We report the detection and follow-up of a superstellar flare GWAC 181229A with an amplitude of ΔR ∼ 9.5 mag on an M9-type star by SVOM/GWAC and the dedicated follow-up telescopes. The estimated bolometric energy E bol is (5.56–9.25) × 1034 erg, which makes the event one of the most powerful flares seen on ultracool stars. The magnetic strength is inferred to be 3.6–4.7 kG. Thanks to sampling with a cadence of 15 s, a new component near the peak time with a very steep decay is detected in the R-band light curve, followed by the two-component flare template given by Davenport et al. An effective temperature of 5340 ± 40 K is measured by fitting a blackbody shape to the spectrum in the shallower phase during the flare. The filling factors of the flare are estimated to be ∼30% and 19% at the peak time and at 54 minutes after the first detection. The detection of this particular event with large amplitude, huge emitted energy, and a new component demonstrates that high-cadence sky monitoring cooperation with fast follow-up observations is very important for understanding the violent magnetic activity.
Gamma-ray burst (GRB) 150910A was detected by Swift/Burst Alert Telescope (BAT), and then rapidly observed by Swift/XRT, Swift/Ultraviolet-Optical Telescope, and ground-based telescopes. We report Lick Observatory spectroscopic and photometric observations of GRB 150910A, and we investigate the physical origins of both the optical and X-ray afterglows, incorporating data obtained with BAT and XRT. The light curves show that the jet-emission episode lasts ∼360 s with a sharp pulse from BAT to XRT (Episode I). In Episode II, the optical emission has a smooth onset bump followed by a normal decay (α R,2 ≈ −1.36), as predicted in the standard external shock model, while the X-ray emission exhibits a plateau (α X,1 ≈ −0.36) followed by a steep decay (α X,2 ≈ −2.12). The light curves show obvious chromatic behavior with an excess in the X-ray flux. Our results suggest that GRB 150910A is an unusual GRB driven by a newly born magnetar with its extremely energetic magnetic dipole (MD) wind in Episode II, which overwhelmingly dominates the observed early X-ray plateau. The radiative efficiency of the jet prompt emission is η γ ≈ 11%. The MD wind emission was detected in both the BAT and XRT bands, making it the brightest among the current sample of MD winds seen by XRT. We infer the initial spin period (P 0) and the surface polar cap magnetic field strength (B p ) of the magnetar as 1.02 × 1015 G ≤ B p ≤ 1.80 × 1015 G and 1 ms ≤ P 0 v ≤ 1.77 ms, and the radiative efficiency of the wind is η w ≥ 32%.
The physical composition of the ejecta of gamma-ray bursts (GRBs) remains an open question. The radiation mechanism of the prompt gamma rays is also in debate. This problem can be solved for the bursts hosting distinct thermal radiation. However, the events with dominant thermal spectral components are still rare. In this work, we focus on GRB 220426A, a recent event detected by the Fermi Gamma-ray Burst Monitor. The time-resolved and time-integrated data analyses yield very hard low-energy spectral indices and rather soft high-energy spectral indices. This means that the spectra of GRB 220426A are narrowly distributed. And the Bayesian inference results are in favor of the multicolor blackbody model. The physical properties of the relativistic outflow are calculated. Assuming a redshift z = 1.4, the bulk Lorentz factors Γ of the shells are found to be between 274 − 18 + 24 and 827 − 71 + 100 , and the corresponding photosphere radii R ph are in the range of 1.83 − 0.50 + 0.52 × 10 11 and 2.97 − 0.15 + 0.14 × 10 12 cm. Similar to GRB 090902B, the time-resolved properties of GRB 220426A satisfy the observed Γ–L and E p –L correlations, where L is the luminosity of the prompt emission and E p is the spectral peak energy.
Assuming that the shallow-decaying phase in the early X-ray lightcurves of gamma-ray bursts (GRBs) is attributed to the dipole radiations (DRs) of a newborn magnetar, we present a comparative analysis for the magnetars born in death of massive stars and merger of compact binaries with long and short GRB (lGRB and sGRB) data observed with the Swift mission. We show that the typical braking index (n) of the magnetars is ∼3 in the sGRB sample, and it is ∼4 for the magnetars in the lGRB sample. Selecting a sub-sample of the magnetars whose spin-down is dominated by DRs (n ≲ 3) and adopting a universal radiation efficiency of 0.3, we find that the typical magnetic field strength (Bp) is 1016 G versus 1015 G and the typical initial period (P0) is ∼20 ms versus 2 ms for the magnetars in the sGRBs versus lGRBs. They follow the same relation between P0 and the isotropic GRB energy as $P_0\propto E_{\rm jet}^{-0.4}$. We also extend our comparison analysis to superluminous supernovae (SLSNe) and stable pulsars. Our results show that a magnetar born in merger of compact stars tends to have a stronger Bp and a longer P0 by about one order of magnitude than that born in collapse of massive stars. Its spin-down is dominated by the magnetic DRs as old pulsars, being due to its strong magnetic field strength, whereas the early spin-down of magnetars born in massive star collapse is governed by both the DRs and gravitational wave (GW) emission. A magnetar with a faster rotation speed should power a more energetic jet, being independent of its formation approach.
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