10827(  6) - 04/18/07 10:52  - [  1]

                  HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 10827

Version: 6    Check-in Time: 18-Apr-2007 14:51:44       STScI Edit Number: 2

Title
Imaging Polarimetry of the Seyfert 1 MCG-6-30-15: Clues to the Structure of Warm
Absorbers
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Type             Cycle     Parallel Pointing Tolerance
GO               15        <none>
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Investigators
                                                                                     Contact?
    PI: Dr. Gerard A. Kriss               Space Telescope Science Institute
   CoI: Prof. Julia C. Lee                Harvard University                            N
   CoI: Dr. Christopher S. Reynolds       University of Maryland                        N
   CoI: Dr. William B. Sparks             Space Telescope Science Institute             N
   CoI: Dr. Claude R. Canizares           Massachusetts Institute of Technology         N

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Abstract

Imaging polarimetry at high spatial resolution, which is only possible with HST,
offers a potentially powerful new tool for determining the orientation and geometry
of AGN containing warm absorbers. These absorbed AGN tend to be more highly
polarized than unabsorbed Type 1s, but less polarized than Type 2s. If the polarized
flux is due to a polar scattering region as seen in polarized flux images of Seyfert
2s, imaging polarimetry of nearby absorbed Type 1 objects using HST can detect and
resolve these scattering regions. We propose to make the first HST imaging
polarimetry study of an absorbed Seyfert 1 by obtaining broad-band polarization
images with WFPC2 of the prototypical "dusty warm absorber" in MCG-6-30-15
(z=0.0077, D~33 Mpc). We will measure the wavelength dependence of the polarized
light free from dilution by the host galaxy starlight in order to assess whether the
polarization is due to a nuclear scattering region or dichroic transmission through
the absorbing dust. These observations will enable us to (1) use the wavelength
dependence of unresolved polarized flux to understand the properties of the
absorbing dust suggested by X-ray spectral features attributed to Fe~I absorption,
and (2) test whether polarization in warm absorbers is due to resolved polar
scattering regions. Resolving the scattering region in a moderately polarized
Seyfert 1 such as MCG-6-30-15 will let us answer the question of whether line-of-
sight inclination can be directly linked to observed outflow characteristics, as
suggested by the most recent unified models of AGN outflows.
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                                                10827(  6) - 04/18/07 10:52  - [  2]

  Observations Description
  ------------------------

     We will use WFPC2 with the broad-band filters F555W and F675W to obtain imaging
     polarimetry of the Seyfert 1 galaxy MCG-6-30-15. We divide our 6-orbit
     allocation into 3 visits of 2 orbits each. To obtain the requisite minimum
     observations through a polarization filter at three different angles, we must
     rely on spacecraft roll to provide the different orientations on the sky of the
     polarizing filter that works best with chip PC1 (POLQ; Biretta \& Sparks 1995,
     ISR WFPC2 95-01). The proposed broadband filters have the highest throughput
     and the lowest systematic calibration errors, $\sim 1$--2\% (Biretta \&
     McMaster 1997, ISR WFPC2 97-11). At z=0.0077 (33 Mpc for $\rm H_0 = 70~km~s^{-
     1}~{Mpc}^{-1}$), the PC's 0.046 arc-second scale will provide us with 7.4 pc
     per pixel. The flux of the unresolved nuclear source in the WFPC2/F606W image
     of MCG--6-30-15 obtained by Malkan \etal (1998) is $1.53 \times 10^{-15}~\rm
     ergs~cm^{-2}~s^{-1}~\AA^{-1}$ at 6060 \AA. We use this flux to normalize the
     spectrum, which we assume is a powerlaw $f_\nu \sim \nu^{-2}$ reddened by E(B$-
     $V)=0.8, following Reynolds \etal (1997). We also include the stellar
     background light within 1 arc sec of the nucleus, which we measure to have a
     surface brightness in the V band of $18.5~\rm mag~arcsec^{-2}$. The
     polarization of MCG--6-30-15, as measured through 4--6 arc second apertures
     using ground-based telescopes, ranges from 3.1--5.1\% from the I band to the B
     band (Thompson \& Martin 1988; Brindle \etal 1990a). Naturally, this is diluted
     by host-galaxy starlight in these large apertures. Our analysis of the
     WFPC2/F606W image of \mcg6\ shows that only 25\% of the flux in the 4--6$''$
     apertures used for the ground-based polarimetry comes from the active nucleus.
     Thus, if the diluting starlight is unpolarized, we expect to see polarizations
     as high as 12--20\% in our HST images. The polarization properties of the
     nucleus in MCG--6-30-15 are well known and well studied. It is the potential
     faint, extended scattering regions that we are searching for. To show that we
     can easily detect a polarized extended source that may represent the scattering
     region in MCG--6-30-15 we calculate the expected S/N and polarization error
     $\sigma_p$ using the approximation $\sigma_p = \sqrt 2 / (S/N)$ (where the S/N
     is for {\it one} of the three observations) with results in Table 1. We assume
     that the scattering region is a $2 \times 2$ pixel region ($15 \times 15$ pc)
     containing 1\% of the nuclear flux with an intrinsic polarization of 25\%
     (typical of dust scattering regions seen in other AGN, e.g., the NE dust knot
     in NGC 1068, Miller \etal 1991). Since the scattering cone in NGC~1068 is
     visible to projected distances of $\sim 1$--3 arc sec, we would expect
     comparable structures in MCG--6-30-15 to lie at distances of $\sim 0.3$--1.0
     arc sec. We must also subtract the bright nuclear source from this region. The
     wings of the PSF at radii of $\sim 0.3\arcsec$ (0.2\% of peak at 0.3$\arcsec$)
     are comparable to the surface brightness of the host galaxy for a source as
     bright as MCG--6-30-15 Section 7.3 of the WFPC2 Instrument Handbook shows that
     sources as faint as $\Delta m = 6.4 - 7.3$ relative to the nuclear source can
     be detected at the $3 \sigma$ level in the region $0.25 - 0.40 \arcsec$. Our
     hypothetical scattering region has $\Delta m = 5.0$, so PSF subtraction should
     pose little difficulty, even with the blooming and saturation expected in our
     individual exposures. This will affect only the central 5 pixels. Table 1. S/N
     Results for Imaging Polarimetry of MCG--6-30-15 Filter P(%) S/N(pt src)
     $\sigma_p$ S/N(ext src) $\sigma_p$ F555W 4% 1121 0.14% 112 1.2% F675W 2% 1086
     0.14% 111 1.2% Integration times are 4 x 260 s in all filters, with GAIN=7, no
     CR-SPLIT, and average backgrounds. Columns 3 and 4 give the S/N and $\sigma_p$
                                                10827(  6) - 04/18/07 10:52  - [  3]

     for an unresolved point source. Columns 5 and 6 give the same for a $2 \times
     2$ pixel extended region with 1\% of the flux of the nuclear source and an
     intrinsic polarization of 25\% superposed on the stellar background of MCG--6-
     30-15 and the PSF of the nuclear point source (0.2\% of peak at 0.3$\arcsec$).
     The bright source and the background provided by the host galaxy light make CTE
     losses negligible ($< 10$\%). Therefore, we propose observations that will
     consist of three 2-orbit visits. Each visit will have a different roll angle to
     appropriately sample the Stokes parameters through the polarized filter. On
     each visit we will use one orbit each to observe through the filters POLQ plus
     F555W and F675W. We will execute a 4-point dither pattern in each filter to
     optimally sample the PSF and to reject bad pixels and cosmic rays. Since
     individual exposures are no more than 260 s, we do not CR-SPLIT the individual
     exposures. The four dithered images will suffice to reject both hot pixels and
     cosmic rays.

  Real Time Justification
  -----------------------


  Calibration Justification
  -------------------------


  Additional Comments
  -------------------

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                                                10827(  6) - 04/18/07 10:52  - [  4]

TARGET LIST    Fixed Targets
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Tar|   Target   |         Target        |            Target              |Coord   | Radial |             Flux data
No |    Name    |       Description     |           Position             |Eqnx    |  Vel.  |
------------------------------------------------------------------------------------------------------------------------------------
1   MCG-6-30-15  GALAXY, NUCLEUS,        RA=13H35M53.8000S +/- 0.1",      J2000    V =      V = 16.0+/-0.2
                 SEYFERT, ELLIPTICAL,    DEC=-34D17'44.00" +/- 0.1"                2323.0   1.5e-15 erg/cm2/s/A at 6060 A
                 DUST LANE
    Reference Frame: Other: NED
    Comments: This object was generated by the targetselector and
              retrieved from the NED database.

                                                10827(  6) - 04/18/07 10:52  - [  5]

Patterns
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Pattern_Number: 1

                    Primary_Pattern                   Secondary_Pattern
Pattern_Type        WFPC2-BOX
Pattern_Purpose     DITHER
Number_Of_Points    4
Point_Spacing       0.559017
Line_Spacing        0.559017
Coordinate_Frame    POS-TARG
Pattern_Orient      26.56505
Angle_Between_Sides 143.130102
Center_Pattern      NO

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                                                10827(  6) - 04/18/07 10:52  - [  6]

Visit: 01
     Visit Priority:      <none>
     Visit Requirements:  SCHED 100%; ORIENT 270.0D TO 285.0 D
     On Hold Comments:    <none>
     Additional Comments: First of three visits at different orientations to fully sample the
                          polarization Stokes parameters.


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exp |   Target  | Instr  | Oper. | Aper    |Spectral|Central|           Optional           |Num|   Time   |       Special
Num |    Name   | Config | Mode  | or FOV  |Element |Waveln.|          Parameters          |Exp|          |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
1    MCG-6-30-15 WFPC2    IMAGE   PC1       F555W,                                          1   260 S      POS TARG 8, 8; PATTERN 1
                                            POLQ                                                           1
------------------------------------------------------------------------------------------------------------------------------------
2    MCG-6-30-15 WFPC2    IMAGE   PC1       F675W,                                          1   260 S      POS TARG 8, 8; PATTERN 1
                                            POLQ                                                           2
------------------------------------------------------------------------------------------------------------------------------------

Sub Exposures
------------------------------------------------------------------------------------------------------------------------------------
Target             | Exp |Instr   | Oper. |  Aper  |Spectral|Cent.|Primary     |Secondary   |Iteration |CR-SPLIT  |Orbit  |Duration
Name               | Num |Config  | Mode  | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num       |Num       |Number |
------------------------------------------------------------------------------------------------------------------------------------
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         1            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         2            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         3            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         4            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         1            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         2            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         3            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         4            none         none       none       2       N/A
                                                    POLQ
                                                10827(  6) - 04/18/07 10:52  - [  7]

Visit: 02
     Visit Priority:      <none>
     Visit Requirements:  SCHED 100%; ORIENT 330.0D TO 359.9 D
     On Hold Comments:    <none>
     Additional Comments: Second of three visits at different orientations to fully sample the
                          polarization Stokes parameters.


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exp |   Target  | Instr  | Oper. | Aper    |Spectral|Central|           Optional           |Num|   Time   |       Special
Num |    Name   | Config | Mode  | or FOV  |Element |Waveln.|          Parameters          |Exp|          |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
1    MCG-6-30-15 WFPC2    IMAGE   PC1       F555W,                                          1   260 S      POS TARG 8, 8; PATTERN 1
                                            POLQ                                                           1
------------------------------------------------------------------------------------------------------------------------------------
2    MCG-6-30-15 WFPC2    IMAGE   PC1       F675W,                                          1   260 S      POS TARG 8, 8; PATTERN 1
                                            POLQ                                                           2
------------------------------------------------------------------------------------------------------------------------------------

Sub Exposures
------------------------------------------------------------------------------------------------------------------------------------
Target             | Exp |Instr   | Oper. |  Aper  |Spectral|Cent.|Primary     |Secondary   |Iteration |CR-SPLIT  |Orbit  |Duration
Name               | Num |Config  | Mode  | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num       |Num       |Number |
------------------------------------------------------------------------------------------------------------------------------------
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         1            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         2            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         3            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         4            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         1            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         2            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         3            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         4            none         none       none       2       N/A
                                                    POLQ
                                                10827(  6) - 04/18/07 10:52  - [  8]

Visit: 03
     Visit Priority:      <none>
     Visit Requirements:  SCHED 100%; ORIENT 300.0D TO 315.0 D
     On Hold Comments:    <none>
     Additional Comments: Third of three visits at different orientations to fully sample the
                          polarization Stokes parameters.


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exp |   Target  | Instr  | Oper. | Aper    |Spectral|Central|           Optional           |Num|   Time   |       Special
Num |    Name   | Config | Mode  | or FOV  |Element |Waveln.|          Parameters          |Exp|          |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
1    MCG-6-30-15 WFPC2    IMAGE   PC1       F555W,                                          1   260 S      POS TARG 8, 8; PATTERN 1
                                            POLQ                                                           1
------------------------------------------------------------------------------------------------------------------------------------
2    MCG-6-30-15 WFPC2    IMAGE   PC1       F675W,                                          1   260 S      POS TARG 8, 8; PATTERN 1
                                            POLQ                                                           2
------------------------------------------------------------------------------------------------------------------------------------

Sub Exposures
------------------------------------------------------------------------------------------------------------------------------------
Target             | Exp |Instr   | Oper. |  Aper  |Spectral|Cent.|Primary     |Secondary   |Iteration |CR-SPLIT  |Orbit  |Duration
Name               | Num |Config  | Mode  | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num       |Num       |Number |
------------------------------------------------------------------------------------------------------------------------------------
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         1            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         2            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         3            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         4            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         1            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         2            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         3            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         4            none         none       none       2       N/A
                                                    POLQ
                                                10827(  6) - 04/18/07 10:52  - [  9]

Visit: 52
     Visit Priority:      <none>
     Visit Requirements:  SCHED 100%; ORIENT 330.0D TO 359.9 D
     On Hold Comments:    <none>
     Additional Comments: Second of three visits at different orientations to fully sample the
                          polarization Stokes parameters.


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exp |   Target  | Instr  | Oper. | Aper    |Spectral|Central|           Optional           |Num|   Time   |       Special
Num |    Name   | Config | Mode  | or FOV  |Element |Waveln.|          Parameters          |Exp|          |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
1    MCG-6-30-15 WFPC2    IMAGE   PC1       F555W,                                          1   260 S      POS TARG 8, 8; PATTERN 1
                                            POLQ                                                           1
------------------------------------------------------------------------------------------------------------------------------------
2    MCG-6-30-15 WFPC2    IMAGE   PC1       F675W,                                          1   260 S      POS TARG 8, 8; PATTERN 1
                                            POLQ                                                           2
------------------------------------------------------------------------------------------------------------------------------------

Sub Exposures
------------------------------------------------------------------------------------------------------------------------------------
Target             | Exp |Instr   | Oper. |  Aper  |Spectral|Cent.|Primary     |Secondary   |Iteration |CR-SPLIT  |Orbit  |Duration
Name               | Num |Config  | Mode  | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num       |Num       |Number |
------------------------------------------------------------------------------------------------------------------------------------
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         1            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         2            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         3            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         1     WFPC2    IMAGE   PC1      F555W,         4            none         none       none       1       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         1            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         2            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         3            none         none       none       2       N/A
                                                    POLQ
MCG-6-30-15         2     WFPC2    IMAGE   PC1      F675W,         4            none         none       none       2       N/A
                                                    POLQ
                                                10827(  6) - 04/18/07 10:52  - [ 10]

            Summary Form for Proposal  10827

  Item                         Used in this proposal
------------------------------------------------------------------------------------------------------------------------------------
  Apertures                    PC1
------------------------------------------------------------------------------------------------------------------------------------
  Configurations               WFPC2
------------------------------------------------------------------------------------------------------------------------------------
  Opmodes                      IMAGE
------------------------------------------------------------------------------------------------------------------------------------
  Special Requirements         SCHED 100%; ORIENT 270.0D TO 285.0 D; POS TARG 8, 8; PATTERN 1 1; PATTERN 1 2; ORIENT 330.0D TO
                               359.9 D; ORIENT 300.0D TO 315.0 D;
------------------------------------------------------------------------------------------------------------------------------------
  Spectral Elements            F555W,POLQ F675W,POLQ
------------------------------------------------------------------------------------------------------------------------------------
  Target Names                 MCG-6-30-15
------------------------------------------------------------------------------------------------------------------------------------