X-shooterX-shooter A Wideband Spectrograph for GRBsA Wideband Spectrograph for GRBs

P. Goldoni, P. Filliatre

On behalf of a larger collaboration

“Call for proposals for 2nd Generation VLTInstruments” (http://www.eso.org/instruments/vlt2ndgenins.html)

The main goal is to get maximum detectivity on stellar or small emission-line objects, while covering the largest possible wavelength range (ideally 0.32 to 2.4 m) in a single observation, presumably leading to a multiple arm ("x-shooter") system. A particularly important requirement is the ability to get spectrographic data on unpredictable/fast varying objects like supernova explosions or gamma ray burst optical counterparts, for the latter if possible in a matter of minutes….

R~ 104 wide-band visible-NIR high-throughput Spectrometer

Specific targets: rapid variation, unpredictable point-like objectsOutput: sky limited U to K spectrum at R~5000-10000 in one shot

Main scientific goals of X-shooter

• Cosmic explosions– GRB afterglows– Supernovae

• High-z Universe– Faint emission-line galaxies– SFR(z)– Tomography of the intergalactic medium

• Stellar physics– Brown dwarfs

X-shooter key points for GRBs

• High sensitivity (R=21.5, 1h S/N=10)

• U to K spectrum in one shot

• Sufficient resolution

• Can be operated quickly (5-15 mins after having the position)

• Sturdy design

Very well designed for exploring GRBs environment and line of sight up to high z

Probing GRB environmentUVES spectrum of GRB 021004Degraded to X-shooter resolution (5000-10000)

The brightest cosmic lighthouses visible up to z ~ 16Stars and Structure formation in the early UniverseLine of sight aborption

Wavelength position of absorption lines and Lyman- forest as a function of redshift. To the right X-shooter spectral range with respect to UVES

Lamb & Reichart, 2000

High-z GRBs with X-shooter

Texp = 2 hr, reionization at z=7, 7 hours post burst.

X-shooter spectrum of GRB 021004 at z=8.5

Sensitivity to a 30 kms-1 line (moderately strong IGMabsorption line) as a function of wavelength: X-shooter, FORS Giraffe and ISAAC

Limiting magnitudes1 hour exp., S/N=10per bin

X-shooter sensitivity (m) Magnitude

0.37 21.0

0.44 21.9

0.54 21.5

0.65 21.4

0.81 20.7

1.26 19.8

1.67 19.6

2.1 17.6

Consortium NL,D,I,F,ESO

• Very Fast realization ! (SWIFT launch 2004).Commisioning in 2007 and operation in 2008 are foreseen

• More than half budget from member states

• First 2nd generation instrument to be operative but very tight budget

• Automatic operations driven by robotic telescopes such as REM (Chile)

Project constraints and characteristics

X-Shooter optical layout

1st dichroic

2nd dichroic

UVB armVIS arm

NIR arm

backbone

Realised by GEPI-Observatoire de Paris (Girafe):DRS responsibility of APC

French contribution I: Integral Field UnitAPC, GEPI, APC

X-shooter FOV & OT positions with IFU

Bloom et al. 2002

X-shooter FOV with IFU (1.8” x 4”) is superposed to the angular distribution of 20 OTs in their galaxy.

100% 66%66%

• Extended obects• Poor pointing• Bad seeing

French contribution II: the data reduction software

Exemple: the VIS arm

Goals:• A robust/quick DRS• Go from raw data to + flux calibrated spectrum• Can be used at the mountain• Can be used by scientists

Echelle spectrographWhat the detector look like in ideal world

ordersSpace axis

Wav

elen

gth

axis

the object

lines

What the detector look like in real world

Curved ordersTilted linesVariable efficiencyOverlapping orders

Conclusions

• X- shooter will be the first 2nd generation instrument operative at VLT

• It will be the most sensitive VLT single object spectrograph

• Fast Operations, high sensitivity, IFU allow to study very faint objects…

• The main scientific aim will be the GRBs with the possibility of detecting the farthest sources at the reionization epoch or beyond