Dean C. Hines Space Telescope Science Institute Precision Imaging Polarimetry with ACS

Polarimetry Polarization can provide vital and otherwise unobtainable constraints on – the origin of light from astronomical sources –the nature of particles involved in emission and scattering –the geometry of the regions producing polarized light Polarization probes asymmetry Provides a “scattering mirror” to “see” around obscuration Science Examples –Synchrotron emission in M87 jet –Unified AGN Model –Aspherical supernovae explosions –Detailed structure, composition, shape/size, or orientation of scattering dust particles 3/2/2016 STScI DCH - 2

Scattering & Polarization 3/2/2016 STScI DCH - 3 Polarization induced by scattering off small particles

Polarimetry 101 3/2/2016 STScI & SSI DCH - 4 Hines et al. (2000) Hines et al. (2000: see also Sparks & Axon 1999) ACS Instrument Handbook (see also Jackson!) p = 100% (Q 2 + U 2 ) 1/2 /I θ instr = 0.5 tan−1 (U/Q) S k = A k I +  k (B k Q + C k U), A k = ½ t k (1+l k ), B k = A k cos (2  k ), C k = sin (2  k ) S k = A k I +  k (B k Q + C k U), A k = ½ t k (1+l k ), B k = A k cos (2  k ), C k = sin (2  k ) I = 2/3(S 0 + S 60 + S 120 ), Q = 2/3(2S 0 + S 60 + S 120 ), U = 2(S 60 – S 120 )/sqrt(3) I = 2/3(S 0 + S 60 + S 120 ), Q = 2/3(2S 0 + S 60 + S 120 ), U = 2(S 60 – S 120 )/sqrt(3) Every Polarimetry Paper…Ever!

Calibration of a Polarimeter Ground-based TV testing with polarized and unpolarized flat field –measure throughputs –polarizing efficiencies –FOV variations –instrumental polarization –fiducial polarization “axis” of the instrument On-orbit using polarized and unpolarized ‘standards” –Multiple “rolls” for separating throughput vs. polarizing efficiency –Measure know extended polarized source to assess field distortions 3/2/2016 STScI & SSI DCH - 5

Updated Calibration ACS internal flat mirrors ACS Cal programs Previous calibration –Cracraft, M., & Sparks, W. B. 2007, ISR ACS 2007–10 –Biretta, J., et al. 2004, ISR ACS 2004–09 New calibrations –Repeatability ~ fractional polarization –Absolute ~ fractional polarization –Position angle ~ 2-3˚ 3/2/2016 STScI & SSI DCH - 6 I cor POL ∗ V = CPOL ∗ V x I obs x POL*V, CPOL0V = , CPOL60V = , CPOL120V = I cor POL ∗ V = CPOL ∗ V x I obs x POL*V, CPOL0V = , CPOL60V = , CPOL120V =

Field Distortion 3/2/2016 STScI & SSI DCH - 7 NICMOSCryogenic Optical Bench KPNO Hines et al. (2000)

Field Distortion 3/2/2016 STScI & SSI DCH - 8 Polarization Position AnglesPerpendiculars Weintraub et al. (2000)

Field Distortion 3/2/2016 STScI & SSI DCH - 9 I Thetap*I p

Field Distortion 3/2/2016 STScI & SSI DCH - 10 I Thetap*I p

Field Distortion 3/2/2016 STScI & SSI DCH - 11 I Thetap*I p

HST/ACS Observations of Comet ISON Observed with Wide Field Camera (WFC) module of the Advanced Camera for Surveys (ACS) Two Orbits DD time May 8, 2013 –F606W (Broad-V) Weak or no emission lines –r h = 3.81 AU from Sun,  = 4.34 AU from Earth –Phase angle was α ≈ 12.25º (“bottom” of negative polarization branch) Three Orbits GO time October 26/27, 2013 –F606W (Broad-V) & F775W (SDSSi) Strong C 2 in F606W Minimal CN in F775W –r h = 1.12 AU,  = 1.37 AU, α ≈ 45.60˚ 3/2/2016 STScI DCH - 12

ACS/WFC F606W Imaging Polarimetry 3/2/2016 STScI DCH - 13 Hines et al. (2014, ApJ, 780, 32L) To Sun 5″

ACS/WFC F606W Imaging Polarimetry Halo p ~ -1.8%, suggest abundance of abundant absorbing particles Central region p ~ + 2.5%, suggests abundant ice-coated grains. Characteristic of cometary jets. 3/2/2016 STScI DCH - 14

ACS/WFC F606W Imaging Polarimetry Halo p ~ -1.8%, suggest abundance of abundant absorbing particles Central region p ~ + 2.5%, suggests abundant ice-coated grains. Characteristic of cometary jets. 3/2/2016 STScI DCH - 15 Hines et al. (2014, ApJ, 780, 32L)

Polarimetry Observations of Comet 67/P Rosetta will measure the properties of the coma of 67P, including the inner coma during Philae lander release by 10 km altitude. 21 Orbit, Cycle 22 HST/ACS F606W (PI: Hines) –Observations bracket the period when Rosetta operates closest to 67P, and will deploy the Philae lander. –2014-Aug-19: r h = 3.52 AU,  = 2.76 AU, α ≈ 12.0˚ –2014-Nov-17: r h = 2.96 AU,  = 3.43 AU, α ≈ 15.7˚ –Planning to propose Post-Perihelion follow-up Contemporaneous observations with remote sensing assets from more distant vantage points will enable linkage of the properties and distribution of dust observed from Earth to the material measured in-situ near the nucleus. 3/2/2016 STScI DCH - 16

Future Plans Refine calibration of supported filters –F775W – already in the works –F435W & F658N Multi-roll calibrations New flat fields CTE effects Striping 3/2/2016 STScI & SSI DCH - 17

3/2/2016 STScI & SSI DCH - 18 Fin