Internal Plateau Research Articles

Quasiperiodic Oscillations in GRB 210514A: a Case of a Newborn Supramassive Precessing Magnetar Collapsing into a Black Hole?

Magnetar is proposed as one of the possible central engines for a gamma-ray burst (GRB). Recent studies show that if a magnetar has a rotational axis misaligned from the magnetic one, a periodic lightcurve pattern is expected with a period of seconds to minutes. Inspired by this unique feature, in this paper, we search for the quasiperiodic oscillation (QPO) signals in the Swift observations of GRBs. Using the Lomb–Scargle periodogram and the weighted wavelet Z-transform algorithms, we find that the Swift Burst Alert Telescope data of GRB 210514A has a QPO signal with a period ∼11 s. The estimated confidence level of the signal is over 3σ. The global lightcurve of this GRB exhibits a double-plateau structure with a sharp decay segment between plateaus. The lightcurve feature resembles those of GRBs that were reported to have internal plateaus. We explain the observations of GRB 210514A with a supramassive magnetar (SMM) model, where the QPO signal in the first plateau is produced via the dipole radiation of the SMM experiencing a precession motion, the sharp decay is due to the collapse of the SMM into a black hole (BH), and the second plateau could be produced via the fallback accretion of the newborn BH. We fit the precession model to the observations using the Bayesian statistic and the best-fit magnetar parameters are discussed. Alternative models concerning a BH central engine may also provide reasonable explanations for this burst, only in this case the QPO signal could merely be a coincidence.

GRB 210323A: Signature of Long-lasting Lifetime of Supra-massive Magnetar as the Central Engine from the Merger of Binary Neutron Star

Theoretically, a supra-massive neutron star or magnetar may be formed after the merger of binary neutron stars. GRB 210323A is a short-duration gamma-ray burst (GRB) with a duration of lasting ∼1 s. The light curve of the prompt emission of GRB 210323A shows a signal-peaked structure and a cutoff power-law model can adequately fit the spectra with E p = 1826 ± 747. More interestingly, it has an extremely long-lasting plateau emission in the X-ray afterglow with a duration of ∼104 s, and then follows a rapid decay with a decay slope ∼3.2. This temporal feature is challenging by invoking the external shock mode. In this paper, we suggest that the observed long-lasting X-ray plateau emission is caused by the energy injection of dipole radiation from supra-massive magnetar, and the abrupt decay following the long-lasting X-ray plateau emission is explained by supra-massive magnetar collapsing into a black hole. It is the short GRB (SGRB) with the longest X-ray internal plateau emission powered by a supra-massive neutron star. If this is the case, one can estimate the physical parameters of a supra-massive magnetar, and compare with other SGRBs. We also discuss the possible gravitational-wave emission, which is powered by a supra-massive magnetar and its detectability, and the possible kilonova emission, which is powered by r-process and magnetar spin-down to compare with the observed data.

GRB 210610B: The Internal and External Plateau as Evidence for the Delayed Outflow of Magnetar

After launching a jet, outflows of magnetar were used to account for the achromatic plateau of afterglow and the early X-ray flux plateau known as “internal plateau”. The lack of detecting magnetic dipole emission together with the energy injection feature in a single observation poses confusion until the long gamma-ray burst (GRB) 210610B is detected. GRB 210610B is presented with an optical bump following an early X-ray plateau during the afterglow phase. The plateau followed by a steep decline flux overlays in the steadily decaying X-ray flux with index α X,1 ∼ 2.06, indicating an internal origin and that can be fitted by the spin-down luminosity law with the initial plateau luminosity log10LX∼48.29ergs−1 and the characteristic spin-down timescale T ∼ 2818 s. A subsequent bump begins at ∼4000 s in the R band with a rising index α R,1 ∼ − 0.30 and peaks at ∼14125 s, after which a decay index α R,2 ∼ 0.87 and finally transiting to a steep decay with α R,3 ∼ 1.77 achieve the closure relation of the external shock for the normal decay phase as well as the magnetar spin-down energy injection phase, provided that the average value of the photon index Γ γ = 1.80 derived from the spectral energy distributions (SEDs) between the X-ray and optical afterglow. The closure relation also works for the late X-ray flux. Akin to the traditional picture of GRB, the outflow powers the early X-ray plateau by dissipating energy internally and collides with the leading decelerating blast burst as time goes on, which could interpret the exotic feature of GRB 210610B. We carry out a Markov Chain Monte Carlo simulation and obtain a set of best parameters: ϵ B ≃ 4.7 × 10−5, ϵ e ≃ 0.15, E K,iso ≃ 4.6 × 1053erg, Γ0 ≃ 832, A * ≃ 0.10, L inj,0 ≃ 3.55 × 1050erg s−1. The artificial light curve can fit the afterglow data well. After that, we estimated the average Lorentz factor and the X-ray radiation efficiency of the later ejecta are 35% and 0.13%, respectively.

Revisiting the Constraint on the Equation of State of Neutron Stars Based on Binary Neutron Star Mergers

The merger of a neutron star (NS)–NS binary can form different productions of compact remnants, among which a supramassive NS (SMNS) can create an internal plateau, and the following steep decay marks the collapse of the SMNS. The proportion of the SMNS and the corresponding collapse time are often used to constrain the NS equation of state (EOS). This paper revisits this topic by considering the effect of an accretion disk on a compact remnant, which is not considered in previous works. Compared with previous works, the collapse-time distribution (peaks ∼100 s) of SMNSs formed from an NS–NS merger is almost unaffected by the initial surface magnetic field (B s,i ) of the NS, but the total energy output of the magnetic dipole radiation from the SMNSs depends on B s,i significantly. Coupling the constraints from the SMNS fraction, we exclude some EOSs and obtain three candidate EOSs, i.e., DD2, ENG, and MPA1. By comparing the distributions of the collapse time and the luminosity of the internal plateau (in the short gamma-ray bursts) for observations obtained based on the three candidate EOSs, it is shown that only the EOS of ENG is favored. Our sample, based on the ENG EOS and a mass distribution motivated by Galactic systems, suggests that approximately 99% of NS–NS mergers collapse to form a black hole (BH) within 107s. This includes scenarios promptly forming a BH (36.5%), an SMNS (60.7%), or a stable NS that transitions into a BH or an SMNS following accretion (2.1%). It also indicates that the remnants for GW170817 and GW190425, and the second object of GW190814, are more likely to be BHs.

Black Hole Activity Imprints on the Internal Plateau and the Subsequent Sharp Decay

A stellar-mass black hole (BH) or a millisecond magnetar is believed to be born as the central engine of gamma-ray bursts (GRBs). The presence of plateaus in the X-ray extended emission or afterglow of GRBs is widely accepted as an indicator of a magnetar central engine, particularly those with a sharp decay (faster than t −3), the so-called internal plateau. However, an alternative model, by taking the evolution of the magnetic flux at the BH horizon into account, suggests that an internal plateau can also arise from a jet powered by the Blandford–Znajek (BZ) mechanism (hereafter, a BZ jet). In this study, we propose that a precessional BZ jet would manifest a quasiperiodic oscillation (QPO) signature on the internal plateau and the subsequent sharp decay. Such lightcurves cannot be readily explained by the activity of a short-lived, supermassive magnetar, thus favoring a Kerr BH as the central engine. The X-ray afterglow of GRB 050904, comprising nine flares, is characterized by a QPO-modulated plateau and sharp decay, which can be well reproduced by a precessional BZ jet model. Therefore, one potential clue for distinguishing between these two engines lies in whether the QPO signature is present throughout the entire plateau and the subsequent sharp decay, as the magnetar scenario suggests a collapse at the end of the plateau.

Sharp Decay of the Internal Plateau in Gamma-Ray Bursts: The Effect of a Cocoon's Thermal Photons on the Jet Emission

Sub-TeV or TeV γ-rays have been convincingly detected in the afterglows of some gamma-ray bursts (GRBs). The relativistic jet associated with GRBs may be surrounded by a cocoon and thus the thermal photons of the cocoon pervading the emission region of the jet can be upscattered to the very-high-energy regime (e.g., sub-TeV/TeV) by the electrons in the jet. In this scenario, the electrons in the jet cool down and thus the jet emission in the low-energy regime, e.g., the X-ray or optical, may be suppressed. In this paper, we study the effect of the cocoon's thermal photons on the jet emission in the low-energy regime. We find that the jet emission in the low-energy regime is significantly suppressed when the emission region of the jet is just encircled by a hot cocoon. Correspondingly, there is a sharp decay in the light curves of the jet emission in the low-energy regime, which can be understood to signify strong TeV emission in GRBs. Since the radius of the cocoon expands with the observer time t obs, the suppression of the jet emission in the low-energy regime may be relieved and thus the light curves of the jet emission would return to those of emission without a cocoon-external inverse-Compton cooling effect. For a long-lasting jet with power , an internal plateau followed by a sharp decay may generally appear.

What constraints can one pose on the maximum mass of neutron stars from multimessenger observations?

ABSTRACT The maximum mass of neutron stars (MTOV) plays a crucial role in understanding their equation of state (EoS). Previous studies have used the measurements for the compactness of massive pulsars and the tidal deformability of neutron stars in binary neutron star (BNS) mergers to constrain the EoS and thus the MTOV. The discovery of the most massive pulsar, PSR J0952−0607, with a mass $\sim 2.35\, {\rm M}_{\odot }$, has provided a valuable lower limit for MTOV. Another efficient method to constrain MTOV is by examining the type of central remnant formed after a BNS merger. Gravitational wave (GW) data can provide the total mass of the system, while accompanying electromagnetic signals can help infer the remnant type. In this study, we combine all the previous constraints and utilize the observational facts that about 24 per cent of the short gamma-ray bursts are followed by an X-ray internal plateau, which indicate that roughly this fraction of BNS mergers yield supermassive neutron stars, to perform (Markov Chain) Monte Carlo simulations. These simulations allow us to explore the probability density distribution of MTOV and other parameters related to BNS mergers. Our findings suggest that MTOV is likely around $2.49\!-\!2.52\, {\rm M}_{\odot }$, with an uncertainty range of approximately [$-0.16$, $0.15\, {\rm M}_{\odot }$] ([$-0.28$, $0.26\, {\rm M}_{\odot }$]) at 1σ (2σ) confidence level. Furthermore, we examine the type of merger remnants in specific events like GW170817 and GW190425 to further constrain MTOV and other relevant parameters, which can help to understand the physical processes involved in BNS mergers.

GRB 160821B late afterglow rebrightening: A new candidate for magnetar-powered Merger-novae

For the first time on August 17, 2017, the LIGO-Virgo scientific collaboration detected a gravitational wave (GW) signal from a binary neutron star (BNS) merger event (i.e., GW170817), along with its multiwavelength electromagnetic counterparts. The detection of GW170817 confirmed that the BNS merger can produce both Gamma-ray Bursts (GRBs) and Merger-novae. Therefore, searching for the Merger-novae signature could help to justify whether those short GRBs without the detected GW parts are indeed resulting from the merger process of double compact stars. In this study, we thoroughly investigate the special characteristics of GRB 160821B's afterglow (including its internal X-ray plateau, late optical, and simultaneous X-ray rebrightening). We found that its multiband afterglow data could be well-interpreted by invoking magnetar dipole radiation, external shock afterglow emission and magnetar-powered Merger-novae radiation, inferring that the rebrightening signature of GRB 160821B is a novel candidate for magnetar-powered Merger-novae. The discovery of Merger-novae candidates, or GRBs, is important as they can provide an important basis for studying the merger rate of compact binary stars at high redshift, which is essential for studying the origin of heavy elements in the universe.

Late Cenozoic Extensional Formation of the Antofalla Depression, Southern Puna Plateau, Argentina: An Effect of Lithospheric Foundering?

AbstractDeformation within orogenic plateaus functions to establish a dynamic equilibrium between tectonic boundary stresses and plateau gravitational potential energy. Temporal changes in deformation kinematics record perturbations to boundary stresses or internal plateau processes, such as lithospheric foundering. We integrate new mapping, field observations, and geochronologic ages with published data to document complex late Cenozoic upper crustal deformation in the region of the Antofalla depression, a ∼125 km long, sublinear basin within the southern Puna orogenic plateau, Argentina. The juxtaposition of stratal ages across the depression requires >900 m vertical offset on a surface‐breaking fault. Regional geologic structure, basin geomorphology, and our observation of a breccia paralleling the depression margin suggest formation of the depression by normal faulting. We interpret published stratigraphic logs to suggest that the depression formed between ca. 16 and 11 Ma following Andean shortening. Folded Late Miocene to Quaternary strata on the eastern depression margin indicate that extension ended and shortening resumed before present, revealing toggling between extensional and contractional kinematic regimes. The kinematic evolution of the Antofalla depression contrasts with the rest of the southern Puna plateau, which underwent shortening until latest Miocene to Quaternary time, followed by extension and strike‐slip deformation. Taken together, the spatial and temporal variations in late Cenozoic deformation of the southern Puna plateau are inconsistent with mechanisms that would affect the entire orogen, such as slowing convergence, but are compatible with lithospheric foundering.

Quaternary uranium mineralization in the Qaidam Basin, northern Tibetan Plateau: Insights from petrographic and C-O isotopic evidences

There are several sandstone-type uraniferous basins around Tibetan Plateau. The Qaidam Basin is the largest sedimentary basin in the internal Tibetan Plateau, hosting important oil and coal resources, and has good potential for sandstone-hosted uranium resources. In this study, we examine the petrographic and mineralogical characteristics of a uranium mineralization interval at the Neogene-Quaternary transition (NQT) in the western Qaidam Basin using optical microscopy and electron probe microanalyzer (EPMA), including back scattered electron (BSE) imaging and energy-dispersive spectroscopy (EDS). The sedimentary lithofacies features and C-O stable isotopes of carbonate cement from two boreholes were analyzed, especially for the uraniferous interval. Whole-rock analyses of ores and barren rocks yielded highly variable U concentrations ranging from 1.5 to 2548 ppm. The main uranium minerals are coffinite and uraninite. The co-occurrence of uranium minerals with calcite indicates that uranium mineralization and calcite cementation were coeval. Coffinite mainly occurs at the interface between calcite cement and other minerals. The carbonates have δ13CV-PDB ranging from −29.67 to +3.93‰, suggesting involvement of organic carbon during uranium mineralization. This is also supported by the general trend of decreasing δ13CV-PDB and increasing U content from the conglomerate/coarse sandstone to the mudstone samples, and the overlap of δ13CV-PDB values between the calcite cement and organic matter in sedimentary rocks. The organic carbon may have been derived from hydrocarbons sourced from the deeper part of the basin and/or biogenic gases at shallow depths. The unconformity at the NQT may represent a favorable interval where oxidizing fluids carrying uranium met with reducing fluids carrying hydrocarbons and resulted in uranium mineralization. Thus, the NQT may serve as a guide for uranium exploration in this basin.

Nonrigid Bookshelf Kinematics of Northeastern Tibet: Constrains from Fault Slip Rates around the Qinghai Lake and Chaka-Gonghe Basins

Abstract The Elashan fault (ELSF) and Qinghainanshan fault (QHNF), two major faults developed around the Qinghai Lake and Chaka-Gonghe basins, are of great importance for investigating the deformation model of the internal northeastern Tibetan Plateau. However, their late Pleistocene slip rates remain poorly constrained. In this study, we combine high-resolution topography acquired from unmanned aerial vehicles (UAV) and geomorphological dating to calculate the slip rates of the two faults. We visited the central ELSF and western QHNF and measured displaced terraces and stream channels. We collected 10Be samples on the surface of terraces to constrain the abandonment ages. The dextral slip rate of the central segment of the Elashan fault is estimated to be 2.6±1.2 mm/yr. The uplift rates since the late Pleistocene of the Elashan and Qinghainanshan faults are 0.4±0.04 mm/yr and 0.2±0.03 mm/yr, respectively. Comparing the geological rates with the newly published global positioning system (GPS) rates, we find that the slip rates of the major strike-slip faults around the Qinghai Lake and Chaka-Gonghe basins are approximately consistent from the late Pleistocene to the present day. The overall NE shortening rates by summing up the geological slip rates on major faults between the East Kunlun and Haiyuan faults are ~3.4 mm/yr, smaller than the geodetic shortening rates (~4.9 to 6.4 mm/yr), indicating that distributed deformation plays an important role in accommodating the regional deformation. By analyzing the geometrical and kinematic characteristics of the major faults surrounding the basins, we suggest that the kinematic deformation of the internal northeastern Tibet is a nonrigid bookshelf model that consists of counterclockwise rotation (~0.8° Myr-1) and distributed thrusting.

The Evolutionary Effects of the Radius and Moment of Inertia of Rapidly Rotating Neutron Stars

Abstract A newly born millisecond magnetar is thought to be the central engine of some gamma-ray bursts (GRBs), especially those that present long-lasting X-ray plateau emissions. By solving the field equations, we find that when the rotational speed of the magnetar is approaching the breakup limit, its radius R and moment of inertia I undergo an obvious evolution as the magnetar spins down. Meanwhile, the values of R and I sensitively depend on the adoption of a neutron star (NS) equation of state (EoS) and the NS baryonic mass. With different EoSs and baryonic mass considered, the magnetic dipole radiation luminosity (L dip) could be variant within one to two orders of magnitude. We thus suggest that when using the X-ray plateau data of GRBs to diagnose the properties of the nascent NSs, EoS and NS mass information should be invoked as simultaneously constrained parameters. On the other hand, due to the evolution of R and I, the temporal behavior of L dip becomes more complicated. For instance, if the spin-down process is dominated by gravitational wave emission due to the NS asymmetry caused by magnetic field distortion ( ϵ ∝ B p 2 ), the segment L dip ∝ t 0 could be followed by L dip ∝ t −γ with γ larger than 3. This case could naturally interpret the so-called internal X-ray plateau feature shown in some GRB afterglows, which means the sharp decay following the plateau unnecessarily corresponds to the collapse of the NS. This may explain why some internal X-ray plateaus are followed by late-time central engine activity, manifested through flares and second shallow plateaus.

Detectability of “Merger-nova” Emission from a Long-lived Magnetar in Short Gamma-Ray Bursts

Abstract One possible progenitor of short gamma-ray bursts (GRBs) is thought to be from a double neutron star (NS) merger, and the remnant of such a merger may be a supramassive NS, which is supported by rigid rotation and through its survival of hundreds of seconds before collapsing into a black hole (BH). If this is the case, an optical/infrared transient (namely merger-nova) is generated from the ejected materials and it is powered by radioactive decay from r-process, spin-down energy from a supramassive NS, as well as the magnetic wind from a newborn BH. In this paper, we systematically search for the signature of a supramassive NS central engine by analyzing the X-ray emission of short GRBs with internal plateau observed by Swift, and we find that five candidates of short GRBs have such a feature with redshift measurement. Then, we calculate the possible merger-nova emission from those candidates given the typical model parameters by considering the above three energy sources, and compare its brightness with the sensitivity of some optical telescopes. We find that the merger-nova emission of GRB 060801 in K-, r-, and U-bands with variations of M ej (10−4–10−2 M ⊙), κ (0.1–10 cm2 g−1), and β (0.1–0.3) is very difficult to detect using the Vera C. Rubin, Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the Zwicky Transient Facility, and the Roman Space Telescope (Roman), except for the case of large ejecta mass M ej = 10−2 M ⊙. However, we are very hopeful to detect the merger-nova emission of GRBs 090515, 100625A, and 101219A using more sensitive instruments, such as Vera C. Rubin, Pan-STARRS, and Roman. Moreover, the merger-nova emission of GRB 160821B is bright enough to detect in our calculations, and it is also consistent with current real observations of merger-nova emission.

A Possible Kilonova Powered by Magnetic Wind from a Newborn Black Hole

Abstract The merger of binary neutron stars (NS–NS) as the progenitor of short gamma-ray bursts (GRBs) has been confirmed by the discovery of the association of the gravitational-wave (GW) event GW170817 with GRB 170817A. However, the merger product of binary NS remains an open question. An X-ray plateau followed by a steep decay (“internal plateau”) has been found in some short GRBs, implying that a supramassive magnetar operates as the merger remnant and then collapses into a newborn black hole (BH) at the end of the plateau. X-ray bump or second plateau following the “internal plateau” are considered as the expected signature from the fallback accretion onto this newborn BH through the Blandford–Znajek mechanism (BZ). At the same time, a nearly isotropic wind driven by the Blandford–Payne mechanism (BP) from the newborn BH’s disk can produce a bright kilonova. Therefore, the bright kilonova observation for a short GRB with “internal plateau” (and followed by X-ray bump or second plateau) provides further evidence for this scenario. In this paper, we find that GRB 160821B is a candidate of such a case, and the kilonova emission of GRB 160821B is possibly powered by the BP wind from a newborn BH. Future GW detection of GRB 160821B–like events may provide further support to this scenario, enable us to investigate the properties of the magnetar and the newborn BH, and constrain the equation of state of neutron stars.

The electromagnetic and gravitational-wave radiations of X-ray transient CDF-S XT2

Binary neutron star (NS) mergers may result in remnants of supra-massive or even stable NS, which have been supported indirectly by observed X-ray plateau of some gamma-ray burst (GRB) afterglows. Recently, Xue et al. () discovered an X-ray transient CDF-S XT2 that is powered by a magnetar from merger of double NS via X-ray plateau and following stepper phase. However, the decay slope after the plateau emission is slightly larger than the theoretical value of spin-down in electromagnetic (EM) dominated by losing its rotation energy. In this paper, we assume that the feature of X-ray emission is caused by a supra-massive magnetar central engine for surviving thousands of seconds to collapse into a black hole. Within this scenario, we present the comparisons of the X-ray plateau luminosity, break time, and the parameters of magnetar between CDF-S XT2 and other short GRBs with internal plateau samples. By adopting the collapse time to constrain the equation of state (EOS), we find that three EOSs (GM1, DD2, and DDME2) are consistent with the observational data. On the other hand, if the most released rotation energy of magnetar is dominated by GW radiation, we also constrain the upper limit of ellipticity of NS for given EOS, and its range is [0.32-1.3] × 10−3. Its GW signal cannot be detected by Advanced LIGO or even for more sensitive Einstein Telescope in the future.

Microstructural origin of ultrahigh damping capacity in Ni50.8Ti49.2 alloy containing nanodomains induced by insufficient annealing and low-temperature aging

We report a Ni50.8Ti49.2 alloy that exhibits an ultrahigh damping capacity with an internal friction value of ~ 0.35 (tan δ) at low frequencies in a wide temperature range. This alloy undergoes low-temperature aging after insufficient annealing from a severely deformed state (~90% cold work). Via phase transformation analyses and microstructure characterization, we demonstrate that this high broad internal friction plateau originates from nanodomains and the stable coexistence of the B19’-phase and R-phase over a wide temperature range. The abundant phase boundaries and movable interfaces between nanodomains contribute significantly to this high broad internal friction plateau. Further analyses validate the critical role of low-temperature aging in inducing nanodomains, i.e., resulting in localized Ni segregation. The abnormal appearance of needle-like B19’ martensites near room temperature is attributed to the defects that remain after insufficient annealing, after which low-temperature aging promotes localized depletion or enrichment of Ni. In addition, the results of this study indicate that high density nanodomains are generated from the two steps of insufficient annealing and low-temperature aging in the severely deformed Ni50.8Ti49.2 alloy. The idea of a high-damping microstructure design based on “domain engineering” is realized.

To Power the X-Ray Plateaus of Gamma-Ray Bursts through Larger Amplitude Electromagnetic Waves

Abstract The origin of the gamma-ray burst (GRB) X-ray plateau, especially the internal plateau, is still unclear, but it could be related to a GRB’s magnetar central engine. It is generally believed that the spin-down power of the magnetar is injected into forward external shock; however, we propose here that most of the power will be dissipated behind the GRB jet through a larger amplitude electromagnetic wave. Based on this proposal, the relevant physical conditions and observational implications are analyzed and discussed, and various kinds of X-ray light curves could be reproduced. Although the chromatic multiband afterglow in the standard external afterglow fireball model is still a matter of debate, we can naturally explain this feature through this proposal, i.e., the electrons generating the X-ray plateau and emitting the optical afterglow are accelerated by different mechanisms. It is emphasized that both the GRB jet and the spin-down wind should have significant observational consequences in the magnetar scenario, and should be focused equally in GRB physics.

Publisher’s Note: Internal x-ray plateau in short GRBs: Signature of supramassive fast-rotating quark stars? [Phys. Rev. D 94 , 083010 (2016)

Publisher’s Note: Internal x-ray plateau in short GRBs: Signature of supramassive fast-rotating quark stars? [Phys. Rev. D <b>94</b> , 083010 (2016)

The Second Plateau in X-Ray Afterglow Providing Additional Evidence for Rapidly Spinning Magnetars as the GRB Central Engine

Abstract Evidence for the central engine of gamma-ray bursts (GRBs) has been collected in the Neil Gehrels Swift data. For instance, some GRBs show an internal X-ray plateau followed by very steep decay, which is difficult interpret within the framework of a black hole (BH) central engine, but is consistent within a rapidly spinning magnetar engine picture. The very steep decay at the end of the plateau suggests a sudden cessation of the central engine, which is explained as the collapse of a supramassive magnetar into a BH when it spins down. Here we propose that some additional evidence, such as a second X-ray plateau feature, would show up if the fallback accretion could activate the newborn BH and sufficient energy could be transferred from the newborn BH to the GRB blast wave. With a systematic data analysis for all long GRBs, we find three candidates in the Swift sample, i.e., GRBs 070802, 090111, and 120213A, whose X-ray afterglow lightcurves contain two plateaus, with the first one being an internal plateau. We find that in a fairly loose and reasonable parameter space, the second X-ray plateau data for all 3 GRBs could be interpreted with our proposed model. Future observations are likely to discover similar events, which could offer more information on the properties of the magnetar, as well as the newborn BH.

Constraining the Nuclear Equation of State via Gravitational-wave Radiation of Short Gamma-Ray Burst Remnants

Abstract The observed internal plateau of X-ray emission in some short gamma-ray bursts (GRBs) suggests the formation of a remnant supramassive magnetar following a double neutron star (NS) merger. In this paper, we assume that the rotational energy is lost mainly via gravitational-wave radiation instead of magnetic dipole (MD) radiation, and present further constraints on the NS nuclear equation of state (EoS) via mass quadrupole deformation and r-mode fluid oscillations of the magnetar. We present two short GRBs with measured redshifts, 101219A and 160821B, whose X-ray light curves exhibit an internal plateau. This suggests that a supramassive NS may survive as the central engine. By considering 12 NS EoSs, within the mass quadrupole deformation scenario we find that the GM1, DD2, and DDME2 models give an M p band falling within the 2σ region of the proto-magnetar mass distribution for ε = 0.01. This is consistent with the constraints from the MD radiation dominated model of rotational energy loss. However, for an r-mode fluid oscillation model with α = 0.1 the data suggest that the NS EOS is close to the Shen and APR models, which is obviously different from the MD radiation dominated and mass quadrupole deformation cases.