Session Y09: Gamma Ray Bursts

Live

The potential existence of two separate classes of Long-duration Gamma-Ray Bursts (LGRBs) with and without radio afterglow emission, corresponding to radio-bright/loud and radio-dark/quiet populations, has been recently argued and favored in the GRB literature. The radio-quiet LGRBs have been found to have, on average, lower total isotropic gamma-ray emission ($E_{iso}$) and shorter intrinsic prompt gamma-ray duration (e.g., $T_{90z}$). Also, an anti-correlation between the intrinsic prompt duration ($T_{90z}$) and redshift ($z$) of radio-loud GRBs has been discovered, which is reportedly missing in the radio-quiet class of GRBs. Here we discuss the significance of the differences between the energetics and temporal properties of the two proposed classes of radio-loud and radio-quiet LGRBs. We show that the proposed evidence in support of the two distinct radio populations of LGRBs can be explained away in terms of selection effects and sample incompleteness. This is based on the recent discovery of the strong positive correlation between $E_{iso}$ and $T_{90z}$, in both populations of short-hard and long-soft GRBs, predicted and quantified by Shahmoradi (2013, ApJ 766:111-133) and Shahmoradi \& Nemiroff (2015, MNRAS 451:4645-4662).   

Live

The observed durations of GRBs are influenced by two competing factors, namely, the elongation of the durations due to cosmological time-dilation and the underestimation of the durations due to the loss of signal into the background noise (i.e., the ``tip of the iceberg” effect). We select a set of GRB lightcurves observed by \textit{The Neil Gehrels Swift Observatory} Burst Alert Telescope (\textit{Swift}/BAT) with a range of features (e.g., structure, short and long duration, brightness) and simulate their observed \textit{Swift}/BAT light curve at varying redshifts in order to determine which of the two factors dominates at increasing redshift. Using Bayesian Block analysis to determine GRB duration for the observed and simulated light curves, we find that signal lost in the noise significantly decreases observed durations leading to an underestimation of GRB durations when only time-dilation corrections are applied. We investigate whether the effects can be used to better characterize GRB durations and improve the confidence of their association with collapsars or NS mergers. Furthermore, as information from GRBs are lost in the noise, there is an underestimation of the total energy emitted from collapsar and binary neutron star merger events.   

Live

Gamma-ray bursts (GRBs) are among the most luminous electromagnetic phenomena in the known universe, but there is still much unknown about them. In particular, their circumstellar environments are likely much more complicated than a simple massive-star wind-density profile, as is commonly assumed. Long GRBs show late time flares in their optical and X-ray light curves that may be a reflection of this rich environment. Using relativistic hydrodynamics simulations, we study a family of initial data with a relativistic blast wave encountering a dense circumstellar shell of matter, similar to what might be generated by an aging star expelling the outer layers of its atmosphere. We test the possibility that some of the late time curve variability may result from these interactions and characterize the profiles of the reverse shocks.   

Live

Despite more than 5,000 detected Gamma-Ray Bursts (GRBs), the nature of the prompt emission and the physical mechanisms powering the GRB relativistic jets are still strongly debated. During the past years, several studies showed that the gamma-ray prompt emission spectra are more complex than the smoothly broken power-law traditionally used. New models emerged, and among them, the three-component model that we propose provides an excellent description of the broadband time-resolved prompt emission of both short and long GRBs from optical to high-energy gamma-rays: (i) a quasi-thermal component interpreted as emission from the jet photosphere, (ii) a non-thermal component interpreted as synchrotron radiation from accelerated electrons within the jet; and (iii) a second non-thermal component, which may be related to magnetic reconnections downstream the jet. Moreover, this model enables a new luminosity/hardness relation suggesting that GRBs may be standard candles; this relation may reveal the underlying physics behind the famous Amati, Ghirlanda, and Yonetoku relations. I will present this three-component model using GRBs detected with Fermi, CGRO/BATSE, Swift$+$Suzaku/WAM, and Wind/Konus. I will discuss the striking similarities of all GRBs using this model and the possible universality of the derived luminosity/hardness relation.   

Live

The Fermi Gamma-ray Burst Monitor (GBM) is an all-sky monitoring instrument sensitive to photon energies from 8 keV to 40 MeV. Its capabilities allow it to observe around 240 gamma-ray bursts (GRBs) each year through on-board triggers alone, making it ideal for providing simultaneous gamma-ray observations of multimessenger transients. This fact was proven through the on-board detection of GRB 170817A and the associated binary neutron star merger event GW170817 which was a major milestone in multimessenger astronomy. Fermi-GBM continues to look for similar multimessenger detections through on-board triggers as well as subthreshold searches for weak transients, performed both in high-time-resolution continuous data and in targeted follow-ups of gravitational-wave and high-energy neutrino events. I will provide an overview of these searches and their recent results.   

Live

The recent detection of GRB 180720B and GRB 190829A by HESS, and GRB 190114C by MAGIC, show that at least in some cases the emission of Gamma-Ray Bursts (GRBs) continue in the Very-High-Energy (VHE) range ($>$100 GeV). Observations during the prompt emission, still undetected at VHE, are crucial to answer important remaining questions. The High-Altitude Water Cherenkov Observatory (HAWC), with its large field of view ($\sim$2 sr) and high duty cycle ($>$95\%), is well suited to study these and other rapid transients. This contribution presents an overview of the efforts of HAWC in the search for short bursts: an untriggered monitoring of the entire overhead sky; the follow-up of GRBs detected by Fermi and Swift; and the search for emission coincident with gravitational-wave events. The results of recent improvements in the reconstruction and analysis of sub-TeV events will be shown, which significantly increased the HAWC sensitivity to these type of phenomena.   

Live

The robotic telescope D50 located at the Ond\v rejov Observatory near Prague, Czech republic routinely follows-up triggers of Gamma-Ray Bursts in order to study their optical counterparts. While the afterglows of long gamma-ray bursts are relatively bright, the optical emission of short bursts is much weaker and more difficult to detect. We present the first-ever optical counterpart of a short burst detected in the Czech Republic by our robotic system.   

On Demand

On August 17th of 2017 the detection of gravitational waves (GWs) and light from GW170817, a Binary Neutron Star (BNS) merger located at a distance of ~40 Mpc, offered us the very first opportunity to investigate directly the nature of the post-merger remnant of BNS mergers. While the nature of the GW170817 remnant is still highly debated, it has been suggested that a NS may have formed in this merger. The idea that long-lived magnetized NS (magnetars) may form in BNS mergers was also previously proposed to explain the 'plateau' feature found in the X-ray afterglow of some short GRBs. The detection of GWs from a leftover magnetar would provide direct proof of the nature of the merger remnant and insights into the equation of state of nuclear matter. Here we present a new data analysis technique based on the Cross Correlation Algorithm (CoCoA) via which the current reach of GW searches for post-merger remnants could be significantly increased, and discuss its implications for current and future observing runs of the LIGO detectors.   

On Demand

The CALorimetric Electron Telescope (CALET) is a payload deployed on the International Space Station to observe high energy cosmic rays and gamma rays. CALET consists of the CALorimeter (CAL), which is the primary instrument of CALET, and the CALET Gamma-ray Burst Monitor (CGBM), which aims to observe gamma-ray bursts (GRBs). CALET has been in nominal on-orbit operation since October 2015. As of the end of 2019, CGBM has detected 181 GRBs including 22 short GRBs over four years and three months. Also, we have searched for high energy gamma-rays from GRBs with CAL. In this work, we present CALET results of GRB observation, including the search for electromagnetic counterparts of gravitational wave events.