1. X-shooter A Wideband Spectrograph for GRBs
P. Goldoni, P. Filliatre
On behalf of a larger collaboration

2. “Call for proposals for 2nd Generation VLT
Instruments” (
R~ 104 wide-band visible-NIR high-throughput Spectrometer
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 mm) 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….
Specific targets: rapid variation, unpredictable point-like objects
Output: sky limited U to K spectrum at R~ in one shot

3. 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

4. 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

5. Probing GRB environment
UVES spectrum of GRB
Degraded to X-shooter resolution ( )

6. High-z GRBs with X-shooter
The brightest cosmic lighthouses visible up to z ~ 16
Stars and Structure formation in the early Universe
Line of sight aborption
Lamb & Reichart, 2000
Wavelength position of absorption lines and Lyman- forest as a function of redshift. To the right X-shooter spectral range with respect to UVES

7. X-shooter spectrum of GRB 021004 at z=8.5
Texp = 2 hr, reionization at z=7, 7 hours post burst.

8. 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
Sensitivity to a 30 kms-1 line (moderately strong IGM
absorption line) as a function of wavelength: X-shooter, FORS Giraffe and ISAAC
Limiting magnitudes
1 hour exp., S/N=10
per bin

9. Project constraints and characteristics
Very Fast realization ! (SWIFT launch 2004).
Commisioning in 2007 and operation in 2008 are foreseen
Automatic operations driven by robotic telescopes such as REM (Chile)
First 2nd generation instrument to be operative
but very tight budget
More than half budget from member states
Consortium NL,D,I,F,ESO

10. X-Shooter optical layout
UVB arm
VIS arm
NIR arm
backbone
1st dichroic
2nd dichroic

11. French contribution I: Integral Field Unit APC, GEPI, APC
Realised by GEPI-Observatoire de Paris (Girafe):
DRS responsibility of APC

12. X-shooter FOV & OT positions with IFU
66%
100%
66%
X-shooter FOV with IFU (1.8” x 4”) is superposed to the angular distribution of 20 OTs in their galaxy.
Extended obects
Poor pointing
Bad seeing
Bloom et al. 2002

13. French contribution II: the data reduction software
Goals:
A robust/quick DRS
Go from raw data to  + flux calibrated spectrum
Can be used at the mountain
Can be used by scientists
Exemple: the VIS arm

14. Echelle spectrograph What the detector look like in real world
What the detector look like in ideal world
the object
lines
Space axis
Wavelength axis
Curved orders
Tilted lines
Variable efficiency
Overlapping orders
orders

15. 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