Pulsar Polarimetry. Roberto P. Mignani. INAF-Istituto di Astrofisica Spaziale, Milan (Italy) Kepler Institute of Astronomy, Zielona Gora (Poland)

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1 Pulsar Polarimetry Roberto P. Mignani INAF-Istituto di Astrofisica Spaziale, Milan (Italy) Kepler Institute of Astronomy, Zielona Gora (Poland) With: P. Moran, A. Sherarer, (NUIG), A. Slowikowska (UZG), B. Rudack (CAMC), S. Bagnulo (Armagh Observatory), G. Kambach (MPE), A. de Luca (INAF)!"#$%&'(')*+),-()).%/"0(1&)2&3"&) Bonn, Pulsar Workshop 2014

2 Introduction Polarisation measurements of pulsars offer unique insights into their highlymagnetised relativistic environments and represent a primary test for neutron star magnetosphere models and radiation emission processes. Besides the radio band, optical observations are best suited to these goals [also thinking of RQ pulsars] Indeed, significant polarisation is expected when the optical emission is produced by synchrotron radiation in the neutron star magnetosphere. Optical polarimetry is a mature technique and applied since decades X-ray polarimetry still moving its first steps (e.g., Forot et al. 2008), with future missions, such GEMS (Ghosh et al. 2013) or XIPE (Soffitta et al. 2013), still being planned.

3 Pulsar Polarisation Observations Polarization of the Crab pulsar optical emission was discovered by Wampler et al. (1969), soon after the discovery of the pulsar optical counterpart (Cocke et al. 1969). Being the brightest (V=16.5) optical pulsar (and the first discovered) the Crab is the only one with repeated phase-resolved polarization measurements More phase-resolved measurements have been obtained by Cocke et al. (1970), Kristian et al. (1970), Jones et al. (1981), Smith et al. (1988), Graham-Smith et al. (1996), Kanbach (2005) ü Measurements as higher time resolution ü Secular changes in the pulsar polarisation ü Polarisation of the inner nebula and features

4 The Crab Knot Point-like emission knot resolved 0.5 SE of the pulsar by HST/WFPC2 images (Hester et al. 1995) The origin of the knot is unclear since then The optical/ir spectrum of the knot was measured with the VLT and HST by Sollerman (2003) Power-law F ν ν α with α~-0.8 (optical/ir) while the pulsar spectrum has α=0.3 (IR) and α=0.11 (optical/uv) Is the knot polarised? Is it affecting polarisation measurements of the pulsar? Knot Crab x10 - See also Hester (2008) ARAA Sollerman (2003)

5 The Crab polarisation Most recent and accurate phase-resolved polarisation measurements for the Crab (Slowikowska et al. 2009) obtained with the Optical Pulsar Timing Analyser (Optima) Optima observations clearly show that the Stoke parameres (I,Q,U) vary the phase PD minimum at peak of the main pulse, strong in the interpulse, and maximum at the off-pulse [see also Smith et al. 1988]. Very strong DC component. Uncertain DC subtraction [depends on off-pulse definition]. Possibly associated with the knot [not resolved by Optima]? Off-pulse Interpulse before DC subtraction after DC subtraction 9.8%±0.1% 5.4%

6 HST optical Polarimetry of the Crab (V~16.5) HST/ACS observations of the Crab (Moran et al. 2013a) 13 epochs ( ) T/epoch=2300s per angle PD=5.2% ± 0.3% PA=104.8 ±1.6 (pulsar) First repeated phase-averaged PD measurements No time variability in both PD and PA The Crab PA is aligned with the PM vector and PWN axis, as seen in the case of the Vela pulsar (Mignani et al. 2007). PA ( )

7 The Knot The knot is the most strongly polarised feature in the Nebula more polarised than the pulsar PD=59.0% ±1.9% PA=124.7 ± The knot and Crab PD are almost aligned The knot varies by >50% in flux but not in PD PA ( ) Being strongly polarised. the knot may be the main source of the pulsar DC polarisation seen in phase-res observations (Slowikowska et al. 2009) Phase-averaged PD of the Crab measured with HST (5.2%±0.3%) with no knot contamination- is consistent with that measured with Optima (5.4%), after subtracting the DC component

8 The Inner Nebula No variation in PD in the wisps (PD~30%-40%) Wisp 1 A 39.8 ± 1.6% Wisp 1 B 43.0 ± 1.3% Wisp 1 C 38.5 ± 1.3% Thin Wisp 36.7 ± 1.4% Counter Wisp 40.6 ± 1.5%

9 Optical Polarimetry of Vela (V~23.6) HST/ACS observations of the Vela pulsar (Moran+ 2014). 5 epochs (2011) T/ PA ( ) epoch=2600s per angle Pulsar ± MJD PD=7.6% ± 0.7% PA=140 ±4.1 (Moran et al. 2014) In agreement with the VLT measurement of Mignani et al. (2007): PD=9.4% ± 4% PA=146 ±11 Comparable to the optical polarisation of other pulsars (5%-10%) Mignani et al. (2010) Alignment between pulsar PD, PM vector and X-ray PWN axis (Mignani et al. 2007) confirmed Common feature of all PSR/PWN systems? MJD 55600

10 PSR B (V~25) Next brightest pulsar (25) after Vela (23.5) No PWN or SNR. Close by (~300 pc) Ten times older than Vela à explore evolutionary effects on PD Observed with in 2013/2014 PD=10.1%±0.6% and PA=75.8 ±6.6 (Mignani et al. 2014) consistent with the average value PD=23%±13% from phaseres polarimetry (Kern et al. 2003) Nearly aligned with the PM vector (PA= ±0.38 ; Brisken et al. 2003).

11 Pulsar optical polarisation, current picture All PD values below model predictions! PD seems to be higher for older and less energetic pulsars No correlation between PD and surface magnetic field B S (nearly constant)

12 Future of Optical Polarimetry Different optical polarimetry facilities on VLT; SALT, GEMINI, GTC, and HST. Phase-resolved polarimetry with guest instruments only Phase-avg polarimetry: confirm doubt results and obtain new ones Instr. Det. Group Res. Type Ref. U-Cam CCD Warwick/Sheffield 5ms imager Dhillon et al.2007 U-Spec emccd Warwick/Sheffield 1ms imager Dhillon et al.2007 OPTIMA SPAD MPE 4 µs photocounter/polarimeter Kanbach et al GASP SPAD Galway 1 ns imager/polarimeter Kyne et al Aqueye SPAD Padua 0.1ns photocounter/polarimeter Barbieri et al Iqueye SPAD Padua 0.1 ns photocounter/polarimeter Naletto et al Phase-res polarimetry only for the Crab and PSR B Next best targets for phase-res polarimetry: PSR B , Vela, PSR B , (all first time) and PSR B (to confirm results) But also the Crab.

13 Giant pulses in radio pulsars Giant Radio Pulses (GRPs) are erratic variation of the peak-to-peak single pulse intensity (few %) So far only observed in radio (~10 pulsars) and in the optical (GOPs) for the Crab pulsar (Shearer et al. 2003; Collins et al. 2012; Strader et al. 2013) GOPs occur in time with GRPs (coherent vs. incoherent radiation) Not yet observed in X (Bilous et al. 2012) and γ-rays (Lewandowska et al. 2011). Window of opportunity for LOFT (ESA M4) Some of the best candidates (B , J ) very faint/undetected in radio + B is also a well known optical pulsar Does phase-resolved polarimetry properties change during a GOP?

14 Moving into a New Era The E-ELT will allow studies of INSs fainter, further away, more absorbed E-ELT would be a powerful tool for pulsar timing AND polarimetry (phaseresolved polarimetry and spectroscopy) See Workshop Speed and Sensitivity Expanding Astronomical Horizons with the ELTs The (E- ELT) Science Working Group also recommended the inclusion of more specific modes, such as spectro- polarimetry and high time resolution astronomy. Allowing for visitor instruments would further enable the E- ELT to react to important emerging niche science. A visitor port will therefore be available for at least the 0irst six to eight years of telescope operation.