NIRSpec Time Series Observations Stephan Birkmann, European Space Agency on behalf of the NIRSpec SOT Enabling Transiting Exoplanet Observations with JWST STScI, 10-12 July 2017

The Near Infrared Spectrograph (NIRSpec) Built for ESA by Airbus Defence and Space, Germany Part of Europe’s contribution to JWST (also Ariane 5 launcher, ½ of MIRI, 15 scientist & engineers for operations support)

NIRSpec Observing Modes Target Type Aperture Mask MOS: Multi-Object Spectroscopy Rich fields or extended objects Selectable from ~250,000 0.2” x 0.46” micro-shutters IFS: Integral-Field Spectroscopy Moderately extended objects 3.0” x 3.0” IFU with 0.1” spaxels FSS: Fixed Slit Spectroscopy Single (compact) object 0.2 x 3.2 slits (3) 0.4 x 3.65 slit 1.6 x 1.6 aperture BOTS: Bright Object Time Series Transit/eclipse spectroscopy NIRSpec’s Focal Plane Assembly (FPA) consists of two closely spaced HAWAII-2RG sensor chip arrays with 5.3 mm cut-off wavelength and ~100 mas pixels on the sky

NIRSpec Aperture Plane Detector gap S200A1 S200A2 S400A1 S1600A1 S200B1 (backup only) S1600A1: (1.6” ☐) aperture for small slit losses and good radiometric stability small enough that background and “spoiler” sources are not an issue

NIRSpec Spectral Configurations All dispersers are available in all NIRSpec modes R ~ 1000 R ~ 30 - 300

NIRSpec BOTS Observations Bright Object Time Series Observations have dedicated template in Astronomer’s Proposal Tool (APT)

NIRSpec BOTS Observations After Template is selected, remaining options become available: Target Acquisition (TA filter, readout pattern, subarray) Science Parameters (subarray, disperser/filter, exposures, readout mode, number of groups and number of integrations) For more details on BOTS template (and all others) refer to the JWST user documentation at https://jwst-docs.stsci.edu/

NIRSpec BOTS Observations Target selection is next. Visit splitting distance will depend on target position (availability of guide stars)

NIRSpec BOTS Observations – Subarrays Size Frame Time Comment SUB2048 32 x 2048 0.90156 s Full wavelength coverage for gratings SUB1024A 32 x 1024 0.45100 s Partial coverage for gratings (low/hi) SUB1024B Partial coverage for gratings (center) SUB512 32 x 512 0.22572 s Full wavelength coverage for prism SUB512S 16 x 512 0.14364 s No pixels outside aperture

NIRSpec BOTS Observations – Disperser/Filter 1.27 mm 0.97 mm 1.87 mm 1.66 mm 3.12 mm 2.87 mm 5.18 mm

NIRSpec BOTS Saturation Limits Use the ETC (https://jwst.etc.stsci.edu/) to check for saturation of your target Sources below lines can be observed without saturation anywhere in science band SUB2048 SUB512

NIRSpec BOTS Observations – Exposure Setup Number of exposures: use 1 single exposure with many integrations (max 65,535) if possible (best for detector stability); should be possible for all but the longest observations of very bright targets with the PRISM using SUB512(S) subarrays

NIRSpec BOTS Observations – Exposure Setup Readout pattern: use NRSRAPID Groups/Int: governs integration time (together with subarray and readout pattern); use ETC to determine number of groups before saturation Integrations/Exp: use as many as needed to cover full event, plus ~15 minutes (detector stabilization) For exposure lengths >10,000 sec will get warning (High Gain Antenna might move during exposure); maximum exposure duration is 172800 sec (2 days)

NIRSpec BOTS Data Many integrations (up to 65,535) per exposure One spectrum per integration PRISM/CLEAR example (SUB512 subarray, ground testing): G235H/F170LP example (SUB2048 subarray, both detectors): 0.6 mm 1.0 mm 2.0 mm 3.0 mm 3.5 mm 4.0 mm 4.5 mm 5.0 mm 5.3 mm 1.7 mm 1.8 mm 1.9 mm 2.0 mm 2.1 mm 2.2 mm 2.3 mm 2.4 mm 2.5 mm 2.6 mm 2.7 mm 2.8 mm 2.9 mm 3.0 mm

NIRSpec BOTS Data (Ground Test) 600 integrations per temporal bin – 15 bins Data taken during ISIM CV3 campaign 9000 integrations with 3 groups in SUB512 and PRISM/CLEAR (~0.9 s per integration) OSIM source showed jitter and drift (~5 times higher than expected in orbit) and some flux variation After correction, came down to expected noise floor (photon + readout noise model) Systematic noise sources (due to e.g. intra-pixel variations, detector persistence) likely <200 pm

NIRSpec BOTS SNR Estimates SNR per spectral pixel for 1 hr transit – 2 hr baseline (3 hour total) Use PandExo (http://pandexo.science.psu.edu:1111/) to do your own SNR calculations See also APT and PandExo presentations by Denise Taylor and Natasha Batalha

Target Acquisition APT 25.2 NIRSpec is very sensitive in imaging mode (used for TA) Targets brighter than ~12 ABmag (~11 Jmag) are too bright for TA APT 25.4 will support Wide Aperture Target Acquisition (WATA) for NIRSpec: will allow use of fainter reference sources (~12 to ~26 ABmag) for TA Reference target will need to be within visit splitting distance (same guide star) Use the ETC (https://jwst.etc.stsci.edu/) to determine suitable filter / subarray / readout pattern for TA

Timing BOTS (e.g. Transit) Observations Timing constraints are specified using special requirements

Summary NIRSpec offers a dedicated template for (bright object) time series observations 0.6 to 5.3 mm wavelength coverage with different spectral resolutions For fainter targets (>10.5 Jmag), PRISM covers full range in one go Bright targets (~6-7 Jmag) can be observed with the high resolution gratings and acquired using a reference target