1. Host Galaxy of Dark Gamma-Ray Burst GRB 051008 Host Galaxy of Dark Gamma-Ray Burst GRB 051008 A. Volnova (SAI MSU), A. Pozanenko (ISR RAS), V. Rumyantsev (CrAO), M. Ibragimov (Institute of Astronomy of UzAS), D. Sharapov (Institute of Astronomy of UzAS), D. Kann (Tautenburg Observatory)

2. Dark gamma-ray bursts: definition Bursts which have no optical afterglow or the afterglow is anomaly faint. about 30% of all bursts Model: F ~ ν -β F X-ray (t = 11 h ) + F optical (t = 11 h ) => β OX β OX < 0.5 – dark gamma-ray bursts (Jakobsson, et al., 2004)

3. Dark gamma-ray bursts: causes insufficiently deep observations; significant extinction of the afterglow radiation in the host-galaxy; high redshift of the source (z ≥ 5 ); intrinsically dark bursts.

4. Host galaxies of gamma-ray bursts First detected was the host of GRB 970228, confirmed a cosmological nature of GRBs (Bloom, et al., 1998). GRB hosts have z from 0,0085 (GRB 980425, Fynbo, 2000 ) to 3,91 (GRB 060210A, Stanek, et al., 2007 ). GRB hosts are galaxies of different types: elliptical, spiral, irregular, mergerers. GRB hosts typically are young galaxies with high star formation rate. A study of gamma-ray burst host galaxy is extremely useful for determination of the distance to burst progenitor, for accurate determination of afterglow light curve, it helps to investigate the burst environment, to understand the processes in galaxies with high redshift.

5. GRB 051008: brief history detection by Swift/BAT: T 0 =05.10.08 16:33:21 UT, X-ray counterpart by XRT/Swift: Т 0 + 50 min, optical counterpart is not detected by UVOT up to 15 m, Т 0 + 50 min (Marshall, et al. (2005) GCN Circ. 4069) ; radio afterglow is not detected (>80 μJy at 8.5 GHz, Cameron, (2005) GCN Circ. 4074) ; also detected in gamma-rays by KONUS-Wind (Golenetskii, et al. (2005) GCN Circ. 4078), Suzaku/WAM (Ohno, et al. (2005) GCN Circ. 4297) ; CrAO, 2.6-m Shain telescope: Т 0 + 32 min: not catalogued source of 21 m.4 is detected, it was not the optical afterglow (Rumyantsev, (2005) GCN Circ. 4081 & Circ. 4087) ; Tautenburg 1.34-m telescope: Т 0 + 96 min: optical afterglow is not detected up to 19 m (Kann, et al. (2005) GCN Circ. 4246) ; April 2006, CrAO, 2.6-m Shain telescope: detection of possible host galaxy of GRB 051008;

6. GRB 051008 is a dark burst (Figure from Zheng, et al., 2009) F X (t = 11 h ) = 0.48 × 10 -7 Jy F ~ t -1 F O (t = 11 h ) = 0.068 × 10 -7 Jy β ОХ < -0.29

7. GRB 051008: deep observations 20.06.2006, Nordic Optical Telescope, R-band, seeing 0".95, upper limit 25 m.2, scale 0.19"/pix, Id3: RA 13 h 31 m 29 s.51 ± 1 s.20, Dec +42°05'53".67  0".08 (J2000), solid lines: ВАТ and XRT preliminary and reduced error boxes.

8. Detection of possible host galaxy. ZTSh 2006-04-28NOT 2006-06-20ZTSh 2006-07-25 Id 121.15 ± 0.1221.15 ± 0.0421.15 ± 0.12 Id 222.22 ± 0.1922.30 ± 0.0922.44 ± 0.23 Id 3 22.76 ± 0.21 23.92 ± 0.0723.98 ± 0.55 Possible host galaxy of GRB 051008 was discovered on Shain Telescope due to a significant rebrightening of unknown nature (possible supernova). This was the reason for further deep observation of this region.

9. GRB 051008: deep observations filterId 1Id 2Id 3Id 4Uplim (3σ) Total exposure, s B 21.80 ± 0.2023.63 ± 0.2124.67 ± 0.2325.34 ± 0.27 25.903300 V 21.50 ± 0.1322.92 ± 0.1424.18 ± 0.1524.68 ± 0.20 25.664800 R 21.15 ± 0.0422.30 ± 0.0923.92 ± 0.0724.51 ± 0.11 25.734800 i 20.38 ± 0.0521.53 ± 0.0623.91 ± 0.1723.89 ± 0.18 23.898400 Photometrical observations obtained on Nordic Optical Telescope (LaPalma) on June and August 2006 and May 2009.

10. Chance probability of projection of Id3 on XRT error box XRT: RA 13 h 31 m 29 s.55, Dec +42° 05' 53".3 (J2000) with radius 1".2, in 3'x3' box there are 52 galaxies: ρ = 0.0016 galaxy by square arcsecond. Id1-4 are located in 16"x16" box P Id3 = 0.72% P no cluster = 0.08%

11. Photometrical redshift estimation Photometrical redshift is determined using HyperZ code, Bolzonella, et al., 2000 IdzP, %Host typeage, GyAVAV MRMR Scale, kpc/”d, kpc 10.92 ± 0.1029.58Im4.500.0-23.507.86355.11 ± 1.57 11.67 ± 0.05>99.99B0.0041.4-24.838.55759.98 ± 1.71 20.70 ± 0.50>99.99B1.431.2-21.707.14446.01 ± 2.57 21.07 ± 0.3299.94Im1.431.2-23.468.16852.60 ± 2.94 30.38 ± 0.2199.81B0.090.0-17.295.1756.31 ± 3.11 31.07 ± 0.13>99.99B0.0091.2-19.618.1689.96 ± 4.90 40.90 ± 0.2598.99Im4.50.0-19.697.8127.89 ± 1.56 41.76 ± 0.4199.51E0.360.2-22.858.5488.63 ± 1.71

12. SN1998bw as template: solid line is a brightness of SN peak as viewed from different redshifts (Zeh et al. astro-ph/0311610). Leftsided hatch is an 1-sigma region of z derived from observed host and peak SN magnitudes. Rightsided hatch is an 1 sigma region of possible z derived from photo-z estimation. Vertical line is the best fits of photo-z estimation respectively. Red color corresponds to the 1 st solution and SN 1998bw z range, blue color corresponds to 2 nd solution and SN 1999as z range.

13. X-ray afterglow lightcurve broken power law approximation:

14. Properties of GRB 051008 progenitor equivalent isotropic energy E iso (1) = (1.93 ± 1.05) × 10 52 erg; E iso (2) = (16.2 ± 0.38) × 10 52 erg time of jet-break t jb = 0.17 ± 0.01 days jet opening angle – a solid angle of the burst outflow cone spread θ j (1) = 2˚.30 ± 0˚.35; θ j (2) = 1˚.51 ± 0˚.09 full gamma-ray energy E γ (1) = 1.94 ± 0.57 × 10 48 erg; E γ (2) = 7.06 ± 0.95 × 10 48 erg peak energy (KONUS-Wind) E p = 865 ± 178 keV

15. Amati diagram Amati diagram (Amati, et al., 2002)– empirical diagram of tight correlation between spectral peak energy Е р and equivalent isotropic energy of burst radiation E iso. Red squares marks GRB 051008 (1 st and 2 nd solutions). The GRB 051008 is well within the Amati correlation. 1 2

16. Comparison of GRB 051008 with other dark gamma- ray bursts with known hosts GRBXTOTRT β OX z Host type R magcolorAVAV MSFRref. 970828 980329 990506 000210 020819 030115 051008 051022 051109b* 060210* + n/o + n/o + -+-+-+---+-+-+-+---+ +++-++-+--+++-++-+-- <0.05 0,54 <0.06 <0.54 - <-0.29 <-0.11 <0.56 0,13 0.96 2-3,9 1.3 0.85 0.41 ~2.5 0.38 1.07 0.81 0.08 3,91 Im - Burst Sc S0/Sb Burst SBa - 25.2 - 25.0 23.46 19.5 25.58 24.15 21.5 14.5 - R-K~3.7 V-I=-0.18 - R-K~2.5 R-K~2.7 R-K~5 V-I~1.9 - Ks=20.2 - >1 - ~0 ~1 1.0 ~0 1.2 1.0 - -19.63 - -19.8 -20.16 -22.25 -20.97 -17.29 -19.61 -21.8 -23.3 - mergerer high SFR ~13 M  /yr ~2.1 M  /yr HII clouds, dust ~4.4 M  /yr - > 25 M  /yr high SFR - Djorgovski, 2001 Lamb, 1999 Bloom, 2003 Gorosabel, 2003 Jakobsson, 2005 Levan, 2006 - Castro-Tirado, 2007 Perley, 2006, GCN5387 Hearty, 2006, GCN 4753

17. Dark gamma-ray bursts: causes insufficiently deep observations; significant extinction of the afterglow radiation in the host-galaxy; high redshift of the source (z ≥ 5 ); intrinsically dark bursts.

18. ConclusionsConclusions No optical afterglow up to 23.3 m at T 0 +32 min; only X-ray afterglow was detected; β ОХ < -0.29: one of the darkest bursts; host galaxy detected on ZTSh, with redshift either 0.38 ± 0.21 or 1.07 ± 0.13, probably located in a cluster; small jet angle in comparison with other known bursts; nature of dark GRB 051008 is not associated with high redshift of its progenitor and, probably, is not associated with high extinction in its host galaxy (A v ~1.2).