Presentation on theme: "X-ray Polarimetry and Radio-Quiet AGN"— Presentation transcript:
1X-ray Polarimetry and Radio-Quiet AGN Giorgio Matt( Dipartimento di Fisica, Università Roma Tre)
2In Radio-quiet AGN, X-ray Polarimetry may: Probe strong gravity effectsDetermine the geometry of the emitting coronaProbe the geometry of the torusDisclose the past activity ofthe Black Hole in the GalaxyUrry & Padovani (1995)
3Strong gravity effects on polarization 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 anglesOrbiting spot with:a=0.998; R=11.1 Rgi=75.5 deg(Phase=0 when the spot is behind the BH).(Connors, Stark & Piran 1980)The PA of the net(i.e. phase-averaged) radiation is also rotated!Depolarization….Newtonian
4Part of the primary emission illuminates the disc and is reflected Strong Gravity in AGNIn Active Galactic Nuclei the primary X-ray emission is dueto Inverse Compton by electrons in a hot Corona of theUV/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) viaCompton Scattering
5Polarization of reflected flux 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.
6A test case: MCG-6-30-15 (Dovciak, Karas & Matt 2004) Variations of h have been suggested to be the cause of the puzzling temporal behaviour of the iron line in MCG (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 is about 30 degrees, Tanaka et al. 1995)Variation of h with timeimplies a time/flux variation of the degree and angle of polarization(Dovciak, Karas & Matt 2004)
8Primary emission from the hot corona The geometry of the hot corona is unknown. Emission is expectedto be polarized if the corona OR the radiation field are not sphericalPolarimetry will helpunderstanding the geometry of the emitting regionNB: in ADAF models, no significant polarization is expected (LLAGN shouldbe unpolarized)Haardt (1997)
9Primary emission from the hot corona Tamborra & Matt (2012)See also Schnittman & Krolik (2010)
10Reflection from the torus Is the torus really a 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)
11The orientation of the torus Geometry of the 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)11
12The orientation of the torus Below a few keV emission is dominated by reflection from the ionization cones, athigher energies from the torus.Goosmann & Matt (2011)12
13The strange case of Sgr B2 SgrB2 is a giant molecular cloud at ~100pc projected distance from the Black HoleThe spectrum of SgrB2 is a pure reflection spectrum (Sunyaev et al. 1993)But no bright enough source is there !!!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)INTEGRAL Image of GC (Revnivtsev 2004)
14Was the GC an AGN a few hundreds years ago? X-ray polarimetry can definitively proof or reject this hypothesis.SgrB2 should be highly polarized with the electric vector perpendicular to the line connecting the two sources.The degree of polarization would measure the angle and provide a full 3-d representation of the clouds (Churazov et al. 2002)
15Unfortunately, the flux of SgrB2 is decreasing with time (Inui et al From Chandra web siteBut 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.
16The observational landscape 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 perspectivesOne mission (GEMS, a Small Explorer NASA mission) is to be launched in 2014, providing a fresh new start to X–ray polarimetryHopefully, other and more ambitious missions will follow
17Observational perspectives Strong gravity effects: challenging (butX-ray binaries can help)Hot corona: probably easyReflection from the torus: feasibleGalactic Centre: challenging (but interpretation straightforward)
18SummaryX-ray polarization measurements will provide valuable information onthe physical conditions and thegeometry of the emitting regions inradio-quiet AGN, and will permitto test the hypothesis thatour own Galaxy was an AGNabout 300 years ago.
19CollaborationsA 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.19