ApJ Letters PreprinttypesetusingLATEXstyleemulateapjv.5/2/11 FIVE NEW MILLISECOND PULSARS FROM A RADIO SURVEY OF 14 UNIDENTIFIED FERMI-LAT GAMMA-RAY SOURCES M. Kerr1,2,3, F. Camilo4,5, T. J. Johnson6,7, E. C. Ferrara8, L. Guillemot9, A. K. Harding8, J. Hessels10,11, S. Johnston12, M. Keith12, M. Kramer13,9 S. M. Ransom14, P. S. Ray15 J. E. Reynolds12, J. Sarkissian16, and K. S. Wood15 ApJL accepted 2 ABSTRACT 1 Wehavediscoveredfivemillisecondpulsars(MSPs)inasurveyof14unidentifiedFermi-LATsources 0 inthe southernskyusing the Parkesradiotelescope. PSRs J0101–6422,J1514–4946,andJ1902–5105 2 reside in binaries, while PSRs J1658–5324 and J1747–4036 are isolated. Using an ephemeris derived n from timing observations of PSR J0101–6422(P=2.57ms, DM=12pccm−3), we have detected γ-ray a pulsations and measured its proper motion. Its γ-ray spectrum (a power law of Γ=0.9 with a cutoff J at 1.6GeV) and efficiency are typical of other MSPs, but its radio and γ-ray light curves challenge 5 simple geometric models of emission. The high success rate of this survey—enabled by selecting γ- 2 ray sources based on their detailed spectral characteristics—and other similarly successful searches indicate that a substantial fraction of the local population of MSPs may soon be known. ] Subject headings: gamma rays: general — pulsars: individual (PSR J0101–6422) E H . 1. INTRODUCTION cially MSPs17. Gamma-ray pulsars—both young and h recycled—arestable emitters and nearly allhave a char- p The Large Area Telescope (LAT, Atwood et al. 2009) acteristic spectrum dN/dE ∝ E−Γexp(−E/E ) with - of the Fermi Gamma-ray Space Telescope is a pulsar c o detector par excellence. Supported by a radio cam- Γ < 2 and 1 < Ec/GeV < 10 (Abdo et al. 2010c), r making unidentified, nonvariable LAT sourceswith such t paign (Smith et al. 2008) providing ephemerides for co- s herent folding of LAT photons, the LAT has identified spectragoodpulsarcandidates. The fine angularresolu- a tion and large effective area of the LAT allow a typical γ-raypulsations from many normal(unrecycled)pulsars [ sourcelocalizationof∼10′,aboutthe1GHzbeamsizeof (e.g. Abdo et al. 2010c) and millisecond pulsars (MSPs, 100mclass radiotelescopes. This happy coincidence en- 1 Abdo et al. 2009a). Additionally, tens of new pulsars ablesdeeppulsationsearcheswithasinglepointing. Dis- v have been discovered in “blind” searches for periodicity 0 in the LAT data (e.g. Abdo et al. 2009b; Pletsch et al. coveryofapulsarandasubsequenttimingcampaigncan 6 2012). “closetheloop”byprovidinganephemeriswithwhichto 1 A third, indirect method of detection has been ex- fold LAT photons and resolve γ-ray pulsations. Indeed, 5 tremely successful in discovering new pulsars, espe- since most MSPs reside in binaries, initial detection and . characterizationofthe orbitatlongerwavelengthsis the 1 only way γ-ray pulsations can be detected from these 0 1W. W. Hansen Experimental Physics Laboratory, Kavli pulsars. 2 Institute for Particle Astrophysics and Cosmology, Department 1 ofPhysicsandSLACNationalAcceleratorLaboratory,Stanford Duetosensitivitylimitations,LAT-guidedsurveysnat- : University,Stanford,CA94305,USA urally target the nearly-isotropic population of nearby v 2email: [email protected] MSPs. The deep radio exposures afforded by the effi- Xi 34ECionlustmeibniaFeAllostwrophysics Laboratory, Columbia University, cient target selection offer the tantalizing possibility of NewYork,NY10027, USA completely cataloguingthe localfield MSPs whose radio r 5email: [email protected] and γ-ray beams cross the Earth. This relatively com- a 6National Research Council Research Associate, National pletesamplewillbevaluableinconstrainingtheemission AcademyofSciences,Washington, DC20001, residentatNaval mechanismsofMSPsandincharacterizingthe evolution ResearchLaboratory,Washington, DC20375,USA 7email: [email protected] of their binary progenitors. 8NASAGoddardSpaceFlightCenter,Greenbelt,MD20771, Towardsthis end, we performed a small, targeted sur- USA vey at the CSIRO Parkestelescope, complementary to a 9Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel Parkes search described in Keith et al. (2011) that had 69,53121Bonn,Germany 10Netherlands Institute for Radio Astronomy (AS- different source selection criteria. TRON),Postbus2,7990AADwingeloo,Netherlands 11Astronomical Institute “Anton Pannekoek” University of 2. TARGETSELECTION Amsterdam,Postbus942491090GEAmsterdam,Netherlands 12CSIRO AstronomyandSpaceScience, AustraliaTelescope We began with the set of southern (Decl. < −40◦; NationalFacility,EppingNSW1710,Australia northern sources are visible to the more sensitive Green 14National Radio Astronomy Observatory (NRAO), Char- Bank Telescope), nonvariable sources in a preliminary lottesville,VA22903, USA 13Jodrell Bank Centre for Astrophysics, School of Physics versionofthe1FGLcatalogofγ-raysources(Abdo et al. andAstronomy,TheUniversityofManchester, M139PL,UK 15Space ScienceDivision,NavalResearchLaboratory, Wash- 17Seehttps://confluence.slac.stanford.edu/display/GLAMCOG/Public+List+of+LA ington, DC20375-5352, USA Detected+Gamma-Ray+Pulsars for an up-to-date list of LAT- 16CSIROParkesObservatory,ParkesNSW2870,Australia detected pulsars 2 KERR ET AL. 2010b). We further restricted consideration to sources quency only differs by a maximum of 3K for different whose LAT position estimates had a 95% confidence locations, out of a total system equivalent temperature error radius ≤7′, the HWHM of the 1.4GHz beam of ofnearly30K,sothatweprovideanaveragefluxdensity the Parkes telescope, and which had no plausible blazar limit. Two of the non-detections have limits of 0.12mJy counterpart. We classified the remaining sources by vi- and0.14mJy owingto shorterintegrationtimes (see Ta- sualinspectionoftheγ-rayspectrum. Sourceswithspec- ble 1). Because of scintillation, a pulsar with a larger tralshapesresemblingknownpulsars(seeabove)werese- average flux than our nominal limits might not be de- lectedforobservation,whilethosesourcesbestdescribed tectable in a particular observation, and vice versa. In by power laws or with significant spectral breaks but any case, the nominal flux density represents the most Γ > 2 were deemed likely to be blazars and were not sensitive searches for MSPs done at Parkes, especially selected. Due to the difficulty of spectral modeling and considering that these LAT-selected targets are largely localizationofLATsourcesintheGalacticplane,wedis- expected to be relatively nearby. cardedsources with |b|<5◦. Our final list comprised14 Characterizationoffour ofthe MSPsrequiresa longer goodcandidates. Thesourcepositions(seeTable1)were radio timing campaign and/or LAT dataset than avail- computed from the same analysis used to estimate the able for this work. We defer discussion of these MSPs spectral shape, but in all cases the difference in position (and of scintillation effects on sensitivity, which like- with 1FGL is well within both the 1FGL 95% errorcon- wise requires repeatedobservations)to Camilo et al. (in tour and the Parkes beam. preparation). The remaining pulsar presents a peculiar light curve and we describe it further below. 3. RADIOSEARCHES We observedthe 14selectedsources(§2)atthe Parkes 4. PSRJ0101–6422 64-m radio telescope between 2009 November 25 and 4.1. Timing Solution December 8. Each source was observed for between After the discoveryobservationson2009November25 1 and 2 hours (see Table 1) using the center beam and confirmation on December 8, we began regular tim- of the multibeam receiver. We recorded 1-bit-digitized ing observations of PSR J0101–6422 at Parkes. We ob- total-power samples every 125µs from each of 512 fre- served the pulsar on 35 days through 2011 November 9, quency channels spanning 256MHz of band centered on detecting it on 28 occasions; on the remaining 7 days it 1390MHz, writing the data to disk for off-line analysis was too faint to detect, due to interstellar scintillation (see Manchester et al. (2001) for more details on the re- (DM=12pccm−3.) Each observation was typically one ceiver and data-acquisition system.) The data were analyzed on the koala computer hour,withthesamereceiveranddataacquisitionsystem used in the search observations. Using TEMPO218 with cluster at Columbia University using standard pulsar the 28 times-of-arrival, we obtained a phase-connected searchtechniques implemented in the PRESTO package timing solution whose parameters are given in Table 2. (Ransom2001). We dedispersedeachdatasetideallyup to a dispersionmeasure ofDM=270pccm−3, andwhen The Shklovskii effect (Shklovskii 1970) increases the searching for periodic signals we allowed for accelerated measured value of P˙ = 5.2×10−21 beyond that intrin- signalscausedbypulsarmotioninabinarysystem. This sic to the pulsar by ∆P˙ = Pv⊥2/dc, with v⊥ the source was parameterized within PRESTO by the parameter velocity transverse to the line-of-sight. For nearby ob- zmax = 50, which is related to the maximum number jects,itmaydominatethetruespindownrateandleadto of bins a signal can drift within the Fourier spectrum overestimates of the spindown luminosity (Camilo et al. andstill be properlydetected by the searchmethod. All 1994). The observedproper motion, at the 0.55kpc DM ofthe data wereanalyzedin this manner within about a distance (Cordes & Lazio 2002), implies a true P˙ = i weekofcollection,andon2009December4wefoundour (4.3 ± 0.1) × 10−21. At this distance, the indicated first MSP. Within a few more days we confirmed it and v⊥ =41kms−1 istypicalforanMSP(seeNice & Taylor four other new MSPs. A 6th MSP was detected unbias- 1995). edlyinoursample,butithadbeenpreviouslydiscovered by Keith et al. (2011, see Table 1). 4.2. Radio Profile and Polarimetry In a few cases, the pulsars were not confirmed on the first attempt and required subsequent observations, ow- Wemadepolarimetricandflux-calibratedobservations ing to their faintness and scintillation in the interstellar of PSR J0101–6422,using the PDFB3 digital filterbank. medium. We reanalyzed the data by increasing the ac- The data were analyzed with PSRCHIVE (Hotan et al. celeration search space to zmax = 200 and by analyzing 2004). The best full-Stokes profile that we obtained is only the first 30 minutes of each data file. The latter shown in Figure 1 (the flux density of this profile is not approach improves our sensitivity to highly accelerated necessarilyrepresentativeoftheaveragepulsarintensity, pulsars (e.g., for anMSP in a 1-day binary,a 2-hour ob- which varies greatly owing to interstellar scintillation). servation cannot be corrected ideally under the assump- The main pulse is linearly polarized at the ∼15% level, tion of a constant acceleration, no matter how large the but the low signal-to-noise ratio prevents a useful de- zmax used), but no new pulsars were detected. termination of rotation measure (the best-fit value is The nominal sensitivity of our searches, for a 2hr ob- ∼ +10radm−2, but within the uncertainties is consis- servation and assuming a pulsar duty cycle of 20%, is tent with zero). These data show that the subsidiary 0.11mJy for P &2ms and DM . 40pccm−3, degrading pulse component is composed of two outer peaks joined gradually for shorter periods and larger DMs. While in by a low bridge of radio emission. detail this depends on the sky backgroundtemperature, the contribution from the Galaxy at our observing fre- 18 http://www.atnf.csiro.au/research/pulsar/tempo2 5 Millisecond Pulsars in Fermi Sources 3 Table 1 ResultsofRadioSearches of141FGLSources atParkes 1FGLName R.A.a Decl.a l b Γ Ec Obs. Time Period DM Distanceb Binary (J2000.0) (J2000.0) (deg) (deg) (GeV) (hr) (ms) (pccm−3) (kpc) J0101.0–6423 01h00m58s.1 −64◦24′03′′ 301.2 –52.7 1.3 2.3 1.0 2.57 11.93 0.55 yes J0603.0–4012 06h03m04s.9 −40◦11′02′′ 246.8 –25.9 1.9 6.5 2.0 — — — — J0933.9–6228 09h33m58s.5 −62◦27′54′′ 282.2 –7.8 0.1 1.2 2.0 — — — — J1036.2–6719 10h36m16s.9 −67◦20′34′′ 290.4 –7.8 1.5 2.2 2.0 — — — — J1227.9–4852 12h27m50s.5 −48◦51′54′′ 298.9 13.8 1.8 1.6 2.0 — — — — J1232.2–5118 12h31m49s.1 −51◦18′50′′ 299.8 11.4 1.8 2.3 2.0 — — — — J1514.1–4945 15h14m05s.7 −49◦45′32′′ 325.2 6.8 1.7 5.9 2.0 3.59 31.5 0.9 yes J1624.0–4041 16h24m06s.2 −40◦40′48′′ 340.6 6.2 2.1 3.3 2.0 — — — — J1658.8–5317 16h58m43s.2 −53◦17′45′′ 335.0 –6.6 2.1 1.8 1.3 2.44 30.9 0.9 no J1743.8–7620 17h43m44s.6 −76◦20′42′′ 317.1 –22.5 1.2 2.0 1.5 — — — — J1747.4–4035 17h47m29s.1 −40◦36′07′′ 350.2 -6.4 1.5 3.4 1.4 1.65 152.9 3.3 no J1902.0–5110 19h02m05s.5 −51◦09′43′′ 345.6 –22.4 1.7 4.4 1.2 1.74 36.3 1.2 yes J2039.4–5621 20h39m30s.5 −56◦20′42′′ 341.2 -37.1 1.6 2.7 1.2 — — — — J2241.9–5236c 22h41m52s.4 −52◦37′37′′ 337.4 -54.9 1.6 3.6 2.0 2.19 11 0.5 yes a Parkestelescope pointingposition. b DistancescomputedfrommeasuredDMusingtheNE2001model(Cordes&Lazio2002). c SeeKeithetal.(2011). 60 P2 1.0 50 P1 b.) n 0.8 Ar er Bi40 0.6 sity ( P n Counts 30 0.4 Hz Inte G (cid:0) 0.2 4 20 1. 0.0 10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Figure 1. Polarimetric pulse profile of PSR J0101–6422 at Rotation Phase 1.4GHz, displayed with 128 phase bins, based on 6hr of Parkes Figure 2. Phase-aligned γ-ray (blue histogram) and radio (red PDFB3datausing256MHzofbandwidth. Theblacktracecorre- trace) pulse profiles of PSR J0101–6422. The radio profile is spondstototalintensity,redtolinearpolarization,bluetocircular. summed from 28 timing detections, while the γ-ray profile cor- In the upper window, the position angle of linear polarization is respondstotheoptimalaperturedescribedinthemaintext. The plotted for bins with linear signal-to-noise ratio > 3. The pulse backgroundestimation(dashedhorizontalline)isderivedbycount- peakisdisplayedwitharbitraryphase, andthe meanfluxdensity ingphotons intheoff-pulsephasewindow(§4.4)andisconsistent is∼0.2mJy. withtheoff-pulsebackground levelobtainedviaspectralanalysis. Thevertical dashedlineindicates thephase(φ≡0.5)fromwhich theoffsetof“P1”ismeasured. 4.3. Gamma-ray Profile To characterize the γ-ray profile, we selected photons hemisphere (see Romani & Yadigaroglu 1995). In this collected between 2008 Aug 4 and 2011 Aug 1 with scenario, the line-of-sight to PSR J0101–6422 would in- reconstructed energies 0.3–3GeV lying within 1.1◦ of tercept a cone of radio emission in one hemisphere to the timing position. This selection balances complete- produce the weaker radio peak at φ∼0.5 and a cone of ness (including as many pulsed photons as possible) and radio and γ emission in the other to produce the two γ pulsed signal-to-noise ratio as measured by the H-test peaks and bright radio peak at φ ∼ 1. In keeping with (de Jager et al. 1989). We apply the same data process- this picture and the conventionestablished in the litera- ing as described in §4.4, omitting the horizon cut to in- ture,we identify the γ peak atφ∼0.8 as “P1”and that crease the livetime by about 20%, at the expense of a at φ∼0.1 as “P2”. slightly increased background. The relative phasing of the peaks is of interest (e.g. The light curve corresponding to this extraction ap- Watters et al. 2009). The radio-to-γ offset, δ, is mea- pears in Figure 2. If γ rays are produced above the null suredfromthecenteroftheleadingradiopeak(takenas charge surface (the locus of points where the magnetic φ ≡ 0.5, see Figure 2). We estimated the phase of the field is orthogonal to the pulsar spin axis), they appear γ-ray peaks using unbinned maximum likelihood with to an observer in the opposite hemisphere, whereas ra- a two-sided Lorentzian model for the peaks. These esti- dio emission from low altitudes is beamed into the same matesyieldδ =0.33±0.01 ±0.03 andaγ-raypeak- stat syst 4 KERR ET AL. kpc. Best-fitvalues off fromthe TPC andOGmodels Ω 60 PSR J0101-6422 (§5.1) are 0.7 and 0.9. This γ-ray efficiency is in accord 50 with other LAT-detected MSPs (Abdo et al. 2009a). n Bi 40 nts/ 30 4.5. X-ray and Optical Observations u o C 20 A3.3ks Swift observationwiththe X-rayTelescope in 10 PC mode was obtained on 2009 Nov 21 to search for an 0 X-raycounterparttothethen-unidentified1FGLsource. 12 LAT ≥ 0.1 GeV No significant counterpart is detected, with a 3σ upper nits) 10 1400 MHz Radio Data limit at the pulsar position of 2.0×10−3ctss−1 (0.5–8 u b. 8 TPC Model keV).Assumingapower-lawspectrumwithphotonindex de (ar 6 OG Model Γ = 1.5 for a column with NH = 3.7×1020cm−2 (10 u plit 4 HI atoms per free electron along the line-of-sight) yields Am 2 an unabsorbed flux limit of 1.1×10−13erg cm−2s−1, or 00 0.2 0.4 0.6 0.8Rotatio1n Phase1.2 1.4 1.6 1.8 2 efficiency LX/E˙ < 5.0×10−4 at 0.55kpc. This limit is comparable to the detected X-ray flux from other MSPs Figure 3. The best-fit model light curves forthe two-pole caus- (e.g. Marelli et al. 2011). tic (TPC) and outer gap (OG) models (see §5.1.) The best fit geometries(α,ζ)forTPCandOGare(26◦,79◦)and(90◦,36◦). We searched four DSS plates and found no optical counterpart consistent with the pulsar’s position, sug- to-peak separationof∆=0.26±0.03stat±0.02syst. The gesting m>21. DM-induced uncertainty in δ is < 0.001P. The system- atic uncertainty for both δ and ∆ is estimated from the 5. DISCUSSION scatter obtained by fitting alternative functional forms 5.1. PSR J0101–6422 for the peaks. At first blush, PSR J0101–6422 is an unremarkable 4.4. Gamma-ray Spectrum member of the growing population of γ-ray MSPs. The orbital characteristics suggest an epoch of Roche lobe Tomeasurethespectrumofpsr,weanalyzedlowback- groundDIFFUSEclass“Pass6”(Atwood et al.2009)LAT overflowfromanevolved1–2M⊙ companionwhichlater became the ∼0.2M⊙ He white dwarf currently inferred datacollectedbetween2008Aug4and2011Apr17. We inthesystem(e.g.Tauris & Savonije1999). However,as filteredperiodswheretheobservatory’srockingangleex- we discuss below, PSR J0101–6422’s light curve is chal- ceeded 52◦ or the Earth’s limb impinged upon the field lengingto explainwith simple geometric models that tie of view (horizon cut, requires zenith angle < 100◦). We theradioandγ emissiontoparticularregionsofthemag- modelled the sensitivity to these events with the flight- netosphere. correctedP6 V11 DIFFUSEinstrumentresponsefunction. Remarkably, a simple picture of γ-ray emission aris- Tocomputebackgroundcontributions,weusedaprelim- ing in the outer magnetosphere appears to describe the inary version of the 2FGL catalog and its accompany- majority of both young (though see Romani et al. 2011) ing diffuse emissionmodels19. We selected eventswithin and recycled pulsars (e.g. Venter et al. 2009). On the 10◦ and re-fit the spectra of point sources within 8◦ and other hand, while radio emission in young pulsars and the normalizations of the diffuse models. Fits employed some MSPs appears to come from lower altitudes, sev- pointlike (Kerr 2011), a binned maximum likelihood al- eral MSPs have phase-aligned γ and radio peaks (e.g. gorithm. Abdo et al.2010a),implyingajointemissionsiteathigh The phase-averaged spectrum of PSR J0101–6422 is altitude. These observations raise the prospect of MSPs well described by an exponentially cutoff power law, whose radio emission combines “traditional” polar cap dN/dE = N (E/E )−Γ exp(−E/E ), whose parame- 0 0 c emission with a higher-altitude component. ters appear in Table 2. We also considered separately To determine which scenario best describes the emis- the spectra of the three major components, viz. P2 sion pattern of PSR J0101–6422, we performed joint (0 < φ < 0.33), the off-pulse (0.33 < φ < 0.63), and fits to the radio and γ-ray data following the method P1 (0.63 < φ < 1). The parameter uncertainties are of Johnson (2011). We modelled the radio as a sim- relatively large, and accordingly we found no significant ple “cone” component (Story et al. 2007) and the γ difference in the spectral shapes of the two peaks. Fur- rays using both the “two-pole caustic” (TPC) model ther, we detected no significant emission in the off-pulse of Dyks & Rudak (2003, N.B. we adopt a larger max- phase window and derived a 95% confidence upper limit imum cylindrical radius of 0.95 of the light cylinder) on the energy flux of a point source with a power law and the “outer gap” (OG) model (Cheng et al. 1986; spectrum with Γ = 2.2, obtaining 6×10−12 erg cm−2 Romani & Yadigaroglu 1995). The angle between the s−1 (>100 MeV; scaled up by 3.3 to the full phase win- pulsar spin axis and the line-of-sight, ζ, is unknown a dow.) priori,butiftheradioemissionarisesfromlowaltitudes, The observed flux is related to the γ-ray luminosity thenthepresenceoftwopeaksseparatedbyroughlyhalf by F =L /4πf d2, where f is an unknown beaming γ γ Ω Ω of a rotation indicates the inclination of the magnetic factor (typically of order unity, e.g. Watters et al. 2009) axisfromthe spinaxis(α) cannotbe small. The best-fit implying a phase-averaged, isotropic (f = 1) γ-ray lu- Ω lightcurvesforthetwoγ-raymodels appearinFigure3. minosity Lγ = 4.0×1032ergs−1= 0.04E˙ at d = 0.55 The low α = 26◦ preferred for the TPC model cannot produce the radio interpulse and produces too much off- 19http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.hptumlsle γ-ray emission. The OG model, with a best fit at 5 Millisecond Pulsars in Fermi Sources 5 Table 2 MeasuredandDerivedParametersforPSRJ0101–6422 Parameter Value Rightascension,R.A.(J2000.0).................. 01h01m11s.1163(2) Declination,decl. (J2000.0)...................... −64◦22′30′.′171(2) PropermotioninR.A.×cosdecl. (masyr−1).... 10(1) Propermotionindecl. (masyr−1)............... –12(2) Galacticlongitude,l(deg.)....................... 301.19 Galacticlatitude,b(deg.)........................ –52.72 Spinperiod,P (ms).............................. 2.5731519721683(2) Periodderivative(apparent), P˙................... 5.16(3)×10−21 Epochofperiod(MJD).......................... 55520 Orbitalperiod,Pb (days)......................... 1.787596706(2) Epochofperiastron(MJD)....................... 55162.4011764(3) Semi-majoraxis(lts)............................ 1.701046(2) Eccentricity...................................... <1.5×10−5 Dispersionmeasure,DM(pccm−3)............... 11.926(1) Spin-downluminositya,E˙ (ergs−1)............... 1.0×1034 –16% Characteristicagea,τc (yr)....................... 9×109 +20% Surfacedipolemagneticfieldstrength(Gauss)a... 1.1×107 –9% Meanfluxdensityb at1.4GHz,S1.4 (mJy)....... 0.28±0.06 Radio–γ-rayprofileoffset, δ (P).................. 0.35±0.01±0.05 γ-rayprofilepeak-to-peakseparation,∆(P)...... 0.26±0.03±0.02 γ-ray(>0.2GeV)photonindex,Γ............... 0.9±0.3±0.2 γ-raycut-offenergy,Ec (GeV)................... 1.6±0.4±0.3 Photonflux(>0.1GeV)(10−9cm−2s−1)........ 9.5±1.8+−12..50 Energyflux(>0.1GeV),Fγ (10−11ergcm−2s−1) 1.1±0.1±0.1 Note. —TimingsolutionparametersgiveninTDBrelativetotheDE405plane- taryephemeris. Numbersinparenthesesrepresentthemeasured1σTEMPO2timing uncertaintiesonthelastdigitsquoted. Forγ-rayparameters,thefirstuncertaintyis statisticalandthesecondsystematic. aCorrectedforShklovskiieffectat0.55kpc;thepercentagechangefromthenominal valueisgiven. b Averagefluxfor28timingdetections. α = 90◦ (orthogonal rotator), produces two radio peaks |b| > 5◦, their efficiency jumps to 3/8. Keith et al. but fails to produce two clear γ peaks and the radio in- (2011)employedsomespectralinformationanddetected terpulse morphology. MSPs unbiasedly in 1/11 pointings (1/4 for |b| > 5◦). Since neither modelfaithfully reproducesthe observed Ashortlistof7 spectrally-exceptionalcandidatesdrawn light curves, we propose that one or both of the radio upforaNan¸cayradiotelescopesurveyincluded5MSPs, peaks may originate at higher altitude. Emission over a three of which were discovered from Nan¸cay observa- range of altitudes would also explain the low observed tions (Cognard et al. 2011; Guillemot et al. 2011). And, linear polarization of PSR J0101–6422 (e.g. Dyks et al. Hessels et al.(2011)usednearlyidenticalselectioncrite- 2004)andadmitsmallerαvalues. Alternatively,thefail- riaasthisworkinaGreenBankTelescopesurveyat350 ure of geometric models may indicate that the physical MHz and found MSPs in 13/49 pointings, all but one at details of the MSP magnetosphere—e.g., currents, mul- |b|>5◦.the tipolar fields, and plasma loading of the field lines—and Targeting pulsar-like sources is clearly efficient. How- the radio and γ emission mechanisms play an important ever,newLATsourceswilllieatthesensitivitythreshold role. and the limited statistics will not admit classification of spectral shape and variability. Nonetheless, we argue 5.2. Survey Implications that future radio searches should omit detailed ranking schemesandtargetanyLATsourceofftheGalacticplane With 6/14pointings resulting in unbiasedMSP detec- that has not been associated with a known blazar. tions,thissurveywasfruitful,thoughthishighefficiency We believe this substantiallyincreasedtargetlist(and maybeluck. Indeed,inourtimingcampaign,weonlyde- telescope time) is justified. In the 2FGL source catalog tectedPSRJ0101–6422on28of35attempts,andseveral (Abdo et al. 2011), there are ∼350 unassociated sources observations required prior knowledge of the ephemeris more than 5◦ off the Galactic plane. MSPs account for fordetection. Thus,becauseofscintillation,weestimate roughly 5% of associated LAT sources, so 15–20 MSPs that PSR J0101–6422is only detectable in 2/3 observa- may remain to be found in these unassociated 2FGL tions, depending somewhat on integration time. Many sources. With only 350 positions to monitor, multiple field MSPs are in eclipsing binaries, further diminishing deepobservationscanamelioratetheconfoundingeffects the probability of detection in a single observation. of scintillation and eclipses while requiring a fraction of We believe the strongest factor in our search’s success the time of a comparably complete all-sky survey. By was the source selection criteria. E.g., Ransom et al. comparison, the otherwise prodigious Parkes multibeam (2011) selected nonvariable LAT sources (but includ- survey found “only” 17 MSPs in over 40,000 pointings ing no spectral shape information) and found MSPs in (e.g. Faulkner et al. 2004). In addition to the intrinsic 3/25 pointings. But when considering only targets with 6 KERR ET AL. interest in discoveringnew MSPs, by thoroughly search- —.2009b, Science,325,840 ingallsuchLATsources,wewillgainconfidencethatthe —.2010a, ApJ,712,957 MSPs we have found can truly provide a volume-limited —.2010b, ApJS,188,405 —.2010c, ApJS,187,460 sampleoftheenergetic,γ-loudMSPs,aninvaluablestep —.2011, arXiv:1108.1435 in the study of the MSP population of the Milky Way. Atwood,W.B.,etal.2009, ApJ,697,1071 Camilo,F.,Thorsett,S.E.,&Kulkarni,S.R.1994,ApJ,421,L15 Cheng,K.S.,Ho,C.,&Ruderman,M.1986, ApJ,300,500 We thank the marvelous staff at Parkes that make it Cognard,I.,etal.2011,ApJ,732,47 Cordes,J.M.,&Lazio,T.J.W.2002,ArXivAstrophysics suchawonderfulresearchfacility,WillemvanStratenfor e-printsarXiv:astro-ph/0207156 help with PSRCHIVE, and Jules Halpern for help with deJager,O.C.,Raubenheimer,B.C.,&Swanepoel, J.W.H. optical analysis. The Parkes Observatory is part of the 1989,A&A,221,180 Australia Telescope, which is funded by the Common- Dyks,J.,Harding,A.K.,&Rudak,B.2004,ApJ,606,1125 wealth of Australia for operation as a National Facility Dyks,J.,&Rudak,B.2003, ApJ,598,1201 Faulkner,A.J.,etal.2004,MNRAS,355,147 managed by CSIRO. Guillemot,L.,etal.2011,MNRAS,submitted The Fermi LAT Collaboration acknowledges support Hessels,J.W.T.,etal.2011, arXiv:1101.1742 from a number of agencies and institutes for both de- Hotan,A.W.,vanStraten, W.,&Manchester, R.N.2004, velopment and the operation of the LAT as well as sci- PublicationsoftheAstron.Soc.ofAustralia,21,302 entific data analysis. These include NASA and DOE Johnson,T.2011,PhDThesis,UniversityofMaryland, arXiv:TBD in the United States, CEA/Irfu and IN2P3/CNRS in Keith,M.J.,etal.2011,MNRAS,414,1292 France,ASIandINFNinItaly,MEXT,KEK,andJAXA Kerr,M.2011,PhDThesis,UniversityofWashington, in Japan, and the K. A. Wallenberg Foundation, the arXiv:1101.6072 SwedishResearchCouncilandtheNationalSpaceBoard Manchester,R.N.,etal.2001,MNRAS,328,17 in Sweden. Additional support from INAF in Italy and Marelli,M.,DeLuca,A.,&Caraveo, P.A.2011,ApJ,733,82 Nice,D.J.,&Taylor,J.H.1995,ApJ,441,429 CNES in France for science analysis during the opera- Pletsch,H.J.,etal.2012,ApJ,744,105 tions phase is also gratefully acknowledged. Ransom,S.M.2001,PhDthesis,HarvardUniversity Support for this workwas providedby NASA through Ransom,S.M.,etal.2011,ApJ,727,L16 Einstein Postdoctoral Fellowship Award Number PF0- Romani,R.W.,Kerr,M.,Craig,H.A.,Johnston,S.,Cognard,I., 110073issuedbytheChandraX-rayObservatoryCenter, &Smith,D.A.2011,ApJ,738,114 Romani,R.W.,&Yadigaroglu,I.-A.1995,ApJ,438,314 whichisoperatedbytheSAOforandonbehalfofNASA Shklovskii,I.S.1970,SovietAst.,13,562 under contract NAS8-03060. Smith,D.A.,etal.2008,A&A,492,923 Facilities: Fermi (LAT), Parkes (PMDAQ, PDFB3) Story,S.A.,Gonthier,P.L.,&Harding,A.K.2007,ApJ,671, 713 Tauris,T.M.,&Savonije,G.J.1999,A&A,350,928 REFERENCES Venter,C.,Harding,A.K.,&Guillemot,L.2009,ApJ,707,800 Watters,K.P.,Romani,R.W.,Weltevrede, P.,&Johnston, S. 2009,ApJ,695,1289 Abdo,A.A.,etal.2009a, Science, 325,848