Search

The milestone discovery of GW170817-GRB 170817A-AT 2017gfo has shown that gravitational waves (GWs) could be produced during the merger of a neutron star–neutron star/black hole and that in electromagnetic (EM) waves, a gamma-ray burst (GRB) and a kilonova (KN) are generated in sequence after the merger. Observationally, however, EM properties before the merger phase are still unclear. Here we report a peculiar precursor in a KN-associated long-duration GRB 211211A, providing evidence of the EM before the merger. This precursor lasts ∼0.2 s, and the waiting time between the precursor and the main burst is ∼1 s, comparable to that between GW170817 and GRB 170817A. The spectrum of the precursor could be well fit with a nonthermal cutoff power-law model instead of a blackbody. In particular, a ∼22 Hz quasiperiodic oscillation candidate (∼3 σ) is detected in the precursor. These temporal and spectral properties indicate that this precursor is probably produced by a catastrophic flare accompanied with magnetoelastic or crustal oscillations of a magnetar in a binary compact merger. The strong magnetic field of the magnetar can also account for the prolonged duration of GRB 211211A. However, it poses a challenge to reconcile the rather short lifetime of a magnetar with the rather long spiraling time of a binary neutron star system only by the GW radiation before the merger. [ABSTRACT FROM AUTHOR]

The formation of black hole–neutron star (BH–NS) or BH–BH systems may be accompanied by special supernova (SN) signals, due to the accretion feedback from the companion BH. The additional heating, which is mainly attributed to the Blandford–Payne mechanism, would disrupt the isotropic nature of the luminosity distribution on the surface of the SN ejecta, leading to the appearance of polarization. Here we develop a three-dimensional Monte Carlo polarization simulation code to conduct simulations for these special SNe. We find that the maximum polarization level of ∼2% occurs at the peak time of SN emission in the "close-binary" scenario, while in the "faraway-binary" case maximum polarization (i.e., ∼0.7%) is observed at a considerably later time than the peak of the SN. The magnitude of polarization is dependent on the degree of unevenness in the luminosity distribution and the angle between the line of sight and the equatorial direction. When considering the geometric distortion of SN ejecta at the same time, the magnitude of polarization may either increase (for a oblate ellipsoidal shape) or decrease (for a prolate ellipsoidal shape). The polarization signatures represent a promising auxiliary instrument to facilitate the identification of the companion-fed SNe. Moreover, comparing the event rate of these special SNe with the event rate density of LIGO-Virgo-detected BH–NS/BH systems could further help to distinguish the BH–NS/BH formation channel. [ABSTRACT FROM AUTHOR]

Early optical observations of gamma-ray bursts can significantly contribute to the study of the central engine and physical processes therein. However, of the thousands observed so far, only a few have data at optical wavelengths in the first minutes after the onset of the prompt emission. Here we report on GRB 190106A, whose afterglow was observed in optical bands just 36 s after the Swift/BAT trigger, i.e., during the prompt emission phase. The early optical afterglow exhibits a bimodal structure followed by a normal decay, with a faster decay after ∼ T 0 + 1 day. We present optical photometric and spectroscopic observations of GRB 190106A. We derive the redshift via metal absorption lines from Xinglong 2.16 m/BFOSC spectroscopic observations. From the BFOSC spectrum, we measure z = 1.861 ± 0.002. The double-peak optical light curve is a significant feature predicted by the reverse-forward external-shock model. The shallow decay followed by a normal decay in both the X-ray and optical light curves is well explained with the standard forward-shock model with late-time energy injection. Therefore, GRB 190106A offers a case study for GRB emission from both reverse and forward shocks. [ABSTRACT FROM AUTHOR]

Recently, some gamma-ray bursts (GRBs) whose light curves consist of repeating emission episodes with similar temporal profiles have attracted extensive attention. They are proposed to be candidates of millilensing events, although smoking gun evidence is lacking, since there are no redshift measurements and no angular offset detections for any of these candidates. Here we show that without invoking gravitational lensing, the repeating light-curve properties of these GRBs could also be interpreted under the jet precession model, as long as the detectable period in every precession circle is less than the precession period, and the precession period is close to the jet emission duration. By fitting the gamma-ray light curves of these GRBs, we suggest that the jet precession angle for these bursts should be relatively small (e.g., θ p < 5.°3), and the jet structure for these bursts are more likely Gaussian. The results suggest us to be careful when identifying millilensing GRBs. Multiband afterglow data and especially angular offset detections are essential to provide comprehensive justification for this identification. [ABSTRACT FROM AUTHOR]

GRB 221009A is a bright gamma-ray burst (GRB) with isotropic energy larger than 1054 erg. Its fairly low redshift makes it a promising candidate for high-energy neutrino detection. However, a neutrino search for this GRB reported by the IceCube collaboration yielded a null result. In this paper, we utilize the upper limit from the IceCube observation to test different GRB prompt emission models. We find that, at least for this specific burst, the dissipative photosphere model could be ruled out in a large parameter space. The internal-shock model can survive only with a large bulk motion Lorentz factor Γ, where the most stringent and conservative constraints are Γ > ∼ 450 and Γ > ∼ 200, respectively. Also, the ratio of the total dissipated energy that goes into the protons and electrons (ϵ p / ϵ e ) can be constrained with a given Γ. For Γ < 400, ϵ p / ϵ e < 10 is required. For the Internal-collision-induced Magnetic Reconnection and Turbulence (ICMART) model, the constraint from GRB 221009A is modest. Under the ICMART model, only for extreme situations when most dissipated energy deposit into protons and all accelerated protons are suitable for producing neutrinos, a slightly large bulk motion (Γ > ∼ 250) is required. [ABSTRACT FROM AUTHOR]

Millilensing of gamma-ray bursts (GRBs) is expected to manifest as multiple emission episodes in a single triggered GRB with similar light-curve patterns and similar spectrum properties. Identifying such lensed GRBs could help improve constraints on the abundance of compact dark matter. Here we present a systemic search for millilensing among 3000 GRBs observed by the Fermi GBM up to 2021 April. Eventually we find four interesting candidates by performing an autocorrelation test, hardness test, and time-integrated/resolved spectrum test. GRB 081126A and GRB 090717A are ranked as the first-class candidates based on their excellent performance in both temporal and spectrum analysis. GRB 081122A and GRB 110517B are ranked as the second-class candidates (suspected candidates), mainly because their two emission episodes show clear deviations in part of the time-resolved spectrum or in the time-integrated spectrum. Considering a point-mass model for the gravitational lens, our results suggest that the density parameter of lens objects with mass M L ∼ 106 M ⊙ is larger than 1.5 Ă— 10âˆ'3. [ABSTRACT FROM AUTHOR]

As the only known intelligent civilization, human beings are always curious about the existence of other communicating extraterrestrial intelligent civilizations (CETIs). Based on the latest astrophysical information, we carry out Monte Carlo simulations to estimate the number of possible CETIs within our Galaxy and the communication probability among them. Two poorly known parameters have a great impact on the results. One is the probability of life appearing on terrestrial planets and eventually evolving into a CETI (f c ), and the other determines at what stage of their host star’s evolution CETIs would be born (F). In order to ensure the completeness of the simulation, we consider a variety of combinations of f c and F. Our results indicate that for optimistic situations (e.g., F = 25% and f c = 0.1%), there could be 42,777 âˆ' 369 + 267 CETIs and they need to survive for 3 âˆ' 2 + 17 yr ( 2000 âˆ' 1400 + 2000 yr) to achieve one-way communication (two-way communication). In this case, human beings need to survive 0.3 âˆ' 0.298 + 0.6 Myr to receive one alien signal. For pessimistic situations (e.g., F = 75% and f c = 0.001%), only 111 âˆ' 17 + 28 CETIs could be born and they need to survive for 0.8 âˆ' 0.796 + 1.2 Myr ( 0.9 âˆ' 0.88 + 4.1 Myr) to achieve one-way communication (two-way communication). In this case, human beings need to survive 50 âˆ' 49.6 + 250 Myr to receive one signal from other CETIs. Our results may quantitatively explain why we have not detected any alien signals so far. The uncertainty of the results has been discussed in detail and would be alleviated with the further improvement of our astronomical observation ability in the future. [ABSTRACT FROM AUTHOR]

Over the course of the third observing run of the LIGOâ€"Virgoâ€"KAGRA Collaboration, several gravitational-wave (GW) neutron starâ€"black hole (NSBH) candidates have been announced. By assuming that these candidates are real signals with astrophysical origins, we analyze the population properties of the mass and spin distributions for GW NSBH mergers. We find that the primary BH mass distribution of NSBH systems, whose shape is consistent with that inferred from the GW binary BH (BBH) primaries, can be well described as a power law with an index of α = 4.8 âˆ' 2.8 + 4.5 plus a high-mass Gaussian component peaking at ∼ 33 âˆ' 9 + 14 M ⊙ . The NS mass spectrum could be shaped as a nearly flat distribution between ∼1.0 and 2.1 M ⊙. The constrained NS maximum mass agrees with that inferred from NSs in our Galaxy. If GW190814 and GW200210 are NSBH mergers, the posterior results of the NS maximum mass would be always larger than ∼2.5 M ⊙ and significantly deviate from that inferred in Galactic NSs. The effective inspiral spin and effective precession spin of GW NSBH mergers are measured to potentially have near-zero distributions. The negligible spins for GW NSBH mergers imply that most events in the universe should be plunging events, which support the standard isolated formation channel of NSBH binaries. More NSBH mergers to be discovered in the fourth observing run would help to more precisely model the population properties of cosmological NSBH mergers. [ABSTRACT FROM AUTHOR]

The possibility that primordial black holes (PBHs) form some part of dark matter has been considered for a long time but poorly constrained over a wide mass range. Fast radio bursts (FRBs) are bright radio transients with millisecond duration. Their lensing effect has been proposed to be one of the cleanest probes for constraining the presence of PBHs in the stellar-mass window. In this paper, we first apply the normalized cross-correlation algorithm to search and identify candidates for lensed FRBs in the latest public FRB observations, i.e., 593 FRBs, which mainly consist of the first Canadian Hydrogen Intensity Mapping Experiment FRB catalog, and then derive constraints on the abundance of PBHs from the null search result of the lensing signature. For a monochromatic mass distribution, the fraction of dark matter made up of PBHs could be constrained to ≤87% for ≥500 M ⊙ at the 95% confidence level by assuming signal-to-noise ratios dependent on the flux ratio threshold for each FRB and that apparently one-off events are intrinsic single bursts. This result would be improved by a factor of 3 when a conventional constant flux ratio threshold is considered. Moreover, we derive constraints on PBHs with a log-normal mass function naturally predicted by some popular inflation models and often investigated with gravitational-wave detections. We find that, in this mass distribution scenario, the constraint from the current public FRB observations is relatively weaker than the one from gravitational-wave detections. It is foreseen that upcoming complementary multimessenger observations will yield considerable constraints on the possibilities of PBHs in this intriguing mass window. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) at high redshifts are expected to be gravitationally lensed by objects of different mass scales. Other than a single recent claim, no lensed GRB has been detected so far by using gamma-ray data only. In this paper, we suggest that multiband afterglow data might be an efficient way to search for lensed GRB events. Using the standard afterglow model, we calculate the characteristics of the lensed afterglow lightcurves under the assumption of two popular analytic lens models: the point-mass and singular isothermal sphere models. In particular, when different lensed images cannot be resolved, their signals would be superimposed together with a given time delay. In this case, the X-ray afterglows are likely to contain several X-ray flares of similar width in linear scale and similar spectrum, and the optical afterglow lightcurve will show re-brightening signatures. Since the lightcurves from the image arriving later would be compressed and deformed in the logarithmic timescale, the larger time delay (i.e., the larger mass of the lens), the easier it is to identify the lensing effect. We analyzed the archival data of optical afterglows and found one potential candidate of the lensed GRB (130831A) with time delay ∼500 s; however, observations of this event in gamma-ray and X-ray bands seem not to support the lensing hypothesis. In the future, with the cooperation of the all-sky monitoring gamma-ray detectors and multiband sky survey projects, the method proposed in this paper would be more efficient in searching for strongly lensed GRBs. [ABSTRACT FROM AUTHOR]

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 (Ldip) 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 Ldip 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 (), the segment Ldip ∝ t0 could be followed by Ldip ∝ 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. [ABSTRACT FROM AUTHOR]

We present detailed simulations of the kilonova and gamma-ray burst (GRB) afterglow and kilonova luminosity function from black hole–neutron star (BH–NS) mergers, and discuss the detectability of an electromagnetic (EM) counterpart in connection with gravitational wave (GW) detections, GW-triggered target-of-opportunity observations, and time-domain blind searches. The predicted absolute magnitude of BH–NS kilonovae at 0.5 days after the merger falls in the range [−10, −15.5]. The simulated luminosity function contains potential information on the viewing-angle distribution of the anisotropic kilonova emission. We simulate the GW detection rates, detectable distances, and signal duration for future networks of 2nd/2.5th/3rd generation GW detectors. BH–NSs tend to produce brighter kilonovae and afterglows if the BH has a higher aligned spin, and a less massive NS with a stiffer equation of state. The detectability of kilonovae is especially sensitive to the BH spin. If BHs typically have low spins, the BH–NS EM counterparts are hard to discover. For 2nd generation GW detector networks, a limiting magnitude of mlimit ∼ 23–24 mag is required to detect kilonovae even if high BH spin is assumed. Thus, a plausible explanation for the lack of BH–NS-associated kilonova detection during LIGO/Virgo O3 is that either there is no EM counterpart (plunging events) or the current follow-ups are too shallow. These observations still have the chance to detect the on-axis jet afterglow associated with a short GRB or an orphan afterglow. Follow-up observations can detect possible associated short GRB afterglows, from which kilonova signatures may be studied. For time-domain observations, a high-cadence search in redder filters is recommended to detect more BH–NS-associated kilonovae and afterglows. [ABSTRACT FROM AUTHOR]

Model-independent estimation of H0 and ΩK can provide clues about the origin of the intractable Hubble constant tension and the current cosmic-curvature crisis. Strongly lensed fast radio bursts (FRBs) have been proposed as precision probes of the universe since the time delay ∼ (10 days) between images could be precisely measured due to short durations ∼ of this kind of bright radio pulses. Here, on the basis of the distance sum rule, we investigate the capacity of model-independently estimating these two parameters from time-delay distances of strongly lensed FRBs and luminosity distances of the upcoming Wide Field InfraRed Survey Telescope (WFIRST) type Ia supernovae observations. Considering the expected FRB detection rate of upcoming facilities, we find that H0 could be determined to a ∼1% precision and ΩK could be constrained to ∼0.1 simultaneously from 10 lensed FRBs. These estimations, which are independent of the energy contents of the universe and validity of Einstein's equation on cosmological scales, will be complementary to popular probes and of great importance for clarifying the current crises of cosmology. [ABSTRACT FROM AUTHOR]

Gravitational wave (GW) events, produced by the coalescence of binary neutron stars (BNSs), can be treated as the standard sirens to probe the expansion history of the universe, if their redshifts can be determined from electromagnetic (EM) observations. For the high-redshift (z ≳ 0.1) events, the short γ-ray bursts (sGRBs) and the afterglows are always considered as the primary EM counterparts. In this paper, by investigating various models of sGRBs and afterglows, we discuss the rates and distributions of the multimessenger observations of BNS mergers using GW detectors in the second-generation (2G), 2.5G, and 3G era with detectable sGRBs and afterglows. For instance, for the Cosmic Explorer GW detector, the rate is about 300–3500 yr−1 with a GECAM-like detector for γ-ray emissions and an LSST/WFST detector for optical afterglows. In addition, we find that these events have redshifts z ≲ 2 and inclination angles ι ≲ 20°. These results justify the rough estimation in previous works. Considering these events as standard sirens to constrain the equation-of-state parameters of dark energy w0 and wa, we obtain the potential constraints of Δw0 ≃ 0.02–0.05 and Δwa ≃ 0.1–0.4. [ABSTRACT FROM AUTHOR]

The discovery of early bumps in some type-I superluminous supernovae (SLSNe-I) before the main peaks offers an important clue to their energy source mechanisms. In this paper, we updated an analytic magnetar-powered model for fitting the multiband light curves of double-peaked SLSNe-I. The early bump is powered by magnetar-driven shock-breakout thermal emission, and the main peak is powered by a radiative diffusion through the supernova (SN) ejecta as in the standard magnetar-powered model. Generally, the diffusive luminosity is greater than the shock-breakout luminosity at the early time, which usually makes the shock-breakout bumps unclear to observe. To obtain a clear double-peaked light curve, inefficient magnetar heating at early times is required. This model is applied to three well-observed double-peaked SLSNe-I (i.e., SN2006oz, LSQ14bdq, and DES14Xtaz). We find that a relatively massive SN ejecta with Mej ≃ 10.2–18.1M⊙ and relatively large kinetic energy of SN ejecta erg are required, and the thermalization efficiency of the magnetar heating is suppressed before tdelay, which is in the range of ≃15–43 days. The model can reproduce the observed light curves well, with a reasonable and similar set of physical parameters for both the early bump and the main peak, strengthening support for the magnetar-powered model. In the future, modeling of the double-peaked SLSNe-I will become more feasible as more events are discovered before the early bump. [ABSTRACT FROM AUTHOR]

A rapidly rotating and highly magnetized neutron star (NS) could be formed from explosive phenomena such as superluminous supernovae and gamma-ray bursts. This newborn NS can substantially influence the emission of these explosive transients through its spin-down. The spin-down evolution of the NS can sometimes be affected by fallback accretion, although it is usually regulated by the magnetic dipole radiation and gravitational wave radiation of the NS. Under appropriate conditions, the accreting material can be first ejected and subsequently recycled back, so that the accretion disk can remain in a quasi-steady state for a long time. Here we describe the interaction of the NS with such a propeller-recycling disk and their coevolution. Our result shows that the spin-down of the NS can be initially dominated by the propeller, which prevents the disk material from falling onto the NS until hundreds or thousands of seconds later. It is suggested that the abrupt fall of the disk material onto the NS could significantly suppress the magnetic dipole radiation and then convert the NS from a normal magnetar to a low-field magnetar. This evolution behavior of the newborn NS can help us understand the very different influence of the NS on the early GRB afterglows and the late supernova/kilonova emission. [ABSTRACT FROM AUTHOR]

The successful operation of dedicated detectors has brought us valuable information for understanding the central engine and the progenitor of gamma-ray bursts (GRBs). For instance, the giant X-ray and optical bumps found in some long-duration GRBs (e.g., GRBs 121027A and 111209A) imply that some extended central engine activities, such as the late X-ray flares, are likely due to the fall-back of progenitor envelope materials. Here we systemically search for long GRBs that consist of a giant X-ray or optical bump from the Swift GRB sample, and eventually we find 19 new possible candidates. The fall-back accretion model could interpret the X-ray and optical bump for all candidates within a reasonable parameter space. Six candidates showing simultaneous bump signatures in both X-ray and optical observations, which could be well fitted at the same time when scaling down the X-ray flux into optical by one order of magnitude, are consistent with the standard Fν ∝ ν1/3 synchrotron spectrum. The typical fall-back radius is distributed around 1010–1012 cm, which is consistent with the typical radius of a Wolf–Rayet star. The peak fall-back accretion rate is in the range of ∼10−11–10−4M⊙ s−1 at time ∼102–105 s, which is relatively easy to fulfill as long as the progenitor's metallicity is not too high. Combined with the sample we found, future studies of the mass supply rate for the progenitors with different mass, metallicity, and angular momentum distribution would help us to better constrain the progenitor properties of long GRBs. [ABSTRACT FROM AUTHOR]

The spectral lags of gamma-ray bursts (GRBs) have been viewed as the most promising probes of the possible violations of Lorentz invariance (LIV). However, these constraints usually depend on the assumption of the unknown intrinsic time lag in different energy bands and the use of a single highest-energy photon. A new approach to test the LIV effects has been proposed by directly fitting the spectral-lag behavior of a GRB with a well-defined transition from positive lags to negative lags. This method simultaneously provides a reasonable formulation of the intrinsic time lag and robust lower limits on the quantum-gravity energy scales (EQG). In this work, we perform a global fitting to the spectral-lag data of GRB 190114C by considering the possible LIV effects based on a Bayesian approach. We then derive limits on EQG and the coefficients of the standard model extension. The Bayes factor output in our analysis shows very strong evidence for the spectral-lag transition in GRB 190114C. Our constraints on a variety of isotropic and anisotropic coefficients for LIV are somewhat weaker than existing bounds, but they can be viewed as comparatively robust and have the promise to complement existing LIV constraints. The observations of GRBs with higher-energy emissions and higher temporal resolutions will contribute to a better formulation of the intrinsic time lag and more rigorous LIV constraints in the dispersive photon sector. [ABSTRACT FROM AUTHOR]

Fast radio bursts (FRBs) are a mysterious astrophysical phenomenon of bright pulses emitted at radio frequencies, and they are expected to be frequently detected in the future. The dispersion measures of FRBs are related to cosmological parameters, thus FRBs have the potential to be developed into a new cosmological probe if their data can be largely accumulated in the future. In this work, we study the capability of future FRB data to improve cosmological parameter estimation in two dynamical dark energy models. We find that the simulated FRB data can break the parameter degeneracies inherent in the current cosmic microwave background (CMB) data. Therefore, the combination of CMB and FRB data can significantly improve the constraints on the Hubble constant and dark energy parameters, compared to those using CMB or FRB alone. If 10,000 FRB events with known redshifts are detected in the future, they would perform better than the baryon acoustic oscillation (BAO) data in breaking the parameter degeneracies inherent in the CMB data. We also find that the combination of FRB and gravitational-wave (GW) standard siren data provides an independent low-redshift probe to verify the results from the CMB and BAO data. For the data combination of CMB, GW, and FRB, it is found that the main contribution to the constraints comes from the CMB and GW data, but the inclusion of the FRB data still can evidently improve the constraint on the baryon density. [ABSTRACT FROM AUTHOR]

The LIGO/Virgo and Fermi collaborations recently reported a possible joint detection of a subthreshold gravitational-wave (GW) event and a subthreshold gamma-ray burst (GRB), GBM-190816, that occurred 1.57 s after the merger. We perform an independent analysis of the publicly available data and investigate the physical implications of this potential association. By carefully studying the following properties of GBM-190816 using Fermi/GBM data, including signal-to-noise ratio, duration, f-parameter, spectral properties, energetic properties, and its compliance with some GRB statistical correlations, we confirm that this event is likely a typical short GRB. Assuming its association with the subthreshold GW event, the inferred luminosity is erg s−1. Based on the available information of the subthreshold GW event, we infer the mass ratio q of the compact binary as (90% confidence interval) according to the reported range of luminosity distance. If the heavier compact object has a mass >3 solar masses, q can be further constrained to. The leading physical scenario invokes an NS–BH merger system with the NS tidally disrupted. Within this scenario, we constrain the physical properties of such a system (including mass ratio q, the spin parameters, and the observer's viewing angle) to produce a GRB. The GW data may also allow an NS–BH system with no tidal disruption of the NS (the plunge events) or a BH–BH merger. We apply the charged compact binary coalescence theory (for both a constant charge and an increasing charge for the merging members) to derive the model parameters to account for GBM-190816 and found that the required parameters are extreme. Finally, we argue that the fact that the observed GW–GRB delay timescale is comparable to that of GW170817/GRB 170817A suggests that the GW–GRB time delay of these two cases is mainly defined by the timescale for the jet to propagate to the energy dissipation/GRB emission site. [ABSTRACT FROM AUTHOR]

In this paper, we explore the viewing angle effect on black hole–neutron star (BH–NS) merger kilonova lightcurves. We extrapolate the fitting formulae for the mass and velocity of dynamical ejecta across a wide mass ratio range validated with 66 simulations and use them in kilonova lightcurve calculations. The calculated peak luminosity of a BH–NS merger kilonova is typically about a few times 1041 erg s−1, which is always ≲4.5 × 1041 erg s−1. This corresponds to AB absolute magnitudes fainter than ∼−15 mag in the optical and ∼−16 mag in the infrared. The dynamical ejecta usually contribute to the majority of the kilonova emission, as its projected photosphere area is much larger than that of the disk wind outflows. The fitted blackbody temperature and the observed multiband lightcurve shape are insensitive to the line of sight. The peak time of the observed multiband lightcurves, affected by the light-propagation effect, is related to the relative motion direction between the dynamical ejecta and the observer. The predicted peak luminosity, which changes with the projected photosphere area, only varies by a factor of ∼(2–3) (or by ∼1 mag) for different viewing angles. When taking the short-duration gamma-ray burst afterglow into account, for an on-axis geometry, the kilonova emission is usually outshone by the afterglow emission and can only be observed in the redder bands, especially in the K band at late times. Compared with GW 170817/AT 2017gfo, BH–NS merger kilonovae are optically dim but possibly infrared bright, and have lower fitting temperature at the same epoch after the merger. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The post-merger product of the first binary neutron star merger event detected in gravitational waves, GW170817, depends on neutron star equation of state (EoS) and is not well determined. We generally discuss the constraints one may pose on the maximum mass of a non-spinning neutron star, MTOV, based on the observations and some EoS-independent universal relations of rapidly spinning neutron stars. If the merger product is a black hole after a brief hypermassive neutron star (HMNS) phase, we derive at the 2σ (1σ) level. The cases for a massive neutron star (MNS), neither a supramassive neutron star (SMNS) nor even a stable neutron star (SNS) are also allowed by the data. We derive for the SMNS case and for the SNS case, at the 2σ (1σ) confidence level. In the MNS cases, we also discuss the constraints on the neutron star parameters (the dipolar magnetic field strength at the surface Bp and the ellipticity ϵ) that affect the spin down history, by considering different MNS survival times, e.g., 300 s, 1 day, and 155 days after the merger, as suggested by various observational arguments. We find that once an SMNS is formed, without violating the electromagnetic observational constraints, there always exist a set of () parameters that allow the SMNS to survive for 300 s, 1 day, 155 days, or even longer. [ABSTRACT FROM AUTHOR]

We continue our systematic statistical study of various components of gamma-ray burst (GRB) optical light curves. We decompose the early onset bump and the late re-brightening bump with empirical fits and analyze their statistical properties. Among the 146 GRBs that have well-sampled optical light curves, the onset and re-brightening bumps are observed in 38 and 26 GRBs, respectively. It is found that the typical rising and decaying slopes for both the onset and re-brightening bumps are ~1.5 and ~-1.15, respectively. No early onset bumps in the X-ray band are detected to be associated with the optical onset bumps, while an X-ray re-brightening bump is detected for half of the re-brightening optical bumps. The peak luminosity is anti-correlated with the peak time Lp α t -1.81±0.32 p for the onset bumps and Lp ℵ t -0.83±0.17 p for the re-brightening bumps. Both Lp and the isotropic energy release of the onset bumps are correlated with Eγ,iso, whereas no similar correlation is found for the re-brightening bumps. These results suggest that the afterglow onset bumps are likely due to the deceleration of the GRB fireballs. Taking the onset bumps as probes for the properties of the fireballs and their ambient medium, we find that the typical power-law index of the relativistic electrons is 2.5 and the medium density profile behaves as n ℵ r -1 within the framework of the synchrotron external shock models. With the medium density profile obtained from our analysis, we also confirm the correlation between the initial Lorentz factor (Γ0) and Eiso,γ in our previous work. The jet component that produces the re-brightening bump seems to be on-axis and independent of the prompt emission jet component. Its typical kinetic energy budget would be about one order of magnitude larger than the prompt emission component, but with a lower Γ0, typically several tens. [ABSTRACT FROM AUTHOR]

In view of the recent Fermi observations of gamma-ray burst (GRB) prompt emission spectra, we develop a theory of photosphere emission of a hybrid relativistic outflow with a hot fireball component (defined by dimensionless entropy η) and a cold Poynting-flux component (defined by magnetization σ0 at the central engine). We consider the scenarios both without and with sub-photospheric magnetic dissipations. Based on a simplified toy model of jet dynamics, we develop two approaches: a “bottom-up” approach to predict the temperature (for a non-dissipative photosphere) and luminosity of the photosphere emission and its relative brightness for a given pair of (η, σ0); and a “top-down” approach to diagnose central engine parameters (η and σ0) based on the observed quasi-thermal photosphere emission properties. We show that a variety of observed GRB prompt emission spectra with different degrees of photosphere thermal emission can be reproduced by varying η and σ0 within the non-dissipative photosphere scenario. In order to reproduce the observed spectra, the outflows of most GRBs need to have a significant σ, both at the central engine and at the photosphere. The σ value at 1015 cm from the central engine (a possible non-thermal emission site) is usually also greater than unity, so that internal-collision-induced magnetic reconnection and turbulence (ICMART) may be the mechanism to power the non-thermal emission. We apply our top-down approach to GRB 110721A and find that the temporal evolution behavior of its blackbody component can be well interpreted with a time-varying (η, σ0) at the central engine, instead of invoking a varying engine base size r0 as proposed by previous authors. [ABSTRACT FROM AUTHOR]

Recently, both theoretical arguments and observational evidence suggested that a small fraction of fast radio bursts (FRBs) could be associated with gamma-ray bursts (GRBs). If such FRB/GRB association systems are commonly detected in the future, the combination of dispersion measures (DM) derived from FRBs and redshifts derived from GRBs makes these systems a plausible tool to conduct cosmography. We quantify uncertainties in deriving the redshift-dependent as a function of z and test how well dark energy models can be constrained with Monte Carlo simulations. We show that with several tens of FRB/GRB systems potentially detected in a decade or so, one may reach reasonable constraints on wCDM models. When combined with Type Ia supernova (SN Ia) data, unprecedented constraints on the dark energy equation of state may be achieved, thanks to the prospects of detecting FRB/GRB systems at relatively high redshifts. The ratio between the mean value and luminosity distance (DL(z)) is insensitive to dark energy models. This gives the prospect of applying SN Ia data to calibrate using a relatively small sample of FRB/GRB systems, allowing a reliable constraint on the baryon inhomogeneity distribution as a function of redshift. The methodology developed in this paper can also be applied if the FRB redshifts can be measured by other means. Some caveats of putting this method into practice are also discussed. [ABSTRACT FROM AUTHOR]

The continued observations of Sw J1644+57 in X-ray and radio bands accumulated a rich data set to study the relativistic jet launched in this tidal disruption event. The X-ray light curve of Sw J1644+57 from 5-30 days presents two kinds of quasi-periodic variations: a 200 s quasi-periodic oscillation (QPO) and a 2.7 day quasi-periodic variation. The latter has been interpreted by a precessing jet launched near the Bardeen-Petterson radius of a warped disk. Here we suggest that the ∼200 s QPO could be associated with a second, narrower jet sweeping the observer line-of-sight periodically, which is launched from a spinning black hole in the misaligned direction with respect to the black hole's angular momentum. In addition, we show that this two-component jet model can interpret the radio light curve of the event, especially the re-brightening feature starting ∼100 days after the trigger. From the data we infer that inner jet may have a Lorentz factor of Γj ∼ 5.5 and a kinetic energy of Ek, iso ∼ 3.0 × 1052 erg, while the outer jet may have a Lorentz factor of Γj ∼ 2.5 and a kinetic energy of Ek, iso ∼ 3.0 × 1053 erg. [ABSTRACT FROM AUTHOR]

Well-sampled optical light curves of 146 gamma-ray bursts (GRBs) are compiled from the literature. By empirical fitting, we identify eight possible emission components and summarize the results in a “synthetic” light curve. Both optical flare and early shallow-decay components are likely related to long-term central engine activities. We focus on their statistical properties in this paper. Twenty-four optical flares are obtained from 19 GRBs. The isotropic R-band energy is smaller than 1% of Eγ, iso. The relation between the isotropic luminosities of the flares and gamma rays follows LFR, iso∝L1.11 ± 0.27γ, iso. Later flares tend to be wider and dimmer, i.e., wF ∼ tFp/2 and LFR, iso∝[tFp/(1 + z)]–1.15 ± 0.15. The detection probability of the optical flares is much smaller than that of X-ray flares. An optical shallow-decay segment is observed in 39 GRBs. The relation between the break time and break luminosity is a power law, with an index of –0.78 ± 0.08, similar to that derived from X-ray flares. The X-ray and optical breaks are usually chromatic, but a tentative correlation is found. We suggest that similar to the prompt optical emission that tracks γ-rays, the optical flares are also related to the erratic behavior of the central engine. The shallow-decay component is likely related to a long-lasting spinning-down central engine or piling up of flare materials onto the blast wave. Mixing of different emission components may be the reason for the diverse chromatic afterglow behaviors. [ABSTRACT FROM AUTHOR]