16926( 5) - 19-May-2022 13:35:02 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16926 Version: 5 Check-in Time: 19-May-2022 13:35:02 STScI Edit Number: 1 Title UV Spectroscopy and Host Galaxy Imaging of a Possibly Coalescing Binary Supermassive Black Hole ------------------------------------------------------------------------------------ Type Cycle GO 29 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Xin Liu University of Illinois at Urbana - Y Champaign ------------------------------------------------------------------------------------ Abstract Binary supermassive black holes (SMBHs) are expected to be common from galaxy mergers. Their final inspirals and coalescences should produce gravitational waves detectable by pulsar-timing arrays and future space interferometers, making them exciting targets for multi-messenger astronomy. Subsequent to the most recent proposal deadline, the first candidate coalescing binary SMBH has just been discovered with rapidly decaying periods revealed by the remarkable optical and X-ray light curves (with the periods decreasing from ~one year to ~one month in the past three years). Multiple lines of evidence suggest that the system may be an uneven mass-ratio, highly eccentric binary SMBH that will likely merge within ~one to three years, presenting an exciting opportunity for multi-wavelength and potentially multi-messenger observations to witness the first ever binary SMBH merger event, justifying the mid-cycle request. New STIS UV spectroscopy may reveal evidence (or lack thereof) for a cutoff or notch in the NUV spectrum, which may arise from gap opening in the circumbinary accretion disk, and to measure the UV emission lines as tracers of accretion disk winds. Multi-band high-resolution WFC3 imaging is proposed to quantify the structural properties of the host galaxy, enabling bulge stellar mass measurement through robust bulge-disk decomposition for better (total) SMBH mass estimation, and to search for morphological evidence of a past merger. The program will independently test the binary hypothesis and provide the anchor for future follow-up campaigns. It has broad implications for multi-messenger astronomy and for understanding the cosmic formation and evolution of SMBHs. ------------------------------------------------------------------------------------ Observations Description ------------------------ The target is a redshift z=0.081 Serfect 1 AGN (i.e., the nuclear point source) hosted by an extended galaxy with an effective radius of ~10" (5") along the semi-major (semi-minor) axis. It has been found to host an unusual nuclear transient flare whose period and amplitude are both decreasing rapidly. While the leading hypothesis is a coalescing binary SMBH, complementary observations are needed to fully test the binary hypothesis as well as alternative scenarios. This program will conduct STIS NUV-MAMA first-order spectroscopy with the G230L grating (3 orbits) and the CCD first-order spectroscopy with the G430L grating (1 orbit) for the nuclear transient flare (point source). This setup will obtain a low-resolution (R~500) spectrum of the AGN flare covering the observed 16926( 5) - 19-May-2022 13:35:02 - [ 2] wavelengths ~1570--5700 Anstrom (rest-frame 1452--5272 Angstrom). It will test if there is any NUV flux deficit as expected from circumbinary accretion disk gap opening assuming the fiducial binary model parameters. The secondary goal with the STIS UV/optical spectroscopy is to measure the UV broad emission line (C IV and Mg II) profiles to test accretion disk wind models. The MAMA G230L grating is preferred over the CCD G230LB grating for its better throughput at the bluer end to measure C IV. The tertiary goal is to identify any spectroscopic post-starburst signatures in the circumnuclear emission as independent evidence for a past galaxy merger (e.g., the Balmer break and strong high-order Balmer absorption). We use the 0.2" wide slit as a good compromise between resolution and throughput. We do not require any specific PA for the slit to minimize scheduling constraints. This program will also conduct WFC3 three-band (UVIS F336W and F606W, and IR F105W) imaging (3 orbits in total at 1 orbit per filter) of the host galaxy of the unusual transient flare. The goal is to perform AGN-host and bulge-disk decompositions, enabling robust bulge stellar mass and total galaxy stellar mass estimates from three-band imaging to independently infer an estimate of the (total) SMBH mass, complementary to the broad-emission-line-based virial mass estimate. The deep, high-resolution imaging will also be used to search for morphological evidence of ongoing nuclear starbursts (with F336W imaging) and low-surface-brightness tidal features (with F105W imaging) as independent evidence of a past galaxy merger event. The F336W filter is chosen to cover the rest-frame U band, sampling ongoing star formation and potential nuclear starbursts in the host. The F606W filter is chosen to cover the rest-frame B band to probe young stellar populations in the host. The F105W is chosen to sample the rest-frame Y band to probe the old stellar populations in the host. The central AGN is expected to be minimal at this wavelength to facilitate AGN-host decomposition. Subarray is used to avoid saturation of the bright galaxy while covering an enough field of view to include the large host galaxy and nearby sources to register absolute astrometry. The 4-point box dither pattern is used to facilitate cosmic-ray and bad-pixel rejection and to better sample the PSF to achieve the best possible image quality. ------------------------------------------------------------------------------------ 16926( 5) - 19-May-2022 13:35:02 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SDSSJ1430+23 GALAXY, SEYFERT RA=14H30M16.0500S, J2000 Z = V = 14.7 +/- 0.3 SDSS r = 15.63, SDSS u 03 DEC=+23D03'44.40" 0.0811 = 18.44, GALEX NUV = 21.0 (pre-flare), GALEX FUV = 23.5 (AB, model magnitudes), Swift UVW1 = 18.8+/-0.2 (flare) Reference Frame: ICRS Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 16926( 5) - 19-May-2022 13:35:02 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-IR-DITHER-BOX-MIN Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.572 Line_Spacing 0.365 Coordinate_Frame POS-TARG Pattern_Orient 18.528 Angle_Between_Sides 74.653 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 3 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.15 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 90.0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 4 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.15 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 90.0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 16926( 5) - 19-May-2022 13:35:02 - [ 5] Visit: 01 (SDSSJ1430+2303_IR_F105W) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SDSSJ1430+2 WFC3/IR MULTIAC IRSUB512 F105W NSAMP=10, SAMP-SEQ=STEP25 4 DEF PATTERN 1 1 303 CUM (2055.034 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SDSSJ1430+2 WFC3/IR MULTIAC IRSUB512 F105W NSAMP=11, SAMP-SEQ=RAPID 1 DEF PATTERN 1 2 303 CUM (37.533 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 1 none 1 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 1 none 2 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 1 none 3 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 1 none 4 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 2 none 1 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 2 none 2 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 2 none 3 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 2 none 4 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 3 none 1 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 3 none 2 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 3 none 3 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 3 none 4 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 4 none 1 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 4 none 2 none 1 N/A CUM 16926( 5) - 19-May-2022 13:35:02 - [ 6] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 4 none 3 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 4 none 4 none 1 N/A CUM SDSSJ1430+2303 2 WFC3/IR MULTIAC IRSUB512 F105W 1 none none none 1 N/A CUM SDSSJ1430+2303 2 WFC3/IR MULTIAC IRSUB512 F105W 2 none none none 1 N/A CUM SDSSJ1430+2303 2 WFC3/IR MULTIAC IRSUB512 F105W 3 none none none 1 N/A CUM SDSSJ1430+2303 2 WFC3/IR MULTIAC IRSUB512 F105W 4 none none none 1 N/A CUM 16926( 5) - 19-May-2022 13:35:02 - [ 7] Visit: 02 (SDSSJ1430+2303_UVIS_F336W) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SDSSJ1430+2 WFC3/UVI ACCUM UVIS2-C1K F336W FLASH=16 1 800 S PATTERN 2 1 303 S 1C-SUB see below (2459 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ SDSSJ1430+2303 1 WFC3/UVI ACCUM UVIS2-C1 F336W 1 none none none 1 140.0 S S K1C-SUB SDSSJ1430+2303 1 WFC3/UVI ACCUM UVIS2-C1 F336W 2 none none none 1 140.0 S S K1C-SUB SDSSJ1430+2303 1 WFC3/UVI ACCUM UVIS2-C1 F336W 3 none none none 1 140.0 S S K1C-SUB SDSSJ1430+2303 1 WFC3/UVI ACCUM UVIS2-C1 F336W 4 none none none 1 2039.0 S S K1C-SUB 16926( 5) - 19-May-2022 13:35:02 - [ 8] Visit: 03 (SDSSJ1430+2303_UVIS_F606W) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SDSSJ1430+2 WFC3/UVI ACCUM UVIS2-C1K F606W 1 500 S PATTERN 2 1 303 S 1C-SUB see below (2496 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ SDSSJ1430+2303 1 WFC3/UVI ACCUM UVIS2-C1 F606W 1 none none none 1 624.0 S S K1C-SUB SDSSJ1430+2303 1 WFC3/UVI ACCUM UVIS2-C1 F606W 2 none none none 1 624.0 S S K1C-SUB SDSSJ1430+2303 1 WFC3/UVI ACCUM UVIS2-C1 F606W 3 none none none 1 624.0 S S K1C-SUB SDSSJ1430+2303 1 WFC3/UVI ACCUM UVIS2-C1 F606W 4 none none none 1 624.0 S S K1C-SUB 16926( 5) - 19-May-2022 13:35:02 - [ 9] Visit: 04 (SDSSJ1430+2303_STIS_G430L_G230L) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SDSSJ1430+2 STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=DIFFUSE, CHECKBOX=51, 1 20 S (20 303 DIFFUSE-CENTER=FLUX-CENTROID S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SDSSJ1430+2 STIS/CCD ACCUM 52X0.2E1 G430L 4300 CR-SPLIT=2 1 1200 S PATTERN 3 2 303 see below (1536 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SDSSJ1430+2 STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=685 1 3600 S PATTERN 4 3 303 -MAMA G see below (8211 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ SDSSJ1430+2303 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SDSSJ1430+2303 2 STIS/CCD ACCUM 52X0.2E1 G430L 4300 1 none none 1 1 384.0 S SDSSJ1430+2303 2 STIS/CCD ACCUM 52X0.2E1 G430L 4300 1 none none 2 1 384.0 S SDSSJ1430+2303 2 STIS/CCD ACCUM 52X0.2E1 G430L 4300 2 none none 1 1 384.0 S SDSSJ1430+2303 2 STIS/CCD ACCUM 52X0.2E1 G430L 4300 2 none none 2 1 384.0 S SDSSJ1430+2303 3 STIS/NUV TIME-TA 52X0.2 G230L 2376 1 none none none 2 2737.0 S -MAMA G SDSSJ1430+2303 3 STIS/NUV TIME-TA 52X0.2 G230L 2376 2 none none none 3 2737.0 S -MAMA G SDSSJ1430+2303 3 STIS/NUV TIME-TA 52X0.2 G230L 2376 3 none none none 4 2737.0 S -MAMA G 16926( 5) - 19-May-2022 13:35:02 - [ 10] Visit: 51 (SDSSJ1430+2303_IR_F105W) Visit Requirements: On Hold Comments: Additional Comments: HOPR repeat of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SDSSJ1430+2 WFC3/IR MULTIAC IRSUB512 F105W NSAMP=10, SAMP-SEQ=STEP25 4 DEF PATTERN 1 1 303 CUM (2055.034 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SDSSJ1430+2 WFC3/IR MULTIAC IRSUB512 F105W NSAMP=11, SAMP-SEQ=RAPID 1 DEF PATTERN 1 2 303 CUM (37.533 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 1 none 1 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 1 none 2 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 1 none 3 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 1 none 4 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 2 none 1 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 2 none 2 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 2 none 3 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 2 none 4 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 3 none 1 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 3 none 2 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 3 none 3 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 3 none 4 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 4 none 1 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 4 none 2 none 1 N/A CUM 16926( 5) - 19-May-2022 13:35:02 - [ 11] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 4 none 3 none 1 N/A CUM SDSSJ1430+2303 1 WFC3/IR MULTIAC IRSUB512 F105W 4 none 4 none 1 N/A CUM SDSSJ1430+2303 2 WFC3/IR MULTIAC IRSUB512 F105W 1 none none none 1 N/A CUM SDSSJ1430+2303 2 WFC3/IR MULTIAC IRSUB512 F105W 2 none none none 1 N/A CUM SDSSJ1430+2303 2 WFC3/IR MULTIAC IRSUB512 F105W 3 none none none 1 N/A CUM SDSSJ1430+2303 2 WFC3/IR MULTIAC IRSUB512 F105W 4 none none none 1 N/A CUM 16926( 5) - 19-May-2022 13:35:02 - [ 12] Summary Form for Proposal 16926 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, 52X0.2E1, F28X50LP, IRSUB512, UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/NUV-MAMA, WFC3/IR, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ, MULTIACCUM, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=DIFFUSE, BUFFER-TIME=685, CHECKBOX=51, CR-SPLIT=2, DIFFUSE-CENTER=FLUX-CENTROID, FLASH=16, NSAMP=10, NSAMP=11, SAMP-SEQ=RAPID, SAMP-SEQ=STEP25 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2, PATTERN 2 1, PATTERN 3 2, PATTERN 4 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F105W, F336W, F606W, G230L, G430L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names SDSSJ1430+2303 ------------------------------------------------------------------------------------------------------------------------------------