Multi-wavelength study of GRBs detected by Fermi and Swift

Abstract

Gamma-ray bursts (GRBs) are the most violent explosions in the Universe. Although they were first discovered over half a century ago, yet many problems remain unsolved. Since the successful launch of GRB dedicated missions such as Swift and Fermi, multi-wavelength ground-based observations have provided a new approach to better characterize these events, their host galaxies and understand the underlying physics around the newly born compact objects following the GRB itself. GRB 140629A was a long burst that triggered the Swift satellite and many facilities at different wavelengths followed up this event, including the optical (Swift/UVOT and various facilities worldwide), infrared (Spitzer) and X-rays (Swift/XRT). These data were taken between 40 seconds and 3 yr after the burst trigger, which made this burst a good case to investigate the properties of the dominant jet and its host galaxy. The absorption features displayed in the optical spectrum, taken with the 6.0m BTA telescope, confirmed the redshift of this GRB (z = 2.276 0.001). We found no strong spectral evolution when fitting the spectral energy distribution of the afterglow from the X-rays to optical wavelengths. The hydrogen column density NH was constrained to be 7.2×1021cm−2 along the line of sight. The afterglow in this burst could be explained by a blast wave jet with a long-lasting central engine expanding into a uniform medium in the slow cooling regime. At the end of energy injection, a normal decay phase was observed in both the optical and X-ray bands. An achromatic jet break was also found in the afterglow light curves 0.4 d after the trigger. We fitted the multi-wavelength data simultaneously with a model (based on numerical simulations) and found that the observations could be explained by a narrow uniform jet in a dense environment with a half-opening angle of 6:7 viewed 3:8 off-axis, implying a total released energy of 1:4 1054 erg. Using the redshift and opening angle, we found that GRB 140629A followed both the Ghirlanda and Amati relations. The peak time of the light curve was identified as the onset of the forward shock (181 s after trigger) and we could constrain the initial Lorentz factor (Γ0) in the range 82-118. After fitting the host galaxy spectral energy distribution, we found the host to be a low mass, star-forming galaxy with a star formation rate (SFR) of log(SFR) = 1.1+0.9−0.4 M yr−1. We also obtained a value for the neutral hydrogen density NHI by fitting the optical spectrum, from which we derived logNHI = 21:0 0:3, classifying this host as a damped Lyman-alpha system. High ionisation lines (N V,Si IV) were also detected in the optical spectrum. Furthermore, polarisation observations by the MASTER network indicated that this burst was weakly polarised. GRB 190829A was detected by both Fermi and Swift but what made a unique event out of it was the detection of very high energy (VHE) gamma-rays by the High-Energy Stereoscopic System telescopes (HESS). The prompt gamma-ray emission displayed two emission episodes separated by a quiescent gap of 40 s. We followed it up with the 10.4 m Gran Telescopio CANARIAS (GTC) and gathered observations of the afterglow of GRB 190829A and its underlying supernova during the following days. We determined the redshift of this event (z = 0.0785 0.005) and compared GRB 190829A to GRB 180728A, another GRB with similar prompt behaviour at VHE energies, and discussed the implications regarding the underlying physical mechanisms producing these two GRBs. Together with the prompt emission data, the 10.4 m GTC data was used to better understand the emission mechanisms and possible progenitors. In the detailed analysis of the multi-band observations of the afterglow, we found the observational properties of the multi-wavelength afterglow could be explained by the cooling frequency passing between the optical and X-ray bands at the early epoch. A few days after, we saw the transition from the afterglow spectrum to the underlying supernova (dubbed SN 2019oyw) spectrum, which dominated the light curve at later times. Although the prompt emission temporal properties of GRB 190829A and GRB 180728A were similar, the two gamma-ray pulses were different in the spectral domain. We also found that the SN 2019oyw associated with GRB 190829A is powered by Ni decay and could be classified as a Type Ic-BL (broad line) supernova. The spectroscopic and photometric properties of this supernova were consistent with those observed for SN 1998bw (also related to another burst, GRB 980425) but SN 2019oyw evolved much faster than SN 1998 bw. Besides these above mentioned two long-duration GRBs, we also investigated the prompt emission and the afterglow properties of a sample of shortduration gamma-ray bursts (sGRBs) including GRB 130603B and another eight sGRB events during 2012-2015. They were observed by several multi-wavelength facilities, including the 10.4m GTC telescope. Prompt emission high energy data of those events were obtained by INTEGRAL-SPI-ACS, Swift-BAT and Fermi- GBM satellites. The prompt emission data by INTEGRAL in the 0.1–10 MeV energy range for sGRB 130603B, sGRB 140606A, sGRB 140930B, sGRB 141212A, and sGRB 151228A did not show signs of the extended emission or the precursor activity and their spectral and temporal properties were found to be similar to those seen in case of other short-duration bursts. For sGRB 130603B, our new afterglow photometric data constrained the pre-jet-break temporal decay due to denser temporal coverage. Its afterglow light curve, containing both our new data as well as previously published photometric data, was broadly consistent with the interstellar medium (ISM) afterglow model. Modelling the host galaxies of sGRB 130603B and sGRB 141212A using the LePHARE software supported a scenario where the burst environment was undergoing moderate star formation activity. From the inclusion of our late-time data for the additional eight sGRBs, we were able to place tight constraints on the non-detection of the afterglow, host galaxy or any underlying ‘kilonova’ emission. Finally, our late-time afterglow observations of the short-duration GRB 170817A (related to the gravitational wave GW 170817) are also discussed and compared with the sub-set of short-duration GRBs.