6645( 5) - 02/06/96 09:15 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6645 Version: 5 Check-in Date: 06-Feb-1996 09:12:42 1.Proposal Title: UV-OPTICAL SPECTRA OF MIDDLE-AGE PULSARS: THERMAL VS. NONTHERMAL ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle HOT STARS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: George Pavlov Pennsylvania State University CoI: France Cordova NASA HQ and Penn State N ------------------------------------------------------------------------------------ 6. Abstract Deep spectral observations of isolated pulsars are critical for understanding the nature and behavior of neutron stars. In principle, such observations could distinguish between thermal radiation from the neutron star surface and nonthermal radiation from its magnetosphere. We propose to investigate the ultraviolet and optical radiation from the radio/soft-X-ray pulsar PSR B0656+14. We have previously detected B0656+14 with the FOC long-pass filter F130LP. A comparison of these HST data and the ROSAT data with model spectra of neutron star atmospheres shows that the optical-UV flux is mainly provided by nonthermal radiation which exceeds by several orders of magnitude predictions of the theoretical models. To better understand the origin of the nonthermal component, we propose to obtain deep imaging of this object with several filters. Measuring the far-UV flux is essential for the detection of the thermal component and evaluation of the effective temperature of the neutron star atmosphere. ------------------------------------------------------------------------------------ 6645( 5) - 02/06/96 09:15 - [ 2] Observations Description ------------------------ We propose to obtain deep images in several FOC filters for the radio and soft X-ray pulsar PSR B0656+14. Because of its greater sensitivity, the FOC is essential for the far-UV through B band imaging. PSR B0656+14 was the brightest pulsar in our Cycle 4 observations, which allows its spectral distribution to be studied with greater spectral resolution. In analyzing our Cycle 4 data we have used IRAF photometry routines to measure the count rates in the summed F130LP images, which incorporate the standard pipeline processing. To evaluate the count rates expected for other filters, we fitted our data along with the marginal detection of V~eq 25 mag of Caraveo et al. (1994) using SYNPHOT. We assumed that the nonthermal component of the spectral flux behaves as F_nu\\propto Nu^-Alpha, and the thermal component is given by one of the extreme models (`bb' and `atm' in Fig. 1 of Pavlov et al. 1996) obtained by extrapolating the ROSAT spectrum to the UV-optical domain. The best fits correspond to Alpha = 0.45 and Alpha=0.85 for `bb' and `atm' thermal components, respectively. Since attenuation of radiation by interstellar extinction may be potentially important, especially in the far-UV range, we estimated the expected extinction E(B-V) for B0656+14 from optical and IUE observation of nearby stars and from attenuation of the soft X-ray radiation of B0656+14. This estimates show that E(B-V) most likely does not exceed 0.02- 0.03; such low extinction can be neglected even in the far-UV band, at least for preliminary estimates of the expected sourse count rates. For the background rate in the FOC, needed to compute exposure times for different filters, we assume R_B=n* 1* 10^-3 s^-1, a typical value obtained in our Cycle 4 observations (n is the number of pixels). Based on these estimates, we propose to observe the pulsar with three FOC filters: F195W, F342W (U) and F430W (B). The expected source count rate in the U filter (3060-3760 Angstrom at half maximum) is R_S=0.20-0.24 s^-1; a S/N=10-12 can be achieved in a 35 min orbit after initial aquisition. The source count rate in the B filter (3520 - 4350 Angstrom ) is less definite, R_S=0.15-0.25; a 40 min (one orbit) exposure would provide sufficiently high S/N=9-12. The filter F195W (the transparency band at half maximum is 1640 - 2580 Angstrom), is the most important for evaluation of the thermal component of the pulsar radiation. Its efficiency is lower than that for U and B although higher than for other far-UV filters. The source count rate estimate for this filter is R_S=0.05- 0.10 s^-1, depending on the (unknown) contribution of the thermal component. This gives S/N =4-7 for one orbit, S/N=6- 10 for two orbits, and S/N=7-12 for three orbits. The latter S/N values would allow us to measure the far UV fluxes with sufficient accuracy. Thus, observation of B0656+14 (one visit) would require a total of 5 FOC orbits. Real Time Justification ----------------------- None. None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6645( 5) - 02/06/96 09:15 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6645( 5) - 02/06/96 09:15 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 PSR-B0656+14 Star,Pulsar,Neutron RA=6H 59M 48.13S +/- 0.05S, J2000 V = 25+/-1 Star DEC=+14D 14' 21.2" +/- 0.1", B-V = 0.3+/-0.3 PLATE-ID=02UU E(B-V) = 0.03+/-0.03 6645( 5) - 02/06/96 09:15 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: (1) LOW-SKY is imperative to minimize the zodiacal background and Earth shine in the F430W and F342W filters (exposures #1 and #2), and to reduce the airglow background in the F195W filter (exposure #3). (2) The most favorable observational periods for the exposure #3 are when the telescope is in the Earth shadow during a substantial part of the orbit (within and around the periods 5 Mar - 9 Mar 1996 and 1 Feb - 22 Feb 1997). (3) Notice that a substantial fraction of the schedulable period in 1997 overlaps with the servicing mission. (4) It is imperative for different iterations (subexposures) of each of the three exposures be executed with the same guide stars. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 PSR- FOC/96 IMAGE 512X512 F342W 2 19M LOW-SKY B0656+14 ------------------------------------------------------------------------------------------------------------------------------------ 2 PSR- FOC/96 IMAGE 512X512 F430W 2 20M LOW-SKY B0656+14 ------------------------------------------------------------------------------------------------------------------------------------ 3 PSR- FOC/96 IMAGE 512X512 F195W 6 20M LOW-SKY B0656+14 ------------------------------------------------------------------------------------------------------------------------------------ 6645( 5) - 02/06/96 09:15 - [ 6] Summary Form for Proposal 6645 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOC/96 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category HOT STARS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements LOW-SKY ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F342W F430W F195W ------------------------------------------------------------------------------------------------------------------------------------ Target Names PSR-B0656+14 ------------------------------------------------------------------------------------------------------------------------------------