X-ray Polarimetry X-ray Polarimetry and Radio-Quiet AGNand Radio-Quiet AGN

GiorgioGiorgio MattMatt ( Dipartimento di Fisica, Università Roma Tre)( Dipartimento di Fisica, Università Roma Tre)

In In Radio-quiet AGNRadio-quiet AGN, X-ray Polarimetry , X-ray Polarimetry may:may: Probe Probe strong gravity effectsstrong gravity effects

Determine the geometry of the Determine the geometry of the emitting emitting coronacorona

Probe the geometry of the Probe the geometry of the torustorus

Disclose the past activity of Disclose the past activity of the the Black Hole in the GalaxyBlack Hole in the Galaxy

Urry & Padovani (1995)

Strong gravity effects on Strong gravity effects on polarizationpolarization

General and Special Relativity effects around a compact object (here-in-after collectively indicated as “strong gravity effects”)

significantly modifies the polarization properties of the radiation (ax explained in M. Dovciak’ talk).

In particular, the Polarization Angle (PA) as seen at infinity is rotated due to aberration (SR) and light bending (GR) effects (“Gravitational Faraday rotation”, e.g. Connors & Stark 1977; Pineault 1977, Ishihara et al. 1988). The rotation is larger for

smaller radii and higher inclination angles(Connors, Stark & Piran 1980)

Newtonian- - - - - - - - - - - - - - - - - - - - - - -

Orbiting spot with:a=0.998; R=11.1 Rg

i=75.5 deg

(Phase=0 when the spot is behind the BH).

The PA of the net (i.e. phase-averaged)

radiation is also rotated!

Depolarization….

Strong Gravity in AGNStrong Gravity in AGN

In Active Galactic Nuclei the primary X-ray emission is dueto Inverse Compton by electrons in a hot Corona of the UV/Soft X-ray disc photons. It is likely to be significantly polarized (e.g. Haardt & Matt 1993, Poutanen & Vilhu 1993),because the system is unlikely to have a spherical symmetry. Part of the

primary emissionilluminates the

disc and is reflected

(and polarized) via

Compton Scattering

Polarization of reflected Polarization of reflected fluxflux

Polarization of reflected (continuum) radiation is large. For instance, it isup to 20% (Matt et al. 1989) assuming isotropic illumination, a plane-parallel reflecting slab and unpolarized illuminating radiation. The exact values depend on the actual geometry of the system and on the polarization degree of the primary radiation.

A test case: MCG-6-30-15 A test case: MCG-6-30-15

Variations of h have been suggested to be the cause of the puzzling temporal behaviour of the iron line in MCG-6-30-15 (Miniutti et al. 2003), where the line flux varies much less than the primary power law flux. This situation is what expected in the aborted jet model for the corona (Ghisellini et al. 2004).

The polarization degree and angle depend on both h and the

incl. angle (the latter may be estimated from the line profile;

for MCG-6-30-15 is about 30 degrees, Tanaka et al. 1995)

Variation of h with timeVariation of h with timeimplies a time/flux variation of implies a time/flux variation of

the degree and angle of the degree and angle of polarizationpolarization

(Dovciak, Karas & Matt 2004)

Polarization AnglePolarization Angle

Schwarzschild, a=0 Extreme Kerr, a=1

Dovciak et al. (2011)

Primary emission from the hot Primary emission from the hot coronacorona

The geometry of the hot corona is unknown. Emission is expected

to be polarized if the corona OR the radiation field are if the corona OR the radiation field are not spherical not spherical Polarimetry will help

understanding the geometry of the emitting

region

Haardt (1997)

NB: in ADAF models, no NB: in ADAF models, no significant polarization is significant polarization is expected (LLAGN should expected (LLAGN should

be unpolarized)be unpolarized)

Primary emission from the hot Primary emission from the hot coronacorona

Tamborra & Matt (2012)

See also Schnittman & Krolik (2010)

Reflection from the torusReflection from the torus

Is the torus really a Is the torus really a torus?torus?

The actual geometry of the obscuring

matter is not well known.

X-ray polarimetry can help in this respect:

the polarization degree is a measure

of the asphericity, the polarization angle will tell us the orientation

of the “torus”.Urry & Padovani (1995)

The orientation of the torusThe orientation of the torus

Geometry of the Geometry of the torus:torus:

the polarization angle will give us the orientation of the torus, to be compared with IR results, and with the ionization cones (Goosmann & Matt 2011) Raban et al.

(2009)

The orientation of the torusThe orientation of the torus

Goosmann & Matt (2011)

Below a few keV emission is dominated by reflection from

the ionization cones, athigher energies from the

torus.

The strange case of The strange case of Sgr B2Sgr B2SgrB2 is a giant molecular cloud at

~100pc projected distance from the Black HoleThe spectrum of SgrB2 is a pure The spectrum of SgrB2 is a pure reflection spectrum (Sunyaev et al. 1993)reflection spectrum (Sunyaev et al. 1993)

But no bright enough source is there !!!

INTEGRAL Image of GC (Revnivtsev 2004)

The emission from SgrB2 is extended and brighter in the direction of the BH (Murakami 2001). It is also varying in time (Inui et al. 2008).

Is SgrB2 echoing past emission from the BH, which was therefore one million time more active (i.e. a LLAGN) ~300 years ago ??? (e.g. Koyama et al. 1996)

Was the GC an AGN a few hundreds Was the GC an AGN a few hundreds years ago?years ago?

X-ray polarimetry can definitively proof or reject this hypothesis.

SgrB2 should be highly polarized with the electric SgrB2 should be highly polarized with the electric vector perpendicular to the line connecting the two vector perpendicular to the line connecting the two sources. sources.

The degree of polarization would measure the angle and provide a full 3-d representation of the clouds (Churazov et al. 2002)

Unfortunately, the flux of SgrB2 is decreasing with time (Inui et al. 2009)

But there are many other time-varying molecular clouds, so we can hope that at the time of the first polarimetry mission (soon, hopefully!) there will be at least one bright enough to test the AGN scenario for the GC.

From Chandra web site

The observational The observational landscapelandscape

No X-ray polarimetry since the 70’s (OSO-8)

New detectors, based on the photoelectric effect (Costa et al. 2001)

and coupled with large area X-ray mirrors provide completely new

perspectives

One mission (GEMS, a Small Explorer NASA mission) is to be launched in

2014, providing a fresh new start to X–ray polarimetry

Hopefully, other and more ambitious missions will follow

Observational Observational perspectivesperspectives

Strong gravity effects: challenging (but

X-ray binaries can help)

Hot corona: probably easy

Reflection from the torus: feasible

Galactic Centre: challenging (but interpretation straightforward)

SummarySummary

X-ray polarization measurements will provide valuable information

on the physical conditions and the the physical conditions and the

geometry of the emitting regions geometry of the emitting regions in in

radio-quiet AGNradio-quiet AGN, and will permit to test the hypothesis that our own Galaxy was an AGN our own Galaxy was an AGN

about 300 years ago.about 300 years ago.

CollaborationsCollaborationsA close collaboration between theoretical groups

(Univ. Roma Tre, Academy of Science of Czech Republic, Observatory of Strasbourg) is behind

many of the results presented here.

A strong connection with the X-ray polarimetry experimental group at INAF/IAPS, Rome (see E.

Costa’s talls) does also exist, with good results.

Expanding these collaborations towards other theoreticians and/or experimental/observational

groups in other bands (where real data do exist!) would be highly beneficial for at least some of the scientific goals presented here.