A detailed study of Gamma-Ray Burst afterglows
Final Report Abstract
The project has focused on three topics: (1) Gamma-Ray Burst (GRB) supernovae (SNe) related to long GRBs, (2) short GRBs and their host galaxies, and (3) relativistic outflow velocities in GRBs. (1) More than 20 years ago, GRB 980425/SN 1998bw revealed that long bursts are associated with broad-line type-Ic SNe. Since then more than 1000 long GRBs have been localized to high angular precision, but only in about 50 cases has the underlying SN component been identified, either photometrically and/or spectroscopically. Using the multi-channel imager GROND mounted at the MPG 2.2m telescope on ESO/La Silla, we analyzed several unprecedented data sets of a number of GRB-SNe. In doing so, we spent a substantial effort in the study of four GRB-SNe at redshifts between 0.4 and 0.8, which were followed-up with GROND. We studied their afterglow light curves, followed the associated photometric SN bumps over several weeks, and characterized their host galaxies. Using the U BRI light curves of SN 1998bw as a template, the derived SN explosion parameters are fully consistent with the corresponding properties of the GRB-SN ensemble at much smaller distance. There is no evidence for an evolution of their properties with redshift. In two cases (GRB 120714B/SN 2012eb at z = 0.398 and GRB 130831A/SN 2013fu at z = 0.479) our additional ESO/VLT spectroscopy of the associated SNe revealed a photospheric expansion velocity at maximum light of about 40,000 and 20,000 km s^−1 , respectively. For GRB 120714B, we found additional evidence for the shock break-out during the SN explosion. These studies also entered the doctoral thesis of a PhD student. Part (2) contains the results of an extensive radio-continuum observing campaign of short-GRB hosts and the results of two detailed studies of such hosts using the Integral Field Unit MUSE at the ESO/VLT. These MUSE observations were devoted to the hosts of GRB 050709 (z=0.16) and GRB 080905 (z=0.12), which are among the nearest short-GRB host galaxies known. The high-resolution MUSE data allowed us to perform a detailed mapping of these galaxies in several emission lines. The goal of the radio survey was to search for optically obscured star formation activity in short-GRB hosts, possibly indicative of a population of young short-GRB progenitors. Our sample comprises the hosts and host-galaxy candidates of 16 short bursts from 2005 to 2015, corresponding to roughly 1/3 of the at that time known ensemble of well-localized short bursts. Eight GRB fields were observed with ATCA, and eight fields with the VLA. The observations typically achieved a 1σrms of 5 to 8 µJy. In most cases they were performed years after the corresponding burst. Only one host galaxy was detected, the host of GRB 100206A at z=0.407. Its measured radio flux implies a star formation rate (SFR) of 60 M yr^−1 . The 15 non-detections constrain the SFRs of the suspected hosts and provide upper limits on late-time luminosities of the associated radio afterglows. Moreover, our non-detections imply that in the time window between about 1 and 10 years (host frame time) the predicted luminous kilonova radio flares do not exist. These studies also entered the doctoral thesis of a PhD student. Finally, part (3) has focused on the very early phase of GRB afterglows. Here we made use of the technical capability of GROND to be operated in Rapid Response Mode (RRM) and to be on target within minutes after the onset of a GRB. As such, in several cases the GROND data show the transition from the rising to the fading optical transient, which is the signature of the onset of the GRB afterglow phase. Within the framework of the standard fireball model, the Lorentz factor of the GRB outflow first increases with time, then undergoes a phase where it is constant (Γ0 ), and is finally progressively decreasing. The so-called initial Lorentz factor Γ0 can be determined based on an afterglow light curve when it shows a transition from a rising to a falling behaviour at very early times, usually within the first 1 ksec after the GRB trigger. In part (3) we have worked on GROND data of 50 GRB afterglows where GROND was on target within 500 seconds after the onset of the corresponding burst. Based on the light curves obtained with GROND, we could determine Γ0 for about 10 bursts and provide upper limits for about 20 events. While this is not the most comprehensive data set ever published, it is unique in the sense that all our data are based on observations in seven photometric bands simultaneously. In this respect, in all cases we could prove that the evolution of the light curve was achromatic. This allowed us to exclude that the light curve peak was related to the passage of a spectral break through the photometric bands.
Publications
- 2017: Multi-messenger Observations of a Binary Neutron Star Merger. Astroph. J. Lett. 848, 12
Abbott, B. P. et al.
(See online at https://doi.org/10.3847/2041-8213/aa91c9) - 2018: The optical/NIR afterglow of GRB 111209A: Complex yet not unprecedented. Astron. Astroph. 617, 122
Kann, D. A. et al.
(See online at https://doi.org/10.1051/0004-6361/201731292) - 2019: Deep ATCA and VLA Radio Observations of Short-GRB Host Galaxies. Constraints on Star Formation Rates, Afterglow Flux, and Kilonova Radio Flares. Astroph. J. 887, 206
Klose, S. et al.
(See online at https://doi.org/10.3847/1538-4357/ab528a) - 2019: Four GRB supernovae at redshifts between 0.4 and 0.8. The bursts GRB 071112C, 111228A, 120714B, and 130831A. Astron. Astroph. 622, A138
Klose, S. et al.
(See online at https://doi.org/10.1051/0004-6361/201832728) - 2019: Highly luminous supernovae associated with gamma-ray bursts. I. GRB 111209A/SN 2011kl in the context of stripped-envelope and superluminous supernovae. Astron. Astroph. 624, 143
Kann, D. A. et al.
(See online at https://doi.org/10.1051/0004-6361/201629162) - 2019: Signatures of a jet cocoon in early spectra of a supernova associated with a GRB. Nature 565, 324
Izzo, L. et al.
(See online at https://doi.org/10.1038/s41586-018-0826-3) - 2020: Observational constraints on the optical and near-infrared emission from the neutron star-black hole binary merger candidate S190814bv. Astron. Astroph. 643, 113
Ackley, K. et al.
(See online at https://doi.org/10.1051/0004-6361/202037669) - 2021: The host galaxy of the short GRB 050709. Astron. Astroph. 650, A117
Nicuesa Guelbenzu, A. M. et al.
(See online at https://doi.org/10.1051/0004-6361/202039689) - 2021: VLT/MUSE and ATCA Observations of the Host Galaxy of the Short GRB 080905A at z = 0.122. Astroph. J. 923, 38
Nicuesa Guelbenzu, A. M. et al.
(See online at https://doi.org/10.3847/1538-4357/ac2faa) - 2022: The Peculiar Short-duration GRB 200826A and Its Supernova. Astroph. J. 932, 1
Rossi, A. et al.
(See online at https://doi.org/10.3847/1538-4357/ac60a2)
