Aybuke Kupcu Yoldas

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.

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.

Photometric redshifts for gamma-ray burst afterglows from GROND and Swift/UVOT

Aims. We present a framework to obtain photometric redshifts (photo-zs) for gamma-ray burst afterglows. Using multi-band photometry from GROND and Swift/UVOT, photo-zs are derived for five GRBs for which spectroscopic redshifts are not available. Methods. We use UV/optical/NIR data and synthetic photometry based on afterglow observations and theory to derive the photometric redshifts of GRBs and their accuracy. Taking into account the afterglow synchrotron emission properties, we investigate the application of photometry to derive redshifts in a theoretical range between z ∼ 1t oz ∼ 12. Results. The measurement of photo-zs for GRB afterglows provides a quick, robust and reliable determination of the distance scale to the burst, particularly in those cases where spectroscopic observations in the optical/NIR range cannot be obtained. Given a sufficiently bright and mildly reddened afterglow, the relative photo-z accuracy η =Δ z/(1 + z) is better than 10% between z = 1. 5a nd z ∼ 7 and better than 5% between z = 2a ndz = 6. We detail the approach on 5 sources without spectroscopic redshifts observed with

The bright optical/NIR afterglow of the faint GRB 080710 – evidence of a jet viewed off-axis

Aims. We investigate the optical/near-infrared light curve of the afterglow of GRB 080710 in the context of rising afterglows. Methods. Optical and near-infrared photometry was performed using the seven-channel imager GROND and the Tautenburg Schmidt telescope. X-ray data were provided by the X-ray Telescope onboard the Swift satellite. We construct an empirical light curve model using the available broadband data, which is well-sampled in the time and frequency domains. Results. The optical/NIR light curve of the afterglow of GRB 080710 is dominated by an initial increase in brightness, which smoothly turns over into a shallow power law decay. At around 10 ks post-burst, there is an achromatic break from shallow to steeper decline in the afterglow light curve with a change in the power law index of Δα ∼ 0.9. Conclusions. The initially rising achromatic light curve of the afterglow of GRB 080710 can be accounted for with a model of a burst viewed off-axis or a single jet in its pre-deceleration phase and in an on-axis geometry. A unified picture of the afterglow light curve and prompt emission properties can be obtained with an off-axis geometry, suggesting that late and shallow rising optical light curves of GRB afterglows might be produced by geometric effects.

First results of GROND

Here we present the first results of GRB afterglow observations with GROND. Gamma‐Ray burst Optical and Near‐infrared Detector (GROND) is an instrument with 4 optical CCDs and 3 near‐infrared detectors with the main aim of observing GRB afterglows and determining their redshift. With its dynamic scheduling and on‐the‐fly data analysis system, GROND combines the sensitivity of the 2.2 m telescope with a robotic observation and analysis scheme. GROND has been mounted on ESO/MPI 2.2 m telescope at La Silla since May 2007. It automatically observed 10 of the 35 GRBs that happened before October 30. Here we will present an overview of GROND observations, concentrating on the preliminary results of two GRBs: the putative host galaxy of the short GRB 070729 and the complex optical/near‐infrared light‐curve of GRB 070802.

A photometric redshift of z = 1.8+0.4 -0.3 for the AGILE GRB 080514B

The AGILE gamma-ray burst GRB 080514B is the first detected to have emission above 30 MeV and an optical afterglow. However, no spectroscopic redshift for this burst is known. We report on our ground-based optical/NIR and millimeter follow-up observations of this event at several observatories, including the multi-channel imager GROND on La Silla, supplemented by Swift UVOT and Swift XRT data. The spectral energy distribution (SED) of the optical/NIR afterglow is found to decline sharply bluewards to the UV bands, which can be utilized in estimating the redshift. Fitting the SED from the Swift UVOT uvw2 band to the H band, we estimate a photometric redshift of z = 1.8 +0.4-0.3, which is consistent with the reported pseudo-redshift based on gamma-ray data. We find that the afterglow properties of GRB 080514B do not differ from those exhibited by the global sample of long bursts. Compared with the long burst sample, we conclude that this burst was special because of its high-energy emission properties, even though both its afterglow and host galaxy are not remarkable in any way. Obviously, high-energy emission in the gamma-ray band does not automatically correlate with the occurrence of special features in the corresponding afterglow light.

GRB 071028B, a burst behind large amounts of dust in an unabsorbed galaxy

Aims. We report on the discovery and properties of the fading afterglow and underlying host galaxy of GRB 071028B, thereby facilitating a detailed comparison between these two. Methods. Observations were performed with the Gamma-ray Burst Optical and Near-infrared Detector at the 2.2 m telescope on the La Silla Paranal Observatory in Chile. We conducted five observations from 1.9 d to 227.2 d after the trigger and obtained deep images

Dark bursts in the Swift era

The nature of the dark bursts is still not fully understood. Here, we report on some of our recent studies on Swift dark bursts. Thereby, we pay particular attention to GRB 050717, for which we obtained a deep imaging of the X‐ray error circle with the newly commissioned GROND camera mounted at the 2.2 m telescope on La Silla.

A photometric redshift of z ∼9.4 for GRB 090429B

Gamma-ray bursts (GRBs) serve as powerful probes of the early universe, with their luminous afterglows revealing the locations and physical properties of star-forming galaxies at the highest redshifts, and potentially locating first-generation (Population III) stars. Since GRB afterglows have intrinsically very simple spectra, they allow robust redshifts from low signal-to-noise spectroscopy, or photometry. Here we present a photometric redshift of z ~ 9.4 for the Swift detected GRB 090429B based on deep observations with Gemini-North, the Very Large Telescope, and the GRB Optical and Near-infrared Detector. Assuming a Small Magellanic Cloud dust law (which has been found in a majority of GRB sight lines), the 90% likelihood range for the redshift is 9.06 7. The non-detection of the host galaxy to deep limits (Y(AB) ~ 28, which would correspond roughly to 0.001L* at z = 1) in our late-time optical and infrared observations with the Hubble Space Telescope strongly supports the extreme-redshift origin of GRB 090429B, since we would expect to have detected any low-z galaxy, even if it were highly dusty. Finally, the energetics of GRB 090429B are comparable to those of other GRBs and suggest that its progenitor is not greatly different from those of lower redshift bursts.