Title: Out of the darkness: the infrared afterglow of the INTEGRAL burst GRB 040422 observed with the VLT
1Out of the darkness the infrared afterglow of
the INTEGRAL burst GRB 040422 observed with the
VLT
- P. Filliatre
- filliatr_at_cea.fr
- Astroparticule et Cosmologie
- CEA/Service dAstrophysique
Based on Filliatre, DAvanzo, Covino et al., to
appear in AA
2The 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
3Summary of the observations
- T0 GRB 040422 detected by INTEGRAL
- l 3337, b 259, error 2.5 (IBAS)
- ? Strong Milky Way absorption
- T1.90 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
- T 64.82 d VLT ISAAC
- Ks (1800 s) deep observation
4INTEGRAL results (I)
SPI (20 - 200 keV) Duration 4 sec. Peak flux
2.8 ph cm-2 s-1
5INTEGRAL 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
6Detection of the afterglow with ISAAC
- Semi-automatic selection
- Criteria
- stellar-like
- variation gt 2 mag
- not blended
- Detected only in Ks
- Huge Galactic absorption
- AR 4.4
- AI 3.8
- AK 0.5
7Detection of the host galaxy
T 1.90 h
2
T 13.04 d
T 64.82 d
8The lightcurve
Ks 18.00.1
Ks gt 20.2
Ks 20.30.2 (host)
I gt 23.4
R gt 24.2
9Comparisons with other afterglows
- The game
- Correct for Galactic absorption
- Extrapolate to T1.90 with F ? t-?
- Extrapolate to K with F ? ?--?
- Assumptions
- Power law is valid
- Intrinsic absorption small enough
10Comparisons with other afterglows
Lowest delay Nearly the faintest
11An 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 ?
Ks14.6) - 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
12How 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