Keywords

GRB 200612A could be classified as an ultralong gamma-ray burst due to its prompt emission lasting up to ∼1020 s and the true timescale of the central engine activity t_burst ≥ 4 × 10⁴ s. The late X-ray light curve with a decay index of α = 7.53 is steeper than the steepest possible decay from an external shock model. We propose that this X-ray afterglow can be driven by dipolar radiation from the magnetar spindown during its early stage, while the magnetar collapsed into the black hole before its spindown, resulting in a very steep decay of the late X-ray light curve. The optical data show that the light curve is still rising after 1.1 ks, suggesting a late onset. We show that GRB 200612A's optical afterglow light curve is fitted with the forward shock model by Gaussian structured off-axis jet. This is a special case among GRBs, as it may be an ultralong gamma-ray burst powered by a magnetar in an off-axis observation scenario.

Recently another long period radio pulsar GPM J1839−10 has been reported, similar to GLEAM-X J162759.5−523504.3. Previously, the energy budget and rotational evolution of long period radio pulsars had been considered. This time, the death line and pulse width for neutron star and white dwarf pulsars are investigated. The pulse width is included as the second criterion for neutron star and white dwarf pulsars. It is found that: (1) PSR J0250+5854 and PSR J0901−4046 etc. should be normal radio pulsars. They have narrow pulse width and they lie near the radio emission death line. (2) The two long period radio pulsars GLEAM-X J162759.5−523504.3 and GPM J1839−10 are unlikely to be normal radio pulsars. Their possible pulse width is relatively large. They lie far below the fiducial death line on the $P-\dot{P}$ diagram. (3) GLEAM-X J162759.5−523504.3 and GPM J1839−10 may be magnetars or white dwarf radio pulsars. At present, there are many parameters and uncertainties in both of these possibilities.

In this paper, we study five luminous supernovae (LSNe) Ibc (SN 2009ca, ASASSN-15mj, SN 2019omd, SN 2002ued, and SN 2021bmf) whose peak absolute magnitudes M_peak are ≈ −19.5 to −21 mag by fitting their multi-band light curves (LCs) with different energy source models. We find that SN 2009ca might be powered by the ⁵⁶Ni model since the required ⁵⁶Ni mass (0.56 M_⊙) is comparable to those of energetic SNe Ic, while the rest four SNe cannot be accounted for the ⁵⁶Ni model since their derived ⁵⁶Ni masses are ≳1 M_⊙ or the ratios of the ⁵⁶Ni mass to the ejecta mass are larger than 0.2. This indicates that some LSNe might be powered by ⁵⁶Ni decay, while most of them need additional energy sources. We then use the magnetar plus ⁵⁶Ni model and the fallback plus ⁵⁶Ni model to fit the LCs of the four LSNe that cannot be explained by the ⁵⁶Ni model, finding that the two models can account for the four SNe, and the derived parameters are comparable to those of LSNe or superluminous SNe in the literature, if they were (mainly) powered by magnetars or fallback. We suggest that the magnetar plus ⁵⁶Ni model is more reasonable than the fallback plus ⁵⁶Ni model, since the validity of the fallback plus ⁵⁶Ni model depends on the value of accretion efficiency (η) and favors a large η value, and the magnetar plus ⁵⁶Ni model yields smaller χ²/dof values. It should be pointed out that, however, the fallback plus ⁵⁶Ni model is still a promising model that can account for the four SNe in our sample as well as other LSNe.

As one class of the most important objects in the universe, magnetars can produce a lot of different frequency bursts including X-ray bursts. In Cai et al., 75 X-ray bursts produced by magnetar SGR J1935+2154 during an active period in 2020 are published, including the duration and net photon counts of each burst, and waiting time based on the trigger time difference. In this paper, we utilize the power-law model, ${dN}{(x)/{dx}\propto (x+{x}_{0})}^{-{\alpha }_{x}}$, to fit the cumulative distributions of these parameters. It can be found that all the cumulative distributions can be well fitted, which can be interpreted by a self-organizing criticality theory. Furthermore, we check whether this phenomenon still exists in different energy bands and find that there is no obvious evolution. These findings further confirm that the X-ray bursts from magnetars are likely to be generated by some self-organizing critical process, which can be explained by a possible magnetic reconnection scenario in magnetars.

The physical mechanism of fast radio bursts (FRBs) is still unknown. On 2020 April 28, a special radio burst, FRB 200428, was detected and believed to be associated with the Galactic magnetar SGR 1935+2154. It confirms that at least some of the FRBs were generated by magnetars, although the radiation mechanism continues to be debated. To this end, we study in detail the multiband afterglows of FRB 200428 described by the synchrotron fireball shock model. We find the prediction for the optical and radio afterglows of FRB 200428 is consistent with the observations when considering the post-FRB optical and radio upper limits from the literature. We also show that the follow up detection of the afterglows from fast radio bursts like—FRB 200428 is possible at the radio band, though challenging. Based on our model, one can obtain information about the energy of the fireball, the radiation zone, and the nature of the surrounding medium. That may shed light on the physical mechanism of FRBs.

Fast radio bursts (FRBs) are extragalactic radio transients with millisecond duration and brightness temperature. An FRB-associated X-ray burst (XRB) was recently found to arise from the Galactic magnetar SGR J1935+2154. Following the model of Dai, in which an FRB may originate from a magnetar encountering an asteroid, we focus on explaining the spectrum of the XRB associated with FRB 200428 from SGR J1935+2154. Collisions between asteroidal fragments and the magnetar surface produce a fireball, which further expands relativistically. Due to the velocity difference among some shells in the fireball, internal shocks would form far away from the magnetar, and further emit X-ray emission. We propose that the FRB-associated XRB can be produced by synchrotron emission from the internal shocks, and then constrain the physical parameters by the observed XRB spectrum.

Quasi-periodic oscillation (QPO) signals are discovered in some fast radio bursts (FRBs) such as FRB 20191221A, as well as in the X-ray burst associated with the galactic FRB from SGR 1935+2154. We revisit the intermediate-field FRB model where the radio waves are generated as fast-magnetosonic waves through magnetic reconnection near the light cylinder. The current sheet in the magnetar wind is compressed by a low frequency pulse emitted from the inner magnetosphere to trigger magnetic reconnection. By incorporating the wave dynamics of the magnetosphere, we demonstrate how the FRB frequency, the single pulse width, and luminosity are determined by the period, magnetic field, QPO frequency and quake energetics of the magnetar. We find that this model can naturally and self-consistently interpret the X-ray/radio event from SGR 1935+2154 and the QPO in FRB 20191221A. It can also explain the observed wide energy range of repeating FRBs in a narrow bandwidth.

The magnetar SGR 1935+2154 is reported to have an anti-glitch, accompanied by fast radio bursts, and transient pulsed radio emission. In the wind braking model, this triplet event tells people that (1) SGR 1935+2154 does not have a strong particle wind and can be approximated by magnetic dipole braking in the persistent state; (2) its anti-glitch is due to an enhanced particle wind, similar to the first anti-glitch in magnetars; (3) its transient pulsed radio emission may be due to a decreasing emission beam during the outburst; (4) the enhanced particle acceleration potential and pulsar death line may not be the dominate factor.

The observed electromagnetic radiation from some long and short gamma-ray bursts, and neutron stars (NSs), and the theoretical models proposed to interpret these observations together point to a very interesting but confusing problem, namely, whether fall-back accretion could lead to dipole field decay of newborn NSs. In this paper, we investigate the gravitational wave (GW) radiation of newborn magnetars with a fall-back disk formed in both the core-collapse of massive stars and the merger of binary NSs. We make a comparison of the results obtained with and without fall-back accretion-induced dipole-field decay (FADD) involved. Depending on the fall-back parameters, initial parameters of newborn magnetars, and models used to describe FADD, FADD may indeed occur in newborn magnetars. Because of the low dipole fields caused by FADD, the newborn magnetars will be spun up to higher frequencies and have larger masses in comparison with the non-decay cases. Thus the GW radiation of newborn accreting magnetars would be remarkably enhanced. We propose that observation of GW signals from newborn magnetars using future GW detectors may help to reveal whether FADD could occur in newborn accreting magnetars. Our model is also applied to the discussion of the remnant of GW170817. From the post-merger GW searching results of Advanced LIGO and Advanced Virgo we cannot confirm the remnant is a low-dipole-field long-lived NS. Future detection of GWs from GW170817-like events using more sensitive detectors may help to clarify the FADD puzzle.

As the third paper in the multiple-part series, we report the statistical properties of radio bursts detected from the repeating fast radio burst (FRB) source FRB 20201124A with the Five-hundred-meter Aperture Spherical radio Telescope during an extremely active episode between the 25th and 28th of September 2021 (UT). We focus on the polarization properties of 536 bright bursts with S/N > 50. We found that the Faraday rotation measures (RMs) monotonically dropped from −579 to −605 rad m⁻² in the 4 day window. The RM values were compatible with the values (−300 to −900 rad m⁻²) reported 4 months ago. However, the RM evolution rate in the current observation window was at least an order of magnitude smaller than the one (∼500 rad m⁻² day⁻¹) previously reported during the rapid RM-variation phase, but is still higher than the one (≤1 rad m⁻² day⁻¹) during the later RM no-evolution phase. The bursts of FRB 20201124A were highly polarized with the total degree of polarization (circular plus linear) greater than 90% for more than 90% of all bursts. The distribution of linear polarization position angles (PAs), degree of linear polarization (L/I) and degree of circular polarization (V/I) can be characterized with unimodal distribution functions. During the observation window, the distributions became wider with time, i.e., with larger scatter, but the centroids of the distribution functions remained nearly constant. For individual bursts, significant PA variations (confidence level 5σ) were observed in 33% of all bursts. The polarization of single pulses seems to follow certain complex trajectories on the Poincaré sphere, which may shed light on the radiation mechanism at the source or the plasma properties along the path of FRB propagation.