X-ray polarimetry as diagnostic tool of the past activity of Sgr A* Paolo Soffitta IAPS/INAF (Rome, Italy) IAPS/INAF : Enrico Costa, Sergio Fabiani , Fabio Muleri , AldaRubini, Paolo Soffitta. INFN-Pisa : Ronaldo Bellazzini, Alessandro Brez, Massimo Minuti, Michele Pinchera, Gloria Spandre. The Galactic Center Black Hole Laboratory
900 x 400 Ly of the Galactic Center as seen in X-rays by Chandra 117 arcmin; color code : 1-3 keV ; 3-5 keV;5-8 keV 36 arcmin Sunyaev et al., 1993 Center Nebular Zone Art-P on GRANAT Sunyaev et al., suggested that part of the emission in the galactic center region could be due to Thomson scattering by dense molecular clouds of rtheadiation coming from the galactic center. Sgr B2 Sgr C Sgr A Complex
The strange case of Sgr B2 SgrB2 is a giant molecular cloud at ~100pc projected distance from the Black Hole The spectrum of SgrB2 is a pure reflection spectrum (Sunyaev et al. 1993) But no bright enough source is there !!! Revnivtsev, 2004 The emission from SgrB2 isextended and brighter in the direction of the BH (Murakami 2001). Itisalsovarying in time (Inui et al. 2008). Is SgrB2 echoingpastemission from the BH, whichwasthereforeonemillion time more active~300 years ago ??? (e.g. Koyama et al. 1996) INTEGRAL Image of GC (Revnivtsev 2004) The Galactic Center Black Hole Laboratory
Was the GC an AGN a fewhundredsyears ago? X-ray polarimetry can definitivelyproof or rejectthishypothesis. SgrB2 should be highlypolarized with the electricvectorperpendicular to the line connecting the twosources. The polarization direction of the scattered radiation is perpendicular to the scattering plane. The degree of polarizationwouldmeasure the angle and provide a full 3-d representation of the clouds (Churazov et al. 2002)
Fundamental parameters Fit function: Φis the azimuthal angle, thatis the angle with respect to the electricvector of the ‘carrier’ of the polarization information = Modulation: Modulation factor, µ : the modulation for 100% linearly polarized radiation is the key parameter of a polarimeter and ranges between 0 (no polarimetricsensitivity) to 1 (maximum sensitivity to polarization). Degree of Polarization is the modulation divided by the modulation factor. P =
Fundamental parameters Minimum Detectable Polarization (99%): S is the source rate, B is the background rate, T is the observing time, m is the modulation factor. If background is negligible like is the case for focal plane X-ray polarimeters: m = 736 103Counts Counts To reach MDP=1% with m=0.5: Source detection: > 10 counts Source spectrum slope: > 100 counts Source polarimetry: > 100,000 counts
Modern polarimeters dedicated to X-ray Astronomy exploit the photoelectric effect resolving most of the problems connected with Thomson/Bragg polarimeter. The exploitation of the photoelectric effect was tempted very long ago, but only since five-ten years it was possible to devise photoelectric polarimeters mature for a space mission. An X-ray photon directed along the Z axis with the electric vector along the Y axis, is absorbed by an atom. The photoelectron is ejected at an angle θ (the polar angle) with respect the incidentphotondirection and at an azimuthal angle φ with respect to the electricvector. If the ejected electron is in ‘s’ state (as for the K–shell) the differential cross sectiondepends on cos2 (φ),thereforeitispreferentiallyemitted in the direction of the electricfield. Being the cross sectionalwaysnull for φ= 90o the modulationfactor µ equals 1 for anypolar angle. HeitlerW.,The Quantum Theory of Radiation Costa, Nature, 2001 β =v/c By measuring the angular distribution of the emission direction of the ejected photoelectrons (the modulation curve) it is possible to derive the X-ray polarization.
X-raypolarimetry with a Gas Pixel Detector GEM electric field X photon (E) conversion GEM gain collection pixel PCB E a 20 ns The principle of detection To efficiently image the track at energies typical of conventional telescopes IASF-Rome and INFN-Pisa developed the Gas Pixel detector. The tracks are imaged by using the charge. A photon cross a Beryllium window and it is absorbed in the gas gap, the photoelectron produces a track. The track drifts toward the multiplication stage that is the GEM (Gas Electron Multiplier) which is a kapton foil metallized on both side and perforated by microscopic holes (30 um diameter, 50 um pitch)and it is then collected by the pixellated anode plane that is the upper layer of an ASIC chip. Costa et al., 2001, Bellazzini et al.2006, 2007 Polarization information is derived from the angular distribution of the emission direction of the tracks produced by the photoelectrons. The detector has a very good imaging capability. The Galactic Center Black Hole Laboratory
Tracksreconstruction 1) The track is collected by the ASIC 2) Baricenterevaluation (using all the triggered pixels) 3) Reconstruction of the principal axis of the track: maximization of the second moment of charge distribution 4) Reconstruction of the conversion point: third moment along the principal axis (asymmetry of charge distribution to select the lower density end) + second moment (length) to select the region for conversion point determination). 5) Reconstruction of emission direction: (maximization of the second moment with respect to the conversion point ) but with pixels weighted according to the distance from it. SPIE Optics + Photonics, San Diego 25-29 August 2013
ASIC features 105600 pixels 50 μm pitch • Peaking time: 3-10 ms, externally adjustable; • Full-scale linear range: 30000 electrons; • Pixel noise: 50 electrons ENC; • Read-out mode: asynchronous or synchronous; • Trigger mode: internal, external or self-trigger; • Read-out clock: up to 10MHz; • Self-trigger threshold: 2200 electrons (10% FS); • Frame rate: up to 10 kHz in self-trigger mode • (event window); • Parallel analog output buffers: 1, 8 or 16; • Access to pixel content: direct (single pixel) or serial • (8-16 clusters, full matrix, region of interest); • Fill fraction (ratio of metal area to active area): 92%) 1.5 cm The chip is self-triggered and low noise. It is not necessary to readout the entire chip since it is capable to define the sub-frame that surround the track. The dead time downloading an average of 1000 pixels is 100 time lower with respect to a download of 105 pixel. The Galactic Center Black Hole Laboratory
Extensively tested, with thermal-vacuum cycles, it has been vibrated, irradiated with Fe ions and calibrated with polarized and unpolarizedX-rays.. The real implementation of a working GPD prototype. HE-DME mixture: sensitive range 2-10 keV Electronics Titanium Frame Beryllium window 9 cm DME = (CH3)2O 60 µm/√cm diffusion Weight of the GPD + Lab Electronics = 2 kg Power Consumption of the GPD + Lab Electronics = 5 W The Galactic Center Black Hole Laboratory
IASF-Rome facility for the production of polarized X-rays. Close-up view of the polarizer and the Gas Pixel Detector Facility at IASF-Rome/INAF keV Crystal Line Bragg angle 1.65 ADP(101) CONT 45.0 2.01 PET(002) CONT 45.0 2.29 Rh(001) Mo Lα 45.3 2.61 Graphite CONT 45.0 3.7 Al(111) Ca Kα 45.9 4.5 CaF2(220) Ti Kα 45.4 5.9 LiF(002) 55Fe 47.6 8.05 Ge(333) Cu Kα45.0 9.7 FLi(420) Au Lα45.1 17.4 Fli(800) Mo Kα44.8 Aluminum and Graphite crystals. Capillary plate (3 cm diameter) Spectrum of the orders of diffraction from the Ti X-ray tube and a PET crystal acquired with a Si-PiN detector by Amptek PET (Muleri et al., SPIE, 2008) The Galactic Center Black Hole Laboratory
Not only MonteCarlo: Our predictions are based on data Eachphotonproduces a track. From the track the impact point and the emission angle of the photoelectronisderived. The distribution of the emission angle is the modulation curve. Muleri et al. 2007 Impact point The modulation factor measured 2.6 keV, 3.7 keV and 5.2 keV has been compared with the Monte Carlo previsions. The agreement is very satisfying. By rotating the polarization vector the capability to measure the polarization angle is shown by the shift of the modulation curve. Present level of absence of systematic effects (5.9 keV). Bellazzini 2010 Soffitta et al., 2010 The Galactic Center Black Hole Laboratory
More energies, more mixtures Pure DME (CH3)2O Modulation curve at 2.0 keV μ = 13.5% We performed measurement at more different energies and gas mixtures. (Muleri et al., 2008, 2010).
The imaging properties of the GPD. Panter X-ray facility (MPE, Germany): JET-X (Telescope, same as Swift, ~1mm/arcmin) Focal Length (3.5 m) JET-X HEW (4.5 keV) : 18’’ JET-X + GPD (HEW) : 23.2’’ (394 m ) IAPS/INAF laboratory : Very narrow pencil beam. Detector shifts : 300 m. Position resolution : 30 m (rms). Half Energy Width : 93 m Spiga et al., 2013, Fabiani et al. 2013 Imaging properties are mainly driven by the optics.
A Gas Pixel Detector for higher energies (6-35 keV) Ar-DME 2-atm; 2-bar Efficiency (dashed) and modulation Factor (solid) with Monte Carlo and measurement for the low energy (2-10 keV) polarimeter and medium energy (6-35 keV) polarimeter.
XIPE The X-ray Imaging Polarimetry Explorer. ESA (Small size) S1 (proposed 2012) (Soffitta et al., Exp.Ast. 2013) Dedicated instrument total 300 cm2optics. 2-10 keV, 23’’ HEW Athena+ ESA (Large) L2 (Medium size) M3 Prop. 2010 NHXM Tagliaferri et al, ExpAst 2010 Large instrument : 10000 cm2 2 keV, HEW 5’’ 2500 cm2 6 keV 1000 cm2 10 keV HEW 20’’ The Galactic Center Black Hole Laboratory
Different Mission, Different effective area Spiga et al., 2013 Tagliaferri et al., 2010 Area of three modules. For polarimetry (6-35 keV) we consider one mirror module GPD FOV 5.2’, HEW 20’’. For XIPE Area to be multiplied by 2 (two telescopes, GPD FOV 15’x15’, HEW 23.2’’, 2-10 keV) Athena+ 2-10 keV, FOV 4.3’ HEW 5’’ The Galactic Center Black Hole Laboratory
Suzaku view of Sgr B2 emission (Ryu, 2013) (Ryu 2009) (3) (1) Rear (1) Front (4,5) NGCE (2) Position of Sgr B2 along the line of sight. Best fit of Sgr B2 spectrum embedded in hot plasma (Ryu 2013) : 16.9 pc in the front. From VLBI radio parallax (Reid 2009) 130 +/- 60 pc in the front. NGCE Non Galactic center emission Sgr C is a complex of three sources distant from each others (Ryu, et al, 2013). The Galactic Center Black Hole Laboratory
Study of Sgr A complex Region from Sgr A* and the radio arc Recent studies with XMM and Chandra revealed the presence of molecular clouds (MC) possibly reflecting and reprocessing radiation perveningSgr A* Ponti et al. 2010 Clavel et al. 2013 • MCs are traced by line molecular emission like CS. • MCs are not in circular motion => Position and velocity do not constrain the distance. • MCs are identified by having similar velocity. The Galactic Center Black Hole Laboratory
Time and space resolved of neutral Fe K line emission with XMM Ponti et al., 2012 • Apparent superluminal propagation west to east appears in XMM data of the bridge. • Constant emission from MC1 and MC2. The position of the molecular clouds result from the hypothesis of a single flare seen by SgrB2 and G0.11-0.11, and the same luminosity of SgrA* as seen by the Bridge, MC1 and MC2. XMM Space and time resolved spectroscopy (Fe K) • Sketch of the positions of the molecular clouds as seen face on with SgrA* as black star on the vertex. • The position of Sgr B2 is measured by parallax (Reid 2009. • SINGLE FLARE The parabola connecting G0.11-0.11 and SgR B2 is the light front emitted 100 years ago. The further parabola represent a light front emitted 400 years ago. 100 pc 50 pc 100 pc
Time and space resolved of neutral Fe K line emission and continuum emission. Chandra Clavel et al. 2013 • Fe K line smoothed at 9 arcsec. • Stongly varying emission in the Sgr A complex with a clear trend from west to east in MC1 and MC2 a late illumination in BR1 and BR2 and a more complex variation in G0.11-0.11. 4-8 keV linear fit slope of the light-curve. • Double flares. Two illumination event would produce the observation pattern. The time behavior of the clouds emission is the convolution of their structure, their positions and the illuminating light-curve. The Galactic Center Black Hole Laboratory
An example for X-ray polarimetryto probe the absolute positions of the molecular clouds in the Sgr A complex and solve the Sgr A* light-curve. Sgr B2 is wide about 3’ smaller than NHXM and Athena+ field of view. Sgr C Complex is 15’x 12’ wide smaller than the XIPE field of view. It can be observed with a single pointing. Sgr A complex is wide about 8’x8’ smaller than XIPE field of view. It can be observed with a single pointing Absolute position taken from : (*) Reid 2009, (**) Ryu 2013, (***) Ponti 2012
The precision with which the measurement of the angle of polarization pinpoints the source of the primary emission. Constraints on the direction of the primary emission source with polarimetry on-board NHXM (500 ks of observation). The Galactic Center Black Hole Laboratory
Conclusions • X-ray polarimetry with the Gas Pixel Detector at the focus of an X-ray optics pinpoints the eventual illuminating source of the molecular nebulae by measuring the polarization angle. It is a viable tool to proof if SgA* is this source. A different result is probably expected in case cosmic rays produce the observed X-rays from molecular clouds. • X-ray polarimetry can fix, with the measure of the degree of polarization, the distance of the reflecting nebulae to Sgr A* helping defining the correct light-curve of its flare. • Measurement is possible in 2-10 keVenergy band by taking care of the Galactic Center Plasma Emission that dilute the polarization degree due to scattering. • Due to the presence of this thermal emission a harder energy band (6-35 keV) could provide a more clean measurement. The Galactic Center Black Hole Laboratory
Fluxes from : => 'SgrB2' -> Sidoli 2001, BeppoSAX, Continuum + Fe Line, 2-10 keV 'SgrB2 continuum' -> Murakami 2000, ASCA, Continuum Emission, 4-10 keV => 'SgrC continuum' -> Murakami 2001, ASCA, Continuum Emission, 2-10 keV • 'SgrB2 - Hard' -> Terrier et al. (2010) ApJ719:143 rescaled in the MEP band • Ponti 2013: • G0.11-0.11 • Bridge • M1 • M2 Extended formula of the MDP The Galactic Center Black Hole Laboratory
Not so easy in 2-10 keV Polarized Unpolarized • A diffuse emission typical of a hot plasma (6.5 keV) is present in the Galactic center region possibly providing not polarized component. • The expected polarization due to reflection must be diluted considering this contribution to be estimated in different regions may be using the ionized Fe line strength. • The not polarized fluorescence lines should be also taken into account when measuring polarization in a large energy range (2-10) . The Galactic Center Black Hole Laboratory
2.2 At the position of the best focus position we measured the Half Energy Width (HEW) at the three chosen energies and we compared the results with that obtained by distinguishing the three contribution : Measured with the GPD at PANTER (Fabiani et al. 2013) 23’’.2 22’’.7 28’’.9 Measured with TRoPIC at PANTER (Spiga et al 2013) Simulated (by knowing Aeff and inc.angle for each shell) Simulated and measured at 4.51 keV The results of the analysis show that the contribution to the total HEW are of decreasing importance going from the top to the bottom. The Galactic Center Black Hole Laboratory