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) Afterglow T= 13.04 d : VLT ISAAC Ks (150 s) reference frames 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 = 13.04 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

Detection of the host galaxy 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) F  t- = 1 (typical) = 0.4 (minimal slope) I > 23.4 R > 24.2

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 ? 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