Proposal_Information ! Title: THE STRUCTURE, POLARIZATION, AND EVOLUTION OF THE CRAB SYNCHROTRON NEBULA Proposal_Category: GO Scientific_Category: INTERSTELLAR MEDIUM Cycle: 5 Investigators PI_name: Jeff J. Hester PI_Institution: ARIZONA STATE UNIVERSITY CoI_Name: Paul Scowen CoI_Institution: Arizona State University CoI_Name: Curt Michel CoI_Institution: Rice University CoI_Name: James Graham CoI_Institution: University of California at Berkeley CoI_Name: John Gallagher CoI_Institution: University of Wisconsin-Madison CoI_Name: Alan Watson CoI_Institution: University of Wisconsin-Madison CoI_Name: Ravi Sankrit CoI_Institution: Arizona State University Questions Observing_Description: The program will consist of three parts: a temporal study, a spectral index study, and a polarization study. Temporal Study (9 orbits): F547M will be used to image the Crab on each of 9 1-orbit visits. Five of these visits will be separated from each other by three months. If these five visits are referred to as A through E, then four additional visits will be scheduled at C minus 1 month, C minus 1 week, C plus 1 week, and C plus one month. This set of observations will provide a full year's monitor of the Crab at 3 month resolution, in the middle of which the four additional observations will sample changes in the nebula over logarithmically stepped 1 week, 2 week, 1 month, and 2 month time scales. For these observations, the pulsar will be centered in the FOV of the Planetary Camera, providing the full resolution of the HST at 5500Angstrom. A full orbit CR- SPLIT F547M exposure will be taken on each visit. F547M is chosen because it offers both a broad passband and minimal contamination from emission lines. On the basis of the existing WFPC2 images we expect a minimum signal in the field of ~ 100 e^- per pixel in each of the two exposures, so the images will be photon noise limited everywhere. In addition to the PC1 observations of the center of the nebula, we will obtain ``serendipitous'' WFC images of much of the nebula located within ~ 90" of the pulsar as the orientation of the HST changes through the year. Study of Spectral Index Variations (3 orbits): On three of our visits to the Crab (the C minus 1 month visit, the C visit, and the C plus 1 month visit) we will obtain a CR-SPLIT full orbit exposure in F814W. This will allow us to investigate spectral index variations in the Crab at the full resolution of the PC within the physical context established by the monitoring program. It will also provide us with a single 2 month baseline to help us relate spectral index variations to the temporal evolution of the region. Polarization Observations (3 orbits): Within one month of the ``C'' visit above (i.e., during the period when we are closely monitoring changes in the Crab as well as spectral index variations) we will obtain a set of polarization images. Data will be obtained with the polarizer crossed with F547M, which is located in a well behaved part of the transmission curve of the polarizer quad (e.g., Figure 3.5 of the WFPC2 Instrument Handbook). A full orbit CR-SPLIT exposure will be taken with each of the WFC channels centered on the pulsar. This will provide three polarization angles which are adequate for determination of Stokes parameters. The Polarizer Quad on WFPC2 is not well calibrated. However, the Crab is one of the most strongly polarized sources in the sky, with peak polarizations upwards of 50-60\ van den Bergh 1990, and references therein). Thus, the quoted 3\ uncertainties are quite adequate for this program. In addition, we intend to propose for ground based time to measure the polarization of the Crab concurrently with the HST observations. By smoothing the HST data to ground based resolution, we will then be able to use the ground based polarizations to ``self calibrate'' the WFPC2 data. The polarization data will be reduced in much the way we did in Michel et al. (1991), allowing generation of both Stokes parameter maps and E \cdot \nabla ``flux tube'' maps. Real_Time_Justification: This program includes a temporal study of the evolution of the Crab Nebula. Hence scheduling is important. This study is built around 4 visits to the Crab, each separated from its neighbors by as close to 3 months as possible. The Crab emerges from the Sun avoidance zone on 95.216 (August 4th, 1995) and will remain accessible until 96.116 (April 28th, 1996), again emerging from the zone on 96.216. Meeting the timing requirements for these 4 visits then requires that observations be scheduled as soon as possible after emergence from the Sun avoidance zone on 95.216 and 96.216, and as late as possible prior to the Crab's entry into the avoidance zone on 96.116. Uniform temporal sampling then requires that the other two observations take place 1/3 and 2/3 of the way though the 95.216 to 96.116 window. The four orbits above provide a one-year monitor of what is going on in the nebula. These will provide an essential scientific context for interpretation of data taken to sample more rapid variations in the Crab, and to investigate spectral index variations and the polarization structure of the nebula. The four additional temporal monitor exposures should be taken 1 month before, 1 week before, 1 week after, and 1 month after the ~ 96.25 visit. The three F814W observations must be taken on consecutive orbit with the 95.216, the 96.25 minus 1 week, and the 96.25 F547M observations. The polarization observations should be taken within 2 weeks of the 96.25 visit. The three polarization observations must be taken on consecutive orbits due to the extremely rapid evolution of the object. We place no constraints on spacecraft orientation for most of the observations, as the region to be studied fits reasonably within the inscribed circle in PC1. While not specifically tagged as a long term proposal, we anticipate that on the basis of the extent, type, and time scales of evolution observed here, we will propose to continue the monitoring program in Cycle VI. Keck: An informal monitoring program has been in effect at the W. M. Keck telescope since November, 1993. Observations have been, and will continue to be, obtained at 2.3 Mum in seeing of typically ~ 0\farcs6. These data will nicely complement the WFPC2 spectral index observations. When tied to the HST F547M data, the Keck observations will offer a spectral index monitor that is more complete and has a longer wavelength baseline, but lower spatial resolution, than the WFPC2 F814W data. (Graham has access to Keck as Co-I of the team that developed the Keck NIRC.) WIYN / Steward / Lowell: Programs will be initiated at these three observatories to carry out as complete a ground based monitor of the Crab as possible over the course of the 1 year HST program. In particular, this monitoring will include frequent photometric monitoring from Lowell (where Watson will be beginning this fall), and polarization observations as closely coincident with the WFPC2 polarization observations as possible. (Hester has access to Steward facilities. Gallagher has access to WIYN.) Calibration_Justification: Additional_Comments: Abstract: The morphology, evolution, polarization, and spectral index variations in the synchrotron nebula surrounding the Crab pulsar have been the subject of intensive study for decades. It is generally accepted that the structure and activity in this region are due to wave phenomena near the termination shock of the pulsar wind, observations of which hold unique promise of leading to more complete models of the pulsar and its magnetosphere. Unfortunately, this promise has not been fulfilled, largely because of the low spatial resolution and uneven temporal coverage of existing studies. Recent HST WFPC2 observations of the Crab synchrotron nebula (Scowen et al 1994; Hester et al 1994a,b) offer new hope in this quest. These data, which reach the natural size scale defined by the Larmor radius of energetic electrons, resolve the majority of the the known features in the Crab. For the first time it is possible to reliably establish the physical conditions (e.g., emissivities, equipartition fields, and pressures) of features associated with the wind and its termination shock. In addition the HST has probed the structure of the nebula very close to the pulsar itself, leading to the exciting discovery of a bright knot located only 1300 AU above the SE pole of the pulsar. We propose to use the WFPC2 to continue this investigation by exploring the time evolution, polarization, and spectral index variations within the pulsar's environment on what current theory indicates are the relevant spatial and temporal scales. Fixed_Targets Target_Number: 1 Target_Name: CRAB-PULSAR Alternate_Names: Description: STAR, PULSAR, HALO Position: RA=5H 34M 32.28S +/- 0.010S, DEC= 22D 0' 55.2" +/- 0.10" Equinox: J2000 Rv_or_z: Flux: Comments: Visits Visit_Number: 01 Visit_Requirements: ORIENT 261D TO 271D BEFORE 20-AUG-95 Visit_Comments: FIRST MONITOR EXPOSURE. FIRST MEASUREMENT OF SPECTRAL INDEX. SHOULD RUN ASAP IN CYCLE 5, BEFORE AUGUST 20 Exposure_number: 10 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:END ORBIT POS TARG -2.7, -3.7 Exposure_number: 20 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F300W Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 1600S Special_Requirements:POS TARG -2.7, -3.7 Exposure_number: 25 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F814W Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 700S Special_Requirements:END ORBIT POS TARG -2.7, -3.7 Exposure_number: 30 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F300W Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 1600S Special_Requirements:POS TARG -2.7, -3.7 Exposure_number: 35 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F814W Optional_Parameters: CLOCKS=NO, CR-SPLIT=NO, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 700S Special_Requirements:END ORBIT POS TARG -2.7, -3.7 Visit_Number: 02 Visit_Requirements: ORIENT 290D TO 299D BETWEEN 5-NOV-95 AND 25-NOV-95 Visit_Comments: SECOND MONITOR EXPOSURE. PROGRAM CONDUCTS TEMPORAL MONITORING. BETWEEN REQ REFLECTS ONLY SCHEDULABLE SEQ. ORIENT IS NOMINAL FOR THIS PERIOD Exposure_number: 40 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:POS TARG -7.7, -4.7 Visit_Number: 03 Visit_Requirements: SAME ORIENT AS 05 AFTER 02 BY 53D TO 67D Visit_Comments: MONITOR AT -1 MONTH. ORIENT AND POS MUST BE CONSTANT FOR VISITS 03-08 TO ALLOW DETECTION OF SUBTLE CHANGES. POSITION OF TARGET ON PC MUST BE THE SAME AS VISITS 03-08 Exposure_number: 50 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:POS TARG -2.7, 8.3 Visit_Number: 04 Visit_Requirements: SAME ORIENT AS 05 AFTER 03 BY 20D TO 26D Visit_Comments: MONITOR AND SPECTRAL INDEX AT -1 WEEK. POSITION OF TARGET ON PC MUST BE THE SAME AS VISITS 03-08 Exposure_number: 60 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:POS TARG -2.7, 8.3 Exposure_number: 70 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F814W Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2400S Special_Requirements:POS TARG -2.7, 8.3 Visit_Number: 05 Visit_Requirements: ORIENT 83D TO 92D AFTER 04 BY 6D TO 8D Visit_Comments: MONITOR, SPECTRAL INDEX - CENTER OF GROUP. POSITION OF TARGET ON PC MUST BE THE SAME AS VISITS 03-08 Exposure_number: 80 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:POS TARG -2.7, 8.3 Exposure_number: 90 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F814W Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2400S Special_Requirements:POS TARG -2.7, 8.3 Visit_Number: 06 Visit_Requirements: SAME ORIENT AS 05 AFTER 05 BY 6D TO 8D Visit_Comments: MONITOR AT +1 WEEK. POSITION OF TARGET ON PC MUST BE THE SAME AS VISITS 03-08 Exposure_number: 100 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:POS TARG -2.7, 8.3 Visit_Number: 07 Visit_Requirements: SAME ORIENT AS 05 AFTER 06 BY 20D TO 26D Visit_Comments: MONITOR AT +1 MONTH. POSITION OF TARGET ON PC MUST BE THE SAME AS VISITS 03-08 Exposure_number: 110 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:POS TARG -2.7, 8.3 Visit_Number: 08 Visit_Requirements: SAME ORIENT AS 05 AFTER 07 BY 53D TO 67D Visit_Comments: MONITOR. POSITION OF TARGET ON PC MUST BE THE SAME AS VISITS 03-08 Exposure_number: 120 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:POS TARG -2.7, 8.3 Visit_Number: 09 Visit_Requirements: GROUP 05, 09-11 WITHIN 1D ORIENT 71.5D TO 72.5D SEQ 09-11 WITHIN 5H Visit_Comments: INTENDED TO PLACE CENTRAL PART OF THE CRAB WITHIN THE UNVIGNETTED FOV OF THE POLQ IN WF2. ORIENT ROTATES THE POLQ BY -15 DEGREE WRT NOMINAL TO ALLOW 3 POLARIZATIONS SPACED BY 60 DEGREE Exposure_number: 130 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: WF2-FIX Sp_Element: F547M, POLQ Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:END ORBIT POS TARG -8.8, 9.0 Visit_Number: 10 Visit_Requirements: ORIENT 86.5D TO 87.5D Visit_Comments: INTENDED TO PLACE TARGET REGION WITHIN UNVIGNETTED FOV OF POLQ IN WF3. ORIENT IS NOMINAL Exposure_number: 140 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: WF3-FIX Sp_Element: F547M, POLQ Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:END ORBIT POS TARG -7.2, -5.5 Visit_Number: 11 Visit_Requirements: ORIENT 101.5D TO 102.5D Visit_Comments: INTENDED TO PLACE TARGET REGION IN UNVIGNETTED FOV OF POLQ IN WF4. ORIENT ROTATES POLQ BY +15 DEGREE WRT NOMINAL TO ACHIEVE 60 DEGREE ROTATION IN POLARIZATION DIRECTION Exposure_number: 150 Target_Name: CRAB-PULSAR Config: WFPC2 Opmode: IMAGE Aperture: WF4-FIX Sp_Element: F547M, POLQ Optional_Parameters: CLOCKS=NO, CR-TOLERANCE=0.0, CR-SPLIT=0.5, ATD-GAIN=7 Number_of_iterations:1 Time_Per_Exposure: 2000S Special_Requirements:END ORBIT POS TARG 8.6, -6.4 Data_Distribution Medium: 8MM Blocking_Factor: 10 Ship_To: PI_Address Ship_Via: UPS Recipient_Email: