Host Galaxy of Dark Gamma-Ray Burst GRB 051008

1. Host Galaxyof Dark Gamma-Ray BurstGRB 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 FX-ray(t = 11h)+ Foptical(t = 11h) => βOX βOX < 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 detectedwas the host ofGRB 970228, confirmed a cosmological nature of GRBs (Bloom, et al., 1998). • GRB hosts have z from 0,0085 (GRB 980425, Fynbo, 2000) to3,91 (GRB 060210A, Stanek, et al., 2007). • GRB hostsare 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 bySwift/BAT: T0=05.10.08 16:33:21 UT, X-ray counterpart by XRT/Swift: Т0 + 50 min , optical counterpart is not detectedby UVOT up to 15m, Т0 + 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: Т0 + 32 min:not catalogued source of 21m.4 is detected, it was not the optical afterglow (Rumyantsev, (2005) GCN Circ. 4081 & Circ. 4087); • Tautenburg 1.34-mtelescope: Т0 + 96 min:optical afterglow is not detected up to 19m (Kann, et al. (2005) GCN Circ. 4246); • April 2006, CrAO, 2.6-m Shain telescope: detection of possible host galaxy ofGRB 051008;

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

7. GRB 051008: deep observations 20.06.2006, Nordic Optical Telescope, R-band, seeing 0".95, upper limit 25m.2, scale 0.19"/pix, Id3: RA 13h31m29s.51 ± 1s.20, Dec +42°05'53".67  0".08 (J2000), solid lines: ВАТ andXRT preliminary and reduced error boxes.

8. Detection of possible host galaxy. 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 Photometrical observations obtained on Nordic Optical Telescope (LaPalma) on June and August 2006 and May 2009.

10. Chance probability of projection ofId3 on XRT error box • XRT: RA 13h 31m 29s.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 in16"x16" box PId3 = 0.72% Pno cluster = 0.08%

11. Photometrical redshift estimation Photometrical redshift is determined using HyperZ code, Bolzonella, et al., 2000

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 1st solution and SN 1998bw z range, blue color corresponds to 2nd solution and SN 1999as z range.

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

14. Properties of GRB 051008 progenitor • equivalent isotropic energy Eiso (1) = (1.93 ± 1.05) × 1052erg; Eiso (2) = (16.2 ± 0.38) × 1052erg • time of jet-break tjb = 0.17 ± 0.01 days • jet opening angle – a solid angle of the burst outflow cone spread θj (1) = 2˚.30 ± 0˚.35;θj (2) = 1˚.51 ± 0˚.09 • full gamma-ray energy Eγ (1) = 1.94 ± 0.57 × 1048erg;Eγ (2) = 7.06 ± 0.95× 1048erg • peak energy (KONUS-Wind) Ep = 865 ± 178 keV

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

16. Comparison ofGRB 051008 with other dark gamma-ray bursts with known hosts

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. Conclusions • No optical afterglow up to 23.3m at T0+32 min; • only X-ray afterglow was detected; • βОХ< -0.29: one of the darkest bursts; • host galaxy detected on ZTSh, withredshift 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 darkGRB 051008is not associated with high redshift of its progenitor and, probably, is not associated with high extinction in its host galaxy (Av~1.2).