C. Clemens

The SEDs and Host Galaxies of the dustiest GRB afterglows

Context. The afterglows and host galaxies of long gamma-ray bursts (GRBs) offer unique opportunities to study star-forming galaxies in the high-z Universe. Until recently, however, the information inferred from GRB follow-up observations was mostly limited to optically bright afterglows, biasing all demographic studies against sight-lines that contain large amounts of dust. Aims. Here we present afterglow and host observations for a sample of bursts that are exemplary of previously missed ones because of high visual extinction (A GRB 1 mag) along the sight-line. This facilitates an investigation of the properties, geometry, and location of the absorbing dust of these poorly-explored host galaxies, and a comparison to hosts from optically-selected samples. Methods. This work is based on GROND optical/NIR and Swift/XRT X-ray observations of the afterglows, and multi-color imaging for eight GRB hosts. The afterglow and galaxy spectral energy distributions yield detailed insight into physical properties such as the dust and metal content along the GRB sight-line and galaxy-integrated characteristics such as the host’s stellar mass, luminosity, color-excess, and star-formation rate. Results. For the eight afterglows considered in this study, we report for the first time the redshift of GRB 081109 (z = 0.9787±0.0005), and the visual extinction towards GRBs 081109 (A GRB = 3.4 +0.4 −0.3 mag) and 100621A (A GRB V = 3.8 ± 0.2 mag), which are among the largest ever derived for GRB afterglows. Combined with non-extinguished GRBs, there is a strong anti-correlation between the afterglow’s metal-to-dust ratio and visual extinction. The hosts of the dustiest afterglows are diverse in their properties, but on average redder (� (R − K)AB �∼ 1.6 mag), more luminous (� L �∼ 0.9L ∗ ), and massive (� log M∗[M� ] �∼ 9.8) than the hosts of optically-bright events. Hence, we probe a different galaxy population, suggesting that previous host samples miss most of the massive and metal-rich members. This also indicates that the dust along the sight-line is often related to host properties, and thus probably located in the diffuse ISM or interstellar clouds and not in the immediate GRB environment. Some of the hosts in our sample, are blue, young, or of low stellar mass illustrating that even apparently non-extinguished galaxies possess very dusty sight-lines owing to a patchy dust distribution. Conclusions. The afterglows and host galaxies of the dustiest GRBs provide evidence of a complex dust geometry in star-forming galaxies. In addition, they establish a population of luminous, massive, and correspondingly chemically evolved GRB hosts. This suggests that GRBs trace the global star-formation rate better than studies based on optically selected host samples indicate, and that the previously claimed deficiency of high-mass hosts was at least partially a selection effect.

Optical and near-infrared follow-up observations of four Fermi/LAT GRBs: redshifts, afterglows, energetics, and host galaxies

Aims. Fermi can measure the spectral properties of gamma-ray bursts over a very large energy range and is opening a new window on the prompt emission of these energetic events. Localizations by the instruments on Fermi in combination with follow-up by Swift provide accurate positions for observations at longer wavelengths leading to the determination of redshifts, the true energy budget, host galaxy properties and facilitate comparison with pre-Fermi bursts. Methods. Multi-wavelength follow-up observations were performed on the afterglows of four bursts with high energy emission detected by Fermi/LAT: GRB 090323, GRB 090328, GRB 090510 and GRB 090902B. They were obtained in the optical/near-infrared bands with GROND mounted at the MPG/ESO 2.2 m telescope and additionally of GRB 090323 in the optical with the 2 m telescope in Tautenburg, Germany. Three of the events are classified as long bursts while GRB 090510 is a well localized short GRB with GeV emission. In addition, host galaxies were detected for three of the four bursts. Spectroscopic follow-up was initiated with the VLT for GRB 090328 and GRB 090510. Results. The afterglow observations in 7 bands are presented for all bursts and their host galaxies are investigated. Knowledge of the distance and the local dust extinction enables comparison of the afterglows of LAT-detected GRBs with the general sample. The spectroscopic redshifts of GRB 090328 and GRB 090510 were determined to be z = 0.7354 ± 0.0003 and z = 0.903 ± 0.001 and dust

The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C

Context. The detection of GeV photons from gamma-ray bursts (GRBs) has important consequences for the interpretation and modelling of these most-energetic cosmological explosions. The full exploitation of the high-energy measurements relies, however, on accurate knowledge of the distance to the events. Aims. Here we report on the discovery of the afterglow and subsequent redshift determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray Space Telescope with high significance detection of photons at energies >0.1 GeV. Methods. Observations were done with the 7-channel “Gamma-Ray Optical and Near-infrared Detector” (GROND) at the 2.2 m MPI/ESO telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4 m telescope in South Africa, and the GMOS instrument at Gemini-S. Results. The afterglow photometric redshift of z = 4.35 ± 0.15, based on simultaneous 7-filter observations with GROND, places GRB 080916C among the top 5% most distant GRBs and makes it the most energetic GRB known to date. The detection of GeV photons from such a distant event is unexpected because of the predicted opacity due to interaction with the extragalactic background light. The observed gamma-ray variability in the prompt emission, together with the redshift, suggests a lower limit for the Lorentz factor of the ultra-relativistic ejecta of Γ > 1090. This value rivals any previous measurements of Γ in GRBs and strengthens the extreme nature of GRB 080916C.

Supersolar metal abundances in two galaxies at z ∼ 3.57 revealed by the GRB 090323 afterglow spectrum★

We report on the surprisingly high metallicity measured in two absorption systems at high redshift, detected in the Very Large Telescope spectrum of the afterglow of the gamma-ray burst GRB090323. The two systems, at redshift z = 3.5673 and z = 3.5774 (separationv � 660 km s −1 ), are dominated by the neutral gas in the interstellar medium of the parent galaxies. From the singly ionized zinc and sulfur, we estimate oversolar metallicities of (Zn/H) = +0.29±0.10 and (S/H) = +0.67±0.34, in the blue and red absorber, respectively. These are the highest metallicities ever measured in galaxies at z > 3. We propose that the two systems trace two galaxies in the process of merging, whose star formation and metallicity are heightened by the interaction. This enhanced star formation might also have triggered the birth of the GRB progenitor. As typically seen in star-forming galaxies, the fine-structure absorption Siii ∗ is detected, both in G0 and G1. From the rest-frame UV emission in the GRB location, we derive a relatively high, not corrected for dust extinction, star-formation rate SFR � 6 M⊙ yr −1 . These properties suggest a possible connection between some high-redshift GRB host galaxies and high-z massive sub-millimeter galaxies, which are characterized by disturbed morphologies and high metallicities. Our result provides additional evidence that the dispersion in the chemical enrichment of the Universe at high redshift is substantial, with the existence of very metal rich galaxies less than two billion years after the Big Bang.

A Strong Optical Flare Before the Rising Afterglow of GRB 080129

We report on GROND observations of a 40 s duration (rest-frame) optical flare from GRB 080129 at redshift 4.349. The rise and decay times follow a power law with indices +12 and –8, respectively, inconsistent with a reverse shock and a factor 105 faster than variability caused by interstellar material interaction. While optical flares have been seen in the past (e.g., GRB 990123, 041219B, 060111B, and 080319B), for the first time, our observations not only resolve the optical flare into subcomponents, but also provide a spectral energy distribution (SED) from the optical to the near-infrared once every minute. The delay of the flare relative to the gamma-ray burst (GRB), its SED as well as the ratio of pulse widths suggest it to arise from residual collisions in GRB outflows. If this interpretation is correct and can be supported by a more detailed modeling or observation in further GRBs, the delay measurement provides an independent determination of the Lorentz factor Γ of the outflow.

A deep search for the host galaxies of gamma-ray bursts with no detected optical afterglow

Gamma-Ray Bursts can provide information about star formation at high redshifts. Even in the absence of a optical/near-infrared/radio afterglow, the high detection rate of X-ray afterglows by swift/XRT and its localization precision of 2-3 arcsec facilitates the identification and study of GRB host galaxies. We focus on the search for the host galaxies of a sample of 17 bursts with XRT error circles but no detected long-wavelength afterglow. Three of these events can also be classified as truly dark bursts: the observed upper limit on the optical flux of the afterglow was less than expected based on the X-ray flux. Our study is based on deep R and K-band observations performed with ESO/VLT instruments, supported by GROND and NEWFIRM. To be conservative, we searched for host galaxies in an area with a radius twice the 90% swift/XRT error circle. For 15 of the 17 bursts we find at least one galaxy inside the doubled XRT error circle. In seven cases we discover extremely red objects in the error circles. The most remarkable case is the host of GRB 080207 which as a colour of R-K~4.7 mag (AB), one of the reddest galaxies ever associated with a GRB. As a by-product of our study we identify the optical afterglow of GRB 070517A. Optically dim afterglows result from cosmological Lyman drop out and dust extinction, but the former process is only equired for a minority of cases (<1/3). Extinction by dust in the host galaxies might explain all other events. Thereby, a seemingly non-negligible fraction of these hosts are globally dust-enshrouded, extremely red galaxies. This suggests that bursts with optically dim afterglows trace a subpopulation of massive starburst galaxies, which are markedly different from the main body of the GRB host galaxy population, namely the blue, subluminous, compact galaxies.

The Swift/Fermi GRB 080928 from 1 eV to 150 keV

We present the results of a comprehensive study of the gamma-ray burst 080928 and of its afterglow. GRB 080928 was a long burst detected by Swift/BAT and Fermi/GBM. It is one of the exceptional cases where optical emission had already been detected when the GRB itself was still radiating in the gamma-ray band. For nearly 100 s simultaneous optical, X-ray and gamma-ray data provide a coverage of the spectral energy distribution of the transient source from about 1 eV to 150 keV. In particular, we show that the SED during the main prompt emission phase agrees with synchrotron radiation. We constructed the optical/near-infrared light curve and the spectral energy distribution based on Swift/UVOT, ROTSE-IIIa (Australia), and GROND (La Silla) data and compared it to the X-ray light curve retrieved from the Swift/XRT repository. We show that its bumpy shape can be modeled by multiple energy-injections into the forward shock. Furthermore, we investigate whether the temporal and spectral evolution of the tail emission of the first strong flare seen in the early X-ray light curve can be explained by large-angle emission (LAE). We find that a nonstandard LAE model is required to explain the observations. Finally, we report on the results of our search for the GRB host galaxy, for which only a deep upper limit can be provided.

The two-component jet of GRB 080413B

Aims. The quick and precise localization of GRBs by the Swift telescope allows the early evolution of the afterglow light curve to be captured by ground-based telescopes. With GROND measurements we can investigate the optical/near-infrared light curve of the afterglow of gamma-ray burst 080413B in the context of late rebrightening. Methods. Multi-wavelength follow-up observations were performed on the afterglow of GRB 080413B. X-ray emission was detected by the X-ray telescope onboard the Swift satellite and obtained from the public archive. Optical and near-infrared photometry was performed with the seven-channel imager GROND mounted at the MPG/ESO 2.2 m telescope and additionally with the REM telescope, both in La Silla, Chile. The light curve model was constructed using the obtained broad-band data. Results. The broad-band light curve of the afterglow of GRB 080413B is well fitted with an on-axis two-component jet model. The narrow ultra-relativistic jet is responsible for the initial decay, while the rise of the moderately relativistic wider jet near its deceleration time is the cause of the rebrightening of the light curve. The later evolution of the optical/NIR light curve is then dominated by the wide component, the signature of which is almost negligible in the X-ray wavelengths. These components have opening angles of θn ∼ 1.7 ◦ and θw ∼ 9 ◦ , and Lorentz factors of Γn > 188 and Γw ∼ 18.5. We calculated the beaming-corrected energy release to be Eγ = 7.9 × 10 48 erg.

The fast evolution of SN 2010bh associated with XRF 100316D

Context. The first observational evidence of a connection between supernovae (SNe) and gamma-ray bursts (GRBs) was found about a decade ago. Since then, only half a dozen spectroscopically confirmed associations have been discovered and XRF 100316D/SN 2010bh is among the latest. Aims. We constrain the progenitor radius, the host-galaxy extinction, and the physical parameters of the explosion of XRF 100316D and its associated SN 2010bh at z = 0.059. We study the brightness and colours of SN 2010bh in the context of GRB-SNe. Methods. We began observations 12 h after the GRB trigger and continued until 80 days after the burst. The Gamma-Ray burst Optical and Near-infrared Detector (GROND) provided excellent photometric data of XRF 100316D/SN 2010bh in six filter bands covering a wavelength range from approximately 350 to 1800 nm, significantly expanding the pre-existing data set for this event. Combining GROND and Swift data, the early broad-band spectral energy distribution (SED) is modelled with a blackbody and afterglow component attenuated by dust and gas absorption. The temperature and radius evolution of the thermal component are analysed and combined with earlier measurements available from the literature. Templates of SN 1998bw are fitted to the SN itself to directly compare the light-curve properties. Finally, a two-component parametrised model is fitted to the quasi-bolometric light curve, which delivers physical parameters of the explosion. Results. The best-fit models to the broad-band SEDs imply moderate reddening along the line of sight through the host galaxy (AV,host = 1.2 ± 0.1 mag). Furthermore, the parameters of the blackbody component reveal a cooling envelope at an apparent initial radius of 7 × 10 11 cm, which is compatible with a dense wind surrounding a Wolf-Rayet star. A multicolour comparison shows that SN 2010bh is 60–70% as bright as SN 1998bw. It proves to be the most rapidly evolving GRB-SNe to date, reaching maximum brightness at 8–9 days after the burst in the blue bands. Modelling of the quasi-bolometric light curve yields MNi = 0.21 ± 0.03 M� and Mej = 2.6 ± 0.2 M� , typical of values within the GRB-SN population. The kinetic energy is Ek = (2.4 ± 0.7) × 10 52 erg, which is making this SN the second most energetic GRB-SN after SN 1998bw. Conclusions. This supernova has one of the earliest peaks ever recorded and thereafter fades more rapidly than other GRB-SNe, hypernovae, or typical type-Ic SNe. This implies that a thin envelope is possibly expanding at very high velocities and is, therefore, unable to retain the γ-rays that would prolong the duration of the SN event.

GRB 091127: the cooling break race on magnetic fuel

Aims. Using high-quality, broad-band afterglow data for GRB 091127, we investigate the validity of the synchrotron fireball model for gamma-ray bursts (GRBs), and infer physical parameters of the ultra-relativistic outflow. Methods. We used multi-wavelength (NIR to X-ray) follow-up observations obtained with GROND simultaneously in the grizJH filters and the XRT onboard the Swift satellite in the 0.3 to 10 keV energy range. The resulting afterglow light curve is of excellent accuracy with relative photometric errors as low as 1%, and the spectral energy distribution (SED) is well-sampled over 5 decades in energy. These data present one of the most comprehensive observing campaigns for a single GRB afterglow and allow us to test several proposed emission models and outflow characteristics in unprecedented detail. Results. Both the multi-color light curve and the broad-band SED of the afterglow of GRB 091127 show evidence of a cooling break moving from high to lower energies. The early light curve is well described by a broken power-law, where the initial decay in the optical/NIR wavelength range is considerably flatter than at X-rays. Detailed fitting of the time-resolved SED shows that the break is very smooth with a sharpness index of 2.2 ± 0.2, and evolves towards lower frequencies as a power-law with index −1.23 ± 0.06. These are the first accurate and contemporaneous measurements of both the sharpness of the spectral break and its time evolution. Conclusions. The measured evolution of the cooling break (νc ∝ t ∼−1.2 ) is not consistent with the predictions of the standard model, wherein νc ∝ t ∼−0.5 is expected. A possible explanation for the observed behavior is a time dependence of the microphysical parame- ters, in particular the fraction of the total energy in the magnetic fieldB. This conclusion provides further evidence that the standard fireball model is too simplistic, and time-dependent micro-physical parameters may be required to model the growing number of well-sampled afterglow light curves.