X-ray polarimetry in astrophysics Paolo Soffitta IAPS-INAF COST Action 1 st WG meeting Warsaw : 7-9 May 2012

X-ray polarimetry in astrophysics

Paolo SoffittaIAPS-INAF

COST Action 1st WG meeting Warsaw : 7-9 May 2012

Why X-ray Astrophysical Polarimetry ?Why X-ray Astrophysical Polarimetry ?

Polarization from celestial sources in X-rays may derive from:

•Emission processes themselves : Emission processes themselves :

cyclotron, synchrotron, non-thermal bremmstrahlungcyclotron, synchrotron, non-thermal bremmstrahlung

(Westfold, 1959; Gnedin & Sunyaev, 1974; Rees, 1975)

•Scattering on aspherical accreting plasmas : Scattering on aspherical accreting plasmas :

disks, blobs, columns. disks, blobs, columns. ( Rees, 1975 Sunyaev & Titarchuk, 1985; Mészáros, P. et al. 1988, Sazonov 2002).

• Vacuum polarization and birefringence through extreme magnetic fieldsVacuum polarization and birefringence through extreme magnetic fields

(Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura, 1979)


Polarization from non thermal Bremmstrahlung.

100 keV electrons

500 keV electrons

2.5 MeVelectrons


Gluckstern & Hull, 1953

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.

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

Beams of electrons producing non-thermal Bremmstrahlung are present in solar flares:

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 uniformly distributed in a cone with 30o aperture. Positive values the polarization is parallel to the radiation plane (p0k). Negative values P is parallel to the normal plane.

The morphology of a solar flare at hard X-rays is complex with a top source and two foot-prints.

Thermal Bremm.

Non Thermal Bremm.


Observational results

Hard- X ray polarimetry from the mission RHESSI (100-350 keV) shows inconclusive results.

Errors are 1s; E. Suarez-Garcia et al., 2006


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.

Synchrotron radiation


• Pulsar wind nebulae.• Blazars.• GRB prompt and afterglow emission


Pulsar Wind NebulaePulsar 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).

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.

Bucciantini et al., 2009


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).

BLAZARSThe 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)• Inverse Compton of ambient radiation (EC) by jet (from disk or Cosmic Microwave Background ).

Poutanen , 1993.

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

Synchrotron

Synchrotron self-Compton or External Compton

Celotti&Matt, 1993

- The polarization of EC is generally smaller than that expected by SSC and has been investigated by simulation (McNamara et al., 2009, 2010).

The polarization of the scattered radiation is about 50 % of the initial synchrotron polarization.

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.

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).

BAT

KONUS-WIND

SUZAKU WAM


Different models with different polarization signatures

•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

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)

Tentative to measure the Gamma-ray polarization of GRBs.


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.

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 %.

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 measured (Steele at al., Nature, 2009). COST Action 1st WG meeting Warsaw :

7-9 May 2012

Homogeneous jet : the emissivity is constant within the jet opening angle.

Structured jet: the emissivity drops off axis.

Homogeneous jet

Afterglow polarimetry

Scattering polarizes radiation

The radiation after the scattering is partially polarized perpendicularly to the plane of scattering.


Accretion physics: magnetic NSAccretion physics: magnetic NS

Meszaros et al. (1988)

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.

Energies (keV). 1.1.6 keV 5. 29.1 keV2. 3.8 keV 6. 38.4 keV 3. 9.0 keV 7. 51.7 keV 4. 18.4 keV 8. 84.7 keV


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.


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) .

Scattering in accretion diskIn 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>>to

2 , i.e. the Comptonized tail of the radiation spectrum.

Scattering in millisecond X-ray pulsars

Sazonov&Sunyaev, 2001.

Poutanen&Gierlinsky, 2003Thermal Comptonizazion

Black-body

Compton reflection plus iron-line

Disk scattered radiation can lead or lag the direct main pulse, because of Doppler boosting.

Solid : radiation from both spot; dashed primary spot; dotted secondary spot

Viironen&Poutanen,2004COST Action 1st WG meeting Warsaw : 7-9 May 2012

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.

Total

Primary spotSecondary spot

Polarization is positive normally to the n k plane (meridional plane).

Disk corotates Disk counter-rotates

X-ray polarimetry and strong gravity effects around black-holes.

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.

(Talks of G. Matt and M. Dovciak)

Galactic black holes

Matter very close to the black hole experiences General and Special Relativity (due to the large velocities involved) called Strong Gravity.

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.


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

High Field. Polarization angle is constant with energy.

Van Adelsberg&Lai,2006Niemiec&Bulik,2006

What can be observed with modern photoelectric polarimeters :

Focal plane polarimeter (With standard multi-layer optics)Effective area :600 cm2 2-8 keV100 cm2 at 30 keV


Experiment with collimators (no optics)

MDP is the minimum detectable polarization at 99 % confidence level

1 mCrab (2-10 keV) = 2.4 10-11 erg/s/cm2

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.


End presentation


Documents

X-ray polarimetry in astrophysics Paolo Soffitta IAPS-INAF COST Action 1 st WG meeting Warsaw : 7-9 May 2012