pulsars besides those in the
powered pulsar visible at optical, X-ray and gamma-ray
Magellanic Clouds. If observed face-on and located away
wavelengths,servesasagoodexampleforapopulationof
from the Galactic disk, the scatter broadening to an ex-
neutron stars that may be detectable only with LOFAR.
ternal galaxy will be relatively low and thus a LOFAR
Withitssensitivitytotheseverydifferenttypesofneutron
surveywillhaveexcellentsensitivityforpulsarsofallspin
stars,aLOFARGalacticpulsarsurveywillbe instrumen-
periods (van Leeuwen & Stappers 2006). For a relatively
tal in providing a complete picture of neutron-star radio
close galaxy like M33, LOFAR could detect all pulsars
emission. more luminous than 50Jy kpc2. Our Galaxy hosts 10
We have carried out sophisticated simulations using pulsars which are brighter than that. There are at least
realistic population and scattering models to determine 20 Galaxies for which LOFAR will have good sensitivity
the optimum observing configuration, observing strategy to their pulsar population. Complementary to this nor-
and frequency for a pulsar survey with LOFAR (Figure 1 malpulsaremission,somepulsarsshowultra-bright’giant
andvan Leeuwen& Stappers 2006).If we extrapolatethe pulses’ that could be visible in evenmore remote galaxies
known low-luminosity tail, we find that a LOFAR all-sky (e.g. McLaughlin & Cordes 2003).
pulsarsurvey(thefirstsurveyoftheNorthernhemisphere A survey for extragalactic pulsars would allow us to
in10years)willfindaround1500newpulsars,almostdou- investigate if the bright end of the pulsar distribution in
bling the total number of pulsars known.The survey pro- other galaxies differs from our galaxy, and how that ties
vides a complete local census of radio-emitting neutron into galaxy type and star formation history. Such pulsars
stars, such as radio pulsars, AXPs and previous ”radio- can also help the understanding of the missing baryon
quiet” pulsars. Using the derived population properties problem, the history of massive star formation in these
102 Stappers et al.: LOFAR: A Powerful Tool for Pulsar Studies
Fig.3.AstackofsinglepulsesofPSRB0329+54obtained
with the Westerbork Synthesis Radio Telescope at a fre-
quency of 143 MHz with a bandwidth of 2.5 MHz. The
Fig.2. Coherently formed beams from the compact core
typical signal-to-noise ratio for each individual pulse in
and the inner 12-km of LOFAR, projected on the core of
these data is 30. LOFAR will have a factor of at least
M81,thesecondbestcandidategalaxyforanextragalactic
30 greater sensitivity, up to 12 times the bandwidth and
survey and also the home of supernova 1993J.
greaterrobustnessto interferencetherebygreatlyincreas-
ing the number of systems for which such studies can be
galaxies and also if sufficient numbers can be found they undertaken.
canbeusedtoprobetheturbulentinter-galacticmedium.
Single pulse studies atlowfrequenciesareparticularly
interesting,becausemicrostructure(seePopovetal.2002
4. Pulsar emission physics
and references therein) and subpulses tend to be stronger
The sensitivity and frequency range of LOFAR open up there.Indeed,wewouldexpectthedensityimbalancesand
the low-frequency window of pulsar emission to exciting plasma dynamics to be the most noticeable at low radio
newstudies.Itispreciselyinthe LOFARfrequencyrange frequencies.Thedurationandseparationtimescalesofmi-
wheresomeofthemostinterestingchangesinpulsarradio crostructure and subpulse emission are similar to those
emission can be observed, including the very significant predictedbytheoreticalmodelsofpulsaremissionphysics.
broadening of the pulse profile (Cordes 1978), changes As the exact timescales are depending on the physics the
in the form of the pulse profile (Kuzmin et al. 1998, detailedstudy ofthistypeofemissioncanbetterdifferen-
Rankin 1983), a turn-over in the flux density spectrum, tiate between models of pulsar emission. While the high
and it is also the frequency range where propagation ef- time resolutionrequirementofthese observationsand the
fects in the pulsar magnetosphere may be expected to existence of interstellar scattering will limit the number
belargest(e.g.Cheng & Ruderman 1979 ,Petrova 2001). of sources that can be studied, the remaining number of
Studyingthelatter,inparticularwithsimultaneousmulti- manytensofsourcesforwhichstudiescanbemade,makes
frequency observations of single pulses, will reveal inter- thisaparticularexcitingaspectsofLOFARradiostudies.
estingcharacteristicsofthepulsarmagnetosphere,suchas Combining simultaneousmulti-frequency observations
particledensitiesandbirefringenceproperties,thatwillul- of radio pulsars using multiple telescopes has provedvery
timatelyleadtoabetterunderstandingoftheworkingsof successfulatrevealingmuchabouttheprocessesdiscussed
pulsars. above (e.g. Karastergiouet al. 2001, Kramer et al. 2003).
Our calculations show that in the LOFAR high band Unfortunately such studies at low frequencies have been
single pulses can potentially be detected with reasonable hampered by the fact the majority of low-frequency facil-
signaltonoisefromuptoone-thirdofpulsarswhileinthe ities available for this work are transit instruments and
low banditis aboutone-quarterofpulsars.This is a very so only short simultaneous observations are possible. Us-
large increase on what has previously been possible, and ing the frequency range and tracking ability of LOFAR
whencombinedwithLOFAR’sabilitytotracksources,al- in observations simultaneous with high frequency instru-
lowsfortherichstudyoftheemissionphysicsofradiopul- ments will further open up this extremely rich source of
sars.FurthermoreLOFARwillalsobeabletodetectsingle information on the way in which pulsars work.
pulses from millisecond pulsars (MSPs), something which
has so far been done for less than a handful of sources
5. Interstellar medium studies
(e.g.Edwards & Stappers 2003), andwill allow us for the
firsttimetocomparethemodechanging,nullinganddrift- Pulsars are excellent probes of the ionized compo-
ingsubpulsepropertiesofMSPswiththeiryounger,higher nent of the interstellar medium through scintillation
magnetic field, slower spinning brethren. (Rickett 2001), dispersion measure, and Faraday rotation
Stappers et al.: LOFAR: A Powerful Tool for Pulsar Studies 103
studies (e.g. Han et al. 2003). Scintillation studies have frequency window for pulsar emission studies. The wide
been revolutionized in the last five years by the discov- fieldsofviewandsensitivitywillalsoallowforanefficient
ery of faint halos of scattered light extending outward and sensitive survey of the whole Northern sky enabling
10–50 times the width of the core of the scattered image thediscoveryofhundredsofnewpulsarsincludinganum-
(Stinebring et al. 2001). This, in turn, gives a wide-angle ber ofexoticobjects andsystems.Asurveyoflocalgroup
view of the scattering medium with milliarcsecond reso- galaxies will also likely detect the first truly extragalac-
lution, andthe illuminated patchscansrapidly acrossthe tic pulsars. Pulsars observed with LOFAR will provide
scatteringmaterialbecauseofthehighpulsarspaceveloc- excellent probes of the interstellar, and potentially inter-
ity.LOFARwillbeanoutstandinginstrumentwithwhich galactic,mediumandwillbeespeciallyusefulforgaininga
to studythis phenomenon,particularlyforstrong,nearby betterunderstandingofthelocalinterstellarmedium.Fur-
pulsars.The dynamic nature ofthese phenomena also fits thermore the exciting possibilities of multibeaming and
wellwith LOFAR’s excellentmonitoring capabilities.Ina radio-sky monitoring open up new avenues of pulsar and
sense, this scintillation imaging will allow us to monitor radio emitting neutron star research.
the range of interstellar conditions encountered along a
particular sight line. This monitoring will not be particu-
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quency range and unprecedented sensitivity, the low-