Pulsar Winds in High EnergyAstrophysics
Dmitry Khangulyan
Institute of Space and Astronautical Science (ISAS/JAXA)The extreme Universe viewed in very high energy gamma-rays, Kashiwa 09/25/2012

D.Khangulyan

Pulsar Winds in High Energy Astrophysics

OUTLINE

Introduction to the pulsar winds
VHE pulsed signal from the Crab pulsar
GeV gamma-ray line
GeV flare from binary pulsar system

D.Khangulyan

Pulsar Winds in High Energy Astrophysics

Broad band emission from pulsars
Crab Nebula with ChandraPulsar(Massaro+)

PWN(Aharonian&Atoyan)

Crab is the canonical pulsar and one of the best studied
astrophysical objects displaying a few radiation components ofdifferent origin
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Pulsar Winds in High Energy Astrophysics

Conventional view on the CrabPulsar Related Emitting Zones

Pulsar(Massaro+)

PWN(Aharonian&Atoyan)

There are a few plausible emissions sites, which are related toeach other, but characterized by very different conditions,
therefore radiate very differently
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Pulsar Winds in High Energy Astrophysics

Pulsar Nebula is a calorimeter of the wind
Pulsar Related Emitting Zones

Pulsar(Massaro+)

PWN(Aharonian&Atoyan)

Wind is not expected to emit any obvious radiation component,
however it is possible to infer its properties through thenon-thermal emission of the nebula
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Pulsar Winds in High Energy Astrophysics

Kennel&Coroniti1984
Pulsar wind

Sketch of the model

Total Luminosity
Lw ' 6 × 1038 erg/s
Bulk Lorentz Factor
Γw ' 3 × 106
Wind Magnetization
σ ' 3 × 10−3

1D MHD model by Kennel&Coroniti favored an ultrarelativistic
weakly magnetized wind: this allows optical-to-VHE emission tobe explainedD.Khangulyan

Pulsar Winds in High Energy Astrophysics

2D modeling of the Crab nebula
Bogovalov&Khangulyan2002

Crab Nebula

The observed structure in the Crab Nebula is likely related toanisotropy of the pulsar wind, and can be checked by 2D
modeling
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Pulsar Winds in High Energy Astrophysics

2D numerical modeling of the Crab nebula
Del Zanna+2006

Crab Nebula

A few groups achieved a very impressive progress in 2Dmodeling of the Crab nebula: Amato, Bucciantini, Del Zanna+;
Komissarov+; Bogovalov+
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Pulsar Winds in High Energy Astrophysics

What is the pulsar wind
Pulsar and Nebula

1D MHD
Bulk Lorentz FactorΓw ∼ 106Wind Magnetizationσ ∼ 3 · 10−3
2D MHD
Bulk Lorentz Factor(?)
Wind Magnetizationσ ∼ 10−2

The properties of pulsar winds are not yet firmly established, however
the requirement of ultrarelativistic wind apparently remains
unavoidableD.Khangulyan

Pulsar Winds in High Energy Astrophysics

Is that the final theory?
Objections against the “Conventional Wind”
Lack of any consistent models for the wind formation
Despite very large efforts, there is still no (even preliminary) model for the wind acceleration

Simplification employed in the nebula modeling, e.g.,
Ideal MHD
Because of the striped structure of the wind, magnetic reconnection may play a very important role

Phenomenological acceleration mechanism
It is unclear how the pulsar termination shock can accelerate particles

Uniformed properties of the wind
The wind should have strong angular dependence, both in energy and in magnetic field strength

Radio emission excess
In the frameworks of the conventional scenario, the modeling significantly under-estimate the flux level in theradio bandThe concept of cold ultra-relativistic pulsar wind has strong
advantages, however, is also challenged in a few different aspects,
thus needs further observational and theoretical proofs.
D.Khangulyan

Pulsar Winds in High Energy Astrophysics

VHE observations of the Crab pulsar
MAGIC Collaboration(2008)

Veritas Collaboration(2011)

MAGIC Collaboration(2011)
Currently there is a generalagreement between different
experiments regarding the detection of the pulsed VHE emission from the Crab pulsar

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Pulsar Winds in High Energy Astrophysics

Where is the pulsed VHE emission produced?
Calculations by K.Hirotani

PulsarMagnetosphere(e.g. Aleksić+2012)
PWN
Pulsar Wind(Bogovalov&Aharonian2001)

The magnetospheric origin of the pulsed VHE signal is strongly
favored, however the modeled lightcurve is not yet inacceptable agreement
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Pulsar Winds in High Energy Astrophysics

Where is the pulsed VHE emission produced?
PulsarMagnetosphere(e.g. Aleksić+2012)
PWN
Pulsar Wind
(Bogovalov&Aharonian2001)

Production of pulsed emission
in the nebula is impossible, buttill recently the Crab flares werealso impossible...

Crab Flare (Buehler+2012)

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Pulsar Winds in High Energy Astrophysics

Where is the pulsed VHE emission produced?
However, it was predicted that the pulsar can also emit
potentially detectable emission, which under certain conditions
can be pulsed
Signal produced by cold pulsar wind

PulsarMagnetosphere(e.g. Aleksić+2012)
PWN
Pulsar Wind(Bogovalov&Aharonian2001)

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Pulsar Winds in High Energy Astrophysics

Bulk Comptonization of the wind
Sketch of the scenario (Aharonian+2012)

If the pulsar wind is accelerated enough close to themagnetosphere, a quite strong IC emission can be generated
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Pulsar Winds in High Energy Astrophysics

Is the wind IC emission pulsed?
Sketch of the scenario (Aharonian+2012)

The X- and gamma-ray signals are perfectly synchronized if thewind is formed NOT very close to the pulsar

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Pulsar Winds in High Energy Astrophysics

Inverse Compton interaction angle is determined fromthe first principles (Bogovalov&Aharonian2001)
Energy–Angular Momentum losses of pulsar:
Ėsd = ΩṀsdEnergy carried by an electron:
Γw mc 2Angular Momentum carried by an electron:
Γw mr⊥ v
Pulsar wind trajectory (if σ  1 ):cr⊥ = = R LΩ
It is plausible that during the acceleration stage the wind
trajectories are bended leading to a non-zero IC interaction
angle
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Pulsar Winds in High Energy Astrophysics

Spectrum can be explained
SED accounting for the wind signal (Aharonian+2012)

The flux level appears to depend strongly on the both model
parameters: wind formation distance and wind Lorentz factor
D.Khangulyan

Pulsar Winds in High Energy Astrophysics

Is the wind IC emission pulsed?
Sketch of the scenario (Aharonian+2012)

The X- and gamma-ray signals are perfectly synchronized if thewind is formed NOT very close to the pulsar

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Pulsar Winds in High Energy Astrophysics

Wind signal lightcurveLight Curve (Aharonian+2012)

Although the VHE lightcurve is consistent with the data points if
the wind is formed not very close to the pulsar, the ratio of thelightcurve peaks indicates on presence of some wind
anisotropy
D.Khangulyan

Pulsar Winds in High Energy Astrophysics

Is that the final theory?
Objections against the “Conventional Wind”
Lack of any consistent models for the wind formation
Despite very large efforts, there is still no (even preliminary) model for the wind acceleration

Simplification employed in the nebula modeling, e.g.,
Ideal MHD
Because of the striped structure of the wind, magnetic reconnection may play a very important role

Phenomenological acceleration mechanism
It is unclear how the pulsar termination shock can accelerate particles

Uniformed properties of the wind
The wind should have strong angular dependence, both in energy and in magnetic field strength

Radio emission excess
In the frameworks of the conventional scenario, the modeling significantly under-estimate the flux level in theradio band

The concept of cold ultra-relativistic pulsar wind has strong
advantages, however, is also challenged in a few different aspects,
thus needs further observational and theoretical proofs.
D.Khangulyan

Pulsar Winds in High Energy Astrophysics

Does ultrarelativistic wind violate radio fluxes?
What one needs
νLν = 1033

−1
Ntot = 2 × 1050 B0.1mG

ν
108 Hz0.7erg s−1−1.5
Etot = 7 × 1047 B0.1mG
erg

What one has
Ninj = 5 × 1049 Γ−1
6b = 4 × 1049 erg
Einj−1
Γ6 B0.1mG≤ 0.25The obtained value of the wind Lorentz factor (Γ6 = 0.5) isconsistent with the total number of electrons in the nebula for avery “comfortable” magnetic field strength of B = 200µG
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Pulsar Winds in High Energy Astrophysics

What can CTA do?

Precisemeasurements
around 50GeV are crucial

SED of pulsed emission

Current detectors operatefor ∼ 10 years
CTA maybe can improve in> 10years (given the expected start of operation)
Since a long collection of data is required to measure pulsed
signal, no immediate results from CTA are expected...

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Pulsar Winds in High Energy Astrophysics

Dark Matter Annihilation Line @ 130GeV
GeV excess in Fermi data (Su&Finkbeiner, 2012)

A very narrow GeV emission component was reported byseveral groups. This emission was interpreted as DM
annihilation
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Pulsar Winds in High Energy Astrophysics

Dark Matter Annihilation Line @ 130GeV
Further results by Boyarsky+(2012)

There are some hints on a several “hot spots” with slightlydifferent emitted energy
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Pulsar Winds in High Energy Astrophysics

Dark Matter Annihilation Line @ 130GeV

Is the “cosmological” interpretation doubtful?
Why the center of mass of strongest excess is shifted inrespect to the GC? (e.g. Su&Finkbeiner2012)
What is the origin of the other excesses?
yarsky+2012)(e.g., Bo-

Why are excesses detected at different energies? (e.g.
Su&Finkbeiner2012)
Standard cosmological interpretation faces a few severe
challenges, thus an astrophysical solution is wanted...

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Pulsar Winds in High Energy Astrophysics

Astrophysical GeV Lines
Klein-Nishina is the only mechanism (Aharonian+2012)

Typically non-thermal radiation mechanisms result in apower-law distribution, but there is one exception
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Pulsar Winds in High Energy Astrophysics

Astrophysical GeV Lines

But the photon field must be very hot > 5 × 104 K (Aharonian+2012)

GeV line emission can be produce by pulsar wind, but in thiscase the source is expected to be point-likeD.Khangulyan

Pulsar Winds in High Energy Astrophysics

Astrophysical GeV Lines: Point Like Sources?
Currently available data (Aharonian+2012)

The “diffuse” components are in fact consistent with point-like
contributions
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Pulsar Winds in High Energy Astrophysics

What can CTA do?
Astronomy Picture of the Day, July 27

HESS II inauguration on September 27-28

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Pulsar Winds in High Energy Astrophysics

Binary pulsar PSR B1259

There are a few systems where non-accretion pulsar is orbitinga massive star
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Pulsar Winds in High Energy Astrophysics

Fermi/LAT Flare from PSR B1259-63

High Energy lightcurve has shown unexpected flare
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Pulsar Winds in High Energy Astrophysics

Fermi/LAT Flare from PSR B1259-63
Very Fast Rise

Nearly SD luminosity

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Pulsar Winds in High Energy Astrophysics

Hydrodynamic modeling by Bogovalov+(2007)

Discovered an instability in the structure of the termination
shock – the transition from closed to unclosed shape. Is thatthe source of the flare?!

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Pulsar Winds in High Energy Astrophysics

Wind Signal from PSR B1259-63

The wind signal is expected to be detectable with Fermi
D.Khangulyan

Pulsar Winds in High Energy Astrophysics

Many Impacting Factors, e.g. GeometryHowever, a precise prediction can be hardly made given theuncertainties in the system parameters
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Pulsar Winds in High Energy Astrophysics

D.Khangulyan

Pulsar Winds in High Energy Astrophysics

Fermi/LAT Flare from PSR B1259-63

The model predictions are roughly consistent with theobservations, if the disk can be strongly perturbed to provide
enough dense target field
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Pulsar Winds in High Energy Astrophysics

PSR B1259-63 flare in VHE (from talk by Sushch@GAMMA2012)

The flare epoch was nicely covered with observations withHESSD.Khangulyan

Pulsar Winds in High Energy Astrophysics

PSR B1259-63 flare in VHE (from talk by Sushch@GAMMA2012)

There were no changes in the VHE emission. As expected inthe suggested scenario.
D.Khangulyan

Pulsar Winds in High Energy Astrophysics

Summary
VHE pulsations
The pulsed VHE signal is the best interpreted as IC emission from the wind
Current measurements of the pulsed VHE signal allows to constraint the key properties of the pulsar wind
Bulk Lorentz factor Γw ' 5 × 105
Wind formation distance Rw ' 30RL
Currently available lightcurve doesn’t allow any robust conclusions, however hints on some wind anisotropy
If the future data confirm the wind origin of the signal, this provides important information about the physicsof the pulsar winds

GeV Line
The reported GeV-line excess has a few puzzling features, which are complicated to be address in thescenario of the DM annihilation
The only astrophysical interpretation of the GeV-line is the signal produced by cold ultrarelativistic pulsar
wind
The pulsar interpretation is (in principle) consistent with the signal morphology (in energy and spatially)

GeV Flare from PSR B1259-63
The suggested model naturally allows to address a few very puzzling features of the flare
Provides another measurement of the pulsar wind bulk Lorentz factor, which suggests (if compared to thevalue inferred in the case of the Crab pulsar) that the number of pairs generated by different pulsars is similar
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Pulsar Winds in High Energy Astrophysics