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Fermi – LAT for Pulsars

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Fermi – LAT for Pulsars 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 (B1706-44) T E J1057-5226 R G (B1055-52) E J952+3252 (B1952+32) T A L - i m Preliminary r e F Preliminary Preliminary 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.
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