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Issue
A&A
Volume 538, February 2012
Article Number A44
Number of page(s) 23
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201117848
Published online 31 January 2012

© ESO, 2012

1. Introduction

Gamma-ray bursts (GRBs) are the brightest explosions in the Universe, releasing isotropic equivalent energies in the range of 1051–1054 erg (of the order of 1051 erg once corrected for collimation) within a few seconds. They were serendipitously discovered in 1967 (Klebesadel et al. 1973) in gamma-rays, but it was not until 1997 that the first counterparts were detected at other wavelengths (van Paradijs et al. 1997; Frail et al. 1997; Costa et al. 1997; Bremer et al. 1998). Thanks to the observation of the fading X-ray emission, which follows the more energetic gamma-ray photons, and the rapid distribution of the X-ray coordinates (much more precise than the gamma-ray ones), it is possible to carry out multiwavelength observations of the counterparts associated with GRBs within seconds of their occurrence. The late time emission, detectable at all wavelengths and observable for days after the burst onset (and some times significantly longer), is what we call GRB afterglow.

It is widely accepted that long GRBs (those with durations above  ~2 s) are related to the collapse of massive stars (Woosley 1993; Paczyński 1998; Hjorth et al. 2003) while short GRBs (shorter than  ~2 s) are probably originated by the coalescence of two compact objects, such as two neutron stars or a neutron star and a black hole (Paczynski 1990; Narayan et al. 1992; Levan et al. 2006; Chattopadhyay et al. 2007; King et al. 2007). In both scenarios, the result is the ejection of material at ultrarelativistic speeds, most probably through jets. This is what we call the relativistic fireball model (Rees & Meszaros 1992; Sari et al. 1998). In this framework, a compact source releases  ~1051 erg of energy within dozens of seconds in a region smaller than 10 km. When the ejecta run into the surrounding medium, a forward shock sweeps the surroundings of the progenitor, producing an afterglow as the ejecta interact with the interstellar matter. A reverse shock, colliding with the ejecta, can also produce an additional emission during early times (Piran 1999).

Until now, millimeter and especially submillimeter observations of GRBs have been scarce due to the small number of available observatories and limited sensitivity, as compared to other wavelength ranges. As examples of large samples of afterglow data see, for example, Evans et al. (2009) in X-rays, Kann et al. (2010) in optical, and more recently Chandra & Frail (2012) in radio. The mm/submm range suffers strongly from atmospheric absorption, mostly due to water vapour. This means that observatories have to be placed in high and dry mountain sites. Nevertheless, the study of GRBs in these wavelengths is of great interest, as the peak (in flux density) of the synchrotron afterglow emission, and even the one due to the reverse shock (see Sect. 3.1 for a description of the emission mechanisms), is expected to be located in this wavelength range during the first days. It has also the benefit of being a wavelength range that is not affected by interstellar extinction, as are the optical or soft X-rays, allowing us to detect highly-extinguished and high-redshift bursts. This range is also normally above the self-absorption frequency of the spectrum, below which the flux is strongly suppressed. Finally, as compared to centimeter and decimeter wavelengths, this range is not affected by interstellar scintillation, allowing more accurate modelling with fewer observations.

Pioneering submm observations of GRB afterglows, made with JCMT/SCUBA (e.g. Smith et al. 1999, 2001) in the submm and with various facilities in the mm (e.g. Bremer et al. 1998; Galama et al. 2000), showed that emission in these wavelengths could be detected in some cases, and provided the first tests of the fireball model using full radio to X-ray SEDs (e.g. Wijers & Galama 1999). However, detections were generally of low significance, and progress in the Swift era was restricted until recently by the dearth of submm bolometer array instruments (following the retirement of SCUBA on JCMT, and until the advent of LABOCA on APEX).

On the 30th September 2011 ALMA came into operation, to revolutionise mm/submm astronomy. ALMA is at an altitude of 5000 m in the Llano de Chajnantor, in northern Chile, one of the best sites for this kind of observations in the world. In its early science phase it has 16 antennas of 12 m, and in full operations (expected for 2013) it will increase to a total of 50 antennas of 12 m in diameter plus a compact array (the Atacama Compact Array – ACA) of 12 7 m antennas and four 12 m antennas. This will be the largest facility for mm/submm astronomy and will improve sensitivity and spatial resolution in over an order of magnitude with respect to previous observatories.

In this paper we collect all the data, to our best knowledge, that have been published for GRBs in the mm/submm range, to which we add observations from our observing programmes from APEX and the SMA. The observations are completed with the first ALMA observation of a GRB, during the commissioning phase of the observatory, as a test of the target of opportunity procedures. Using this sample, we give a review of the past experience and put it into context of the forthcoming ALMA era. We give some examples of single-epoch spectral energy distributions but do not attempt full broadband modelling for individual bursts as it is beyond the scope of this paper.

Section 2 describes the data of 11 bursts collected with our observing programmes. Section 3 presents the catalogue of mm/submm GRB data. In Sect. 4 we describe the capabilities of ALMA compared with previous observatories and discuss its future role in the field. Finally, Sect. 5 lists our conclusions. Throughout the paper we assume a cosmology with Ωm = 0.30, ΩΛ = 0.70, and H0 = 70.

2. Observations

On March 2009 we performed the first observation of a GRB (GRB 090313) using the 12 m, single-dish, APEX telescope (in Chajnantor, Chile; Güsten et al. 2006) at 345 GHz, as a feasibility test, during a technical night through a Director Discretionary Time programme. Since then, we have had several observing programmes at APEX, with which we have followed-up 6 GRBs (plus an additional Galactic X-ray binary, initially identified as a GRB). All these observations were performed using the Large Apex BOlometer CAmera (LABOCA; Siringo et al. 2009), a multi-channel bolometer array with 295 elements designed for continuum observations, with a field of view (FOV) of and an angular resolution of 19.5 ± 1′′. Observations until the end of 2009 were performed using a spiral raster mapping, whereas later observations used the photometric mode. The mapping mode results in a fully sampled and homogeneously covered map which is less sensitive than the photometric mode (which lacks the spatial resolution) that was made available in 2010. We chose to use the photometric mode when it became available, as we are more interested in the sensitivity than in the spatial coverage, as our sources are unresolved. Data reduction of LABOCA/APEX observations was done using BoA1, as well as CRUSH and miniCRUSH (Kovács 2008) software packages.

Since November 2009 we have an observing project for GRB afterglows from the northern hemisphere at the 8-antenna SMA interferometer (Hawaii, USA) with which we have followed-up 4 other bursts. Observations from SMA were reduced using the MIR-IDL2 and MIRIAD (Sault et al. 1995) packages.

In the following, we provide details on each of our GRB observations. The data are listed, together with the complete catalogue in Table 1.

GRB 090313: This burst was a 78 s long event with a bright afterglow at a redshift of z = 3.3736 (de Ugarte Postigo et al. 2010c). After the discovery of a bright mm counterpart (Bock et al. 2009a), we obtained continuum observations at 345 GHz using LABOCA/APEX bolometer array during technical time. Data were acquired on 2009 March 17 and 24 under good weather conditions (zenith opacity values ranged from 0.24 to 0.33 at 345 GHz). Observations were performed using a spiral raster mapping, providing a fully sampled and homogeneously covered map in an area of diameter 12′, centered at the coordinates of the optical afterglow of GRB 090313. The total on-source integration time of the two combined epochs was 4.6 h. Calibration was performed using observations of Saturn as well as IRAS 09452+1330, HD 82385, G10.6-0.4, and IRAS 17574-2403 as secondary calibrators. The absolute flux calibration uncertainty is estimated to be 11%. The telescope pointing was checked every hour, finding a root mean square (rms) pointing accuracy of . The individual maps were co-added and smoothed to a final angular resolution of . We obtained a 3σ upper limit of 14 mJy for each of the two epochs and a limit of 9 mJy in the coadded maps (see Fig. 1). These data were presented, together with an analysis of the evolution of the afterglow emission by Melandri et al. (2010).

thumbnail Fig. 1

Observation of GRB 090313 with APEX (contours are 1, 2, 3 and 4σ detection levels) in March 2009 plotted over an optical X-shooter acquisition image showing the position of the afterglow. The field of view is 90′′ × 70′′. Only a field galaxy, unassociated with the GRB is detected, and we impose a 3-sigma upper limit of 9 mJy for the afterglow (adapted from de Ugarte Postigo et al. 2010c; Melandri et al. 2010).

GRB 091102: This was the first burst that we observed during our regular programme time. It was a 6.6 s long burst, for which no credible optical counterpart was identified and no redshift was obtained. LABOCA/APEX observations were performed on 2009 November 3 under average weather conditions (zenith opacity values ranged from 0.4 to 0.5 at 345 GHz). The total on-source integration time was  ~2 h. Calibration was performed using observations of Uranus and Neptune. The absolute flux calibration uncertainty is estimated to be  ~8%. The telescope pointing was checked every hour towards the source PMNJ0450-8100. We obtained a 3σ point source sensitivity of 20 mJy.

GRB 091127: This was a nearby burst (z = 0.49044; Vergani et al. 2011) with a duration of 7.1 s and a bright optical counterpart that we followed with LABOCA/APEX on 2009 Nov. 28 and 29 at 345 GHz. Weather conditions were average, with zenith opacity values ranging from 0.4 to 0.63 at 345 GHz. Observations were performed using a spiral raster mapping. The total on-source integration time of the two combined epochs was 6.4 h (2.9 and 3.5 h in the first and second epoch, respectively). Pointing was checked regularly on J0050-095 and J0145-276. Calibration was performed using observations of Uranus and the secondary calibrator NGC 2071 IR. The absolute flux calibration uncertainty is estimated to be about 15%. We obtained 3σ upper limits of 14.9 mJy and 13.2 mJy for each of the two epochs, respectively, and 9.6 mJy for the combined epochs. An analysis of the afterglow emission of this burst, including the APEX data, is presented by Vergani et al. (2011).

thumbnail Fig. 2

Observations of GRB 100418A obtained with SMA during the first 5 days after the GRB (adapted from de Ugarte Postigo et al., in prep.).

GRB 100418A: On April, 18th 2010, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100418A. It was an intermediate duration burst (7.0 s) at a redshift of z = 0.62 (Antonelli et al. 2010). It had a peculiar optical light curve peaking several hours after the event. We performed observations of the afterglow using 7 out of the 8 SMA antennas, starting  ~16 h after the burst. Weather conditions were good, with zenith opacities at 225 GHz of τ ~ 0.06 (precipitable water vapour, PWV  ~  1 mm). Titan and Neptune were used as flux calibrators and 3C 454.3 as bandpass calibrator. Atmospheric gain was corrected from observations of the nearby quasar J1751+096 every 15 min. Using these data we discovered a bright counterpart (Martin et al. 2010) at a flux of 13.40 ± 1.60 mJy (see Fig. 2). At the time of the discovery, this was the second brightest mm/submm counterpart detected (after GRB 030329, which peaked at  ~70 mJy; Sheth et al. 2003; Resmi et al. 2005). Observations continued over the following 4 nights, tracking the evolution of the afterglow until it became undetectable on April 23rd (de Ugarte Postigo et al., in prep.).

GRB 100814A: This was a very long burst (150 s) with a bright counterpart at a redshift of z = 1.44 (O’Meara et al. 2010). Continuum observations at 345 GHz were carried out using LABOCA/APEX. Data were acquired on August 15, 2010, starting 26 h after the burst, under good weather conditions (zenith opacity value was 0.28 at 345 GHz). Observations were performed using the photometry mode. The total on-source integration time was 1.5 h. Pointing was checked regularly on J0145-276. Calibration was performed using observations of Uranus and the secondary calibrator V883-ORI. The absolute flux calibration uncertainty is estimated to be about 15%. The formal flux measured at the position of the afterglow was −0.33 ± 1.6 mJy/beam, i.e. a 3σ limit of 4.8 mJy. A radio counterpart was detected by Chandra et al. (2010) several days later from EVLA.

GRB 100901A: This was a bright event, at a redshift of z = 1.41 (Chornock et al. 2010). Observations were carried out with SMA on the 3rd of September 2010. All eight antennas, arranged in the extended array configuration, were tuned to 345.8 GHz. Weather was excellent with a zenith opacity at 225 GHz of τ ~ 0.06 (precipitable water vapour, PWV  ~ 1 mm). Uranus and Callisto were observed as flux calibrators and 3C 454.3 as bandpass. Atmospheric gain calibration was derived from the observations of the nearby quasars J0237+288 and J0319+415 every 15 min. The GRB was observed for  ~10 h, and the observations resulted in a non detection, being the GRB flux constrained down to an rms noise of  ~0.75 mJy, which gives the deepest limit in our sample. A radio counterpart was later detected by WSRT (van der Horst et al. 2010c,b) and EVLA (Chandra & Frail 2010).

GRB 100925A/MAXI J1659-152: On 2010 September 25 we responded to a GRB alert using APEX. Observations were performed 15.5 h after the burst onset and a bright submillimeter afterglow was detected, with a flux density of 12.6 ± 2.4 mJy (de Ugarte Postigo et al. 2010d). Later spectroscopic observations performed with the X-shooter spectrograph at the Very Large Telescope, showed that the source was not due to an extragalactic GRB but to an unusually energetic burst from an X-ray binary (de Ugarte Postigo et al. 2010b). Thanks to the detection from APEX, an extensive radio observation campaign was triggered (van der Horst et al. 2010a, including further observations from APEX).

GRB 110422A: Observations of this burst at z = 1.770 (Malesani et al. 2011; de Ugarte Postigo et al. 2011a) were carried out with the SMA on May 6th, 2011 in search for an emission tentatively detected by the Herschel Space Observatory (Huang et al. 2011). The seven available antennas were tuned to 233.7 GHz. Weather was bad but stable enough with zenith opacity at 225 GHz was τ ~ 0.35 (PWV  > 5 mm). J0721+713 was used as flux calibrator and 3C 273 as bandpass. Atmospheric gain calibration was derived from J0721+713 quasar observations every 15 min. The GRB was observed for 4 h, reaching an rms noise of  ~2.8 mJy but obtained no detection.

GRB 110503A: Observations of this z = 1.613 (de Ugarte Postigo et al. 2011b) burst were carried out with the SMA on May 4th, 2011. The five available antennas were tuned to 225.0 GHz. Weather was mediocre but stable with zenith opacity at 225 GHz was τ ~ 0.25 (PWV  > 4 mm). J0927+390 was used as flux calibrator and 3C273 as bandpass. Atmospheric gain calibration was derived from J0927+390 and J0920+446 quasar observations every 15 min. The GRB was observed for 7 h reaching an rms noise of  ~1.7 mJy.

GRB 110709B: This was a very peculiar burst, that showed two gamma-ray triggers over a period of almost 15 min (Barthelmy et al. 2011). Early optical observations showed that it was a very dark event (Fong & Berger 2011), with no credible afterglow detected or redshift measured. We performed submm observations with APEX to try to localise the counterpart and to constrain the synchrotron spectrum. Observations began on July 11, 2.00 days after the burst, with an on-source time of 106 min and precipitable water vapour of 0.8 mm. We did not detect any significant emission, obtaining a formal flux on the source position of 1.4 ± 2.3 mJy (3-sigma limit of 6.9 mJy). A detection of a radio counterpart at 5.8 GHz was later reported by Zauderer & Berger (2011).

GRB 110715A: This burst had a very bright optical counterpart in spite of being located very close to the plane of the Milky Way from which it suffered from significant dust extinction. Its redshift was measured to be z = 0.82 (Piranomonte et al. 2011). APEX observations began on July 16, 1.42 days after the burst and were performed using the photometric mode. The weather conditions were very good, with a precipitable water vapor of 0.62 mm. Using these observations we discovered a bright submm counterpart at 10.4 ± 2.4 mJy (de Ugarte Postigo et al. 2011e).

As a test of the target of opportunity procedure, GRB 110715A was subsequently observed at Atacama Large Millimeter Array (ALMA). The ALMA Science Team report a preliminary detection from a test observation of this source of 4.9 ± 0.6 mJy at 345 GHz after 25 min on source with 7 antennas. The centroid of the ALMA position is 15:50:44.05 –46:14:06.54 with an uncertainty of at a position angle of 76 degrees. Observations began on July 19 at 02:50 UT (3.57 days after the burst). The weather conditions were very good, with a precipitable water vapor of 0.5 mm. In spite of being obtained during a test observation, with an order of magnitude fewer antennas than will be available with the full observatory and for only 25 min, this is the deepest observation in the complete sample at 345 GHz and provides the most accurate coordinates available for this burst. A complete analysis of the afterglow emission of this burst, will be presented by Sanchez-Ramirez et al. (in prep.).

GRB 110918A: This burst, at a redshift of z = 0.98 (Levan et al. 2011) was one of the brightest GRBs ever detected in gamma-rays, and the brightest ever observed by Konus/WIND (Golenetskii et al. 2011). The large error box generated just with Konus/WIND data did not allow us to observe until 2 days after, when the coordinates were refined thanks to the detection of X-ray (Mangano et al. 2011) and optical counterparts (Tanvir et al. 2011). Continuum observations at 345 GHz were carried out using LABOCA/APEX. Data were acquired on 2011-09-21 between 03:02 and 05:46 under excellent conditions (zenith opacity value was 0.2 at 345 GHz). Observations were performed using the photometry mode. The total on source integration time was 2.41 h. Pointing was checked regularly on J0145-276. Calibration was performed using observations of Uranus and Neptune and and the secondary calibrator V883-ORI. The absolute flux calibration uncertainty is estimated to be about 15%. Because of technical issues, the data did not reach the theoretical noise level, being no flux measured at the position of the afterglow and the upper 3-σ limit was 15 mJy.

3. The pre-ALMA millimeter/submillimeter sample of GRBs

Table 2

Definition of the ALMA bands.

Table 3

Detection ratios in the catalogue for each of the observing bands, considering only 3σ or higher significance detections.

thumbnail Fig. 3

Light curves of GRB afterglows in the different bands (see Table 2 for a definition of the bands). Coloured dots indicate detections, while gray triangles are 3σ detection limits. Observations of an individual burst are connected by a thick coloured line in case of detections and a thin dotted line in case of detection limits. GRBs with only one observation are shown as individual symbols with no connecting lines. Detections of GRB 010222 in bands 6 and 7 are due to the host galaxy and not the afterglow. The detection of XT 080109 in band 6 is due to the host galaxy.

thumbnail Fig. 4

Histograms of peak fluxes and 3σ upper limits in each of the three main bands in our sample. Detection limits for early and full ALMA (3σ, derived from the rms calculated in Table 5) are indicated with vertical dotted and dashed lines, respectively. The dashed-dotted line indicates the median detection limit of the early observations (see Table 4).

In order to put our observations into context, we have collected, in Table 1, the most complete sample of continuum observations that have been published to date of GRB afterglows and their host galaxies in the ALMA wavelength range, covering from early 1997 until the 30th of September 2011 (starting day of the early science operations with ALMA). The data are ordered chronologically by GRB, considering as detections only those with higher significance than 3σ, and otherwise providing detection limits. The observations are separated into ALMA bands 3 (84–116 GHz), 4 (125–163 GHz), 6 (211–275 GHz), 7 (275–373 GHz) and 9 (602–720 GHz). See Table 2 for a definition of all the ALMA bands3, which we will use throughout the paper for simplicity. The complete sample includes observations of 102 bursts. There have been 88 searches for GRB afterglows, of which 22 have reports of detections. Separating into individual bands, we have 18 detections in band 3, one in band 4, six in band 6, five in band 7 and none in band 9. Table 3 displays a list of the detection ratios for each of the observing bands. As for the host galaxies, there have been specific host galaxy searches for 36 cases, although limits can be provided for the 102 bursts that have been followed. Host galaxy detections have only been achieved in four cases: GRB 000210, GRB 000418, GRB 010222 and XT 080109, this last one cannot be considered a normal GRB as the burst was in X-rays and not in gamma (Soderberg et al. 2008). There have been further claims of host detections but we consider only those with signal to noise ratio greater than 3.

Figure 3 shows a compilation of the mm/submm GRB afterglow light curves for ALMA bands 3, 6 and 7. Figure 4 shows histograms of the peak detections and detection limits. The peak detections are, strictly speaking, only lower limits to the peak flux density in each band, as only a few light curves cover the maximum of the light curve. The detection limits are the single-epoch deepest 3σ limits for each burst, independently of the time of the observation, so they can only be indicative of the capabilities of pre-ALMA observatories.

The afterglow peak flux for each observing band in the mm/submm range is expected to be very similar, with only the time at which the peak is reached being different. According to the fireball model (Sari et al. 1998, see aslo Sect. 3.1), the peak is expected to cross from higher to lower energies, so that the higher frequency bands are expected to peak first. Hence, the ratio of afterglow detections is mostly determined by the different observing sensitivities of each of the telescopes in each band, given a reasonably prompt reaction time. Table 4 displays the median time between the burst onset and the first observation for the bursts in the sample, together with the median 3σ limiting flux of the earliest observations. We do not include Band 4, as it has only 2 observations and the statistics are not significant.

Table 4

Median time between the burst and the first observation in each band and median limiting flux of those first observations (3σ).

3.1. The physics of GRBs and their environments

GRB afterglows can be described, in the simplest case, using the fireball model (Sari et al. 1998). According to this model, material is ejected at ultrarelativistic velocities through collimated jets (with opening angle θj). When this material interacts with the medium surrounding the progenitor, the accelerated particles emit a synchrotron spectrum that is characterised by three break frequencies: νm is the characteristic synchrotron frequency and is the maximum of the emission; νc is the cooling frequency, above which radiative cooling is significant; νa is the synchrotron self-absorption frequency.

The location of the spectral breaks at a given time, and the spectral slopes, shown in Fig. 5, are determined by the following five parameters: E – isotropic equivalent kinetic energy of the shock; n – particle number density in the surroundings of the GRB, which can be considered constant or following a stellar wind profile; ϵB – fraction of the shock energy that goes into magnetic energy density; ϵe – fraction of the shock energy that goes to electron acceleration; p – slope of the relativistic electron energy distribution in the shock. Detailed multi-wavelength modelling of the spectral energy distribution allows us to derive these micro- and macro-physical parameters of the emission.

thumbnail Fig. 5

The synchrotron afterglow spectrum expected from a simple fireball model in the slow-cooling regime, assuming a constant density environment and spherical expansion. The figure shows the characteristic frequencies, their evolution and the spectral slopes of the spectrum (adapted from Sari et al. 1998).

thumbnail Fig. 6

Light curves of what could be a typical GRB afterglow using the standard fireball model, plotted for the different ALMA bands and at different redshifts.

Figure 6 shows an example of what we would expect the light curves of a typical GRB to look like in the different ALMA bands and at different redshifts using the standard fireball model. It assumes an homogeneous jet, adiabatic dynamics (based on Rhoads 1999), constant density environment, and integration over equal arrival time surfaces. The physical parameters of the fireball are the following: E = 1053 erg s-1, n = 0.1 cm-3, ϵB = 0.01, ϵe = 0.088, p = 2.1 and θj = 5 deg. The light curves are characterised by a shallow rise during the first hours to days after the burst followed by a decay, as seen in most of the light curves of Fig. 3, when there are enough data points. The figure also shows how the peak is reached later at lower frequencies for a given redshift, while it is delayed for high redshift events due to cosmological time dilation.

A reverse shock, produced inside the ejecta, can produce additional emission at early times (Piran 1999). This emission has been rarely observed in the optical wavelengths (e.g. Akerlof et al. 1999; Blake et al. 2005; Jelínek et al. 2006; Racusin et al. 2008; Zheng et al. 2011) but is expected to have a significant contribution in the early mm/submm emission (Inoue et al. 2007). For example, the mm detection of GRB 090423, at a redshift of 8.2, seems to show excess emission possibly due to a reverse shock (Castro-Tirado et al. 2009b; Chandra et al. 2010).

This model is, however, known to be too simplistic to explain the complex evolution that we can see in the densely sampled X-ray and optical light curves of Swift GRBs. Several modifications have been suggested to explain some of the observed fluctuations, flares, bumps and wiggles. Such modifications may include different density profiles and fluctuations (Wang & Loeb 2000; Ramirez-Ruiz et al. 2001; Dai & Lu 2002; Nakar & Piran 2003), energy injections (Rees & Meszaros 1998; Sari & Mészáros 2000; Granot et al. 2003; Björnsson et al. 2004; Jóhannesson et al. 2006), jets with complex structure (Mészáros et al. 1998; Kumar & Piran 2000; Rossi et al. 2002), late engine activity (Dai & Lu 1998; Zhang & Mészáros 2002; Ramirez-Ruiz 2004), microlensing (Loeb & Perna 1998; Garnavich et al. 2000; Ioka & Nakamura 2001) or dust echoes (Esin & Blandford 2000; Mészáros & Gruzinov 2000; Reichart 2001). Discerning the different scenarios is only possible through the analysis of the SEDs obtained with multi-wavelength observations (see Sect. 4) and, in some cases, with the aid of polarimetry.

3.2. Redshift distribution and luminosities

In spite of the strong limitation imposed by the detection limits of current observatories, the sample includes detections of GRBs at most redshifts, ranging from z = 0.168 (GRB 030329 Greiner et al. 2003) all the way up to z = 8.2 (GRB 090423 Tanvir et al. 2009; Salvaterra et al. 2009). XT 080109 at a redshift of 0.0065 (Malesani et al. 2008) had no gamma-ray emission, so we will exclude it from the analysis. This shows that GRBs can be great tools to study the evolution of the star formation across the complete history of the Universe also in this frequency range. Figure 7 shows the distribution of the redshifts of GRBs detected in the mm/submm range as compared to the distribution obtained from optical observations (Jakobsson et al. 2012). The mm/submm sample has a larger percentage of low redshift detections but, on the other hand, the average redshift was compensated by deep searches of the high redshift events. We note that the optical sample has its own biases, for example the limited capability to detect dark bursts (see Sect. 4.1 for a definition of dark burst). A Kolmogorov-Smirnov test gives a probability of 11% for both mm/submm detected bursts and the Swift sample of GRBs with redshift of coming from the same population. We obtain a median (mean) redshift for our sample of detections of 1.48 (1.99), as compared to 1.92 (2.19) of the sample of Swift bursts (Jakobsson et al. 2012).

thumbnail Fig. 7

The cumulative fraction of GRBs as a function of redshift for the optical sample of Swift GRBs (dotted curve; Jakobsson et al. 2012) and mm/submm-detected bursts (solid curve).

thumbnail Fig. 8

Peak flux density measured in mm/submm (as in Fig. 4) vs. redshift. Filled dots indicate detections whereas empty triangles are detection limits. The dotted lines indicate the flux density levels for equal luminosity objects at varying redshifts. Some interesting bursts are indicated in the figure.

In Fig. 8 we show the peak flux density measured in the mm/submm range as a function of the redshift. The dotted lines indicate the expected flux at different redshifts for objects of equal peak luminosities. The figure shows that the brightest burst ever detected, GRB 030329 (Hjorth et al. 2003; Stanek et al. 2003; Lipkin et al. 2004; Sheth et al. 2003; Resmi et al. 2005), is mainly so due to its low redshift. GRB 090423, the furthermost detected in mm/submm, at a redshift of z = 8.2 (Salvaterra et al. 2009; Tanvir et al. 2009), was not extremely luminous, and was only detected thanks to very deep observations (it is, in fact, the deepest detection in the sample; Castro-Tirado et al., in prep.). GRB 080319B, also known as the “naked-eye burst” due to its extreme optical brightness (Racusin et al. 2008; Bloom et al. 2009; Pandey et al. 2009) is found in the faint end of the distribution, indicating that most of the emission was concentrated in the optical bands.

On the upper part of the diagram, we identify a family of very luminous events, peaking at  ~1033erg s-1Hz-1, composed by GRB 980329 (Smith et al. 1999; Jaunsen et al. 2003), GRB 050904 (Tagliaferri et al. 2005; Haislip et al. 2006), GRB 080129 (Greiner et al. 2009) and GRB 090313 (Melandri et al. 2010), all beyond redshift 3. If those bursts were to be found at redshift of  ~1 they would reach peak fluxes of 40 mJy and easily over 100 mJy at redshifts of  ~0.5.

On the fainter side, GRB 060218 (z = 0.03) is be the burst for which we would have the most constraining peak luminosity limits. However, this burst is known to be peculiar, being its optical emission dominated by a supernova component and not the afterglow (Campana et al. 2006; Soderberg et al. 2006; see also Thöne et al. 2011). GRB 050509B an extremely faint short burst, most probably hosted by a giant elliptical galaxy at z = 0.22 (Hjorth et al. 2005; Gehrels et al. 2005; Castro-Tirado et al. 2005) is the following dimmest limit. The least luminous burst with detection in the mm/submm range is GRB 041219A, which curiously was one of the longest and brightest GRBs detected, for which a redshift of z = 0.31 has been recently suggested (Götz et al. 2011).

In the figure we also identify GRB 100418A and GRB 110715A, whose counterparts were discovered with our programmes. They are the 3rd and 4th brightest bursts ever detected in the mm/submm bands.

Using all the detections, we obtain an average peak spectral luminosity of 10(32.1 ± 0.7)erg s-1Hz-1. This is roughly an order of magnitude brighter than the value found by Chandra & Frail (2012) in cm wavelengths, as expected by afterglow models. However, we note that these values must be used with care, as both samples strongly biased due to a low detection rate (25% in our case and 30% in the case of the cm range study).

thumbnail Fig. 9

Afterglow light curves transformed to the rest frame of the central engine. The data points of the different bands have been shifted using an offset based on the fireball model, as explained in the text.

Finally, we collect the light curves of GRBs with at least 2 detections and a well determined redshift, and transform them to the rest frame of the central engine to compare them directly. In Fig. 9 we plot the light curves (only detections) in luminosity and corrected of cosmological time dilation. To be able to use all the bands together, we introduce a temporal offset assuming that the light curves are described by a fireball model in which the peak frequency is evolving with t−3/2 (see Sect. 3.1). This is a simplistic approach but enough for a qualitative view. The intrinsic light curves appear much more clustered than the observed ones in Fig. 3 where the dispersion in fluxes was mostly due to redshift: The brightest light curve was from GRB 030329, the nearest burst and the faintest from GRB 090423, the furthest one, whereas in Fig. 9 they are both normal luminosity bursts. On the other hand, the light curves appear to be reasonably well described by the expectations of the theory described in Sect. 3.1, reaching a maximum between a few hours and a few days and then decaying.

3.3. Estimation of the peak flux density distribution

We can try to make a rough estimate of the distribution of peak flux densities using the data from our sample and information extrapolated from other spectral ranges. We assume that the peak flux density of GRB afterglows can be described by a Gaussian function in the logarithmic space as is seen for the flux density at specific times in other wavelengths. This Gaussian can be normalised using the fact that in band 3 we are detecting 30% of the afterglows down to a limit of  ~0.9 mJy. The largest uncertainty here comes from the dispersion of the Gaussian. Using the complete catalogue of data from Kann et al. (2010) we find that the dispersion in the brightness of observed optical R-band magnitudes, corrected of extinction, at a specific time (1.0 days) is of 0.49, whereas for the X-rays, using the data from the burst analyser (Evans et al. 2010) find a dispersion of 0.56. Both values are quite similar, however, as the X-ray constitutes a more complete sample we will use this estimate. We must also consider that the sample of mm/submm follow-up mainly includes brighter bursts. If we calculate the average X-ray flux densities at one day of all the GRBs in the Swift sample and compare it with the subsample of the ones that were followed in mm/submm we find that the one first is dimmer by a factor of 1.7, which we apply to the average of the estimated distribution in mm/submm. Figure 10 shows a histogram with the detections and detection limits of all bands combined, together with the distribution of peak fluxes that has been estimated here, with an average of 0.3 mJy. Using these values we estimate that ALMA should be able to detect, using band 3, 87% of all the afterglows already in its early configuration with 16 antennas, and 98% in its full setup of 66. Using band 6, the values would be 72% (early) and 94% (full), and in band 7, 50% (early) and 82% (full).

thumbnail Fig. 10

Histogram showing the detections and detection limits of all the bands combined and an estimate of the real peak flux density distribution (indicated with a thick trace), compared to the detection limits of ALMA (from Table 5). The dashed-dotted Gaussian is the distribution derived from our sample, before correcting for the selection bias, as explained in the text.

3.4. Comparison with X-ray flux densities

Thanks to the X-Ray Telescope (XRT; Burrows et al. 2005) onboard Swift, nowadays we have early X-ray detections of the vast majority of GRB afterglows within the first minutes after the burst. These observations give us not only a precise position of the afterglow, but also a spectrum from which we can derive the unextinguished flux density, the spectral slope and the extinction due to metals in the line of sight. All this information can be used to estimate the expected flux in other wavelengths on a case to case basis assuming a synchrotron model as described in Sect. 3.1.

In this section we make a general comparison of the flux densities measured in the X-ray and mm/submm bands for those bursts that were observed by Swift. For the X-rays, we use the flux densities measured 0.5 days after the burst (when we can assume that the emission is dominated by the afterglow) at 10 keV (Evans et al. 2010). The scarce and heterogenous mm/submm coverage does not allow us to obtain a sample of flux densities at a given epoch. In consequence, we use, as an approximation the peak flux densities, as explained in Sect. 3. The two quantities are plotted in Fig. 11, where we can see, as expected, a correlation between the flux density measured in X-rays and in mm/submm, although with a significant dispersion.

thumbnail Fig. 11

Comparison of the peak flux density measured in mm/submm with the X-ray flux density 0.5 days after the burst (at 10 keV). Filled dots indicate detections, whereas empty triangles are 3σ detection limits. Dashed lines indicate some characteristic slopes of the fireball model. The dotted line is the average observed in the sample of detections and the dashed-dotted line is an estimation of the real average.

Evans et al. (2009) found that the typical spectral slope measured in X-rays was, on average βX ~ −1.0 (where Fν ∝ νβ), which would be the slope expected for an electron index of p = 2.0 if X-rays were above the cooling break (νc). The extrapolation of the X-ray measurement to mm/submm using this slope would give us the maximum flux that we could expect to measure for a synchrotron spectrum and is indicated in the figure by the upper dashed line. As expected, there is no GRB that has been measured above this extrapolation. On the other side, if νc was just below X-rays, the lowest extrapolated flux would be obtained using a spectral index of β = −0.5, also indicated in the figure. This is what Jakobsson et al. (2004) used to define the limit for dark bursts (see Sect. 4.1). In the case of mm/submm, due to the lack of extinction or due to dust or high redshift, we would only expect to find below this threshold those bursts where νm would be above the mm/submm range. In Fig. 11 we can see that all our detected bursts are located between these two limits, and mostly clustered around β = −0.75. On the other hand, the average flux density derived from X-rays 0.5 days after the burst (derived in a similar way as we did for 1 day in Sect. 3.3) at 10 keV is 1.04 × 10-5 mJy, whereas we estimated at typical peak flux density of 0.3 mJy. This would imply a typical βmm−X = 0.65, slightly lower than the typical value for detected bursts, as expected due to the abundant detection limits.

4. GRBs in the ALMA era

Table 5

Continuum flux density sensitivity (1σ) with 1 h on-source observations.

As shown in Figs. 3 and 4, until now the mm/submm observations of GRBs have been limited by the low sensitivity of the observations in this range as compared to other wavelengths. This is changing with the start of operations of the ALMA observatory. Even in its early configuration, ALMA will outperform the sensitivity of all previous observatories. Table 5 shows a comparison of the sensitivities (assuming equal observing conditions) of some of the current mm/submm observatories with ALMA, both in its early and final configuration. In its final setup, the sensitivities will improve between 1 and 2 orders of magnitude. If we consider that the atmospheric conditions at the ALMA site are commonly better than the average of other observatories, the difference will be even more significant.

However, it will not only be the sensitivity, but also the spatial resolution that will allow ALMA to make breakthroughs in GRB studies. In its extended configuration, ALMA in its full array capabilities will allow resolutions of 0.050′′ in band 3 or 0.013′′ in band 7 (4.8′′ and 1.7′′, respectively, in the most compact configuration), allowing precise localisation of the afterglows and detailed studies of the host galaxies.

The observation of GRB 110715A that we present here, obtained during the commissioning phase of ALMA, is an advance of what this observatory will be capable of. Already the commissioning phase it was proved that ALMA can reliably perform ToO observations. Furthermore, with only 25 minutes on target, it obtained the deepest observation of a GRB in the 345 GHz band to date and allowed us to derive the most precise coordinates that are available for this afterglow.

4.1. GRB afterglows

ALMA observations will play a significant role in understanding the physics of GRBs and their environment. The increased sensitivity will allow us to probe almost complete samples of afterglows, not just limited to the brightest events as we do now, eliminating most of the observational biases. In the case of very bright events ALMA will allow us to attempt, for the first time, studies of absorption spectral features, from which we will be able to derive the molecular content of distant galaxies and the chemical enrichment of the interstellar medium along the history of the Universe.

Thanks to its large collecting area, ALMA will also be able to provide polarimetric measurements for a sample or relatively bright events. As shown by Toma et al. (2008), this can give a unique view on some fundamental physics of shockwaves (though ALMA polarimetric calibration is unlikely to get better than  ~1 percent). Furthermore, simple linear polarisation measurements in submm and optical, will tell us about the plasma properties in the forward shock and the geometry of the ejecta.

One of the key areas where ALMA will play an important role will be the study of dark bursts, which strongly bias the optically selected samples of GRBs (see for example Fynbo et al. 2009). Dark bursts are those that are less luminous in the optical than what we would expect them to be assuming a synchrotron spectrum as the one explained in Sect. 3.1 based on the extrapolation of X-ray observations. Jakobsson et al. (2004) define them assuming that the electron index is p2 (Sari et al. 1998), and that νm is below optical frequencies (as is normally the case). Under this assumption, we would never expect a synchrotron spectrum with a slope between optical and X-rays βOX > −0.5 (note that in this paper we use the convention Fν ∝ νβ as in Sari et al. 1998, β having opposite sign to the one used by Jakobsson et al. 2004). This definition was extended by van der Horst et al. (2009) not limiting the value of p and, instead, requiring βOX > βX + 0.5 (being βX the spectral slope measured in X-rays), given that the X-ray spectral slope is normally well constrained. This reduced optical emission can be due to optical extinction by dust, a high-redshift event (where the Lyman break is beyond the optical, at z5) or a deviation from the simple synchrotron spectrum (like an additional inverse Compton component or a distribution of the emitting electrons not described by a single power law). Below we give an example of each of the two main families of dark bursts.

Optically selected samples of GRBs are limited by the amount of extinction in the host galaxy that, if large, can attenuate the optical emission and make it undetectable. Hence, there is a strong bias against high extinction bursts, which limits our capability to study the star forming regions where GRBs are produced. However, the negligible effect of dust extinction in the mm/submm range allows us to detect afterglows independently of this effect, and consequently study a more complete sample. As an example of highly extinguished burst, we can look at the case of GRB 051022, with βOX > 0.05 (Castro-Tirado et al. 2007; Rol et al. 2007) one of the darkest bursts detected to date, for which an optical counterpart was not found. Precise location was obtained thanks to mm observations (Cameron & Frail 2005; Bremer et al. 2005; Castro-Tirado et al. 2007), which allowed identification of the host galaxy, for which a redshift of z = 0.8 was derived (Castro-Tirado et al. 2007). A broadband study allows us to impose a lower limit on the host galaxy extinction of AV > 15 (see Fig. 12, and for other estimates also Castro-Tirado et al. 2007; Rol et al. 2007), far beyond what is typically seen in optically selected GRB afterglows.

thumbnail Fig. 12

SED of the afterglow of the dark GRB 051022. Filled dots mark detections while empty dots and arrow mark detection limits. In blue we draw a synchrotron spectrum that can explain the radio and X-ray emission, while the optical/nIR data (1014−15 Hz) lie well bellow. Adding a Small Magellanic Cloud (SMC) extinction of AV ~ 15 results in a SED (red dotted line) consistent with all the data (data obtained from Cameron & Frail 2005; Bremer et al. 2005; Castro-Tirado et al. 2007).

thumbnail Fig. 13

SED of GRB 050904 at redshift z = 6.29 (Kawai et al. 2006) using the detection from PdBI 5.896 days after the burst (Castro-Tirado et al., in prep.). The X-ray flux is estimated from the XRT light curve, the radio data are from Frail et al. (2006), while optical/NIR are extrapolations from Tagliaferri et al. (2005); Haislip et al. (2006). For the rough synchrotron spectrum drawn in blue, we have assumed an electron energy distribution index of p = 2.30, while an SMC extinction with AV = 0.12 (consistent with the value derived by Stratta et al. 2011; but see also Zafar et al. 2010) and Lyman blanketing (Madau 1995) have been considered in the red dotted line to match the optical/NIR observations. With these data we can constrain the position of νa < 2 × 1011 Hz and 2 × 1011 < νc < 1013 Hz.

The other main cause for optically dark GRBs is a high redshift. In these cases the absorption produced at frequencies higher than the Lyman limit does not allow us to obtain optical detections (in R-band) of GRB afterglows beyond redshifts of 6. These events are important to understand the formation of the first stars in the Universe. Proof that they can be detected in the mm/submm range is the fact that already in the pre-ALMA era, out of the three GRB afterglows observed at z > 6, two have been detected (GRB 050904 at z = 6.3 and GRB 090423 at z = 8.2). In Fig. 13 we have drawn the SED of GRB 050904 at z = 6.3, including radio limits (Frail et al. 2006), a detection in band 3 (and a limit in band 6) from Plateau de Bure Interferometer (Castro-Tirado et al., in prep.), near-infrared and optical data (Tagliaferri et al. 2005; Haislip et al. 2006) and X-ray data from Swift. In spite of its high redshift, this burst was detected with a peak flux density of 1.47 mJy in band 3, 80 times above ALMA’s detection threshold for 1 h of observations in band 3. In the case of bright events it has been suggested that the study of the HD deuterium-molecule rotational-transition in absorption at 112 μm could give us important clues on the formation of population III stars at very high redshift (Inoue et al. 2007). CO absorption lines, which have been already reported through optical spectroscopy of the afterglow of GRB 080607 (Prochaska et al. 2009; Sheffer et al. 2009) are also good candidates to produce absorption features in the mm/submm range.

4.2. GRB host galaxies with ALMA

Up to now the majority of GRB hosts has not been detected at mm/submm wavelengths (Tanvir et al. 2004; Berger et al. 2003; Priddey et al. 2006; Watson et al. 2011) even when dark GRBs (which are supposed to be dusty) were targeted (Barnard et al. 2003). This was interpreted as an indication that they are not heavily dust-obscured. Except for the host of the local GRB 980425 (Michałowski et al., in prep.), only three z ~ 1 GRB hosts (GRB 000210, GRB 000418 and GRB 010222) were detected at mm and submm wavelengths (Frail et al. 2002; Berger et al. 2003; Tanvir et al. 2004). Their derived dust masses of a few times 108M⊙ (Michałowski et al. 2008) are comparable to that of z ~ 2–3 dusty starbursts known as submm galaxies (Kovács et al. 2006; Michałowski et al. 2010). Despite these significant amounts of dust, these GRB hosts exhibit properties consistent with the rest of the GRB host population, namely blue optical colours, low extinction and low optical star formation rates (Gorosabel et al. 2003a,b; Galama et al. 2003; Berger et al. 2003; Savaglio et al. 2003, 2009; Christensen et al. 2004; Castro Cerón et al. 2006, 2010; Kann et al. 2006). This discrepancy was explained by Michałowski et al. (2008) by invoking very young stellar population with optical emission completely extinguished by dust. This indicates that it is virtually impossible to predict the dust emission of GRB hosts based on their optical properties and advocates for the use of ALMA to target a significant and unbiased sample of these galaxies.

Despite numerous attempts, CO emission has not been detected for any GRB host galaxy (Kohno et al. 2005; Endo et al. 2007; Hatsukade et al. 2007, 2011; Stanway et al. 2011); we refer to Table 1 in Hatsukade et al. (2011) for a homogeneous overview of all available limits on CO line luminosities of GRB hosts. The derived upper limits on molecular gas masses of 108–109M⊙ indicate that GRB hosts are not as gas-rich as submm galaxies (Greve et al. 2005; Ivison et al. 2011).

We further note that the recent discovery of strong CO and H2 features in absorption in an optical spectrum of the afterglow of GRB 080607 (Prochaska et al. 2009; Sheffer et al. 2009) highlights the value of ALMA molecular line observations in tandem with optical spectroscopy. The detection of vibrationally excited transitions (H) in the afterglow of GRB 080607 (Sheffer et al. 2009), likely excited by the UV flux of the afterglow, show the importance of repeated spectroscopic observations by ALMA to probe the ISM properties of the host galaxies through time variable lines.

thumbnail Fig. 14

Spectral lines observable at each ALMA band as a function of the redshift. Different colours have been used to indicate different elements.

The unprecedented sensitivity of ALMA in the mm and sub-mm wavelengths, will allow the study of GRB host galaxies through both continuum and emission line observations. ALMA is sensitive enough to probe the dust properties of representative GRB hosts, not only the most star-forming ones. Moreover, the excellent angular resolution of ALMA will allow spatially-resolved studies of these galaxies. This is important as there are indications that the immediate environments (1–3 kpc) of GRBs are the most star-forming sites within their host galaxies (Bloom et al. 2002; Fruchter et al. 2006; Thöne et al. 2008; Östlin et al. 2008; Michałowski et al. 2009).

To date, measured spectroscopic GRB redshifts range between 0.0085 and 8.2. Figure 14 shows a number of spectral features that will be covered by the different ALMA bands for a range of redshifts between z = 0 up to z = 10. Taking into account the weather constraints at the highest bands, we consider that bands 3 to 7 will be better suited for host detections. For the nearer events (z4) the carbon monoxide ladder of rotational transitions up to J = 11–10 could be feasible as reported for the submillimeter galaxy SMM J16359+6612 (at z = 2.52; Weiß et al. 2005b) and the broad absorption line quasar APM08279+5255 (at z = 3.87; Weiß et al. 2007). Additionally, the fine structure transitions of neutral carbon at 492 and 809 GHz were also detected towards APM08279+5255 (Weiß et al. 2005a; Wagg et al. 2006). Moreover, [CI] at 809 GHz and CO J = 7–6, separated  ~2.4 GHz in the rest frame, could be observed simultaneously for redshifts of z ~ 2–8 provided the large bandwidth of ALMA receivers. At redshifts above z = 4–5, the higher-J CO transitions will still be visible mostly within bands 3 to 5. Above that redshift, the fine structure lines of the most abundant elements, O, C, and N, will be the best suited ISM tracers. In particular the fine structure line of [CII] at 158μm is likely the brightest emission line observable at high-z (Maiolino et al. 2009). The lines of [OIII] at 52 μm and 88 μm and [OI] at 63 μm have also been reported towards high-redshift lensed systems (Ferkinhoff et al. 2010; Sturm et al. 2010).

5. Conclusions

This paper presents the most complete catalogue of GRB observations in the mm/submm range to date, including observations of 11 GRBs from our programmes at APEX and SMA. We include a short observation of a GRB obtained with the ALMA observatory during its commissioning phase and with only 7 antennas, that gives an early idea of the potential of the observatory. The total sample contains data from 102 GRBs. Among this collection of observations, there are 88 observations of GRB afterglows, of which 22 (25%) were detected (two of them discovered within our programmes) and 35 of host galaxies, of which 4 were detected. Observations until now have been strongly limited by the sensitivity of the instruments in this range, imposing important biases to any statistical study performed with them. Even with these limitations, we have proved that it is already possible to detect counterparts to GRBs in the mm/submm range with redshifts ranging from 0.168 all the way up to 8.2. This spectral range has also proved to be a very useful tool to localise and characterise the counterparts of dark GRBs, which are elusive in optical wavelengths.

Within the sample bursts have peak luminosities spanning over 2.5 orders of magnitude, with the most luminous reaching 1033erg s-1Hz-1. If such a burst happened at redshift of  ~1 they would reach peak fluxes of 40 mJy and over 100 mJy at redshifts of  ~0.5. Comparing with X-rays, we find a correlation of the peak flux density in mm/submm with the X-ray flux density at 0.5 days that can serve to make rough predictions of mm/submm brightness from the early X-ray data. Using data from the sample and assumptions based on samples at other wavelengths, an estimate of the real peak flux density distribution of GRBs is made, from which an average peak flux density value of 0.33 mJy can be expected.

On the 30th September 2011, the ALMA observatory started scientific operations with a limited setup of 16 antennas. Already then, it became the most sensitive observatory in the mm/submm range, increasing the sensitivity and resolution by around an order of magnitude with respect to the previous instrumentation, being even more significant in the highest frequencies. The final full observatory, with 66 antennas will set completely new standards in the field. In addition to the improved sensitivity and resolution, ALMA will be capable of observing in a very wide range of frequencies, that was never covered before by a single observatory. Using our peak flux density estimates and the detection limits calculated for ALMA, we can expect that early ALMA will detect around 87% of the bursts, while full ALMA should be able to detect 98%. In the case of bright GRB afterglows, like GRB 030329, GRB 100621A, GRB 100418A or GRB 110715A, where fluxes reached tens of mJy, ALMA will be able to study spectral features and perform polarimetric studies of the afterglow, which have been out of reach until now.

With ALMA we will be, for the first time, in position to undertake studies of samples of GRB host galaxies. We will be able to perform studies of the continuum emission to characterise the dust content and determine the unextinguished star formation rate of the hosts. Through the study of emission features from the host, we will be able to understand the molecular content and the chemical enrichment of the strong star-forming regions in which GRBs are found, at redshifts that go back to the epoch in which the first stars were formed.

Online material

Table 1

Pre-ALMA observations of GRBs in the millimeter and submillimeter range.

Acknowledgments

A.d.U.P. acknowledges support from ESO’s Scientific Visitor Programme in Chile, where a significant part of this work was done. The Dark Cosmology Centre is funded by the Danish National Research Foundation. The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. This paper makes use of the following ALMA Science Verification data: ADS/JAO.ALMA#2011.0.00006.SV. We are grateful for the support from the ALMA commissioning team. APEX is a collaboration between the Max-Planck-Institut fur Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. This paper made use of observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. We acknowledge partial support from the Spanish Ministry of Science and Innovation through programmes AYA20082008-03467/ESP and AYA2009-14000-C03-01. S.S. acknowledges support by a Grant of Excellence from the Icelandic Research Fund.

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All Tables

Table 2

Definition of the ALMA bands.

In the text
Table 3

Detection ratios in the catalogue for each of the observing bands, considering only 3σ or higher significance detections.

In the text
Table 4

Median time between the burst and the first observation in each band and median limiting flux of those first observations (3σ).

In the text
Table 5

Continuum flux density sensitivity (1σ) with 1 h on-source observations.

In the text
Table 1

Pre-ALMA observations of GRBs in the millimeter and submillimeter range.

In the text

All Figures

thumbnail Fig. 1

Observation of GRB 090313 with APEX (contours are 1, 2, 3 and 4σ detection levels) in March 2009 plotted over an optical X-shooter acquisition image showing the position of the afterglow. The field of view is 90′′ × 70′′. Only a field galaxy, unassociated with the GRB is detected, and we impose a 3-sigma upper limit of 9 mJy for the afterglow (adapted from de Ugarte Postigo et al. 2010c; Melandri et al. 2010).
In the text
thumbnail Fig. 2

Observations of GRB 100418A obtained with SMA during the first 5 days after the GRB (adapted from de Ugarte Postigo et al., in prep.).
In the text
thumbnail Fig. 3

Light curves of GRB afterglows in the different bands (see Table 2 for a definition of the bands). Coloured dots indicate detections, while gray triangles are 3σ detection limits. Observations of an individual burst are connected by a thick coloured line in case of detections and a thin dotted line in case of detection limits. GRBs with only one observation are shown as individual symbols with no connecting lines. Detections of GRB 010222 in bands 6 and 7 are due to the host galaxy and not the afterglow. The detection of XT 080109 in band 6 is due to the host galaxy.
In the text
thumbnail Fig. 4

Histograms of peak fluxes and 3σ upper limits in each of the three main bands in our sample. Detection limits for early and full ALMA (3σ, derived from the rms calculated in Table 5) are indicated with vertical dotted and dashed lines, respectively. The dashed-dotted line indicates the median detection limit of the early observations (see Table 4).
In the text
thumbnail Fig. 5

The synchrotron afterglow spectrum expected from a simple fireball model in the slow-cooling regime, assuming a constant density environment and spherical expansion. The figure shows the characteristic frequencies, their evolution and the spectral slopes of the spectrum (adapted from Sari et al. 1998).

In the text
thumbnail Fig. 6

Light curves of what could be a typical GRB afterglow using the standard fireball model, plotted for the different ALMA bands and at different redshifts.
In the text
thumbnail Fig. 7

The cumulative fraction of GRBs as a function of redshift for the optical sample of Swift GRBs (dotted curve; Jakobsson et al. 2012) and mm/submm-detected bursts (solid curve).
In the text
thumbnail Fig. 8

Peak flux density measured in mm/submm (as in Fig. 4) vs. redshift. Filled dots indicate detections whereas empty triangles are detection limits. The dotted lines indicate the flux density levels for equal luminosity objects at varying redshifts. Some interesting bursts are indicated in the figure.
In the text
thumbnail Fig. 9

Afterglow light curves transformed to the rest frame of the central engine. The data points of the different bands have been shifted using an offset based on the fireball model, as explained in the text.

In the text
thumbnail Fig. 10

Histogram showing the detections and detection limits of all the bands combined and an estimate of the real peak flux density distribution (indicated with a thick trace), compared to the detection limits of ALMA (from Table 5). The dashed-dotted Gaussian is the distribution derived from our sample, before correcting for the selection bias, as explained in the text.
In the text
thumbnail Fig. 11

Comparison of the peak flux density measured in mm/submm with the X-ray flux density 0.5 days after the burst (at 10 keV). Filled dots indicate detections, whereas empty triangles are 3σ detection limits. Dashed lines indicate some characteristic slopes of the fireball model. The dotted line is the average observed in the sample of detections and the dashed-dotted line is an estimation of the real average.
In the text
thumbnail Fig. 12

SED of the afterglow of the dark GRB 051022. Filled dots mark detections while empty dots and arrow mark detection limits. In blue we draw a synchrotron spectrum that can explain the radio and X-ray emission, while the optical/nIR data (1014−15 Hz) lie well bellow. Adding a Small Magellanic Cloud (SMC) extinction of AV ~ 15 results in a SED (red dotted line) consistent with all the data (data obtained from Cameron & Frail 2005; Bremer et al. 2005; Castro-Tirado et al. 2007).

In the text
thumbnail Fig. 13

SED of GRB 050904 at redshift z = 6.29 (Kawai et al. 2006) using the detection from PdBI 5.896 days after the burst (Castro-Tirado et al., in prep.). The X-ray flux is estimated from the XRT light curve, the radio data are from Frail et al. (2006), while optical/NIR are extrapolations from Tagliaferri et al. (2005); Haislip et al. (2006). For the rough synchrotron spectrum drawn in blue, we have assumed an electron energy distribution index of p = 2.30, while an SMC extinction with AV = 0.12 (consistent with the value derived by Stratta et al. 2011; but see also Zafar et al. 2010) and Lyman blanketing (Madau 1995) have been considered in the red dotted line to match the optical/NIR observations. With these data we can constrain the position of νa < 2 × 1011 Hz and 2 × 1011 < νc < 1013 Hz.

In the text
thumbnail Fig. 14

Spectral lines observable at each ALMA band as a function of the redshift. Different colours have been used to indicate different elements.

In the text

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