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Appendix A: Additional notes on individual targets: observations by Swift and other facilities

A.1. GRB 050717

GRB 050717 triggered Swift/BAT at 10:30:52 UT (Hurkett et al. 2005b). It was a long burst with a duration of T₉₀(15−350 keV) = (86 ± 2) s (Cummings et al. 2005a) that was also detected by Konus-WIND (Golenetskii et al. 2005). Swift/XRT began observing 79 s after the trigger and found a bright, fading X-ray source, while simultaneous SwiftUltra-Violet/Optical Telescope (UVOT, Roming et al. 2005) observations started 78 s after the trigger and resulted only in upper limits (Hurkett et al. 2005b; Blustin et al. 2005). Unfortunately, XRT was unable to automatically centre on the burst, leading to a delay of 2.5 h in the determination of the X-ray position (error circle radius 60; Kennea et al. 2005; see also Hurkett et al. 2005a). The burst is discussed by Krimm et al. (2006b) in detail; it was very luminous, has one of the highest-ever measured peak energies, and a probable redshift z > 2.7. Deep ground-based K-band follow-up observations were performed with the du Pont 100-inch telescope at Las Campanas Observatory with a first run starting 37.7 h after the burst. No fading NIR source was detected (Berger & Lopez-Morales 2005; Berger et al. 2005). Optical observations with the Tenagra 0.35-m telescope at Perth, Australia, did not find a new source down to the limit of the DSS2 red survey (Luckas et al. 2005). In addition, PROMPT-5 at Cerro Tololo Inter-American Observatory in Chile automatically observed the field starting 13 h after the burst. No fading source was found down to R_C = 21.7 and I_C = 21.5 (MacLeod et al. 2005).

Swift/UVOT obtained an upper limit of v > 19.0 for any afterglow at 420 s (mid-time) after the onset of the burst (Blustin et al. 2005), corresponding to v > 18.3 after correction for Galactic extinction. Using the observed constraint on the spectral slope β_OX < 0.40 at the time of the UVOT observations, this corresponds to an upper limit of R_AB > 18.2. In the same way, following Rol et al. (2005), at the time of the optical observation the observed (mean) X-ray flux together with the observed (mean) spectral slope β_X predicts a non-extinguished R_AB-band magnitude of between 14.5 ± 1.5 and 18.2 ± 1.5, where the brighter magnitude corresponds to ν_c = ν_opt and the fainter magnitude to ν_c = ν_X (ν_c is the cooling frequency; Sari et al. 1998). According to the criterion of J04, which uses β_OX, the burst is dark, while according to the criterion of V09, which uses β_OX and β_X, the burst is not dark (Table 2).

A.2. GRB 050922B

Swift/BAT detected the burst at 15:02:00 UT. It was an image trigger lasting for 168 s (Norris et al. 2005). Cummings et al. (2005b) measured T₉₀(15−150 keV) = (250 ± 20) s. Because of the image trigger history, Norris et al. (2005) speculated that it could be a high-redshift event similar to GRB 050904. Swift/XRT started observing 342 s after the trigger, and UVOT one second later (Norris et al. 2005). A decaying X-ray afterglow was detected (Godet et al. 2005) but no optical counterpart (Pasquale et al. 2005). Several ground-based small telescopes responded to the trigger but also failed to find any afterglow candidate, namely ROTSE IIIa (upper limit CR = 17.3 at 3 min; Schaefer et al. 2005), the 14-inch Automated Response Telescope at the University of Osaka, Japan (upper limit CR = 15.1 at 3 min; Torii 2005), the 0.4-m telescope of Ussuriysk Astrophysical Observatory, Russia (upper limit CR = 16.0 at 15 min; Kornienko et al. 2005), and the 30-cm telescope at University of Miyazaki, Japan (upper limit CR = 16.1 at 21 min; Sonoda et al. 2005).

The INT 2.5-m telescope at Observatorio del Roque de los Muchachos on La Palma obtained an upper limit to the afterglow of r′ > 22.5 at 49 ks (mid-time) after the onset of the burst (Guziy et al. 2005), corresponding to r′ > 22.4 after correction for Galactic extinction. There are no X-ray data for the time between about t = 10 ks and 100 ks after the burst, but there are for observations from t ~ 100 ks to about 1 Ms. The latter data can be used to extrapolate the X-ray flux to t = 49 ks. The spectral slope is then β_OX < 0.39, corresponding to an upper limit of about R_AB > 22.3. Similarly, the observed X-ray flux together with the observed spectral slope β_X at t = 49 ks predicts a non-extinguished R_AB-band magnitude between 11.1 ± 2.9 and 14.8 ± 3.0. Using β_OX and β_X, the burst is dark according to the criterion of J04 as well as V09 (Table 2).

A.3. GRB 060211A

Swift/BAT was triggered by GRB 060211A at 09:39:11 UT (Hurkett et al. 2006). It was a long burst with a duration of T₉₀ (15–350 keV)  = 126 ± 5 s (Sato et al. 2006a; Krimm et al. 2006a). The spacecraft slewed promptly to the BAT position and Swift/XRT found a bright, fading X-ray source, while Swift/UVOT started observing 183 s after the trigger but did not detect any afterglow candidate (Hurkett et al. 2006). ROTSE IIIa, located at Siding Spring Observatory, Australia, and the Moscow Union “Optic” MASTER robotic system responded to GRB 060211 immediately. ROTSE’s automated response took the first image 147 s after the burst, under twilight conditions, while MASTER started 202 s after the GRB trigger. Only upper limits could be reported (Rujopakarn et al. 2006; Lipunov et al. 2006; see also Urata et al. 2006). In addition, the 2-m Faulkes Telescope North robotically followed-up GRB 060211 starting 5.4 min after the trigger. No fading optical counterpart down to R ≈ 18.5 was found (Gomboc et al. 2006). Deep upper limits were also reported by Norris et al. (2006), J > 19.1 at 17 h after the burst, and Sharapov et al. (2006), R > 22 at 5.5 h after the burst.

The 1.5-m telescope of Maidanak Astronomical Observatory obtained for the afterglow an upper limit of R = 22.0 at  ~ 20 ks (mid-time) after the onset of the burst (Sharapov et al. 2006), corresponding to R = 21.6 after correction for Galactic extinction. This corresponds to an upper limit of R_AB > 21.8. Among all available upper limits, this observation provides the tightest constraints on the spectral properties of the afterglow from the optical to the X-ray band. According to these data, however, GRB 060211A cannot be classified as a dark burst (Table 2).

A.4. GRB 060805A

The burst triggered Swift/BAT on May 8, 2006 at 04:47:49 UT (Ziaeepour et al. 2006). It had a duration of T₉₀(15−350 keV)  = 5.4 ± 0.5 s (Barbier et al. 2006a). Swift/XRT began taking data 93 s after the BAT trigger. A ground analysis revealed a faint, uncatalogued X-ray source. Swift/UVOT started observing 97 s after the trigger but no afterglow candidate was detected in any band (Ziaeepour et al. 2006; Pandey et al. 2006). Additional ground-based observations could only provide upper limits. The robotic 0.76-m Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory started observing the field 119 s after the BAT trigger but no afterglow was found (V > 16.8,I > 16.7; Li 2006). The automated Palomar 60-inch telescope responded to GRB 060805A and started observing 3 min after the burst trigger. No source down to R > 19 was found in the XRT error circle (Cenko 2006). Additional upper limits were obtained by the 1.3-m Skinakas Observatory (University of Crete, Heraklion, Greece) of R > 21.5 at 14 h after the burst (Muehlegger et al. 2006) and by the 2-m Liverpool Telescope on La Palma of r′ > 22.9 and i′ > 22.6 at 0.725 days and 0.748 days, respectively, after the burst (Rol & Page 2006).

The 2-m Liverpool Telescope observations correspond to r′ > 22.7, after correction for Galactic extinction. Using β_OX < 1.00, this corresponds to an upper limit of R_AB > 22.7. Among all available upper limits for this burst, this observation provides the tightest constraint on β_OX and β_X (Table 2). However, these constraints do not qualify GRB 060805A as a dark burst, especially because the X-ray afterglow itself was very subluminous.

A.5. GRB 060919

GRB 060919 triggered Swift/BAT at 07:48:38 UT (Guidorzi et al. 2006a). It was a long burst with a duration of T₉₀ = (15−350 keV)  = 9.1 ± 0.2 s (Sato et al. 2006b). Swift/XRT began taking data 87 s after the BAT trigger. Ground analysis revealed a faint X-ray source with an revised error circle of (Guidorzi et al. 2006a,b). Swift/UVOT started observing the field 73 s after the burst but did not detect an optical counterpart in any band down to deep flux limits (Breeveld & Guidorzi 2006). The robotic TAROT telescope on La Silla started observing 28 s after the trigger. No optical transient was found down to R > 15.4 in the first 60 s of observations. An upper limit of R > 15.8 could be set for any transient up to 382 s after the trigger (Klotz et al. 2006). The Faulkes Telescope South started observing about 2.8 h after the event. No optical transient was detected down to a limiting magnitude of R > 19.5 (Melandri et al. 2006).

The UVOT upper limit at 918 s corresponds to v > 20.0, after correction for Galactic extinction. Using β_OX < 0.68, this corresponds to an upper limit of R_AB > 19.8. As for to the previous two bursts, this observation provides the tightest constraints achieved to date on the spectral properties of the afterglow. On the basis of these data, GRB 060919 is not a dark burst (Table 2).

A.6. GRB 060923B

Swift/BAT triggered on GRB 060923B at 11:38:06 UT (Stamatikos et al. 2006). It was a single-peaked burst with a duration of T₉₀(15−350 keV)  = 8.8 ± 0.1 s (Barbier et al. 2006b). Swift/XRT began observing the field 114 s after the BAT trigger and found an uncatalogued X-ray source with a positional accuracy of . Swift/UVOT started observing 122 s after the burst with the white filter but could not detect an afterglow candidate (Stamatikos et al. 2006; Holland & Cucchiara 2006). No further ground-based follow-up observations were reported in the literature.

Swift/UVOT obtained an afterglow upper limit of v > 18.1 at 295 s (mid-time) after the burst (Holland & Cucchiara 2006), corresponding to v > 18.0 after correction for Galactic extinction. Using β_OX < 0.62, this corresponds to an upper limit of R_AB > 17.9. Among all available upper limits for this burst this observation provides the tightest constraint on β_OX and β_X (Table 2). These constraints do not classify GRB 060923B as a dark event.

A.7. GRB 061102

GRB 061102 triggered Swift/BAT at 01:00:31 UT (Holland et al. 2006). It was a long burst with a duration of T₉₀(15−350 keV)  = 17.6 ± 1 s (Tueller et al. 2006). Swift/XRT began observing the field 100 s after the BAT trigger and found an uncatalogued, fading X-ray source (Holland et al. 2006; Starling et al. 2006). Swift/UVOT started observing 110 s after the trigger with the white filter but no afterglow candidate was seen down to a 3σ upper limit of white <18.5 (Holland et al. 2006). Continued observations provided only upper limits in all UVOT bands (Holland 2006). No further ground-based follow-up observations of this event are reported in the literature.

Swift/UVOT obtained an even deeper upper limit of v > 20.5 at 1480 s (mid-time) after the onset of the burst (Holland 2006), corresponding to v > 20.4 after correction for Galactic extinction. Using the observed β_OX < 1.10, this corresponds to an upper limit of R_AB > 20.1. Among all available upper limits, this observation provides the tightest constraints on the afterglow SED, which do not classify GRB 061102 as dark (Table 2).

A.8. GRB 070429A

The burst 070429A triggered Swift/BAT at 01:35:10 UT (Barthelmy et al. 2007). It was a long burst with T₉₀(15−350 keV)  = 163 ± 5 s (Cannizzo et al. 2007). Swift/XRT started observing 153 s after the trigger and found a fading, uncatalogued X-ray source, while Swift/UVOT started observing 211 s after the trigger but did not detect an optical counterpart in any band (Schady & Cannizzo 2007). The ROTSE-IIIc telescope, located at Mt. Gamsberg, Namibia, started observing 97 s after the burst. No afterglow candidate was found down to CR > 17.3 (unfiltered images) for images taken within 3 min after the trigger and down to CR > 18.0 within 8 min (Rykoff et al. 2007). Additional data were obtained with the 0.6-m BOOTES-IR/T60 robotic telescope (Castro-Tirado et al. 2006), starting 3.25 h after the burst but no afterglow was found (de Ugarte Postigo et al. 2007). Deep K-band observations with the 4.2-m William Herschel Telescope on La Palma, beginning 4.1 h after the burst, detected a faint source in the XRT error circle, but no fading behavior was found (de Ugarte Postigo et al. 2007). Within its astrometric errors, this source corresponds to object C detected in our observations (see Table 3).

The Gemini North telescope mounted with the GMOS camera observed the field in i′ and z′ 44 ks (mid-time) after the burst. No afterglow candidate was found (Price 2007). Unfortunately, no magnitude limits were reported. Therefore, we used a conservative upper limit of R > 24.0 based on the original Gemini data available in the Gemini archive¹⁴. This corresponds to an upper limit of R_AB > 23.8. Together with the measured X-ray flux at the same time, this leads to β_OX < 0.42 and β_X − β_OX − 0.5 > 0.14. According to J04 as well as V09, the burst is dark. The observed X-ray flux predicts a non-extinguished R_AB-band magnitude between 17.2 ± 1.5 and 21.0 ± 1.5. Since the Swift/XRT light curve shows a constant decay with a constant spectral slope during the time when the optical upper limit was obtained, GRB 070429A is one of three events in our sample that can be securely classified as dark.

A.9. GRB 070517

This burst triggered Swift/BAT at 11:20:58 UT (Vergani et al. 2007a) and T₉₀ was 9 ± 1 s (Vergani et al. 2007b). Swift/XRT clearly detected an afterglow and could even see evidence of a break in the X-ray light curve. Swift/UVOT could not observe because of a 4 mag bright star in the field of view. Ground-based optical follow-up was only reported by Gilmore (2007) (UL = DSS2 Infrared at 2.7 h after the burst) and Fox et al. (2007) using Gemini-South  ~16 h after the burst. The latter authors suggested that there were two afterglow candidates in the XRT error circle but no further observations of these sources were reported in the literature. Therefore, we used their faintest detection (i′ > 24.5) as an upper limit at 57 600 s. Using the corresponding β_OX < 0.56, this translates into an upper limit of R_AB > 24.3, which does not classify GRB 070517 as a dark burst. However, in our late-time follow-up observations with VLT/FORS1 the brighter object reported by Fox et al. (2007) (r′ = 22.1) is no longer visible. Thus, we conclude that this was the optical afterglow of GRB 070517.

A.10. GRB 080207

GRB 080207 triggered Swift/BAT at 21:30:21 UT (Racusin et al. 2008) and had a duration of T₉₀ = 340 ± 20 s (Stamatikos et al. 2008). The XRT started observing the field 124 s after the BAT trigger and detected a bright source in WT mode. After  ~5000 s, it continued observing in PC mode, producing a light curve with a constant decay index. Swift/UVOT observations did not detect the afterglow both in early observations after 140 s in a white finding chart and later deeper observations (>1.5 h, Cucchiara & Racusin 2008). Several limiting magnitudes based on ground-based observations were reported: R > 14.3 at 1607 s (0.45 h), and R > 19.0 at 5049 s (1.45 h) (TAROT, Calern observatory, Klotz et al. 2008); J > 16.7, H > 15.9, K > 13.9 at 7.8 h, 7.7 h, and 10.1 h after the trigger, respectively (60-cm REM telescope, La Silla, D’Avanzo et al. 2008); R > 21.8 at 0.759 h (RTT150 1.5-m telescope, TUBITAK National Observatory, Khamitov et al. 2008); R > 20.8 at 13.7 h (Super-LOTIS, Kitt Peak observatory, Updike et al. 2008b); GMOS camera on the Gemini South telescope did not detect the afterglow down to g′r′i′z′ = 24.1, 24.5, 24.2, 25.0 at 9.8 h (Cucchiara & Fox 2008); R > 23.5 at 9.75 h (MOSCA mounted at NOT, La Palma; Marìn et al. 2008); J > 23.5, H > 22.8, and K > 21.5 (VLT/SINFONI, Fugazza et al. 2008); R > 20.3 at 1.49 h and R > 21.0 at 4.94 h (Zeiss-600 at Mt.Terskol observatory, Andreev et al. 2008). Based on GROND data we did not detect the afterglow in any band down to deep flux limits (Table A.3).

The Zeiss-600 telescope upper limit at 1.69 h corresponds to an upper limit of R_AB > 20.5. The observed X-ray flux predicts a non-extinguished R_AB-band magnitude between 11.3 ± 1.4 and 15.0 ± 1.4. According to the criterion of V09 as well as J04, GRB 080207 is a dark burst (Table 2). It is one of three events in our sample that can be securely classified as dark.

A.11. GRB 080218B

GRB 080218B triggered Swift/BAT at 23:57:47 UT and had a duration of T₉₀ = 6.2 ± 1.2 s (Schady et al. 2008b). Swift slewed immediately to the burst and XRT found a bright, uncatalogued X-ray source that could be localized with an uncertainty of . The Swift/UVOT started observing 551 s after the BAT trigger using the white filter. No afterglow candidate was found down to a 3σ limiting magnitude of 20.6 (Schady et al. 2008a). Several limiting magnitudes based on ground-based observations were then reported: CR > 16, starting 60 s after the trigger (unfiltered, 0.4-m Watcher telescope, South Africa, French et al. 2008; I > 21 and J > 18.7 at 3.1 h after the burst (1.3-m SMARTS telescope equipped with ANDICAM at CTIO, Cobb 2008a); H > 13.7 at 2 min and K > 12.6 at 8 min after the trigger (60-cm REM telescope on La Silla, Covino et al. 2008a); B > 22.1, V > 22.7, R > 22.9, and I > 22.6 at about 1 h and J > 20.6, H > 20.1, and K_s > 19.4 at about 3 h after the trigger using VLT/FORS2 and NTT/SOFI (Vreeswijk et al. 2008). Finally, no transient radio source was detected in the XRT error circle two weeks after the burst (Australia Telescope Compact Array, ATCA; Moin et al. 2008). Most importantly, the GROND imager did not detect the afterglow down to deep limits in all seven bands in spite of a rapid response time (Table A.3).

Based on GROND data we obtained an afterglow upper limit of r′ > 24.7 at 11520 s (mid-time) after the onset of the burst (Rossi et al. 2008b), corresponding to r′ > 24.3 after correction for Galactic extinction. Using the observed spectral slope β_OX < 0.18, this corresponds to an upper limit of R_AB > 24.3. The observed X-ray flux predicts a non-extinguished R-band magnitude between 15.1 ± 1.5 and 18.9 ± 1.5. The burst is dark according to the criterion of J04 as well as V09. It is one of three events in our sample that can be securely classified as dark.

A.12. GRB 080602

Swift/BAT triggered the burst at 01:30:28 UT (Beardmore et al. 2008a) and T₉₀ was 74 ± 7 s (Beardmore et al. 2008e). The burst was also detected by Konus-WIND, observations of this satellite allowing the peak energy to be constrained to be higher than 226 keV (Golenetskii et al. 2008). Swift/XRT found a bright, uncatalogued X-ray source resulting in a 58 error circle. Evidence of substantial X-ray absorption in excess of the Galactic value was found. Swift/UVOT started observing 123 s after the trigger but no afterglow candidate was detected. The XRT error circle was finally reduced to only 17 and 18, respectively (Beardmore et al. 2008c,d). The only optical follow-up observation was reported by Malesani et al. (2008b) about 3.4 h after the trigger using the NOT telescope on La Palma. No afterglow candidate was found down to R > 22.3 (Malesani et al. 2008c).

The Swift/UVOT imager obtained an upper limit of v > 20.3 at 504 s (mid-time) after the onset of the burst (Beardmore et al. 2008e), corresponding to v > 20.2 after correction for Galactic extinction. Using the observed spectral slope of β_OX < 0.05, this corresponds to an upper limit of R_AB > 20.2. Following Rol et al. (2005), the observed X-ray flux and spectral slope predicts a non-extinguished R_AB-band magnitude of between 13.0 ± 0.7 and 16.7 ± 0.7. According to the criterion of J04 as well as V09 the burst is dark (Table 2). However, because the X-ray light curve is rather flat instead of decaying during the time when the optical upper limit was obtained, the burst cannot be securely classified as dark. Unfortunately, no X-ray data was taken contemporaneously with the deep NOT observations.

A.13. GRB 080727A

Swift/BAT triggered on the burst at 05:57:39 UT with a duration (T₉₀) of 4.9 ± 1.0 s. About 109 s later, Swift/XRT began observing the field (Immler et al. 2008), unveiling a light curve with constant decay and evolving spectral index (see the XRT repository, Evans et al. 2007, 2009). The UVOT imager started observing at 113 s, no afterglow being found (Landsman & Immler 2008). In addition, UKIRT on Mauna Kea did not detect the afterglow down to K > 19.8 at 0.63 h after the trigger (Levan & Wiersema 2008). FORS1 on ESO/Paranal observed the field at 17.5 h and did not detect the afterglow down to the very deep upper limit of R > 26 (Malesani et al. 2008a).

Using the observed spectral slope β_OX < 0.85, the UKIRT upper limit corresponds to an upper limit of R_AB > 22.8. Following Rol et al. (2005), the observed X-ray flux and spectral slope at the time when the optical upper limit was obtained predicts a non-extinguished R_AB-band magnitude of between 17.9 ± 2.2 and 21.6 ± 2.2. According to the criterion of V09, the burst lies at the boundary region between dark and non-dark events (Δ_min = −0.05; Table 2). Unfortunately, no X-ray data was taken contemporaneously with the deep VLT observations.

A.14. GRB 080915A

GRB 080915A triggered Swift/BAT at 00:02:49 UT (Oates et al. 2008a). It was a long burst with a duration of T₉₀ = (15 − 350 keV)  = 14 ± 5 s (Ukwatta et al. 2008). Unfortunately, owing to an observing constraint, Swift could not slew to the burst during the first hour after the event, therefore XRT and UVOT could start observing only 3.9 ks after the trigger. Starting at this time, Swift/UVOT did not detect the optical afterglow (Oates et al. 2008b). The ROTSE-IIIc telescope located on Mt. Gamsberg, Namibia, responded to GRB 080915A automatically and took unfiltered images starting 52 s after the GRB trigger (cloudy conditions, full Moon). No afterglow candidate was found in the BAT error circle down to about CR > 14 (Rujopakarn et al. 2008). The robotic 60-cm REM telescope on La Silla started observing 2 min after the trigger. No afterglow candidates fainter than the 2MASS limits were seen in J,H,K (Covino et al. 2008b). Beginning 4.9 ks after the trigger, Swift/XRT and Swift/UVOT started observing. XRT found a faint, fading X-ray source with an error circle of (Evans & Oates 2008). Only upper limits could be reported for the UVOT bands (Breeveld & Oates 2008). Deep ground-based observations with ANDICAM on the SMARTS 1.3-m telescope at CTIO provided only upper limits of I > 21.9 and J > 20.1 (mid-time of 1.9 h after the burst; Cobb 2008b).

Deep prompt follow-up observations of the field were performed with GROND (Rossi et al. 2008c). They started 4.9 min after the trigger and lasted for 130 min. No evidence of a variable source was found when splitting these observations into two data sets (Table A.3). Second-epoch observations were performed with GROND the following night. Again, no afterglow candidate was found. Using the GROND upper limit of at 6840 s (mid-time; Rossi et al. 2008c), and β_OX < 0.62 (Table 2), this corresponds to an upper limit of R_AB > 22.0. Following Rol et al. (2005), we can use the observed X-ray flux as well as the X-ray slope to predict the non-extinguished R_AB-band magnitude. However, in this case owing to the small number statistics we can only give an upper limit of R_AB < 21. According to the criterion of V09, the burst is dark (Table 2), but the X-ray light curve is faint and very uncertain. Therefore, this burst cannot be securely classified as dark.

A.15. GRB 081012

Swift/BAT triggered on the burst at 13:10:23 UT. T₉₀ (15–350 keV) was 29 ± 4 s. The burst was also seen by Fermi/GBM, the peak energy was 320 ± 80 keV (Bissaldi 2008). The XRT began observing the field 49 min after the BAT trigger, an X-ray afterglow was found (Kennea & Stroh 2008), the error circle is just 18 in size (Evans et al. 2008). The UVOT started observing 3 min after the XRT; no afterglow candidate was detected (Kuin & Stroh 2008). Deep ground-based follow-up observations were performed using ROTSE IIIa (starting 39 s after the burst), the 2.5-m NOT telescope (de Ugarte Postigo & Malesani 2008).

The GROND imager obtained an upper limit on any optical afterglow of at  ~70 ks (mid-time) after the onset of the burst (Filgas et al. 2008; Table A.3), corresponding to r′ = 23.5 after correction for Galactic extinction. Using the observed spectral slope of β_OX < 0.83, this corresponds to an upper limit of R_AB > 23.4. Among all available optical upper limits, this observation provides the tightest constraint on the SED of the afterglow (Table 2). On the basis of these data, this burst is not dark.

A.16. GRB 081105

This burst triggered Konus-WIND, Swift, AGILE, Suzaku, and INTEGRAL at 13:26:12 UT. It was localized only via IPN. The burst had a single peak, about 10 s long (Cummings et al. 2008). Swift/XRT and UVOT started observing the field about 16 h later. An X-ray afterglow candidate was detected with an original uncertainty of 48 (Beardmore & Cummings 2008) and later confirmed (Beardmore et al. 2008b). Observations with UVOT could only provide upper limits (Curran et al. 2008).

The GROND imager obtained an afterglow upper limit of r′ > 23.0 at  ~46 ks (mid-time) after the burst (Clemens et al. 2008; Table A.3), corresponding to r′ > 22.9 after correction for Galactic extinction. Using β_OX < 0.61, this corresponds to an upper limit of R_AB > 22.8. This observation provides the tightest constraints on the SED. On the basis of these data, the burst is not dark (Table 2).

A.17. GRB 081204

The burst was detected by the INTEGRAL satellite at 16:44:55 UT. It lasted for about T₉₀ = 20 s (Götz et al. 2008). Swift reacted to the Integral alert, and started observing the field about 2.7 h after the burst, and found an uncatalogued X-ray source (Mangano et al. 2008a,b). Swift/UVOT started observing 3 h after the trigger in the white filter but no source was detected. Berger & Rest (2008) suggested an r = 23.5 ± 0.3 afterglow candidate based on observations with the Magellan/Clay telescope about 9 h after the trigger.

The field was also observed with GROND, which also detected the afterglow candidate observed by Berger & Rest (2008), together with another object, without finding evidence of fading in either source (Updike et al. 2008a). Both objects are discussed in this paper as host candidates (see Sect. 3.3). Stacking the highest quality GROND data, we obtained the revised upper limits reported in Table A.3, centered at a mid-time of 9.6 h. The GROND upper limit of r′ > 24.1 corresponds to r′ > 24.0 after correction for Galactic extinction. Using the observed β_OX < 0.55, this corresponds to an upper limit of R_AB > 23.9. Following Rol et al. (2005), we can use the observed X-ray flux as well as the X-ray slope to predict the non-extinguished R_AB-band magnitude. However, in this case owing to the small number statistics we can only give an upper limit of R_AB < 23 in the worse case of a break between optical and X-ray bands. The burst is dark according to V09 (Table 2), but owing to the faint XRT light curve and the poorly determined high X-ray spectral slope () this burst cannot be securely classified as dark.

© ESO, 2012