1. Out of the darkness: the infrared afterglow of the INTEGRAL burst GRB 040422 observed with the VLT
P. Filliatre
Astroparticule et Cosmologie
CEA/Service d’Astrophysique
Based on Filliatre, D’Avanzo, Covino et al.,
to appear in A&A

2. The dark burst problem
No optical afterglow detected for a fraction of GRBs : 10% (Lamb et al. 2004) - 60% (Lazzati et al. 2002)
Popular physical explanations:
High z burst + Lyman  cut (Lamb 2000)
Population of intrinsically faint afterglows (e.g. Fynbo et al. 2001)
Dust-enshrouded afterglows (e.g. Reichart & Price 2002)
 An issue for cosmology and understanding of the GRB environments
Or maybe the search is not efficient (not quick enough, no infrared…)
The afterglow of GRB was detected thanks to a quick observation in the NIR with VLT

3. Summary of the observations
T=0 : GRB detected by INTEGRAL :
l = 33°37’, b = 2°59’, error = 2.5’ (IBAS)
 Strong Milky Way absorption
T=1.90 h (!): VLT ISAAC/FORS 2
Ks (2.2 m 150 s)
R (0.7 m 120 s), I (0.9 m 120 s)
Afterglow
T= d : VLT ISAAC
Ks (150 s) reference frames
T= d : VLT ISAAC
Ks (1800 s) deep observation
Host galaxy

4. INTEGRAL results (I) SPI (20 - 200 keV) Duration: 4 sec.
Peak flux: 2.8 ph cm-2 s-1

5. INTEGRAL results (II) IBIS/ISGRI (15 - 200 kev)
Power law (N(E)  E ):
= -2.2  0.4,  = 86.4/36
Broken power law (Band model):
Ep = 41  3 keV,  =  0.03,
 = -4 (fixed),  = 39.9/35
Similar spectral properties to GRB
Nothing really special…

6. Detection of the afterglow with ISAAC
T = d
IBAS,  5’
~7000 objects Ks
Semi-automatic selection
Criteria:
stellar-like
variation > 2 mag
not blended
Detected only in Ks
Huge Galactic absorption:
AR = 4.4
AI = 3.8
AK = 0.5

7. Detection of the host galaxy2”
T = d
T = d

8. The « lightcurve » Ks = 18.0±0.1 Ks > 20.2 Ks = 20.3±0.2 (host)
F  t-
= 1
(typical)
= 0.4
(minimal slope)
I > 23.4
R > 24.2

9. Comparisons with other afterglows
The game:
Correct for Galactic absorption
Extrapolate to T=1.90 with F  t-
Extrapolate to K with F  --
Assumptions:
Power law is valid
Intrinsic absorption small enough

10. Comparisons with other afterglows
Lowest delay
Nearly the faintest

11. An almost dark burst
Shows that 8-m telescopes can lower the proportion of dark burst with :
Quick follow-up (less than 2 hours, but 5min  Ks=14.6)
Multiband including NIR
Why GRB is faint ?
Remote ?
 Unlikely, because the host is rather bright
A normal afterglow enshrouded by dust ?
 Maybe
Belongs to a class of intrinsically faint afterglows ?
These observations are all we have…

12. How to gather more data ? Need to have very quickly a 1” position
Use it to quickly have a large band spectrum with an high efficiency
REM, a VIS/NIR robotic telescope
La Silla
X-shooter
an U to K spectrograph
VLT 2007