Pulsars in the Fermi Era

Ozlem Celik

NASA- GSFC / CRESST-UMBC Outline

PART I Gamma-ray Astronomy Pulsars – Introduction Pulsars – Why? Pulsars – HE Pulsed Emission Pulsars – Open Questions PART II Pulsars in the EGRET Era Fermi Gamma-ray Space Telescope EGRET Pulsars in the Fermi Era New Radio-loud gamma-ray pulsars Surprise! Gamma-ray only pulsars Millisecond pulsars in gamma-rays! And more...

PART III Fermi population of gamma-ray pulsars What have we learned? What is next?

05/12/10 Gamma-ray Astronomy

Gamma-ray Energy Band: Probes Non-thermal emission processes, astrophysical particle accelerators : Pulsars, PWN, SNR, AGNs, GRB VERITAS, CANGAROO, EGRET, Fermi HESS, MAGIC Eg: Synchrotron Radiation up to ~300MeV Space-based from Crab Nebula is from an accelerated Ground-based electron population with energy spectrum from 100 MeV to 1 PeV. (potentially) probes exotic physics (dark matter, massive relics) 05/12/10 Gamma-Ray Astronomy

Supernova Remnants Micro-quasars Gamma-ray Bursts Pulsars & PWNe

Low E High E Cosmological gamma-ray horizon Active Galactic Nuclei

Testing Cold Dark Lorentz Inv. Matter Fermi-LAT 1-year sky map >1000 LAT Sources

05/12/10 4 Pulsars - Basics

Pulsars are rapidly rotating, highly magnetized neutron stars. They born in supernova explosions of massive stars when the neutron degeneracy pressure prevents further gravitational collapse. The core is like a gigantic atomic nucleus, with density ~ nuclear matter. Typically, M ~ 1.4 M , R ~ 10 km and B ~ 1012 Gauss. sun surf A dense plasma is co-rotating with the star. The magnetosphere extends to the“light cylinder”, where the rotation reaches the speed of light. Emission (radio, optical, X-ray …) can be produced in beams around the pulsar, which acts like a cosmic light-house.

05/12/10 Pulsars - History

• 1934 – Walter Baade & Fritz Zwicky: Existence of neutron starts 1967 – Franco Pacini: Energy from a rotating neutron star E = B 2 Ω 4 R6/ 6c3 , • rot 0 emitted in the form of pulsating radiation and produce energy to the surrounding nebula • 1968 – Jocelyn Bell, Antony Hewish: Serendipitous discovery of radio pulsars: Little Green Man-1, -2, -3 → Oh, no, they are fast rotating stars! • 1968 – Tommy Gold: Lighthouse model of pulsars. • 1968 – Franco Pacini: Spin-down energy from Crab pulsar powers the Crab Nebula!

05/12/10 Radio Pulsars

Currently, >1800 radio pulsars are known Surface dipole field: B = 6.4 х 1019 G (P ) 0 Ṗ Characteristic age: τ = P / (2 Ṗ)

Two distinct populations: Young pulsars Recycled millisecond pulsars: Very different characteristics from the normal gamma-ray pulsars Spinning 100 times faster Magnetic fields ~10,000 times lower ~10,000 times older “Recycled” pulsars spun-up by binary companion stars 05/12/10 7 High Energy Pulsed Emission

Std. magnetosphere model for HE emission. Rotating dipole-->Induced E fields. Particles ripped from the surface. Gaps where E-fields cannot be shorted. Particles accelerated there and radiate curvature radiation and pair produce Three main HE emission models:

Polar Cap Models: Acceleration through the polar caps near the surface, low altitude emission. Strong B-fields near surface: Sharp super-exponential cutoff at a few GeV due to magneto- pair production Outer Gap Models: Acceleration between the region of null-charge surface, the last closed & first open field lines, high altitude emission. Slot Gap Models: Acceleration along the first open field lines, high altitude emission. Weaker B-fields, gentler simple-exponential cut-offs due to photon-photon pair production

05/12/10 8 Predictions of HE models

Energy Spectrum Different cutoff shapes due to different underlying physical process.

Pulse Profiles: All models can produce double-peak pulse profiles Different origin in the magnetosphere → Different emission geometry → Different #of peaks, peak separation, radio/gamma lag, ratio of radio-loud/radio-quiet.

PC: double peaks only for small observer and inclination angles. SG & OG: High altitude, double peaks up to larger angles PC & SG models predict off-pulse emission throughout the entire phase, OG models do not. 05/12/10 9 Pulsars – Open Questions

What mechanisms produce the emission of pulsars, from radio to gamma rays? Where are the locations of the acceleration regions? What does the pulse profiles in the gamma-ray region look like? What is the highest energy pulsars can accelerate particles. What is the shape of the spectral cut-off in their spectrum? Are there gamma-ray millisecond pulsars? What is the ratio of the radio-quiet to radio-loud pulsars? Dependency of the pulsar characteristics on the age, magnetic field, pulsed period.

05/12/10 10 Why Pulsars are Interesting?

Unique laboratory for strong B fields and relativistic plasmas They are relatively close-by. Prototypes of other astrophysical objects: accretion disks, jets, black hole magnetospheres Fascinating electromagnetic machines Not understood for > 40 yrs Fermi is probing where most of the energy is.

05/12/10 First Gamma-Ray Pulsars

{ SAS-2 (1973) First Radio-quiet Pulsar: { Vela Pulsar Geminga Discovered as an UnID point source with SAS2 COS-B (1980)} Optical and X-ray Crab Pulsar} counterpart were found but no radio source X-ray pulsations discovered in 1992 CGRO -EGRET (1991 ): Gamma-ray pulsations detected by EGRET in 1992 B1706-44 B1055-52 B1952+32

05/12/10 EGRETEGRET PulsarsPulsars

6 EGRET high-confidence gamma-ray pulsars

05/12/10 EGRET Unidentified Sources

05/12/10 Fermi Launch

• Launch from Cape Canaveral Air Station 11 June 2008 at 12:05 PM EDT • Circular orbit, 565 km altitude (96 min period), 25.6 05/12/10deg inclination. Fermi Gamma-Ray Space Telescope

Large AreaTelescope (LAT) 20 MeV - >300 GeV

Gamma-ray Burst Monitor (GBM) NaI and BGO Detectors 8 keV - 30 MeV

KEY FEATURES Huge field of view LAT: 20% of the sky at any instant; in sky survey mode, expose all parts of sky for ~30 minutes every 3 hours. GBM: whole unocculted sky at any time. Huge energy range: including largely unexplored band 10 GeV - 100 GeV. Total of >7 energy decades! Large leap in all key capabilities. Great discovery potential.

05/12/10 OverviewOverview ofof LAT:LAT: HowHow itit worksworks γ • Precision Si-strip Tracker (TKR) Measure the photon direction; Tracker gamma ID. • Hodoscopic CsI Calorimeter (CAL) Measure the photon energy; image the shower. • Segmented Anticoincidence Detector (ACD) Reject background of charged cosmic rays; segmentation removes self- veto effects at high energy. • Electronics System Includes flexible, robust hardware trigger ACD e+ e– and software filters. [surrounds 4x4 Calorimeter array of TKR towers] Atwood et al, ApJ 2008 Systems work together to identify and measure the flux of cosmic gamma rays with energy 20 MeV - >300 GeV. 05/12/10 Fermi – LAT for Pulsars

●LAT instrument on Fermi Gamma-ray Space Telescope is >30 times more sensitive than EGRET

●Energy Range 20 MeV to ~300 GeV → Relevant E band for pulsars

●Energy Resolution: <10% → Better spectra

2 -9 -2 -1 ●Superior Area: >8000 cm , Sensitivity: 6 x 10 cm s → More photons collected

●Low deadtime ~ 20ms → finer pulsar light curves

o ●Angular resolution <0.15 (>10GeV), Field of view > 2sr, ~20% of the sky at any time →Discoveries of many new sources!

●Expected to discover >100 new pulsars in comparison to 6 pulsars discovered by EGRET

05/12/10 Detecting Gamma-Ray Pulsars

PROBLEMS Very low rate of gamma-ray photons (4 ph/min for Vela!) Collecting enough photons can require MONTHS to YEARS Young pulsars spin down rapidly and have glitches in rotation and spin-down rate

SOLUTIONS Use known pulsation parameters (ephemeris) from radio or X-rays. All 6 EGRET pulsars have been found this way. Need supporting observations from other telescopes Search for pulsations in gamma-rays Need good search algorithm And lots of computer time • Radio pulsar searches of LAT unidentified sources – Sensitivity to MSPs, binaries, very noisy pulsars

05/12/10 Pulsar Timing Campaign

Large campaign organized to provide radio (and X-ray) timing models for all pulsars with Ė > 1 x 1034 erg/s (Smith et al. 2008 A&A, 492, 923)

RXTE (in space) Jodrell Bank (UK) Nançay (France) + other contributions: Arecibo, Hartebeesthoek, etc.

Provide ephemerides for 762 pulsars

Parkes (Australia) Green Bank (USA) 05/12/10 7 EGRET Pulsars with Fermi - I

The 6 EGRET pulsars are prime targets for spectral analyses with unprecedented details, because of their brightness. High signal-to-noise and good timing models allow study of fine features in the light curve and evolution of profile shapes with energy. Many features seen with EGRET confirmed, eg: P1/P2 ratio decrease with energy. New features seen with high resolution Fermi data, eg: shift of Vela P3 with energy. Vela Geminga Crab

inary Prelim

05/12/10 EGRET Pulsars with Fermi - II

J1709-4429 J1057-5226 J952+3252 (B1706-44) (B1055-52) (B1952+32) T E R G E

inary Prelim T A L

- y i inar relim m ary P r elimin e Pr F

05/12/10 EGRET Pulsars with Fermi - III

It is possible to make detailed spectral analysis for these pulsars, thanks to the large number of photons collected with Fermi LAT. Cutoffs in the energy spectrum of the pulsed emission detected for the first time and the cutoff energies were measured. In general, pulsar spectra are consistent with simple exponential cutoffs, around 1.5 to 5.8 GeV, indicative of absence of magnetic pair attenuation and high altitude emission. Phase-resolved spectroscopy reveals rapid changes is spectral parameters (e.g. cutoff energy) within gamma-ray peaks, perhaps due to variation in emission altitude Cutoff energy and spectral index vs. pulse phase, for the Vela pulsar Vela: complex P1 and P2 behaviors. A shift of P3 with energy has been observed 05/12/10 (Abdo et al., ApJ 696, 1084, 2009)! 8 Young Radio-Loud Pulsars

Fermi detected 24 radio-loud gamma-ray pulsars so far. Many of them are young and highly energetic (Ė > 3x1033 erg/s). Many are seen as unidentified EGRET sources!

PSR J1028-5819 (Abdo et al., ApJL 695, 72, 2009)

PSR J1420-6048 (Weltevrede et al., ApJ 2009 submitted)

PSR J2021+ 3651 (Abdo et al., ApJ 700, 1059, 2009)

PSR J1048-5832 & J2229+ 6114 PSR J0205+ 6449 (Abdo et al., ApJL 699, 102, 2009) 05/12/10 (Abdo et al., ApJ 2009 accepted) 9 Fermi LAT Detects Millisecond Pulsars!

The LAT detected pulsed gamma-ray emission from J0030+0451, making it the first firm detection of an MSP in gamma rays (Abdo et al., ApJ 699, 1171, 2009).

After 9 months of data taking, the LAT had detected 8 gamma-ray MSPs (Abdo et al. Science 325, 848, 2009).

At least 6 more detected recently, papers in preparation.

For the first time, a population of gamma-ray MSPs has been observed.

05/12/10 10 Fermi detection of 47 Tuc

Fermi 95 %

47 Tuc is a globular cluster (GC) in which 23 MSPs are known. The Fermi LAT detects 47 Tuc as a point source. We might be seeing the collective emission from MSPs in 47 Tuc. Individual detections ? We'll see. 47 Tuc is 4.9 kpc away: comparable to

J0218+423205/12/10 (~ 3 kpc) Surprise! First Gamma-ray-Only Pulsar

3EG J0010 +7309 95% error box

RX J00070+7302 + Fermi 95% error box

CTA1 Pulsar CTA 1 supernovae remnant Exhibits all characteristics of a young high- energy pulsar (P = 316 ms, characteristic age ~1.4 x 104 yr) 95% error circle radius =0.038° contains the X-ray source RX J00070+7302 Spin-down luminosity ~1036 erg s-1, sufficient to supply the PWN with magnetic Central to the PWN superimposed on the fields and energetic electrons. radio map at 1420 MHz. The γ-ray flux from the CTA1 pulsar Pulsar off-set from center of radio SNR; rough corresponds to about 1-10% of Erot estimate of the lateral speed of the pulsar is (depending05/12/10 on beam geometry) ~450 km/s 16 Pulsars Found in Blind Searches

After 4 months of data taking, 16 pulsars have been found with the same technique! (Abdo et al., Science 325, 840, 2009). 13 were unidentified sources for EGRET 6 of 16 were found using well localized counterparts at other wavelengths.

05/12/10 8 More Pulsars Found in Blind Search

After 1 year of data taking, 8 more pulsars have been found, They were among LAT unidentified J1022-5746 J1044-5737 sources. 5 young and energetic 1 very energetic and associated with J1413-6205 J1429-5911 a TeV source. ry a in 2 old (~1 Myr) and off the plane. lim re P Locations can be refined to as J1846+0919 J1954+2836 precise as several arcsec by timing (Ray et al. 2009)

J1957+5036 J2055+2539 8 new detections in blind search! (Abdo et al., in prep) 05/12/10 The Pulsing Gamma-ray Sky

P uls es at 1/10 th true rate

05/12/10 The Population of Fermi Pulsars

First LAT Pulsar Catalog: 46 pulsars included 17 gamma-ray selected (blue squares) 6 EG RET 8 M SPs (red triangles) 16 other young radio pulsars (green circles)

Multi-band light curves, spectral fits, etc... all done in a uniform way. First cut at population statistics, correlations (A bdo et al. 2010, ApJS, 187, 460)

Since then: 18 more pulsars: 7 gamma-ray selected 7 M SPs

05/12/104 new young radio-loud pulsars What have we learned ?

As for EGRET, the detected pulsars are BLC vs. characteristic age for the catalog PSRs relatively close and highly energetic. (High Edot1/2/D2) The detected pulsars also have the highest values of magnetic field at the light cylinder, BLC. Both detected normal PSRs and MSPs have comparable BLC values. Similar emission mechanisms operating? Luminosities are affected by distance uncertainties. However, the luminosity seems to grow with spin-down energy; with a L ∝ Ė at low Ė, L ∝ √Ė at high Ė. Gamma-ray luminosity vs. spin-down energy for the 05/12/10 catalog PSRs What have we learned?

• Majority have double gamma-ray peaks with phase separation 0.2 – 0.5 • Gamma-ray peaks are not aligned with radio peak(s) • Gamma-ray beams are must be larger that radio beams: Fan Beams! • Spectra are power-laws with simple exponential cutoffs at 1-6 GeV

High-energy emission comes from the outer magetosphere

Emission mechanism is likely curvature radiation from continuously accelerated particles 05/12/10 What have we learned?

Impressive fits can be achieved with both TPC and OG models based on the geometric considerations

Discrimination will come from phase averaged and phase resolved spectra..

05/12/10 What is next: Radio Follow-up All the new gamma-ray selected pulsars (discovered by blindly searching LAT data) radio quiet? Some (CTA1, 3EG J1835+5918) already have stringent radio limits Radio observations gives: distance from DM, geometry from polarization, radio-gamma lag etc. PSR J1741-2054 • Radio pulsar found in archival Parkes data • A low luminosity pulsar (L~0.025 mJy kpc2) • Extremely low DM (4.7 pc cm-3), implies D=400pc PSR J2032+4127 • Pulsations discovered at GBT • DM=115 implies D=3.6 kpc, but may be at half that distance (associated with Cyg OB2?) PSR J1907+06: New Detection! Pulsations discovered at Arecibo Very low luminosity radio pulsar (L ~3.5 μJ!!) DM05/12/10 82 pc cm-3 gives distance of 3.1 kpc What is Next: Search for Radio Pulsars in LAT UnID Sources • 9 LAT source locations searched so far • 3 new millisecond pulsars found! 0FGL J2214.8+3002 --> PSR J2214+30 ●‘Black Widow’ pulsar 0FGL J0614.3-3330 is ●3.12 ms spin period PSR J0614-33 ●10 hour orbit ●3 Ms spin period ●Unknown orbit

0FGL J1231.5-1410 is PSR J1231-14 3.68 ms spin 1.86 day orbit

Bright and stable millisecond pulsars are in high demand to complete timing arrays searching for gravitational radiation 05/12/10 SummarySummary

We are finally answering fundamental questions of gamma-ray pulsar astrophysics – but raising new ones High-energy emission comes from outer magnetosphere The mystery of unidentified Galactic gamma-ray sources from the EGRET era has largely been solved – they’re pulsars Radio-loud, radio-quiet and millisecond pulsars have similar gamma- ray light curves and spectra Similar emission mechanisms and geometry Fermi has so far detected 64 gamma-ray pulsars - including ms pulsars – many radio-quiet – more to come! Fermi is aiding discovery of new millisecond pulsars perfect for nanosecond timing arrays – first direct detection of gravitational radiation may be sooner that we thought!

05/12/10