Pulsars in the Fermi Era - NASA- GSFC / CRESST-UMBC Ozlem Celik - CERN Indico
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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
Eg: Synchrotron Radiation up to ~300MeV HESS, MAGIC
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 Msun , R ~ 10 km and Bsurf ~ 1012 Gauss. 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 Erot = B02 Ω 4 R6/ 6c3 ,
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:
B0 = 6.4 х 1019 G (P 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
05/12/10
the entire phase, OG models do not. 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 10Why 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
05/12/10EGRET Pulsars 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.
05/12/10Overview of LAT: How it works
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
Atwood et al, ApJ 2008
towers]
Systems work together to identify and measure the flux
of cosmic gamma rays with energy 20 MeV - >300 GeV.
05/12/10Fermi – 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: 8000 cm2 , Sensitivity: 6 x 10-9 cm-2s-1
→ More photons collected
● Low deadtime ~ 20ms → finer pulsar light curves
●
Angular resolution 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
05/12/10Detecting 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
05/12/10Pulsar 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
7EGRET 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
l i mi n ary
Pre
05/12/10EGRET Pulsars with Fermi - II
J1709-4429 J1057-5226 J952+3252
(B1706-44) (B1055-52) (B1952+32)
EGRET
l i mi n ary
Pre
Fermi-LAT
l i mi n ary
Pre
el i minary
Pr
05/12/10EGRET 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
Cutoff energy and spectral index vs. pulse phase,
altitude 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)! 8Young 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) 9Fermi 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
10Fermi 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+4232
05/12/10 (~ 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
05/12/10
(depending on beam geometry) ~450 km/s16 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
a TeV source. J1413-6205 J1429-5911
a ry
i n
2 old (~1 Myr) and off the plane. lim
e
Pr
Locations can be refined to as J1954+2836
J1846+0919
precise as several arcsec by timing
(Ray et al. 2009)
J1957+5036 J2055+2539
8 new detections in blind search!
05/12/10
(Abdo et al., in prep)The Pulsing Gamma-ray Sky
P uls es a t
1/10 th true
ra te
05/12/10The Population of Fermi Pulsars
First LA T Pulsar C atalog:
46 pulsars included
17 g amma-ray selected (blue
squares)
6 EG RET
8 M S Ps (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, A pJS, 187, 460)
S ince then:
18 more pulsars:
7 gamma-ray selected
7 M S Ps
4 new young radio-loud pulsars
05/12/10What have we learned ?
As for EGRET, the detected pulsars are B LC 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 PSRsWhat 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
05/12/10What 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!!) 05/12/10 DM 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
●3.12 ms spin period 0FGL J0614.3-3330 is
●10 hour orbit PSR J0614-33
●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
05/12/10Summary
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!
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