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The radiation mechanism (thermal photosphere or magnetic synchrotron) and the progenitor of gamma-ray burst (GRB) are under hot debate. Recently discovered, the prompt long-duration ($\sim$ 10 s, normally from the collapse of massive stars) property of GRB 211211A strongly conflicts with its association with a kilonova (normally from the merger of two compact objects, NS-NS, NS-BH, or NS-WD, duration $\lesssim$ 2 s). In this paper, we find the probability photosphere model with a structured jet can satisfactorily explain this peculiar long duration, through the duration stretching effect ($\sim$ 3 times) on the intrinsic longer ($\sim$ 3 s) duration of NS-BH (neutron star and black hole) merger, the observed empirical 2SBPL spectrum (with soft low-energy index $\alpha$ of $\sim$ -1) and its evolution. Also, much evidence of the NS-BH merger origin is found, especially the well fit of the afterglow-subtracted optical-NIR light curves by the significant thermal cocoon emission and the sole thermal red kilonova component. Finally, a convincing new explanation for the X-ray afterglow plateau is revealed., Comment: 38 pages, 17 figures, 2 tables. Under review at Nature Communications

It is generally believed that long-duration gamma-ray bursts (GRBs) are associated with massive star core-collapse, whereas short-duration GRBs are associated with mergers of compact star binaries. However, growing observations have suggested that oddball GRBs do exist, and multiple criteria (prompt emission properties, supernova/kilonova associations, and host galaxy properties) rather than burst duration only are needed to classify GRBs physically. A previously reported long-duration burst, GRB 060614, could be viewed as a short GRB with extended emission if it were observed at a larger distance and was associated with a kilonova-like feature. As a result, it belongs to the Type-I (compact star merger) GRB category and is likely of the binary neutron star merger origin. Here we report a peculiar long-duration gamma-ray burst, GRB 211211A, whose prompt emission properties in many aspects differ from all known Type-I GRBs, yet its multi-band observations suggest a non-massive-star origin. In particular, significant excess emission in both optical and near-infrared wavelengths has been discovered, which resembles kilonova emission as observed in some Type-I GRBs. These observations point towards a new progenitor type of GRBs. A scenario invoking a white dwarf-neutron star merger with a post-merger magnetar engine provides a self-consistent interpretation for all the observations, including prompt gamma-rays, early X-ray afterglow, as well as the engine-fed kilonova emission., Comment: Updated; Authors' version. 44 pages, 8 figures, 6 tables

The Chinese CubeSat Mission, Gamma Ray Integrated Detectors (GRID), recently detected its first gamma-ray burst, GRB 210121A, which was jointly observed by the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM). This burst is confirmed by several other missions, including \fermi and \textit{Insight}-HXMT. We combined multi-mission observational data and performed a comprehensive analysis of the burst's temporal and spectral properties. Our results show that the burst is relatively special in its high peak energy, thermal-like low energy indices, and large fluence. By putting it to the $E_{\rm p}$-$E_{\rm\gamma, iso}$ relation diagram with assumed distance, we found this burst can be constrained at the redshift range of [0.3,3.0]. The thermal spectral component is also confirmed by the direct fit of the physical models to the observed spectra. Interestingly, the physical photosphere model also constrained a redshift of $z\sim$ 0.3 for this burst, which help us to identify a host galaxy candidate at such a distance within the location error box. Assuming the host galaxy is real, we found the burst can be best explained by the photosphere emission of a typical fireball with an initial radius of $r_0\sim$ 3.2 $\times 10^7$ cm., Comment: ApJ accpeted; 12 pages, 11 figures, 3 tables

Gamma-ray bursts have been phenomenologically classified into long and short populations based on whether the observed duration is longer or shorter than two seconds. Multi-wavelength and multi-messenger observations in recent years have revealed that in general long GRBs originate from massive star core collapse events, whereas short GRBs originate from binary neutron star mergers. It has been known that the duration criterion is sometimes unreliable, and multi-wavelength criteria are needed to identify the physical origin of a particular GRB. Some apparently long GRBs have been suggested to have a neutron star merger origin, whereas some apparently short GRBs have been attributed to genuinely long GRBs whose short, bright emission is above the detector's sensitivity threshold. Still, there has been no known case that a GRB is genuinely short but originates from death of a massive star. Here we report the comprehensive analysis of the multi-wavelength data of a bright short GRB 200826A. This burst has a sharp 1-second spike, which is not part of an underlying long-duration event. Its other observational properties are, however, inconsistent with those of other short GRBs believed to originate from binary neutron star mergers. Rather, these properties resemble those of long GRBs. This burst presents a challenge to the traditional GRB classification scheme and reveals a class of core-collapse-origin GRBs with genuinly short durations.

Millilensing of Gamma-Ray Bursts (GRBs) is expected to manifest as multiple emission episodes in a single triggered GRB with similar light-curve patterns and similar spectrum properties. Identifying such lensed GRBs could help improve constraints on the abundance of compact dark matter. Here we present a systemic search for millilensing among 3000 GRBs observed by the \textit{Fermi} GBM up to 2021 April. Eventually we find 4 interesting candidates by performing auto-correlation test, hardness test, and time-integrated/resolved spectrum test to the whole sample. GRB 081126A and GRB 090717A are ranked as the first class candidate based on their excellent performance both in temporal and spectrum analysis. GRB 081122A and GRB 110517B are ranked as the second class candidates (suspected candidates), mainly because their two emission episodes show clear deviations in part of the time-resolved spectrum or in the time-integrated spectrum. Considering a point mass model for the gravitational lens, our results suggest that the density parameter of lens objects with mass $M_{\rm L}\sim10^{6} M_{\odot}$ is larger than $1.5\times10^{-3}$., Comment: Accepted for publication in The Astrophysical Journal

The spectral lags of gamma-ray bursts (GRBs) have been viewed as the most promising probes of the possible violations of Lorentz invariance (LIV). However, these constraints usually depend on the assumption of the unknown intrinsic time lag in different energy bands and the use of a single highest-energy photon. A new approach to test the LIV effects has been proposed by directly fitting the spectral lag behavior of a GRB with a well-defined transition from positive lags to negative lags. This method simultaneously provides a reasonable formulation of the intrinsic time lag and robust lower limits on the quantum-gravity energy scales ($E_{\rm QG}$). In this work, we perform a global fitting to the spectral lag data of GRB~190114C by considering the possible LIV effects based on a Bayesian approach. We then derive limits on $E_{\rm QG}$ and the coefficients of the Standard Model Extension. The Bayes factors output in our analysis shows a very strong evidence for the spectral-lag transition in GRB~190114C. Our constraints on a variety of isotropic and anisotropic coefficients for LIV are somewhat weaker than existing bounds, but they can be viewed as comparatively robust and have the promise to complement existing LIV constraints. The observations of GRBs with higher-energy emissions and higher temporal resolutions will contribute to a better formulation of the intrinsic time lag and more rigorous LIV constraints in the dispersive photon sector., 13 pages, 6 figures, 2 tables, the version accepted for publication in the Astrophysical Journal (ApJ)