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If double neutron star mergers leave behind a massive magnetar rather than a black hole, then a bright early afterglow can follow the gravitational wave burst (GWB) even if there is no short gamma-ray burst (SGRB)-GWB association or if there is an association but the SGRB does not beam toward Earth. Besides directly dissipating the proto-magnetar wind, as suggested by Zhang, herewe suggest that themagnetar wind could push the ejecta launched during the merger process and, under certain conditions, would reach a relativistic speed. Such a magnetar-powered ejecta, when interacting with the ambient medium, would develop a bright broadband afterglow due to synchrotron radiation. We study this physical scenario in detail and present the predicted X-ray, optical, and radio light curves for a range of magnetar and ejecta parameters.We show that the X-ray and optical light curves usually peak around the magnetar spin-down timescale (~103-105 s), reaching brightnesses readily detectable by wide-field X-ray and optical telescopes, and remain detectable for an extended period. The radio afterglow peaks later, but is much brighter than the case without a magnetar energy injection. Therefore, such bright broadband afterglows, if detected and combined with GWBs in the future, would be a probe of massive millisecond magnetars and stiff equations of state for nuclear matter. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) have variable light curves. Although most models attribute the observed variability to one physical origin (e.g., central engine activity, clumpy circumburst medium, or relativistic turbulence), some models invoke two physically distinct variability components. We develop a method, namely, the stepwise filter correlation method, to decompose the variability components in a GRB light curve. Based on a low-pass filter technique, we progressively filter the high-frequency signals from the light curve, and then perform a correlation analysis between each adjunct pair of filtered light curves. Our simulations suggest that if a mock light curve contains a "slow" variability component superposed on a rapidly varying time sequence, the correlation coefficient as a function of the filter frequency would display a prominent "dip" feature around the frequency of the slow component. Through simulations, we demonstrate that this method can identify significant clustering structures of a light curve in the frequency domain, and we prove that it can catch superposed signals that are otherwise not easy to retrieve based on other methods (e.g., the power density spectrum analysis method).We apply this method to 266 Burst and Transient Source Experiment bright GRBs. We find that the majority of the bursts have clear evidence of such a superposition effect. We perform a statistical analysis of the identified variability components and discuss the implications for GRB physics. [ABSTRACT FROM AUTHOR]

The joint detection of GW170817 and GRB 170817A indicated that at least a fraction of short gamma-ray bursts (SGRBs) originate from binary neutron star (BNS) mergers. One possible remnant of a BNS merger is a rapidly rotating, strongly magnetized neutron star, which has been discussed as one possible central engine for gamma-ray bursts. For a rapidly rotating magnetar central engine, the deposition of the rotation energy into the ejecta launched from the merger could lead to bright radio emission. The brightness of radio emission years after an SGRB would provide an estimate of the kinetic energy of ejecta and, hence, a possible constraint on the BNS merger product. We perform a more detailed calculation on the brightness of radio emission from the interaction between the merger ejecta and circumburst medium in the magnetar scenario, invoking several important physical processes such as generic hydrodynamics, relativistic effects, and the deep Newtonian phase. We use the model to constrain the allowed parameter space for 15 SGRBs that have late radio observations. Our results show that an injection energy of Einj ∼ 1052 erg is allowed for all the cases, which suggests that the possibility of a supramassive or hypermassive neutron star remnant is not disfavored by the available radio data. [ABSTRACT FROM AUTHOR]

Based on the early-year observations from Neil Gehrels Swift Observatory, Liang et al. performed a systematic analysis for the shallow decay component of gamma-ray bursts (GRBs) X-ray afterglow, in order to explore its physical origin. Here we revisit the analysis with an updated sample (with Swift/XRT GRBs between 2004 February and 2017 July). We find that with a larger sample, (1) the distributions of the characteristic properties of the shallow decay phase (e.g., tb, SX, ΓX,1, and αX,1) still accord with normal or lognormal distribution; (2) ΓX,1 and Γγ still show no correlation, but the tentative correlations of durations, energy fluences, and isotropic energies between the gamma-ray and X-ray phases still exist; (3) for most GRBs, there is no significant spectral evolution between the shallow decay segment and its follow-up segment, and the latter is usually consistent with the external-shock models; (4) assuming that the central engine has a power-law luminosity release history as , we find that the value q is mainly distributed between −0.5 and 0.5, with an average value of 0.16 ± 0.12; (5) the tentative correlation between and disappears, so that the global three-parameter correlation () becomes less significant; (6) the anticorrelation between LX and and the three-parameter correlation () indeed exist with a high confidence level. Overall, our results are generally consistent with Liang et al., confirming their suggestion that the shallow decay segment in most bursts is consistent with an external forward shock origin, probably due to a continuous energy injection from a long-lived central engine. [ABSTRACT FROM AUTHOR]

The excessive dispersion measure (DM) of fast radio bursts (FRBs) has been proposed to be a powerful tool to study intergalactic medium (IGM) and to perform cosmography. One issue is that the fraction of baryons in the IGM, fIGM, is not properly constrained. Here, we propose a method of estimating fIGM using a putative sample of FRBs with the measurements of both DM and luminosity distance dL. The latter can be obtained if the FRB is associated with a distance indicator (e.g., a gamma-ray burst or a gravitational-wave event), or the redshift z of the FRB is measured and dL at the corresponding z is available from other distance indicators (e.g., SNe Ia) at the same redshift. As dL/DM essentially does not depend on cosmological parameters, our method can determine fIGM independent of cosmological parameters. We parameterize fIGM as a function of redshift and model the DM contribution from a host galaxy as a function of star formation rate. Assuming fIGM has a mild evolution with redshift with a functional form and by means of Monte Carlo simulations, we show that an unbiased and cosmology-independent estimate of the present value of fIGM with a ∼12% uncertainty can be obtained with 50 joint measurements of dL and DM. In addition, such a method can also lead to a measurement of the mean value of DM contributed from the local host galaxy. [ABSTRACT FROM AUTHOR]

An X-ray plateau followed by a steep decay (“internal plateau”) has been observed in both long and short gamma-ray burst (GRBs), implying that a millisecond magnetar operates in some GRBs. The sharp decay at the end of the plateau, marking the abrupt cessation of the magnetar’s central engine, has been considered the collapse of a supra-massive magnetar into a black hole (BH) when it spins down. If this “internal plateau” is indeed evidence of a magnetar central engine, the natural expectation in some candidates would be a signature from the newborn BH. In this work, we find that GRB 070110 is a particular case which shows a small X-ray bump following its “internal plateau.” We interpret the plateau as a spin-down supra-massive magnetar and the X-ray bump as fallback BH accretion. This indicates that a newborn BH is likely active in some GRBs. Therefore, GRB 070110-like events may provide further support to the magnetar central engine model and enable us to investigate the properties of the magnetar as well as the newborn BH. [ABSTRACT FROM AUTHOR]

Zhang et al. propose to redefine the true γ-ray burst (GRB) central engine activity duration, , by considering the contributions from the prompt γ-ray emission, X-ray flare, and internal plateau features. With a comprehensive study of a large sample of Swift GRBs, it is shown that the distribution in the observer frame consists of a bimodal feature, suggesting the existence of a new population of ultra-long GRBs. In this work, we make a series of further studies on : we update the Swift GRB sample up to 2016 June; we investigate the properties of distribution in the rest frame; we redefine by involving external plateau contributions; and we make a multiple parameter analysis to investigate whether the bursts within the ultra-long population are statistically different in the sense of other features besides the duration distribution. We find that for all situations, the distribution of requires two normal distributions in logarithmic space to provide a good fit both in the observer frame and in the rest frame. Considering the observational gap effect would not completely erase the bimodal distribution feature. However, the bursts within the ultra-long population may have no statistical difference in the sense of other features besides the duration term. We thus suggest that if the ultra-long population of GRBs indeed exists, their central engine and radiation mechanisms should be similar to those of the normal population, but they have a longer central engine activity timescale. [ABSTRACT FROM AUTHOR]

Neutron stars may sustain a non-axisymmetric deformation due to magnetic distortion and are potential sources of continuous gravitational waves (GWs) for ground-based interferometric detectors. With decades of searches using available GW detectors, no evidence of a GW signal from any pulsar has been observed. Progressively stringent upper limits of ellipticity have been placed on Galactic pulsars. In this work, we use the ellipticity inferred from the putative millisecond magnetars in short gamma-ray bursts (SGRBs) to estimate their detectability by current and future GW detectors. For ∼1 ms magnetars inferred from the SGRB data, the detection horizon is ∼30 Mpc and ∼600 Mpc for the advanced LIGO (aLIGO) and Einstein Telescope (ET), respectively. Using the ellipticity of SGRB millisecond magnetars as calibration, we estimate the ellipticity and GW strain of Galactic pulsars and magnetars assuming that the ellipticity is magnetic-distortion-induced. We find that the results are consistent with the null detection results of Galactic pulsars and magnetars with the aLIGO O1. We further predict that the GW signals from these pulsars/magnetars may not be detectable by the currently designed aLIGO detector. The ET detector may be able to detect some relatively low-frequency signals (<50 Hz) from some of these pulsars. Limited by its design sensitivity, the eLISA detector seems to not be suitable for detecting the signals from Galactic pulsars and magnetars. [ABSTRACT FROM AUTHOR]

The jet compositions, central engines, and progenitors of gamma-ray bursts (GRBs) remain open questions in GRB physics. Applying broadband observations, including GRB prompt emission and afterglow properties derived from Fermi and Swift data, as well as from Keck host-galaxy observations, we address these questions for the peculiar, bright GRB 110731A. By using the pair-opacity method, we derive Γ0 > 190 during the prompt emission phase. Alternatively, we derive Γ0 ≈ 580 and Γ0 ≈ 154 by invoking the early-afterglow phase within the homogeneous density and wind cases, respectively. On the other hand, nondetection of a thermal component in the spectra suggests that the prompt emission is likely powered by dissipation of a Poynting-flux-dominated jet leading to synchrotron radiation in an optically thin region. The nondetection of a jet break in the X-ray and optical bands allows us to place a lower limit on the jet opening angle θj > 5.°5. Within a millisecond magnetar central engine scenario, we derive the period P0 and polar magnetic field strength Bp, which have extreme (but still allowed) values. The moderately short observed duration (7.3 s) and relatively large redshift (z = 2.83) place the burst as a “rest-frame short” GRB, so the progenitor of the burst is subject to debate. Its relatively large parameter (ratio of the 1 s peak flux of a pseudo-GRB and the background flux) and a large physical offset from a potential host galaxy suggest that the progenitor of GRB 110731A may be a compact-star merger. [ABSTRACT FROM AUTHOR]

The merger of a double neutron star (NS–NS) binary may result in a rapidly rotating massive NS with an extremely strong magnetic field (i.e., a millisecond magnetar). In this case, the magnetic spin-down of the NS remnant provides an additional source of sustained energy injection, which would continuously power the merger ejecta. The thermal emission from the merger ejecta would give rise to a bright optical “magnetar-powered merger-nova.” In this work, we carry out a complete search for magnetar-powered merger-nova from a Swift short gamma-ray burst sample. We focus on short GRBs with extended emission or internal plateau, which may signify the presence of magnetars as the central engine. We eventually find three candidates of magnetar-powered merger-nova from the late observations of GRB 050724, GRB 070714B, and GRB 061006. With standard parameter values, the magnetar remnant scenario could well interpret the multi-band data of all three bursts, including the extended emission and their late chromatic features in the optical and X-ray data. The peak luminosities of these merger-novae reach several times , more than one order of magnitude brighter than the traditional “kilo-novae” with peak luminosity of . Intense, multi-color, late-time observations of short GRBs are encouraged to identify more merger-novae in the future. [ABSTRACT FROM AUTHOR]

Zhang proposed a type of GRB-less X-ray transient associated with double neutron star (NS–NS) mergers under the conjecture of a rapidly spinning magnetar merger product with the line of sight off the short gamma-ray burst (GRB) jet. We investigate possible light curves of these transients by considering different observers’ viewing angles. We perform Monte Carlo simulations to calculate the peak luminosity function (LF) and event rate density of these X-ray transients. By considering that a fraction of massive neutron stars may be supra-massive and later collapse into black holes after spinning down, we investigate how the predicted LF depends on the equation of state (EoS) of the central object and the geometry of the system. In general, the LF can be fit by two log-normal distributions peaking around and , corresponding to the trapped and free zones, respectively. For the majority of the EoS models, the current non-detection is consistent with having a free zone solid angle, at most a few times the solid angle of the short GRB jet. The event rate density of these X-ray transients is around a few tens of for luminosity above 1045 . We predict that future X-ray telescopes (such as Einstein Probe) with sensitivity would detect as many as several tens of such transients per year per steradian. Within 200 Mpc, the aLIGO average range for NS–NS mergers, the estimated event rate of these transients is about 1 transient per year all sky. [ABSTRACT FROM AUTHOR]

Recently, the first association between an ultra-long gamma-ray burst (GRB) and a supernova was reported, i.e., GRB 111209A/SN 2011kl, enabling us to investigate the physics of central engines or even progenitors for ultra-long GRBs. In this paper, we inspect the broadband data of GRB 111209A/SN 2011kl. The late-time X-ray light curve exhibits a GRB 121027A-like fallback bump, suggesting a black hole (BH) central engine. We thus propose a collapsar model with fallback accretion for GRB 111209A/SN 2011kl. The required model parameters, such as the total mass and radius of the progenitor star, suggest that the progenitor of GRB 111209A is more likely a Wolf–Rayet star instead of a blue supergiant, and the central engine of this ultra-long burst is a BH. The implications of our results are discussed. [ABSTRACT FROM AUTHOR]

Within the framework of the external shock model of gamma-ray burst (GRB) afterglows, we perform a morphological analysis of the early-optical light curves to directly constrain model parameters. We define four morphological types, i.e., the reverse shock-dominated cases with/without the emergence of the forward shock peak (Type I/Type II), and the forward shock-dominated cases without/with νm crossing the band (Type III/IV). We systematically investigate all of the Swift GRBs that have optical detection earlier than 500 s and find 3/63 Type I bursts (4.8%), 12/63 Type II bursts (19.0%), 30/63 Type III bursts (47.6%), 8/63 Type IV bursts (12.7%), and 10/63 Type III/IV bursts (15.9%). We perform Monte Carlo simulations to constrain model parameters in order to reproduce the observations. We find that the favored value of the magnetic equipartition parameter in the forward shock () ranges from 10−6 to 10−2, and the reverse-to-forward ratio of ϵB () is about 100. The preferred electron equipartition parameter value is 0.01, which is smaller than the commonly assumed value, e.g., 0.1. This could mitigate the so-called “efficiency problem” for the internal shock model, if ϵe during the prompt emission phase (in the internal shocks) is large (say, ∼0.1). The preferred value is in agreement with the results in previous works that indicate a moderately magnetized baryonic jet for GRBs. [ABSTRACT FROM AUTHOR]

The Einstein Equivalence Principle (EEP) can be probed with astrophysical sources emitting simultaneously different types of neutral particles, or particles with varying energies, by testing their time of flight through the same gravitational field. Here we use the time delays between correlated photons from cosmological transients to constrain the accuracy of the EEP. We take data from two gamma-ray bursts as an example and, as a lower limit to the theoretical time delays between different energies, we use delays arising from only the gravitational field of our own galaxy. We then show that the parameterized post-Newtonian parameter γ is the same for photons over energy ranges between eV and MeV and between MeV and GeV to a part in 10−7, which is at least one order of magnitude better than previous limits. Combining this bound on the wavelength dependence of γ with the absolute bound from light-deflection measurements at optical (eV) wavelengths, we thus extend this absolute bound on γ to GeV energies. [ABSTRACT FROM AUTHOR]

In the quantum theory of gravity, for photons we expect the Lorentz Invariance Violation (LIV) and the modification of the dispersion relation between energy and momentum. The effect of the energy-dependent velocity due to the modified dispersion relation for photons was studied in the standard cosmological context by using a sample of gamma-ray bursts (GRBs). In this paper we mainly discuss the possible LIV effect of using different cosmological models for the accelerating universe. Due to the degeneracies among model parameters, the GRBs’ time delay data are combined with the cosmic microwave background data from the Planck first-year release, the baryon acoustic oscillation data at six different redshifts, and Union2 Type Ia supernovae data to constrain both the model parameters and the LIV effect. We find no evidence of the LIV. [ABSTRACT FROM AUTHOR]

GRB 080503 is a short gamma-ray burst (GRB) detected by Swift and has been classified as a GRB originating from a compact star merger. The soft extended emission and the simultaneous late re-brightening in both the X-ray and optical afterglow light curves raise interesting questions regarding its physical origin. We show that the broadband data of GRB 080503 can be well explained within the framework of the double neutron star merger model, provided that the merger remnant is a rapidly rotating massive neutron star with an extremely high magnetic field (i.e., a millisecond magnetar). We show that the late optical re-brightening is consistent with the emission from a magnetar-powered “merger-nova.” This adds one more case to the growing sample of merger-novae associated with short GRBs. The soft extended emission and the late X-ray excess emission are well connected through a magnetar dipole spin-down luminosity evolution function, suggesting that direct magnetic dissipation is the mechanism to produce these X-rays. The X-ray emission initially leaks from a hole in the merger ejecta pierced by the short GRB jet. The hole subsequently closes after the magnetar spins down and the magnetic pressure drops below ram pressure. The X-ray photons are then trapped behind the merger-nova ejecta until the ejecta becomes optically thin at a later time. This explains the essentially simultaneous re-brightening in both the optical and X-ray light curves. Within this model, future gravitational-wave sources could be associated with a bright X-ray counterpart along with the merger-nova, even if the short GRB jet beams away from Earth. [ABSTRACT FROM AUTHOR]

The gamma-ray burst (GRB) central engine intrinsic activity time is usually described through either the γ-ray duration T90 or through a generalized burst duration that includes both the γ-ray emission and (when present) an extended flaring X-ray plateau. Here, we define a more specific operational description of , and within the framework of the internal–external shock model, we develop a numerical code to study the relationship between T90 and , as well as between and , for different initial conditions. We find that when s, late internal collisions or refreshed external collisions result in values of and larger than , usually by factors of 2–3. For s, the is always a good estimator for , while T90 can underpredict when the late central engine activity is moderate. We find a clear bimodal distribution for , based on our simulations as well as on the observational data for T90 and . We suggest that is a reliable measure for defining “ultra-long” GRBs. Bursts with T90 of order 103 s need not belong to a special population, while bursts with s, where the late central engine activity is more moderate and shows up in X-rays, may represent a new population. These conclusions are insensitive to the initial conditions assumed in the models. [ABSTRACT FROM AUTHOR]