7294( 5) - 12/12/97 14:25 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7294 Version: 5 Check-in Date: 12-Dec-1997 14:25:00 1.Proposal Title: Observations of a BL Lacertid with a Unique Absorption Line System ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: ROSS COHEN University of California at San Diego CoI: Edward Beaver University of California, San Diego N CoI: Margaret Burbidge University of California, San Diego N CoI: Vesa Junkkarinen University of California, San Diego N CoI: Ron Lyons University of California, San Diego N CoI: Greg Madejski NASA, Goddard Space Flight Center N ------------------------------------------------------------------------------------ 5. Abstract AO 0235+164 is a BL Lac object, the line of sight to which passes close to two luminous objects. One of these objects (or both) produces a unique variable 21 cm absorption line system. Observations of the Ly-alpha absorption line at 1853Angstrom in the spectrum of AO 0235+164 will yield the H I column density, providing a measurement of spin temperature in the absorber. When combined with observations of weak metal lines obtained at the Keck Observatory, an absolute measurement of the gas phase metal abundances in a galactic disk at z=0.524 will be obtained. This can be compared with the total metal abundance measured from X-ray absorption to yield a dust-to-gas ratio. Comparison of the continuum shape with new X-ray continuum observations (which can be scheduled simultaneously due to the flexibility of the Asca scheduling system) will measure the extinction. A comparison of the extinction with both the total abundance of metals and the gas phase abundance will be unique. HST spectroscopic observations are required because the Ly-alpha line lies in the UV. ------------------------------------------------------------------------------------ 7294( 5) - 12/12/97 14:25 - [ 2] Observations Description ------------------------ The H I column density at z=0.524 will be measured by observing the damped LyAlpha line with STIS, the near-UV MAMA, and G230L. Although the prism is marginally more sensitive, the dispersion is too low at the appropriate wavelength. We will perform a target acquisition on a star from the guide star catalog located approximately 30" away, and make our spectroscopic observations with the 0.5" slit. Because the offset star is saturated in the wfc image on which the offsets were measured, we will not try to offset into a narrower slit, but if we are able to refine the offset measurement, we will peak up on the offset star and then make the observation with the 0.2" slit. We will not peak up on the target itself because it is highly variable. The narrower slit would provide a slight advantage because there is a faint object 1" away which could provide a small contamination of the target spectrum depending on the (unknown) extent to which it is extended. A somewhat brighter companion 2" away will not contaminate the spectrum because it will be well separated along the slit at the orientations at which this observation will be made. Schramm et al. (1994) give R=16.23 with a total range of almost +/- 2 magnitudes over more than 4 years. Translated to V with a slope of alpha=3 (F_Nu propto nu^-alpha) determined from our optical data suggests a maximum V magnitude of 19.25 over a many year period. Extrapolated to B, this is actually slightly fainter than the faintest state observed by Webb et al. (1988) over a 10 year span. We estimate that AO 0235+164 was actually slightly fainter than this during an attempt to make this measurement with GTO observations, which failed when AO 0235+164 was equal in brightness to the optical companion. Subsequent optical observations show that it returned almost immediately to a brighter state. The more than 14 years of optical monitoring show that such a faint state is extremely rare, and we base our calculations on that monitoring, rather than on our one observation which apparently took place at a very rare state for this object. While it is not impossible that the object could become this faint again, it appears extremely unlikely. The UV observation is designed so that, in 5 orbits of integration, we will achive a signal--to--noise of 5 per resolution element for the faintest state in which this object is likely to be found (see above). This was calculated with the STIS Spectroscopic Exposure Time Calculator assuming average background levels (which should be faint at UV wavelengths in any case). As shown in figure 1, this will provide a measurement of the H I column density accurate to better than 25%. In brighter states, the determination will be much better. A measurement superior to the IUE measurement can be made even in a fainter state. Because of the low redshift, contamination by Ly- alpha forest lines will be minimal even at the relatively low resolution we will use. This observations should be done in one visit to ensure simultaneity. However, because of the restrictons on the number of orbits with MAMA observations, this is set up as two visits. The first visit will be devoted to the target acq. and observations with the CCD and G430L and G750L. This will be adequate to achieve a S/N of 10 per resolution element over the entire optical spectrum from 3000 Angstrom to 1 micron. A contemporaneous flat should be taken with these observations. The second visit will include a target acq. and 5 orbits with MAMA and G230L. The MAMA obs. could precede the CCD obs. if that made scheduling easier. Real Time Justification ----------------------- This is a highly variable object (+/- 2 magnitudes; see above). Variability occurs in optical spectral shape as well as magnitude and occurs at high energies as well. Observations of this object with ROSAT and Asca (Madejski et al. 1996) revealed that the continuum is highly variable; the ROSAT flux essentially doubled in 3 days, followed by a slower quasi-exponential decay. The second portion of this project requires measurement of the slope in the optical/UV as well as X-ray. Because of the variability, these observations must be made simultaneously. If the observations are scheduled anywhere during the Asca observing window, simultaneous Asca observations will be possible (see below). The observing windows for the two telescopes do have significant overlap. An application for observations with Asca has been approved to measure the X-ray power-law slope for comparison with the optical/near-UV measurements. Provided that the HST observations are made during the appropriate time period, it will be possible to schedule the Asca observations simultaneously. The Asca observing windows for both 1997 and 1998 are January 11--February 10 and July 19--August 26 which overlap nicely with the HST observing windows determined with RPS2. It would probably be safer to exclude the three days at the ends of the windows. Asca scheduling is flexible, and changes to the Asca observation schedule can be made after the HST schedule is finalized. If scheduling is not feasible in 1997, we would like to have it scheduled in 1998, as in the past it was generally possible to postpone approved ASCA observations for reasons of simultaneity. We will apply for observing time with the HIRES spectrograph on the Keck Telescope. This time will not need to be simultaneous. Our experience is that we will be awarded the relatively small amounts of time necessary for this project. I would like to suggest that the observations not be left until the last remaining window, as we then have no safety margin if anything fails. Calibration Justification ------------------------- We request a real time flat for the G750L spectrum, because the fringing residuals described in the instrument update would make our observations unusable at the red end of the spectrum, especially in the faint state. Additional Comments ------------------- ------------------------------------------------------------------------------------ 7294( 5) - 12/12/97 14:25 - [ 3] Data Distribution Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7294( 5) - 12/12/97 14:25 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AO0235+164 GALAXY,BL Lac,EXT- RA=02H 38M 38.9306S +/- 0.5", 2000 Z=0.94 V = 17.0+/-2.0 MEDIUM,Damped Lyman DEC=16D 37' 0.035" +/- 0.5", B-V = 1.0+/-0.5 Alpha Cloud,Absorption PLATE-ID=01G9 Line System 2 AO0235+164- STAR,K V-IV RA=02H 38M 39.86S +/- 0.3", 2000 V = 14.36+/-0.5 OFFSET DEC=16D 37' 20.3" +/- 0.3", B-V = 1.0+/-0.2 PLATE-ID=01G9 TYPE=K Comments: This is the offset star. The exact spectral type is unknown, and B-V is a rough estimate. 7294( 5) - 12/12/97 14:25 - [ 5] Visit: 01 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: This is meant to overlap in time with an ASCA observation of the same object. The observing windows are 14-JAN-98 to 15-FEB-98 and 22-JUL-98 to 23 -AUG-98. We have identified the days with the greatest possible ASCA/HST overlap in the first window as Feb 11, Feb 6, Feb 10, and Feb 12, in decreasing order of overlap. Ideally, a portion of each visit should overlap with the ASCA observations. Please call me with any questions. If only one visit can overlap with ASCA, it is more critical that the HST/ASCA overlap is with Visit 02. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 AO0235+164- STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 1 S ONBOARD ACQ FOR 2-4 OFFSET CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 AO0235+164 STIS/C ACCUM 52X0.5 G430L 4300 1 2500 S CD ------------------------------------------------------------------------------------------------------------------------------------ 3 AO0235+164 STIS/C ACCUM 52X0.5 G750L 7751 1 1800 S CD ------------------------------------------------------------------------------------------------------------------------------------ 4 CCDFLAT STIS/C ACCUM 0.3X0.0 G750L 7751 2 DEF CD 9 ------------------------------------------------------------------------------------------------------------------------------------ 7294( 5) - 12/12/97 14:25 - [ 6] Visit: 02 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Because of the extreme variability of the object, the visits should be as close together in time as possible, but the order of visits does not matter. Ideally, the visits would be contiguous, and a gap of three orbits or less between visits would be excellent. Any schedule which allows overlap between the ASCA observation and a portion of each visit would ensure an unambiguous scientific result. In our original proposal we used the GROUP 1,2 WITHIN 15 HOURS visit requirement, based on estimates of the variability time scale. While far from ideal, this is still acceptable if a shorter time between visits cannot be arranged. If only one visit can be overlapped with ASCA, it is more critical that the HST/ASCA overlap be for Visit 02. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 AO0235+164- STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 1 S ONBOARD ACQ FOR 2-6 OFFSET CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 AO0235+164 STIS/N ACCUM 52X0.5 G230L 2376 1 2300 S EXPAND END ORBIT UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 3 AO0235+164 STIS/N ACCUM 52X0.5 G230L 2376 1 2300 S EXPAND END ORBIT UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 AO0235+164 STIS/N ACCUM 52X0.5 G230L 2376 1 2300 S EXPAND END ORBIT UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 5 AO0235+164 STIS/N ACCUM 52X0.5 G230L 2376 1 2300 S EXPAND END ORBIT UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 6 AO0235+164 STIS/N ACCUM 52X0.5 G230L 2376 1 2300 S EXPAND END ORBIT UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 7294( 5) - 12/12/97 14:25 - [ 7] Summary Form for Proposal 7294 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2-4 ONBOARD ACQ FOR 2-6 EXPAND END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G430L G750L G230L ------------------------------------------------------------------------------------------------------------------------------------ Target Names AO0235+164-OFFSET AO0235+164 CCDFLAT ------------------------------------------------------------------------------------------------------------------------------------