Transcript X-ray polarimetry in astrophysics

X-ray polarimetry in astrophysics
Paolo Soffitta
IAPS-INAF
COST Action 1st WG meeting Warsaw : 7-9 May 2012
Why X-ray Astrophysical Polarimetry ?
Polarization from celestial sources in X-rays may derive
from:
•Emission processes themselves :
cyclotron, synchrotron, non-thermal bremmstrahlung
(Westfold, 1959; Gnedin & Sunyaev, 1974; Rees, 1975)
•Scattering on aspherical accreting plasmas :
disks, blobs, columns. (
1975 Sunyaev & Titarchuk, 1985; Mészáros, P. et al. 1988, Sazonov 2002).
• Vacuum polarization and birefringence through extreme magnetic fields
(Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura, 1979)
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Rees,
Polarization from non thermal Bremmstrahlung.
When a beam of electrons is deflected by the nuclei of the target medium the produced radiation is polarized. The polarization
dependent cross section for a given a photon momentum and electron emerging direction can be integrated over the direction of the
emerging electrons. A non null net linear polarization is present at both ends of the photon energy spectrum except when the beam
axis is on the same direction of the line of sight and in this case the polarization is null.
100 keV
electrons
Qo is the angle between the direction of the
electron beam and the direction of the
considered photon beam (radiation plane).
The polarization is ‘negative’ if it is in the
radiation plane
500 keV
electrons
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2.5 MeV
electrons
Gluckstern & Hull, 1953
Beams of electrons producing non-thermal
Bremmstrahlung are present in solar flares:
Thermal Bremm.
Non Thermal
Bremm.
Bai and Ramaty, 1978
Degree of Polarization of primary hard X-rays due to accelerated electrons with a
power-law energy distribution moving toward the photosphere with velocity
The morphology of a solar flare at hard X-rays is
uniformly distributed in a cone with 30o aperture. Positive values the
complex with a top source and two foot-prints.
polarization is parallel to the radiation plane (p0k). Negative values P is parallel to
the normal plane.
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Observational results
Errors are 1s; E. Suarez-Garcia et al., 2006
Hard- X ray polarimetry from the mission RHESSI (100-350 keV) shows inconclusive results.
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Synchrotron radiation
The polarization of the single electron does depends
on the frequency
The polarization of a power-law distributed electrons (N(E) ˜ E-p)
does not depends on the frequency it is linear and it is perpendicular
to the projection of the magnetic field onto sky.
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• Pulsar wind nebulae.
• Blazars.
• GRB prompt and afterglow emission
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Pulsar Wind Nebulae
Pulsar Wind Nebulae are bubbles of relativistic particles and magnetic field (10-5-10-3
Gauss) created when the ultra-relativistic wind from a pulsar interacts with the ambient
medium (Supernova Remnants or Interstellar Medium) in a termination shock (TS).
Bucciantini et al., 2009
The pulsar wind is slowed down and the toroidal magnetic field of the wind is
compressed, the plasma is heated and particles are accelerated to high energies and
produce synchrotron radiation.
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A PWN was first observed in Crab SNR, and then detected in many others X-ray sources
X-rays emitting particles have a short lifetime for synchrotron losses, and they are
present only in the vicinity of the TS. Not imaging X-ray polarimetry from the Crab Nebula
shows a polarization of 19 % (2.6 keV, 5.2 keV; Weisskopf et al., 1975, 1978). Main
components of PWNs are :
• a main emission torus, corresponding to the equatorial plane of the pulsar rotation
• multiple arcs or rings
• a central knot, located in the vicinity of the pulsar,
• one or two opposite jets along the polar axis.
Relativistic MHD models are capable to reproduce the main features of the PWN including
the jets (Del Zanna et al., 2004).
X-ray imaging polarization maps permit to deduce the magnetic field direction, model the
acceleration of particle at TS and reveal the presence of an additional poloidal field
considered responsible for the presence of jets.
(Bucciantini et al., 2005; Volpi et al., 2009).
BLAZARS
The class of AGN called Blazars (BL Lac and OVV) are characterized by a high degree of
optical polarization and rapid variability. They are interpreted as AGN with a jet pointing close
to the observer.
Two models are proposed to explain the observed multi-wavelength spectrum :
• Synchrotron-self Compton (SSC)
Synchrotron
• Inverse Compton of ambient radiation (EC) by jet
(from disk or Cosmic Microwave Background ).
The polarization of
the
scattered
radiation is about
50 % of the initial
synchrotron
polarization.
•
Poutanen , 1993.
- The polarization of EC is generally smaller
than that expected by SSC and has been
investigated by simulation (McNamara et al.,
2009, 2010).
•
Synchrotron
self-Compton
or External
Compton
Fossati et al., 1998
Characteristic of the polarization vector for SSC :
The polarization vector is always parallel for the
synchrotron and synchrotron self-Compton component.
The polarization is normal to the Jet axis (normal to B) for
an uniform B . In case of a axis-symmetric magnetic field
configuration it depends whether dominates the
component parallel or perpendicular to the symmetry axis
Celotti&Matt, 1993
Gamma Ray Bursts (GRB)
Gamma-ray bursts are the most energetic events in the Universe, with radiated energy up to 1054 erg. The
collapse of a massive rotating star produces a hot fireball which is ejected at relativistic velocity in form of a jet.
Internal shocks are responsible for the observed variability (prompt emission). When the fireball interacts with
the ambient medium it is produced an afterglow. At first the jet is tightly collimated with an opening angle q. The
relativistic aberration allows for seeing most of the energy in a narrow beam, more collimated than q. As the
fireball slows-down the beaming spreads exceeding q at the jet-break time with an achromatic light-curve
bending.
SUZAKU WAM
KONUS-WIND
BAT
GRB 050904 was one the most distant event ever observed at z=6.295 or 13 billion
years ago. The peak energy was 314 keV the peak intensity was ~ 10-7 erg/s/cm2
GRB 090423 was observed at z = 8.1 or 14.4 billions years ago (4 % age of Universe).
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Different models with different polarization signatures
The outflow that produces the prompt event may contain magnetic fields and synchrotron emission could be
responsible for the observed radiation. While on the afterglow is well modeled for the prompt event there
are many theories that cannot be understood with only timing and spectral measures.
Lyutikov, Pariev & Blanford, 2003; Waxman, 2003; Gruzinov & Waxman, 1999; Heinz &
Begelman, 1999; Dado & Dar, 2009;Yamazachi et al., 2006)
• Toroidal magnetic field : The whole radiation does not produce a net polarization but due to
light aberration the observer probes a small region with the magnetic field ordered in a
specific direction. Therefore the polarization is high.
• Random magnetic field : The polarization can still be high if the line of sight is 1/G with
respect to the edge of the jet.
• Magnetic field domain: If the magnetic field is patchy the polarization is the maximum
obtainable from synchrotron process divided by the number of patches.
• Fragmented fireball (each pulse in the light-curve is due to the emission of different blobs):
The flux and polarization depends on the orientation of the individual blobs
Tentative to measure the Gamma-ray polarization of GRBs.
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Afterglow polarimetry
Afterglow radiation is known to be produced by synchrotron from relativistic electrons. The
magnetic field is tangled and compressed in the plane perpendicular to the motion.
Measurements of polarization as a function of time can reveal if the jet is homogeneous or
structured.
Homogeneous jet : the emissivity is constant within
Homogeneous jet
the jet opening angle.
The polarization is initially vanishingly small. At times
before the jet breaks a small polarization is expected
becoming null at the jet break. Afterwords it reappears
rotated by 90 degree with a maximum of 10 %.
Structured jet: the emissivity drops off axis.
Uniform jet
If the fireball is structured the polarization is instead expected to have a constant angle. The
degree of polarization reaches a maximum at the break time.
The measure of the polarization of the X-ray afterglows is difficult because of the few %
expected by optical measurements and because they are faint and the flux decrease with time.
In optical light an early afterglow (161 s after the trigger) polarization of 10 ±1 % has been
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measured (Steele at al., Nature, 2009).
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Scattering polarizes radiation
The radiation after the scattering is partially polarized perpendicularly to the
plane of scattering.
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Accretion physics: magnetic NS
In X-ray pulsators the plasma from a companion accretes on the magnetic poles of a neutron star following
the magnetic (B ~1012 gauss) field lines. Cyclotron lines of different orders are produced by the transition
between the Landau levels. Scattering in such an accreting plasma is anisotropic since the cross section for
polarization perpendicular to B is smaller than that for polarization parallel to B.
Phase resolved X-ray polarimetry compared to phase resolved X-ray photometry may constraint the
accretion geometry determining if the accretion is in form a ‘pencil’ or a ‘fan’ beam and direct measuring
the angle between the dipole axis and the rotation axis.
1.
2.
3.
4.
Energies (keV).
1.6 keV
5. 29.1 keV
3.8 keV
6. 38.4 keV
9.0 keV
7. 51.7 keV
18.4 keV
8. 84.7 keV
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Meszaros
al.2012
(1988)
Cyclotron lines from X0115+63
Shown is the energy spectrum by BeppoSAX (Santangelo et al., 1999) of X0115+63 with four
cyclotron harmonics. It is shown the Minimum Detectable Polarization (MDP) expected
(above 10 keV) by a conventional photoelectric polarimeter at the focus of a multi-layer optics
(200 ks of observation) .
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Scattering in accretion disk
In case of a disk dominated by Thomson scattering with infinite optical depth, the classical
result foresees that the polarization increases from 0 % (disk seen face-on) to 11.7 % (disk seen
edge-on) with the polarization vector parallel to the disk surface. This is the so called
Chandrasekhar limit (Chandrasekhar, 1960). In case of small optical depth the hard X-rays
obtained by Comptonization bring memory of the last scattering and they overcome by far this
limit.
Polarization of the total radiation (scattered
plus un-scattered).
Polarization of the hard radiation, which has
undergone a number of scattering k>>to2 , i.e. the
Comptonized tail of the radiation spectrum.
Scattering in millisecond X-ray pulsars
Millisecond X-ray pulsar are the link between Low Mass X-ray Binaries and millisecond radio pulsar with the pulsed emission originating on two
antipodal spots. Scattering on the inner accretion disk may be important (as in Sazonov & Sunyaev) with polarized flux leading or trailing the
primary pulse of direct emission or may not be important since it prevails thermal Comptonization at the accreting slab (Viironen & Poutanen)
on the neutron star magnetic poles. In this case the direct emission is in phase with the polarized flux. Pulse and polarization profile can also
help to determine the compactness of the neutron star due to gravity effects.
Poutanen&Gierlinsky, 2003
Thermal
Comptonizazion
Black-body
Sazonov&Sunyaev, 2001.
Disk corotates
Disk counter-rotates
Compton reflection plus
iron-line
Total
Primary spot
Secondary spot
Polarization is positive
normally to the n k plane
(meridional plane).
Solid : radiation from both spot; dashed primary spot; dotted secondary spot
Disk scattered radiation can lead or lag the
direct main pulse, because of Doppler boosting.
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Viironen&Poutanen,2004
X-ray polarimetry and strong gravity
effects around black-holes.
(Talks of G. Matt and M. Dovciak)
Matter very close to the black hole experiences General and Special Relativity
(due to the large velocities involved) called Strong Gravity.
Galactic black holes
In galactic black-hole the disk emits in X-ray. The thermal emission by scattering becomes
polarized and the polarization signature of strong gravity is a continuous variation with
energy of the polarization degree and angle.
AGN
In AGNs the disk emits in UV and the X-ray polarization signature of strong gravity is
possible thanks to variability of the reflection component of the observed radiation.
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QED effects on magnetars and X-ray
polarization
A magnetar is a type of neutron star with an extremely powerful
magnetic field. The decay of magnetic fields powers the emission
of high-energy electromagnetic radiation (Thomson and Duncan
1993).
The polarization signatures of the emerging radiation with
respect to energy strongly depends on the magnetic field. QED
effects of vacuum polarization modify the behavior of
polarization with energy.
Low Field. Polarization angle
changes 90o with energy
Van Adelsberg&Lai,2006
Niemiec&Bulik,2006
High Field. Polarization angle is
constant with energy.
What can be observed with modern
photoelectric polarimeters :
MDP is the minimum detectable polarization at 99 % confidence level
Focal plane polarimeter
(With standard multi-layer optics)
Effective area :
600 cm2 2-8 keV
100 cm2 at 30 keV
Experiment with
collimators (no optics)
1 mCrab (2-10 keV) = 2.4 10-11 erg/s/cm2
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What COST can do for X-ray
polarimetry
• The theoretical expectation are within the
reach of X-ray polarimeters as dedicated
instruments (see talk of Enrico Costa).
• What is needed is that COST put an action on
ESA for considering X-ray polarimetry in a
future X-ray missions.
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May 2012
End presentation
COST Action 1st WG meeting Warsaw : 7-9
May 2012