Free Access
Issue
A&A
Volume 532, August 2011
Article Number A143
Number of page(s) 31
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201116663
Published online 08 August 2011

Online material

Appendix A: Details on the spectroscopic GRB afterglow sample

A.1. GRB 050401

The optical spectrum of the afterglow of GRB 050401 was secured with the VLT/FORS2 instrument (Watson et al. 2006; F09). The NIR photometry in the J, H and K bands from Watson et al. (2006) is interpolated to the time of the SED using the R-band decay rate (α = 0.86 ± 0.1). We find that the SED of the afterglow can be well reproduced by a broken power-law with substantial amount of SMC-type dust extinction with a value of AV = 0.65 ± 0.04 mag. The best-fit parameters are given in Table 3.

GRB 050401 at z = 2.8983 is categorized as a “dark burst” as defined by Jakobsson et al. (2004a). Previously Watson et al. (2006) found the afterglow SED is compatible with a broken power-law model and SMC type dust reddening with AV = 0.62 ± 0.06, in agreement with our results. This burst has remarkably one of the highest H   i column densities with log NH   i/cm-2 = 22.60 ± 0.3 (Watson et al. 2006).

A.2. GRB 050730

The optical spectrum of the afterglow of GRB 050730 (z = 3.9693) presented in this paper was carried out with the VLT/FORS2 (F09). The afterglow SED is constructed at 4.1 h after the burst. The SED can be explained by a broken power-law, consistent with a Δβ ~ 0.5, plus a small amount of SMC type extinction with AV = 0.12 ± 0.02 mag.

Optical spectra of GRB 050730 were also acquired by the Magellan Inamori Kyocera Echelle (MIKE) echelle spectrograph on the Magellan Telescopes (Chen et al. 2005), the Intermediate-dispersion Spectroscopic and Imaging System (ISIS) on the William Herschel Telescope (WHT; Starling et al. 2005) and VLT UV-visual Echelle Spectrograph (UVES; D’Elia et al. 2007; Ledoux et al. 2009). The derived H i column density is log NH   i/cm-2 = 22.1 ± 0.1 (Starling et al. 2005; D’Elia et al. 2007; Ledoux et al. 2009). A foreground QSO is also detected in the 2D spectrum at an impact parameter of 20 arcsec (F09). Previously (Oates et al. 2009) derived AV = 0.15 from a fit of UVOT and X-ray data. Kann et al. (2010) find AV = 0.10 ± 0.02 from their optical/NIR SED fitting. Both results are consistent with our findings. Starling et al. (2005) find AV = 0.01 from spectral fitting. Schady et al. (2010) find that no extinction model is capable to fit of the NIR through X-ray SED, and derive AV ≈ 0.16 − 0.23 for the SMC model using single and broken power-laws.

A.3. GRB 050824

The optical spectrum of the afterglow of GRB 050824 (z = 0.8278) was taken with the VLT/FORS2 instrument (Sollerman et al. 2007; F09). GRB 050824 is an X-ray flash (XRF) (Sollerman et al. 2007). The optical and X-ray spectrum are normalized to the level of acquisition image photometry taken at 9.5 h after the burst trigger. The SED of the afterglow of GRB 050824 is well explained by a broken power-law and SMC extinction with AV = 0.14 ± 0.04 mag.

Schady et al. (2007b) find SMC dust fits the data best, but they are not able to discriminate among the single and broken power-law models. For both power-law models Schady et al. (2010) find AV ≈ 0.12 − 0.16, in excellent agreement with our values. Kann et al. (2010) also find that SMC model well fits the optical/NIR SED with an insignificant amount of dust (AV = 0.14 ± 0.13).

A.4. GRB 060115

The optical spectrum and the R-band photometry of the afterglow of GRB 060115 (z = 3.5328) were carried out with the VLT/FORS1 (Piranomonte et al. 2006b; F09). The SED is generated at t0 + 8.9 h after the burst. The afterglow SED of GRB 060115 is described by a single power-law and SMC type extinction with a very small amount of dust (AV = 0.10 ± 0.02 mag). The optical spectrum has very low S/N and this can be seen clearly in the extinction curve (see Fig. A.2). F09 derived an H i column density of log NH   i/cm-2 = 21.5 ± 0.1.

A.5. GRB 060512

The spectrum of the afterglow of GRB 060512 was carried out with the VLT/FORS2 (Starling et al. 2006a; F09). For this afterglow there is uncertainty about the redshift in the GCN circulars. The redshift of z = 2.1 comes from a single absorption line detected in the Telescopio Nazionale Galileo (TNG; Starling et al. 2006b) and the FORS1 spectra (F09). This redshift is also consistent with the spectral break detected by UVOT (de Pasquale & Cummings 2006; Oates et al. 2009). The R-band afterglow light curve is constructed by using magnitudes given by Mundell et al. (2006); Cenko (2006a); Milne (2006); Cenko & Baumgartner (2006). The Ks-band photometry is interpolated at the time of the SED by using observations given in Hearty et al. (2006); Tanaka et al. (2006). The J-band observations of the afterglow are taken from Sharapov et al. (2006). We reduced the UVOT data and the afterglow is clearly detected in the v, b and u bands. The interpolated lightcurve for each band is obtained by using the R-band lightcurve decay rate of α = 1.05 ± 0.1. The SED of the afterglow of GRB 060512 fits well with a broken power-law and no dust reddening (see Table 3). We estimated the 2σ upper limit for extinction in the rest-frame V-band ( < 0.08 mag).

Previously Schady et al. (2010) find that no dust model is capable of fitting the NIR through X-ray data and derive high extinction using SMC model (AV ≈ 0.47−0.66). The extinction corrected data presented in Schady et al. (2010) do not match with their given intrinsic slope of the afterglow SED.

A.6. GRB 060614

The optical spectrum of the afterglow of GRB 060614 was obtained with the VLT/FORS2 (Della Valle et al. 2006; F09). GRB 060614 has a redshift of z = 0.1257, the lowest redshift in our flux calibrated GRB sample. The redshift is estimated from the emission lines from the GRB host galaxy (Della Valle et al. 2006). GRB 060614 is a nearby, long duration GRB but not accompanied by a bright supernova (Fynbo et al. 2006c; Della Valle et al. 2006; Gal-Yam et al. 2006; Gehrels et al. 2006). The UVOT data were previously published in Xu et al. (2009) and Mangano et al. (2007). We re-reduced the UVOT data for v, b, u, uvw1, uvm2 and calibrated the afterglow fluxes in these bands using the photometric technique given in Poole et al. (2008). The optical to the X-ray SED of the afterglow of GRB 060614 is well modeled with a broken power-law and low SMC type extinction with AV = 0.1 ± 0.03 mag.

Mangano et al. (2007) find that SMC dust well fits the optical through X-ray SED, with AV = 0.05 ± 0.02, values comparable to ours. Kann et al. (2011) find SMC dust model fits the optical data, and even higher extinction is obtained (AV = 0.28 ± 0.07). The SED is not provided in Kann et al. (2011), therefore a detailed comparison could not be made.

A.7. GRB 060707

The optical spectrum of the afterglow of GRB 060707 (z = 3.4240) together with R-band photometry are taken with the VLT/FORS1 (Jakobsson et al. 2006a; F09). The afterglow SED is constructed at 34.4 h after the burst trigger. The optical to X-ray SED of the afterglow of GRB 060707 can be nicely reproduced with a single power-law and SMC type dust extinction (AV = 0.08 ± 0.02 mag). The derived H i column density is log NH   i/cm-2 = 21.0 ± 0.2 (Jakobsson et al. 2006a).

A.8. GRB 060708

The optical spectrum and R-band photometry of the afterglow of GRB 060708 are obtained with the VLT/FORS2 (Jakobsson et al. 2006c; F09). A precise redshift of the burst is not obtained due to the low S/N of the optical spectrum (F09). The reported redshift of z = 1.92 was based on spectral break seen in UVOT data due to Lyα break (Schady & Moretti 2006). Together with the X-ray and optical spectrum we reduced the UVOT data in all bands obtained from the Swift science archive facility. The data were reduced using the standard UVOT photometric calibration technique described in Poole et al. (2008). The afterglow is clearly detected in the UVOT v, b and u photometric bands. The optical to X-ray SED of the afterglow is well fit with a broken power-law together with SMC type dust extinction, AV = 0.13 ± 0.02 mag.

A.9. GRB 060714

The optical spectrum of the afterglow of GRB 060714 (z = 2.7108) was carried out with the VLT/FORS1. The R-band lightcurve is obtained from the circulars spanning from 0.04 to 4 days after the burst. The I and J-band photometric magnitudes were obtained from the CTIO A Novel Dual Imaging CAMera (ANDICAM) instrument (Cobb 2006). The UVOT data of the afterglow is reduced and the source is well detected in the v and b bands. We modeled the intrinsic SED of the afterglow and found that the afterglow can be well explained by a broken power-law together with SMC type extinction with restframe extinction of AV = 0.22 ± 0.02 mag.

Previously Jakobsson et al. (2006a) obtained the redshift and the H   i column density of log NH   i/cm-2 = 21.8 ± 0.1 from the optical afterglow spectrum. Moreover extended Lyα emission in the center of the DLA trough is clearly detected (F09). Schady et al. (2010) find that no dust model is able to fit the optical/X-ray data. The least bad is the SMC-type dust and power-law with dust extinction AV = 0.46 ± 0.17, comparable to our results. The afterglow SED in Schady et al. (2010) is constructed from the Rvb band and X-ray data. While b band is clearly affected by the Lyα absorption at z = 2.7108 and might causes high amount of extinction.

A.10. GRB 060729

The optical spectrum of the afterglow of GRB 060729 at z = 0.5428 is obtained with the VLT/FORS2 (Thöne et al. 2006b; F09). The acquisition image photometry was carried out using the I-band filter as reported in Table 1. The UVOT data of the afterglow of GRB 060729 is previously published in Grupe et al. (2007). The Swift UVOT data of the afterglow of GRB 060729 have been re-reduced and the source is clearly detected in the v, b, u, uvw1 and uvm2 filters. The optical spectrum is not of good quality and has broad undulations, therefore, the true shape of the underlying continuum cannot be seen from the spectrum. The optical spectrum has very low S/N and is affected by the high airmass at the time of the observation (F09). The afterglow SED can be fitted by a single power-law and SMC type dust extinction, and resulted in a very small amount of dust (AV = 0.07 ± 0.02 mag).

Schady et al. (2010) find that SMC/power-law is the best model, resulting in low amount of extinction with AV = 0.03 ± 0.01 and consistent with our value.

A.11. GRB 060904B

The optical spectrum of the afterglow of GRB 060904B (z = 0.7029) was obtained with the VLT/FORS1 instrument (Fugazza et al. 2006; F09). In our sample GRB 060904B has the highest amount of foreground extinction and absorption (see Table 1). The SED of the afterglow is constructed at 5.1 h after the burst. The SED fits well with a single power-law and SMC type extinction with extinction of AV = 0.31 ± 0.02 mag. The SED seems like a broken power-law case with softer X-ray slope. However, due to large uncertainties on the X-ray spectrum the fitting routine cannot minimize both models accurately and prefers a single power-law.

Previously Schady et al. (2010) find no good fit for any dust model but find that single power-law is less likely, suggesting low extinction with . Kann et al. (2010) find that their optical-NIR SED is consistent with no dust reddening using any dust model. The SED is not provided in Kann et al. (2010), therefore a detailed comparison could not be made.

A.12. GRB 060906

The optical spectrum of the afterglow of GRB 060906 (z = 3.6856) was obtained with the VLT/FORS1 spectrograph (Jakobsson et al. 2006a; F09). The I-band observation was taken at the New Mexico Skies Observatory (Torii 2006) and corrected for the time using the R-band lightcurve power-law with shallow decay index α = 0.56 ± 0.07 (Cenko et al. 2006; Cenko 2006b). The z′ band observation is taken from the lightcurve presented in Cenko et al. (2009). The optical to X-ray SED can be fitted with a broken power-law with no dust extinction. The derived 2σ extinction upper limit is  < 0.09 mag. The X-ray to optical slope differ by Δβ ~ 0.5 which corresponds to the change in slope caused by the cooling frequency (Sari et al. 1998) lying within the observed frequency range.

The derived H i column density is log NH   i/cm-2 = 21.85 ± 0.1 (Jakobsson et al. 2006a). Previously Cenko et al. (2009) fitted the SED and found dust extinction with . Kann et al. (2010) fit the optical afterglow SED and find insignificant dust extinction AV = 0.05 ± 0.05, in agreement with our results.

A.13. GRB 060926

The optical afterglow of GRB 060926 (z = 3.2086) was observed with the VLT/FORS1 (Piranomonte et al. 2006a; F09). The Lyα emission from the host galaxy of the GRB is detected in the trough of the DLA (F09). The estimated H i column density is log NH   i/cm-2 = 22.7 ± 0.1 (Jakobsson et al. 2006a). The optical to X-ray SED is normalized to the acquisition image photometry at 7.7 h after the burst. The optical to X-ray SED is well described by a single power-law and SMC type dust with a moderate extinction of AV = 0.31 ± 0.02 mag.

A.14. GRB 060927

The optical spectrum of the afterglow of GRB is carried out with the VLT/FORS1 spectrograph (Ruiz-Velasco et al. 2007; F09). GRB 060927 with a redshift of z = 5.4636 is the second highest redshift GRB in our spectroscopic GRB sample. The redshift is based on a single Si   ii absorption line (Ruiz-Velasco et al. 2007). The K, J and I band observations of the afterglow were carried out with VLT filters (Ruiz-Velasco et al. 2007). We took NIR photometry and spectroscopy to construct the composite SED at 12.5 h after the burst trigger. The SED of the afterglow can be fitted with a single power-law without any dust extinction. The estimated 2σ upper limit for the AV is  < 0.12 mag.

The optical spectrum of the afterglow is previously published in Ruiz-Velasco et al. (2007) reporting the H i column density of log NH   i/cm-2 = 22.50 ± 0.15. Kann et al. (2010) fit the NIR SED of the afterglow and found  ≤ 2.5σ significant dust with AV = 0.21 ± 0.08 for SMC dust. Due to the large uncertainties on the NIR data, no model could be able to fit the NIR segment alone.

A.15. GRB 061007

GRB 061007 is an extremely bright burst detected by Swift accompanied by a very luminous afterglow and similar decay rate in the X-ray and optical bands (Schady et al. 2007a). The spectrum of the optical afterglow of GRB 061007 at z = 1.2622 was obtained with the VLT/FORS1 (Jakobsson et al. 2006b; Paper). The I-band photometry is obtained from the lightcurve presented by Mundell et al. (2007). We reduced the UVOT data of the GRB afterglow and detected source in the v, b and u photometric bands. The SED of the afterglow is nicely fitted with a single power-law together with SMC extinction with a value of AV = 0.35 ± 0.03 mag.

Oates et al. (2009) and Schady et al. (2010) find high values of extinction varying from AV ≈ 0.66 − 0.75, while Schady et al. (2010) prefer LMC dust model. The results of Mundell et al. (2007) with AV = 0.48 ± 0.19 and Kann et al. (2010) with AV = 0.48 ± 0.10 are comparable with our values. There is an indication of a 2175 Å bump in the blue end of the optical spectrum. The central wavelength of the bump (i.e. λobs ~ 4520 Å) does not match with the redshift of the GRB. An intervening absorber at redshift z = 1.066 is also seen in the spectrum with strong Mg   ii and Ca   ii lines. This absorption feature could be associated with the intervening absorber. Apart from the optical spectrum, no other sign of the intervening absorber is seen at this redshift.

A.16. GRB 061021

The spectrum of the GRB 061021 was secured with the VLT/FORS1 spectrograph (Thöne et al. 2006a; F09). The redshift of the burst (i.e. z = 0.3463) is obtained from Mg   ii absorption lines (Hjorth et al, in prep). This is the second lowest redshift of our flux calibrated GRB afterglow sample. The UVOT photometric data have been reduced and the afterglow is well detected in the v, b, u, uvw1 and uvm2 filters (see Fig. A.1). The intrinsic SED of the afterglow can be explained by a broken power-law consistent with a cooling break as explained by Sari et al. (1998) and with no dust extinction. The estimated 2σ extinction upper limit is  < 0.1 mag.

A.17. GRB 061110A

The optical spectrum of the afterglow of GRB 061110A at z = 0.7578 was obtained with the VLT/FORS1 (Fynbo et al. 2007; F09). The intrinsic SED of the afterglow is scaled to the acquisition image photometry. The afterglow spectrum of this burst has low S/N. The optical to X-ray SED can be modeled with a single power-law with no dust reddening. We derived the 2σ extinction upper limit of AV < 0.1 mag.

A.18. GRB 061110B

The VLT/FORS1 carried out the optical observation of the afterglow of GRB 061110B at z = 3.4344 (Fynbo et al. 2006a; F09). The afterglow SED is constructed at t0 + 2.5 h epoch scaling with the R-band flux. The estimated H i column density is log NH   i/cm-2 = 22.35 ± 0.1 (F09). The intrinsic SED of the afterglow can be explained by a broken power-law all the way from optical to the X-ray with SMC type extinction with AV = 0.22 ± 0.03 mag.

A.19. GRB 070110

The optical spectrum of GRB 070110 (z = 2.3521) was obtained with the VLT/FORS2 spectrograph (Jaunsen et al. 2007b; F09). The UVOT data of this burst have been reduced by using UVOT calibration technique given by Poole et al. (2008). The afterglow is detected only in the v and b bands. The optical to X-ray SED of the afterglow is fitted with a broken power-law indicating a cooling break as suggested by Sari et al. (1998). We find no dust extinction in this burst. The derived 2σ extinction upper limit is AV < 0.11 mag.

The estimated H i column density is log NH   i/cm-2 = 21.7 ± 0.1 (F09). Previously Schady et al. (2010) find that broken power-law well fits the data, but they could not distinguish between the MW, LMC and SMC dust models, finding dust extinction of with SMC model.

A.20. GRB 070125

The optical spectrum of the afterglow of GRB 070125 (z = 1.5471) was carried out with the VLT/FORS2 and previously published in F09. The broad undulations in the optical spectrum are due to systematics in the flux calibration. The redshift of the GRB is determined from the data obtained with Gemini North Telescope equipped with GMOS (Cenko et al. 2008). The Rc and I band observations have been collected by the MITSuME Telescope (Yoshida et al. 2007). The J, H and Ks band observations of the afterglow have been obtained with the robotic Peters Automatic Infrared Imaging Telescope (PAIRITEL; Bloom et al. 2007b) and scaled to Rc-band flux level using the decay rate from a decent R-band lightcurve obtained from the circulars. The R-band lightcurve exhibits a plateau phase and a break to a steeper decay with decay indices α1 = 1.4 ± 0.1 and α2 = 2.55 ± 0.07 and break time at  ~4 days after the burst. The optical spectrum is normalized to Rc-band flux level. The X-ray to optical/NIR SED can be fitted with a broken power-law and SMC-type linear extinction with AV = 0.27 ± 0.03 mag.

Previously Kann et al. (2010) carried out a fit to the optical/NIR data and found that SMC dust best fits the data with little amount of dust (AV = 0.11 ± 0.04). We find that the errors on the photometry are large and the NIR/optical X-ray joint fit suggests a cooling break in the SED and moderate extinction.

A.21. GRB 070129

The optical spectrum of the afterglow of GRB 070129 was obtained with the VLT/FORS2 spectrograph together with the R-band imaging (F09). We collected the I-band detection of the afterglow from Michigan-Dartmouth-MIT (MDM) Hiltner telescope (Halpern et al. 2007), which is close to the time of the SED. The redshift (z = 2.338) reported here was estimated from the host galaxy [O   iii] emission line (Bo Milvang-Jensen private comm.). The intrinsic SED of the afterglow is scaled to the level of the R-band observation. The X-ray to optical SED of the afterglow is reproduced well with a broken power-law with SMC type extinction with a typical value of AV = 0.30 ± 0.02 mag.

A.22. GRB 070318

The optical spectrum of the afterglow of GRB 070318 (z = 0.8397) was carried out with the VLT/FORS1 spectrograph (Jaunsen et al. 2007a; F09). The spectrum shows an unusual sharp break in the optical spectrum around λobs ~ 5000 Å. A spectrum simultaneously obtained at the Magellan telescope, showed the same optical break (Chen et al. 2007). The Swift UVOT data have been reduced together with the NIR data from ANDICAM. The UVOT slope is consistent with the downturn in the optical spectrum. The SED of the afterglow is generated at t0 + 16.7 h after the burst. There is clear evidence for dust attenuation in the NIR to X-ray afterglow SED. Due to the sharp break in the optical spectrum and peculiar dust reddening the data are not consistent with SMC-origin extinction curve. We fit the SED with FM-dust induced model and found that the SED fits with a sharp break in the optical spectrum. The extinction curve is very unusual due to the sharp optical break. The GRB is not included in our extinction sample due to non-consistency with the synchrotron model. This sharp break could be due to destruction of dust grains by the GRB. A more detailed and comprehensive analysis of the SED will be given in Watson et al. (in prep).

Previously Schady et al. (2010) implemented UVOT-XRT joint fit and found that a broken power-law is a slightly better fit to the data with dust extinction of .

A.23. GRB 070506

The optical spectrum of the afterglow of GRB 070506 (z = 2.3090) was taken with the VLT/FORS2 instrument (Thöne et al. 2007b; F09). The estimated H   i column density is log NH   i/cm-2 = 22.0 ± 0.3 (F09). The optical spectrum redwards of λobs ≈ 7000 Å  is affected by strong fringing, therefore, this part is not included in our SED analysis. The intrinsic SED of the afterglow is modeled and we find a broken power-law to be a reasonable fit to the data. In addition to the broken power-law the data require dust using SMC type dust grains with AV = 0.44 ± 0.05 mag.

A.24. GRB 070611

The optical spectrum of the afterglow of GRB 070611 (z = 2.0394) was obtained with the VLT/FORS2 (Thöne et al. 2007a; F09). The derived neutral hydrogen column density of the DLA is log NH   i/cm-2 = 21.3 ± 0.2 (F09). The SED is scaled to the time of the R-band photometry at 7.7 h. The X-ray to optical SED of the afterglow is defined by a single power-law and SMC type dust extinction (AV = 0.06 ± 0.02 mag).

A.25. GRB 070721B

The optical spectrum of the afterglow of GRB 070721B (z = 3.6298) is carried out with the VLT/FORS2 spectrograph (Malesani et al. 2007; F09). The estimated H i column density is log NH   i/cm-2 = 21.5 ± 0.2 (F09). The SED is scaled to the flux level of the R-band photometry. The intrinsic SED of the afterglow is nicely reproduced by using a broken power-law and SMC-type extinction with AV = 0.2 ± 0.02 mag (see Fig. A.1).

A.26. GRB 070802

The optical observations of GRB 070802 (z = 2.4541) were obtained with the VLT/FORS2 spectrograph. The 2175 Å dust extinction feature is clearly seen in the optical spectrum of the afterglow (F09; Elíasdóttir et al. 2009). The burst is categorized as dark burst as defined by Jakobsson et al. (2004a). The J, H and K band observations are taken at t0 + 2.0 h from the lightcurves presented in Krühler et al. (2008) for each band. We reduced UVOT data using the calibration techniques defined by Poole et al. (2008). The afterglow is well detected in the v and b bands (see Fig. A.1). Due to the presence of a strong 2175 Å bump feature, the SMC extinction curve gives the worst fit to the data. The observed bump is shallower than the MW, therefore, CCM is also not a good fit to the data. We fitted the SED using a FM extinction model with both single and broken power-laws. The intrinsic SED is well described with a single power-law and a large amount of dust observed in rest-frame V-band (AV = 1.19 ± 0.15 mag) with total-to-selective extinction of RV = 2.81 ± 0.68. The complete log of the best fit FM parameters is given in Table 4.

The neutral hydrogen column density of log NH   i/cm-2 = 21.5 ± 0.2 is determined from the optical spectrum (Elíasdóttir et al. 2009). The rest-frame extinction curve of the afterglow is shown in Fig. A.2. The NIR to X-ray SED has been previously fitted by Elíasdóttir et al. (2009); Krühler et al. (2008); Cenko et al. (2009); Greiner et al. (2011), finding high dust extinction (AV ≳ 1.0) with a single power-law, similar to our results.

A.27. GRB 071020

There was no PC mode data available near the time of the optical spectrum of GRB 071020 (z = 2.1462), therefore, we reduced early and late time PC mode X-ray data for this burst using the procedure described in Sect. 2.4. We further checked that the X-ray spectral slope for early and late time are similar. The optical spectrum of the afterglow was obtained with the VLT/FORS2 instrument (Jakobsson et al. 2007c; F09). The derived H i column density is log NH   i/cm-2 < 20.30 (Nardini et al. in prep.; F09). In our spectroscopic sample, this is the lowest column density obtained from the optical spectrum. The Lyα is in the very blue end of the spectrum and is not seen clearly (see F09). The R, J and K band observations are taken from Nardini et al. (in prep.). The H-band photometry is carried out with PAIRITEL (Bloom et al. 2007a) and scaled to the time of the SED. A comprehensive analysis of the optical spectrum of GRB 071020 will be discussed in Nardini et al. (in prep.). The SED of the afterglow of GRB 071020 can be modeled using a broken power-law with SMC type extinction with AV = 0.40 ± 0.04 mag. The change in slope is consistent with Δβ = 0.5 (see Table 3).

The best fit of Kann et al. (2010) implies SMC-type dust with AV = 0.28 ± 0.09, comparable to our results within uncertainties.

A.28. GRB 071031

The acquisition image photometry and the spectroscopy of the afterglow of GRB 071031 (z = 2.6918) were carried out with the VLT/FORS2 instrument (Ledoux et al. 2007; F09). The i′, z′, J, H and Ks band photometry have been obtained from GROND (Krühler et al. 2009). We also compared acquisition camera photometry to the r′ band photometry presented in Krühler et al. (2009) and taken at similar time, finding consistent values. The obtained column density of the GRB-DLA is log NH   i/cm-2 = 22.15 ± 0.05 (Ledoux et al. 2009). The SED of the afterglow of GRB 071031 fits well with a broken power-law and no dust reddening. The estimated 2σ reddening upper limit in the V-band is  < 0.07 mag.

Previously Kann et al. (2010) modeled the optical-NIR data and found that SMC model well fits the data with insignificant amount of dust AV = 0.14 ± 0.13. The SED presented in this paper clearly indicate no dust. Greiner et al. (2011) find no dust extinction from the GROND-XRT joint fit, consistent with our results.

A.29. GRB 071112C

The optical spectrum of the afterglow of GRB 071112C at z = 0.8227 was carried out with the VLT/FORS2 instrument together with the acquisition image photometry (Jakobsson et al. 2007b; F09). The only available H band photometry has been taken with the MAGNUM (Minezaki et al. 2007). The R-band lightcurve is retrieved from the GRBlog and fitted with a decay index of α = 0.92 ± 0.06. We used the R-band lightcurve decay rate to obtain the NIR photometry at 10 h after the burst. The SED and optical/NIR observations of the afterglow have been previously discussed in Uehara et al. (2010). The intrinsic SED of the afterglow is modeled well with a broken power-law and presents no dust extinction. The derived upper limit for dust reddening is  <0.08 mag.

Previously Kann et al. (2010) studied the optical-NIR afterglow SED and found that SMC dust best fit the data with an insignificant amount of dust (AV = 0.23 ± 0.21). On the contrary, NIR to X-ray afterglow SED studied in this paper suggests no dust extinction (see Fig. A.1).

A.30. GRB 071117

The optical spectrum of the afterglow of GRB 071117 (z = 1.3308) was obtained with the VLT/FORS1 (Jakobsson et al. 2007a; F09). The estimated redshift of the afterglow comes from [O   ii] emission line. The optical and X-ray spectrum are normalized to the R-band acquisition camera photometry at 9 h after the burst trigger. The NIR to X-ray data fit well with a single power-law and SMC type with dust reddening of AV = 0.26 ± 0.02 mag.

A.31. GRB 080210

The optical spectrum of the afterglow of GRB 080210 at z = 2.6419 was carried out using the VLT/FORS2 instrument (Jakobsson et al. 2008b; De Cia et al. 2011; F09). The early and late time R-band photometry has been acquired with FORS2 (see De Cia et al. 2011). We selected the R-band photometry at 1.69 h which is near the mid time of the optical spectrum and scaled the afterglow SED to that level. The optical spectrum is corrected for slit losses and kindly provided by Annalisa De Cia (see also De Cia et al. 2011). In addition to the optical spectrum the optical/NIR data have been obtained by using the GROND telescope (Greiner et al. 2011). We obtained the g′, r′, i′, z′, J, H and Ks band photometry at the time of the SED (Krühler, private comm.). The i′ and z′ data show a spectral plateau and are clearly down compared to the rest of the photometry (see Fig. A.1), therefore, the GROND photometry is not included in our SED fitting. The optical-XRT SED is modeled nicely with a broken power-law and SMC-type dust reddening. The dust extinction is found to be AV = 0.33 ± 0.03. The change in the spectral slope for this afterglow is Δβ = 1.14 ± 0.13 (see Fig. 1).

The estimated H i column density for the DLA is of log NH   i/cm-2 = 21.90 ± 0.10 (De Cia et al. 2011). Kann et al. (2010) fitted the GROND SED and assumed the i′ and z′ data as an indication of 2175 Å bump, resulting in some deviation from the dust model and give quite large extinction (AV = 0.7). An independent analysis of De Cia et al. (2011) also confirms that the spectral change is around 1.0 and is inconsistent with the fireball model. This is the only outlier with twice the spectral slope change as compared to the other break frequency cases. The X-ray slope for this case is much softer than the other cases (see Fig. 1). We checked that the X-ray spectrum becomes gradually softer (Γ = β + 1 > 2.0) in the time interval ranging from 1000 to 10 000 s, suggesting that energy injection from the central engine is likely to be decreasing at these times. For SMC type extinction De Cia et al. (2011) found AV = 0.18 ± 0.03. Greiner et al. (2011) employed a joint GROND-XRT fit to the data with fixed Δβ = 0.5 and find AV = 0.24 ± 0.03.

A.32. GRB 080319B

The optical spectrum of the afterglow of GRB 080319B (z = 0.9382) was obtained with the VLT/FORS2 spectrograph almost one day after the burst trigger (F09). The GRB is commonly referred to as the “naked eye burst” due to its peak visual magnitude of 5.3 mag (Racusin et al. 2008; Woźniak et al. 2009; Bloom et al. 2009). The g′, r′, i′ and z′ band photometry has been obtained from the lightcurves for these filters presented in Tanvir et al. (2010). The UVOT data have been reduced and the magnitudes obtained for the v, b, u and uvw1 filters, where the afterglow is clearly detected. We generated the SED at 26 h after the burst and modeled it using power-laws and dust extinction. We found the observed data fit very well with a single power-law all the way from optical to the X-ray and no dust reddening. The upper limit on the amount of dust is estimated to be  < 0.11 mag.

Kann et al. (2010) find no dust extinction from optical-NIR SED fitting, in excellent agreement with our result.

A.33. GRB 080520

The optical spectrum of the afterglow of GRB 080520 (z = 1.5457) was obtained with the VLT/FORS2 spectrograph at the time when the afterglow was very faint. The UVOT data have been reduced and the afterglow is only detected in the v band. The SED of the afterglow is constructed and the data are well fitted with a single power-law and SMC-type dust extinction with AV = 0.23 ± 0.02 mag.

Greiner et al. (2011) fit the XRT through optical/NIR data and find weakly constrained high amount of extinction (). Our results are consistent to Greiner et al. (2011) within 1σ.

A.34. GRB 080605

The optical spectrum of the afterglow of GRB 080605 (z = 1.6403) was carried out with the VLT/FORS2 in different grisms (Jakobsson et al. 2008c; F09). The 2175 Å absorption dip is clearly seen in two FORS2 grism spectra. Being at redshift z < 2 the Lyα is not detected for this burst due to the sensitivity range of the spectrograph. Therefore metal abundances cannot be obtained for this interesting case. We reduced the UVOT data of the afterglow and found that the source is contaminated by a nearby object. After subtraction the afterglow is not visible in any UVOT bands. The 2175 Å absorption dip is very weak and narrow. The SED is modeled with the FM extinction law. The SED is well described by a single power-law and large amount of dust extinction in restframe V band of  mag. The extinction curve of the afterglow suggests RV consistent with its value in the MW (see Fig. A.2). The metallicity obtained from the equivalent width of Si   ii (1526 Å) suggests a fairly high metallicity for this burst (see Table 6).

Greiner et al. (2011) find that the MW dust model best-fit the data with moderate amount of extinction (AV = 0.47 ± 0.03). The SED presented in this paper is not complete and lacking the NIR data, which could be in the restframe V-band. This particular case with other 2175 Å bump cases will be discussed in Zafar et al. (in prep.).

A.35. GRB 080607

The optical spectrum of the afterglow of GRB 080607 (z = 3.0368) was obtained with the Keck telescope and previously published in Prochaska et al. (2009); Perley et al. (2011b) and F09. Because of being not observed with the VLT, this burst is not a part of our spectroscopic GRB sample. The derived neutral hydrogen column density of GRB-DLA is log NH   i/cm-2 = 22.70 ± 0.15 (Prochaska et al. 2009; F09). Due to the occurrence of GRBs in star-forming molecular clouds, molecular hydrogen has been searched for many GRBs and resulted in non-detections (Vreeswijk et al. 2004; Tumlinson et al. 2007; Sheffer et al. 2009). Fynbo et al. (2006b) tentatively interpreted an absorption feature as H2 in their spectrum of GRB 060206. For the first time, strong unambiguous H2 and CO molecular absorption lines were detected in the optical afterglow spectrum of GRB 080607 (Prochaska et al. 2009; Perley et al. 2011b). Most of the 2175 Å extinction bump is clearly detected in the optical spectrum. The remaining part of the bump is covered by using the z and i filters. The afterglow is detected in the NIR J, H and Ks bands with PAIRITEL. The I and V band observations have been performed with the Katzman Automatic Imaging Telescope (KAIT) and the R, i and z band detection are from the robotic Palomar 60 inch telescope (see Table:1 Perley et al. 2011b). This burst classifies as dark by the definition of Jakobsson et al. (2004a). The GRB afterglow is remarkably luminous in optical and NIR wavelengths. Since the optical data have strong H2 molecular absorption lines which makes it hard to see the underlying continuum, therefore, we fit with the binned spectrum where lines are already taken out (see Table 4; Perley et al. 2011b). We fit the data with the FM parameterization and the best fit parameters are given in Table 4. The intrinsic SED of the afterglow is nicely reproduced with  mag and a very high RV = 3.75 ± 1.04. The 2175 Å is the widest compared to all other bump cases (see Table 4). The host galaxy of the GRB afterglow is also extremely red in the optical and NIR bands (Chen et al. 2010).

Previously Prochaska et al. (2009) found AV ≈ 3.2 and Perley et al. (2011b) estimated AV = 3.3 ± 0.4 the optical/NIR data fit. Both authors found RV ~ 4 which is consistent with our result of a shallower extinction curve.

A.36. GRB 080707

The optical spectrum of the afterglow of GRB 080707 (z = 1.2322) is obtained with the VLT/FORS1 under bad conditions (Fynbo et al. 2008b; F09). The SED is normalized to the R-band observation taken at 1.1 h after the burst. The intrinsic SED of the afterglow is fitted with a broken power-law and no dust reddening. The upper limit on the dust content is AV < 0.13 mag.

Greiner et al. (2011) implemented the GROND-XRT joint fit to the data and find that the data are consistent with no dust with an insignificant value of  mag.

A.37. GRB 080721

The optical spectrum of the afterglow of GRB 080721 at z = 2.5914 was obtained with the VLT/FORS1 spectrograph (Starling et al. 2009; F09). We extracted the UVOT data and the afterglow is detected in the v and b band. The I-band photometry is obtained at 10.2 h using the I-band observations given by Starling et al. (2009). The neutral hydrogen column density of the DLA is log NH   i/cm-2 = 21.6 ± 0.1 (F09). The intrinsic SED is scaled to the R-band acquisition camera photometry. The data is well described by using a single power-law and no dust reddening. The estimated upper limit for dust reddening is  < 0.12 mag.

Previously Starling et al. (2009) found no cooling break between the X-ray and optical wavelengths. Despite the large errors on the photometry, Kann et al. (2010) fit the optical SED and find AV = 0.35 ± 0.07.

A.38. GRB 080805

The optical spectrum of the afterglow of GRB 080805 (z = 1.5042) was obtained with the VLT/FORS2 (Jakobsson et al. 2008a; F09). The spectrum clearly show a 2175 Å absorption feature at the redshift of the GRB. Due to the relative faintness in the optical, the afterglow is not detected in any of the UVOT filters. The spectrum shows a pretty clear absorption dip and the extinction curve resembles that of the MW. The intrinsic SED of the afterglow is generated 1.0 h after the burst trigger. The X-ray to optical SED is reproduced nicely with a single power-law plus FM extinction curve with a large amount of dust reddening of AV = 1.53 ± 0.13 mag and total-to-selective extinction of RV = 2.48 ± 0.39.

Previously Greiner et al. (2011) modeled the GROND-XRT data and found that MW dust model, with a 2175 Å bump, provides the best explanation to the data with . The value is consistent with the results we find from our analysis.

A.39. GRB 080905B

The optical spectroscopy of the afterglow of GRB 080905B at z = 2.3739 was taken with the VLT/FORS2 together with the R band acquisition image photometry (Vreeswijk et al. 2008b; F09). We implemented MIDAS package FITLYMAN (Fontana & Ballester 1995) to determine the neutral hydrogen column density. The derived H   i column density of the afterglow of GRB 080905B is log NH   i/cm-2 < 22.15. Since the afterglow flux is contaminated by the light from another object on the slit, we suggest the column density to be an upper limit. We modeled the intrinsic afterglow SED and the data are well fitted with a single power-law and dust extinction with AV = 0.41 ± 0.03 mag

A.40. GRB 080913

The spectrum of the afterglow of GRB 080913 was taken with the VLT/FORS2 (Greiner et al. 2009; F09; Zafar et al. 2011). GRB 080913 is the highest redshift GRB in our spectroscopic sample. The acquisition image was taken with z-Gunn filter. The magnitude reported in Table 1 is strongly affected by Lyα absorption. We corrected the magnitude for Lyα absorption using spectro-photometric analysis (see Zafar et al. 2011 for detail). A comprehensive description of the SED analysis is given in Zafar et al. (2011). The neutral H   i column density of the DLA is NH   i/cm-2 < 21.14 (Greiner et al. 2009).

A.41. GRB 080916A

The optical spectrum of the optical afterglow of GRB 080916A (z = 0.6887) was carried out with the VLT/FORS2 (Fynbo et al. 2008a; F09). The SED is constructed at 17.1 h i.e. the time of R-band acquisition image photometry. The intrinsic SED is nicely reproduced with a broken power-law and SMC type dust extinction with AV = 0.15 ± 0.04 mag.

A.42. GRB 080928

The optical spectrum of the afterglow of GRB 080928 at z = 1.6919 was obtained with the VLT/FORS2 (Vreeswijk et al. 2008a; F09). We collected the optical/NIR photometry at the time of the SED taken with GROND from Rossi et al. (2011). We scaled the spectrum to the level of the r′ band photometry taken at 15.5 h. In addition we reduced the UVOT data for this burst and fluxes are obtained for the v, b and u bands at the common SED epoch. The intrinsic SED is well fitted with a single power-law and SMC extinction. The best fit AV value is 0.3 ± 0.03 mag.

Kann et al. (2010) find that a MW dust model best fits the optical-NIR data with a small amount of extinction (AV = 0.14 ± 0.08). We do not find any evidence for the 2175 Å bump in the optical spectrum.

thumbnail Fig. A.1

Afterglow SEDs of the spectroscopic GRB sample in νFν and ν space. In each figure, we plot the best fit extinguished (solid lines) and extinction corrected spectral model (dashed lines). The grey curve represents the optical spectrum. The X-ray spectra are indicated by blue points. The black triangle corresponds to acquisition camera photometry used for scaling the SED. Black open circles are not included in the spectral fitting because of the optical spectrum wavelength coverage in that region while solid circles represent the data points included in the SED modeling. The red curve shows the best fit single power-law model and the green curve illustrates the broken power-law model.

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thumbnail Fig. A.2

Absolute extinction curves of the spectroscopic GRB sample. The extinction curves are based on the best fit models given in Table 2. The grey curve represents the optical spectrum. The black triangles correspond to acquisition camera photometry used for scaling the afterglow SED. Black open circles are not included in the spectral fitting because of the optical spectrum wavelength coverage in that region while solid circles represents the photometric data points included in the SED modeling. The red solid curve corresponds to the best dust model for each GRB. Also shown are the Milky Way (green dashed line) and SMC (blue dotted line) extinction models taken from Pei (1992).

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© ESO, 2011

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