Keywords

Fast radio bursts (FRBs) are short pulses observed in radio frequencies usually originating from cosmological distances. The discovery of FRB 200428 and its X-ray counterpart from the Galactic magnetar SGR J1935+2154 suggests that at least some FRBs can be generated by magnetars. However, the majority of X-ray bursts from magnetars are not associated with radio emission. The fact that only in rare cases can an FRB be generated raises the question regarding the special triggering mechanism of FRBs. Here we report long time spin evolution of SGR J1935+2154 until the end of 2022. According to ν and $\dot{\nu }$, the spin evolution of SGR J1935+2154 could be divided into two stages. The first stage evolves relatively steady evolution until 2020 April 27. After the burst activity in 2020, the spin of SGR J1935+2154 shows strong variations, especially for $\dot{\nu }$. After the burst activity in 2022 October, a new spin-down glitch with Δν/ν = (–7.2 ± 0.6) × 10⁻⁶ is detected around MJD 59876, which is the second event in SGR J1935+2154. At the end, spin frequency and pulse profile do not show variations around the time of FRB 200428 and radio bursts 221014 and 221021, which supply strong clues to constrain the trigger mechanism of FRBs or radio bursts.

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 detection of quasi-periodic oscillations (QPOs) in magnetar giant flares (GFs) has brought a new perspective to studies of the mechanism of magnetar bursts. Due to the scarcity of GFs, searching for QPOs in magnetar short bursts is reasonable. Here we report the detection of a narrow QPO at approximately 110 Hz and a wide QPO at approximately 60 Hz in the short magnetar burst SGR 150228213, with a confidence level of 3.35σ. This burst was initially attributed to 4U 0142+61 by Fermi/GBM on location, but we have not detected such QPOs in other bursts from this magnetar. We also found that there was a repeating fast radio burst associated with SGR 150228213 on location. Finally, we discuss the possible origins of SGR 150228213.

We present the timing and spectral studies of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 giant outbursts using observations with the SXT and LAXPC instruments on the AstroSat satellite. Unlike the 1998 and 2010 outbursts, where a giant outburst was followed by several low intensity periodic outbursts, the 2018 and 2021 outbursts were single outbursts. The X-ray pulsations are detected over a broad energy band covering 0.5–80 keV from the compact object. We construct the spin evolution history of the pulsar over two decades and find that the pulsar spins-up during the outbursts but switches to spin-down state in the quiescent periods between the outbursts. Energy resolved pulse profiles generated in several bands in 0.5–80 keV show that the pulse shape varies with the energy. The energy spectrum of the pulsar is determined for the 2018 and 2021 outbursts. The best fit spectral models require presence of cyclotron resonant scattering feature at about 43 keV in the energy spectra of both the outbursts. We find indication of possible reversal in the correlation between the cyclotron line energy and luminosity which needs to be ascertained from future observations. Using the best fit spectra the X-ray luminosity of XTE J1946+274 is inferred to be 2.7 × 10³⁷ erg s⁻¹ for the 2018 observations and 2.3 × 10³⁷ erg s⁻¹ for the 2021 observations. We discuss possible mechanisms which can drive outbursts in this transient Be X-ray binary.

The fast blue optical transients (FBOTs) are a new population of extragalactic transients of unclear physical origin. A variety of mechanisms has been proposed including failed supernova explosion, shock interaction with a dense medium, young magnetar, accretion onto a compact object and stellar tidal disruption event, but none is conclusive. Here we report the discovery of a possible X-ray quasi-periodicity signal with a period of ∼250 s (at a significance level of 99.76%) in the brightest FBOT AT2018cow through the analysis of XMM-Newton/PN data. The signal is independently detected at the same frequency in the average power density spectrum from data taken from the Swift telescope, with observations covering from 6 to 37 days after the optical discovery, though the significance level is lower (94.26%). This suggests that the quasi-periodic oscillation (QPO) frequency may be stable over at least 1.1 × 10⁴ cycles. Assuming the ∼250 s QPO to be a scaled-down analog of that typically seen in stellar mass black holes, a black hole mass of ∼10³–10⁵ solar masses could be inferred. The overall X-ray luminosity evolution could be modeled with a stellar tidal disruption by a black hole of ∼10⁴ solar masses, providing a viable mechanism to produce AT2018cow. Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes.

Crust cooling of soft X-ray transients has been observed after outbursts, but an additional shallow heating during accretion in outburst is needed to explain the quiescent light curve. However, shallow heating is significantly different between sources and even within one source between different outbursts, and the source of shallow heat is as yet unknown. Using the open source code "dStar" which solves the fully general relativistic heat diffusion equation for the crust, we investigate the effect of magnitude and depth of shallow heating on crust cooling and find that some exceptional sources (Swift J174805.3-244637, MAXI J0556-332 during outburst II and GRO J1750-27) in which shallow heating may be inactive could be explained by a deeper shallow heating mechanism. We compare our results with those from previous works and find that the shallow heating is model dependent. In addition, the effects of mass and radius of a neutron star on shallow heating are studied, and it is shown that the more compact the star, the less shallow heating will be needed to fit the crust cooling light curves.

We describe a blind uniform search for thermonuclear burst oscillations (TBOs) in the majority of Type I bursts observed by the Rossi X-ray Timing Explorer (RXTE) (2118 bursts from 57 neutron stars). We examined 2–2002 Hz power spectra from the Fourier transform in sliding 0.5–2 s windows, using fine-binned light curves in the 2–60 keV energy range. The significance of the oscillation candidates was assessed by simulations which took into account light-curve variations, dead time, and the sliding time windows. Some of our sources exhibited multi-frequency variability at ≲15 Hz that cannot be readily removed with light-curve modeling and may have an astrophysical (non-TBO) nature. Overall, we found that the number and strength of potential candidates depends strongly on the parameters of the search. We found candidates from all previously known RXTE TBO sources, with pulsations that had been detected at similar frequencies in multiple independent time windows, and discovered TBOs from SAX J1810.8−2658. We could not confirm most previously reported tentative TBO detections or identify any obvious candidates just below the detection threshold at similar frequencies in multiple bursts. We computed fractional amplitudes of all TBO candidates and placed upper limits on non-detections. Finally, for a few sources we noted a small excess of candidates with powers comparable to fainter TBOs, but appearing in single independent time windows at random frequencies. At least some of these candidates may be noise spikes that appear interesting due to selection effects. The potential presence of such candidates calls for extra caution if claiming single-window TBO detections.

Type I X-ray bursts are thermonuclear burning events that occur on the surfaces of accreting neutron stars. Burning begins in a localized spot in the star's ocean layer before propagating across the entire surface as a deflagration. On the scale of the entire star, the burning front can be thought of as discontinuity. To model this, we investigated the reactive Riemann problem for relativistic deflagrations and detonations and developed a numerical solver. Unlike for the Newtonian Riemann problem, where only the velocity perpendicular to the interface is relevant, in the relativistic case the tangential velocity becomes coupled through the Lorentz factor and can alter the waves present in the solution. We investigated whether a fast tangential velocity may be able to cause a deflagration wave to transition to a detonation. We found that such a transition is possible, but only for tangential velocities that are a significant fraction of the speed of light or for systems already on the verge of transitioning. Consequently, it is highly unlikely that this transition would occur for a burning front in a neutron star ocean without significant contributions from additional multidimensional effects.

We report for the first time below 1.5 keV, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass X-ray binary 4U 1608–52. Our time-resolved spectroscopy of the burst is consistent with a model consisting of a varying-temperature blackbody, and an evolving persistent flux contribution, likely attributed to the accretion process. The dip in the burst intensity before the secondary peak is also visible in the bolometric flux. Prior to the dip, the blackbody temperature reached a maximum of ≈3 keV. Our analysis suggests that the dip and secondary peak are not related to photospheric expansion, varying circumstellar absorption, or scattering. Instead, we discuss the observation in the context of hydrodynamical instabilities, thermonuclear flame spreading models, and reburning in the cooling tail of the burst.

Particle acceleration in solar flares remains an outstanding problem in solar physics. It is currently unclear which of the acceleration mechanisms dominates and how exactly the excessive magnetic energy is transferred to nonthermal and other forms of energy. We emphasize that the ultimate acceleration mechanism must be capable of efficiently working in the most extreme conditions, such as the shortest detected timescales and the highest acceleration efficiency. Here we focus on a detailed multiwavelength analysis of the initial phase of the SOL2011-08-04 flare, which demonstrated prominent short subpeaks of nonthermal emission during filament eruption associated with the flare. We demonstrate that the three-dimensional configuration of the flare, combined with timing and spectral behavior of the rapidly varying component, put very stringent constraints on the acceleration regime. Specifically, the rapid subpeaks are generated by short injections of nonthermal electrons with a reasonably hard, single power-law spectrum and a relatively narrow spread of pitch-angles along the mean magnetic field. The acceleration site is a compact volume located near the top of the extended coronal loop(s). The electrons are promptly accelerated up to several hundreds of keV, with the characteristic acceleration time shorter than 50 ms. We show that these properties are difficult to reconcile with widely adopted stochastic acceleration models, while the data inescapably require acceleration by a super-Dreicer electric field, whether regular or random.