Central Engine Of Gamma-ray Bursts Research Articles

Neutrino Oscillation Effects on the Luminosity of Neutrino-dominated Accretion Flows around Black Holes

A stellar-mass black hole surrounded by the neutrino-dominated accretion flow (NDAF) has been proposed as the central engine of gamma-ray bursts. In this work, we investigate the neutrino/antineutrino luminosity and annihilation luminosity from the NDAF and pair annihilation model, taking into account the neutrino oscillation above the accretion disk. The disk's hydrodynamical properties are modeled using an empirical solution previously derived with boundary conditions, including the effect of electron degeneracy and neutrino trapping. Our key parameters are the mass accretion rate and the black hole spin given in the ranges of Ṁ=0.1 –10 M ⊙ s−1 and 0 ≤ a < 1, respectively. Without neutrino oscillation, the obtained neutrino/antineutrino luminosity is ∼1051–1053 erg s−1, while the neutrino annihilation luminosity is found to be ∼1046–1051 erg s−1. In the presence of neutrino oscillation in the vacuum limit, the electron neutrino annihilation luminosity decreases by ≲22% through the flavor transformation, while the muon and tau neutrino luminosity can increase by up to ∼45% and 60%, respectively. As a result, the total annihilation luminosity can be reduced by up to ∼19% due to the oscillation process above the disk. Finally, we also investigate the case whereby the charge-conjugation-parity-symmetry-violating (CP-violating) phase is changed from δCP = 0° to 245°. However, our results reveal that the CP-violating phase has minimal impact on the neutrino annihilation luminosity.

GRB 211211A: The Case for an Engine-powered over r-process-powered Blue Kilonova

The recent gamma-ray burst (GRB) GRB 211211A provides the earliest (∼5 hr) data of a kilonova (KN) event, displaying bright (∼1042 erg s−1) and blue early emission. Previously, this KN was explained using simplistic multicomponent fitting methods. Here, in order to understand the physical origin of the KN emission in GRB 211211A, we employ an analytic multizone model for r-process-powered KNe. We find that r-process-powered KN models alone cannot explain the fast temporal evolution and the spectral energy distribution (SED) of the observed emission. Specifically, (i) r-process models require high ejecta mass to match early luminosity, which overpredicts late-time emission, while (ii) red KN models that reproduce late emission underpredict early luminosity. We propose an alternative scenario involving early contributions from the GRB central engine via a late low-power jet, consistent with plateau emission in short GRBs and GeV emission detected by Fermi-LAT at ∼104 s after GRB 211211A. Such late central engine activity, with an energy budget of ∼a few percent of that of the prompt jet, combined with a single red KN ejecta component, can naturally explain the light curve and SED of the observed emission, with the late-jet–ejecta interaction reproducing the early blue emission and r-process heating reproducing the late red emission. This supports claims that late low-power engine activity after prompt emission may be common. We encourage early follow-up observations of future nearby GRBs and compact binary merger events to reveal more about the central engine of GRBs and r-process events.

GRB Afterglows with Energy Injections in AGN Accretion Disks

Active galactic nucleus (AGN) disks are widely considered potential hosts for various high-energy transients, including gamma-ray bursts (GRBs). The reactivation of GRB central engines can provide additional energy to shocks formed during the interaction of the initially ejected GRB jets with the circumburst material, commonly referred to as energy injections. In this paper, we study GRBs occurring in AGN disks within the context of energy injections. We adopt the standard external forward shock (EFS) model and consider both short- and long-duration GRB scenarios. Light curves for two types of radiation, namely, the radiation from the heated disk material (RHDM) and GRB afterglows, are computed. We find that the energy injection facilitates the EFS to break out from the photosphere of the low-density AGN disk at relativistic velocity. Moreover, the energy injection almost does not affect the RHDM but significantly enhances the peak flux of the GRB afterglows.

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.

Ending the prompt phase in photospheric models of gamma-ray bursts

The early steep decay, a rapid decrease in X-ray flux as a function of time following the prompt emission, is a robust feature seen in almost all gamma-ray bursts with early enough X-ray observations. This peculiar phenomenon has often been explained as emission from high latitudes of the last flashing shell. However, in photospheric models of gamma-ray bursts, the timescale of high-latitude emission is generally short compared to the duration of the steep decay phase, and hence an alternative explanation is needed. In this paper we show that the early steep decay can directly result from the final activity of the dying central engine. We find that the corresponding photospheric emission can reproduce both the temporal and spectral evolution observed. This requires a late-time behaviour that should be common to all gamma-ray burst central engines, and we estimate the necessary evolution of the kinetic power and the Lorentz factor. If this interpretation is correct, observation of the early steep decay can give us insights into the last stages of central activity, and provide new constraints on the late evolution of the Lorentz factor and photospheric radius.

Signatures of the Self-organized Criticality Phenomenon in Precursors of Gamma-Ray Bursts

Precursors provide important clues to the nature of gamma-ray burst (GRB) central engines and can be used to contain GRB physical processes. In this Letter, we study the self-organized criticality in precursors of long GRBs in the third Swift/Burst Alert Telescope catalog. We investigate the differential and cumulative size distributions of 100 precursors, including peak flux, duration, rise time, decay time, and quiescent time with the Markov Chain Monte Carlo technique. It is found that all of the distributions can be well described by power-law models and understood within the physical framework of a self-organized criticality system. In addition, we inspect the cumulative distribution functions of the size differences with a q-Gaussian function. The scale-invariance structures of precursors further strengthen our findings. Particularly, similar analyses are made in 127 main bursts. The results show that both precursors and main bursts can be attributed to a self-organized criticality system with the spatial dimension S = 3 and driven by a similar magnetically dominated process.

Magnetar Central Engines in Gamma-Ray Bursts Follow the Universal Relation of Accreting Magnetic Stars

Gamma-ray bursts (GRBs), both long and short, are explosive events whose inner engine is generally expected to be a black hole or a highly magnetic neutron star (magnetar) accreting high-density matter. Recognizing the nature of GRB central engines, and in particular the formation of neutron stars (NSs), is of high astrophysical significance. A possible signature of NSs in GRBs is the presence of a plateau in the early X-ray afterglow. Here we carefully select a subset of long and short GRBs with a clear plateau, and look for an additional NS signature in their prompt emission, namely a transition between the accretion and propeller phases in analogy with accreting, magnetic compact objects in other astrophysical sources. We estimate from the prompt emission the minimum accretion luminosity below which the propeller mechanism sets in, and the NS magnetic field and spin period from the plateau. We demonstrate that these three quantities obey the same universal relation in GRBs as in other accreting compact objects switching from accretion to propeller. This relation provides also an estimate of the radiative efficiency of GRBs, which we find to be several times lower than radiatively efficient accretion in X-ray binaries and in agreement with theoretical expectations. These results provide additional support to the idea that at least some GRBs are powered by magnetars surrounded by an accretion disk.

Gamma-Ray Burst Prompt Emission Spectrum and E p Evolution Patterns in the ICMART Model

Abstract In this paper, we simulate the prompt emission light curve, spectrum, and E p evolution patterns of gamma-ray bursts (GRBs) within the framework of the Internal-collision-induced Magnetic Reconnection and Turbulence (ICMART) model. We show that this model can produce a Band-shaped spectrum, whose parameters (E p, α, β) could follow the typical distribution of GRB observations, as long as the magnetic field and the electron acceleration process in the emission region are under appropriate conditions. On the other hand, we show that for one ICMART event, E p evolution is always a hard-to-soft pattern. However, a GRB light curve is usually composed of multiple ICMART events that are fundamentally driven by the erratic activity of the GRB central engine. In this case, we find that if one individual broad pulse in the GRB light curve is composed of multiple ICMART events, the overall E p evolution could be disguised as an intensity-tracking pattern. Therefore, mixed E p evolution patterns can coexist in the same burst, with a variety of combined patterns. Our results support the ICMART model as a competitive model to explain the main properties of GRB prompt emission. The possible challenges faced by the ICMART model are also discussed in detail.

Jet launching from merging magnetized binary neutron stars with realistic equations of state

We perform general relativistic, magnetohydrodynamic (GRMHD) simulations of binary neutron stars in quasi-circular orbit that merge and undergo delayed or prompt collapse to a black hole (BH). The stars are irrotational and modeled using an SLy or an H4 nuclear equation of state. To assess the impact of the initial magnetic field configuration on jet launching, we endow the stars with a purely poloidal magnetic field that is initially unimportant dynamically and is either confined to the stellar interior or extends from the interior into the exterior as in typical pulsars. Consistent with our previous results, we find that only the BH + disk remnants originating from binaries that form hypermassive neutron stars (HMNSs) and undergo delayed collapse can drive magnetically-powered jets. We find that the closer the total mass of the binary is to the threshold value for prompt collapse, the shorter is the time delay between the gravitational wave peak amplitude and jet launching. This time delay also strongly depends on the initial magnetic field configuration. We also find that seed magnetic fields confined to the stellar interior can launch a jet over $\sim 25\,\rm ms$ later than those with pulsar-like magnetic fields. The lifetime of the jet [$\Delta t\lesssim 150\,\rm ms$] and its outgoing Poynting luminosity [$L_{\rm EM}\sim 10^{52\pm 1}\rm erg/s$] are consistent with typical short gamma-ray burst central engine lifetimes, as well as with the Blandford--Znajek mechanism for launching jets and their associated Poynting luminosities. Our numerical results also suggest that the dynamical ejection of matter can be enhanced by the magnetic field. Therefore, GRMHD studies are required to fully understand kilonova signals from GW170818-like events.

X-ray flares raising upon magnetar plateau as an implication of a surrounding disk of newborn magnetized neutron star

The X-ray flares have usually been ascribed to long-lasting activities of the central engine of gamma-ray bursts (GRBs), e.g., fallback accretion. The GRB X-ray plateaus, however, favor a millisecond magnetar central engine. The fallback accretion can be significantly suppressed due to the propeller effect of a magnetar. Therefore, if the propeller regime cannot resist the mass flow onto the surface of the magnetar efficiently, the X-ray flares raising upon the magnetar plateau would be expected. In this work, such peculiar cases are connected to the accretion process of the magnetars, and an implication for magnetar-disc structure is given. We investigate the repeated accretion process with multi-flare GRB 050730, and give a discussion for the accretion-induced variation of the magnetic field in GRB 111209A. Two or more flares exhibit in the GRB 050730, 060607A and 140304A; by adopting magnetar mass M = 1.4 M ⊙ and radius R = 12 km, the average mass flow rates of the corresponding surrounding disk are 3.53 × 10−4 M ⊙ s−1, 4.23 × 10−4 M ⊙ s−1, and 4.33 × 10−4 M ⊙ s−1, and the corresponding average sizes of the magnetosphere are 5.01 × 106 cm, 6.45 × 106 cm, and 1.09 × 107 cm, respectively. A statistic analysis that contains eight GRBs within 12 flares shows that the total mass loading in single flare is ∼ 2 × 10−5 M ⊙. In the lost mass of a disk, there are about 0.1% used to feed a collimated jet.

Evidence of X-Ray Plateaus Driven by the Magnetar Spindown Winds in Gamma-Ray Burst Afterglows

Abstract The central engine of gamma-ray bursts (GRBs) remains an open and cutting-edge topic in the era of multimessenger astrophysics. X-ray plateaus appear in some GRB afterglows, which are widely considered to originate from the spindown of magnetars. According to the stable magnetar scenario of GRBs, an X-ray plateau and a decay phase ∼t −2 should appear in X-ray afterglows. Meanwhile, the “normal” X-ray afterglow is produced by the external shock from a GRB fireball. We analyze the Neil Gehrels Swift GRB data, then find three gold samples that have an X-ray plateau and a decay phase ∼t −2 superimposed on the jet-driven normal component. Based on these features of the lightcurves, we argue that the magnetars should be the central engines of these three GRBs. Future joint multimessenger observations might further test this possibility, which can then be beneficial to constrain GRB physics.

Constraining properties of GRB central engines with X-ray flares

ABSTRACT X-ray flares in gamma-ray bursts (GRBs) are believed to be generated by the late activities of central engine, and thus provide an useful tool to diagnose the properties of central objects. In this paper, we work on a GRB X-ray flare sample whose bulk Lorentz factors are constrained by two different methods and the jet opening angles are determined by the jet breaks in afterglow light curves. Considering a hyperaccreting stellar-mass black hole (BH) as the central engine of GRBs and the Blandford &amp; Znajek process (BZ) as the jet production mechanism, we constrain the parameters of central engine by using the X-ray flare data. We find that the BZ mechanism is so powerful making it possible to interpret both GRB prompt emissions and bright X-ray flares. The wind parameter (p) and accreted mass (Md) fall into reasonable ranges. Our result is also applied to GRB 170817A. The late X-ray flare in GRB 170817A, if it is true, might not be a BH origin.

Energy Injection Driven by Precessing Jets in Gamma-Ray Burst Afterglows

Abstract Jet precession is considered to universally exist in different-scale astronomical phenomena, including gamma-ray bursts (GRBs). For the long-lived GRB central engine, the relativistic precessing jets will periodically inject kinetic energy into the external shocks, then significantly modulate the shapes of the light curves (LCs) in GRB afterglows. In this paper, we adopt the standard external shock model to investigate the effects of jet precession on GRB X-ray afterglows in cases with different parameters, i.e., the steady or time-dependent jet powers, precession periods, precession angles, and viewing angles. In the case where the jet powers are in steady or slow decay and the jet can sweep across the line of sight, shallow decay (or plateau) segments should appear; otherwise, a giant bump will emerge in the GRB afterglow LCs. We show that jet precession is a new plausible mechanism of the energy injection in GRBs. Moreover, some observed X-ray transients without GRB associations might be powered by the precessing jets.

Two X-Ray Plateaus of Gamma-Ray Bursts: Energy Injection from Nascent Magnetars with an Evolving Magnetic Inclination Angle

Abstract As the central engine of gamma-ray bursts (GRBs), a nascent magnetar can release its rotational energy by magnetic dipole radiation. This energy is injected into the outflow of the GRB jet. With the rapid evolution of the magnetic inclination angle, the perpendicular component of the magnetar’s surface magnetic field decreases rapidly. The energy loss rate of the magnetar is characterized by two different timescales. This peculiar two-stage energy injection model will affect the shape of the X-ray afterglow light curve. It is shown that a new shallow decay phase will emerge following the initial shallow decay phase. As a result, there could be two plateaus in the X-ray light curve. The effects of various physical parameters on the light curves are studied in detail. It is found that the observed afterglows of GRB 161217A and GRB 190114A, both having two plateaus in the light curve, can be well fitted in our framework.

Identifying Black Hole Central Engines in Gamma-Ray Bursts

Abstract The nature of the gamma-ray burst (GRB) central engine still remains an enigma. Entities widely believed to be capable of powering the extreme jets are magnetars and black holes. The maximum rotational energy that is available in a millisecond magnetar to form a jet is ∼1052 erg. We identify eight long GRBs whose jet-opening angle-corrected energetics of the prompt emission episode are &gt;1052 erg with high confidence level and, therefore, their central engines are expected to be black holes. The majority of these GRBs present significant emission in the sub-GeV energy range. The X-ray afterglow light curves of these bursts do not show any shallow decay behavior such as a plateau; however, a few cases exhibit flares and multiple breaks instead of a single power-law decay. For a minimum mass of the black hole (∼2 M ⊙), we find the efficiency of producing a jet from its rotational energy to range between 2% and 270%. Highly energetic jets requiring high efficiencies implies that either the mass of these black holes are much larger or there are, in addition, other sources of energy that power the jet. By considering the Blandford–Znajek mechanism of jet formation, we estimate the masses of these black holes to range between ∼2 and 60 M ⊙. Some of the lighter black holes formed in these catastrophic events are likely candidates to lie in the mass-gap region (2–5 M ⊙).

Testing Blandford–Znajek Mechanism in Black Hole Hyperaccretion Flows for Long-duration Gamma-Ray Bursts

Abstract Long-duration gamma-ray bursts (GRBs) are generally related to the core-collapse of massive stars. In the collapsar scenario, a rotating stellar-mass black hole (BH) surrounded by a hyperaccretion disk has been considered as one of the plausible candidates of GRB central engines. In this paper, we work on a sample including 146 long GRBs with significant jet break features in the multiband afterglows. The jet opening angles can then be obtained by the jet break time. By assuming GRB jets powered by the Blandford–Znajek (BZ) mechanism in the BH hyperaccretion system, we analyze the distributions of the long GRB luminosities and durations in the samples, and constrain the accretion rates for the different BH spins. As a result, we find that the BZ mechanism is so powerful that it is possible to interpret the long GRB prompt emissions within reasonable accretion rates.

Nondetection of the Gamma-Ray Burst X-Ray Emission Line: The Down-Comptonization Effect

Abstract The detection of the gamma-ray burst (GRB) X-ray emission line is important for studying GRB physics and constraining the GRB redshift. Since the line-like feature in the GRB X-ray spectrum was first reported in 1999, several works on line searching have been published over the past two decades. Even though some observations on the X-ray line-like feature were performed, its significance remains controversial to date. In this paper, we utilize the down-Comptonization mechanism and present the time evolution of the Fe Kα line emitted near the GRB central engine. The line intensity decreases with the evolution time, and the time evolution depends on the electron density and the electron temperature. In addition, the initial line with a larger broadening decreases less over time. For instance, when the emission line penetrates material with an electron density above 1012 cm−3 at 1 keV, it generally becomes insignificant enough after 100 s for it not to be detected . The line-like profile deviates from the Gaussian form, and it finally changes to be similar to a blackbody shape at the time of the thermal equilibrium between the line photons and the surrounding material.

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.

Magnetar as Central Engine of Gamma-Ray Bursts: Quasi-universal Jet, Event Rate, and X-Ray Luminosity Function of Dipole Radiations

Abstract Early shallow-decaying X-ray afterglows of gamma-ray bursts (GRBs) may be attributed to the dipole radiations of newly born magnetars. Assuming that the GRB jets powered by magnetars are quasi-universal, we find that the jet structure can be parameterized as a uniform jet with a luminosity of (1σ) and an opening angle (50% confidence level), surrounding by a power-law decay component with an index of (1σ). The inferred local GRB rate is ρ = 9.6 Gpc−3 yr−1 by including both the typical GRBs and LL-GRBs as the same population. The typical viewing angle is 3.3°, and may be 20° ∼ 30° for LL-GRBs. The X-ray luminosity function of the dipole radiation wind can be empirically described by a broken power-law function with indices and broken at . In the case where the wind outflow is collimated and coaxial with the GRB jet, we find that the wind structure is similar to the GRB jet, i.e., , , and . The observed correlation between the prompt gamma-ray luminosity and X-ray luminosity of the wind may result from the viewing angle effect in such a jet–wind system. A discussion on surveys using the X-ray instruments on board the Einstein Probe mission in the soft X-ray band for the jet and wind emission is also presented.

Viscous Evolution of Magnetized Clumps: A Source for X-Ray Flares in Gamma-Ray Bursts

Abstract X-ray flares may indicate the late-time activity of the central engines of gamma-ray bursts. Such long-term activity has been described through some models, one of which is the viscous evolution of the outer disk’s fragments proposed by Perna et al., and developed quantitatively by Dall’Osso et al. Here, we reconstruct the framework of Dall’Osso et al. by taking both small- and large-scale effects of magnetic field into account. To consider the magnetic barrier as a possible mechanism that might govern the accretion process of each magnetized clump, we construct a simple pattern in boundary conditions through which this mechanism might act. Regarding various model parameters, we probe for their influence and follow some key analogies between our model predictions and previous phenomenological estimates, for two different choices of boundary conditions (with and without a magnetic barrier). Our model is remarkably capable of matching the bolometric and X-ray light curves of flares, as well as reproducing their statistical properties, such as the ratios between rise and decay time, width parameter and peak time, and the power-law correlation between peak luminosity and peak time. Combining our results with the conclusions of previous studies, we are led to interpret a magnetic barrier as a less probable mechanism that might control the evolution of these clumps, especially those created later (or viscously evolved ones).