Interpreting the X-ray afterglows of gamma-ray bursts with radiative losses and millisecond magnetars

ABSTRACT The spin-down energy of millisecond magnetars has been invoked to explain X-ray afterglow observations of a significant fraction of short and long gamma-ray bursts. Here, we extend models previously introduced in the literature, incorporating radiative losses with the spin-down of a magnetar central engine through an arbitrary braking index. Combining this with a model for the tail of the prompt emission, we show that our model can better explain the data than millisecond-magnetar models without radiative losses or those that invoke spin-down solely through vacuum dipole radiation. We find that our model predicts a subset of X-ray flares seen in some gamma-ray bursts. We can further explain the diversity of X-ray plateaus by altering the radiative efficiency and measure the braking index of newly born millisecond magnetars. We measure the braking index of GRB061121 as $n=4.85^{+0.11}_{-0.15}$ suggesting the millisecond-magnetar born in this gamma-ray burst spins down predominantly through gravitational-wave emission.

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Gravitational waves from mountains in newly born millisecond magnetars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab307 ◽

2021 ◽

Vol 502 (4) ◽

pp. 4680-4688

Author(s):

Ankan Sur ◽

Brynmor Haskell

Keyword(s):

Gravitational Wave ◽

Gravitational Waves ◽

Massive Star ◽

Gamma Ray ◽

Dipole Radiation ◽

X Ray ◽

Binary Neutron Star ◽

Spin Period ◽

Lower Maximum ◽

Wave Emission

ABSTRACT In this paper, we study the spin-evolution and gravitational-wave luminosity of a newly born millisecond magnetar, formed either after the collapse of a massive star or after the merger of two neutron stars. In both cases, we consider the effect of fallback accretion; and consider the evolution of the system due to the different torques acting on the star, namely the spin-up torque due to accretion and spin-down torques due to magnetic dipole radiation, neutrino emission, and gravitational-wave emission linked to the formation of a ‘mountain’ on the accretion poles. Initially, the spin period is mostly affected by the dipole radiation, but at later times, accretion spin the star up rapidly. We find that a magnetar formed after the collapse of a massive star can accrete up to 1 M⊙, and survive on the order of 50 s before collapsing to a black hole. The gravitational-wave strain, for an object located at 1 Mpc, is hc ∼ 10−23 at kHz frequencies, making this a potential target for next-generation ground-based detectors. A magnetar formed after a binary neutron star merger, on the other hand, accretes at the most 0.2 M⊙ and emits gravitational waves with a lower maximum strain of the order of hc ∼ 10−24, but also survives for much longer times, and may possibly be associated with the X-ray plateau observed in the light curve of a number of short gamma-ray burst.

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Intermittent hydrodynamic jets in collapsars do not produce GRBs

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1216 ◽

2020 ◽

Vol 495 (1) ◽

pp. 570-577 ◽

Cited By ~ 2

Author(s):

Ore Gottlieb ◽

Amir Levinson ◽

Ehud Nakar

Keyword(s):

High Efficiency ◽

Gamma Ray ◽

Heavy Baryon ◽

Stellar Envelope ◽

X Ray ◽

Radiative Efficiency ◽

Engine Model ◽

Central Engine ◽

Radio Transients ◽

Prompt Emission

ABSTRACT Strong variability is a common characteristic of the prompt emission of gamma-ray bursts (GRB). This observed variability is widely attributed to an intermittency of the central engine, through formation of strong internal shocks in the GRB-emitting jet expelled by the engine. In this paper, we study numerically the propagation of hydrodynamic jets, injected periodically by a variable engine, through the envelope of a collapsed star. By post-processing the output of 3D numerical simulations, we compute the net radiative efficiency of the outflow. We find that all intermittent jets are subject to heavy baryon contamination that inhibits the emission at and above the photosphere well below detection limits. This is in contrast to continuous jets that, as shown recently, produce a highly variable gamma-ray photospheric emission with high efficiency, owing to the interaction of the jet with the stellar envelope. Our results challenge the variable engine model for hydrodynamic jets, and may impose constraints on the duty cycle of GRB engines. If such systems exist in nature, they are not expected to produce bright gamma-ray emission, but should appear as X-ray, optical, and radio transients that resemble a delayed GRB afterglow signal.

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Statistical Analyses of the Energies of X-Ray Plateaus and Flares in Gamma-Ray Bursts

The Astrophysical Journal ◽

10.3847/1538-4357/ac35e7 ◽

2022 ◽

Vol 924 (2) ◽

pp. 69

Author(s):

Shuang-Xi Yi ◽

Mei Du ◽

Tong Liu

Keyword(s):

Gamma Ray ◽

Gamma Ray Bursts ◽

Physical Origin ◽

Radiation Mechanisms ◽

Gaussian Distributions ◽

Emission Energy ◽

X Ray ◽

Central Engine ◽

Positive Correlations ◽

Prompt Emission

Abstract Distinct X-ray plateau and flare phases have been observed in the afterglows of gamma-ray bursts (GRBs), and most of them should be related to central engine activities. In this paper, we collect 174 GRBs with X-ray plateau phases and 106 GRBs with X-ray flares. There are 51 GRBs that overlap in the two selected samples. We analyze the distributions of the proportions of the plateau energy E plateau and the flare energy E flare relative to the isotropic prompt emission energy E γ,iso. The results indicate that they well meet the Gaussian distributions and the medians of the logarithmic ratios are ∼−0.96 and −1.39 in the two cases. Moreover, strong positive correlations between E plateau (or E flare ) and E γ,iso with slopes of ∼0.95 (or ∼0.80) are presented. For the overlapping sample, the slope is ∼0.80. We argue that most of X-ray plateaus and flares might have the same physical origin but appear with different features because of the different circumstances and radiation mechanisms. We also test the applicabilities of two models, i.e., black holes surrounded by fractured hyperaccretion disks and millisecond magnetars, on the origins of X-ray plateaus and flares.

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From the earliest pulses to the latest flares in long gamma-ray bursts

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732270 ◽

2018 ◽

Vol 615 ◽

pp. A80

Author(s):

A. Pescalli ◽

M. Ronchi ◽

G. Ghirlanda ◽

G. Ghisellini

Keyword(s):

Spectral Properties ◽

Gamma Ray ◽

Gamma Ray Bursts ◽

Peak Time ◽

Emission Pulse ◽

X Ray ◽

Temporal Properties ◽

Central Engine ◽

Prompt Emission ◽

Different Origin

The prompt emission of gamma-ray bursts extends from the early pulses observed in γ-rays (>15 keV) to very late flares of X-ray photons (0.3–10 keV). The duration of prompt γ-ray pulses is rather constant, while the width of X-ray flares correlates with their peak time, suggesting a possibly different origin. However, pulses and flares have similar spectral properties. Considering internal and external shock scenarios, we derive how the energy and duration of pulses scale with their time of occurrence, and we compare this with observations. The absence of an observed correlation between the prompt emission pulse duration and its time of occurrence favours an “internal” origin and confirms earlier results. We show that the energetic and temporal properties of X-ray flares are also consistent with being produced by internal shocks between slow fireballs with a small contrast between their bulk Lorentz factors. These results relax the requirement of a long-lasting central engine to explain the latest X-ray flares.

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X-ray flares in early GRB afterglows

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2006.1970 ◽

2007 ◽

Vol 365 (1854) ◽

pp. 1213-1226 ◽

Cited By ~ 33

Author(s):

D.N Burrows ◽

A Falcone ◽

G Chincarini ◽

D Morris ◽

P Romano ◽

...

Keyword(s):

Gamma Ray ◽

Gamma Ray Bursts ◽

Late Time ◽

Emission Mechanism ◽

Time Activity ◽

X Ray ◽

Central Engine ◽

Prompt Emission

The Swift X-ray Telescope (XRT) has discovered that flares are quite common in early X-ray afterglows of gamma-ray bursts (GRBs), being observed in roughly 50% of afterglows with prompt follow-up observations. The flares range in fluence from a few per cent to approximately 100% of the fluence of the prompt emission (the GRB). Repetitive flares are seen, with more than four successive flares detected by the XRT in some afterglows. The rise and fall times of the flares are typically considerably smaller than the time since the burst. These characteristics suggest that the flares are related to the prompt emission mechanism, but at lower photon energies. We conclude that the most likely cause of these flares is late-time activity of the GRB central engine.

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Optical Afterglows as Probes for the Central Engine and Fireball of Gamma-Ray Bursts

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312019928 ◽

2012 ◽

Vol 8 (S290) ◽

pp. 263-264

Author(s):

Liang Li ◽

En-Wei Liang ◽

He Gao ◽

Bing Zhang

Keyword(s):

Statistical Analysis ◽

Gamma Rays ◽

Density Profile ◽

Gamma Ray ◽

Ambient Medium ◽

Gamma Ray Bursts ◽

Prompt Gamma ◽

Medium Density ◽

Central Engine ◽

Prompt Emission

AbstractWell-sampled optical lightcurves of 146 gamma-ray bursts (GRBs) are compiled from literature. We identify possible emission components based on our empirical fits and present statistical analysis for these components. We find that the flares are related to prompt emission, suggesting that they could have the same origin in different episodes. The shallow decay segment is not correlated with prompt gamma-rays. It likely signals a long-lasting injected wind from GRB central engines. Early after onset peak is closely related with prompt emission. The ambient medium density profile is likely n ∝ r−1. No correlation between the late re-brightening bump and prompt gamma-rays or the onset bump is found. They may be from another jet component.

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

The Astrophysical Journal ◽

10.3847/1538-4357/ac2c74 ◽

2021 ◽

Vol 922 (2) ◽

pp. 102

Author(s):

Shu-Jin Hou ◽

Shuang Du ◽

Tong Liu ◽

Hui-Jun Mu ◽

Ren-Xin Xu

Keyword(s):

Gamma Ray ◽

Normal Component ◽

Gamma Ray Bursts ◽

Cutting Edge ◽

Decay Phase ◽

Gamma Ray Burst ◽

X Ray ◽

Central Engine

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.

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THE INTERPLAY OF PROMPT AND AFTERGLOW EMISSION IN GRB 060418

International Journal of Modern Physics D ◽

10.1142/s0218271808012899 ◽

2008 ◽

Vol 17 (09) ◽

pp. 1343-1349 ◽

Cited By ~ 2

Author(s):

S. D. VERGANI ◽

D. MALESANI ◽

E. MOLINARI

Keyword(s):

Gamma Rays ◽

Spectral Properties ◽

Gamma Ray ◽

Early Time ◽

Lorentz Factor ◽

X Ray ◽

Central Engine ◽

Low Energies ◽

Prompt Emission

We present observations of the early afterglow emission of GRB 060418. Thanks to the simultaneous coverage at optical, X-ray and gamma-ray wavelengths, we can detect and separate the external shock emission (visible in the optical and late X-ray data) and the central engine activity (early X and gamma rays). The two components are clearly distinguished based on temporal and spectral properties. The detection of the afterglow onset (in the optical) allows the determination of the fundamental fireball properties, namely its bulk Lorentz factor and total energy. The early time X-ray flare closely resembles the prompt emission gamma-ray pulses in its temporal profile, being wider at low energies and showing lags between the hard and soft bands. This provides a strong suggestion that X-ray flares are a continuation of the prompt emission.

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X-ray plateaus in gamma-ray bursts’ light curves from jets viewed slightly off-axis

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa070 ◽

2020 ◽

Vol 492 (2) ◽

pp. 2847-2857 ◽

Cited By ~ 11

Author(s):

Paz Beniamini ◽

Raphaël Duque ◽

Frédéric Daigne ◽

Robert Mochkovitch

Keyword(s):

Gamma Ray ◽

Large Fraction ◽

Gamma Ray Bursts ◽

Point Of View ◽

Light Curves ◽

Late Time ◽

X Ray ◽

Narrow Region ◽

Central Engine ◽

Γ Rays

ABSTRACT Using multiple observational arguments, recent work has shown that cosmological gamma-ray bursts (GRBs) are typically viewed at angles within, or close to the cores of their relativistic jets. One of those arguments relied on the lack of tens-of-days-long periods of very shallow evolution that would be seen in the afterglow light curves of GRBs viewed at large angles. Motivated by these results, we consider that GRBs efficiently produce γ-rays only within a narrow region around the core. We show that, on these near-core lines of sight, structured jets naturally produce shallow phases in the X-ray afterglow of GRBs. These plateaus would be seen by a large fraction of observers and would last between 102–105 s. They naturally reproduce the observed distributions of time-scales and luminosities as well as the intercorrelations between plateau duration, plateau luminosity, and prompt γ-ray energy. An advantage of this interpretation is that it involves no late-time energy injection which would be both challenging from the point of view of the central engine and, as we show here, less natural given the observed correlations between plateau and prompt properties.

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