Observations of pulsars
in high energy gamma-rays
Takayuki Saito
Max-Planck-Institut fuer Physik

Pulsars•Neutron star with strong B ﬁeld
•Surrounded by dense plasma corotating with NS•non-perpendicular-to-B E ﬁeld onlyin Polar Cap and Outer Gap•electrons accelerated along B ﬁeld inPC and OG•high energy gamma-rays produced
via curvature radiation in PC and OG.

Areobserve
PC od pur OGlsat?
ions fromHow to distinguish PC/OG?
•PC case•OG case

ee+e+γγ

Curvaturecoolingmagneticpair creation•High energy gamma-ray interacts with•Acceleration by E-ﬁeld and cooling bystrong B ﬁeld creating a pair
•Very sharp (super-exponential) cutoﬀ•For Crab, no emission above 10 GeV

curvature radiation balance•Mono-energetic electron (Γ~107)•Exponential Cutoﬀ at3!c

E cut =10GeVHigh energy end is thekey!

!

2Rcurv"3

EGRET (1991-2000)•on Compton Gamma-Ray Observatory•20MeV- 30 GeV•Operational 1991- 2000
•Anti-coincidence counter + Spark chamber+ Calorimeter•eﬀective ~0.15 m2•Detected ﬁrst 7 gamma-ray pulsars

EGRET measurement (1991-2000)•7 gamma-ray pulsars discovered•Spectra up to a few GeV•Cutoﬀ shape not clear......
•Both PC and OG possible.....

MAGIC telescope(s)•Imaging Atmospheric Cherenkov Telescope•Located at Canary Islands, LaPalma, 2200m a.s.l.
•World largest reﬂector, 17 m in diameter•Operational since 2004
•Energy threshold 60 GeV (2004-2007)•World lowest energy threshold among IACTs.
•Stereo since 2009

1st MAGIC Observations (Crab)•Observations in 2005-2006•A hint of pulsation above 60 GeV (2.6 sigma)•Only from second pulse•Catching the exponential tail? or just aﬂuctuation?
60GeV is not low enough!
Letʼs reduce further!!

New Trigger System (2007)•Standard Trigger•Pixel Threshold at ~6 phe•4NN Topology•New Analogue Sum Trigger•Summing 18 analog signals•Threshold for summed signal at 27 phe

Energy threshold
25 GeV!

2nd MAGIC Observations (Crab)•Observation in 2007-2008•First pulsar detection by IACT•Discovery of 25 GeV pulsation•PC rejected!
•Spectral shape not clear...
•Combined with EGRET, 17.7GeV
exponential cutoﬀ explains the results

Fermi-LAT (2008-)•EGRET•Fermi-LAT•20MeV- 30 GeV•Operational 1991- 2000
•Anti-coincidence counter + Spark chamber•100MeV- 300 GeV•Operational 2008 •Anti-coincidence counter + Silicon strip ++ Calorimeter•eﬀective ~0.15 m2•Detected ﬁrst 7 gamma-ray pulsars

Calorimeter•eﬀective ~1 m2•10 times higher sensitivity than EGRET• Detected > 100 gamma-ray pulsars

Fermi-LAT observations (2008)•Detected 46 gamma-raypulsars in the ﬁrst year
•Now more than 100 detected•Population study•All spactra basically consistent
with exponential cutoﬀ,supporting the OG model.

Vela pulsar

Fermi-LAT observations (Crab)Exponential cutoﬀ at 5.8 GeV?!

MAGIC/Fermi comparison

Data 2007-2009

5.8 sigma
discrepancy!

Event “SIZE” distribution

2,5 sigma
discrepancy

4.7 sigma
discrepancy

Spectrum doesnʼt follow
exponential cutoﬀ!
Power-law-like tail!
Conventional OG models
questioned.

VERITAS observations (Crab)•Observations 2009-2011•Detect pulsation above 100 GeV•Broken power law

Stereo MAGIC observations (Crab)•No doubt that there is a power-law-like tail•How to explain this? -> next three talks

Light Curve in log•Fermi >200MeV•In log scale, two triangles appear•Slopes decrease with energy except
for falling edge of P1

100-300 MeV!

300-1000 MeV!

•Rise vs Energy

1-3 GeV!

•Fall vs Energy•Currently no explanation•Hope this helps to construct better pulsar models

Pulse Width•Width decreases with energy, consistent with previous slide•See more in Kisaka sanʼs talkConclusion and Prospect• In the last 5 years, pulsar physics made rapid progress thanks to consecutive
important measurements by MAGIC, Fermi and VERITAS
PC model rejected (for gamma-ray emission)➡ conventional OG model forced to modify (for Crab)➡

• However, in-depth study could be done only with Crab, which is unique/special
in many senses. To understand pulsars further, more observations/detections inVHE are necessary• CTA will make another “revolution” in pulsar physics :)