Gamma-ray Burst Afterglow Light Curves Research Articles

Multi-messenger prospects for black hole – neutron star mergers in the O4 and O5 runs

The existence of merging black hole-neutron star (BHNS) binaries has been ascertained through the observation of their gravitational wave (GW) signals. However, to date, no definitive electromagnetic (EM) emission has been confidently associated with these mergers. Such an association could help unravel crucial information on these systems, for example, their BH spin distribution, the equation of state (EoS) of the neutron star and the rate of heavy element production. We modeled the multi-messenger (MM) emission from BHNS mergers detectable during the fourth (O4) and fifth (O5) observing runs of the LIGO-Virgo-KAGRA (LVK) GW detector network in order to provide detailed predictions that can help enhance the effectiveness of observational efforts and extract the highest possible scientific information from such remarkable events. Our methodology is based on a population synthesis approach, which includes the modeling of the signal-to-noise ratio of the GW signal in the detectors, the GW-inferred sky localization of the source, the kilonova (KN) optical and near-infrared light curves, the relativistic jet gamma-ray burst (GRB) prompt emission peak photon flux, and the GRB afterglow light curves in the radio, optical, and X-ray bands. The resulting prospects for BHNS MM detections during O4 are not promising, with an LVK GW detection rate of 15.0−8.8+15.4 yr−1, but joint MM rates of ∼10−1 yr−1 for the KN and ∼10−2 yr−1 for the jet-related emission. In O5, we found an overall increase in expected detection rates by around an order of magnitude, owing to both the enhanced sensitivity of the GW detector network and the coming online of future EM facilities. Considering variations in the NS EoS and BH spin distribution, we find that the detection rates can increase further by up to a factor of several tens. Finally, we discuss direct searches for the GRB radio afterglow with large field-of-view instruments during O5 and beyond as a new possible follow-up strategy in the context of ever-dimming prospects for KN detection due to the recession of the GW horizon.

Scaling relations for gamma-ray burst afterglow light curves and centroid motion independent of jet structure and dynamics

ABSTRACT Models for gamma-ray burst afterglow dynamics and synchrotron spectra are known to exhibit various scale invariances, owing to the scale-free nature of fluid dynamics and the power-law shape of synchrotron spectra. Since GRB 170817A, off-axis jet models including a lateral energy structure in the initial outflow geometry have gained in prominence. Here, we demonstrate how the scale invariance for arbitrary jet structure and dynamical stage can be expressed locally as a function of jet temporal light-curve slope. We provide afterglow flux expressions and demonstrate their use to quickly assess the physical implications of observations. We apply the scaling expressions to the Swift X-ray Telescope sample, which shows a spread in observed fluxes, binned by light-curve slope at time of observation, that increases with increasing light-curve slope. According to the scaling relations, this pattern is inconsistent with a large spread in environment densities if these were the dominant factor determining the variability of light curves. We further show how the late deep Newtonian afterglow stage remains scale-invariant but adds distinct spectral scaling regimes. Finally, we show that for given jet structure a universal curve can be constructed of the centroid offset, image size, and ellipticity (that can be measured using very large baseline interferometry) versus observer angle, in a manner independent of explosion energy and circumburst density. Our results apply to any synchrotron transient characterized by a release of energy in an external medium, including supernova remnants, kilonova afterglows, and soft gamma-repeater flares.

The emergence of diffused gamma-ray burst afterglows from the discs of active galactic nuclei

ABSTRACT The discs of active galactic nuclei (AGNs) have emerged as rich environments for the production and capture of stars and the compact objects that they leave behind. These stars produce long gamma-ray bursts (GRBs) at their deaths, while frequent interactions among compact objects form binary neutron stars and neutron star–black hole binaries, leading to short GRBs upon their merger. Predicting the properties of these transients as they emerge from the dense environments of AGN discs is key to their proper identification and to better constrain the star and compact object population in AGN discs. Some of these transients would appear unusual because they take place in much higher densities than the interstellar medium. Others, which are the subject of this paper, would additionally be modified by radiation diffusion, since they are generated within optically thick regions of the accretion discs. Here, we compute the GRB afterglow light curves for diffused GRB sources for a representative variety of central black hole masses and disc locations. We find that the radiation from radio to ultraviolet and soft X-rays can be strongly suppressed by synchrotron self-absorption in the dense medium of the AGN disc. In addition, photon diffusion can significantly delay the emergence of the emission peak, turning a beamed, fast transient into a slow, isotropic, and dimmer one. These would appear as broad-band correlated AGN variability with a dominance at the higher frequencies. Their properties can constrain both the stellar populations within AGN discs and the disc structure.

Robust features of off-axis gamma-ray burst afterglow light curves

ABSTRACT The ultra-relativistic outflows powering gamma-ray bursts (GRBs) acquire angular structure through their interaction with external material. They are often characterized by a compact, nearly uniform narrow core (with half-opening angle θc,{ϵ, Γ}) surrounded by material with energy per unit solid angle ($\epsilon =\epsilon _{\rm c}\Theta _{\epsilon }^{-a}$, where $\Theta _{\lbrace \epsilon ,\Gamma \rbrace }=[1+\theta ^2/\theta _{{\rm c},\lbrace \epsilon ,\Gamma \rbrace }^2]^{1/2}$) and initial specific kinetic energy ($\Gamma _0-1=[\Gamma _{\rm c}-1]\Theta _\Gamma ^{-b}$) declining as power laws. Multiwavelength afterglow light curves of off-axis jets (with viewing angle θobs > θc) offer robust ways to constrain a, b, and the external density radial profile (ρ ∝ R−k), even while other burst parameters may remain highly degenerate. We extend our previous work on such afterglows to include more realistic angular structure profiles derived from three-dimensional hydrodynamic simulations of both long and short GRBs (addressing also jets with shallow angular energy profiles, whose emission exhibits unique evolution). We present afterglow light curves based on our parametrized power-law jet angular profiles for different viewing angles θobs and k = {0, 1, 2}. We identify a unique evolutionary power-law phase of the characteristic synchrotron frequencies (νm and νc) that manifests when the light curve is dominated by emission sensitive to the angular structure of the outflow. We calculate the criterion for obtaining single or double peaked light curves in the general case when θc,Γ ≠ θc,ϵ. We emphasize how the shape of the light curve and the temporal evolution of νm and νc can be used to constrain the outflow structure and potentially distinguish between magnetic and hydrodynamic jets.

A Bayesian Inference Framework for Gamma-ray Burst Afterglow Properties

In the field of multi-messenger astronomy, Bayesian inference is commonly adopted to compare the compatibility of models given the observed data. However, to describe a physical system like neutron star mergers and their associated gamma-ray burst (GRB) events, usually more than ten physical parameters are incorporated in the model. With such a complex model, likelihood evaluation for each Monte Carlo sampling point becomes a massive task and requires a significant amount of computational power. In this work, we perform quick parameter estimation on simulated GRB X-ray light curves using an interpolated physical GRB model. This is achieved by generating a grid of GRB afterglow light curves across the parameter space and replacing the likelihood with a simple interpolation function in the high-dimensional grid that stores all light curves. This framework, compared to the original method, leads to a ∼90× speedup per likelihood estimation. It will allow us to explore different jet models and enable fast model comparison in the future.

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.

GRB Fermi-LAT Afterglows: Explaining Flares, Breaks, and Energetic Photons

Abstract The Fermi-LAT collaboration presented the second gamma-ray burst (GRB) catalog covering its first 10 years of operations. A significant fraction of afterglow-phase light curves in this catalog cannot be explained by the closure relations of the standard synchrotron forward-shock model, suggesting that there could be an important contribution from another process. In view of the above, we derive the synchrotron self-Compton (SSC) light curves from the reverse shock in the thick- and thin-shell regime for a uniform-density medium. We show that this emission could explain the GeV flares exhibited in some LAT light curves. Additionally, we demonstrate that the passage of the forward shock synchrotron cooling break through the LAT band from jets expanding in a uniform-density environment may be responsible for the late time (≈102 s) steepening of LAT GRB afterglow light curves. As a particular case, we model the LAT light curve of GRB 160509A that exhibited a GeV flare together with a break in the long-lasting emission, and also two very high energy photons with energies of 51.9 and 41.5 GeV observed 76.5 and 242 s after the onset of the burst, respectively. Constraining the microphysical parameters and the circumburst density from the afterglow observations, we show that the GeV flare is consistent with an SSC reverse-shock model, the break in the long-lasting emission with the passage of the synchrotron cooling break through the Fermi-LAT band, and the very energetic photons with SSC emission from the forward shock, when the outflow carries a significant magnetic field (R B ≃ 30) and it decelerates in a uniform-density medium with a very low density ( ).

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

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.

The First Day in the Life of a Magnetar: Evolution of the Inclination Angle, Magnetic Dipole Moment, and Braking Index of Millisecond Magnetars during Gamma-Ray Burst Afterglows

Abstract The afterglow emission of some gamma-ray bursts (GRBs) shows a shallow decay (plateau) phase implying continuous injection of energy. The source of this energy is very commonly attributed to the spin-down power of a nascent millisecond magnetar. The magnetic dipole radiation torque is considered to be the mechanism causing the spin-down of the neutron star. This torque has a component working for the alignment of the angle between rotation and the magnetic axis, i.e., the inclination angle, which has been neglected in modeling GRB afterglow light curves. Here, we demonstrate the evolution of the inclination angle and magnetic dipole moment of nascent magnetars associated with GRBs. We constrain the initial inclination angle, magnetic dipole moment, and rotation period of seven magnetars by modeling the seven long-GRB afterglow light curves. We find that, in its first day, the inclination angle of a magnetar decreases rapidly. The rapid alignment of the magnetic and rotation axis may address the lack of persistent radio emission from mature magnetars. We also find that in three cases the magnetic dipole moments of magnetars decrease exponentially to a value a few times smaller than the initial value. The braking index of nascent magnetars, as a result of the alignment and magnetic dipole moment decline, is variable during the afterglow phase and always greater than three.

On the Deceleration and Spreading of Relativistic Jets. I. Jet Dynamics

Abstract Jet breaks in gamma-ray burst (GRB) afterglows provide a direct probe of their collimation angle. Modeling a jet break requires an understanding of the jet spreading process, whereby the jet transitions from a collimated outflow into the spherical Sedov–Taylor solution at late times. Currently, direct numerical calculations are the most accurate way to capture the deceleration and spreading process, as analytical models have previously given inaccurate descriptions of the dynamics. Here (in paper I) we present a new, semi-analytical model built empirically by performing relativistic numerical jet calculations and inferring the relationship between the Lorentz factor, opening angle, and shock radius. We then use the analytical model to calculate the Lorentz factor and jet opening angle as a function of the shock radius and compare to the numerical solutions. Our analytic model provides efficient means of computing synthetic GRB afterglow light curves and spectra, which is the focus of paper II.

Inhomogeneities in the light curves of gamma-ray bursts afterglow

We discuss the inhomogeneous behavior of gamma-ray burst afterglow light curves in optic. We use well-sampled light curves based on mostly our own observations to find and identify deviations (inhomogeneities) from broken power law. By the inhomogeneous behavior we mean flashes, bumps, slow deviations from power law (wiggles) in a light curve. In particular we report parameters of broken power law, describe phenomenology, compare optical light curves with X-ray ones and classify the inhomogeneities. We show that the duration of the inhomogeneities correlates with their peak time relative to gamma-ray burst (GRB) trigger and the correlation is the same for all types of inhomogeneities.

Gamma-Ray Burst Jet Breaks Revisited

Abstract Gamma-ray Burst (GRB) collimation has been inferred with the observations of achromatic steepening in GRB light curves, known as jet breaks. Identifying a jet break from a GRB afterglow light curve allows a measurement of the jet opening angle and true energetics of GRBs. In this paper, we re-investigate this problem using a large sample of GRBs that have an optical jet break that is consistent with being achromatic in the X-ray band. Our sample includes 99 GRBs from 1997 February to 2015 March that have optical and, for Swift GRBs, X-ray light curves that are consistent with the jet break interpretation. Out of the 99 GRBs we have studied, 55 GRBs are found to have temporal and spectral behaviors both before and after the break, consistent with the theoretical predictions of the jet break models, respectively. These include 53 long/soft (Type II) and 2 short/hard (Type I) GRBs. Only 1 GRB is classified as the candidate of a jet break with energy injection. Another 41 and 3 GRBs are classified as the candidates with the lower and upper limits of the jet break time, respectively. Most jet breaks occur at 90 ks, with a typical opening angle θ j = (2.5 ± 1.0)°. This gives a typical beaming correction factor f b − 1 ∼ 1000 for Type II GRBs, suggesting an even higher total GRB event rate density in the universe. Both isotropic and jet-corrected energies have a wide span in their distributions: log(E γ,iso/erg) = 53.11 with σ = 0.84; log(E K,iso/erg) = 54.82 with σ = 0.56; log(E γ /erg) = 49.54 with σ = 1.29; and log(E K/erg) = 51.33 with σ = 0.58. We also investigate several empirical correlations (Amati, Frail, Ghirlanda, and Liang–Zhang) previously discussed in the literature. We find that in general most of these relations are less tight than before. The existence of early jet breaks and hence small opening angle jets, which were detected in the Swfit era, is most likely the source of scatter. If one limits the sample to jet breaks later than 104 s, the Liang–Zhang relation remains tight and the Ghirlanda relation still exists. These relations are derived from Type II GRBs, and Type I GRBs usually deviate from them.

MODELING THE MULTI-BAND AFTERGLOW OF GRB 091127: EVIDENCE OF A HARD ELECTRON ENERGY SPECTRUM WITH AN INJECTION BREAK

The afterglows of gamma-ray bursts (GRBs) are believed to originate from the synchrotron emission of shock-accelerated electrons produced by the interaction between the outflow and the external medium. The accelerated electrons are usually assumed to follow a power-law energy distribution with an index of p. Observationally, although most GRB afterglows have a p larger than 2, there are still a few GRBs suggestive of a hard () electron spectrum. The well-sampled broad-band afterglow data for GRB 091127 show evidence of a hard electron spectrum and strong spectral evolution, with a spectral break moving from high to lower energies. The spectral break evolves very fast and cannot be explained by the cooling break in the standard afterglow model, unless evolving microphysical parameters are assumed. In addition, the multi-band afterglow light curves show an achromatic break at around 33 ks. Based on the model of a hard electron spectrum with an injection break, we interpret the observed spectral break as the synchrotron frequency corresponding to the injection break, and the achromatic break as a jet break caused by the jet-edge effect. It is shown that the spectral evolution and the multi-band afterglow light curves of GRB 091127 can be well reproduced by this model.

EXTREMELY SOFT X-RAY FLASH AS THE INDICATOR OF OFF-AXIS ORPHAN GRB AFTERGLOW

We verified the off-axis jet model of X-ray flashes (XRFs) and examined a discovery of off-axis orphan gamma-ray burst (GRBs) afterglows. The XRF sample was selected on the basis of the following three factors: (1) a constraint on the lower peak energy of the prompt spectrum $E^{src}_{obs}$, (2) redshift measurements, and (3) multi-color observations of an earlier (or brightening) phase. XRF020903 was the only sample selected basis of these criteria. A complete optical multi-color afterglow light curve of XRF020903 obtained from archived data and photometric results in literature showed an achromatic brightening around 0.7 days. An off-axis jet model with a large observing angle (0.21 rad, which is twice the jet opening half-angle, $\theta_{jet}$) can naturally describe the achromatic brightening and the prompt X-ray spectral properties. This result indicates the existence of off-axis orphan GRB afterglow light curves. Events with a larger viewing angle ($>\sim2\theta_{jet}$) could be discovered using an 8-m class telescope with wide field imagers such as Subaru Hyper-Suprime-Cam and the Large Synoptic Survey Telescope.

DISCERNING EMISSION COMPONENTS IN EARLY AFTERGLOW DATA AND CONSTRAINING THE INITIAL LORENTZ FACTOR OF LONG GRB FIREBALL

We prove that both the canonical and single power-law decay X-ray afterglow lightcurves of gamma-ray bursts (GRBs) observed with the Swift X-ray telescope may be an emission component radiated by external shocks prior to the GRB trigger. Our systematical analysis on both the early optical and X-ray afterglow data also indicates that they might be from different components. The detected optical emission possibly is dominated by the afterglow of the GRB fireball. The X-ray afterglows may be detected for some GRBs, but most of the detected X-rays for most GRBs are likely dominated by the prior X-ray component. With the deceleration feature in the early optical afterglow data, we estimate the initial Lorentz factors of the GRBs and discover a tight relation of the Lorentz factor to the isotropic gamma-ray energy.

Afterglow from GRB 070610/Swift J195509.6+261406: An explanation using the fireball model

GRB 070610, which is also named Swift J195509.6+261406, is a peculiar Galactic transient with significant variability on short timescales in both X-ray and optical light curves. One possible explanation is that GRB 070610/Swift J195509.6 + 261406 is a soft gamma-ray repeater (SGR) in our Galaxy. Here, we use the fireball model, which is usually recognized as the standard model of gamma-ray burst (GRB) afterglows, and the energy injection hypothesis to interpret the X-ray and optical afterglow light curves of GRB 070610/Swift J195509.6 + 261406. It is found that the model is generally consistent with observations.

EFFICIENCY OF MAGNETIC TO KINETIC ENERGY CONVERSION IN A MONOPOLE MAGNETOSPHERE

Unconfined relativistic outflows from rotating, magnetized compact objects are often well modeled by assuming that the field geometry is approximately a split-monopole at large radii. Earlier work has indicated that such an unconfined flow has an inefficient conversion of magnetic energy to kinetic energy. This has led to the conclusion that ideal magnetohydrodynamical (MHD) processes fail to explain observations of, e.g., the Crab pulsar wind at large radii where energy conversion appears efficient. In addition, as a model for astrophysical jets, the monopole field geometry has been abandoned in favor of externally confined jets since the latter appeared to be generically more efficient jet accelerators. We perform time-dependent axisymmetric relativistic MHD simulations in order to find steady-state solutions for a wind from a compact object endowed with a monopole field geometry. Our simulations follow the outflow for 10 orders of magnitude in distance from the compact object, which is large enough to study both the initial 'acceleration zone' of the magnetized wind as well as the asymptotic 'coasting zone'. We obtain the surprising result that acceleration is actually efficient in the polar region, which develops a jet despite not being confined by an external medium. Our models contain jets thatmore » have sufficient energy to account for moderately energetic long and short gamma-ray burst (GRB) events ({approx}10{sup 51}-10{sup 52} erg), collimate into narrow opening angles (opening half-angle {theta} {sub j} {approx} 0.03 rad), become matter-dominated at large radii (electromagnetic energy flux per unit matter energy flux {sigma} < 1), and move at ultrarelativistic Lorentz factors ({gamma} {sub j} {approx} 200 for our fiducial model). The simulated jets have {gamma} {sub j}{theta} {sub j} {approx} 5-15, so they are in principle capable of generating 'achromatic jet breaks' in GRB afterglow light curves. By defining a 'causality surface' beyond which the jet cannot communicate with a generalized 'magnetic nozzle' near the axis of rotation, we obtain approximate analytical solutions for the Lorentz factor that fit the numerical solutions well. This allows us to extend our results to monopole wind models with arbitrary magnetization. Overall, our results demonstrate that the production of ultrarelativistic jets is a more robust process than previously thought.« less

THE DYNAMICS AND AFTERGLOW RADIATION OF GAMMA-RAY BURSTS. I. CONSTANT DENSITY MEDIUM

Direct multidimensional numerical simulation is the most reliable approach for calculating the fluid dynamics and observational signatures of relativistic jets in gamma-ray bursts (GRBs). We present a two-dimensional relativistic hydrodynamic simulation of a GRB outflow during the afterglow phase, which uses the fifth-order weighted essentially nonoscillatory scheme and adaptive mesh refinement. Initially, the jet has a Lorentz factor of 20. We have followed its evolution up to 150 years. Using the hydrodynamic data, we calculate synchrotron radiation based upon standard afterglow models and compare our results with previous analytic work. We find that the sideways expansion of a relativistic GRB jet is a very slow process and previous analytic works have overestimated its rate. In our computed light curves, a very sharp jet break is seen and the postbreak light curves are steeper than analytic predictions. We find that the jet break in GRB afterglow light curves is mainly caused by the missing flux when the edge of the jet is observed. The outflow becomes nonrelativistic at the end of the Blandford–McKee phase. But it is still highly nonspherical, and it takes a rather long time for it to become a spherical Sedov–von Neumann–Taylor blast wave. We find that the late-time afterglows become increasingly flatter over time. But we disagree with the common notion that there is a sudden flattening in light curves due to the transition into the Sedov–von Neumann–Taylor solution. We have also found that there is a bump in light curves at very late times (∼1000 days) due to radiation from the counter jet. We speculate that such a counter jet bump might have already been observed in GRB 980703.

ABSOLUTE MAGNITUDE DISTRIBUTION AND LIGHT CURVES OF GAMMA-RAY BURST SUPERNOVAE

Photometry data were collected from the literature and analyzed for supernovae (SNe) that are thought to have a gamma-ray burst (GRB) association. There are several GRBs afterglow light curves that appear to have an SN component. For these light curves, the SN component was extracted and analyzed. An SN light-curve model was used to help determine the peak absolute magnitudes as well as estimates for the kinetic energy, ejected mass, and nickel mass in the explosion. The peak absolute magnitudes are, on average, brighter than those of similar SNe (stripped-envelope SNe) that do not have a GRB association, but this can easily be due to a selection effect. However, the kinetic energies and ejected masses were found to be considerably higher, on average, than those of similar SNe without a GRB association.

Gamma-ray burst afterglows: luminosity clustering at infrared wavelengths?

Context. Clustering in the luminosity of the afterglows of gamma-ray burst has been reported in the optical and X-ray. Aims. We investigate the possibility that a clustering in the luminosity of the afterglows of gamma-ray burst exists in the near infrared ( J , H , K bands). We use observations of events from 1997 to the end of 2007. Methods. We correct the gamma-ray burst afterglow light curve for distance effects and time dilation, and rescale all light curves to a common distance of $z=1$. We used only observations of signals emitted in the near infrared (in the burst frame). Results. We observe a clustering identical to the one observed in the optical and similar to the one observed in X-rays. We thus confirm previous work made in optical wavelengths and set a constraint on the total energy of the fireball.