1. GROND an optical/NIR 7-channel imager:
Capabilities for transient follow-up
Jochen Greiner Max-Planck Institut für extraterrestrische Physik Garching, Germany

2. GRB Afterglow and Light Curve
Afterglow emission at radio, optical, and NIR wavelengths
~2 hrs
~12 hrs
~48 hrs

3. Quickly finding high-redshift GRBs
Due to characteristic powerlaw spectrum of GRB Afterglow: Derive redshifts from simultaneous photometry in 7 filters.

4. GROND=GRB Optical/NIR Detector
Imaging in 7 channels simultaneously K J H g r i z

5. GROND @ 2.2m MPI telescope (La Silla)
History:
First light: Apr 30, First GRB: May 21, Photometric calibration: Jul Routine observations: since Sep fastest response time: 2 min H K J
Electronics M3
cryostat
δ-twister
cryostat
GROND

6. GROND: General Design
7 bands: Sloan g’, r’, i’, z’ and J, H, K One detector for one filter band (no movable filters!) 3 HAWAII 1K*1K Arrays E2V 2K*2K CCDs
Field-of-view: Visual: 5.4´x5.4´ (0.16´´/pixel) NIR: ´x10´ (0.59´´/pixel)
Dichroics tuned to minimize intrinsic polarization effects
2 shutters, i.e. g’r’ and i’z’ pairs of CCDs have same exposure
Combined telescope and intrinsic mirror (K-band) dithering
Sensitivity: 4 min hr g’r’ mag mag (AB) i’z’ mag mag (AB) J/H/K 20/19.5/19 mag 23/22.5/22 mag (AB)

7. Example of GROND sensitivity
7 filter bands cover Lyman-α for large z-range: 3<z<13
GROND has proper z-band sensitivity to define drop-out for z~8
Advantage of separate detector per filter: can add sensibilization for spectral band:  detector/filter is 4x more sensitive than FORS2  total efficiency only 4x less than FORS2, not 16 (8.22/2.22)

8. Photometric redshifts for GRBs
GRB at redshift 6.7
1st exposure starts T0+6 min
T0+15 min: XRT position
T0+35 min: first SED (Σ of 3 OBs = 16 min expo)
T0+90 min: VLT starts taking spectrum with proper grism (5 mag fainter than )
Greiner et al. 2009

9. Combined GROND-Swift/XRT SEDs
Some GRB afterglow SEDs extend from 240 nm to 10 keV with single power law; most with slope break of 0.5
in both cases: sensitive to extinction AV and/or Ly-break
Greiner et al. 2011
KHJ z’i’r’g’
KHJ
z’i’r’g’
keV
GRB
GRB

10. Seeing dust in SEDs GRB 070802: first 2200 A bump at z=2.45
Krühler et al. 2008
SED at 2000 sec after GRB

11. SEDs every minute
GRB
First image: 4 min after GRB onset, ~80 sec after trigger
1st night: 6 hrs continuous exposure; 350 data points
ß-evolution (instead of one for all AG)
Krühler et al. 2008

12. Chromatic flares GRB 080129 GRB 081029 Nardini et al. 2012
Greiner et al. 2009

13. Multi-filter light curves
SN 2009mg
Multi-filter light curves
Pulsar Fermi J
WASP-4
Porb = 93 min
Romani+2012
Nikolov+2012

14. Recognize ‘thermal’ transients
With 7 filter bands you always get an SED
Transient classification according to SED: allows to distinguish other types of transients, e.g. accretion-disk dominated galactic transients
“GRB” A  likely galactic transient

15. Contemporaneous UV-NIR SEDs of blazars
>100 observed in 1 yr
doubled # of known BL Lac at z>1.2
High-z BL Lacs interesting for various “applications”, e.g. EBL studies
Rau et al

17. Transient follow-up GROND is built for rapid (min), automatic response
Simultaneous 7-filter broad-band coverage is helpful for identification
Particularly advantageous for color changes along with intensity changes
GROND science goals include transients of all kinds; use for GRBs is being reduced
Presently contracted telescope access until 9/2016