Gamma-ray bursts (GRBs) have been phenomenologically divided into long- and short-duration populations, generally corresponding to collapsar and compact merger origins, respectively. Here, we collect three unique bursts, GRBs 060614, 211211A, and 211227A, all of which are characterized by a long-duration main emission (ME) phase and a rebrightening extended emission (EE) phase, to study their observed properties and their potential origins as neutron star–black hole (NSBH) mergers. NS-first-born (BH-first-born) NSBH mergers tend to contain fast-spinning (nonspinning) BHs that more easily (hardly) allow tidal disruption to occur, while (without) forming electromagnetic signals. We find that NS-first-born NSBH mergers can well interpret the origins of these three GRBs, supported by the following. (1) Their X-ray MEs and EEs show unambiguous fallback accretion signatures, decreasing as ∝ t ^−5/3, which might account for their long durations. The EEs could result from the fallback accretion of r-process heating materials, which is predicted to occur after NSBH mergers. (2) The beaming-corrected local event-rate density for these types of merger-origin long-duration GRBs is  0 ∼ 2.4 − 1.3 + 2.3 Gpc − 3 yr − 1 , consistent with that of NS-first-born NSBH mergers. (3) Our detailed analysis of the EE, afterglow, and kilonova of the recent high-impact event GRB 211211A reveals that it could be a merger between a ∼ 1.23 − 0.07 + 0.06 M ⊙ NS and a ∼ 8.21 − 0.75 + 0.77 M ⊙ BH, with an aligned spin of χ BH ∼ 0.62 − 0.07 + 0.06 , supporting an NS-first-born NSBH formation channel. A long-duration burst, with a rebrightening fallback accretion signature after the ME, and a bright kilonova, might be commonly observed features for on-axis NSBH mergers. We estimate the multimessenger detection rate between gravitational waves, GRBs, and kilonova emissions from NSBH mergers in O4 (O5) to be ∼0.1 yr⁻¹ (∼1 yr⁻¹).

Fast radio bursts (FRBs) have excessive dispersion measures (DMs) and an all-sky distribution, which point toward an extragalactic or even a cosmological origin. We develop a method to extract the mean host galaxy DM ( ) and the characterized luminosity (L) of FRBs using the observed DM–flux data, based on the assumption of a narrow luminosity distribution. Applying Bayesian inference to the data of 21 FRBs, we derive a relatively large mean host DM, i.e., with a large dispersion. A relatively large DM_HG of FRBs is also supported by the millisecond scattering times of some FRBs and the relatively small redshift z = 0.19273 of FRB 121102 (which gives ). The large host galaxy DM may be contributed by the interstellar medium (ISM) or a near-source plasma in the host galaxy. If it is contributed by the ISM, the type of the FRB host galaxies would not be Milky Way–like, consistent with the detected host of FRB 121102. We also discuss the possibility of having a near-source supernova remnant, pulsar wind nebula, or H ii region that gives a significant contribution to the observed DM_HG.

In the internal shock model for gamma-ray bursts (GRBs), the synchrotron spectrum from the fast cooling electrons in a homogeneous downstream magnetic field (MF) is too soft to produce the low-energy slope of GRB spectra. However, the MF may decay downstream with distance from the shock front. Here we show that the synchrotron spectrum becomes harder if electrons undergo synchrotron and inverse-Compton cooling in a decaying MF. To reconcile this with the typical GRB spectrum with low-energy slope νF νν, the postshock MF decay time must be comparable to the cooling time of the bulk electrons (corresponding to a MF decaying length typically of ~105 skin depths); that the inverse-Compton cooling should dominate synchrotron cooling after the MF decay time; and/or that the MF decays with comoving time roughly as Bt –1.5. An internal shock synchrotron model with a decaying MF can account for the majority of GRBs with low-energy slopes not harder than ν4/3.

Cassiopeia A (Cas A), as the nearby young remnant of a core-collapse supernova, is the best candidate for astrophysical studies in supernova explosion and its environment. We studied the hard X-ray emission from Cas A using the 10 year data of INTEGRAL observations, and first detected non-thermal continuum emission from the source up to 220 keV. The ⁴⁴Ti line emission at 68 and 78 keV is confirmed by our observations with a mean flux of ∼(2.2 ± 0.4) × 10⁻⁵ ph cm⁻² s⁻¹, corresponding to a ⁴⁴Ti yield in Cas A of (1.3 ± 0.4) × 10⁻⁴ M _⊙. The continuum emission from 3 to 500 keV can be fit with a thermal bremsstrahlung of kT ∼ 0.79 ± 0.08 keV plus a power-law model of Γ ∼ 3.13 ± 0.03. The non-thermal emission from Cas A is well fit by a power-law model without a cutoff up to 220 keV. This radiation characteristic is inconsistent with diffusive shock acceleration models with a remnant shock velocity of only 5000 km s⁻¹. The central compact object in Cas A cannot significantly contribute to the emission above 80 keV. Some possible physical origins of the non-thermal emission above 80 keV from the remnant shock are discussed. We deduce that the asymmetrical supernova explosion scenario of Cas A is a promising scenario for the production of high-energy synchrotron radiation photons, where a portion of the ejecta with a velocity of ∼0.1c and opening angle of ∼10° can account for the 100 keV emission, as is consistent with the “jet” observed in Cas A.