More Like this

1. A Comprehensive Study of Bright Fermi-GBM Short Gamma-Ray Bursts: II. Very Short Burst and Its Implications

   https://doi.org/10.3390/universe8100512  

   Hu, Ying-Yong ; Huang, Yao-Lin ; Huang, Jia-Wei ; Zhu, Zan ; Tang, Qing-Wen ( October 2022 , Universe)

   A thermal component is suggested to be the physical composition of the ejecta of several bright gamma-ray bursts (GRBs). Such a thermal component is discovered in the time-integrated spectra of several short GRBs as well as long GRBs. In this work, we present a comprehensive analysis of ten very short GRBs detected by Fermi Gamma-Ray Burst Monitor to search for the thermal component. We found that both the resultant low-energy spectral index and the peak energy in each GRB imply a common hard spectral feature, which is in favor of the main classification of the short/hard versus long/soft dichotomy in the GRB duration. We also found moderate evidence for the detection of thermal component in eight GRBs. Although such a thermal component contributes a small proportion of the global prompt gamma-ray emission, the modified thermal-radiation mechanism could enhance the proportion significantly, such as in subphotospheric dissipation. 

   more » « less
2. GRB 231129C: Another Thermal Emission Dominated Gamma-Ray Burst

   https://doi.org/10.3847/1538-4357/ad5f93  

   Chen, Jia-Ming ; Zhu, Ke-Rui ; Peng, Zhao-Yang ; Zhang, Li ( September 2024 , The Astrophysical Journal)

   This study presents detailed time-integrated and time-resolved spectral analysis of the Fermi Gamma-ray Burst Monitor observations of the bright GRB 231129C. The results reveal its distinct spectral characteristics, featuring a hard low-energy spectral index (α) and soft high-energy spectral index (β), similar to GRB 090902B, suggesting a possible dominance of thermal emission. Further analysis indicates that 92% of the spectral indices exceed the synchrotron “line of death,” with the hardest index atα∼ +0.44. Simultaneously, 53% of the spectra can be well fitted by the nondissipative photosphere model, supporting a potential origin from a nondissipative photosphere. Additionally, we observe strong correlations between the spectral indexαand peak energyE_pwith flux. For theα−Frelationship, we employF=F₀e^{(3.00±0.10)α}to describe it, whereas theE_p−Frelationship requires a smoothly bending power-law function. Based on the framework proposed by Hascoët et al. and Gao & Zhang, the jet characteristics of this burst were studied, revealing that both methods support the suitability of a pure fireball model for this GRB at small initial jet radii.

    

   more » « less
3. Time-resolved Spectral Properties of Fermi-GBM Bright Long Gamma-Ray Bursts

   https://doi.org/10.1088/1674-4527/ad16af  

   Wang, Wan-Kai ; Xie, Wei ; Gao, Zhi-Fu ; Xiao, Shuo ; Dong, Ai-Jun ; Zhang, Bin ; Zhi, Qi-Jun ( January 2024 , Research in Astronomy and Astrophysics)

   The prompt emission mechanism of gamma-ray bursts (GRBs) is still unclear, and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes. We performed a detailed time-resolved spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray Burst Monitor. A total of 1490 spectra were obtained and their properties were studied using a typical Band-shape model. First, the parameter distributions of the time-resolved spectrum are given as follows: the low-energy spectral indexα∼ − 0.72, high-energy spectral indexβ∼ − 2.42, the peak energyE_p∼ 221.69 keV, and the energy fluxF∼ 7.49 × 10⁻⁶erg cm⁻²s⁻¹. More than 80% of the bursts exhibit the hardest low-energy spectral index${\alpha }_{\max }$exceeding the synchrotron limit (−2/3). Second, the evolution patterns ofαandE_pwere statistically analyzed. The results show that for multi-pulse GRBs the intensity-tracking pattern is more common than the hard-to-soft pattern in the evolution of bothE_pandα. The hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial pulse of multi-pulse GRBs. Finally, we found a significant positive correlation betweenFandE_p, with half of the samples exhibiting a positive correlation betweenFandα. We discussed the spectral evolution of different radiation models. The diversity of spectral evolution patterns indicates that there may be more than one radiation mechanism occurring in the GRB radiation process, including photospheric radiation and synchrotron radiation. However, it may also involve only one radiation mechanism, but more complicated physical details need to be considered.

    

   more » « less
4. Revealing the characteristics of the dark GRB 150309A: Dust extinguished or high- z ?

   https://doi.org/10.1051/0004-6361/202346042  

   Castro-Tirado, A J ; Gupta, R ; Pandey, S B ; Nicuesa_Guelbenzu, A ; Eikenberry, S ; Ackley, K ; Gerarts, A ; Valeev, A F ; Jeong, S ; Park, I H ; et al ( March 2024 , Astronomy & Astrophysics)

   Context.Dark gamma-ray bursts (GRBs) constitute a significant fraction of the GRB population. In this paper, we present a multi-wavelength analysis (both prompt emission and afterglow) of an intense (3.98  ×  10⁻⁵erg cm⁻²usingFermi-Gamma-Ray Burst Monitor) two-episodic GRB 150309A observed early on until ∼114 days post burst. Despite the strong gamma-ray emission, no optical afterglow was detected for this burst. However, we discovered near-infrared (NIR) afterglow (K_S-band), ∼5.2 h post burst, with the CIRCE instrument mounted at the 10.4 m Gran Telescopio Canarias (hereafter, GTC).

   Aims.We aim to examine the characteristics of GRB 150309A as a dark burst and to constrain other properties using multi-wavelength observations.

   Methods.We usedFermiobservations of GRB 150309A to understand the prompt emission mechanisms and jet composition. We performed early optical observations using the BOOTES robotic telescope and late-time afterglow observations using the GTC. A potential faint host galaxy was also detected in the optical wavelength using the GTC. We modelled the potential host galaxy of GRB 150309A in order to explore the environment of the burst.

   Results.The time-resolved spectral analysis ofFermidata indicates a hybrid jet composition consisting of a matter-dominated fireball and magnetic-dominated Poynting flux. The GTC observations of the afterglow revealed that the counterpart of GRB 150309A was very red, withH − K_S > 2.1 mag (95% confidence). The red counterpart was not discovered in any bluer filters ofSwiftUVOT/BOOTES, which would be indicative of a high redshift origin. Therefore we discarded this possibility based on multiple arguments, such as spectral analysis of the X-ray afterglow constrainz < 4.15 and a moderate redshift value obtained using the spectral energy distribution (SED) modelling of the potential galaxy. The broadband (X-ray to NIR bands) afterglow SED implies a very dusty host galaxy with a deeply embedded GRB (suggestingA_V ≳ 35 mag).

   Conclusions.The environment of GRB 150309A demands a high extinction towards the line of sight. Demanding dust obscuration is the most probable origin of optical darkness as well as the very red afterglow of GRB 150309A. This result establishes GRB 150309A as the most extinguished GRB known to date.

    

   more » « less
5. Probing into emission mechanisms of GRB 190530A using time-resolved spectra and polarization studies: Synchrotron Origin?

   https://doi.org/10.1093/mnras/stac015  

   Gupta, Rahul ; Gupta, S ; Chattopadhyay, T ; Lipunov, V ; Castro-Tirado, A J ; Bhattacharya, D ; Pandey, S B ; Oates, S R ; Kumar, Amit ; Hu, Y-D ; et al ( January 2022 , Monthly Notices of the Royal Astronomical Society)

   Abstract Multi-pulsed GRB 190530A, detected by the GBM and LAT onboard Fermi, is the sixth most fluent GBM burst detected so far. This paper presents the timing, spectral, and polarimetric analysis of the prompt emission observed using AstroSat and Fermi to provide insight into the prompt emission radiation mechanisms. The time-integrated spectrum shows conclusive proof of two breaks due to peak energy and a second lower energy break. Time-integrated (55.43 ± 21.30 %) as well as time-resolved polarization measurements, made by the Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat, show a hint of high degree of polarization. The presence of a hint of high degree of polarization and the values of low energy spectral index (αpt) do not run over the synchrotron limit for the first two pulses, supporting the synchrotron origin in an ordered magnetic field. However, during the third pulse, αpt exceeds the synchrotron line of death in few bins, and a thermal signature along with the synchrotron component in the time-resolved spectra is observed. Furthermore, we also report the earliest optical observations constraining afterglow polarization using the MASTER (P < 1.3 %) and the redshift measurement (z= 0.9386) obtained with the 10.4m GTC telescopes. The broadband afterglow can be described with a forward shock model for an ISM-like medium with a wide jet opening angle. We determine a circumburst density of n0 ∼ 7.41, kinetic energy EK ∼ 7.24 × 1054 erg, and radiated γ-ray energy Eγ, iso ∼ 6.05 × 1054 erg, respectively. 

   more » « less