Unusual gamma-ray bursts and X-ray flashes

Unusual GRBs, including X-ray Flashes, ultra-long GRBs and low-luminosity GRBs and the precursors component of GRB emission, are recent hot topics in the GRB field. Understanding such phenomena can help us unveil some key physics about the progenitor, central engine, and radiation mechanisms of Gamma-ray Bursts. Thanks to its large field of view (1.1 sr) and dedicated energy coverage and detailed spectral resolution in soft X-rays (0.5 – 4 keV), the Einstein Probe is an ideal mission that possesses a unique capacity of observing such unusual GRBs.

X-ray Flashes: The so-called X-ray Flashes (XRFs) are a subset of GRBs which show an abundance of X-ray (X-ray-rich GRBs; XRR) or dominated by X-ray emission with few or no gamma rays detected at all. They account for 20-30% of all GRBs. Like normal GRBs, XRFs are distributed homogeneously, lasting from 10 to 200 s, associated with afterglows at lower energies and located in star-forming regions at high redshifts.  The significant difference between X-ray flashes and classic GRBs is that their peak energies are lower. The explanation of the reduced peak energy remains a matter of debate. The popular explanations include: (1) they are just normal GRBs but are observed off-axis so only the “wings” of the uniform or structured jet are observed with mildly relativistic speed (2) the rich X-ray are formed in a dirty fireball in which the ejecta is slower because it consists of more baryonic matter than the case in normal GRBs.

Ultra-long GRBs: Ultra-long GRBs (ULGRBS) last much longer (~hours) in γ-rays than typical long GRBs (~minutes), and it has recently been proposed that these "ultra-long GRBs" may form a distinct population, probably with a different (e.g., blue supergiant) progenitor than typical GRBs.  Interestingly, Swift observations show that the X-ray emission of ULGRBs are strong correlated with the γ-rays and can be regarded as the prompt emission in X-ray band.  Thus, identifying such ULGRBs with large FOV X-ray telescopes is possible and promising to study the prompt emission properties of those special GRBs.  

Low-luminosity GRBs: Low-Luminosity GRBs (LLGRBS) are those GRBs whose isotropic luminosity (1046-1048 erg s-1) are much lower than those (1050-1053 erg s-1) of typical GRBs. Although the current sample of LLGRBs is small (only a total of six such LLGRBs have been observed), they are curial to understand the GRB-SNe connection as well as the GRB progenitors and evolution of massive stars. LLGRBs seem to be very different from those of normal long GRBs in terms of luminosity, rates, and prompt light curves. A "failed" jet has been involved to explain the LL-GRBs. In such a scenario, an LLGRB can arise when a relativistic shock breaks out from a stellar envelope. Such a shock breakout naturally releases its most energy in X-ray range. 

GRB precursors: Precursors, by definition, are less intense episodes of emission before the main bursts.  Comparing to the main burst, a precursor is weaker in flux and softer in spectrum. It is usually separated by a significant amount of time from the main burst and exhibits a simpler shape (Fast Rising Exponential Decay; FRED) in light curve. Theoretically a thermally-dominant precursor is expected when the GRB ejecta break out of the star surface. At such a closer distance, a precursor is expected to reveal more information of the central engine. About 10% of Swift GRBs are associated with precursors. However due to their early occurring time and low significant level, precursors have never been observed in the multi-wavelength context.

Proto-type GRBs: Some GRBs have a precursor, while some others are characterized by extended emission. The recently observed GRB 160625B consists of three distinct and isolated emission episodes. Interestingly the spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within a single GRB.  Although the case like GRB 160625B is rarely observed, it has been proposed that GRB 160625B is the proto-type of at least one sub-group of long GRBs. If such a proto-type burst had been less energetic or had been placed at a larger distance, one with Gamma-ray only detectors would only detect two episodes or even only one, and the event would be ascribed as a “normal” GRB.  On the other hand, such proto-type GRBs can be easily caught in their early phase by EP thanks to its large FOV and high sensitivity in soft X-rays. Observing a large sample of such sample will further help us understand the physical process of gamma-ray burst in general.