Out of the darkness:
the infrared afterglow of the
INTEGRAL burst GRB 040422
observed with the VLT
P. Filliatre
filliatr@cea.fr
Astroparticule et Cosmologie
CEA/Service d’Astrophysique
Based on Filliatre, D’Avanzo, Covino et al.,
to appear in A&A
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 040422 was detected thanks
to a quick observation in the NIR with VLT
Summary of the observations
• T=0 : GRB 040422 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)
• T= 13.04 d : VLT ISAAC
Ks (150 s) reference frames
Afterglow
• T= 64.82 d : VLT ISAAC
Ks (1800 s) deep observation
Host galaxy
INTEGRAL results (I)
SPI (20 - 200 keV)
Duration: 4 sec.
Peak flux: 2.8 ph cm-2 s-1
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,  = -1.26  0.03,
 = -4 (fixed),  = 39.9/35
Similar spectral properties to GRB 030131
Nothing really special…
Detection of the afterglow with ISAAC
T = 1.90
13.04hd
Ks
IBAS,  5’
~7000 objects
• 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
Detection of the host galaxy
T = 1.90 h
2”
T = 13.04 d
T = 64.82 d
The « lightcurve »
Ks = 18.0±0.1
Ks > 20.2
Ks = 20.3±0.2
(host)
I > 23.4
R > 24.2
F  t-
=1
(typical)
 = 0.4
(minimal slope)
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
Comparisons with other afterglows
Lowest delay
Nearly the faintest
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 040422 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 ?
 Maybe
• These observations are all we have…
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