! Proposal 6635, submission 1 ! PI: Francesco Bertola ! Received Fri Feb 9 19:30:34 EST 1996 ! From: mcappell@eso.org ! +====================+ ! | PED OUTPUT PRODUCT | ! +====================+ ! ! /diskc/visitor/mcappell/6635.prop ! Generated by PREPROCESSOR, version 6.0e ! Date: Sat Feb 3 15:33:43 MET 1996 Proposal_Information Title: Unveiling the Nature of the Ultraviolet Spike at the Center of the Sa Galaxy NGC 2681 Proposal_Category: GO Scientific_Category: GALAXIES & CLUSTERS Cycle: 6 Investigators PI_Name: Francesco Bertola PI_Institution: University of Padova CoI_Name: David Burstein CoI_Institution: Arizona State University Contact: N CoI_Name: Lucio Buson CoI_Institution: Padova Observatory Contact: N CoI_Name: Michele Cappellari CoI_Institution: University of Padova Contact: Y CoI_Name: Jose Funes CoI_Institution: University of Padova Contact: N CoI_Name: Laura Greggio CoI_Institution: University of Bologna Contact: N CoI_Name: Alvio Renzini CoI_Institution: ESO Contact: N Abstract: We propose to investigate the nature of the central UV-bright spike in the bulge-dominated Sa galaxy NGC 2681, first detected in the course of our previous HST/FOC observations of this galaxy. The feature appears point-like in our Pre-Costar images and is located within ~ 2 pc from the galaxy center. New combined FOS and FOC/96 observations will provide its UV/optical SED and higher resolution imaging. These data will allow to determine the physical nature of the source (a compact cluster of hot stars, or a non-thermal, perhaps transient nuclear source). In particular, these data should allow to establish whether the UV spike in NGC 2681 is the same kind of transient phenomenon we have detected in NGC 4552, an otherwise normal elliptical galaxy in which a similar UV-bright central spike underwent a flare, brightening by over a factor of 7 between 1991 and 1993. Questions Observing_Description: The proposed observations are almost identical to those requested in our DD Cycle 5 proposal (ID 6309) aimed at imaging the central spike of NGC 4552. As a consequence, we expect to be able to reduce the data in a homogeneous way, though taking advantage of a higher S/N ratio for NGC 2681., Using as reference our own previous (epoch 1993) FOC/96 images of the same galaxy, the expected background subtracted flux from the spike in NGC 2681 is ~ 3*10^-16 erg cm^-2 s^-1 AA^-1 at Lambda ~ 2700 Angstrom. Its SED appears consistent -- in the 1700-2800 Angstrom range -- with that of a T ~ 10000 K Black Body., As in the case of NGC 4552, we plan to spend about two orbits for the acquisition of the target using the FOS ACQ/PEAK mode. The remaining two orbits will be devoted to the science exposures: we will acquire the spectra using the gratings G270H (3rd orbit) as well as G780H and G650L (4th orbit). Given the flux, the expected S/N ratio over 6 Angstrom intervals (3 diodes) for the G270H spectrum is ~20 with the spike contributing about 4/5 of the total count rate in one of the 0.25-PAIR aperture. The contaminating galaxy light will be removed using the existing IUE spectrum. The G780H grating will possibly allow us to detect HAlpha emission, as already detected by Eracleous et al. (1995, ApJ 438, 610) at the 7*10^-16 erg cm^-2 s^-1 ang^-1 level in the nucleus of NGC 1097, and attributed to accretion around a massive black hole in that galaxy. At the distance of NGC 2681 this emission would appear at the 1.1*10^-15 erg cm^-2 s^-1 ang^-1 level and would be easily detectable with good S/N over the galaxy continuum. Finally, the G650L grating will permit us to complete the full SED of the spike with little additional overhead. Our FOS observations will be supplemented by contemporary FOC imaging, primarily to try to determine the spatial extent of the spike in NGC 2681. The F175W image permits extension of the SED of the spike from 2200 down to 1600 Angstrom, using known sensitivity curves and our FOS spectra (to account for the red--leak). Together with the F275W image it will be used to check for variability of the spike. The F342W image will be used, together with the other images, to determine the UV color of any additional structure that may exist around the spike., Using our previous FOC observations and owing to the improved performance of the Post-COSTAR HST, we estimate that we will be able to get a better S/N in less than half the exposure time, so we can efficiently pack all FOC observations within one orbit. Using the formulas for nonlinearity correction of point sources over a bright background (Greenfield, P., ISR FOC-074, 1994), we estimate that the Post-COSTAR count rate will remain in the linear range of the FOC 512*512 mode both in the F175W and F275W filter, whereas we will need an additional neutral F1ND filter for the F342W exposure. Real_Time_Justification: Calibration_Justification: Additional_Comments: Fixed_Targets Target_Number: 1 Target_Name: NGC2681-NUC Alternate_Names: Description: GALAXY,SPIRAL, Position: RA=8H 53M 32.80S +/- 0.1S,DEC=51D 18' 49.4" +/- 1" Equinox: J2000 RV_or_Z: RA_PM: 0.0 Dec_PM: 0.0 Epoch: Annual_Parallax: 0.0 Flux: F(1750)=1.5+/-0.5e-16 F(2750)=3.0+/-0.5e-16 F(3400)=3.0+/-0.5e-16 W-LINE(6600)=200+/-100 F-LINE(6600)=1+/-0.5e-15 Comments: The target luminosity might be variable. The fluxes at specified wavelength refer to the last values measured with FOC in october 1993. The line flux is a very uncertain extrapolation based on theoretical considerations. Solar_System_Targets Generic_Targets Scan_Data Visits Visit_Number: 01 Visit_Requirements: SEQ 1-2 WITHIN 24H On_Hold_Comments: Visit_Comments: FOC photometry will be used in conjunction with FOS spectra to determine the Spectral Energy Distribution of the flare. This proposal includes FOS observations which have to be done before the 2nd Servicing Mission in Feb 1997. Exposure_Number: 1 Target_Name: NGC2681-NUC Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F342W,F2ND Wavelength: 3410 Optional_Parameters: Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: Comments: Exposure_Number: 2 Target_Name: NGC2681-NUC Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F275W,F1ND Wavelength: 2770 Optional_Parameters: Number_of_Iterations: 1 Time_Per_Exposure: 550S Special_Requirements: SAME POS AS 1 Comments: Exposure_Number: 3 Target_Name: NGC2681-NUC Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F175W Wavelength: 1730 Optional_Parameters: Number_of_Iterations: 1 Time_Per_Exposure: 1025S Special_Requirements: END ORBIT EXPAND SAME POS AS 1 Comments: Visit_Number: 02 Visit_Requirements: On_Hold_Comments: Visit_Comments: Exposure_Number: 1 Target_Name: NGC2681-NUC Config: FOS/RD Opmode: ACQ/PEAK Aperture: 4.3 Sp_Element: G270H Wavelength: Optional_Parameters: SCAN-STEP-Y=1.23,SEARCH-SIZE-X=1,SEARCH-SIZE-Y=3 Number_of_Iterations: 1 Time_Per_Exposure: 12S Special_Requirements: ONBOARD ACQ FOR 2 Comments: Exposure_Number: 2 Target_Name: NGC2681-NUC Config: FOS/RD Opmode: ACQ/PEAK Aperture: 1.0-PAIR-A Sp_Element: G270H Wavelength: Optional_Parameters: SCAN-STEP-X=.61,SCAN-STEP-Y=.61,SEARCH-SIZE-X=6,SEARCH-SIZE-Y=2 Number_of_Iterations: 1 Time_Per_Exposure: 12S Special_Requirements: ONBOARD ACQ FOR 3 Comments: Exposure_Number: 3 Target_Name: NGC2681-NUC Config: FOS/RD Opmode: ACQ/PEAK Aperture: 0.25-PAIR-A Sp_Element: G270H Wavelength: Optional_Parameters: SCAN-STEP-X=.17,SCAN-STEP-Y=.17,SEARCH-SIZE-X=5,SEARCH-SIZE-Y=5 Number_of_Iterations: 1 Time_Per_Exposure: 12S Special_Requirements: ONBOARD ACQ FOR 4 Comments: Exposure_Number: 4 Target_Name: NGC2681-NUC Config: FOS/RD Opmode: ACQ/PEAK Aperture: 0.25-PAIR-A Sp_Element: G270H Wavelength: Optional_Parameters: SCAN-STEP-X=.052,SCAN-STEP-Y=.052,SEARCH-SIZE-X=5,SEARCH-SIZE-Y=5 Number_of_Iterations: 1 Time_Per_Exposure: 68S Special_Requirements: ONBOARD ACQ FOR 5 Comments: Critical ACQ/PEAK stage Exposure_Number: 5 Target_Name: NGC2681-NUC Config: FOS/RD Opmode: ACCUM Aperture: 0.25-PAIR-A Sp_Element: G270H Wavelength: Optional_Parameters: STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 2590S Special_Requirements: END ORBIT EXPAND Comments: Exposure_Number: 6 Target_Name: NGC2681-NUC Config: FOS/RD Opmode: ACCUM Aperture: 0.25-PAIR-A Sp_Element: G650L Wavelength: Optional_Parameters: STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 600S Special_Requirements: SAME POS AS 5 Comments: Exposure_Number: 7 Target_Name: NGC2681-NUC Config: FOS/RD Opmode: ACCUM Aperture: 0.25-PAIR-A Sp_Element: G780H Wavelength: Optional_Parameters: STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 1720S Special_Requirements: END ORBIT EXPAND SAME POS AS 5 Comments: Data_Distribution ! 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