9177( 4) - 04/23/01 08:15 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9177 Version: 4 Check-in Date: 23-Apr-2001 08:15:07 1.Proposal Title: Tidal Disruption of Stars by Massive Black Holes in Galaxy Nuclei: After the Flare ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Jules Halpern Columbia University in the City of New York CoI: Prof. Karen Leighly The University of Oklahoma N CoI: Dr. Dirk Grupe Max-Planck-Institut N CoI: Dr. Stephanie Komossa Max-Planck-Institut N ------------------------------------------------------------------------------------ 5. Abstract A supermassive black hole will rip apart a star that strays within its tidal radius, causing an Eddington-limited UV/X-ray flare for several months as the orbiting debris accretes. While such events are predicted to occur at most once in 10^4 yr per galaxy, an experiment was performed in 1990-91 which sampled hundreds of thousands of galaxies in the ideal wavelength band, i.e., the ROSAT All-Sky Survey. Three galaxies had unusual X-ray flares, but no evidence for nuclear activity in ground-based spectra. To establish beyond a reasonable doubt that these were tidal disruption events, we propose to make a sensitive search for permanent Seyfert activity, the only possible alternative to the disruption hypothesis. Nuclear optical spectra obtained through a narrow slit will reject most of the starlight and place limits on AGN-like emission line activity below those of the weakest Seyferts. If necessary, we could return over the next decade to ensure that any weak emission lines are decaying. Establishing the UV/X-ray properties of genuine tidal disruption events is important for the next generation of sky surveys that will monitor millions of galaxies. Growing evidence that supermassive black holes are ubiquitous in galaxy nuclei means that the frequency of tidal disruption as a function of galaxy mass and type could be determined. Masses of black holes could be studied by monitoring the outburst light curves and the spectra of the tidal debris. ------------------------------------------------------------------------------------ 9177( 4) - 04/23/01 08:15 - [ 2] Observations Description ------------------------ The main observational goal of this proposal is to obtain spectra of sufficient sensitivity to either detect or rule out the presence of Seyfert-like emission lines in the nuclei of galaxies which have had luminous X-ray flares in the recent past. These galaxies have nuclei dominated by starlight in ground-based spectra taken through slits of width 1.0"-1.7", and they present no indication of Seyfert-like emission lines. However, HST can provide a significant improvement by rejecting starlight by up to a factor of 100 in comparison with the best ground-based observations. Accordingly, we plan to use the 0.1" wide slit, and the STIS/CCD with gratings G750L (or G750M) and G430L in order to cover the important emission lines in the visible region of the spectrum. The 0.1" wide slit provides the best compromise between spatial resolution and throughput for a point source surrounded by diffuse emission. The principal wavelength region of interest in the G750 grating includes [O I]6300, [Fe X]6375, H-alpha, [N II]6548, 6583, and [S II]6716, 6730. It is standard practice to distinguish between Seyfert activity and ordinary stellar processes using this complement of lines. The G750M provides high spectral resolution, ~0.85 Angstrom, which is useful for obtaining velocity information that can help to discriminate among various sources of emission lines. It will be used to study NGC 5905, for which there are strong H II region lines present. If any weak Seyfert emission lines are present, they will be readily apparent from their velocity widths, as is commonly observed in many "composite" Seyfert/H II galaxies that have been studied from the ground. For the higher redshift galaxies, which have weak or no H II region emission, we will use the lower- resolution G750L grating. The signal-to-noise for these fainter objects would be prohibitively weak at higher resolution, where read noise and dark count starts to dominate. With a velocity resolution of ~320 km s^-1, the G750L will be able to resolve Seyfert-like emission lines, which are almost always broader than 300 km s^-1. Since we expect any nuclear emission lines, if present at all, to be extremely weak, we also want to obtain coverage of their H-eta/[O III]5007 region, since this line ratio is an excellent diagnostic of Seyfert activity. [O III]5007 is often the brightest emission line in a Seyfert nucleus, and it averages over decades of activity, while a single tidal disruption event would not produce much of this line. The G430L grating will provide the ideal resolution and adequate signal in just one or two orbits per galaxy, as well as provide information about any blue nonstellar continuum that may be present from residual accretion or AGN activity. While extra reddening is often a concern in studying galactic nuclei, there is no possibility that these particular nuclei are obscured, precisely because they emitted ultrasoft X-ray flares without significant absorption (N_H < 3 * 10^20 cm^-2). Thus, there cannot be any intrinsic obscuration in the optical. A conservative upper limit is A_V < 0.2. We have also ruled out the possibility that these variable soft X-ray sources are interlopers, i.e., chance superpositions of foreground Galactic objects such as high B-field AM Her stars or supersoft binaries. This conclusion comes from optical observations and from study of their X-ray light curves. Their soft X-ray spectra and relatively steady peak flux on a time scale of days also demonstrates that they are not GRB afterglows. Fluxes and signal-to-noise ratios are estimated in the accompanying Table. These were calculated assuming a slit throughput of 70%, and a nucleus extraction of 3 rows, which contain 65%-70% of the flux of a point source. Sky background is negligible, but dark count and read noise were included assuming CR-SPLIT sub- exposures of 10 minutes each. The continuum fluxes that we plan to see in the neighborhood of the important H-alpha and H-beta/[O III] emission lines are listed, together with the source count rate and S/N per pixel for the exposure times that can be obtained after all overheads are taken into account. These expected fluxes are approximately 10 times fainter than seen in our ground based spectra of the nuclei. Although the count rates and signal- to-noise ratios are relatively small, these are values per individual continuum pixel (or two pixels in the case of the G750M grating). Of principal scientific relevance is the sensitivity to emission lines, which for H-alpha is approximately 1 * 10^-16 erg cm^-2 s^-1 for a line 1000 km s^-1 wide. In Seyfert nuclei, the broad H-alpha emission is typically 10% of the soft X-ray flux. So these HST spectra will be sensitive to optical lines that we would expect to accompany a Seyfert X-ray source of luminosity 1 * 10^40 erg s^-1 at z = 0.050. Thus, we will be sensitive to the weakest known Seyferts. Limiting Sensitivities and Estimated Exposure Times------------------------------------- -------------------------------- Object z Grating Lambda Limiting Count Exp. S/N f-lambda Rate Time per (A) (erg/cm2/s/A) (ct/s/pix) (min) pix-------------- ------------------------------------------------------- NGC 5905 0.011 G430L 4914 1.0 * 10^-16 0.159 32 14 G750M 6635 1.2 * 10^-16 0.124 44 12 RXJ1624.9 0.064 G430L 5172 1.4 * 10^-17 0.023 82 7 +7554 G750L 6983 1.9 * 10^-17 0.051 46 5 RXJ1242.6 0.050 G430L 5104 4.5 * 10^-17 0.067 39 8-1119A G750L 6891 4.0 * 10^ -17 0.121 27 10 RXJ1242.6 0.050 G430L 5104 0.7 * 10^-17 0.012 68 3-1119B G750L 6891 0.6 * 10^-17 0.018 39 3--------------------------------------------------- ------------------ Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9177( 4) - 04/23/01 08:15 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5905 GALAXY, SPIRAL RA=15H 15M 23.32S +/- 0.5", J2000 Z = V = 15.5+/-0.5 DEC=55D 31' 2.18" +/- 0.5", 0.011308 B-V = 0.7+/-0.2 PLATE-ID=01RT Coordinate Source: GSC_SURVEY_PLATE 2 RXJ1624.9+75 GALAXY, SPIRAL RA=16H 24M 56.49S +/- 0.5", J2000 Z = V = 17.5+/-0.5 54 DEC=75D 54' 56.09" +/- 0.5", 0.0636 B-V = 1.0+/-0.2 PLATE-ID=034H Coordinate Source: GSC_SURVEY_PLATE 3 RXJ1242-11A GALAXY, SPIRAL RA=12H 42M 38.46S +/- 0.5", DEC= J2000 Z = V = 17.5+/-0.5 -11D 19' 20.31" +/- 0.5", PLATE- 0.050 B-V = 1.0+/-0.2 ID=02NU Coordinate Source: GSC_SURVEY_PLATE 4 RXJ1242-11B GALAXY, SPIRAL RA=12H 42M 38.14S +/- 1.0", DEC= J2000 Z = V = 19.4+/-0.5 -11D 19' 12.95" +/- 1.0", PLATE- 0.050 B-V = 1.0+/-0.2 ID=02NU Coordinate Source: IMAGE_TIED_TO_GSC_FRAME 9177( 4) - 04/23/01 08:15 - [ 4] 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 NGC5905 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 5.0 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5905 STIS/C ACQ/PEA 52X0.1E MIRROR 1 5.0 S CD K 1 ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5905 STIS/C ACCUM 52X0.1E G430L 4300 CR-SPLIT=3 1 2235 S CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC5905 STIS/C ACCUM 52X0.1E G750M 6581 CR-SPLIT=4 1 2980 S CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 9177( 4) - 04/23/01 08:15 - [ 5] 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 RXJ1624.9+7 STIS/C ACQ 50CCD MIRROR ACQTYPE=DIFFUSE,CHECKBOX=13, 1 30 S 554 CD DIFFUSE-CENTER=FLUX-CENTROID ------------------------------------------------------------------------------------------------------------------------------------ 30 RXJ1624.9+7 STIS/C ACCUM 52X0.2E G750L 7751 CR-SPLIT=7 1 5334 S 554 CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 40 CCDFLAT STIS/C ACCUM 52X0.2E G750L 7751 2 DEF CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 50 RXJ1624.9+7 STIS/C ACCUM 52X0.2E G430L 4300 CR-SPLIT=8 1 6080 S 554 CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 9177( 4) - 04/23/01 08:15 - [ 6] Visit: 03 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 RXJ1242-11A STIS/C ACQ 50CCD MIRROR ACQTYPE=POINT 1 30 S CD ------------------------------------------------------------------------------------------------------------------------------------ 30 RXJ1242-11A STIS/C ACCUM 52X0.2E G750L 7751 CR-SPLIT=7 1 4655 S CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 40 CCDFLAT STIS/C ACCUM 52X0.2E G750L 7751 2 DEF CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 50 RXJ1242-11A STIS/C ACCUM 52X0.2E G430L 4300 CR-SPLIT=8 1 5488 S CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 9177( 4) - 04/23/01 08:15 - [ 7] Summary Form for Proposal 9177 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.1E1 50CCD 52X0.2E1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT CR-SPLIT=3 CR-SPLIT=4 DIFFUSE-CENTER=FLUX-CENTROID CR-SPLIT=7 CR-SPLIT=8 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G430L G750M G750L ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC5905 RXJ1624.9+7554 CCDFLAT RXJ1242-11A ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 4300 6581 7751 ------------------------------------------------------------------------------------------------------------------------------------