9123( 2) - 02/16/01 13:48 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9123 Version: 2 Check-in Date: 16-Feb-2001 13:48:23 1.Proposal Title: Synchrotron self-Compton Emission from the Radio Hot Spots of Cygnus A ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Andrew Wilson University of Maryland CoI: Dr Andy Young University of Maryland N CoI: Dr Patrick Shopbell Caltech N ------------------------------------------------------------------------------------ 5. Abstract We have recently studied the radio galaxy Cygnus A in X-rays with Chandra. All four of the radio hot spots are detected with similar morphologies to the radio maps. The X-ray emission is almost certainly synchrotron self- Compton emission from the radio synchrotron emitting electrons. The predictions of this model are in excellent agreement with the Chandra spectra and also imply a magnetic field strength close to equipartition. We propose here to image the predicted SSC emission from the hot spots in the optical with HST. Our goals are a) to confirm the optical fluxes predicted by the SSC model, and b) to use the radio synchrotron and optical SSC brightness distributions at ~eq 0.1 arc sec resolution to determine the internal structures of the magnetic fields and relativistic particles within the two brighter hot spots (A and D). ------------------------------------------------------------------------------------ 9123( 2) - 02/16/01 13:48 - [ 2] Observations Description ------------------------ 1. The need for HST HST is needed for two reasons: a) Cygnus A is at low Galactic latitude (b = 6^degrees) and so there are many foreground stars. As can be seen in Fig. 3, hot spots A and B lie close to faint (below the limit of the Sky Survey) stars. The infrared - optical broad band energy distribution indicates they are both K stars (Meisenheimer et al. 1997). It is important to emphasise that hot spot A - the primary target in the NW lobe - is not coincident with a star. HST is needed to separate cleanly the expected optical emission of the hot spots from such foreground stars. The nearest known stars to hot spots D and E are about 2.5 arc secs away; however, extrapolation of the star counts to the expected magnitude (see below) of hot spot D indicates a significant probability of confusion at ground-based (1 arc sec) resolutions. b) The two brighter hot spots - A and D - have diameters in the radio of about 2 arc secs. Thus they could be just spatially resolved by ground-based optical observations. However, as described in the Scientific Justification, one of our goals is to make a detailed comparison between the synchrotron radio and optical SSC emission. The hot spots have fine scale structure on the angular scale of the highest resolution (0.1 - 0.2 arc secs) VLA maps, so the similar resolution of HST is needed to make this comparison. beginfigure epsscale1.0 plottwofig4a.epsfig4b.eps captionThe STIS 50CCD field of view superposed on the ESO/ECF Sky Survey by means of the Visual Target Tuner (VTT). Left: the region of the NW hot spots. The cross marks hot spot A. Right: the region of the SE hot spots. The cross marks hot spot D. labelfig-4 endfigure \medskip 2. Instrument, Filter, Intrinsic Fluxes, Exposure Time and Signal to Noise We need to observed with maximum sensitivity because of the predicted faintness of the hot spots. The maximum throughput appears to be obtained with STIS and clear filter (50CCD). The project could also be done with WFPC2 and F606W (see last paragraph of this section). The predicted SSC intrinsic flux densities of hot spots A and D are similar - 1.46 and 1.84 MuJy, respectively, at 7, 000Angstrom. Allowing for the obscuration A_V = 1.6 mag indicated by the column density to the X-ray sources associated with the hot spots, the observed fluxes will be a factor of 3.0 below these values - i.e. 0.48 and 0.61 MuJy, which correspond to R = 24.4 and 24.2 mag respectively, about 2 mag fainter than Meisenheimer et al's upper limit to hot spot D from 3.5m telescope ground-based observations (the ground-based upper limit to hot spot A is higher due to the proximity of a Galactic star - see Fig. 3). We have assumed this emission is uniformly spread over a 2 arc sec diameter in each case. Use of the STIS imaging exposure time calculator then gives a S/N = 2 for an area of 4 unbinned STIS CCD pixels in an exposure time of 16, 600 secs for hot spot A and 10, 820 secs for hot spot D (the background is of course dominated by the sky and we have assumed low Zodiacal light in this simulation). Over the whole hot spot, there are are about 300 of these 4 pixel regions, so the S/N for detection of the whole source would be about 35. The S/N for the 4 unbinned STIS pixels assumes, of course, that the brightness distribution over the hot spot is uniform. In reality, the radio images show very bright ridges and compact regions within each hot spot (e.g. Carilli et al. 1999), so some regions will have higher S/N than 2 and others lower. It will be possible to obtain similar details of the SSC brightness distribution, as needed to compare with the synchrotron distribution for our second scientific goal (see above). Allowing for some loss of time through our Low-sky preference, we thus request 5 orbits for the western hot spots (A and B) and 5 orbits for the eastern hot spots (D and E), giving a total of 10 orbits. Fig. 4 shows the superposition of the STIS 50CCD field on the ESO/ECF Sky Survey. This demonstrates that the stars detected by Roeser et al. (1996) near hot spots A and B, and the closest star to hot spots D and E (see Fig. 3), are all below the limit of the Sky Survey. The absence of nearby, bright stars makes it unlikely that charge bleeding or the spikes of a star's PSF will limit our sensitivity. Nevertheless, if this proposal is accepted, we shall, in phase 2, look closely at the star field to determine whether any special orientation is desirable to minimise the above effects. Even if a certain orientation is found to be desirable, the range of possible orientations should be quite large, so scheduling will not be difficult. The astrometric (r.m.s. deviation of primary reference stars = 0.24 arc secs) positions of the stars in the vicinity of the hot spots have been measured and listed by Roeser et al. (1996), so we know precisely where the radio hot spots lie in the star field (Fig. 3). In phase 2, we will also solicit STScI expertise on dithering strategies. We have also performed the exposure time calculation for WFPC2 with F606W. For the same S/N = 2 per 0.1 arc sec WF pixel, the exposure time needed is about twice that required with STIS and the clear filter. The hot spots are separated by 2.1 arc min, so both could be done in a single pointing with WFPC2, while two pointings are needed with STIS. Thus, the total exposure times would be about the same for the two instruments. The advantage of STIS is the smaller pixel size, which will allow us to obtain better resolution on bright regions and better discrimination against faint stars. However, if STIS were unavailable for any reason, we could switch to WFPC2. Real Time Justification ----------------------- The observations are limited by the sky background and would benefit from scheduling during Low-sky, which we request. None Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9123( 2) - 02/16/01 13:48 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 WESTERN- EXT-MEDIUM, HOTSPOT, RA=19H 59M 22.75S +/- 0.01S, J2000 Z = R = 24.4 HOTSPOTS GALAXY, JET DEC=40D 44' 25.5" +/- 0.1" 0.056075 Coordinate Source: OTHER_SOURCE 2 EASTERN- EXT-MEDIUM, HOTSPOT, RA=19H 59M 32.97S +/- 0.01S, J2000 Z = R = 24.2 HOTSPOTS GALAXY, JET DEC=40D 43' 40" +/- 0.1" 0.056075 Coordinate Source: OTHER_SOURCE 9123( 2) - 02/16/01 13:48 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 10 Primary_Pattern Secondary_Pattern Pattern_Type STIS-CCD-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.2835 Coordinate_Frame POS-TARG Pattern_Orient 26.6 Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ 9123( 2) - 02/16/01 13:48 - [ 5] Visit: 01 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 WESTERN-HOT STIS/C ACCUM 50CCD MIRROR CR-SPLIT=3 1 2625 S LOW-SKY PATTERN 10 10 SPOTS CD ------------------------------------------------------------------------------------------------------------------------------------ 20 WESTERN-HOT STIS/C ACCUM 50CCD MIRROR CR-SPLIT=3 1 2625 S LOW-SKY SPOTS CD ------------------------------------------------------------------------------------------------------------------------------------ 9123( 2) - 02/16/01 13:48 - [ 6] Visit: 02 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 EASTERN-HOT STIS/C ACCUM 50CCD MIRROR CR-SPLIT=3 1 2625 S LOW-SKY PATTERN 10 10 SPOTS CD ------------------------------------------------------------------------------------------------------------------------------------ 20 EASTERN-HOT STIS/C ACCUM 50CCD MIRROR CR-SPLIT=3 1 2625 S LOW-SKY SPOTS CD ------------------------------------------------------------------------------------------------------------------------------------ 9123( 2) - 02/16/01 13:48 - [ 7] Summary Form for Proposal 9123 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=3 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements LOW-SKY PATTERN 10 10 LOW-SKY ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names WESTERN-HOTSPOTS EASTERN-HOTSPOTS ------------------------------------------------------------------------------------------------------------------------------------